Coupling Influence Between Recycled Ceramics and Grazed Hollow

materials

Article Coupling Inﬂuence between Recycled Ceramics and Grazed Hollow Beads on Mechanical Properties and Thermal Conductivity of Recycled Thermal Insulation Concrete

Ying Yu *,†, Ben Li *,† and Dongmei Luo

Advanced and Sustainable Infrastructure Materials Group, School of Transportation, Civil Engineering and Architecture, Foshan University, Foshan 528000, China; [email protected] * Correspondence: [email protected] (Y.Y.); [email protected] (B.L.) † Ying Yu and Ben Li contributed equally to this work as co-ﬁrst author.

Abstract: This paper investigated the influence of recycled ceramics and grazed hollow beads on the mechanical, thermal conductivity and material properties of concrete. The results showed that the concentration of recycled ceramics and grazed hollow beads has significant optimization on the workability and thermal properties of the concrete. However, the superabundant concentration can reduce the hydration degree of the concrete, which results in the suppressed production of C-S-H gel and the increase of material defects. In summary, considering the coordinated development of key factors such as thermal insulation properties, mechanical properties and microstructure, 10% RCE and 60% GHB are the optimal material system design methods. Keywords: recycled ceramics (RCE); grazed hollow beads (GHB); recycled thermal insulation con- Citation: Yu, Y.; Li, B.; Luo, D. crete (RTIC); mechanical properties; thermal conductivity; materials properties Coupling Influence between Recycled Ceramics and Grazed Hollow Beads on Mechanical Properties and Thermal Conductivity of Recycled 1. Introduction Thermal Insulation Concrete. Materials 2021, 14, 4695. https:// Environmental pollution and resource and energy shortages have become the main doi.org/10.3390/ma14164695 problems hindering the development of industrialization and urban construction [1–5]. With the continual improvement of industrialization processes, the accumulated storage Academic Editor: Dolores Eliche volume of industrial waste has rapidly increased, causing irreversible environmental Quesada damage, such as soil erosion, air pollution, landscape destruction, deterioration of wildlife habitat and serious personal and property losses [6–8]. As the largest consumption carrier Received: 3 July 2021 of industrial waste, concrete is gradually being recognized by government departments Accepted: 18 August 2021 and the public for its role in the sustainable development of green ecology. Simultaneously, Published: 20 August 2021 with the supersaturated population growth rate and the development of urbanization, the demand for concrete still maintains a relatively high volume. Recycling waste materials Publisher’s Note: MDPI stays neutral into concrete may be a necessary means for solving the above problems. On the one with regard to jurisdictional claims in hand, it absorbs recyclable industrial waste to prepare new green building materials. published maps and institutional affil- On the other hand, it saves the non-renewable raw materials consumed in the traditional iations. concrete preparation process [9–16]. As the world’s largest ceramic producer and consumer, Foshan had produced more than 9 million tons of ceramics with a scrap rate of 5–25% by 2020 [17–20]. However, China’s disposal of waste ceramics is still in the stage of centralized burial, which has largely caused a waste of resources and environmental pollution. The Copyright: © 2021 by the authors. resource utilization methods and methods of waste ceramic disposal have become the Licensee MDPI, Basel, Switzerland. hotspots and key points in the current research in the field. In recent years, many scholars This article is an open access article have carried out research on recycled ceramic aggregate concrete. The research results can distributed under the terms and be summarized as: (1) Due to the effect of the characteristics of ceramic aggregates, the conditions of the Creative Commons replacement of natural coarse aggregates with ceramic coarse aggregates will lead to a Attribution (CC BY) license (https:// decrease in the relative volume and fluidity of the mortar in the concrete. (2) Simultaneously, creativecommons.org/licenses/by/ the compressive strength decreases as the replacement rate of ceramic coarse aggregate 4.0/).

Materials 2021, 14, 4695. https://doi.org/10.3390/ma14164695 https://www.mdpi.com/journal/materials Materials 2021, 14, 4695 2 of 22

increases. (3) The flexural strength of recycled ceramic coarse aggregate concrete decreases with the increase of the ceramic coarse aggregate replacement rate because the crushing index of ceramic coarse aggregate is higher than that of natural coarse aggregate [21–25]. Based on the characteristics that the mechanical properties of recycled ceramic coarse aggregate concrete are lower than ordinary concrete, recycled ceramic concrete is more widely used in the preparation of enclosure structures. The engineering application of the envelope structure with heat preservation and heat insulation properties can effectively alleviate the problem of energy shortage in China which conforms to the fundamental national policy of energy saving and emission reduction [26–30]. Based on ceramic coarse aggregates, lightweight thermal insulation concrete structures can be prepared. However, due to the complex material properties of ceramic coarse aggregates, thermal insulation concrete has the disadvantages of uneven temperature distribution and high thermal conductivity. At present, the preparation of thermal insulation concrete structures with ceramic coarse aggregates is still unable to fully meet engineering needs. How to improve the thermal insulation performance of recycled ceramic coarse aggregate concrete and meet the needs of engineering applications is still an urgent problem to be solved. In recent years, as a new type of lightweight aggregate, grazed hollow beads (GHB) have the characteristics of a porous inner surface and a glass-like outer surface, and have the function of a micro pump, which is easy to combine with cementitious materials. At the same time, the concrete prepared with GHB as lightweight aggregate has good mechanical and thermal insulation properties [16,31–38]. However, the research on the mechanical properties, thermal conductivity and material properties of cement-based material systems prepared by combining GHB and ceramic coarse aggregates is insufficient, and it is still necessary to enrich the mechanism research of the basic material system to support its engineering applications. The goal of this study is to fill these research gaps by investigating the mechanical properties, thermal conductivity and material characteristics of recycled thermal insulation concrete (RTIC) that incorporates grazed hollow beads and recycled ceramics as replacement fine and coarse aggregates. Slump, density, splitting strength, compressive strength, axial compressive strength, elastic modulus and thermal conductivity were measured to evaluate the impact of grazed hollow beads and recycled ceramics on the macroperfor- mance. Material morphology, hydration products and functional groups were investigated to determine the mesoscopic properties of recycled thermal insulation concrete. In summary, based on the above research results, new ideas are provided for the actual use of lightweight thermal insulation cement-based materials in civil engineering, such as the construction of thermal insulation structural floor slabs and the preparation of special concrete structures.

2. Materials and Methods 2.1. Raw Materials and Mixing Proportions Ordinary Portland Cement (P.O. 42.5 N), local nature sand (NSD), grazed hollow beads (GHB), recycled ceramics (RCE) and nature stone (NSE) were used throughout the experiment. The mechanical properties of cement and chemical composition of cement are shown in Tables1 and2. The particle size distribution curves of nature sand and nature stone are shown in Figures1 and2. The physical properties of local nature sand, grazed hollow beads, recycled ceramics and nature stone are shown in Table3. In this paper, the grazed hollow beads were used for replacing nature sand in different volume ratios as 0, 20, 40, 60% and the recycled ceramics were used for replacing nature stone in different mass ratios as 0, 10, 20, 30%. The replacement methods for grazed hollow beads and recycled ceramics are shown in Figure3. The water-to-cement ratio ( w/c) was 0.5 and the mixing proportions of the concrete mixtures are listed in Table4. In the process of preparing concrete, the use of traditional mixing preparation methods caused the concrete slump to fail to meet the speciﬁcation requirements due to the high water absorption of RCE and GHB. In order to solve this problem, the RCE was pre-wetted during the preparation Materials 2021, 14, x FOR PEER REVIEW 3 of 23

cled ceramics are shown in Figure 3. The water-to-cement ratio (w/c) was 0.5 and the mixing proportions of the concrete mixtures are listed in Table 4. In the process of preparing concrete, the use of traditional mixing preparation methods caused the concrete slump to Materials 2021, 14, 4695 3 of 22 fail to meet the specification requirements due to the high water absorption of RCE and GHB. In order to solve this problem, the RCE was pre-wetted during the preparation process, and then the GHB was pretreated by the free water-cement ratio method, and the process,moisture and required then the to GHBbe absorbed was pretreated by the byGHB the was free calculated. water-cement Finally, ratio method,the actual and water the moisturedemand during required concrete to be absorbedpreparation by was the GHBcalculated wascalculated. for final mixing Finally, preparation. the actual water demand during concrete preparation was calculated for ﬁnal mixing preparation. Table 1. Mechanical properties of Portland cement (MPa).

Table 1. Mechanical properties ofCompressive Portland cement Strength (MPa). Flexural Strength (MPa) Fineness Setting Time (min) Flexural Strength (MPa) Compressive Strength(MPa) (MPa) Fineness Setting Time (min) 3 days 28 days 3 days 28 days InitialInitial setting set- Final setting 4.23 days± 0.2 7.5 28± days0.5 23.5 3 ±days0.8 43.2 28± days0.4 1.2 186Final 252 setting 1.2 ting 4.2 ± 0.2 7.5 ± 0.5 23.5 ± 0.8 43.2 ± 0.4 186 252 Table 2. Chemical properties of Portland cement (%). Table 2. Chemical properties of Portland cement (%). Compounds SiO2 Al2O3 Fe2O3 CaO MgO SO3 Loss Compounds SiO2 Al2O3 Fe2O3 CaO MgO SO3 Loss Content 22.53 4.42 2.06 61.71 4.55 2.23 2.86 Content 22.53 4.42 2.06 61.71 4.55 2.23 2.86

100 90 80 70 60 Upper range 50 Sand 40 Lower range 30 20 Cumulative Sieve Margin ( % ) 10 0 012345 Materials 2021, 14, x FOR PEER REVIEW Filter dimension ( mm ) 4 of 23

Figure 1. The particle size distributiondistribution curves of nature sand.sand.

100 90 80 70 60 50 Upper range 40 Stone 30 Lower range 20 Cumulative Sieve Margin ( % ) Margin Sieve Cumulative 10 0 0 5 10 15 20 25 30 35 40 Filter dimension ( mm ) Figure 2. The particle size distribution curves of naturenature stone.stone.

Grazed hollow Recycled beads ceramic

Fine Coarse Water Cement aggregate aggregate

Figure 3. The replacement methods for RCE and GHB.

Table 3. Properties of nature sand, grazed hollow beads, nature stone and recycled ceramic.

Physical Properties Fine Aggregate Coarse Aggregate Type NSD GHB NSE RCE Accumulation density (kg/m3) 2050 99.52 1389 972 Apparent density (kg/m3) 2512 174.7 3045 1819 Water content (%) 0.81 0.5 0.12 2.51 Mass water absorption (%) 7.58 246 0.16 15.47 Crushing value (%) - - 5.32 15.95 Thermal conductivity (W/m·K) - 0.072 - - Note: The crushing value is the ultimate compressive strength of the coarse aggregate used in the concrete preparation process. It is one of the required attributes in raw material performance research.

2.2. Specimen Casting and Curing Conditions The recycled thermal insulation concrete (RTIC) specimens had dimensions of 150 mm × 150 mm × 150 mm (144 pieces) and 300 mm × 300 mm × 30 mm (72 pieces) according to the Chinese standard GB/T 50081-2002 [39] and GB/T 10294-2008 [40]. All experimental specimens were demolded after 24 h and maintained for 28 days in a steam curing room with a temperature of 20 ± 2 °C and a relative humidity of 95%.

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100 90 80

Materials 2021, 14, 4695 70 4 of 22 60 50 Table 3. Properties of nature sand, grazed hollow beads, nature Upper stone range and recycled ceramic. 40 Stone Physical Properties Fine Aggregate Coarse Aggregate 30 Lower range Type NSD GHB NSE RCE Accumulation density (kg/m320) 2050 99.52 1389 972 Cumulative Sieve Margin ( % ) Margin Sieve Cumulative 3 Apparent density (kg/m ) 10 2512 174.7 3045 1819 Water content (%) 0.81 0.5 0.12 2.51 Mass water absorption (%)0 7.58 246 0.16 15.47 Crushing value (%)0 5 10- 15 20 25 - 30 35 40 5.32 15.95 Thermal conductivity (W/m·K) - 0.072 - - Filter dimension ( mm ) Note: The crushing value is the ultimate compressive strength of the coarse aggregate used in the concrete preparation process. It is one of the required attributes in rawFigure material 2. performanceThe particle research. size distribution curves of nature stone.

Grazed hollow Recycled beads ceramic

Fine Coarse Water Cement aggregate aggregate

Figure 3. The replacement methods for RCE and GHB. Figure 3. The replacement methods for RCE and GHB. Table 4. Mixing proportions of concrete mixtures (kg/m3). Table 3. Properties of nature sand, grazed hollow beads, nature stone and recycled ceramic. W/C = 0.5 Cement Water NSD NSE RCE GHB SP Physical Properties Fine Aggregate Coarse Aggregate RCE0 GHB0 370 185 628 1218 0 0 1.9 RCE0 GHB20Type 370 202.8NSD 628 1218GHB 0NSE 19.9 1.9 RCE AccumulationRCE0 GHB40 density 370 (kg/m3) 220.52050 628 1218 99.52 0 1389 39.8 1.9 972 RCE0Apparent GHB60 density (kg/m 3703) 238.32512 628 1218 174.7 0 3045 59.7 1.9 1819 RCE10 GHB0 370 185 628 1096 122 0 1.9 RCE10Water GHB20 content (%) 370 202.8 0.81 628 1096 0.5 122 0.12 19.9 1.9 2.51 MassRCE10 water GHB40 absorption 370 (%) 220.5 7.58 628 1096 246 122 0.16 39.8 1.9 15.47 RCE10Crushing GHB60 value (%) 370 238.3- 628 1096- 1225.32 59.7 1.915.95 RCE20 GHB0 370 185 628 974 244 0 1.9 ThermalRCE20 GHB20 conductivity (W/m·K) 370 202.8- 628 9740.072 244 19.9- 1.9 - Note:RCE20 The crushing GHB40 value is 370 the ultimate 220.5 compressive strength 628 of the 974 coarse aggregate 244 used in the 39.8 concrete preparation 1.9 process.RCE20 It is GHB60 one of the required 370 attributes 238.3 in raw material 628 performance 974 research. 244 59.7 1.9 RCE30 GHB0 370 185 628 853 365 0 1.9 RCE30 GHB20 370 202.8 628 853 365 19.9 1.9 RCE30 GHB40 3702.2. Specimen 220.5 Casting and 628Curing Conditions 853 365 39.8 1.9 RCE30 GHB60 370The recycled 238.3 thermal 628 insulation 853concrete (RTIC) 365 specimens 59.7 had dimensions 1.9 of 150 mm × Note:150 mm SP is the× 150 high-efﬁciency mm (144 polycarboxylic pieces) and acid 300 water mm reducer. × 300 mm × 30 mm (72 pieces) according to the Chinese standard GB/T 50081-2002 [39] and GB/T 10294-2008 [40]. All experimental 2.2. Specimen Casting and Curing Conditions specimens were demolded after 24 h and maintained for 28 days in a steam curing room with a temperatureThe recycled thermal of 20 ± insulation 2 °C and concretea relative (RTIC) humidity specimens of 95%. had dimensions of 150 mm × 150 mm × 150 mm (144 pieces) and 300 mm × 300 mm × 30 mm (72 pieces) according to the Chinese standard GB/T 50081-2002 [39] and GB/T 10294-2008 [40]. All experimental

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specimens were demolded after 24 h and maintained for 28 days in a steam curing room with a temperature of 20 ± 2 ◦C and a relative humidity of 95%. 2.3. Experimental Methods 2.3. Experimental Methods 2.3.1. Workability Workability Properties Test The workability workability of of recycled recycled thermal thermal insulati insulationon concrete concrete based based on on the the coupling coupling influ- in- enceﬂuence between between GHB GHB and and RCE RCE was was determined determined and andconducted conducted through through the slump the slump and den- and sitydensity test testaccording according to Chinese to Chinese standard standard GB/T50080-2016 GB/T50080-2016 [41] [ 41and] and GB/T50080-2002 GB/T50080-2002 [42]. [42 ].

2.3.2. Mechanical Mechanical Properties Test A total total of of 144 144 RTIC RTIC specimens specimens (150 (150 mm mm × 150× 150 mm mm × 150× mm)150 mm)were prepared were prepared for series for experimentsseries experiments of compressive of compressive strength strength (72 pieces (72) pieces)and splitting and splitting strength strength (72 pieces) (72 accord- pieces) ingaccording to Chinese to Chinese standard standard GB/T50081-2002 GB/T50081-2002 [39]. [39].

2.3.3. Thermal Insulation Properties Test The thermal insulationinsulation propertyproperty ofof concrete concrete is is usually usually reﬂected reflected by by thermal thermal conductiv- conduc- tivity.ity. In thisIn this paper, paper, a total a total of 72of specimens72 specimens (300 (300 mm mm× 300 × 300 mm mm× 30× 30 mm) mm) were were prepared prepared to todetermine determine the the thermal thermal conductivity conductivity of concreteof concrete specimens specimens based based on the on steady-statethe steady-state and anddouble-plate double-plate method method (shown (shown in Figure in Figure4). 4).

Thermal Conductivity

Sample Preparation

105 ± 5 ℃，48h Dry to Constant Weight

Mass Weighing Flatness < 0.5mm Thickness Measurement unit weight ＜5%

Smooth Surface Starting Setting Instrument Parameters

4h Data Acquisition

Figure 4. Measurement of thermal conductivityconductivity on the RTIC.

Table2.3.4. 4. Materials Mixing proportions Characterization of concrete Tests mixtures (kg/m3).

W/C = 0.5 Cement To further Water explore NSD the effect of RCE NSE and GHB RCE on the mechanical GHB properties SP and thermal conductivity early performance of recycled thermal insulation concrete, several RCE0 GHB0 370 185 628 1218 0 0 1.9 macro-experimental methods were conducted. A total of 80 experimental samples were RCE0 GHB20 370 202.8 628 1218 0 19.9 1.9 subjected to scanning electron microscopy (Hitachi, Shenzhen, China) (SEM, 33 samples), RCE0 GHB40 370X-ray diffraction220.5 (Haoyuan,628 Dandong,1218 China) (XRD,0 24 samples),39.8 and Fourier-transform1.9 RCE0 GHB60 370infrared 238.3 spectroscopy (BRUKER,628 Shanghai,1218 China)0 (FTIR, 24 samples)59.7 to analyze1.9 the RCE10 GHB0 370changes in185 the micromorphology,628 hydration1096 products,122 and chemical 0 bonding or molecules. 1.9 RCE10 GHB20 370 202.8 628 1096 122 19.9 1.9 RCE10 GHB40 370 220.5 628 1096 122 39.8 1.9

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RCE10 GHB60 370 238.3 628 1096 122 59.7 1.9 RCE20 GHB0 370 185 628 974 244 0 1.9 RCE20 GHB20 370 202.8 628 974 244 19.9 1.9 RCE20 GHB40 370 220.5 628 974 244 39.8 1.9 RCE20 GHB60 370 238.3 628 974 244 59.7 1.9 RCE30 GHB0 370 185 628 853 365 0 1.9 RCE30 GHB20 370 202.8 628 853 365 19.9 1.9 RCE30 GHB40 370 220.5 628 853 365 39.8 1.9 RCE30 GHB60 370 238.3 628 853 365 59.7 1.9 Note: SP is the high-efficiency polycarboxylic acid water reducer.

2.3.4. Materials Characterization Tests To further explore the effect of RCE and GHB on the mechanical properties and thermal conductivity early performance of recycled thermal insulation concrete, several macro-experimental methods were conducted. A total of 80 experimental samples were

Materials 2021, 14, 4695 subjected to scanning electron microscopy (Hitachi, Shenzhen, China) (SEM, 33 samples),6 of 22 X-ray diffraction (Haoyuan, Dandong, China) (XRD, 24 samples), and Fourier-transform infrared spectroscopy (BRUKER, Shanghai, China) (FTIR, 24 samples) to analyze the changes in the micromorphology, hydration products, and chemical bonding or molecules.The preparation The preparation and drying and drying conditions conditions of these of samples these samples were following were following Chinese Chinese standard standardGB/T 16594-2008 GB/T 16594-2008 [43], GB/T [43], 30904-2014 GB/T 30904-2014 [44], and [44], ISO and 19618-2017 ISO 19618-2017 [45]. In [45]. summary, In sum- the mary,experimental the experimental ﬂowcharts flowcharts in this paper in this are paper shown are in shown Figure 5in. Figure 5.

SEM Sand Grazed hollow Compressive Splitting 10×10mm beads strength test strength test Slump

XRD Stone Recycle ceramics Axial compres- Elastic mod- Size:<10µm

sion test ulus test

Density Thermal conductivity test FTIR Curing conditions Size:<2.5–25µm

1: materials and curing equipment 2: workability 3 (a): mechanical test 3(b): material property test and Thermal conductivity test and equipment

Figure 5. Experimental ﬂowchart. Figure 5. Experimental flowchart. 3. Results and Discussion

3.1. Coupled Inﬂuence of RCE and GHB on Macroscopic Comprehensive Characteristics of RTIC In this section, the macroscopic comprehensive performance of RTIC is studied. This section mainly explores the inﬂuence of GHB and RCE on the working performance, lightweight performance, mechanical properties and thermal conductivity of RTIC under the coupling effects of different doping amounts.

3.1.1. Workability and Density Properties of RTIC The experimental results and changes of slump subjected to the coupling influence between GHB and RCE on fresh concrete are shown in Figure6a,b. The results showed that, (1) when the content of RCE was constant, the slump degree of concrete increased with the increase of GHB content. Compared with RCE0GHB0 (45 mm), RCE0GHB20 (80 mm), RCE0GHB40 (110 mm) and RCE0GHB60 (115 mm) increased by 77.8, 144.4 and 115.6%, respectively. (2) However, when the content of GHB was constant, the influence of RCE on the slump properties of concrete was not obvious. Compared with the reference concrete, the change of collapse degree was 11.1, 1.0 and −11.1% within the content range of RCE from 10 to 30%. (3) Similarly, the combined effect of GHB and RCE could better improve the workability performance of RTIC. When the blending contents were RCE0GHB60 (115 mm) and RCE30GHB60 (115 mm), the material system could obtain the best slump value. (4) The improved workability performance of RTIC was mainly attributed to the influence of GHB. This is because GHB has good water retention properties and can reduce the friction between the aggregates, which can eventually lead to provide the required water phase during the material preparation process. Nevertheless, RCE also has good water absorption, it has little effect on the workability of concrete. This is mainly because Materials 2021, 14, x FOR PEER REVIEW 7 of 23

3. Results and Discussion 3.1. Coupled Influence of RCE and GHB on Macroscopic Comprehensive Characteristics of RTIC In this section, the macroscopic comprehensive performance of RTIC is studied. This section mainly explores the influence of GHB and RCE on the working performance, lightweight performance, mechanical properties and thermal conductivity of RTIC under the coupling effects of different doping amounts.

3.1.1. Workability and Density Properties of RTIC The experimental results and changes of slump subjected to the coupling influence between GHB and RCE on fresh concrete are shown in Figure 6a,b. The results showed that, (1) when the content of RCE was constant, the slump degree of concrete increased with the increase of GHB content. Compared with RCE0GHB0 (45 mm), RCE0GHB20 (80 mm), RCE0GHB40 (110 mm) and RCE0GHB60 (115 mm) increased by 77.8, 144.4 and 115.6%, respectively. (2) However, when the content of GHB was constant, the influence of RCE on the slump properties of concrete was not obvious. Compared with the reference concrete, the change of collapse degree was 11.1, 1.0 and −11.1% within the content range of RCE from 10 to 30%. (3) Similarly, the combined effect of GHB and RCE could better improve the workability performance of RTIC. When the blending contents were RCE0GHB60 (115 mm) and RCE30GHB60 (115 mm), the material system could obtain the best slump value. (4) The improved workability performance of RTIC was mainly attributed to the influence of GHB. This is because GHB has good water retention properties Materials 2021, 14, 4695 7 of 22 and can reduce the friction between the aggregates, which can eventually lead to provide the required water phase during the material preparation process. Nevertheless, RCE also has good water absorption, it has little effect on the workability of concrete. This is mainly becauseRCE needs RCE to needs be pre-wetted to be pre-wetted in advance in adva to reachnce saturation,to reach saturation, which reduces which thereduces effect the of effectimproving of improving flow performance. flow performance. Figure7a,b Figu showre 7a,b the experimental show the experimental results and changesresults and of changesdensity subjectof density to the subject coupling to the influence coupling between influence GHB between and RCE GHB on and concrete. RCE on The concrete. results Theshow results that: (1)show compared that: (1) withcompared RCE0GHB0, with RCE0 the densityGHB0, the of thedensity concrete of the with concrete the increasing with the increasingsingle incorporation single incorporation of RCE (10~30%) of RCE (10 and~30%) GHB and (20~60%) GHB (20 decreased~60%) decreased by 1.6, 3.7, by 5.31.6, and3.7, 5.30.8, and 1.6, 0.8, 2.5%, 1.6, respectively. 2.5%, respectively. (2) RCE (2) has RCE a significant has a significant effect on effect reducing on reducing the density the density of the ofconcrete. the concrete. Although Although the density the density of GHB of is muchGHB is lower much than lower that than of natural that of sand, natural it absorbs sand, it a absorbslot of water a lot during of water the during concrete the preparation concrete pr process,eparation which process, is not obviouswhich is for not the obvious reduction for theof concrete reduction density. of concrete (3) In density. the compound (3) In the incorporation compound incorporation of GHB and RCE,of GHB the and negative RCE, thedensity negative change density of RCE10GHB20 change of RCE10GHB20 was the smallest, was whichthe smallest, had the which best combined had the best effect com- on binedreducing effect the on density reducing of concrete. the density of concrete.

160 0% GHB 20% GHB 140 40% GHB 60% GHB 120

100

Slump (mm) 60

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RCE (%) (a) 180

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140

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40 Change in slump (%) 20

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0 0 0 0 0 0 0 0 0 0 4 6 4 6 2 6 B 2 B B B20 B40 H B4 H H HB H HB H GH G G 0G 0G 0 0G 0G 3 0G 1 10 2 3 RCE0GHB2RCE RCE RCE10GHB0 RCE20GHB0 RCE RCE RCE10GHBRCE RCE20GHBRCE RCE20 RCE30GHBRCE RCE30GHB60 (b)

FigureFigure 6.6. TheThe experimentalexperimental results results and and changes changes of slumpof slump subjected subjected to the to couplingthe coupling inﬂuence influence between be- GHBtween and GHB RCE and on RCE fresh on concrete.fresh concrete. (a) The (a) experimental The experimental results results of slump of slump on fresh on fresh concrete; concrete; (b) the (b) changesthe changes of slump of slump on fresh on fresh concrete. concrete.

2500 0% GHB 20% GHB 2450 40% GHB 60% GHB 2400 ) 3 2350

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180

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−20

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Materials 2021, 14, 4695 Figure 6. The experimental results and changes of slump subjected to the coupling influence8 of be- 22 tween GHB and RCE on fresh concrete. (a) The experimental results of slump on fresh concrete; (b) the changes of slump on fresh concrete.

2500 0% GHB 20% GHB 2450 40% GHB 60% GHB 2400 ) 3 2350

2300

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Change in density (%) density in Change -8

-9

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RCE0GHB20RCE0GHB40RCE0GHB60RCE10GHB0RCE10GHB20RCE10GHB40RCE10GHB60RCE20GHB0RCE20GHB20RCE20GHB40RCE20GHB60RCE30GHB0RCE30GHB20RCE30GHB40RCE30GHB60 (b)

Figure 7. Figure 7. The experimental resultsresults andand changeschanges ofof densitydensity subjectsubject toto thethe couplingcoupling influenceinfluence betweenbetween GHBGHB and and RCE RCE on on RTIC. RTIC. (a) ( Thea) T experimentalhe experimental results results of density of density on concrete; on concrete; (b) the (changesb) the chan of densityges of ondensity fresh on concrete. fresh concrete. 3.1.2. Analysis of the Compressive Strength of RTIC 3.1.2. Analysis of the Compressive Strength of RTIC The experimental results and changes of compressive strength are shown in Figure8a,b. The experimental results and changes of compressive strength are shown in Figure It can be seen that, (1) GHB had a decisive influence on the compressive strength of concrete. 8a,b. It can be seen that, (1) GHB had a decisive influence on the compressive strength of Compared with RCE0GHB0, the compressive strength of RCE0GHB20, RCE0GHB40 and concrete. Compared with RCE0GHB0, the compressive strength of RCE0GHB20, RCE0GHB60 decreased by 5.7, 15.6 and 22.7%, respectively. With the increase of GHB RCE0GHB40 and RCE0GHB60 decreased by 5.7, 15.6 and 22.7%, respectively. With the content, the compressive strength of concrete was further reduced. That is because GHB increase of GHB content, the compressive strength of concrete was further reduced. That is a brittle material, which is prone to brittle fracture and decomposition due to stress concentrationis because GHB under is a brittle the action material, of load, which thereby is prone forming to brittle more fracture pore and defects decomposition in the con- cretedue to [46 stress,47]. Inconcentration summary, GHB under has the no action significant of load, contribution thereby forming to the compressive more pore def strengthects in the concrete [46,47]. In summary, GHB has no significant contribution to the compressive strength development of concrete. (2) Furthermore, incorporating ceramic waste aggregate as a partial replacement of natural coarse aggregate also reduced the compressive strength. The compressive strength reduction of RCE10GHB0, RCE20GHB0 and RCE30GHB0 was 5.9, 11.1 and 13.3% compared with RCE0GHB0. The concrete prepared with RCE was prone to through cracks due to aggregate damage in the process of compressive load failure due to low crush value and porous brittle material properties [7,48,49]. (3) The coupling of GHB and RCE had no positive effect on the compressive strength of RTIC. Comprehensive comparison of the change in compressive strength, RCE30GHB60 had the largest decrease in compressive strength, while RCE10GHB20 had the smallest decrease.

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development of concrete. (2) Furthermore, incorporating ceramic waste aggregate as a partial replacement of natural coarse aggregate also reduced the compressive strength. The compressive strength reduction of RCE10GHB0, RCE20GHB0 and RCE30GHB0 was 5.9, 11.1 and 13.3% compared with RCE0GHB0. The concrete prepared with RCE was prone to through cracks due to aggregate damage in the process of compressive load failure due to low crush value and porous brittle material properties [7,48,49]. (3) The coupling of GHB and RCE had no positive effect on the compressive strength of RTIC. Comprehensive Materials 2021, 14, x FOR PEER REVIEW 10 of 23 comparison of the change in compressive strength, RCE30GHB60 had the largest decrease in compressive strength, while RCE10GHB20 had the smallest decrease.

55 0% GHB 50 20% GHB 45 40% GHB 60% GHB 40

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−8

−12

−16

−20

−24

−28

Change in compressive strength (%) strength compressive in Change −32

−36

−40

0 0 0 6 B0 6 20 60 B40 H B40 B20 H HB HB HB HB H HB40 0G GHB0 G 0GH 0GHB 0G G 0 G G 30 G G E E E1 10GHB20 10 20 20 E 30 30 C E E E E E E RCE0GHB20RC RC R C C RC RCE1 RC RCE2 R RCE20GHB40RC RC R RC RCE30GHB60 (b)

FigureFigure 8. TheThe experimental experimental results results and and changes changes of of compressive compressive strength strength subject subject to tothe the coupling coupling in- inﬂuencefluence between between GHB GHB and and RCE RCE on on RTIC. RTIC. ( (aa)) The The experimental experimental results of compressivecompressive strengthstrength ofof RTIC;RTIC; ((bb)) thethe changeschanges ofof compressivecompressive strengthstrength ofof RTIC.RTIC.

3.1.3. Analysis on Splitting Tensile Strength of RTIC Figure 9a,b shows the experimental results and changes of splitting tensile strength of RTIC. Splitting tensile strength decreased significantly with the increase of the incorporation of GHB and RCE. In the process of splitting failure, the GHB and RCE in the concrete deteriorated (even powdery failure) before the natural aggregates. At the same time, the pore structure and micro cracks in the concrete will increase and form more mechanically weak areas, which will eventually lead to a negative impact on the splitting performance of concrete based on the coupling influence between GHB and RCE. In addition, RCE30GHB60 had the largest reduction in split tensile strength (−1.06 MPa), while RCE10GHB20 had the smallest reduction in split tensile strength (−0.68 MPa), which is the best ratio to ensure split tensile performance.

Materials 2021, 14, 4695 10 of 22

3.1.3. Analysis on Splitting Tensile Strength of RTIC Figure9a,b shows the experimental results and changes of splitting tensile strength of RTIC. Splitting tensile strength decreased signiﬁcantly with the increase of the incorporation of GHB and RCE. In the process of splitting failure, the GHB and RCE in the concrete deteriorated (even powdery failure) before the natural aggregates. At the same time, the pore structure and micro cracks in the concrete will increase and form more mechanically weak areas, which will eventually lead to a negative impact on the splitting performance of concrete based on the coupling inﬂuence between GHB and RCE. In addition, RCE30GHB60 had the largest reduction in split tensile strength (−1.06 MPa), while Materials 2021, 14, x FOR PEER REVIEWRCE10GHB20 had the smallest reduction in split tensile strength (−0.68 MPa),11 of which 23 is the

best ratio to ensure split tensile performance.

4.5 0% GHB 4.0 20% GHB 40% GHB 3.5 60% GHB

3.0

2.5

2.0

1.5

1.0 Splitting tensile strength (MPa) 0.5

0.0 0 102030 RCE (%) (a)

−5

−10

−15

−20

−25 Change in splitting tensile strength (%) −30

−35

0 0 0 0 0 0 0 0 20 40 60 2 4 6 B 2 4 6 B B B B HB HB HB HB HB HB G 0GH 10GHB00GH G G 20GH G G G E0GH E0GH E E 1 10 10 E C C C C E E E E30GHB60 R C C R R R R RC RC RC RCE20 RCE20 RCE20 RCE30RCE30GHB20RCE30GHB40R (b)

FigureFigure 9. 9. TheThe experimental experimental results results and and changes changes of splitti of splittingng tensile tensile strength strength subject to subject the coupling to the coupling influence between GHB and RCE on RTIC. (a) The experimental results of splitting tensile strength inﬂuence between GHB and RCE on RTIC. (a) The experimental results of splitting tensile strength of of RTIC; (b) the changes of splitting tensile strength of RTIC. RTIC; (b) the changes of splitting tensile strength of RTIC. 3.1.4. Insulation Properties of RTIC The experimental results and changes of thermal conductivity and insulation property values of RTIC subject to the coupling influence between GHB and RCE are shown in Figure 10a–c. The results show that, (1) the thermal conductivity decreased as the content of GHB increased. When the GHB content was 60%, RTIC had lower thermal conductivity and performance. The significant decrease in thermal conductivity was mainly attributed to the good thermal insulation properties of GHB. When GHB is distributed in

Materials 2021, 14, 4695 11 of 22

3.1.4. Insulation Properties of RTIC The experimental results and changes of thermal conductivity and insulation property values of RTIC subject to the coupling influence between GHB and RCE are shown in Figure 10a–c. The results show that, (1) the thermal conductivity decreased as the content of GHB increased. When the GHB content was 60%, RTIC had lower thermal conductivity and performance. The significant decrease in thermal conductivity was mainly attributed to the good thermal insulation properties of GHB. When GHB is distributed in concrete, the pore structure and internal spaces are filled with foam material, which hinders the conduction and dispersion of heat, thereby reducing the thermal conductivity of the concrete. (2) However, the thermal conductivity decreased slightly with the increase of RCE, but when

Materials 2021, 14, x FOR PEER REVIEWRCE and GHB were coupled, the thermal conductivity did not change significantly.12 of 23 This is because the thermal conductivity of ceramics is not significantly different from that of natural coarse aggregates. Compared with natural coarse aggregate, ceramic coarse aggregateconcrete, the has pore more structure microcracks, and internal and spac thees porosity are filled of with ceramic foam coarsematerial, aggregate which hin- is greater thanders the that conduction of ordinary and concrete. dispersion Theseof heat, factors thereby have reducing a limited the thermal influence conductivity on reducing of the thermalthe concrete. conductivity (2) However, of concrete. the thermal (3) conducti In summary,vity decreased GHB played slightly a decisive with the roleincrease in reducing theof RCE, thermal but when conductivity RCE and GHB and were improving coupled, the the thermal thermal conductivity insulation performancedid not change of RTIC. Consideringsignificantly. This themechanical is because the properties thermal conductivity and thermal of ceramics insulation is not properties significantly of concrete, dif- the mixingferent from amount that of natural RCE is coarse 10–20%, aggregates. and the mixingCompared amount with natural of GHB coarse is 40–60%. aggregate, ceramic coarse aggregate has more microcracks, and the porosity of ceramic coarse aggre- 3.2.gate Coupled is greater Influence than that of RCEof ordinary and GHB concrete. on Material These Propertiesfactors have of a RTIC limited influence on reducingIn order the thermal to further conductivity explore the of modification concrete. (3) In mechanism summary, ofGHB RTIC played on the a decisive comprehensive propertiesrole in reducing of concrete, the thermal a set ofconductivity meso-experimental and improving methods the thermal to explore insulation the coupled perfor- effects of mance of RTIC. Considering the mechanical properties and thermal insulation properties GHB and RCE on RTIC’s microstructure, material morphology and key hydration products of concrete, the mixing amount of RCE is 10–20%, and the mixing amount of GHB is 40– are60%. discussed in this section.

1.8 0% GHB 1.6 20% GHB 40% GHB 1.4 60% GHB

1.2

1.0

0.8

0.6

0.4 Thermal conductivity (w/(m·k)) 0.2

0.0 0 102030 RCE (%) (a)

Figure5 10. Cont.

−5

−10

−15

−20

−25

−30

−35

Change in thermal (%) conductivity in thermal Change −40

−45

−50

0 0 0 0 0 0 0 20 4 6 B 4 B B HB0 B2 B40 B20 B40 B6 B20 B6 H G H H GH 0 GH GH GH GH GHB GH 0 1 0 0 0 0 0 0 E0G 10GHB60E20GH 20G E30GHB030G CE C CE E1 E1 E C E2 E C E E3 R R RCE0GHBR C C R C R C C R R RC R RC RCE2 R RCE3 R (b)

Materials 2021, 14, x FOR PEER REVIEW 12 of 23

concrete, the pore structure and internal spaces are filled with foam material, which hinders the conduction and dispersion of heat, thereby reducing the thermal conductivity of the concrete. (2) However, the thermal conductivity decreased slightly with the increase of RCE, but when RCE and GHB were coupled, the thermal conductivity did not change significantly. This is because the thermal conductivity of ceramics is not significantly different from that of natural coarse aggregates. Compared with natural coarse aggregate, ceramic coarse aggregate has more microcracks, and the porosity of ceramic coarse aggregate is greater than that of ordinary concrete. These factors have a limited influence on reducing the thermal conductivity of concrete. (3) In summary, GHB played a decisive role in reducing the thermal conductivity and improving the thermal insulation performance of RTIC. Considering the mechanical properties and thermal insulation properties of concrete, the mixing amount of RCE is 10–20%, and the mixing amount of GHB is 40– 60%.

1.8 0% GHB 1.6 20% GHB 40% GHB 1.4 60% GHB

1.2

1.0

0.8

0.6

0.4 Thermal conductivity (w/(m·k)) 0.2

0.0 0 102030 Materials 2021, 14, 4695 12 of 22 RCE (%) (a)

−5

−10

−15

−20

−25

−30

−35

Change in thermal (%) conductivity in thermal Change −40

−45

−50

0 0 0 0 0 0 0 20 4 6 B 4 B B HB0 B2 B40 B20 B40 B6 B20 B6 H G H H GH 0 GH GH GH GH GHB GH 0 1 0 0 0 0 0 0 E0G 10GHB60E20GH 20G E30GHB030G Materials 2021, 14, x FOR PEER REVIEW CE C CE E1 E1 E C E2 E C E E3 13 of 23 R R RCE0GHBR C C R C R C C R R RC R RC RCE2 R RCE3 R (b) 1.6

1.4

1.2

1.0

0.8

0.6

0.4 Insulation properties (w/(m·k)) properties Insulation 0.2

0.0

RCE0GHB0 RCE0GHB20RCE0GHB40RCE0GHB60RCE10GHB0RCE10GHB20RCE10GHB40RCE10GHB60RCE20GHB0RCE20GHB20RCE20GHB40RCE20GHB60RCE30GHB0RCE30GHB20RCE30GHB40RCE30GHB60 (c)

FigureFigure 10. 10. TheThe experimental experimental results results and andchanges changes of thermal of thermal conductivity conductivity and insulation and insulation property property values of RTIC subject to the coupling influence between GHB and RCE. (a) The experimental values of RTIC subject to the coupling influence between GHB and RCE. (a) The experimental results results of thermal conductivity of RTIC; (b) the changes of thermal conductivity of RTIC. (c) ofInsulation thermal propert conductivityy values of of RTIC. RTIC; (b) the changes of thermal conductivity of RTIC. (c) Insulation property values of RTIC. 3.2. Coupled Influence of RCE and GHB on Material Properties of RTIC 3.2.1. Material Morphology Analysis of RTIC In order to further explore the modification mechanism of RTIC on the comprehensiveThe analysis properties of the of material concrete morphology, a set of meso- ofexperim RTICental is shown methods inFigure to explore 11. the Compared withcoupled RCE0GHB0, effects of GHB RCE and affected RCE on theRTIC cured’s microstruct form ofure, C-S-H material gel inmorphology concrete. and As key the content ofhydration RCE increased, products are more discussed flocculent in this C-S-H section gels. were formed and attached to the surface of the unhydrated cement stone. At the same time, RCE affected the internal spatial distribution3.2.1. Material of Morphology the concrete Analysis and caused of RTIC more microcracks or an increase in obvious pores, whichThe explained analysis o whyf the thematerial mechanical morphology properties of RTIC of is the shown concrete in Fig decreasedure 11. Compared due to the RCE aggregate.with RCE0GHB0, In addition, RCE affect GHBed causedthe cured unhydrated form of C-S cement-H gel in particles concrete. to As adhere the content to the of surface of C-S-H.RCE increase Withd the, more increase flocculent in the C- amountS-H gels ofwe GHB,re formed the volumeand attached and poreto the size surface of the of the pores in the cementunhydrated stone cement increased stone. significantly,At the same time, thereby RCE a improvingffected the internal the thermal spatial insulation distribution properties of the concrete and caused more microcracks or an increase in obvious pores, which of the concrete, but impairing the mechanical properties. explained why the mechanical properties of the concrete decreased due to the RCE aggregate. In addition, GHB caused unhydrated cement particles to adhere to the surface of C-S-H. With the increase in the amount of GHB, the volume and pore size of the pores in the cement stone increased significantly, thereby improving the thermal insulation properties of the concrete, but impairing the mechanical properties.

Materials 2021, 14, 4695 13 of 22 Materials 2021, 14, x FOR PEER REVIEW 14 of 23

(a)

(b)

(c)

(d)

Figure 11. Cont.

Materials 2021, 14, 4695 14 of 22 Materials 2021, 14, x FOR PEER REVIEW 15 of 23

(e)

(f)

(g)

FigureFigure 11. TheThe analysis analysis of the of material the material morphology morphology of RTIC of based RTIC on based SEM on(2.0 SEMk to 10.0 (2.0 k). k to(a) 10.0RCE0GHB0; k). (a) RCE0GHB0; (b) RCE10GHB0; (b) RCE10GHB0; (c) RCE20GHB0; (c) RCE20GHB0; (d) RCE30GHB0; (d) RCE30GHB0; (e) RCE10GHB20; (e) RCE10GHB20; (f) RCE10GHB40; (g) RCE10GHB60. (f) RCE10GHB40; (g) RCE10GHB60.

3.2.2.3.2.2. HydrationHydration ProductsProducts AnalysisAnalysis ofof RTICRTIC TheThe experimentalexperimental resultsresults ofof thethe crystalcrystal compositioncomposition andand hydrationhydration productsproducts ofof RTICRTIC areare shownshown inin FigureFigure 12 12.. TheThe resultsresults showshow that,that, (1)(1) withwith thethe increaseincrease of of RCE RCE and and GHB, GHB, the the characteristiccharacteristic peakpeak intensityintensity ofof hydrationhydration productsproducts CH CH and and C-S-HC-S-H decreased.decreased. TheThe increaseincrease ofof RCERCE andand GHBGHB inhibitedinhibited thethe formationformation ofof thethe mainmain hydrationhydration productsproducts inin thethe concrete,concrete, whichwhich waswas not conducive conducive to to the the development development of ofmechanical mechanical properties. properties. (2) RCE (2) RCEand GHB and GHBhad a had significant a signiﬁcant effect effect on the on formation the formation of CaCO of CaCO3. The3 .increase The increase of CaCO of CaCO3 leads3 toleads prem- to prematureature calcification calciﬁcation or aging or aging of concrete, of concrete, which which is very is very unfavorable unfavorable to tothe the development development of ofmechanical mechanical properties. properties. (3) (3) Further Further analysis analysis found found that that GHB GHB had had a promoting effecteffect onon thethe formationformation ofof Tobermorite.Tobermorite. WithWith the the increase increase of of GHB, GHB, the the characteristic characteristic peak peak intensity intensity and and

Materials 2021, 14, x FOR PEER REVIEW 16 of 23 Materials 2021, 14, 4695 15 of 22

crystal yield of Tobermorite increased. Tobermorite is a kind of hydrated calcium silicate crystal yield of Tobermorite increased. Tobermorite is a kind of hydrated calcium silicate with high crystallinity. It has strong thermal insulation performance and can improve the with high crystallinity. It has strong thermal insulation performance and can improve thermal insulation performance of concrete. Therefore, with the increase of GHB, the ther- the thermal insulation performance of concrete. Therefore, with the increase of GHB, the mal insulation performance of concrete is also enhanced. thermal insulation performance of concrete is also enhanced.

1-CH 4-AFt

2-C-S-H 5-CaCO3

3-C2S,C3S 6-Tobermorite

1 2 1 54 3 6 RCE30GHB0

RCE20GHB0

RCE10GHB0

RCE0GHB0

15 20 25 30 35 40 45 50 55 60 65 70 2 Theta (Theta) (a)

CH C-S-H

RCE30GHB0 RCE30GHB0 RCE20GHB0 RCE20GHB0 RCE10GHB0 RCE10GHB0 RCE0GHB0 RCE0GHB0

17.8 17.9 18.0 18.1 18.2 18.3 29.3 29.4 29.5 29.6 29.7 2 Theta (Theta) 2 Theta (Theta)

CaCO3 Tobermorite

RCE30GHB0

RCE20GHB0

RCE10GHB0 RCE30GHB0 RCE20GHB0 RCE0GHB0 RCE10GHB0 RCE0GHB0

39.3 39.4 39.5 39.6 39.7 54.7 54.8 54.9 55.0 55.1 2 Theta (Theta) 2 Theta (Theta) (b)

Figure 12. Cont.

Materials 2021, 14, x FOR PEER REVIEW 17 of 23 Materials 2021, 14, 4695 16 of 22

1-CH 4-AFt

2-C-S-H 5-CaCO3

3-C2S,C3S 6-Tobermorite

1 2 1 5 3 6 4 RCE10GHB60

RCE10GHB40

RCE10GHB20

RCE10GHB0

15 20 25 30 35 40 45 50 55 60 65 70 2 Theta (Theta) (c)

CH C-S-H

RCE10GHB60 RCE10GHB60 RCE10GHB40 RCE10GHB40 RCE10GHB20 RCE10GHB20 RCE10GHB0 RCE10GHB0

17.8 17.9 18.0 18.1 18.2 18.3 29.2 29.3 29.4 29.5 29.6 29.7 2 Theta (Theta) 2 Theta (Theta)

Tobermorite CaCO3

RCE10GHB60

RCE10GHB60 RCE10GHB40 RCE10GHB40 RCE10GHB20 RCE10GHB20 RCE10GHB0 RCE10GHB0

39.3 39.4 39.5 39.6 39.7 54.7 54.8 54.9 55.0 55.1 2 Theta (Theta) 2 Theta (Theta) (d)

FigureFigure 12. 12.The The experimental experimental results results of theof the crystal crystal composition composition and and hydration hydration products products of RTIC of basedRTIC onbased XRD. on ( aXRD.) The crystal(a) The crystal composition and hydration products of RTIC (RCE0GHB0, RCE10GHB0, RCE20GHB0, RCE30GHB0); (b) CH, C- composition and hydration products of RTIC (RCE0GHB0, RCE10GHB0, RCE20GHB0, RCE30GHB0); (b) CH, C-S-H, CaCO3, S-H, CaCO3, Tobermorite (RCE0GHB0, RCE10GHB0, RCE20GHB0, RCE30GHB0); (c) the crystal composition and hydra- Tobermorite (RCE0GHB0, RCE10GHB0, RCE20GHB0, RCE30GHB0); (c) the crystal composition and hydration products tion products of RTIC (RCE10GHB0, RCE10GHB20, RCE10GHB40, RCE10GHB60); (d) CH, C-S-H, CaCO3, Tobermorite d of(RCE10GHB0, RTIC (RCE10GHB0, RCE10GHB20, RCE10GHB20, RCE10GHB40, RCE10GHB40, RCE10GHB60). RCE10GHB60); ( ) CH, C-S-H, CaCO3, Tobermorite (RCE10GHB0, RCE10GHB20, RCE10GHB40, RCE10GHB60).

Materials 2021, 14, 4695 17 of 22 Materials 2021, 14, x FOR PEER REVIEW 18 of 23

3.2.3.3.2.3. Functional GroupGroup ChangesChanges ofof RCTICRCTIC TheThe effecteffect ofof RCERCE and and GHB GHB on on the the functional functional groups groups of of concrete concrete hydration hydration products products is shownis shown in in Figure Figure 13 13.. The The shrinkage shrinkage vibration vibration of of H-O-H H-O-H (3400) (3400) functional functional group group andand thethe bendingbending vibrationvibration ofof H-O-HH-O-H (1640)(1640) significantlysignificantly changedchanged withwith thethe increasingincreasing contentcontent ofof RCERCE andand GHB.GHB. H-O-HH-O-H (3400)(3400) functionalfunctional groupgroup hadhad aa more pronouncedpronounced oscillationoscillation range,range, becausebecause RCERCE andand GHBGHB havehave highhigh waterwater absorptionabsorption andand cancan storestore aa largelarge amountamount ofof waterwater phase.phase. AtAt thethe same time,time, thethe shrinkage vibrationvibration ofof thethe -OH-OH (3637)(3637) functionalfunctional groupgroup inin CHCH increased significantly with the content of RCE and GHB based on the influence of CaCO3 increased significantly with the content of RCE and GHB based on the influence of CaCO3 productionproduction inin concrete.concrete.

85 85 REC0GHB0 80 80 RCE10GHB0 75 RCE20GHB0 75 70 RCE30GHB0

3400 ↑ 65 70 60 65 55 ↑ ↑ 50 60 3400 1640 45 ↑ 878 ↑

693

55 3400 ↑ RCE0GHB0 Transmittance (%) 40 Transmittance (%) RCE10GHB0 ↑ 35 ↑ 878 50 RCE20GHB0 1483 ↑ 30 1419 RCE30GHB0 ↑ 45 ↑ ↑ ↑ ↑ 3400 1086 983 ↑ 3637 3428 25 460 40 20 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 −1 −1 Wavenumbers (cm ) Wavenumbers (cm ) (a) 90 85 RCE10GHB0 85 RCE10GHB20 80 80 RCE10GHB40

3400 75 75 ↑ 70 RCE10GHB60 70 65 ↑ 60 65 3400 55 60 50 45 55 40 35 ↑ 50 1640 ↑ RCE10GHB0 30 ↑ 3400 Transmittance (%) Transmittance Transmittance(%) RCE10GHB20 25 713 45 ↑ RCE10GHB40 20 ↑ 878 1433 ↑ 40 ↑ RCE10GHB60 15 ↑ 460 1086 983 35 ↑ 10 3637 ↑ ↑ 5 3428 3400 30 0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400

−1 −1 Wavenumbers (cm ) Wavenumbers (cm ) (b)

FigureFigure 13.13. TheThe effect effect of of RCE RCE and and GHB GHB on onthe thefunctional functional groups groups of concrete of concrete hydration hydration products. products. (a) The ( afunctional) The functional groups groupsof RTIC of RTIC(RCE0GHB0, (RCE0GHB0, RCE10GHB RCE10GHB0,0, RCE20GHB0, RCE20GHB0, RCE30GHB0); RCE30GHB0); (b) (theb) the functional functional groups groups of of RTIC RTIC (RCE10GHB0,(RCE10GHB0, RCE10GHB20, RCE10GHB40, RCE10GHB60). RCE10GHB20, RCE10GHB40, RCE10GHB60).

Materials 2021, 14, 4695 18 of 22

3.3. Mathematical Representation of the Coupled Inﬂuence of RCE and GHB The effects of RCE and GHB on RTIC workability, compressive strength, splitting tensile strength and insulation properties can be quantitatively determined by the correlation principle. The interaction between these two parameters is determined as follows:

Slump[Combined RCE and GHBContent] k = (1) 1 Slump[Control Group]

Compressive strength[Combined RCE and GHBContent] k = (2) 2 Compressive strength[Control Group] Splitting tensile strength[Combined RCE and GHBContent] k = (3) 3 Splitting tensile strength[Control Group] Insulation properties[Combined RCE and GHBContent] k = (4) 4 Insulation properties[Control Group]

K = <1k1 + <2k2 + <3k3 + <4k4 (5)

where k1 is the coefficient of the influence on liquidity, k2 is the coefficient of influence on compressive strength, k3 is the coefficient of influence on splitting tensile strength, k4 is the coefficient of influence on insulation, < is the correlation parameter between RCE and GHB content, K is the quantitative characterization of the coupled effects of RCE and GHB content on concrete comprehensive characteristics. The calculation results of the influence coefficient of RCE and GHB on concrete performance are shown in Figure 14. The following conclusions can be reached. (1) Considering the mechanical properties and thermal insulation properties of concrete, the best content of RTIC is RCE10GHB60. (2) The allowable content of GHB used for general engineering needs can be relaxed in order to improve thermal insulation performance while ensuring the quality of the concrete. (3) The content of RCE needs to be strictly Materials 2021, 14, x FOR PEER REVIEWcontrolled, and its reasonable replacement rate should not exceed 20% of the natural coarse20 of 23

aggregate.

3.0 1.2 K Value K Value Plain cement paste Plain cement paste 1.1 2.5

1.0

2.0 0.9

0.8 1.5 Relative coefficient, K1 coefficient, Relative K2 coefficient, Relative 0.7

1.0 0.6

0.5 0.5

0 0 0 0 0 0 0 0 0 0 0 0 B0 6 B B0 4 B0 B4 B20 B40 B20 B60 B40 H HB B2 H B2 B4 B6 B6 H H H H H H H HB H H H H GHB0 GH GHB0 G G GHB20 GH GHB 0 0 GHB60 G 0 G G G 0 20 30 GHB20 0 10 0 G 2 0 G 30 GHB230 GHB4 0 G E0 E E1 E E3 E CE0 E E E E RCE0 G C CE0 C C C C R C CE1 CE2 CE3 R R R R RCE10 GRCE10 GRCE10 GHB60R RCE20 GRCE20 GHB40RCE20 GR RC RCE30 GRCE30 GHB60 RCE0 GHB20RCE0 GHB40R RC R RCE10 RCE10 GHB60RCE R RCE20 RCE20 RCE30 RCGHB0 RC R

1.2 Figure 14. 2.5Cont. K Value K Value Plain cement paste Plain cement paste 1.1

2.0 1.0

0.9 1.5

0.8 Relative coefficient, K4 coefficient, Relative Relative coefficient, K3 0.7 1.0

0.6

0.5 0.5

0 0 0 0 0 0 0 0 0 0 0 B B0 6 B 4 B 2 4 B40 B20 B40 B60 H B B60 H B B B60 HB60 GHB0 HB60 HB20 H H H H H G GHB2 GHB2 GHB4 GHB6 G 0 GHB 10 0 0 G 0 0 0 30 0 G 0 G E0 GH E0 GHB0 E 1 3 3 C E0 GH E0 G E10 E20 GHE20 GHB20E20 GHB40 E30 GHB40E30 GHB60 C R C C C C C C C C R CE CE CE CE CE RCE0 GHB20R R R RCE10 GHRCE10 GHRCE10 GR R R RCE20 GHRCE30 RCE30GHB0 GR R RCE0 GHB20RCE0 GHB40RC R RCE1 R RCE10 GRCE20 RCE2GHB0 RCE2 RCE2 R R R RCE30 G

(a)

Materials 2021, 14, x FOR PEER REVIEW 20 of 23

3.0 1.2 K Value K Value Plain cement paste Plain cement paste 1.1 2.5

1.0

2.0 0.9

0.8 1.5 Relative coefficient, K1 coefficient, Relative K2 coefficient, Relative 0.7

1.0 0.6

0.5 0.5

0 0 0 0 0 0 0 0 0 0 0 0 MaterialsB02021, 14, 46956 B B0 4 B0 19 of 22 B4 B20 B40 B20 B60 B40 H HB B2 H B2 B4 B6 B6 H H H H H H H HB H H H H GHB0 GH GHB0 G G GHB20 GH GHB 0 0 GHB60 G 0 G G G 0 20 30 GHB20 0 10 0 G 2 0 G 30 GHB230 GHB4 0 G E0 E E1 E E3 E CE0 E E E E RCE0 G C CE0 C C C C R C CE1 CE2 CE3 R R R R RCE10 GRCE10 GRCE10 GHB60R RCE20 GRCE20 GHB40RCE20 GR RC RCE30 GRCE30 GHB60 RCE0 GHB20RCE0 GHB40R RC R RCE10 RCE10 GHB60RCE R RCE20 RCE20 RCE30 RCGHB0 RC R

1.2 2.5 K Value K Value Plain cement paste Plain cement paste 1.1

2.0 1.0

0.9 1.5

0.8 Relative coefficient, K4 coefficient, Relative Relative coefficient, K3 0.7 1.0

0.6

0.5 0.5

0 0 0 0 0 0 0 0 0 0 0 B B0 6 B 4 B 2 4 B40 B20 B40 B60 H B B60 H B B B60 HB60 GHB0 HB60 HB20 H H H H H G GHB2 GHB2 GHB4 GHB6 G 0 GHB 10 0 0 G 0 0 0 30 0 G 0 G E0 GH E0 GHB0 E 1 3 3 C E0 GH E0 G E10 E20 GHE20 GHB20E20 GHB40 E30 GHB40E30 GHB60 C R C C C C C C C C R CE CE CE CE CE Materials 2021RCE0, 14 GHB20R, x FORR PEERR RCE10 REVIEW GHRCE10 GHRCE10 GR R R RCE20 GHRCE30 RCE30GHB0 GR R RCE0 GHB20RCE0 GHB40RC R RCE1 R RCE10 GRCE20 RCE2GHB0 RCE2 RCE2 R R 21R ofRCE30 23 G

(a) 2.0

1.8

1.6

1.4 K

1.2

1.0

0.8

0.6

0 0 0 0 0 0 0 0 0 2 40 B 4 6 2 6 2 4 B B20 B B HB0 B B B B HB HB60 H H G H H G G G G 0 G G 0 10 2 20 30 RCE0 GHB0 E10 RCE RCE0 GHRCE0 RCE10 RCGH RCE10 GHRCE RCE RCE RCE20 GHB40RCE20 GHRCE30 RCE30GHB0 GHRCE RCE30 GHB6 (b)

Figure 14. The calculationFigure results 14. The of the calculation inﬂuence results coefﬁcient of the of influence RCE and GHBcoefficient on concrete of RCE performance. and GHB on ( aconcrete) K1, K2, perfor- K3, K4. (b) K. mance. (a) K1, K2, K3, K4. (b) K.

4. Conclusions4. Conclusions This paperThis paperinvestigated investigated the influence the inﬂuence of RCE and of RCE GHB and on GHBthe mechanical, on the mechanical, thermal con- thermal ductivityconductivity and material and properties material propertiesof concrete, of and concrete, the following and the conclusions following can conclusions be drawn: can be drawn: 1. RCE can inhibit the density and mechanical properties of RTIC, but it has little effect on the working and thermal insulation properties of RTIC. GHB can improve the workability and insulation properties of RTIC (155.6 and 91.9% maximum increase), but decreases the density and mechanical properties (9.9 and 33.9% maximum decrease). 2. The incorporation of GHB and RCE inhibited the formation of CH and C-S-H and promoted the increase of CaCO3, which was detrimental to the macroscopic mechanical properties of RTIC. Meanwhile, the increase in GHB led to an increase in the production of Tobermorite, which largely improves the insulation properties of RTIC. 3. GHB and RCE can be used to prepare regenerated thermal insulation concrete due to the synergistic development of mechanical properties and thermal insulation properties. It is considered that 10% RCE and 60% GHB are the best doping amounts. The new thermal insulation concrete material prepared based on this material design system can be used in practical projects such as prefabricated roof structures in civil engineering, special transportation pipelines, and link bridges in equipment and accessory buildings. 4. GHB and RCE can be used to prepare regenerated thermal insulation concrete due to the synergistic development of mechanical properties and thermal insulation properties. It is considered that 10% RCE and 60% GHB are the best doping amounts.

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1. RCE can inhibit the density and mechanical properties of RTIC, but it has little effect on the working and thermal insulation properties of RTIC. GHB can improve the workability and insulation properties of RTIC (155.6 and 91.9% maximum increase), but decreases the density and mechanical properties (9.9 and 33.9% maximum decrease). 2. The incorporation of GHB and RCE inhibited the formation of CH and C-S-H and promoted the increase of CaCO3, which was detrimental to the macroscopic mechanical properties of RTIC. Meanwhile, the increase in GHB led to an increase in the production of Tobermorite, which largely improves the insulation properties of RTIC. 3. GHB and RCE can be used to prepare regenerated thermal insulation concrete due to the synergistic development of mechanical properties and thermal insulation properties. It is considered that 10% RCE and 60% GHB are the best doping amounts. The new thermal insulation concrete material prepared based on this material design system can be used in practical projects such as prefabricated roof structures in civil engineering, special transportation pipelines, and link bridges in equipment and accessory buildings. 4. GHB and RCE can be used to prepare regenerated thermal insulation concrete due to the synergistic development of mechanical properties and thermal insulation properties. It is considered that 10% RCE and 60% GHB are the best doping amounts.

Author Contributions: Conceptualization, Y.Y. and B.L.; methodology, B.L.; software, Y.Y.; validation, Y.Y., B.L.; formal analysis, Y.Y.; investigation, D.L.; resources, D.L.; data curation, Y.Y. and B.L.; writing—original draft preparation, Y.Y.; writing—review and editing, Y.Y. and B.L.; visualization, Y.Y.; supervision, B.L.; project administration, B.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Science and Technology Innovation Platform of Foshan City (Grant No. 2016AG100341, Guangdong Province, China). 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. Conflicts of Interest: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Coupling influence between recycled ceramics and grazed hollow beads on mechanical properties and thermal conductivity of recycled thermal insulation concrete”.

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