Mechanical and Durability Performance Improvement of Natural Hydraulic Lime-Based Mortars by Lithium Silicate Solution

materials

Article Mechanical and Durability Performance Improvement of Natural Hydraulic Lime-Based Mortars by Lithium Silicate Solution

Zijian Song 1,2,*, Zhongyuan Lu 1,* and Zhenyu Lai 1

1 State Key Laboratory for Environment-Friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China; [email protected] 2 Department of Material Engineering, Mianyang Vocational and Technical College, Mianyang 621000, China * Correspondence: [email protected] (Z.S.); [email protected] (Z.L.)

Received: 21 October 2020; Accepted: 19 November 2020; Published: 23 November 2020

Abstract: Natural hydraulic lime (NHL) as a building material has been widely used to restore the historic structure. However, the slow growth rate of strength and durability limits its engineering application. In this work, the NHL-based mortars were pretreated by lithium silicate (LS) solution impregnation and surface spraying. The results show that the compressive strength, surface hardness, and freeze-thaw cycle (FTC) resistance of NHL-based mortar were greatly improved after LS pretreatment. Speciﬁcally, the compressive strengths of the sample increased by about 32.7–52.0%. LS was sprayed on the sample’s surface (about 0.2 kg/m2) and the surface hardness increased by up to 10 grades. Compared with the control samples, the weight loss of treated samples decreased by about 31.6–43.8%. A rehydration process to generate the hydrated calcium silicate gel (C-S-H) was observed between calcium hydroxide (CH) and LS, which can decrease the sample’s porosity and form a silicate coating on the surface. With an increase in the concentration of LS, the macropores (50–10,000 nm) content decreased, while the mesopores (10–50 nm) and nanopores (less than 10 nm) increased. This work reveals that the LS pretreatment provides a potential route to improve NHL-based mortar’s mechanical properties and durability.

Keywords: lithium silicate; natural hydraulic lime; pores structure; compressive strength; durability

1. Introduction Natural hydraulic lime (NHL) is an important inorganic binder that can be hydrated in air and water [1–3]. Three classes of NHL were defined by the new version of the European Standard (EN459-1:2015) based on the compressive strength after 28 days of curing and the content of calcium hydroxide (CH): NHL2, NHL3.5, and NHL5 [4]. In NHL, C2S is the major hydraulic phase, while the calcium hydroxide (CH) is contributed to the carbonation for hardening [5–7]. Due to NHL-based mortars having low shrinkage [8], good permeability, and compatibility with old materials, NHL is widely used to repair historical structures and the preparation of fluid screeds [9,10]. However, certain disadvantages of NHL, such as the slow growth rate of strength and low final strength, limit its wide application. Multiple additives have been used to improve the mechanical performance and its durability. For example, pozzolan’s introduction as a partial substitution of NHL has been studied [11–13]. Grilo et al. [14] reported that the mechanical strength of NHL-metakaolin (NHL-MK) mortar increased with the introduction of metakaolin to replace lime partially. However, with an increase in aging time, the NHL-MK mortar’s compressive strength would decrease due to the increase of kaolin content.

Materials 2020, 13, 5292; doi:10.3390/ma13225292 www.mdpi.com/journal/materials Materials 2020, 13, 5292 2 of 14

A similar result is reported by Luso et al. [15,16] in which the compressive strength of the injection grout with a ternary binder experienced a decrease after 180 days of aging. The poor durability is ascribed to the instability of calcium-aluminum hydrated compounds formed in the pozzolanic reaction [17]. In addition, Luis G. et al. [18] have investigated the effect of silica fume (SF) on the properties of NHL-based grouts. The mechanical strength was increased with an increase of SF dosages. However, the workability decreased gradually. Furthermore, the mechanical and physical properties have been improved by adding graphene oxide (GO) in NHL-based mortar [19]. The above studies mainly focused on the modification of lime as a restoration material for historic buildings. However, the issues of the slow growth of strength and surface hardness remain, which are of critical importance to explore the wide scope of application of NHL in engineering. Lithium silicate (Li O nSiO ) (LS) is one of the typical lithium salt, where n is the modulus of 2 · 2 LS and is defined by the mole ratio of SiO2/Li2O. Since the lithium ion’s radius is only 0.0768 nm, LS solutions with a larger modulus can be prepared without affecting the viscosity of the solution. LS is widely used as an anti-corrosive coating for the surface of steel bars and the surface treatment of cement-based materials [20]. Witzleben [21] has systematically investigated the effects of the addition of LS in cement, including the setting time, strength, heat of hydration, and phase formation of cementitious systems. Abou Sleiman et al. [22] reported that the LS could form insoluble wear and moisture-protective surface on the concrete’s surface. Stepien et al. [23] used LS to modify silicate autoclaved material in which the improved compressive strength was obtained. The corresponding chemical reaction between LS and CH Equation (1) is:

Li O nSiO + mH O + nCa(OH) nCaO SiO (m + n 1)H O + 2LiOH (1) 2 · 2 2 2→ · 2· − 2 It shows that the existence of LS leads to part of CH forming new calcium silicate gel (C-S-H), which gives rise to the improved densification of the hydration products. Previous studies have shown that there is a large amount of CH in NHL, but the carbonation is a long-term process [24,25]. The existence of a large amount of CH in NHL-based mortar would adversely affect its strength, surface hardness, and durability [26,27]. Hence, converting the CH to C-S-H could dramatically improve the serviceability of NHL. In this paper, we focus on the effects of the LS modification on NHL-based mortars’ mechanical and durability. To verify whether LS could improve NHL-based mortar properties, the compressive strength, surface hardness, air permeability, water absorption, and FTC resistance were tested after pretreatment. The micro-morphology, pore structures, and thermogravimetric (TG) analysis were used to investigate the mechanism of the effect of LS on NHL-based mortar.

2. Materials and Methods

2.1. Materials In this study, NHL2 was purchased from Hessler Kalkwerke GmbH (Hessler, Germany) following the European standard EN459-1:2015. Chemical composition and X-ray diﬀraction patterns of NHL2 are shown in Table1 and Figure1, respectively. The LS was supplied by Specchem LLC (Shanghai, China), diluted with deionized water into three diﬀerent concentrations. The concentration was 5.0%, 10.0%, and 15.0%, respectively. The aggregates are quartz sand with particle diameters ranging from 0.5 to 2.0 mm. Tap water was used in the preparation of the NHL-based mortar samples.

Table 1. Chemical composition of NHL2 (wt %).

Chemical Composition SiO2 CaO Fe2O3 MgO Al2O3 NHL2 9.48 78.66 1.98 4.51 3.12 Materials 2020, 13, 5292 3 of 14 Materials 2020, 13, x FOR PEER REVIEW 3 of 15

Figure 1. X-ray diffraction pattern of NHL2. Figure 1. X-ray diﬀraction pattern of NHL2.

Table 1. Chemical composition of NHL2 (wt %). 2.2. Preparation Chemical Composition SiO2 CaO Fe2O3 MgO Al2O3 In this investigation, the massNHL2 ratio of water9.48 /NHL278.66 is1.98 0.5, and4.51 sand3.12/NHL2 is 1:1, 2:1, and 3:1, respectively. The mix proportion and pretreated details are listed in Table2. According to the mixing ratio, the raw2.2. materialsPreparation were mixed in the mortar mixer. The metallic mould is 40 mm 40 mm 40 mm × × for the compressiveIn this investigation, strength, water the mass absorption, ratio of water/NHL2 and FTC is 0.5, tests and (Three sand/NHL2 samples is 1:1,are 2:1, neededand 3:1, for each group, andrespectively. average valuesThe mix proportion are calculated). and pretreated The metallicdetails are listed mould in Table is 20 2.mm According100 to mmthe mixing100 mm for × × the hardnessratio, and the permeabilityraw materials were tests mixed (Three in the samples mortar mixer. are The needed metallic for mould each is group, 40 mm × and 40 mm average × 40 values mm for the compressive strength, water absorption, and FTC tests (Three samples are needed for are calculated). After molding, all the samples were cured for 24 h (RH = 100% and T = 21 1 C), each group, and average values are calculated). The metallic mould is 20 mm × 100 mm × 100 mm for ± ◦ then demolded and cured (RH = 50% and T = 21 1 C). After curing for 3-days, 7-days, and 28-days, the hardness and permeability tests (Three sample± s ◦are needed for each group, and average values the samplesare for calculated). compressive After molding, strength all measurementthe samples were were cured driedfor 24 h to (RH a constant = 100% and weight T = 21 ± (T 1 =°C),50 ◦C) and then submergedthen demolded in LS solution and cured for (RH 8 = h. 50% After and T soaking = 21 ± 1 °C). for After 8 h, curing all samples for 3-days, were 7-days, taken and out 28-days, and dried to a the samples for compressive strength measurement were dried to a constant weight (T = 50 °C) and constant weight in the oven (T = 50 C). The samples for air permeability index (API) and hardness then submerged in LS solution for◦ 8 h. After soaking for 8 h, all samples were taken out and dried to tests are cured for 7-days and 28-days (RH = 50% and T = 21 1 C) and then dried to constant weight a constant weight in the oven (T = 50 °C). The samples for air permeability± ◦ index (API) and hardness before thetests LS solution are cured wasfor 7-days sprayed and 28-days onto the(RH surface= 50% and (about T = 21 ± 0.21 °C) kg and/m then2). Similarly, dried to constant the samples weight for FTC before the LS solution was sprayed onto the surface (about 0.2 kg/m2). Similarly, the samples for FTC and water absorption tests are cured for 28-days (RH = 50% and T = 21 1 ◦C) and then dried to a and water absorption tests are cured for 28-days (RH = 50% and T = 21 ± 1 °C)± and then dried to a constant weight before the LS solution was sprayed onto the surface (about 0.2 kg/m2). When the constant weight before the LS solution was sprayed onto the surface (about 0.2 kg/m2). When the compressivecompressive strength strength test was test completed,was completed, samples samples were were broken broken andand screened screened as required as required for SEM, for SEM, TG test (RemoveTG test (Remove sands from sands samples),from samples), and and MIP MIP tests tests (The particle particle diameter diameter is about is about5 mm). 5 mm).

Table 2. The mix design of NHL-based mortar.

Composition of Mixture (kg/m3) Samples LS Concentration (%) Sand/NHL2 Water/NHL2 NHL2 Sand Water N1-0 0 N1-1 5 1:1 0.5 450 450 225 N1-2 10 N1-3 15 N2-0 0 N2-1 5 2:1 0.5 450 900 225 N2-2 10 N2-3 15 N3-0 0 N3-1 5 3:1 0.5 450 1350 225 N3-2 10 N3-3 15 Materials 2020, 13, x FOR PEER REVIEW 4 of 15

Table 2. The mix design of NHL-based mortar.

2.3.2.3. Test Test Methods Methods TheThe pencil pencil hardness hardness tester tester (Figure (Figure 22)) (3BEVS1301,(3BEVS1301, Huasheng, Guangzhou, China) was used for thethe hardness hardness test test in in accordance accordance with with the the ASTM ASTM D3363-2005 D3363-2005 [28]. [28]. This This test test method method covers covers a a procedure procedure forfor the rapid,rapid, inexpensiveinexpensive determination determination of of the the substrate’s substrate’s surface surface hardness hardness in drawing in drawing leads leads or pencil or pencilleads ofleads known of known hardness. hardness. There There are 14 are grades 14 grades of pencil of pencil hardness hardness tester, tester, from low from to low high, to 6B, high, 5B, 6B, 4B, 5B,3B, 4B, 2B, 3B, B, HB,2B, B, F, HB, H, 2H, F, H, 3H, 2H, 4H, 3H, 5H, 4H, and 5H, 6H, and respectively. 6H, respectively. The weight The weight of the of complete the complete hardness hardness tester testerwas about was about 19.6 N 19.6 and N the and weight the weight transferred transferred by the by tip the of thetip penof the is aboutpen is 6.9about5 6.9 N. When± 5 N. testing,When ± testing,the sample the sample was placed was horizontally,placed horizontally, and the and hardness the hardness tester was tester placed was onplaced the surfaceon the surface of the sample. of the sample.The pencil The formed pencil aformed 45◦ angle a 45° with angle the with tested the surface. tested surface. The pencil The hardness pencil hardness tester was tester pushed was forwardpushed forwardhorizontally horizontally at a speed at ofa speed 1 cm/ sof for 1 cm/s 2 cm. for 2 cm.

FigureFigure 2. 2. PencilPencil hardness hardness tester. tester. ( (aa)) Pencil Pencil hardness hardness tester. tester. ( (bb)) Pencil. Pencil.

TheThe NHL mortars’mortars’ compressive compressive strengths strengths were were evaluated evaluated on the on testing the machinetesting machine (SANS CMT5105, (SANS CMT5105,Shenzhen, Shenzhen, China) at aChina) loading at a rate loading of 2400 rate of200 2400 N/s. ± The200 N/s. treated The samples treated samples and the controland the samplescontrol ± sampleswere cured were for cured 3-days, for 7-days,3-days, and7-days, 28-days and under28-days the under same the conditions. same conditions. The compressive The compressive strength strengthvalues at values different at stagesdifferent were stages obtained were toobtained evaluate to LS’s evaluate effect onLS’s the effect NHL-based on the NHL-based mortar strength. mortar strength.The Autoclam permeability system (Figure3) was developed by Queen’s University Belfast to measureThe Autoclam the sorptivity, permeability air permeability, system and(Figure water 3) permeabilitywas developed of concreteby Queen’s [29 ,University30]. This system Belfast was to measureadopted the to test sorptivity, the air permeability air permeability, of the and NHL-based water permeability mortar. During of concrete the test, [29,30]. the air This was injectedsystem was into adoptedthe Autoclam to test body the air through permeability a syringe. of Whenthe NHL- the airbased pressure mortar. reached During 0.5 the bars, test, the the electronic air was controller injected intostarted the toAutoclam work, and body a pressure through decrease a syringe. was When monitored the air every pressure minute reached until the 0.5 pressurebars, the diminished electronic controllerto zero. A started plot of to the work, natural and logarithma pressure of decreas air pressuree was monitored against time every is linear, minute and until the the slope pressure of the linear regression curve for tests is used as an air permeability index (API). The logarithmic function is calculated by Equation (2). AI = ln P/t (2) where AI is the logarithmic function value, P is the air pressure, and t is the pressure corresponding to time. To evaluate the effect of LS on FTC resistance of NHL-based mortar, the weight loss was investigated after FTC. After 28-days of curing, the NHL-based mortar was initially immersed in tap water until obtaining a saturated state. An FTC includes the following processes. First, the samples were set in a 15 C freezer for 4 h. Then this was followed with a thawing period, in which the samples − ◦ were immersed in water at room temperature for 4 h. The weight loss of samples was calculated by Equation (3). W Wn W = 0 − 100 (3) W0 × Materials 2020, 13, x FOR PEER REVIEW 5 of 15 diminished to zero. A plot of the natural logarithm of air pressure against time is linear, and the slope of the linear regression curve for tests is used as an air permeability index (API). The logarithmic functionMaterials is2020 calculated, 13, 5292 by Equation (2). 5 of 14 AI = ln P/t (2) wherewhere AI W is is the the logarithmic weight loss function of FTC (%)value, for P n is times, the airW pressure,0 is the initial and t weight, is the pressure and Wn correspondingis the weight of tosamples time. after FTC for n times.

Figure 3. Autoclam permeability system. (a) Syringe for applying air pressure, autoclam body an Figure 3. Autoclam permeability system. (a) Syringe for applying air pressure, autoclam body an delectronic controller. (b) The sample being tested delectronic controller. (b) The sample being tested The surface morphology of the samples was characterized by SEM (UItra55, Carl Zeiss NTS To evaluate the effect of LS on FTC resistance of NHL-based mortar, the weight loss was GmbH, Heidenheim, Germany). The matrix’s pore structure was represented by MIP (Auto pore investigated after FTC. After 28-days of curing, the NHL-based mortar was initially immersed in tap IV9500, Micromeritics, Norcross, GA, USA, maximum mercury intrusion pressure: 30,000 Psi). water until obtaining a saturated state. An FTC includes the following processes. First, the samples Thermogravimetry (TG, Jupiter STA449C, Netzsch, Bavaria, Germany) was carried out at 10 ◦C/min in were set in a −15 °C freezer for 4 h. Then this was followed with a thawing period, in which the an atmosphere of ﬂowing N2 (50 mL/min) in alumina crucibles over a temperature ranging from 25 to samples were immersed in water at room temperature for 4 h. The weight loss of samples was 1000 ◦C. calculated by Equation (3).

3. Results and Discussion W = 100 (3) 3.1. The Effect on Mechanical Properties where W is the weight loss of FTC (%) for n times, W0 is the initial weight, and Wn is the weight of samplesNHL-based after FTC for mortars n times. were treated with three different concentrations of LS. The effect of the LS pretreatmentThe surface on morphology the hardness of NHL-basedthe samples mortar was characterized is shown in Figure by SEM4. In (UItra55, Figure4, samples’Carl Zeiss surface NTS GmbH,hardness Heidenheim, at 7-days andGermany). 28-days The increased matrix’s significantly pore structure when was LS represented solution concentration by MIP (Auto gradually pore IV9500,increases Micromeritics, from 5.0% to 15.0%.Norcross, Specifically, GA, USA, after maximum treatment mercury by LS, the intrusion 7-days surface pressure: hardness 30,000 grades Psi). of Thermogravimetrysample N1-3, N2-3, (TG, and Jupiter N3-3 reach STA449C, 4H, 3H, Netzsch, and 3H, Bavaria, respectively. Germany) Similarly, was thecarried surface out hardnessat 10 °C/min of the insample an atmosphere cured for of 28-days flowing can N2 also(50 mL/min) significantly in alumina increase crucibles with the over LS surface a temperature treatment. ranging The 28-days from 25surface to 1000 hardness °C. grades of sample N1-3, N2-3, and N3-3 reach 6H. This phenomenon can be attributed to the fact that a large amount of CH in NHL2 is consumed by LS to form C-S-H, forming a silicate 3.coating Results onand the Discussion sample’s surface [6,21]. The sand/NHL2 ratio has an important effect on NHL-based mortar’s surface hardness. It can be concluded that a lower sand/NHL2 ratio is more likely to improve 3.1.the The surface Effect hardness. on Mechanical It is similar Properties to the results reported by Pan et al. [31]. The magnesium fluorosilicate andNHL-based sodium silicate mortars decreased were treated the carbonation with three depth different and concentrations increased surface of LS. hardness The effect of concrete of the LS due pretreatmentto the formation on the of hardness silicate coating of NHL-based by C-S-H. mortar In addition, is shown Thompson in Figure 4. et In al. Fi [gure32] reported 4, samples’ that surface sodium hardnesssilicate couldat 7-days improve and concrete28-days surfaceincreased properties significantly such aswhen hardness, LS solution permeability, concentration chemical gradually durability, increasesand abrasion from resistance.5.0% to 15.0%. Specifically, after treatment by LS, the 7-days surface hardness grades of sampleAs shown N1-3, N2-3, in Figure and5, N3-3 the compressive reach 4H, 3H, strength and 3H, test respectively. results revealed Similarly, that the the compressive surface hardness strengths ofof the samples sample treatedcured for by 28-days LS were can higher also significantl than that ofy increase the control with samples. the LS surface The compressive treatment. The strength 28- daysincreases surface with hardness an increase grades in of LS sample concentration. N1-3, N2 Specifically,-3, and N3-3 the reach compressive 6H. This strengths phenomenon of the can sample be attributedN1-3 were to increasedthe fact that by a 52.0% large (3-days),amount of 40.0% CH in (7-days), NHL2 is and consumed 32.7% (28-days), by LS to respectively.form C-S-H, forming Similarly, the compressive strengths of the sample N2-3 were increased by 81.0% (3-days), 40.0% (7-days), and 64.3% (28-days). The compressive strength of the sample N3-3 were increased by 94.7% (3-days), 53.3% (7-days), and 75.7% (28-days), respectively. The increased compressive strengths were attributed Materials 2020, 13, x FOR PEER REVIEW 6 of 15

a silicate coating on the sample’s surface [6,21]. The sand/NHL2 ratio has an important effect on NHL- based mortar’s surface hardness. It can be concluded that a lower sand/NHL2 ratio is more likely to improve the surface hardness. It is similar to the results reported by Pan et al. [31]. The magnesium fluorosilicate and sodium silicate decreased the carbonation depth and increased surface hardness of concrete due to the formation of silicate coating by C-S-H. In addition, Thompson et al. [32] reported that sodium silicate could improve concrete surface properties such as hardness, permeability, chemical durability, and abrasion resistance.

Materials 2020, 13, x FOR PEER REVIEW 6 of 15

Materialsa silicate2020 coating, 13, 5292 on the sample’s surface [6,21]. The sand/NHL2 ratio has an important effect on NHL-6 of 14 based mortar’s surface hardness. It can be concluded that a lower sand/NHL2 ratio is more likely to improve the surface hardness. It is similar to the results reported by Pan et al. [31]. The magnesium tofluorosilicate the introduction and sodium of more silicate silicate decreased ions by the the higher carbonation concentration depth an ofd LSincreased solution, surface which hardness consumed of CHconcrete to form due more to the C-S-H. formation This of result silicate agrees coating well by with C-S-H. the In results addition, reported Thompson by Luis et Gal. [ 18[32]] inreported which thethat silica sodium fume silicate (SF) leads could to pozzolanicimprove concrete reactions surface by reaction properties with CH,such resulting as hardness, in the permeability, formation of additionalchemical durability, C-S-H. and abrasion resistance.

Figure 4. Effect of LS on the hardness of NHL-based mortar. (a) Modification after 7-days of curing. (b) Modification after 28-days of curing.

As shown in Figure 5, the compressive strength test results revealed that the compressive strengths of samples treated by LS were higher than that of the control samples. The compressive strength increases with an increase in LS concentration. Specifically, the compressive strengths of the sample N1-3 were increased by 52.0% (3-days), 40.0% (7-days), and 32.7% (28-days), respectively. Similarly, the compressive strengths of the sample N2-3 were increased by 81.0% (3-days), 40.0% (7- days), and 64.3% (28-days). The compressive strength of the sample N3-3 were increased by 94.7% (3-days), 53.3% (7-days), and 75.7% (28-days), respectively. The increased compressive strengths were attributed to the introduction of more silicate ions by the higher concentration of LS solution, which consumed CH to form more C-S-H. This result agrees well with the results reported by Luis G [18] FigureinFigure which 4.4. E Effecttheffect silica ofof LS LS fume on on the the (SF) hardness hardness leads of toof NHL-based pozzolaNHL-basednic mortar. reactionsmortar. (a )( Modificationa )by Modification reaction after with after 7-days CH, 7-days resulting of curing. of curing. (inb) the formationModification(b) Modification of additional after after 28-days C-S-H. 28-days of curing. of curing.

As shown in Figure 5, the compressive strength test results revealed that the compressive strengths of samples treated by LS were higher than that of the control samples. The compressive strength increases with an increase in LS concentration. Specifically, the compressive strengths of the sample N1-3 were increased by 52.0% (3-days), 40.0% (7-days), and 32.7% (28-days), respectively. Similarly, the compressive strengths of the sample N2-3 were increased by 81.0% (3-days), 40.0% (7- days), and 64.3% (28-days). The compressive strength of the sample N3-3 were increased by 94.7% (3-days), 53.3% (7-days), and 75.7% (28-days), respectively. The increased compressive strengths were attributed to the introduction of more silicate ions by the higher concentration of LS solution, which consumed CH to form more C-S-H. This result agrees well with the results reported by Luis G [18] inFigureFigure which 5.5. EffectEtheﬀect silica of of LS LS fumeon on compressive compressive (SF) leads strength to strength pozzola of NHL-based ofnic NHL-based reactions mortar: mortar: by ( areaction) Group (a) Group N1,with (b N1, )CH, group (b )resulting group N2, and N2, in the formationand(c) group (c) of group additional N3. N3. C-S-H.

3.2.3.2. TheThe EEffectffect onon AirAir Permeability AutoclamAutoclam airair permeabilitypermeability testing testing was was adopted adopted to to investigate investigate the the e ffeffectsects of of LS LS pretreatment pretreatment on on the airthe permeability air permeability of NHL-based of NHL-based mortar mortar [33]. As[33]. shown As shown in Figure in 6Figure, the API 6, valuethe API of samplesvalue of showedsamples a decreasingshowed a trenddecreasing when LS’strend concentration when LS’s graduallyconcentrat increasedion gradually from 0%increased to 15.0%. from Specifically, 0% to 15.0%. the API value of N1-3, N2-3, and N3-3 at 7-days were 1.7, 1.7, and 1.6, respectively. A dramatic decrease occurred at 28 days when the API value of N1-3, N2-3, and N3-3 were 1.6, 1.5, and 1.4 after being pretreated with 15.0% LS solutions. It can be ascribed to the formation of a large amount of C-S-H within 8 h. C-S-H formed silicate coating on the samples’ surface that blocks the capillary pores, which could reduce the erosion of harmful substances and improve NHL-based mortar durability [34]. The resultFigure was 5. Effect consistent of LS on with compressive previous strength results reportedof NHL-based by J. mortar: LaRosa (a Thompson,) Group N1, ( whichb) group demonstrated N2, and that silicate(c) group sealers N3. could decrease the air permeability [32].

3.3. Eﬀect of LS on Water Absorption of NHL-Based Mortar 3.2. The Effect on Air Permeability After curing for 28-days, the water absorption of the sample was tested after being treated by Autoclam air permeability testing was adopted to investigate the effects of LS pretreatment on LSthe [ 30air]. permeability The samples of were NHL-based ﬁrst vacuumed mortar for [33]. 4 h, As and shown then immersedin Figure 6, in the deionized API value water of forsamples 48 h. m Aftershowed wiping a decreasing the surface trend water, when the massLS’s (concentrat1) of theion samples gradually were measuredincreased underfrom 0% a surface-dry to 15.0%. Materials 2020, 13, 5292 7 of 14

condition, and then the samples were dried in an oven at 80 ◦C until reaching constant mass (m0). The water absorption of the samples can be calculated using Equation (4).

m m ρ = 1 − 0 100 (4) m × 1

The effect of LS pretreatment on water absorption of NHL-based mortar is shown in Figure7. As shown in Figure7, the water absorption of mortar decreases gradually after pretreatment. This result agreesMaterials well 2020 with, 13, x theFOR API PEER testing. REVIEWWhen compared with the control samples, the water absorptions7 of 15 of the group N1, N2, and N3 decreased by 43.0%, 36.6%, and 31.6% at 28-days after 15.0% LS solution pretreatment.Specifically, Waterthe API absorption value of N1-3, reflects N2-3, the porosityand N3-3 of at concrete. 7-days Thus,were 1.7, it represents 1.7, and 1.6, that respectively. the LS solution A reduceddramatic permeability decrease occurred performance at 28 days by when decreasing the API the value porosity of N1-3, of NHL-basedN2-3, and N3-3 mortar. were 1.6, It has 1.5, beenand reported1.4 after that being inorganic pretreated and organicwith 15.0% materials LS solutions. are used It to can prepare be ascribed coatings to of the cement-based formation of materials. a large Relevantamount literatureof C-S-H [32within] reveals 8 h. that C-S-H the formed sodium silicatessilicate coating (SS) can on prevent the samples’ erosion withsurface 15% that concentration blocks the ofcapillary chloride pores, ion and which that thecould resistance reduce ofthe freezing-thawing erosion of harmful of coated substances samples and is improve improved NHL-based due to the reducedmortar waterdurability absorption [34]. The [35 ].result Although was organicconsistent surface with treatmentprevious agentsresults canreported significantly by J. LaRosa reduce waterThompson, absorption, which they demonstrated may lose their that protective silicate seal effersect could under decrease high temperature the air permeability and weathering [32]. [36].

FigureFigure 6. 6.Effect Effect of LSof onLS API on ofAPI NHL-based of NHL-based mortar. mortar. (a) Modification (a) Modification after 7-days after of curing. 7-days ( bof) Modification curing. (b) MaterialsafterModification 2020 28-days, 13, x FOR of after curing. PEER 28-days REVIEW of curing. 8 of 15

3.3. Effect of LS on Water Absorption of NHL-Based Mortar After curing for 28-days, the water absorption of the sample was tested after being treated by LS [30]. The samples were first vacuumed for 4 h, and then immersed in deionized water for 48 h. After wiping the surface water, the mass (m1) of the samples were measured under a surface-dry condition, and then the samples were dried in an oven at 80 °C until reaching constant mass (m0). The water absorption of the samples can be calculated using Equation (4).

𝑚 𝑚 𝜌 100 (4) 𝑚 The effect of LS pretreatment on water absorption of NHL-based mortar is shown in Figure 7. As shown in Figure 7, the water absorption of mortar decreases gradually after pretreatment. This result agrees well with the API testing. When compared with the control samples, the water absorptions of the group N1, N2, and N3 decreased by 43.0%, 36.6%, and 31.6% at 28-days after 15.0% LS solution pretreatment. Water absorption reflects the porosity of concrete. Thus, it represents that the LS solution reduced permeability performance by decreasing the porosity of NHL-based mortar. FigureFigure 7.7. EEffectﬀect ofof LSLS onon waterwater absorptionabsorption ofof NHL-basedNHL-based mortar.mortar. It has been reported that inorganic and organic materials are used to prepare coatings of cement- 3.4.based The materials.Analysis of Relevant Morphology literature [32] reveals that the sodium silicates (SS) can prevent erosion with 15% concentration of chloride ion and that the resistance of freezing-thawing of coated samples is improvedThe micro-morphology due to the reduced of thewater samples absorption N3-0 [35].cured Although for 3 days organic and samplesurface treatmentN3-3 treated agents with can LS aftersignificantly being cured reduce for water3 days absorption, are shown theyin Figure may lose8. As their shown protective in Figure effect 8a,b, under a large high number temperature of CH andand a weathering few C-S-H are[36]. observed. These products are mainly from the raw materials and partly from the hydration products of C2S. The edges of CH grains are clear and overlapped, which leads to larger micro-pores, microvoids, and micro-cracks in sample N3-0.

Figure 8. SEM images of (a,b) samples N3-0, (c,d) samples N3-3.

In Figure 8c,d, CH almost disappeared, and a large number of amorphous phases were observed. It can be observed that a dense C-S-H coating has been formed on the surface of sample N3-3. The high concentration of silicate ions indicated that a large amount of CH was consumed by Materials 2020, 13, x FOR PEER REVIEW 8 of 15

Materials 2020, 13, 5292 8 of 14 Figure 7. Effect of LS on water absorption of NHL-based mortar.

3.4.3.4. The The Analysis Analysis of of Morphology Morphology TheThe micro-morphology micro-morphology of of the the samples samples N3-0 N3-0 cured cured for for 3 3 days days and and sample sample N3-3 N3-3 treated treated with with LS LS afterafter being being cured cured for for 3 3 days are shown in Figure 88.. AsAs shownshown inin FigureFigure8 8a,b,a,b, a a large large number number of of CH CH andand a a few few C-S-H C-S-H are are observed. observed. These These products products are are mainly mainly from from the the raw raw materials materials and and partly partly from from the the hydrationhydration products products of of C C22S.S. The The edges edges of of CH CH grains grains are are clear clear and overlapped, which which leads leads to to larger larger micro-pores,micro-pores, microvoids, and mi micro-crackscro-cracks in sample N3-0.

FigureFigure 8. 8. SEMSEM images images of of ( (aa,,bb)) samples samples N3-0, N3-0, ( (cc,,dd)) samples samples N3-3. N3-3. In Figure8c,d, CH almost disappeared, and a large number of amorphous phases were observed. In Figure 8c,d, CH almost disappeared, and a large number of amorphous phases were It can be observed that a dense C-S-H coating has been formed on the surface of sample N3-3. The high observed. It can be observed that a dense C-S-H coating has been formed on the surface of sample concentration of silicate ions indicated that a large amount of CH was consumed by LS to form C-S-H. N3-3. The high concentration of silicate ions indicated that a large amount of CH was consumed by These results are consistent with the permeability test. The pores are gradually ﬁlled, giving rise to a decrease in air permeability when more C-S-H gel is formed [30,32].

3.5. Effect of LS Pretreatment on FTC Resistance Figure9a–c show the damage of group N3 after 10, 20, and 30 FTC tests, respectively. As shown in Figure9a, the surface separation occurred on the surface of sample N3-0 after 10 FTC, and the sample N3-1, N3-2, and N3-3 still maintained an intact state. In comparison, the control samples occurred with a cataclastic failure after 20 and 30 FTC, respectively. It is verified by the treated samples to have larger FTC resistance when compared to the control samples. Particularly, the higher concentration of LS was more beneficial to improve the FTC resistance. Similarly, group N1 and group N2 show the same trend after FTC tests. Figure9d–f shows the weight loss of group N1, N2, and N3 after 30 FTC. Compared with the pretreated samples, the weight loss of N1-0, N2-0, and N3-0 after 30 FTC were 23.0%, 19.0%, and 32.0%, respectively, while the weight loss of sample N1-3, N2-3, and N3-3 after 30 FTC were 13.0%, 13.0%, and 18.0%, respectively. These results are very different from previous reports. Shuqiang Xu et al. [37] reported that the compressive strength is much higher after the FTC tests, which could be attributed to the water curing condition that can further improve the mortars’ strength. Materials 2020, 13, 5292 9 of 14 Materials 2020, 13, x FOR PEER REVIEW 10 of 15

Figure 9. PhotographsPhotographs ofof thethe groupgroup N3 N3 after after FTC FTC tests: tests: (a )(a After) After 10 10 FTC, FTC, (b) ( Afterb) After 20 FTC, 20 FTC, (c) After (c) After 30 FTC. 30 FTC.Weight Weight loss with loss FTCwith forFTC NHL-based for NHL-based mortar: mortar: (d) Group (d) Group N1, (e N1,) group (e) group N2, and N2, ( fand) group (f) group N3. N3.

3.6. Pore Pore Structures Structures Figure 10 presents the effect eﬀect of LS modification modiﬁcation on the pore structure distribution of NHL-based mortar. A main peak waswas locatedlocated atat 10001000 nm nm in in the the pore pore size size distribution distribution curves, curves, which which is is the the same same as asthe the previous previous studies studies [5]. [5]. When When the concentrationthe concentratio of LSn of increased LS increased from 5%from to 15.0%,5% to 15.0%, the peaks the around peaks around1000 nm 1000 decreased nm decreased gradually. gradually. As shown As in shown Figure in10 Figureb,c, these 10b,c, peaks these moved peaks to moved the position to the of position 700 nm ofand 700 400 nm nm, and respectively. 400 nm, respectively. It indicates It that indicates the void that ratio the of void macropores ratio of macrop (50–10,000ores nm) (50–10,000 in the samples nm) in thedecreases samples gradually decreases with gradually an increase with in an the increase concentration in the concentration of LS. In contrast, of LS. the In voidcontrast, ratio the of nanoporesvoid ratio of(less nanopores than 10 nm) (less increased than 10 nm) gradually. increased Porosity gradually. and pore Porosity size distribution and pore size through distribution MIP data through are shown MIP datain Table are3 shown and Figure in Table 11, respectively.3 and Figure 11, respectively. Materials 2020, 13, 5292 10 of 14 MaterialsMaterials 2020 2020, ,13 13, ,x x FOR FOR PEER PEER REVIEW REVIEW 1111 of of 15 15

FigureFigureFigure 10.10. 10. Effect EEffectﬀect of of LSLS pretreatmentpretreatment pretreatment on on the the pore pore size size distribution distribution ofof ofNHLNHL NHL mortar:mortar: mortar: ( (aa)) Group (Groupa) Group N1, N1, ( N1, b(b) ) (groupbgroup) group N2, N2, N2, and and and ( (cc)) (group cgroup) group N3.N3. N3.

FigureFigure 11. 11. Percent of macropores, macropores, mesopores, mesopores, and and nanopores nanopores of ofthe the samples samples after after modification: modification: (a) Figure 11. Percent of macropores, mesopores, and nanopores of the samples after modification: (a) (aGroup) Group N1, N1, (b ()b group) group N2, N2, and and (c) ( cgroup) group N3. N3. Group N1, (b) group N2, and (c) group N3. Table 3. Effect of LS on porosity of NHL-based mortar cured for 28-days (wt %). Table 3. Effect of LS on porosity of NHL-based mortar cured for 28-days (wt %). Table 3. Effect of LS on porosity of NHL-based mortar cured for 28-days (wt %). ConcentrationConcentration of LS of LS Group Group N1 N1 Group Group N2 N2 Group GroupN3 N3 Concentration of LS Group N1 Group N2 Group N3 0% 0%25.58 25.58 29.84 37.7137.71 0% 25.58 29.84 37.71 5.0% 5.0%23.58 23.58 29.20 36.7236.72 5.0% 10.0%23.58 23.11 28.3829.20 34.72 36.72 10.0% 23.11 28.38 34.72 10.0% 15.0%23.11 20.12 26.3828.38 31.83 34.72 15.0% 20.12 26.38 31.83 15.0% 20.12 26.38 31.83 TheThe porosity porosity ofof samplessamples with a di differentfferent void void ratio ratio show show a adecreasing decreasing trend trend with with an anincrease increase in The porosity of samples with a different void ratio show a decreasing trend with an increase in inLS LS concentration. concentration. In In the the meantime, meantime, the the number number of of macropores macropores (50–10,000 (50–10,000 nm) nm) decreased decreased while while LS concentration. In the meantime, the number of macropores (50–10,000 nm) decreased while mesoporesmesopores (10–50 (10–50 nm) nm) and and nanopores nanopores (less than(less 10 than nm) increased10 nm) increased with an increase with an in LSincrease concentration. in LS Themesoporesconcentration. void ratio (10–50 The of macropores voidnm) ratioand ofnanopores in macropores the sample (less in decreasesthethan sample 10 gradually.nm)decreases increased grad Compared ually.with Comparedan with increase the with control in the LS samples,concentration.control samples, macropores The macropores void of theratio samples ofof themacropores samples N1-3, N2-3, N1-3, in the and N2-3, sample N3-3 and gradually decreases N3-3 gradually decreased gradually. decreased from Compared 92.9% from 92.9% towith 82.7%, theto 92.5%control82.7%, to samples,92.5% 84.4% ,to and macropores84.4%, 92.6% and to 83.0%,of92.6% the samplesto respectively. 83.0%, N1-3, respectively. OnN2-3, the and contrary, On N3-3 the gradually withcontrary, LS concentration decreased with LS concentration from increasing, 92.9% to the82.7%,increasing, void 92.5% ratio the ofto mesoporesvoid84.4%, ratio and andof 92.6% mesopores nanopores to 83.0%, and in therespectively. nano samplepores gradually inOn the the sample contrary, increases. gradually with The mesoporesLS increases. concentration of The the samplesincreasing,mesopores N1-3, theof N2-3, thevoid samples andratio N3-3 ofN1-3, mesopores gradually N2-3, and increasedand N3-3 nano gradually frompores 6.1% in increased the to 14.3%,sample from 6.5% gradually 6.1% to 12.5%, to 14.3%,increases. and 6.5% 6.3% The to to 13.6%,mesopores12.5%, respectively. and of 6.3% the tosamples Mesopores 13.6%, N1-3,respectively. of theN2-3, samples andMesopore N3-3 N1-3, graduallys N2-3,of the and samples increased N3-3 graduallyN1-3, from N2-3, 6.1% increased and to N3-3 14.3%, from gradually 6.1%6.5% toto 14.3%,12.5%,increased 6.5%and from to6.3% 12.5%, 6.1%to 13.6%, and to 6.3%14.3%, respectively. to 6.5% 13.6%, torespectively. Mesopore12.5%, ands of6.3% Nanopores the tosamples 13.6%, of the N1-3,respectively. samples N2-3, N1-3, and Nanopores N2-3,N3-3 gradually and of N3-3 the graduallyincreasedsamples N1-3, increasedfrom N2-3, 6.1%from andto 14.3%, N3-3 1.0% gradually to6.5% 3.0%, to 1.0%12.5%,increased to an 2.9%, dfrom 6.3% and 1.0% to 1.1% to13.6%, 3.0%, to 3.4%, respectively.1.0% respectively. to 2.9%, Nanoporesand 1.1% The increaseto 3.4%,of the ofsamplesrespectively. mesopores N1-3, andThe N2-3, nanoporesincrease and N3-3 of mesopores with gradually the addition increasedand nanopore of LSfrom mainlys with1.0% theto attributed 3.0%, addition 1.0% to of theto LS 2.9%, filling mainly and eff attributed1.1%ect of to C-S-H 3.4%, to formationrespectively.the filling through effect The of increase rehydration C-S-H formation of mesopores of CH through and and LS [ 21rehydrationnanopore]. It is similars with of CH to the theand addition results LS [21]. reported of It LSis similar mainly by Kai to attributed Luothe results [38,39 to]. thereported filling byeffect Kai of Luo C-S-H [38,39]. formation through rehydration of CH and LS [21]. It is similar to the results 3.7.reported TG Analysis by Kai ResultsLuo [38,39]. 3.7. TG Analysis Results Figure 12 shows the thermogravimetric (TG) curve of group N1 cured for 28-days with and 3.7. TG Analysis Results withoutFigure LS pretreatment. 12 shows the Four thermogravimetric mass loss stages (TG) could curve be observed,of group N1 including cured for 0–100 28-days◦C, 100–400 with and◦C, 400–570withoutFigure◦ C,LS and12pretreatment. shows 570–800 the◦ C,thermogravimetricFour respectively. mass loss stages The (TG) mass could cu loss rvebe stage observed,of group below includingN1 100 cured◦C is 0–100for corresponding 28-days °C, 100–400 with to °C,and the dehydrationwithout400–570 LS °C, pretreatment. ofand adsorption 570–800 °C,Four water. respectively. mass The loss mass stages The loss mass atco 100–400uld loss be stage observed,◦C correspondsbelow including 100 °C to is dehydration 0–100corresponding °C, 100–400 of C-S-H,to the °C, while400–570dehydration the °C, mass and of loss adsorption570–800 at 400–570 °C, water. ◦respectively.C is theThe dehydroxilation mass The loss mass at 100–400 loss ofCH. stage °C The corresponds below mass 100 loss °C atto 570–800isdehydration corresponding◦C is of due C-S-H, to thethe decompositiondehydrationwhile the mass of of adsorptionloss calcium at 400–570 carbonate water. °C Theis (CC)the mass dehydroxilation [40 loss]. Based at 100–400 on eachof CH. °C mineral’s corresponds The mass mass loss to loss at dehydration 570–800 characteristics °C isof dueC-S-H, at to an whilethe decomposition the mass loss atof 400–570calcium carbonate°C is the dehydroxilation (CC) [40]. Based of on CH. each The mineral’s mass loss mass at 570–800 loss characteristics °C is due to the decomposition of calcium carbonate (CC) [40]. Based on each mineral’s mass loss characteristics Materials 2020, 13, 5292 11 of 14 Materials 2020, 13, x FOR PEER REVIEW 12 of 15 elevatedat an elevated temperature, temperature, the content the content of water of boundwater bound to C-S-H to (%),C-S-H CH, (%), and CH CC, and in hardened CC in hardened NHL pastes NHL couldpastes be could evaluated be evaluated [41,42]. [41,42]. As shown As inshown Figure in 12 Figure, with 12, an increasewith an increase in LS concentration, in LS concentration, the content the ofcontent C-S-H of increased, C-S-H increased, while the while content the ofcontent CH and of CH CC decreased.and CC decreased. The detailed The detailed content content of C-S-H, of CH,C-S- andH, CH, CC inand hardened CC in hardened pastes after pastes being after treated being by treated LS are by given LS are in Tablegiven4 in. Table 4.

FigureFigure 12. 12.TG TG curvescurves ofof thethe groupgroup N1. N1.

TableTable 4. 4.The The contentcontent of of the the main main phases phases in in samples samples with with various various concentrations concentrations of of LS LS (%). (%).

GroupGroup Samples Water Water Bound to to C-S-H C-S-H CHCH CC N1-0N1-0 3.43.4 31.431.4 24.5 24.5 N1-1N1-1 4.64.6 28.328.3 23.3 23.3 N1N1 N1-2N1-2 4.84.8 26.526.5 19.2 19.2 N1-3 5.3 23.1 21.0 N1-3 5.3 23.1 21.0 N2-0N2-0 3.53.5 32.432.4 24.6 24.6 N2-1 4.3 29.2 23.7 N2 N2-1 4.3 29.2 23.7 N2 N2-2 4.5 27.3 19.8 N2-3N2-2 5.24.5 23.827.3 19.8 18.0 N2-0N2-3 3.35.2 32.323.8 18.0 24.0 N2-1N2-0 4.73.3 29.332.3 24.0 23.0 N3 N2-2N2-1 4.64.7 27.429.3 23.0 19.1 N3 N2-3N2-2 5.34.6 24.127.4 19.1 18.2 N2-3 5.3 24.1 18.2 It can be observed that the addition of LS can regulate the content of C-S-H. Compared with the controlIt can sample,be observed the C-S-H that the content additi ofon theof LS group can regulate N1, group the N2, cont andent group of C-S-H. N3 increasedCompared by with 55.9%, the 48.6%,control and sample, 60.6%, the respectively. C-S-H content On theof the contrary, group theN1,addition group N2, of LSand can group decrease N3 increased the content by of55.9%, CH and48.6%, CC. and In particular, 60.6%, respectively. CH’s content On decreased the contrary, by 35.9%,the addition 36.1%, andof LS 34.0% can decrease when the the concentration content of CH of LSand was CC. 15.0%, In particular, respectively. CH’s Atcontent the same decreased time, the by CC35.9%, content 36.1%, decreased and 34.0% by when 16.7%, the 36.7%, concentration and 31.9%, of respectively.LS was 15.0%, The respectively. results show At that the LSsame could time, consume the CC exisingcontent ordecreased newly generated by 16.7%, CH 36.7%, to form andC-S-H. 31.9%, Theserespectively. results agreeThe results well with show the that results LS could of pore consum structurese exising analysis or newly and thegenerated air permeability CH to form test. C-S-H. These results agree well with the results of pore structures analysis and the air permeability test. 4. Conclusions 4. Conclusions In this study, NHL-based mortar was pretreated by LS. From the test and analysis results, the followingIn this study, conclusions NHL-based can be mortar drawn: was pretreated by LS. From the test and analysis results, the following conclusions can be drawn: 1. The NHL-based mortar’s compressive strength can be improved after being impregnated in LS Materials 2020, 13, 5292 12 of 14

1. The NHL-based mortar’s compressive strength can be improved after being impregnated in LS solution for 8 h. The growth rate of compressive strength was maintained between 32.7% and 52.0%. 2. After spraying LS on the sample’s surface (about 0.2 kg/m2), the surface hardness increased by up to 10 grades. 3. The sample surface was densiﬁed by LS, and the FTC resistance was improved. In particular, compared with the control samples after 30 FTC tests, the weight loss of sample N1-3, N2-3, and N3-3 decreased by 43.0%, 31.6%, and 43.8%, respectively. 4. LS can consume existing or newly generated CH to form C-S-H, which reﬁnes the NHL-based mortar’s pore structure, leading to a decrease in API value and water absorption.

Author Contributions: Conceptualization, Z.S.; methodology, Z.S.; validation, Z.L. (Zhenyu Lai); investigation, Z.S.; resources, Z.L. (Zhongyuan Lu); data curation, Z.L. (Zhenyu Lai); writing—original draft preparation, Z.S.; writing—review and editing, Z.L. (Zhenyu Lai); project administration, Z.L. (Zhongyuan Lu); funding acquisition, Z.L. (Zhongyuan Lu). All authors have read and agreed to the published version of the manuscript. reported. Funding: This work was supported by the Sichuan Science and Technology Program (No.2019ZDZX0024) and the Doctoral Research Foundation of Southwest University of Science and Technology (18zx7134). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Silva, B.A.; Ferreira Pinto, A.P.; Gomes, A. Natural hydraulic lime versus cement for blended lime mortars for restoration works. Constr. Build. Mater. 2015, 94, 346–360. 2. Arizzi, A.; Viles, H.; Cultrone, G. Experimental testing of the durability of lime-based mortars used for rendering historic buildings. Constr. Build. Mater. 2012, 28, 807–818. 3. Maravelaki-Kalaitzaki, P.; Bakolas, A.; Karatasios, I.; Kilikoglou, V. Hydraulic lime mortars for the restoration of historic masonry in Crete. Cem. Concr. Res. 2005, 35, 1577–1586. 4. CEN. Building Lime. Definitions, Specifications and Conformity Criteria; Part 1; Cen: Brussels, Belgium, 2015. 5. Lanas, J.; Pérez Bernal, J.L.; Bello, M.A.; Alvarez Galindo, J.I. Mechanical properties of natural hydraulic lime-based mortars. Cem. Concr. Res. 2004, 34, 2191–2201. 6. Mertens, G.; Madau, P.; Durinck, D.; Blanpain, B.; Elsen, J. Quantitative mineralogical analysis of hydraulic limes by X-ray diffraction. Cem. Concr. Res. 2007, 37, 1524–1530. 7. Garijo, L.; Zhang, X.; Ruiz, G.; Ortega, J.J.; Wu, Z. The effects of dosage and production process on the mechanical and physical properties of natural hydraulic lime mortars. Constr. Build. Mater. 2018, 169, 325–334. 8. Pozo-Antonio, J.S. Evolution of mechanical properties and drying shrinkage in lime-based and lime cement-based mortars with pure limestone aggregate. Constr. Build. Mater. 2015, 77, 472–478. 9. Elert, K.; Rodriguez-Navarro, C.; Pardo, E.S.; Hansen, E.; Cazalla, O. Lime Mortars for the Conservation of Historic Buildings. Stud. Conserv. 2002, 47, 62–75. 10. Jaafri, R.; Aboulayt, A.; Alam, S.Y.; Roziere, E.; Loukili, A. Natural hydraulic lime for blended cement mortars: Behavior from fresh to hardened states. Cem. Concr. Res. 2019, 120, 52–65. 11. De Azeredo, A.F.N.; Struble, L.J.; Carneiro, A.M.P. Microstructural characteristics of lime-pozzolan pastes made from kaolin production wastes. Mater. Struct. 2014, 48, 2123–2132. 12. Bras, A.; Henriques, F.M.A.; Cidade, M.T. Effect of environmental temperature and fly ash addition in hydraulic lime grout behaviour. Constr. Build. Mater. 2010, 24, 1511–1517.

13. Pozo-Antonio, J.S.; Dionísio, A. Physical-mechanical properties of mortars with addition of TiO2 nanoparticles. Constr. Build. Mater. 2017, 148, 261–272. 14. Grilo, J.; Faria, P.; Veiga, R.; Santos Silva, A.; Silva, V.; Velosa, A. New natural hydraulic lime mortars—Physical and microstructural properties in diﬀerent curing conditions. Constr. Build. Mater. 2014, 54, 378–384. 15. Vavriˇcuk,A.; Bokan-Bosiljkov, V.; Kramar, S. The inﬂuence of metakaolin on the properties of natural hydraulic lime-based grouts for historic masonry repair. Constr. Build. Mater. 2018, 172, 706–716. 16. Luso, E.; Lourenço, P.B. Experimental laboratory design of lime based grouts for masonry consolidation. Int. J. Archit. Herit. 2017, 1–10. [CrossRef] Materials 2020, 13, 5292 13 of 14

17. Grilo, J.; Santos Silva, A.; Faria, P.; Gameiro, A.; Veiga, R.; Velosa, A. Mechanical and mineralogical properties of natural hydraulic lime-metakaolin mortars in different curing conditions. Constr. Build. Mater. 2014, 51, 287–294. 18. Baltazar, L.G.; Henriques, F.M.A.; Jorne, F.; Cidade, M.T. Combined effect of superplasticizer, silica fume and temperature in the performance of natural hydraulic lime grouts. Constr. Build. Mater. 2014, 50, 584–597. 19. Faria, P.; Duarte, P.; Barbosa, D.; Ferreira, I. New composite of natural hydraulic lime mortar with graphene oxide. Constr. Build. Mater. 2017, 156, 1150–1157. 20. Tang, W.; Khavarian, M.; Yousefi, A.; Chan, R.W.K.; Cui, H. Influence of Surface Treatment of Recycled Aggregates on Mechanical Properties and Bond Strength of Self-Compacting Concrete. Sustainability 2019, 11, 1–18. 21. Witzleben, S.T. Acceleration of Portland cement with lithium, sodium and potassium silicates and hydroxides. Mater. Chem. Phys. 2020, 243. 22. Abou Sleiman, C.N.; Shi, X.; Zollinger, D.G. An Approach to Characterize the Wearability of Concrete Pavement Surface Treatments. Transport. Res. Rec. 2019, 2673, 230–239. 23. Stepien, A.; Kostrzewa, P.; Dachowski, R. Influence of barium and lithium compounds on silica autoclaved materials properties and on the microstructure. J. Clean. Prod. 2019, 236, 117507. 24. Forster, A.M.; Szadurski, E.M.; Banfill, P.F.G. Deterioration of natural hydraulic lime mortars, I: Effects of chemically accelerated leaching on physical and mechanical properties of uncarbonated materials. Constr. Build. Mater. 2014, 72, 199–207. 25. Silva, B.A.; Ferreira Pinto, A.P.; Gomes, A. Influence of natural hydraulic lime content on the properties of aerial lime-based mortars. Constr. Build. Mater. 2014, 72, 208–218. 26. Gulotta, D.; Goidanich, S.; Tedeschi, C.; Nijland, T.G.; Toniolo, L. Commercial NHL-containing mortars for the preservation of historical architecture. Part 1: Compositional and mechanical characterisation. Constr. Build. Mater. 2013, 38, 31–42. 27. Gulotta, D.; Goidanich, S.; Tedeschi, C.; Toniolo, L. Commercial NHL-containing mortars for the preservation of historical architecture. Part 2: Durability to salt decay. Constr. Build. Mater. 2015, 96, 198–208. 28. ASTM. Standard Test Method forFilm Hardness by Pencil Test; ASTM D3363-2005; ASTM: Philadelphia, PA, USA, 2005. 29. Yang, K.; Basheer, P.A.M.; Magee, B.; Bai, Y. Investigation of moisture condition and Autoclam sensitivity on air permeability measurements for both normal concrete and high performance concrete. Constr. Build. Mater. 2013, 48, 306–314. 30. Jia, L.; Shi, C.; Pan, X.; Zhang, J.; Wu, L. Effects of inorganic surface treatment on water permeability of cement-based materials. Cem. Concr. Compos. 2016, 67, 85–92. 31. Pan, X.; Shi, C.; Zhang, J.; Jia, L.; Chong, L. Effect of inorganic surface treatment on surface hardness and carbonation of cement-based materials. Cem. Concr. Compos. 2018, 90, 218–224. 32. Thompson, J.; Silsbee, M.; Gill, P.; Scheetz, B. Characterization of silicate sealers on concrete. Cem. Concr. Res. 1997, 27, 1561–1567. 33. Basheer, P.A.M.; Montgomery, F.R.; Long, A.E. Clam’ Tests for Measuring in-Situ Permeation Properties of Concrete. Nondestruct. Test. Eval. 1995, 12, 53–73. 34. Dai, J.-G.; Akira, Y.; Wittmann, F.; Yokota, H.; Zhang, P. Water repellent surface impregnation for extension of service life of reinforced concrete structures in marine environments: The role of cracks. Cem. Concr. Compos. 2010, 32, 101–109. 35. Moon, H.Y.; Shin, D.G.; Choi, D.S. Evaluation of the durability of mortar and concrete applied with inorganic coating material and surface treatment system. Constr. Build. Mater. 2007, 21, 362–369. 36. Levi, M.; Ferro, C.; Regazzoli, D.; Dotelli, G.; Presti, A.L. Comparative evaluation method of polymer surface treatments applied on high performance concrete. J. Mater. Sci. 2002, 4881–4888. 37. Xu, S.; Ma, Q.; Wang, J. Combined effect of isobutyltriethoxysilane and silica fume on the performance of natural hydraulic lime-based mortars. Constr. Build. Mater. 2018, 162, 181–191.

38. Luo, K.; Li, J.; Han, Q.; Lu, Z.; Deng, X.; Hou, L.; Niu, Y.; Jiang, J.; Xu, X.; Cai, P. Inﬂuence of nano-SiO2 and carbonation on the performance of natural hydraulic lime mortars. Constr. Build. Mater. 2020, 235, 117411.

39. Luo, K.; Li, J.; Lu, Z.; Jiang, J.; Niu, Y. Eﬀect of nano-SiO2 on early hydration of natural hydraulic lime. Constr. Build. Mater. 2019, 216, 119–127. Materials 2020, 13, 5292 14 of 14

40. Alarcon-Ruiz, L.; Platret, G.; Massieu, E.; Ehrlacher, A. The use of thermal analysis in assessing the eﬀect of temperature on a cement paste. Cem. Concr. Res. 2005, 35, 609–613. 41. Xu, S.; Wang, J.; Sun, Y. Eﬀect of water binder ratio on the early hydration of natural hydraulic lime. Mater. Struct. 2014, 48, 3431–3441. 42. Tsivilis, S.; Kakali, G.; Chaniotakis, E. A Study on the Hydration of Portland Limestone Cement by Means of TG. J. Therm. Anal. Calorim. 1998, 52, 863–870.

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