Influence of Pozzolan, Slag and Recycled Aggregates on The

materials

Article Inﬂuence of Pozzolan, Slag and Recycled Aggregates on the Mechanical and Durability Properties of Low Cement Concrete

Eliana Soldado 1,*, Ana Antunes 1,2, Hugo Costa 1,2, Ricardo do Carmo 1,2 and Eduardo Júlio 2,3

1 ISEC-Polytechnic Institute of Coimbra, 3045-093 Coimbra, Portugal; [email protected] (A.A.); [email protected] (H.C.); [email protected] (R.d.C.) 2 Civil Engineering Research and Innovation for Sustainability (CERIS), 1049-001 Lisbon, Portugal; [email protected] 3 Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal * Correspondence: [email protected]

Abstract: The sustainability of the construction sector demands the reduction of CO2 emissions. The optimization of the amount of cement in concrete can be achieved either by partially replacing it by additions or by reducing the binder content. The present work aims at optimizing the properties of concrete used in the production of reinforced concrete poles for electrical distribution lines, combining the maximization of compactness with the partial replacement of cement by ﬂy ash, natural pozzolans, and electric furnace slags. Natural aggregates were also partially replaced by recycled ones in mixtures with ﬂy ash. Two types of concrete were studied: a fresh molded one with a dry consistency and a formwork molded one with a plastic consistency. The following properties

were characterized: mechanical properties (flexural, tensile splitting, and compressive strengths, as well as Young’s modulus) and durability properties (capillary water absorption, water penetration depth under pressure, resistance to carbonation, chloride migration, and concrete surface resistivity). Citation: Soldado, E.; Antunes, A.; Costa, H.; do Carmo, R.; Júlio, E. The service life of structures was estimated, taking the deterioration of reinforcement induced by Influence of Pozzolan, Slag and concrete carbonation or chloride attack into account. Results revealed that mixtures with fly ash Recycled Aggregates on the exhibit higher mechanical performance and mixtures with fly ash or pozzolans reveal much higher Mechanical and Durability Properties durability results than the full Portland cement-based mixtures. of Low Cement Concrete. Materials 2021, 14, 4173. https://doi.org/ Keywords: low cement concrete; pozzolan; slag; additions; recycled aggregates; mechanical perfor- 10.3390/ma14154173 mance; durability

Academic Editor: Francisca Puertas

Received: 30 June 2021 1. Introduction Accepted: 23 July 2021 Published: 27 July 2021 In recent years, there has been growing concern about environmental sustainability and global warming causes and effects. The construction sector has a very important role

Publisher’s Note: MDPI stays neutral in reducing the greenhouse effect and promoting sustainability. It is estimated that, in 2018, with regard to jurisdictional claims in cement production reached 3.99 billion tons worldwide [1]. Clinker is responsible for 60 published maps and institutional afﬁl- to 65% of the total emissions of the entire cement production process [2]. The European iations. Cement Association predicts that, with a joint effort by the construction industry, deep CO2 emission cuts can be achieved to lead to carbon neutrality. The same association proposes measures that will lead to a reduction in CO2 emissions of the production process of up to 3.5%, in 2030, and 8%, in 2050. The proposed strategy aims at the use of zero- carbon materials, such as natural, waste materials and by-products from other industries, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. such as pozzolans, ﬂy ash, granulated slag, silica fume, and others, resulting in a good This article is an open access article example of industrial symbiosis [2]. Additionally, the replacement of natural aggregates distributed under the terms and with recycled aggregates is a measure that promotes the sustainability and the circular conditions of the Creative Commons economy, minimizing the environmental impact and excessive extraction of non-renewable Attribution (CC BY) license (https:// resources [3]. This replacement is an urgent measure, as the construction sector is a major creativecommons.org/licenses/by/ consumer of natural resources and the global aggregate production went from 21 billion 4.0/). tons in 2007 to 40 billion tons in 2014 [4].

Materials 2021, 14, 4173. https://doi.org/10.3390/ma14154173 https://www.mdpi.com/journal/materials Materials 2021, 14, 4173 2 of 26

Optimizing concrete’s eco-efficiency should not harm its mechanical and durability properties. Previous studies prove that it is possible to produce concrete with low cement content and good mechanical performance [5,6]. Concrete’s durability can be enhanced by optimizing the packing density, which is highly influenced by the distribution and the size of the particles, reducing the space between them and obtaining a closed and less permeable matrix, also reducing the space needed to be filled with hydration products [6–9]. This process can be simplified through the addition of superplasticizers, reducing the amount of water required by the mixture and thus avoiding water in excess, which is quite harmful to mixtures’ performance [5,10–14]. The most commonly used additions in cement replacement are silica fume, fly ash, ground granulated blast furnace slag, metakaolin, and limestone filler, amongst other industrial by-products. Most studies focus on the addition of fly ash (with a recommended replacement rate of 15–20%, but some studies guarantee up to 50% [15–18]) due to its proven benefits in workability, mechanical performance and economy [5,15,16,18,19]. Being a pozzolanic addition, it has a slow initial reaction and maximum strength at older ages [5,11]. Regarding durability, the literature highlights the good results of fly ash addition in resistance to chlorides, permeability and porosity [15,20–22]. On the contrary, its negative effect on carbonation-induced corrosion is recognized, since it is a sensitive addition and leads to a loss of pH in concrete, suggesting, many times, that it should be used combined with limestone filler [6,11,23]. However, its announced extinction in developed countries underlines the need to develop sustainable and efficient alternatives, such as natural pozzolans or by-products, like industrial slags. In the last decade, research on the properties of concrete with coarse recycled concrete aggregates, as substitutes for coarse natural aggregates, has increased substantially [24–29]. However, the practical use of the recycled aggregates in concrete is still mostly non- structural [4], lacking studies about their variability and reliability, since they are more heterogeneous than natural aggregates [24]. Its use is generally hampered by the higher porosity and reduced stiffness, which also depends on the original concrete quality. These factors may not highly affect mechanical strength for current concrete strength, but may interfere with other parameters, such as Young’s modulus [30]. Nevertheless, the new versions of concrete codes, under development, already contemplate the inclusion of recycled aggregates in concrete, enabling their wide and regulated use [30,31]. The general conclusion from the literature is that there is a decrease in durability with the amount of recycled aggregates included [32]. Though, it is possible to fill this gap by including additions, such as fly ash and silica fume to the concrete [28,33–36]. The aim is to make the matrix more closed and less permeable to aggressive agents. The problems associated with the durability of the reinforced concrete structures, as well as concrete poles, are related with the corrosion risk of the rebars due to permanent exposure to very aggressive environments, such as chloride ions and carbonation. Chloride induced corrosion is one of the most frequent causes of concrete deterioration [33,37], compromising its service life and the safety of the structures, as its load capacity is severely decreased [38–40]. Once corrosion starts, phenomena such as concrete cracking, delami- nation, loss of the steel-concrete bond and loss of the reinforcement section, occur. These phenomena lead to very high repair costs and reduced service life [41,42]. Although the majority of reinforced concrete structures are design to have a service life of 50–100 years, many require sooner intervention and, in severe environments, it may be difficult to avoid steel corrosion within typical service periods of 15–20 years [40,43]. The analysis of capillary absorption in concrete is important because it is through this capillary network that water and aggressive agents are dissolved and transported to the concrete matrix. Carbon dioxide is diffused through this network and dissolved in pore water, forming carbonic acid [44–46], creating a carbonation front that reduces the concrete alkalinity and creates the corrosion risk. In general, 2/3 of the concrete structures are exposed to this aggressive agent, continuously or cyclically [47]. In the literature, durability is usually analyzed in terms of resistance to carbonation, chlorides and permeability [15,16,20,43]. However, that Materials 2021, 14, 4173 3 of 26

analysis is available for concrete with current formulation, but it has not been studied in depth for concretes with high compactness, low cement dosages and specific additions, such as natural pozzolan and electric furnace slag, instead of fly ash. In this sense, this work is part of a research project that aims to improve the formulation, eco-efficiency, performance and durability of a precast reinforced concrete used to produce poles for the electric distribution lines. Its main goal is to develop eco-efficient concrete solutions, combining high compactness with partial cement replacement by natural additions and by waste by-products from other industries and with the inclusion of recycled concrete aggregates. This study intended to develop the following points: (i) assess the influence of the maximum increase in compactness without loss of the required workability; (ii) analyze the effect of different additions (fly ash, natural pozzolan and ground electric furnace slag) as cement substitutes; (iii) evaluate the combination between the considered additions and high compactness in the mechanical and durability properties of concrete; (iv) study the correlation between those properties, namely between different durability tests (capillary absorption, accelerated carbonation, electrical resistivity and chloride ion migration); (v) predict the service life for different exposures to chloride and carbonation ingression. Currently, the production of concrete poles is divided into two types: one with dry consistency for fresh molded concrete (BT series); and other with plastic consistency concrete (MT series), for formwork molded concrete. The mixtures considered as a reference are those currently produced in the industry, with cement content between 360 and 400 kg/m3. For the optimization study, the quantity of binder was established at 350 kg/m3, in order to guarantee the workability and cohesion of the fresh mixtures [6], and the cement content replaced by additions of fly ash, natural pozzolans and electric furnace slag was up to 50%. In the mixtures with fly ash, 20% of the natural aggregates were replaced by recycled concrete aggregates, from industrial waste from the poles production. The presented mixtures are based on previous extensive study performed in mortar matrices, evaluating mechanical and durability performance [48]. Based on the results of that study, the optimized mixtures of each series were then reproduced on concrete, as above mentioned, in which the mechanical properties and durability were evaluated. For instance, when pozzolans are considered, carbonation was not performed in concrete, since it presents reduced resistance in mortar matrices; when electric furnace slags are added, the chlorides migration was not characterized in concrete, since it presents reduced chloride resistance in mortar matrices.

2. Materials and Methods The optimized concrete mixtures with low cement content were formulated and compared with reference fully cement-based mixtures, in order to evaluate the mechanical performance and durability of the optimized concretes. In these mixtures, the following powder materials were selected: CEM II-A/L 42.5R (C); F-type fly ash (FA); Cape Verde natural pozzolans (Poz) and ground electric furnace slag (Slag). The corresponding density values, in kg/dm3, were characterized: 3.08; 2.30; 2.30 and 3.83. The chemical composition of the powders is presented in Table1. The pozzolanic activity indexes, assessed at 90 days [49], resulted in the following values: 0.88; 0.87; 0.70, respectively for fly ash, natural pozzolans and electric furnace slag. The following aggregates were used in the mixtures: fine siliceous sand 0/1 mm; medium siliceous sand 0/4 mm; crushed limestone gravels with two size fractions, 2/5 mm and 6/12 mm; recycled concrete aggregates (RA), from the concrete poles production waste, with two different size fractions (RA1/8 mm, for BT mixtures; RA4/16 mm, for MT mixtures), which were incorporated in 20% of volume replacement. The following densities were characterized for the aggregates, in kg/dm3: 2.63 for sands, 2.66 for gravels and 2.30 for RA. The RA were characterized with absorption of 4%. Tap water from public supply and two types of admixtures were used: HE200P (HE), a water reducer and hardening accelerator for molded cohesive concrete with dry consistency, with density of 1.17 kg/dm3, favoring a better dispersion for the BT Materials 2021, 14, 4173 4 of 26

mixtures; Sky526 (SKY), an ether-polycarboxylate based superplasticizer with a density of 1.06 kg/dm3, to increase plasticity and reduce the water content for the MT mixtures.

Table 1. Chemical composition of the powders, characterized by XRF (X-ray ﬂuorescence).

Test C FA Poz Slag

% SiO2 17.38 54.0 46.18 16.10 % Al2 O3 4.67 22.0 16.44 11.08 % Fe2O3 2.91 8.50 6.30 31.75 % CaO 62.00 6.00 6.11 27.31 % MgO 1.38 1.60 3.09 5.71 % SO3 2.96 0.00 0.01 0.31 % K2O 0.54 1.60 4.19 0.02 % Na2O 0.09 1.00 5.80 0.07 % TiO2 - 1.20 1.91 0.67 %P2O5 - 0.80 0.41 0.50 % MnO - <0.3 (l.q) 0.26 3.53 % SrO - - 0.08 0.03 % CuO - - - 0.04 %V2O5 - - - 0.11 % Cr2O3 - - - 1.92 % ZrO2 - - - 0.03 % Nb2O5 - - - 0.03 % BaO - - - 0.12 % Cl 0.05 - - 0.04 % F - - - 0.65 LOI (%) 8.00 3.90 8.33 −2.08

Regarding concrete mixtures, two types of consistency were considered and divided in two series, BT-dry consistency and MT-plastic consistency. The mixtures were formulated based on the method proposed by Lourenço et al. [50] and developed by Costa [51], which parameters were adopted in order to minimize the cement dosage; however, maximizing compactness, taking a maximum amount of binder of 350 kg/m3 into account. The corresponding amount of cement was between 200 to 250 kg/m3, depending on the selected additions and on the target strength of circa 50 MPa, at 56 days. The resulting proportions of additions varied from 29% to 43% for those mixtures. Admixtures’ dosages were adjusted to achieve the target consistency of each mixture, whose content in relation to the cement weight varied between 2.9% and 4.5% in BT and between 0.7% and 1.1% in MT series. The concrete was mixed with pre-wetting of recycled aggregates, with around 30% of the total water, waiting about 5 min, preventing the admixture from being partially absorbed with water. Then, the remaining solid constituents were placed and mixed, then the remaining water with the diluted admixture was added, mixing until obtaining the pre-defined homogeneity and consistency. The molds were filled and compacted with the vibrating table, being the compacting energy low in MT mixtures and high in BT mixtures. Concrete formulations are presented in Table2, together with water/cement ratio (W/C), water/binder ratio (W/B) and the compactness of each mixture, and compared with the two reference mixtures, BTref and MTref, which have current design parameters of prefabrication concrete company. The air content was targeted to 2.0% in BT and 1.5% in MT mixtures. The effective water of the mixture is calculated from the fluid part of the mixture which, in turn, depends on the established compactness. The absorption water is related to recycled aggregates and with the corresponding amount of water they absorb, which was added taking into account an initial absorption of 4% (previously characterized). Materials 2021, 14, 4173 5 of 26

Table 2. Constituents proportioning and formulation parameters of the studied concrete mixtures.

BT200- BT200- BT250- MT250- MT250- MT250- MT250- Constituents BTref BT200-FA FA-RA Poz Slag MTref FA FA-RA Poz Slag C (kg/m3) 400 200 200 200 250 360 250 250 250 250 FA (kg/m3) - 150 150 - - - 100 100 - - Poz (kg/m3) - - - 150 - - - - 100 - Slag (kg/m3) - - - - 100 - - - - 100 HE (kg/m3) - 7.0 9.1 7.5 7.5 - - - - - Sky (kg/m3) - - - - - 2.2 1.8 2.5 2.5 2.3 1 3 Weff (kg/m ) 160 109 112 114 113 173 143 143 143 143 2 3 Wabs (kg/m ) - - 13 - - - - 13 - - S0/1 (kg/m3) - 402 230 398 430 - 270 221 270 286 S0/4 (kg/m3) 923 403 941 399 431 773 632 579 632 671 C2/5 (kg/m3) 898 1144 368 1138 1120 586 197 440 197 195 C5/12 (kg/m3) - - - - - 457 795 272 795 787 RA1/8 (kg/m3) - - 336 ------RA4/16 (kg/m3) ------329 - - Aggregates 1822 1949 1873 1935 1981 1816 1894 1841 1894 1939 (kg/m3) W/C 0.40 0.55 0.56 0.57 0.45 0.48 0.57 0.57 0.57 0.57 W/B 0.40 0.31 0.32 0.33 0.32 0.48 0.41 0.41 0.41 0.41 Compactness 0.820 0.865 0.860 0.860 0.860 0.805 0.840 0.840 0.840 0.840 1 Effective water; 2 absorption water.

To measure the consistency, the slump test was performed, according to EN 12350-2 [52], or the Vebe test, following EN 12350-3 [53] standard, and the degree of compactability test following EN 12350-4 [54], depending on whether the fresh mixture is plastic or dry, respectively. Along with consistency, in order to adjust the dosages of the constituents, the air content of the mixtures was also measured, according to EN 12350-7 [55] specification. The results of slump tests for MT mixtures resulted in values between 65 and 80 mm, presenting no relevant differences, due to the admixture adjustment. Regarding the compactability test in BT mixtures, it resulted in values between 1.32 and 1.35; the results of Vebe test for the same mixtures ranged between 25 and 33 s, corresponding the longer time to BTref mixture. Figure1 shows the tests to characterize the mechanical properties of the mixtures. The tests to access compressive strength were performed according to EN 12390-3 [56], on cubic specimens with 100 mm of edge, and the results were obtained using three specimens at each age (28 and 90 days). The splitting tensile strength was also characterized according to EN 12390-6 [57] at 28 and 90 days, using three prismatic specimens of 100 × 100 × 200 mm3 and the ﬂexural strength was assessed following EN 12390-5 [58], in two prismatic specimens of 100 × 100 × 500 mm3. The Young’s modulus test was performed at 28 days following the LNEC E 397 [59], using two prismatic specimens of 100 × 100 × 400 mm3. Figure1 shows the tests performed to assess the durability of the mixtures. Capillary water absorption was evaluated in three specimens of 100 × 100 × 200 mm3, according to the LNEC E 393 [60]. The depth of water penetration under pressure was evaluated according to EN 12390-8 [61], using three cubic specimens with 150 mm edge for each mixture, after 28 days of water curing. The determination of carbonation resistance was assessed according LNEC E 391 [62], using two cylindrical samples with 100 mm diameter and 50 mm height, at each considered exposure period (56, 90 and 120 days). It was performed an accelerated laboratory test, exposed to an atmosphere of 5% carbon dioxide and 60% relative humidity. The chloride diffusion coefﬁcient by non-steady state migration was tested according to LNEC E 463 [63], at 56 and 90 days, using three cylindrical specimens also with 100 mm diameter and 50 mm height, for each age. The surface electrical resistivity was evaluated on cylindrical specimens with 100 mm diameter and 200 mm height, at several ages, according to AASHTO T-358 [64]. Materials 2021, 14, 4173 6 of 26 Materials 2021, 14, x FOR PEER REVIEW 6 of 27

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 1. Tests toto characterizecharacterize the mechanicalmechanical properties and durability: ( a) compressivecompressive strength; (b) f ﬂexurallexural strength; strength; ( (cc)) tensile tensile splitting splitting strength; strength; (d ()d Young’s) Young’s modulus; modulus; (e) ( ecapillary) capillary absorption; absorption; (f) water penetration under pressure; (g) carbonation resistance; (h) chloride ion diffusion by non- (f) water penetration under pressure; (g) carbonation resistance; (h) chloride ion diffusion by non- steady state migration; (i) surface electrical resistivity. steady state migration; (i) surface electrical resistivity.

3. Results and D Discussioniscussion 3.1. Mechanical P Propertiesroperties 3.1.1. Tensile SplittingSplitting StrengthStrength The results of average strengths of tensile splitting test, at 28 and 90 days, for BT and MT mixtures, are shown in Figure2 2,, asas wellwell as as the the error error bars. bars. InIn thisthis test,test, onlyonly thethe final final selected mixtures were characterized, since the binding matrix of the previous study was already characterized, characterized, and and no no significant significant variations variations were we registeredre registered depending depending on the on used the usedadditions additions [48]. In[48] BT. In series, BT series, optimized optimized mixtures mixtures with flywith ash fly show ash show lower lower strengths strengths than thanthe reference the reference at 28 at days, 28 days, by 5% by and 5% 18%,and 18%, and and at 90 at days, 90 days, by 7% by and7% and 16%, 16%, respectively respectively for formixtures mixtures BT200-FA BT200-FA and and BT200-FA-RA. BT200-FA-RA. All All the the tensile tensile splitting splitting strengths strengths at at90 90 daysdays are higher than those at 28 days. These results were expected, since BTref has twice the amount higher than those at 28 days. These results were expected, since BTref has twice the of cement of the optimized mixtures. A slight decrease in the mixtures with the inclusion of amount of cement of the optimized mixtures. A slight decrease in the mixtures with the recycled aggregates was noticed, when compared to mixtures with natural aggregates, due inclusion of recycled aggregates was noticed, when compared to mixtures with natural to their lower strength. In MT series, the optimized mixtures stand out for having strengths aggregates, due to their lower strength. In MT series, the optimized mixtures stand out above the reference by 31% and 22%, for MT250-FA and MT250-FA-RA, respectively, at for having strengths above the reference by 31% and 22%, for MT250-FA and MT250-FA- 90 days, reflecting the effect of the increased compactness and of the addition of fly ash. RA, respectively, at 90 days, reflecting the effect of the increased compactness and of the addition of fly ash.

Materials 2021, 14, x FOR PEER REVIEW 7 of 27

Materials 2021, 14, 4173 7 of 26 Materials 2021, 14, x FOR PEER REVIEW 7 of 27

6.0 4.57 4.71 5.06.0 4.38 4.21 4.17 4.00 4.24 4.16 4.57 3.85 4.71 4.38 4.05.0 4.21 3.58 4.173.44 3.26 4.16 4.00 4.24 3.85 3.58

3.04.0 3.44 3.26 (MPa)

2.03.0

(MPa) ctm,sp

f 1.02.0 ctm,sp f 0.01.0 28 days 90 days 0.0 BTref BT200-FA BT200-FA-RA28 days MTref MT250-FA90 daysMT250-FA-RA

BTref BT200-FA BT200-FA-RA MTref MT250-FA MT250-FA-RA Figure 2. Tensile splitting strength for BT and MT mixtures, at 28 and 90 days. Figure 2. Tensile splitting strength for BT and MT mixtures, at 28 and 90 days. FigureThe 2. graph Tensile shown splitting in strength Figure for3 shows BT and the MT ratio mixtures, between at 28 theand tensile90 days. splitting strength test valuesThe and graph the shown codes’ in predicted Figure3 showsvalues. the The ratio prediction between of the this tensile strength splitting followed strength the currenttestThe values design graph and codes shown the for codes’ inall Figure mixtures, predicted 3 shows EC2 values. the[65] ratio and The betweenMC10 prediction [66] the, since oftensile this the strengthsplitting expressions followedstrength of the current design codes for all mixtures, EC2 [65] and MC10 [66], since the expressions tensiletest values splitting and strength the codes’ remains predicted globally values. unaffected The prediction by recycled of this concrete strength aggregates followed in- the of tensile splitting strength remains globally unaffected by recycled concrete aggregates corporatcurrention design [30] . codesThe EC2 for allvalues, mixtures, based EC2 on the[65] compressiveand MC10 [66] strength,, since theconfirm expressions that the of incorporation [30]. The EC2 values, based on the compressive strength, conﬁrm that the testedtensile mixtures splitting present strength consistent remains globallyvalues with unaffected those predicted. by recycled The concrete mixtures aggregates BT200FA, in- tested mixtures present consistent values with those predicted. The mixtures BT200FA, BT200corporat-FA-ionRA [30]and. MTrefThe EC2 have values, a tested based value on ofthe about compressive 10% lower strength, than the confirm predicted that inthe BT200-FA-RA and MTref have a tested value of about 10% lower than the predicted in EC2, EC2,tested whic mixturesh is not presentconsidered consistent significant. values with those predicted. The mixtures BT200FA, BT200which-FA is not-RA considered and MTref signiﬁcant. have a tested value of about 10% lower than the predicted in EC2, which is not considered significant. 1.30

1.201.30

1.101.20 1.04 1.06 1.05 1.02 1.10 1.00 1.031.04 1.06 1.05 ctm,sp_EC2 1.02 1.01 1.02

/f 0.92 0.901.00 1.03 0.88 0.88 ctm,sp_EC2 1.01 1.02 0.90

/f 0.92

ctm,sp 0.88 0.870.88 f 0.800.90 0.90

ctm,sp 0.87 f 0.700.80 BT200- BT200- MT250- MT250- BTref MTref 0.70 FA FA-RA FA FA-RA BT200- BT200- MT250- MT250- BTref 28 days 90 daysMTref FA FA-RA FA FA-RA FigureFigure 3. Ratio 3. Ratio between between tensile tensile splitting splitting28 days strength strength90 days tested tested values values and and those those predicted predicted in inEC2. EC2.

3.1.2. Flexural Strength 3.1.2.Figure Flexural 3. Ratio S betweentrength tensile splitting strength tested values and those predicted in EC2. Figure4 shows graph plots with the flexural strengths, with the error bars, at 28 and Figure 4 shows graph plots with the flexural strengths, with the error bars, at 28 and 3.1.2.90 days, Flexural for the Strength BT and MT series. The optimized mixtures have, in general, higher values 90 days, for the BT and MT series. The optimized mixtures have, in general, higher values than the references, with the exception of the pozzolans addition, which present values than theFigure references, 4 shows with graph the plots exception with the of theflexural pozzolans strengths addition,, with thewhich error present bars, at values 28 and below the reference in 13% and 21%, at 28 and 90 days, respectively, for BT, and similar below90 days, the forreference the BT inand 13% MT and series. 21%, The at optimized28 and 90 days,mixtures respectively, have, in general, for BT, higherand similar values values in MT series, compared to the reference. As mentioned, the opposite effect was valuethans thein MTreferences, series, comparedwith the exception to the reference. of the pozzolans As mentioned, addition, the whichopposite present effect values was found in mixtures with the additions of fly ash and slags. The first ones show increases of foundbelow in the mixtures reference with in the 13% additions and 21%, of atfly 28 ash and and 90 slags. days, The respectively, first ones showfor BT, increases and similar of value3% ands in 20%, MT namelyseries, compared for BT200-FA to the and reference. MT250-FA, As at mentioned, 90 days; the the latter opposite present effect values was 4% foundand 27% in mixtures higher than with the the references, additions of for fly BT250-slag ash and slags. and MT250-slag,The first ones respectively, show increases also of at

Materials 2021, 14, x FOR PEER REVIEW 8 of 27

Materials 2021, 14, 4173 8 of 26 3% and 20%, namely for BT200-FA and MT250-FA, at 90 days; the latter present values 4% and 27% higher than the references, for BT250-slag and MT250-slag, respectively, also at 90 days. As the results also show, the flexural strength was not affected by the incorpo- 90 days. As the results also show, the ﬂexural strength was not affected by the incorporation ration of 20 % of RA, presenting similar values to those of mixtures with fly ash and nat- of 20 % of RA, presenting similar values to those of mixtures with ﬂy ash and natural ural aggregates, showing the same trend of results as those described in other studies aggregates, showing the same trend of results as those described in other studies [67,68]. [67,68].

9.0 9.0 7.99 7.57 7.71 7.38 7.54 7.29 7.14 7.28 6.83 7.5 6.78 7.5 6.64 6.49 6.51 6.26 6.54 5.96 5.85 5.52 5.49 6.0 5.18 6.0

4.5 4.5

(MPa) (MPa)

3.0 3.0

ctm,f ctm,f

ctm,f ctm,f f 1.5 f 1.5

0.0 0.0 28 days 90 days 28 days 90 days

BTref BT200-FA BT200-FA-RA BT200-Poz BT250-Slag MTref MT250-FA MT250-FA-RA MT250-Poz MT250-Slag

(a) (b)

FigureFigure 4. 4. FlexuralFlexural strength strength at at 28 28 and and 90 90 days: days: ( (aa)) BT BT mixtures; mixtures; ( (bb)) MT MT mixtures. mixtures.

AccordingAccording to to EC2 EC2 [65] [65],, the the tensile tensile splitting splitting streng strengthth results results are are usually usually lower lower than than flexural,ﬂexural, a atrend trend also also proved proved with with this this concrete concrete mixtures. mixtures. The Theprediction prediction according according to EC2 to [65]EC2 and [65 ]MC10 and MC10 [66] of [ 66the] oftensile the tensile strength strength is directly is directly related related to the tensile to the tensilesplitting. splitting. Thus, andThus, taking and into taking account into account that the thatresults the of results this test of thisshow test reduced show reduceddispersion, dispersion, it is possible it is topossible conclude to concludethat the tested that the values tested are values in line are with in linethe codes with the prediction. codes prediction.

3.1.3.3.1.3. Compressive Compressive S Strengthtrength TheThe compressive compressive strength strength values values with with the the error error bars bars for for the the BT BT and and MT MT mixtures, mixtures, at at 28 and 90 days, are shown in Figure5. The addition of ﬂy ash stands out, which, due to 28 and 90 days, are shown in Figure 5. The addition of fly ash stands out, which, due to the pozzolanic effect, shows a notable evolution of strengths over time, with high values the pozzolanic effect, shows a notable evolution of strengths over time, with high values at 90 days, 10% and 26% higher than the references, for mixtures BT200-FA and MT250- at 90 days, 10% and 26% higher than the references, for mixtures BT200-FA and MT250- FA, respectively. This pozzolanic effect is less noticeable in mixtures with the addition FA, respectively. This pozzolanic effect is less noticeable in mixtures with the addition of of pozzolans, which have values very close to the references, both in BT and MT series. pozzolans, which have values very close to the references, both in BT and MT series. Mix- Mixtures with slag addition show similar results to the reference, in BT series, and, in MT, tures with slag addition show similar results to the reference, in BT series, and, in MT, show slight improvements, circa 15% and 16%, in relation to the reference, at 28 and 90 days, show slight improvements, circa 15% and 16%, in relation to the reference, at 28 and 90 respectively. Regarding the results of the mixtures with RA, it is proved that the addition days, respectively. Regarding the results of the mixtures with RA, it is proved that the of recycled concrete aggregates decreases the compressive strength, when compared to the addition of recycled concrete aggregates decreases the compressive strength, when com- homonymous mixture with natural aggregates, despite presenting very similar results to pared to the homonymous mixture with natural aggregates, despite presenting very sim- the respective references, in BT series, and increases of 7% and 14% in MT250-FA-RA, at 28 ilar results to the respective references, in BT series, and increases of 7% and 14% in and 90 days, respectively. As several studies concluded [30], considering the same (W/C)eff, MT250the compressive-FA-RA, at strength28 and 90 of days, RAC respectively. is generally lowerAs several than thestudies compressive concluded strength [30], consid- of the eringsame the mixtures same (W/C) only witheff, the natural compressive aggregates. strength However, of RACthis is generally effect depends lower than on the the design compressiveand strength strength of the of concretethe same matrix mixtures and only also onwith the natural quality aggregates. and strength However, of the concrete this effect that dependsorigins the on RA.the Indesign this study, and strength considering of the that concrete the concrete matrix used and to also prepare on the the quality RA has and 20% strengthlower strength of the concrete that the designedthat origins concrete the RA. and In that this is study, mostly considering uncompact that waste the concrete, concrete a usedstrength to prepare lost was the expected. RA has The 20% standard lower strength deviations that of the all mixturesdesigned are concrete reduced, and although that is mostlywith some uncompact exceptions, waste being, concrete, however, a strength within lost the was limits expected. considered The by standard EC 2 [65 deviation]. s of all mixtures are reduced, although with some exceptions, being, however, within the limits considered by EC 2 [65].

MaterialsMaterials 20212021,, 1414,, x 4173 FOR PEER REVIEW 99 of of 27 26 Materials 2021, 14, x FOR PEER REVIEW 9 of 27

80 80 80 71.8 80 71.8 69.2 70 65.3 63.7 63.0 70 69.2 63.2 70 65.3 63.7 60.463.0 70 62.4 63.2 58.5 60.4 62.4 57.458.5 55.8 56.3 55.3 57.4 54.7 56.5 60 57.4 55.8 51.256.3 60 55.3 53.4 57.4 54.7 56.5 60 51.2 60 50.0 53.4 49.6 50.0 49.6 5050 5050

4040 4040

(MPa)

(MPa) (MPa)

3030 (MPa) 3030

f cm

f 20 20 20 f 20 1010 1010 00 00 28 days 90 days 28 days 90 days 28 days 90 days 28 days 90 days BTref BT200-FA BT200-FA-RA BT200-Poz BT250-Slag MTref MT250-FA MT250-FA-RA MT250-Poz MT250-Slag BTref BT200-FA BT200-FA-RA BT200-Poz BT250-Slag MTref MT250-FA MT250-FA-RA MT250-Poz MT250-Slag

((aa)) ((bb)) FigureFigureFigure 5. 5. 5. Compressive CompressiveCompressive strength strength strength at at at 28 28 28 and and and 90 90 90 days: days: days: ( (a (a)a) )BT BT BT mixtures; mixtures; mixtures; ( (b (bb)) )MT MT MT mixtures. mixtures. mixtures.

FigureFigureFigure 66 showsshowsshows thethe evolution evolutionevolution of ofof compressive compressivecompressive strength strengthstrength with withwith age, age,age, of BTofof BT andBT andand MT MT mixtures,MT mix-mixtures,tures,adjusted adjusted adjusted to the to to hardening the the hardening hardening curve curve ofcurve EC2 of of [65 EC2 EC2]. The [65] [65] hardening. .The The hardening hardening curve followedcurve curve followed followed the expressions the the ex- ex- pressionspressionsof current of of codes, current current since codes, codes, the inclusionsince since the the ofinclusioninclusion recycled of of aggregates recycled recycled aggregates aggregates does not have does does a significantnotnot have have a a influ-sig- sig- ence on the prediction of compressive strength [30]. However, the inclusion of pozzolanic nificanificantnt influence influence on on the the prediction prediction of of compressive compressive strength strength [30] [30]. .However, However, the the inclusion inclusion additions on the binder powder does influence the curve shape. For this reason, both series ofof pozzolanicpozzolanic additionsadditions onon thethe binderbinder powderpowder doesdoes influenceinfluence thethe curvecurve shape.shape. ForFor thisthis are divided into two graphs, changing the parameter “s” in the prediction expression for reason,reason, both both series series are are divided divided into into two two graphs, graphs, changing changing the the parameter parameter “s” “s” in in the the pre predic-dic- each: Figure6b,d show the results of mixtures containing pozzolanic additions (fly ash tiontion expressionexpression forfor each:each: FigureFigure 66bb,d,d showshow thethe resultsresults ofof mixturesmixtures containingcontaining pozzolanicpozzolanic and pozzolans), adopting s = 0.38, and the other presenting mixtures without pozzolanic additionsadditions (fly(fly ashash andand pozzolans),pozzolans), adoptingadopting ss == 0.38,0.38, andand thethe otherother presentingpresenting mixturesmixtures additions, with s = 0.20. withoutwithout pozzolanic pozzolanic additions, additions, with with s s = = 0.20. 0.20. 80 80 80 80 70 70 70 70 60 60 60 60 50 50 50 50 40 40 BT200-FA_EC2

BT200-FA_EC2 (MPa)

40 (MPa) 40 (MPa) (MPa) BT200-FA-RA_EC2 BTref_EC2 BT200-FA-RA_EC2

cm 30 30

BTref_EC2 cm BT200-Poz_EC2 f

cm 30 30 f cm BT200-Poz_EC2

f BT250-Slag_EC2 BT250-Slag_EC2 f BT200-FA 20 20 BT200-FA 20 BTref 20 BT200-FA-RA BTref BT200-FA-RA 10 BT250-Slag 10 BT200-Poz 10 BT250-Slag 10 BT200-Poz 0 0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 Age (days) Age (days) Age (days) Age (days)

((aa)) ((bb)) 80 80 80 80 70 70 70 70 60 60 60 60 50 50 50 50 40 40 MT250-FA_EC2

MT250-FA_EC2 (MPa)

(MPa) 40 40 (MPa) (MPa) MTref_EC2 MT250-FA-RA_EC2

30 MTref_EC2 30 MT250-FA-RA_EC2 cm

cm 30 30 MT250-Poz_EC2

f cm

cm MT250-Slag_EC2 MT250-Poz_EC2 f f 20 MT250-Slag_EC2 20 MT250-FA 20 MTref 20 MT250-FA MTref MT250-FA-RA MT250-FA-RA 10 MT250-Slag 10 MT250-Poz 10 MT250-Slag 10 MT250-Poz 0 0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 Age (days) Age (days) Age (days) Age (days)

((cc)) ((dd)) Figure 6. Evolution of average compressive strength with age and the influence of hardening in: (a) BT mixtures without FigFigureure 6. 6. EvolutionEvolution of of average average compressive compressive strength strength with with age age and and the the influence inﬂuence of of hardening hardening in: in: (a ()a )BT BT mixtures mixtures without without pozzolanic addition; (b) BT mixtures with pozzolanic additions; (c) MT mixtures without pozzolanic additions; (d) MT pozzolanic addition; (bb) BT mixtures with pozzolanic additions; (c)c MT mixtures without pozzolanic additions; (dd) MT mixturespozzolanic with addition; pozzolan ( ic) BTadditions. mixtures with pozzolanic additions; ( ) MT mixtures without pozzolanic additions; ( ) MT mixturesmixtures with with pozzolan pozzolanicic additions. additions.

Materials 2021, 14, x FOR PEER REVIEW 10 of 27 Materials 2021, 14, 4173 10 of 26

By analyzing the graphs, it is possible to observe that, in both series, in mixtures withoutBy pozzolanic analyzing additions, the graphs, the it hardening is possible curve to observe is more that, pronounced in both series, at early in ages mixtures and tendswithout to stabilize pozzolanic earlier. additions, On the the contrary, hardening thecurve curves is of more mixtures pronounced with pozzolanic at early ages addi- and tionstends are to less stabilize pronounced earlier. On in early the contrary, ages, developing the curves more of mixtures in later with ages, pozzolanic being a clear additions con- sequenceare less pronounced of the pozzolanic in early effect ages, of developing the fly ash more and pozzolans. in later ages, being a clear consequence of the pozzolanic effect of the ﬂy ash and pozzolans. 3.1.4. Young’s Modulus 3.1.4. Young’s Modulus The measured average values of the Young’s modulus at 28 days are shown in Figure The measured average values of the Young’s modulus at 28 days are shown in Figure7 . 7. It is proved that the higher compactness, combined with the additions in cement re- It is proved that the higher compactness, combined with the additions in cement replace- placement, is advantageous in increasing the Young’s Modulus, comparatively to the ref- ment, is advantageous in increasing the Young’s Modulus, comparatively to the references erences with plain cement. As expected, when recycled aggregates with lower strength with plain cement. As expected, when recycled aggregates with lower strength are in- are included, the values of Young’s modulus decrease, as this recycled concrete aggre- cluded, the values of Young’s modulus decrease, as this recycled concrete aggregates have a gateslower have stiffness a and lower higher stiffness porosity and than higher natural porosity aggregates. than The natural BT200-FA-RA aggregates. and MT250- The BT200FA-RA-FA mixtures-RA and presentMT250- valuesFA-RA approximatelymixtures present 12% values and 13%approximately lower than 12% the respectiveand 13% lowermixtures than with the therespective addition mixtures of ﬂy ash with and the natural addition aggregates of fly ash (BT200-FA and natural and MT250-FA),aggregates (BT200being- theFA reductionand MT250 of-FA), 9% and being 2% the in relationreduction to of the 9% respective and 2% in references. relation to In the BT respective series, the references.addition of In pozzolans BT series, the stands addition out, withof pozzolans 11% increase stands compared out, with 11% to the inc reference.rease compared In MT toseries, the reference. this increase In isMT also series, prominent, this increase reaching is 13%also aboveprominent, the reference, reaching in 13% MT250-Poz above the and reference,MT250-FA in mixtures. MT250-Poz and MT250-FA mixtures.

50.0 44.3 BTref 45.0 41.4 42.6 41.0 40.9 40.0 38.9 BT200-FA 40.0 36.6 36.2 35.6 BT200-FA-RA 35.0 BT200-Poz 30.0 BT250-Slag

(GPa) 25.0

cm MTref

E 20.0 MT250-FA 15.0 MT250-FA-RA 10.0 MT250-Poz 5.0 MT250-Slag 0.0

FigureFigure 7 7.. Young’sYoung’s modulus modulus results results for for BT BT and and MT MT mixtures, mixtures, at at 28 28 days. days.

FigureFigure 88 presentspresents the the ratio ratio betwee betweenn the the results results of of tested Young’s Young’s modulus and the predictedpredicted values values of EC2 [[65]65],, calculatedcalculated through through Equation Equation (1). (1 However,). However, recent recent research research [30 ] [30]proposes proposes an adaptation an adaptation of the of expression the expression to mixtures to mixtures with the with inclusion the inclusion of recycled of recycled concrete aggregates, Equation (2): concrete aggregates, Equation (2): 1/3 Ecm = kE × fcm (1) 1/3 Ecm = kE × fcm 1/3 (1) Ecm_RAC = kE × (1 − 0.25 × αRA) × fcm (2) Ecm_RAC = kE × (1 − 0.25 × αRA) × fcm1/3 (2) where kE is a factor that depends on the type of natural aggregates used in the mixtures, wherewhich k isE is 9500 a factor for siliceousthat depends aggregates; on the type fcm is of the natural average aggregates compressive used in strength; the mixtures,αRA is whichthe substitution is 9500 for ratesiliceous of normal aggregates; by recycled fcm is the concrete average aggregates compressive and isstrength; given by αRA the is ratiothe substitutionbetween the rate quantity of normal of recycled by recycled sand concrete and gravel aggregates and the total and quantityis given by of aggregatesthe ratio be- in tweenthe concrete. the quantity of recycled sand and gravel and the total quantity of aggregates in the concrete.The analysis of the graph allows to conclude that the values obtained in the Young’s modulus test are always higher than the predicted. Mixtures with the addition of pozzolans stand out, with values 26% and 17% higher than those predicted, in the BT200-Poz and MT250-Poz mixtures, respectively.

Materials 2021, 14, x FOR PEER REVIEW 11 of 27 Materials 2021, 14, 4173 11 of 26

1.30 1.26

1.20 1.17 1.17 1.12 1.13 1.09 1.10 1.06 1.06

1.03 1.04 EC2

cm_ 1.00 /E

cm 0.90 E

0.80

0.70 BT200- BT200- BT200- BT250- MT250- MT250- MT250- MT250- BTref MTref FA FA-RA Poz Slag FA FA-RA Poz Slag

FiguFigurere 8. 8. RatioRatio between between Young’s Young’s modulus modulus tested tested values values and and those those predicted predicted in in EC2. EC2. 3.2. Durability The analysis of the graph allows to conclude that the values obtained in the Young’s 3.2.1. Water Absorption through Capillarity modulus test are always higher than the predicted. Mixtures with the addition of pozzolans stTheand characterizationout, with values of26% all and mixtures 17% higher had already than those been predicted, carried out in inthe mortar BT200 matri--Poz andces MT250 [48], which-Poz ismixtures, why not respectively. all the results are presented, but only the final selected mixtures with fly ash addition. The results revealed that mixtures with higher packing density, 3.2.both Durability BT and MT, and combined with slag addition, presented the best results, absorbing down to 60% less water. Fly ash and pozzolan addition also revealed good performance in 3.2.1.reducing Water water Absorption absorption. through Capillarity 2 TheFigure characterization9 shows the evolution of all mixtures of capillary had already absorption been values,carried Souta (mg/mm in mortar )mat withrices the [48]square, which root is ofwhy time, not for all BTthe andresults MT are mixtures, presented, after but 28 only days the curing. final selected By analyzing mixtures the withresults, fly ash performed addition. on The three results consecutive revealed days, that mixtures it can be with concluded higher that packing the capillary density, waterboth BTabsorption and MT, and occurs combined with more with intensity slag addition, in the presented first hours the and best tends resu tolts, stabilize absorbing over down time, topresenting 60% less water. a nonlinear Fly ash evolution. and pozzolan In both addition series, also the revealed decrease good of theperformance powder content in re- ducingcombined water with absorption. the increase of compactness and the partial replacement of cement by additionsFigure proved 9 shows to the be advantageousevolution of capillary in decreasing absorption water absorptionvalues, Sa (mg/mm over time.2) with However, the squarethis fact root is moreof time, prominent for BT and in MTMT series,mixtures, since after the 28 values days ofcuring. optimized By analy mixtureszing the inBT results, series performedremain similar on three to the consecutive reference. Indays, the latter,it can thebe concluded addition of that fly ashthe (50%capillary of replacement water absorp- rate) tionand occurs the inclusion with more of 20% intensity of recycled in the aggregates first hoursdoes and tends not change to stabilize the capillary over time, absorption present- of ingconcrete. a nonlinear Comparing evolution. the twoIn both series, series, it is noticeablethe decrease that of the the amount powder of content cement combined influences withthis parameter, the increase proved of compactness by the lower and values the of partial the BTref replacement (with cement of cement content ofby 400 additions kg/m3), provedin relation to be to advantageous MTref (with in 400 decreasing kg/m3). water However, absorption the optimized over time. mixtures However, with this fly fact ash isshow more thatprominent the increase in MT inseries, compactness since the values and the of replacementoptimized mixtures of cement in BT by series this additionremain similaralso has to athe great reference. influence In the on latter, the reduction the addition of capillarity, of fly ashas (50% the of concrete replacement matrix rate) becomes and themore inclusion compact of and20% less of recycled permeable. aggregates In BT series, does despite not change of the the 50% capillary of cement absorption replacement, of concretthe capillarye. Comparing absorption the two is similar series, to it reference.is noticeable However, that the in amount MT mixtures, of cement the influences optimized thismatrix parameter, with high proved compactness, by the and lower about values 30% of of the cement BTref replacement (with cement by fly content ash, promotes of 400 kg/ma significant3), in relation reduction to MTref in capillarity, (with 400 of kg/m at least3). However, 35%. the optimized mixtures with fly ash showIn MT that mixtures, the increase the partial in compactness cement replacement and the replacement by fly ash and of cement 20% of recycledby this addition concrete alsoaggregates, has a great mixture influence MT250-FA-RA, on the reduction leads toof downcapillarity, to 44% as lessthe waterconcrete absorption, matrix becomes and the moremixture compact with and natural less aggregatespermeable. (MT250-FA)In BT series, presentsdespite of equally the 50% reduced of cement values. replacement,Figure 10 theshows capillary the quality absorption of the isconcrete similar to as reference. a function However, of the capillary in MT absorptionmixtures, the coefficient, optimized Sa . matrixBrowne with [69 high] proposed compactness, the following and about classification 30% of cement for replacement the quality by of fly the ash, concrete, promotes as a function of the S coefficient: above 0.2 mg/mm2 × min1/2 is “low quality”; between 0.1 a significant reductiona in capillarity, of at least 35%. and 0.2 mg/mm2 × min1/2 is “medium quality” and below 0.1 mg/mm2 × min1/2 is “high quality”. According to this classification, all mixtures are considered to be of “high quality”, regarding the water absorption through capillarity. In addition, due to pozzolanic effect of the optimized mixtures, there is great potential to improve the performance for longer ages, beyond 28 days.

Materials 2021, 14, x FOR PEER REVIEW 12 of 27

1.2 1.2

1.0 1.0

)

2 ) 2 0.8 0.8

0.6 0.6

(mg/mm (mg/mm

0.4 a 0.4

S S 0.2 0.2 Materials 20212021,, 14,, x 4173 FOR PEER REVIEW 1212 of 27 26 0.0 0.0 0 15 30 45 60 75 0 15 30 45 60 75 t (min1/2) t (min1/2) 1.2 1.2 BTref BT200-FA BT200-FA-RA MTref MT250-FA MT250-FA-RA 1.0 1.0

(a) (b)

)

2 )

2 0.8 0.8 Figure 9. Capillarity water absorption through square root of time: (a) BT mixtures; (b) MT mixtures. 0.6 0.6

In MT mixtures, the partial cement replacement by fly ash and 20% of recycled con-

(mg/mm (mg/mm

0.4 crete aggregates, mixture MT250-FAa -RA,0.4 leads to down to 44% less water absorption, and

S S 0.2 the mixture with natural aggregates (MT2500.2 -FA) presents equally reduced values. Figure 10 shows the quality of the concrete as a function of the capillary absorption coefficient, 0.0 Sa. Browne [69] proposed the following0.0 classification for the quality of the concrete, as a 0 15 30 45 60 75 0 15 30 45 60 75 function of the Sa coefficient: above 0.2 mg/mm2 × min1/2 is “low quality”; between 0.1 and 1/2 t (min1/2) t (min0.2 mg/mm) 2 × min1/2 is “medium quality” and below 0.1 mg/mm2 × min1/2 is “high quality”. BTref BT200-FAAccording toBT200-FA-RA this classification, all mixtures MTrefare consideredMT250-FA to be ofMT250-FA-RA “high quality”, regarding the water absorption through capillarity. In addition, due to pozzolanic effect of (a) (b) the optimized mixtures, there is great potential to improve the performance for longer FigureFigure 9. 9. CapillarityCapillarityages, water water beyond absorption absorption 28 days. through through square square root of time: ( (aa)) BT BT mixtures; mixtures; ( (b) MT mixtures.

In2.0 MT mixtures, the partial cement replacement by fly ash and 20% of recycled concrete aggregates, mixture MT250-FA-RA, leads to down to 44% less water absorption, and

the mixture with natural aggregates (MT250-FA) presents equallyBTref reduced values. Figure ) 10 2 shows1.5 the quality of the concrete as a function of the capillaryBT200-FA absorption coefficient, Sa. Browne [69] proposed the following classification for the qualityBT200-FA-RA of the concrete, as a 2 1/2 MTref function1.0 of the Sa coefficient: above 0.2 mg/mm × min is “low quality”; between 0.1 and 2 1/2 2 1/2

0.2 mg/mm(mg/mm × min is “medium quality” and below 0.1 mg/mm MT250-FA × min is “high quality”. a

AccordingS to this classification, all mixtures are considered to MT250-FA-RAbe of “high quality”, re- garding0.5 the water absorption through capillarity. In addition, dueS=0.1 to pozzolanic effect of the optimized mixtures, there is great potential to improve theS=0.2 performance for longer ages, beyond 28 days. 0.0 0 15 30 45 60 75 2.0 t (min1/2)

BTref

FigureFigure) 1 10.0. CapillaryCapillary water water absorption absorption through through square square root root of of time. time. 2 1.5 BT200-FA 3.2.2.3.2.2. Water Water P Penetrationenetration D Depthepth under under P Pressureressure BT200-FA-RA Materials 2021, 14, x FOR PEER REVIEW Figure 11 shows the water penetration depth under pressureMTref of MT mixtures,13 of 27 at Figure1.0 11 shows the water penetration depth under pressure of MT mixtures, at 28

28 days.(mg/mm This test was not performed on BT mixtures, since theMT250-FA molded faces have high

days.a This test was not performed on BT mixtures, since the molded faces have high po-

porosity,S which would make the necessary sealing quite difﬁcult.MT250-FA-RA rosity, which would make the necessary sealing quite difficult. 0.5 S=0.1 S=0.2

0.0 0 15 30 45 60 75 t (min1/2)

Figure 10. Capillary water absorption through square root of time.

3.2.2. Water Penetration Depth under Pressure Figure 11 shows the water penetration depth under pressure of MT mixtures, at 28 days. This test was not performed on BT mixtures, since the molded faces have high porosity, which would make the necessary sealing quite difficult.

FigureFigure 11. 11. WaterWater penetration penetration depth depth under under pressure pressure for for MT MT mixtures, mixtures, at at 28 28 days. days.

SimilarSimilar to to what what occurs occurs with with the the absorption absorption of water of water by capillarity, by capillarity, the addition the addition of fly of ashﬂy show ash showss significant signiﬁcant reductions reductions of water of water penetration penetration under under pressure. pressure. The optimized The optimized mixture with higher compactness and addition of fly ash, MT250-FA, presents values down to 65% of less penetration depth when compared to the reference. The substitution of 20% natural aggregates through recycled concrete aggregates, combined with the previous matrix, make its compact matrix noteworthy, showing a 58% reduction in relation to the reference. The mixture with the addition of pozzolans, MT250-Poz, reveals the lowest water penetration result, very close to the value of the mixture with fly ash addition. This analysis reinforces that the higher compactness, and matrix refinement due to fly ash addition, of the optimized mixtures, it is quite important to reduced concrete permeability. In addition, the pozzolanic effect will promote even better results at more advanced ages, where lower penetration results are potentially expected.

3.2.3. Carbonation Resistance The phenomenon of concrete carbonation is mainly associated with the penetration, by diffusion, of carbon dioxide. The decrease in pH is visible through the reaction with phenolphthalein sprayed in concrete, in which the carbonated area is colorless (Figure 12).

56 days

F 90 days

5 120 days

(a) (b) Figure 12. Specimens tested regarding resistance to carbonation over exposure time: (a) BT200-FA- RA; (b) MT250-slag.

Materials 2021, 14, x FOR PEER REVIEW 13 of 27

Materials 2021, 14, 4173 Figure 11. Water penetration depth under pressure for MT mixtures, at 28 days. 13 of 26

Similar to what occurs with the absorption of water by capillarity, the addition of fly ash shows significant reductions of water penetration under pressure. The optimized mix- turemixture with with higher higher compactness compactness and and addition addition of offly fly ash, ash, MT250 MT250-FA,-FA, presents presents values values down down to 65% of less penetration depth when compared to the reference. The substitution of to 65% of less penetration depth when compared to the reference. The substitution of 20% 20% natural aggregates through recycled concrete aggregates, combined with the previous natural aggregates through recycled concrete aggregates, combined with the previous ma- matrix, make its compact matrix noteworthy, showing a 58% reduction in relation to the trix, make its compact matrix noteworthy, showing a 58% reduction in relation to the ref- reference. The mixture with the addition of pozzolans, MT250-Poz, reveals the lowest water erence. The mixture with the addition of pozzolans, MT250-Poz, reveals the lowest water penetration result, very close to the value of the mixture with fly ash addition. This analysis penetration result, very close to the value of the mixture with fly ash addition. This anal- reinforces that the higher compactness, and matrix refinement due to fly ash addition, of ysis reinforces that the higher compactness, and matrix refinement due to fly ash addition, the optimized mixtures, it is quite important to reduced concrete permeability. In addition, of the optimized mixtures, it is quite important to reduced concrete permeability. In ad- the pozzolanic effect will promote even better results at more advanced ages, where lower dition, the pozzolanic effect will promote even better results at more advanced ages, penetration results are potentially expected. where lower penetration results are potentially expected. 3.2.3. Carbonation Resistance 3.2.3. Carbonation Resistance The phenomenon of concrete carbonation is mainly associated with the penetration, by diffusion,The phenomenon of carbon of dioxide. concrete The carbonation decrease is in mainly pH is visible associated through with the the reaction penetration, with byphenolphthalein diffusion, of carbon sprayed dioxide. in concrete, The decrease in which in the pH carbonated is visible areathrough is colorless the reaction (Figure with 12). phenolphthalein sprayed in concrete, in which the carbonated area is colorless (Figure 12).

56 days

F 90 days

5 120 days

(a) (b)

FFigureigure 12. 12. SpecimensSpecimens tested tested regarding regarding resistance resistance to to carbonation carbonation over over exposure exposure time: time: (a (a) )BT200 BT200-FA--FA- RA;RA; ( (bb)) MT250 MT250-slag.-slag.

Figure 13 shows the results evolution of the carbonation depth (Cdi), measured in mm, of the BT and MT series, with the square root of time. The analysis of carbonation depth, Cdepth, of the studied mixtures proved to evaluate that it varies with the square root of time, as expected, with a general trend to be linear. The amount of cement highly influences this parameter, and that effect is more evident in BT mixtures, whose reference contains a higher amount of cement than in MT mixture. However, BT250-slag mixture proves that the slags are the most beneficial additions in this series, showing a slower initial evolution and only 46% of increased carbonation depth in relation to the reference. On the contrary, also in BT series, the increased compactness and the partial replacement of cement by fly ash (BT200-FA and BT200-FA-RA) is not able to avoid the increase of the carbonation depth, up to 122%, when compared to the reference. Through these mixtures, it can also be concluded that the inclusion of 20% of recycled aggregates does not increase carbonation, on the contrary, it is slightly lower. Previously to this study, in mortar matrices, pozzolans additions did not reveal promising results in preventing carbonation [48]. In concrete mixtures, the results proved to follow the same trend. Materials 2021, 14, x FOR PEER REVIEW 14 of 27

Figure 13 shows the results evolution of the carbonation depth (Cdi), measured in mm, of the BT and MT series, with the square root of time. The analysis of carbonation depth, Cdepth, of the studied mixtures proved to evaluate that it varies with the square root of time, as expected, with a general trend to be linear. The amount of cement highly influences this parameter, and that effect is more evident in BT mixtures, whose reference contains a higher amount of cement than in MT mixture. However, BT250-slag mixture proves that the slags are the most beneficial additions in this series, showing a slower initial evolution and only 46% of increased carbonation depth in relation to the reference. On the contrary, also in BT series, the increased compactness and the partial replacement of cement by fly ash (BT200-FA and BT200-FA-RA) is not able to avoid the increase of the carbonation depth, up to 122%, when compared to the reference. Through these mixtures, it can also be concluded that the inclusion of 20% of recycled aggregates does not increase Materials 2021, 14, 4173 carbonation, on the contrary, it is slightly lower. Previously to this study, in mortar14 ofma- 26 trices, pozzolans additions did not reveal promising results in preventing carbonation [48]. In concrete mixtures, the results proved to follow the same trend.

20 20

16 16

12 12

(mm) (mm)

8 8

depth

C C 4 4

0 0 0 1 2 3 4 5 6 7 8 9 10 11 0 1 2 3 4 5 6 7 8 9 10 11 t (days1/2) t (days1/2)

BTref BT200-FA BT200-FA-RA BT250-Slag MTref MT250-FA MT250-FA-RA MT250-Slag

(a) (b)

FigureFigure 13.13. CarbonationCarbonation depthdepth throughthrough squaresquare rootroot ofof time:time: ((aa)) BTBT mixtures;mixtures; ((bb)) MTMT mixtures.mixtures.

InIn thethe MTMT series,series, thethe differencedifference betweenbetween thethe optimizedoptimized mixturesmixtures andand thethe referencereference isis

notnot soso evident, sincesince thethe cement replacement isis lowerlower (30%),(30%), presentingpresenting aa similarsimilar evolutionevolution ofof thethe carbonationcarbonation depthdepth forfor thethe ﬁrstfirst testingtesting ages.ages. ThisThis evolutionevolution isis lessless pronouncedpronounced afterafter 5656 days,days, withwith thethe exceptionexception ofof thethe MT250-slagMT250-slag mixture,mixture, whichwhich showsshows aa moremore pronouncedpronounced increaseincrease ofof thethe carbonationcarbonation depth,depth, presentingpresenting valuesvalues upup toto 69%69% aboveabove reference.reference. ThisThis effecteffect waswas notnot noticed noticed for for younger younger testing testing ages ages on theon preliminarythe preliminary study study of mortar of mortar matrices, matrices, since thesince carbonation the carbonation evolved evolved linearly linearly with with square square root ofroot time of time up to up 90 to days. 90 days. Unlike Unlike the BTthe series,BT series, the optimizedthe optimized mixtures mixtures with with ﬂy ash fly of ash MT of series, MT series, MT250-FA, MT250 present-FA, present values values closer tocloser the reference,to the reference, showing showing an increase an increase of up to 31%,of up at to the 31%, longer at the tested longer age. tested The inclusion age. The ofinclusion recycled of aggregates, recycled aggregates, once again, once did again, not harm did not the harm performance the performance of the concrete of the concrete at this parameter,at this parameter, presenting presenting similar evolutionsimilar evolution with lower with values lower to values the homonymous to the homonymous mixture withmixture natural with aggregates. natural aggregates. Despite additionalDespite additional absorption, absorption water is, consideredwater is considered in concrete in withconcrete RA, thewith probable RA, the absorption probable absorption during setting during time setting may reduce time may the effective reduce the water effective of the

mixtureswater of the with mixtures RA, which with seems RA, which to result seems on a to positive result effecton a positive on carbonation effect on reduction. carbonation reduction. 3.2.4. Chloride Migration Materials 2021, 14, x FOR PEER REVIEW The penetration depth of chlorides into the specimen is visible through the15 silver of 27 3.2.4. Chloride Migration colour area, which corresponds to the silver nitrate precipitation, as is shown in Figure 14. The penetration depth of chlorides into the specimen is visible through the silver colour area, which corresponds to the silver nitrate precipitation, as is shown in Figure 14.

56 days

90 days

(a) (b)

FigureFigure 14. 14. SpecimensSpecimens tested tested for for resistance resistance to to chl chlorideoride migration, migration, at at 56 56 and and 90 90 days: days: (a (a) )BT200 BT200-FA;-FA; ((bb)) MT250 MT250-Poz.-Poz.

TheThe results results shown shown in in Figure Figure 1515 representrepresent the the chloride chloride diffusion diffusion coefficient coefﬁcient (D (D00)) by by

A F 5 7 C P 5 7 1 nonnon-steady-steady state state migration migration test test at at 56 56 and and 90 90 days. days. In In this this test, test,1 the the lower lower the the D D00 coefficient,coefﬁcient,

thethe higher higher t thehe resistance resistance to to chloride chloride migration, migration, the the durability durability performance performance also also being being higher.higher. This This test highlights thethe beneﬁtbenefit of of pozzolanic pozzolanic additions additions (both (both ﬂy fly ash ash and and pozzolans), pozzolans), bringing the pozzolanic refinement to the porous network of the matrix, combined with increased compactness, in reducing the chloride migration. The addition of natural pozzolans stands out positively by presenting very low D values, down to 64% and 81%, below the references for mixtures BT200-Poz and MT250- Poz, respectively, at 90 days. Due to its high content of alumina, when pozzolans are combined with cement, it promotes a reduction of the porous structure of the matrix with time, being beneficial to durability related with to corrosion risk, induced by chloride penetration, and to strength performance. The results reveal similarly low values for the BT200-FA and MT250-FA mixtures, down to 80% and 77%, respectively, at 90 days, when compared to the references. This effect is already scientifically known. However, the inclusion of recycled aggregates affects the increase in the diffusion coefficient, when compared with similar mixtures with only natural aggregates, although it presents values 51% and 70% below the references for BT200-FA-RA and MT250-FA-RA, respectively. It is important to note that, in the BT200-FA-RA mixture, the D0 coefficient maintain its value from 56 to 90 days. Contrarily, the MT250-FA-RA mixture reduces the diffusion coefficient. Apparently, the higher cement content and lower replacement rate, in comparison to BT mixtures, is the probable reason for that difference.

8.2 BTref 10.9 1.6 BT200-FA 2.9 BT200-FA-RA 4.0 90 days 4.0 2.9 56 days BT200-Poz 2.1

12.2 MTref 13.0 2.8 MT250 FA 5.1 3.6 MT250-FA-RA 6.3 2.3 MT250-Poz 2.4 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 D ( 10−12 m2/s) 0

Figure 15. Chloride diffusion coefficient (D0), at 56 and 90 days, for BT and MT mixtures.

Materials 2021, 14, x FOR PEER REVIEW 15 of 27

56 days

90 days

(a) (b) Figure 14. Specimens tested for resistance to chloride migration, at 56 and 90 days: (a) BT200-FA; (b) MT250-Poz.

The results shown in Figure 15 represent the chloride diffusion coefficient (D0) by

A F 5 7 C P 5 7 1 non-steady state migration test at 56 and 90 days. In this test,1 the lower the D0 coefficient, the higher the resistance to chloride migration, the durability performance also being higher. This test highlights the benefit of pozzolanic additions (both fly ash and pozzolans), bringing the pozzolanic refinement to the porous network of the matrix, combined with increased compactness, in reducing the chloride migration. The addition of natural pozzolans stands out positively by presenting very low D values, down to 64% and 81%, below the references for mixtures BT200-Poz and MT250- Poz, respectively, at 90 days. Due to its high content of alumina, when pozzolans are combined with cement, it promotes a reduction of the porous structure of the matrix with time, being beneficial to durability related with to corrosion risk, induced by chloride penetration, and to strength performance. The results reveal similarly low values for the BT200-FA and MT250-FA mixtures, down to 80% and 77%, respectively, at 90 days, when compared to the references. This effect is already scientifically known. However, the inclusion of recycled aggregates affects the increase in the diffusion coefficient, when compared with similar mixtures with only natural aggregates, although it presents values 51% and 70% below the references Materials 2021, 14, 4173 for BT200-FA-RA and MT250-FA-RA, respectively. It is important to note that, 15in of the 26 BT200-FA-RA mixture, the D0 coefficient maintain its value from 56 to 90 days. Contrarily, the MT250-FA-RA mixture reduces the diffusion coefficient. Apparently, the higher cement content and lower replacement rate, in comparison to BT mixtures, is the probable bringingreason for the that pozzolanic difference. reﬁnement to the porous network of the matrix, combined with increased compactness, in reducing the chloride migration.

8.2 BTref 10.9 1.6 BT200-FA 2.9 BT200-FA-RA 4.0 90 days 4.0 2.9 56 days BT200-Poz 2.1

12.2 MTref 13.0 2.8 MT250 FA 5.1 3.6 MT250-FA-RA 6.3 2.3 MT250-Poz 2.4 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 D ( 10−12 m2/s) 0

Figure 15. Chloride diffusion coefﬁcientcoefficient (D00), at 56 and 90 days, for BT and MT mixtures.

The addition of natural pozzolans stands out positively by presenting very low D values, down to 64% and 81%, below the references for mixtures BT200-Poz and MT250-Poz,

respectively, at 90 days. Due to its high content of alumina, when pozzolans are combined with cement, it promotes a reduction of the porous structure of the matrix with time, being beneficial to durability related with to corrosion risk, induced by chloride penetration, and to strength performance. The results reveal similarly low values for the BT200-FA and MT250-FA mixtures, down to 80% and 77%, respectively, at 90 days, when compared to the references. This effect is already scientifically known. However, the inclusion of recycled aggregates affects the increase in the diffusion coefficient, when compared with similar mixtures with only natural aggregates, although it presents values 51% and 70% below the references for BT200-FA-RA and MT250-FA-RA, respectively. It is important to note that, in the BT200- FA-RA mixture, the D0 coefficient maintain its value from 56 to 90 days. Contrarily, the Materials 2021, 14, x FOR PEER REVIEW 16 of 27 MT250-FA-RA mixture reduces the diffusion coefficient. Apparently, the higher cement content and lower replacement rate, in comparison to BT mixtures, is the probable reason for that difference. FigureFigure 1616 showshow the the classification classification of the chloride penetrationpenetration resistanceresistance of of concrete concrete as as a afunction function of of the the chloride chloride diffusion diffusion coefficient, coefficient, D D0,0, as as used used in in [ 33[33]]. .

20.0

17.5 Low

15.0 /s)

2 12.5 Moderated m

−12 10.0

( 7.5 High

0 0 D 5.0 Very high 2.5 Extremely high 0.0 BT ref BT200- BT200- BT200- MTref MT250- MT250- MT250- FA FA-RA Poz FA FA-RA Poz 56 days 90 days FigureFigure 16 16.. ChlorideChloride penetration penetration resistance resistance of ofconcrete concrete depending depending on on the the chloride chloride diffusion diffusion coeffi- coefﬁ- cient (D0). cient (D0).

According to this classification, and by analyzing the plotted graph, it is possible to note that only the references, both BT and MT, present a “moderate” resistance of concrete to chloride penetration, although the BTref mixture achieves a classification of “high” resistance, at 90 days. Mixtures optimized with additions and increased compactness reveal very promising classification results. The mixtures with the addition of pozzolans (more evident in MT250-Poz) stand out for mostly obtaining the classification of “extremely high” resistance to chloride penetration. Mixtures with fly ash also stand out for the same reason, although they are mostly classified as “very high” resistance. The replacement of natural aggregates by recycled concrete aggregates does not greatly harm the mixtures, since, after 90 days, they are maintained, at least, with the classification of “very high” resistance.

3.2.5. Concrete Surface Resistivity Figure 17 shows the results of the surface electrical resistivity of concrete (ρ), of the BT and MT mixtures, over time. This is a fast test allowing to indirectly predict the permeability to fluids and the diffusion of chloride ions in concrete. The presented results were determined in mortar matrixes of the same concrete formulations, since they show the same trend as in concrete.

210 210

180 BTref 180 MTref BT200-FA MT250-FA 150 150

BT200-Poz MT250-Poz cm)

120 cm) 120 -

BT250-Slag - MT250-Slag Ω

90 Ω 90

(k ρ 60 ρ 60

30 30

0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 t (days) t (days)

Figure 17. Concrete surface resistivity over time: BT mixtures (left); MT mixtures (right).

Materials 2021, 14, x FOR PEER REVIEW 16 of 27

Figure 16 show the classification of the chloride penetration resistance of concrete as a function of the chloride diffusion coefficient, D0, as used in [33].

20.0

17.5 Low

15.0 /s)

2 12.5 Moderated m

−12 10.0

( 7.5 High

0 0 D 5.0 Very high 2.5 Extremely high 0.0 BT ref BT200- BT200- BT200- MTref MT250- MT250- MT250- FA FA-RA Poz FA FA-RA Poz 56 days 90 days Figure 16. Chloride penetration resistance of concrete depending on the chloride diffusion coeffi- Materials 2021, 14, 4173 16 of 26 cient (D0).

According to this classification, and by analyzing the plotted graph, it is possible to note thAccordingat only the to references, this classification, both BT and MT, by analyzing present a the“moderate” plotted graph, resistance it is possibleof concrete to tonote chloride that only penetration, the references, although both the BT BTref and MT, mixture present achieves a “moderate” a classification resistance of of“high” concrete resistance,to chloride at 90 penetration, days. Mixtures although optimized the BTref with additions mixture achieves and increased a classification compactness of “high”reveal veryresistance, promising at 90 days.classification Mixtures results. optimized The withmixtures additions with andthe increasedaddition of compactness pozzolans (more reveal evidentvery promising in MT250 classification-Poz) stand results. out for The mostly mixtures obtaining with the the addition classification of pozzolans of “extremely (more high”evident resistance in MT250-Poz) to chloride stand penetration. out for mostly Mixtures obtaining with the fly classification ash also stand of “extremelyout for the same high” reason,resistance although to chloride they penetration. are mostly classified Mixtures withas “very fly ash high” also resistance. stand out forThe the replacement same reason, of naturalalthough aggregates they are mostly by recycled classified concrete as “very aggregates high” resistance. does not greatly The replacement harm the ofmixtures, natural since,aggregates after by90 recycleddays, they concrete are maintained, aggregates at does least, not with greatly the harm classification the mixtures, of “very since, high” after resistance.90 days, they are maintained, at least, with the classification of “very high” resistance.

3.2.5. Concrete Concrete S Surfaceurface R Resistivityesistivity Figure 17 shows the results of the surface electrical resistivity of concrete (ρ), of the BT and and MT MT mixtures, mixtures, over over time time.. This This is a is fast a fast testtest allowing allowing to indirectly to indirectly predict predict the per- the meabilitypermeability to fluids to ﬂuids and and the the diffusion diffusion of ofchloride chloride ions ions in in concrete. concrete. The The presented presented results were determined in mortar matrixes of the same concrete formulations,formulations, since they show thethe same trend as in concrete.

210 210

180 BTref 180 MTref BT200-FA MT250-FA 150 150

BT200-Poz MT250-Poz cm)

120 cm) 120 -

BT250-Slag - MT250-Slag Ω

90 Ω 90

(k ρ 60 ρ 60

30 30

0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 t (days) t (days)

Figure 17. Concrete surface resistivity over time: BT mixtures (left); MT mixtures (right).).

Through the analysis of the graphs, in both series, it is possible to conclude that the slag addition does not lead to positive results in this test, presenting similar values to the references with plain cement binder. The reference mixtures and those with slag addition have no significant variations of electrical resistivity with age. The opposite is found in mixtures with pozzolanic additions, which present results that increase with the testing age. Mixtures with fly ash addition stand out for having higher values, almost 20 times higher than the references, for both BT200-FA and MT250-FA mixtures, measured at 450 days. The pozzolan addition shows resistivity values nearly 6 and 16 times higher than the references, for BT250-Poz and MT250-Poz, respectively, also at 450 days. Despite fly ash and natural pozzolan both being pozzolanic additions, their influence on resistivity is different with time regarding the evolution and the optimal replacement rate. The electrical resistivity evolves faster with age, up to 56 days, for mixtures with pozzolan addition and develops a smoother evolution after that. Contrarily, the mixtures with fly ash addition present smooth evolution at younger ages, but after 120 days the increase in rate with age tends to be faster, probably due to higher pozzolanic reactivity at greater ages. However, at 300 days, mixtures with both additions tend to stabilize, although mixtures with fly ash stabilize more smoothly. In the MT series, the MT250-Poz mixture is very close to MT250- FA, showing closer values of resistivity evolution over time; a lower cement replacement by pozzolans, in comparison to the BT series, seems to suggest a possible optimal for moderate replacement rates (circa 30%). Contrarily, in the BT series, the higher cement replacement of fly ash in the mixture BT200-FA, 50%, suggest that the resistivity increase is more effective for higher fly ash content, mainly for longer ages. In fact, in a previous study Materials 2021, 14, 4173 17 of 26

carried out in mortar matrixes [48], it was possible to draw this conclusion, corroborated also by Kurda et al. [70]. The optimal proportions, for both types of pozzolanic additions, regarding the increase in electrical resistivity and the reduction of chloride migration need to be studied in future research. Further, due to its known relation to the resistance to chloride migration, and as is evident from the analysis of the graphs, the pozzolanic effect promotes an improved behavior at older ages. That effect increases the refinement of the internal porous microstructure with age, both for pozzolan and for fly ash; however, at different rates, the fly ash being lower at shorter ages, but with a greater effect in longer ages, and mainly when high replacement rates are used. The followed standard [64] refers to a correlation between the resistivity test and the chloride ion penetration, establishing limit parameters. At 450 days of age, the mixtures with additions of pozzolan and fly ash of the BT and MT series were classified, regarding the penetration of chloride ion, as being “very low”, with ρ values between 37 and 254 kΩ-cm. For the reference mixtures, at the same age, BTref was classified as “moderate” (12 kΩ-cm < ρ < 21 kΩ-cm) and MTref as “high” (ρ < 12 kΩ-cm). Mixtures with slag additions were classified as “high”, which is why concrete mixtures with that addition are not recommended for exposure to chlorides. Materials 2021, 14, x FOR PEER REVIEW Figure 18 proves the high correlation between the chloride diffusion coefficient,18 of D270 , of the mixtures and the surface resistivity, ρ, of the corresponding matrixes, which is very reliable, and was proved by the high R2 values of the power function.

16 14

12 /s) /s) 2 10 D = 89.348. -0.974 m 0 ρ

R² = 0.933 12

− 8

10 6

(

0 4 D 2 0 0 20 40 60 80 ρ (kΩ-cm)

FigureFigure 18. 18. CorrelationCorrelation between between the the chloride chloride diffusion diffusion coefficient coefﬁcient and and the the surface surface resistivity. resistivity.

3.2.6.3.2.6. Prediction Prediction of of Service Service L Lifeife  • EnvironmentalEnvironmental exposure exposure XC XC (corrosion (corrosion induced induced by by carbonation carbonation)) 5 TheThe determination determination of of carbonation carbonation resistance, resistance, RC65 (kg(kg ×× year/myear/m5), ),of of each each developed developed concreteconcrete was was calculated calculated using using Eq Equationuation (3 (3)) and and the the carbonation carbonation depth, depth, C Cdidi. .These These values values werewere measured measured after after the the concrete concrete was was exposed exposed for for a acertain certain time time to to a ahighly highly concen concentratedtrated −3 3 carboncarbon dioxide dioxide environment, environment, C Cacelacel, 90, 90 × 10× −310 kg/mkg/m3. .

22∙·CCacel∙t·it R == acel i RC65C65 22 (3(3)) XXii

where ti is the time of exposure in years, and Xi has the same meaning as Cdi (carbonation where ti is the time of exposure in years, and Xi has the same meaning as Cdi (carbonation depth). Table 3 presents the average values of carbonation resistance, RC65, at 56, 90, and depth). Table3 presents the average values of carbonation resistance, R C65, at 56, 90, and 120120 days days to to minimize minimize the the error errorss related related with with the the measurement measurement of of the the carbonation carbonation depth. depth.

Table 3. Average values of carbonation resistance, RC65.

Concrete RC65 (kg·year/m5) BTref 1875.6 BT200-FA 160.7 BT200-FA-RA 195.6 BT250-Slag 601.4 MTref 357.8 MT250-FA 238.9 MT250-FA-RA 306.8 MT250-Slag 219.5 The deterioration of reinforced concrete due to steel corrosion can be divided into two periods: the initiation, tic, and the propagation of corrosion, tp. The minimum propagation period, tp, depends of several factors, such as the exposure class. Table 4 presents the propagation period, tp, according to LNEC E-465 [71]. Thus, considering that the intended service life, tg, is equal to 50 years, and considering that the electrical poles can be associated to the reliability class (RC1), where the safety factor of the service life is equal to 2.0 [71], the design period of initiation, tic, can be determined using Equation (4) (Table 4).

tic = γ (tg − tp) (4)

Materials 2021, 14, 4173 18 of 26

Table 3. Average values of carbonation resistance, RC65.

5 Concrete RC65 (kg·year/m ) BTref 1875.6 BT200-FA 160.7 BT200-FA-RA 195.6 BT250-Slag 601.4 MTref 357.8 MT250-FA 238.9 MT250-FA-RA 306.8 MT250-Slag 219.5

The deterioration of reinforced concrete due to steel corrosion can be divided into two periods: the initiation, tic, and the propagation of corrosion, tp. The minimum propagation period, tp, depends of several factors, such as the exposure class. Table4 presents the propagation period, t p, according to LNEC E-465 [71]. Thus, considering that the intended service life, tg, is equal to 50 years, and considering that the electrical poles can be associated to the reliability class (RC1), where the safety factor of the service life is equal to 2.0 [71], the design period of initiation, tic, can be determined using Equation (4) (Table4). tic = γ (tg − tp) (4)

Table 4. Values of tp and tic for each exposure class, XC.

Periods during XC2 XC3 Service Life XC4 (Dry Reg.) XC4 (Wet Reg.)

tp (years) 10 45 15 5 tic (years) 80 10 70 90

The minimum concrete cover required to guarantee the resistance against the reinforcement corrosion due to carbonation, cmin,dur, is determined using Equation (5), considering the RC65 already calculated for each concrete and t is equal to the design period of initiation, tic (Tables4 and5).

s n 2 × 0.0007 × tic p t0 X = × k0 × k1 × k2 × (5) RC65 tic

Table 5. Minimum cover, cmin,dur, to resist corrosion induced by carbonation in electrical concrete poles.

Minimum Cover, cmin,dur (mm) Concrete XC2 XC3 XC4 (Dry Reg.) XC4 (Wet Reg.) BTref 3 4 5 6 BT200-FA 9 14 19 21 BT200-FA-RA 8 12 17 19 BT250-Slag 5 7 10 11 MTref 6 9 12 14 MT250-FA 8 11 15 17 MT250-FA-RA 8 12 16 18 MT250-Slag 80 10 70 90

The meanings of the symbols are: k0 is a factor related with test conditions and is equal to 3, k1 is a factor related with relative humidity, k2 is a factor related with concrete cure conditions and is also 1, n is a factor related with the influence of wetting/drying cycles over time, and t0 is the reference period and is 1 year. The Equation (5) can also be used to predict the service life, tg, when the cover used is already predefined. For this purpose, X must be equal to the cover predefined, and tic Materials 2021, 14, 4173 19 of 26

is the unknown variable that should be calculated using the Equation (5). Usually, the randomness of the factors that affect the service life happens during the initiation period, so after the determination of tic, the service life, tg, is calculated adding to this value the propagation period, tp, presented in Table4. The service life was computed for different exposure classes considering covers equal to 15, 20, and 30 mm, see Figure 19 and Table6. The presented covers only concern durability and were not determined to ensure proper ﬁre resistance nor adequate transmission of bond forces between rebars and concrete. For the standard for concrete poles, the minimum cover is related to the exposure conditions and also to the concrete strength; parameters of composition were not considered.

Table 6. Prediction of the service life, tg, for different cover values (years), classes XC.

Cover Concrete (mm) XC2 XC3 XC4 (Dry Reg.) XC4 (Wet Reg.) 15 BTref 20 >>100 30 15 51 35 25 BT200-FA 20>>100 56 56 46 30 71 >>100 15 53 41 31 BT200-FA-RA 20>>100 59 67 57 30 77 >>100 15 69 117 107 BT250-Slag 20>>100 90 >>100 30 >>100 15 59 69 59 >>100 MTref 20 71 >>100 30 >>100 15 54 48 38 MT250-FA 20>>100 62 82 72 30 85 >>100 15 57 60 50 MT250-FA-RA 20>>100 67 >>100 96 30 96 >>100 15 54 45 35 MT250-Slag 20>>100 61 75 65 30 81 >>100 Materials 2021, 14, x FOR PEER REVIEW 20 of 27

20 59 67 57 30 77 >>100 15 69 117 107 BT250-Slag 20 >>100 90 >>100 30 >>100 15 59 69 59 >>100 MTref 20 71 >>100 30 >>100 15 54 48 38 MT250-FA 20 >>100 62 82 72 30 85 >>100 15 57 60 50 MT250-FA-RA 20 >>100 67 >>100 96 30 96 >>100 15 54 45 35 MT250-Slag 20 >>100 61 75 65 30 81 >>100

It is clear that higher cement content provides a better performance regarding durability related with carbonation, since the two reference concrete mixtures had the highest service life. The results of Table 6 show that the service life expected for the poles, in most cases, was higher than 50 years, expect in environment XC4 (wet regime), meaning that the significant reduction in the amount of cement of the eco-efficient concrete can be com- pensated by using additions. The fly ash effect is similar in both types of concrete, the BT-

Materials 2021, 14, 4173 dry consistency and the MT-plastic consistency, and the incorporation of the recycled20 ag- of 26 gregates does not influence the concrete performance. However, the use of ground electric furnace slag is more beneficial with BT250-slag than MT250-slag.

Service life (years) XC3 50 63 75 88 100

100 BT ref 100 56 cover = 20 mm BT200-FA 71 cover = 30 mm 59 BT200-FA-RA 77 90 BT250-Slag 100 71 MT ref 100 62 MT250-FA 85 67 MT250-FA-RA 96 61 MT250-Slag 81

FigureFigure 19. 19. ServiceService life life expected expected for for poles, poles, class class XC3. XC3. It is clear that higher cement content provides a better performance regarding dura-  Environmental exposure XS (chloride-induced corrosion) bility related with carbonation, since the two reference concrete mixtures had the highest serviceFor environments life. The results exposed of Table to 6chlorides show that, the the minimum service life propagation expected forperiod, the poles, tp, also in dependsmost cases, of several was higher factors, than and 50, according years, expect to LNEC in environment E-465 [71], those XC4 (wetvalues regime), are 0 years meaning for XS1that and the XS3, significant and 40 reductionyears for exposure in the amount class XS2 of cement [71]. The of thechloride eco-efficient diffusion concrete coefficient, can be Dcompensated (m2/s), is determined by using for additions. each developed The fly ash concrete effect isusing similar Equation in both (6 types) and ofusing concrete, the ex- the perimentBT-dryally consistency determined and values the MT-plastic of non-steady consistency,-state migration and the incorporation coefficients, D of0 ( theFigure recycled 15). aggregates does not influence the concrete performance. However, the use of ground electric furnace slag is more beneficial with BT250-slag than MT250-slag. • Environmental exposure XS (chloride-induced corrosion)

For environments exposed to chlorides, the minimum propagation period, tp, also depends of several factors, and, according to LNEC E-465 [71], those values are 0 years for XS1 and XS3, and 40 years for exposure class XS2 [71]. The chloride diffusion coefficient, D (m2/s), is determined for each developed concrete using Equation (6) and using the experimentally determined values of non-steady-state migration coefficients, D0 (Figure 15). Using Equations (6) and (7) [71], and defining the cover thickness, X, it is possible to determine the design period of initiation of poles made with each type of concrete and after the service life, tg. In addition, using the same equations and defining the design period of initiation, tic, it is possible to determine the minimum concrete cover required to guarantee the resistance against the reinforcement corrosion induced by chlorides, cmin,dur. For a service life equal to 50 years and considering the already mentioned reliability class (RC1), the design period of initiation, tic, can be determined using Equation (4) and is 100 years for XS1 and XS3, and 20 years for exposure class XS2.

n D(t) = k D0 × (t0/t) (6) p X = 2 × ξ × D × tic (7) where k is a factor that takes into account the curing conditions, the relative humidity and the temperature; n is a factor that considers the chloride diffusion decrease over time; t0 is the time of the experimental tests to evaluate D0; t is the exposure time in days; ξ is a parameter related with the concentration of chlorides in the binding paste; X is the cover. The minimum concrete cover required to protect the steel reinforcement against corrosion induced by chlorides, cmin,dur, is presented Table7 and the prediction of service life, t g, for 20 and 30 mm is presented in Figure 20 and Table8. Materials 2021, 14, 4173 21 of 26

Table 7. Minimum cover, cmin,dur, to resist corrosion induced by chlorides in electrical concrete poles.

Minimum Cover, cmin,dur (mm) Concrete XS2 2 XS1 1 XS3 3 1 m 1.4–25 m BTref 40 45 50 97 BT200-FA 17 21 24 46 BT200-FA-RA 21 25 29 55 BT200-Poz 15 18 21 40 MTref 48 52 58 112 MT250-FA 28 31 35 67 MT250-FA-RA 31 35 39 74 MT250-Poz 19 21 24 46 1 Poles exposed to the air sea salts located on the coast; 2 permanently submerged; 3 poles in tidal splash and spray zones.

Table 8. Prediction of the service life, tg, for different cover values (years), classes XS. XS2 Concrete Cover (mm) XS1 XS3 1 m 1.4–25 m 20 2 41 40 0 BTref 30 14 42 42 0 20 91 48 45 1 BT200-FA 30 >>100 76 63 7 20 40 44 43 1 BT200-FA-RA 30 >>100 59 52 3 20 >>100 54 49 2 BT250-Poz 30 >>100 101 79 14 20 1 40 40 0 MTref 30 6 41 41 0 20 11 42 41 0 MT250-FA 30 68 49 46 1 20 7 41 41 0 MT250-FA-RA 30 43 46 44 1 20 61 48 45 1 MT250-Poz 30 >>100 75 64 8

It is highly improbable that electric poles will be placed in tidal splash or spray zones or will be permanently submerged, but the results show that poles manufactured with these concretes should not be used in such zones. In environments with air-based sea salts, the reference concretes have a performance that is clearly worse than concretes with additions, which is reﬂected in the values of minimum cover, as well as in the service life expected. Among the concretes with additions, the behavior is relatively similar, showing that the pozzolanic additions can replace the cement dosage in the context of protection against corrosion induced by chlorides. It is also noted that, in this case, the incorporation of recycled aggregates slightly increases the minimum cover and decreases the expected service life. To assure a service life of 50 years for poles manufactured using these eco- efﬁcient concretes, for both types of consistency, the cover requires ranges between 15 and 31 mm, which are the values usually used in the production of this type of structures. Materials 2021, 14, x FOR PEER REVIEW 21 of 27

Using Equations (6) and (7) [71], and defining the cover thickness, X, it is possible to determine the design period of initiation of poles made with each type of concrete and after the service life, tg. In addition, using the same equations and defining the design period of initiation, tic, it is possible to determine the minimum concrete cover required to guarantee the resistance against the reinforcement corrosion induced by chlorides, cmin,dur. For a service life equal to 50 years and considering the already mentioned reliability class (RC1), the design period of initiation, tic, can be determined using Equation (4) and is 100 years for XS1 and XS3, and 20 years for exposure class XS2.

D(t) = k D0 × (t0/t)n (6)

X = 2 × ξ × √D × tic (7) where k is a factor that takes into account the curing conditions, the relative humidity and the temperature; n is a factor that considers the chloride diffusion decrease over time; t0 is the time of the experimental tests to evaluate D0; t is the exposure time in days; ξ is a parameter related with the concentration of chlorides in the binding paste; X is the cover. The minimum concrete cover required to protect the steel reinforcement against corrosion induced by chlorides, cmin,dur, is presented Table 7 and the prediction of service life, tg, for 20 and 30 mm is presented in Figure 20 and Table 8.

Table 7. Minimum cover, cmin,dur, to resist corrosion induced by chlorides in electrical concrete poles.

Minimum Cover, cmin,dur (mm) Concrete XS2 2 XS1 1 XS3 3 1 m 1.4–25 m BTref 40 45 50 97 BT200-FA 17 21 24 46 BT200-FA-RA 21 25 29 55 BT200-Poz 15 18 21 40 MTref 48 52 58 112 MT250-FA 28 31 35 67 MT250-FA-RA 31 35 39 74 MT250-Poz 19 21 24 46 Materials 2021, 14, 4173 22 of 26 1 Poles exposed to the air sea salts located on the coast; 2 permanently submerged; 3 poles in tidal splash and spray zones.

Service life (years) XS1 0.0 25.0 50.0 75.0 100.0

2.0 BT ref 14 BT200-FA 91.0

BT200-FA-RA 40.0

BT200-Poz

1.0 MT ref 6 11.0 MT250-FA 68 cover = 20 mm MT250-FA-RA 7.0 43 cover = 30 mm MT250-Poz 61.0

Figure 20. Service life expected for poles, class XS1. Figure 20. Service life expected for poles, class XS1.

4. Conclusions The experimental study presented herein is focused on optimizing the mechanical and durability properties of concrete with low cement content, maximizing the compactness

and combining the partial replacement of cement with a high content of fly ash, natural pozzolans, and electric furnace slags, with a 20% replacement of natural aggregates by recycled concrete aggregates. Two types of formulations were produced and characterized, BT (with dry consistency) and MT (with plastic consistency). The properties were compared to plain cement-based reference mixtures, commonly used in the prefabrication industry. Based on the analyses, the following conclusions are drawn: 1. Tensile splitting strength: (i) unlike the BT series, the addition of fly ash increases this strength by 31% in plastic concrete with natural aggregates, MT250-FA, and by 22% with recycled concrete aggregates, MT250-FA-RA, compared to the reference; (ii) the addition of recycled aggregates reduces this strength by 9% when compared to the same mixture with natural aggregates; (iii) the test results and the prediction values of EC2 are very similar, with a difference of about 10%. 2. Flexural strength: (i) although cement reductions are high (from 30 to 50%) in optimized mixtures, those with the addition of fly ash and slag show an increase of up to 27% in MT250-slag mixtures, compared to the respective reference, at 90 days; (ii) the inclusion of recycled concrete aggregates have practically no influence on this parameter; (iii) the prediction of EC2 has very similar values to those obtained in the tests, since this strength is directly related to the tensile splitting. 3. Compressive strength: (i) mixtures with additions tend to increase from 28 to 90 days, due to the pozzolanic effect and, despite the cement reduction, it increases up to 26% with fly ash and natural aggregates (MT250-FA), and up to 14% with fly ash and natural and recycled aggregates (MT250-FA-RA), at 90 days, in comparison to the reference; (ii) the hardening curve of EC2 prediction has an R (rapid) evolution at early ages in concretes without pozzolanic additions, contrary to the curve of mixtures with pozzolanic additions, which has an S (slow) beginning, but a more pronounced evolution at later ages. 4. Young’s modulus: (i) adding supplementary cementitious materials (pozzolan, slag and fly ash) to the concrete increases this parameter, mainly in the case of pozzolans addition, that lead to increases of 11% and 13%, for BT200-Poz and MT250-Poz, respectively, comparing to the references; (ii) as expected, the inclusion of recycled concrete aggregates reduces this parameter, due to their lower stiffness; (iii) the test Materials 2021, 14, 4173 23 of 26

results are always higher than those predicted with the codes, with increases of up to 26% for the mixture with the addition of pozzolans, BT250-Poz. 5. Capillary water absorption and water penetration under pressure: (i) in dry BT concrete, the high content of fly ash addition (50% of cement replacement rate) combined with high compactness is beneficial, since it presents the same capillarity as the reference; (ii) concrete with plastic consistency–MT, where 30% of the cement was replaced by fly ash addition, shows even more positive results, absorbing 44% less capillary water than the reference; (iii) the inclusion of recycled concrete aggregates is also beneficial, since it presents similar or lower capillarity values as the mixtures with only natural aggregates; (iv) the addition of fly ash is beneficial to reduce water penetration under pressure, absorbing 65% less water than the reference and the recycled aggregates have little influence on this parameter. 6. Carbonation resistance: (i) the cement dosage has an important influence on reducing the carbonation depth, the reference mixtures being those with less carbonation; (ii) in BT, high compactness combined with slag addition proved to be an efficient mixture regarding this parameter, since despite having high cement replacement, BT250-slag presents only 46% more carbonation depth than the reference, instead of 120% for the mixtures using fly ash; (iii) MT mixtures with fly ash additions have similar effects in reducing carbonation compared to the reference, with or without recycled concrete aggregates; (iv) in MT series, slag addition, after an initial benefit, also tends to change the effect at older ages. 7. Service life of structures exposed to carbonation: (i) higher cement dosages promote longer lifetimes, as the references present the highest values of this parameter; (ii) in XC2 environments, a service life of 100 years is ensured for all mixtures with a 15-mm cover, while in XC3 environments a 50 year service life is ensured for all mixtures with a 20-mm cover; in the XC4 environment, the same service life is, in general, ensured with a cover of 20 mm; (iv) the incorporation of recycled aggregates has a small but positive influence on concrete performance. 8. Chloride diffusion coefficient: (i) mixtures with type II additions show the greatest influence of pozzolanic effects and the corresponding maturity of the matrixes with age; (ii) the addition of fly ash is very advantageous for increasing the resistance to chloride migration, reducing the chloride coefficient up to 80%, in MT250-FA, at 90 days; (ii) natural pozzolan addition is also very beneficial regarding chloride resistance, presenting D0 values of 81% below the reference, in MT250-Poz; (iii) natu- rally, the inclusion of recycled concrete aggregates slightly increase the D0 coefficient; (iv) all optimized mixtures are rated with “very/extremely high” chloride penetration resistance. 9. Electrical resistivity of the concrete surface: (i) since this test allows a quick assessment and prediction of chloride migration, this evaluation proves to be accurate, revealing the same trend in the chloride migration test and considering the high correlation between both results; (ii) concrete with the addition of fly ash shows the highest values of this parameter, followed by concrete with the addition of pozzolans; (iii) all mixtures tend to stabilize after around 300 days. 10. Service life of structures exposed to chloride ions: (i) pozzolanic additions highly influence the resistance to chloride penetration, which is proved by the results of mixtures with fly ash and pozzolans; (ii) to assure a service life of 50 years, both in the BT and MT series, the cover required ranges between 15 and 31 mm; (iii) the incorporation of recycled aggregates slightly increases the minimum cover and decreases the expected service life. Finally, it is concluded that eco-efficient concrete mixtures designed with high compactness, low cement content, and a high replacement rate (up to 50%) of cement through specific additions (fly ash, natural pozzolans and electric furnace slags), combined with 20% recycled concrete aggregates, still generally present improved mechanical properties. Regarding durability, depending on the exposure conditions related to corrosion risk in- Materials 2021, 14, 4173 24 of 26

duced by chloride ions or by carbonation, these concrete mixtures can lead to signiﬁcant improvements in terms of chloride resistance and to a minor inﬂuence, or a slight decrease, in carbonation resistance, even though they have a generally long service life.

Author Contributions: Conceptualization, H.C.; formal analysis, E.S., A.A. and R.d.C.; investigation, H.C.; methodology, H.C. and R.d.C.; project administration, H.C. and E.J.; supervision, E.J.; validation, H.C. and R.d.C.; writing—original draft, E.S. and A.A.; writing—review and editing, H.C., R.d.C. and E.J. All authors have read and agreed to the published version of the manuscript. Funding: This work is co-financed by the European Regional Development Fund (ERDF), through the partnership agreement Portugal2020–Operational Program for Competitiveness and Internation- alization (COMPETE2020), under the project POCI-01-0247-FEDER-033523 “POSTEJO 4.0: Inovação para a diversificação e a exportação”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request due to restrictions eg privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to confidentiality restrictions of the funded project. Acknowledgments: Authors acknowledge the Polytechnic Institute of Coimbra, ISEC, for providing the facilities and all necessary resources to perform the present study and the companies Postejo, Cimpor, Sarendur, Sabril, Omya, Harsco, Basf and Sika, for their support through material supply. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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