# Influence of Recycled Glass on Strength Development of Alkali-Activated High-Calcium Fly Ash Mortar

Authors: Nattapong Damrongwiriyanupap; Sangsan Wongchairattana; Tanakorn Phoo-Ngernkham; Aruz Petcherdchoo; Suchart Limkatanyu; Prinya Chindaprasirt
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

This article presents the development of green and sustainable mortars using alkali-activated high-calcium fly ash (AAFA) and recycled glass (RG) as part of the fine aggregate. RG was used to replace river sand at dosages of 0%, 25%, 50%, 75%, and 100% by weight. Sodium hydroxide (SH) and sodium silicate (SS) solutions were used as liquid alkaline activators in all mixtures. The AAFA samples were prepared with different liquid-to-binder ratios of 0.6 and 0.7, and the ratio of SS-to-SH was fixed at 2.0. Compressive and flexural strengths were determined at the ages of 7, 28, and 60 days. Test results showed that the compressive and flexural strengths of AAFA mortars declined as RG replacement increased; nevertheless, they increased with curing time. The high Na2O concentration derived from RG and the weak interfacial transition zone of RG are reasons for the decrease in strength development. The optimum percentage replacement of fine aggregates with RG was found at 25%. The 28-day compressive strength of AAFA with 25% RG was 32.5 MPa for L/B ratios of 0.6 and 29.5 MPa for L/B ratios of 0.7, which resulted in a strength index higher than 75% while releasing low CO2.


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## Body

## 1. Introduction

Recycled glass (RG) is increasingly used for civil engineering applications such as construction backfill material, subbase and subgrade materials, pavement aggregates, trench bedding material, and landfill material. This obviously illustrates the necessity for an alternative approach to glass recycling. The construction industry offers an interesting application of waste glass for concrete, which is an important factor in building and is extensively employed in a variety of countries. Several studies have been conducted to utilize RG as aggregates in concrete [1] as well as using RG powder as partial replacement of cement in concrete [2–5]. Only a few have studied the use of RG as aggregate replacement and as cement partial replacement in ultra-high performance concrete. For example, Yang et al. [6] used RG with a particle size of less than 600 microns as a replacement for 240 microns of silica sand in ultrahigh-performance fibre-reinforced concrete. They reported that the samples with RG had 12% lower compressive strength than the ones with silica sand when cured at room temperature and 4% lower when cured at 90°C. The average 28-day strength of the samples cured at room temperature was found to be 110 MPa, while those cured at 90°C were 150 MPa. Wille et al. [7] used very fine glass powder (median particle size of 1.7 microns) in the production of ultrahigh-performance concrete with a food-type mixer. They reported that glass powder could be used for cement replacement and that the optimum amount of replacement is 25% of cement by weight. Kou and Xing [8] used 45 microns of RG powder as a cement replacement in ultrahigh-performance concrete. They reported that the replacement of cement with glass powder helped increase the long-term strength of the concrete and that 15% replacement performed better than 30% replacement. Higher strengths were observed for the specimens cured at 90°C in comparison to those cured at 20°C.However, it is well known that concrete manufacturing is a resource- and energy-intensive process, generating about 5–8% of global greenhouse gas emissions [9–12]. As such, alternatives to decrease its environmental impact are necessary. Nowadays, the use of new binding material i.e. alkali-activated binder (AAB) is one of the ways to reduce CO2 emissions due to Portland cement production [11, 13, 14]. The AAB can be chemically produced by combining aluminosilicate compounds with alkaline solutions [14–18]. High-calcium fly ash (FA) from the Mae Moh power plant has been utilized as a precursor for AAB production in Thailand due to its extensive availability and acceptable chemical composition [19]. The reaction between SiO2 and CaO in the FA system assisted in the geopolymerization process when cured at room temperature [20–22]. Hence, a combination of C-S-H (calcium silicate hydrate) and/or C-A-S-H (calcium aluminosilicate hydrate) and N-A-S-H (sodium aluminosilicate hydrate) gels is the main reaction of the high-calcium AAB system [15, 23, 24]. According to Phoo-Ngernkham et al. [25–28], alkali-activated high-calcium FA is a reasonable alternative, especially for repairing works.As previously mentioned, natural aggregates are limited but extensively used every year. Furthermore, the crushed aggregate for use in the building sector is expensive. Alternatively, it can be seen that using RG as an aggregate in structural concrete has the potential to not only improve the environment due to reducing waste and raw material utilization but also benefit the industry due to lowering costs. The outcome of this research could help not only Thailand but also the world to divert a significant quantity of waste material, i.e., waste glass, from landfills and considerably reduce environmental damage caused by carbon emissions due to PC production. Therefore, in this work, the effect of the incorporation of RG as fine aggregate replacement on the strength development of alkali-activated, high-calcium FA mortar was investigated.


## 2. Materials and Testing Analysis

High-calcium fly ash (FA) derived from the Mae Moh lignite power plant in northern Thailand was used as a raw material for producing alkali-activated high-calcium FA (AAFA). The FA had a specific gravity of 2.65, a median particle size of 15.5μm, and Blaine fineness of 3840 cm2/g. The chemical compositions of FA consisted of SiO2 (25.6%), Al2O3 (9.1%), Fe2O3 (27.8%), CaO (25.0%), MgO (3.0%), K2O (3.0%), Na2O (0.2%), SO3 (3.85%), and LOI (2.54%), as shown in Table 1. The sum of SiO2, Al2O3, and Fe2O3 was 62.5%, and the CaO content was high at 25.0%. This FA was Class C fly ash as specified by ASTM C618 [29]. The XRD patterns of FA and RG are illustrated in Figure 1, whereas their SEM images are shown in Figure 2(a) for FA and Figure 2(b) for RG. 10 molar sodium hydroxide (SH) and sodium silicate (SS) with 13.89% Na2O, 32.15% SiO2, and 53.96% H2O were used as alkali activators. The fine aggregates used in this study were local river sand (RS) and crushed recycled glass (RG). The RS had a specific gravity and fineness modulus of 2.62 and 1.41, respectively, whereas the RG had a specific gravity and fineness modulus of 2.51 and 1.67, respectively.Table 1
Chemical compositions of FA, RS, and RG (by % weight).
MaterialsSiO2Al2O3Fe2O3CaOMgOK2ONa2OOtherSO3LOIFA25.569.0527.7924.970.463.010.162.613.852.54RS92.001.600.600.900.102.200.10——2.10RG65.801.330.1712.001.290.1712.905.760.130.45Figure 1
XRD patterns of materials: (a) RG and (b) FA (o = calcium sulfate (CaSO4), M = magnesioferrite (MgFe2O4), and Q = quartz (SiO2)).Figure 2
SEM images of (a) FA and (b) RG.
(a)(b)The abbreviations FA-0RG, FA-25RG, FA-50RG, FA-75RG, and FA-100RG indicate AAFA mortars containing the RS replacement with RG at 0%, 25%, 50%, 75%, and 100% by weight, respectively. The ratios of liquid to solid binder (L/B) were varied between 0.6 and 0.7. While the ratios of SS-to-SH and sand-to-binder were fixed at 2.0.In the mixing process of AAFA mortars, FA and RS/RG were dry-mixed until the mixture was homogeneous, which took approximately 1 minute. The alkali activator solutions were then added and mixed for an additional 3 minutes. After mixing, fresh mortars were then placed into 50 × 50 × 50 mm cube and 75 × 75 × 300 mm prism molds for compressive [30] and flexural [31] strength tests, respectively. To prevent moisture loss, the specimens were covered with a vinyl sheet and stored in a 25°C controlled room for 7, 28, and 60 days prior to the day of testing. In the results, three samples were tested and averaged. Table 2 lists the mix proportions and setting time results of AAFA mortar under different RG contents. According to Table 2, it is found that the setting time of AAFA tended to increase marginally as RG content increased. In addition, an increase in their L/B ratios could prolong their setting times. This is agreed with the works of Sinsiri et al. [20].Table 2
Mix proportions of alkali-activated high-calcium FA mortars with RG (kg/m3).
Mix no.Mix symbolL/B ratioFA (kg)RS (kg)RG (kg)NH (kg)NS (kg)Setting time (min)InitialFinal1FA-0RG0.60862.0862.0—172.4344.820372FA-25RG859.0644.3343.6171.8343.620383FA-50RG856.0428.0428.0171.2342.422384FA-75RG853.0213.3639.8170.6341.223405FA-100RG850.0—850.0170.0340.024426FA-0RG0.70815.0815.0—190.2380.327487FA-25RG813.0609.8203.3189.7379.428508FA-50RG810.0405.0405.0189.0378.030509FA-75RG807.0201.8605.3188.3376.6325210FA-100RG804.0—804.0187.6375.23352


## 3. Results and Discussion

### 3.1. Compressive Strength

The compressive strength of AAFA mortars under different L/B ratios at various amount of RS replacement with RG is summarized in Figure3. As seen in Figure 3, the compressive strength of mortar specimens depends on the L/B ratio and percentage of RG content in the mixtures. At 7 days, it is evident that compressive strength of specimens slightly decreased with increasing the amount of RG replacement for both L/B ratios of 0.6 and 0.7. For example, the compressive strengths of FA-0RG, FA-25RG, FA-50RG, FA-75RG, and FA-100RG mortars at the L/B ratio of 0.60 are 16.9, 16.6, 16.4, and 14.5 MPa, respectively. The highest compressive strength of mortar was obtained at the reference mortar (0% RG replacement), which was 17.8 and 13.2 MPa for L/B ratios of 0.6 and 0.7, respectively. The significant decrease in strength development occurred due to a weaker interfacial transition zone in the mortar with RG. Ling and Poon [32] also reported that the reduction in bonding between aggregates and paste occurred due to the smoother surface of RG compared to that of RS. Also, the effect of the Na2O concentration (see Table 1) played a significant role in the development of its strength. Detphan et al. [33] claimed that the excess Na2O content in the matrix induced a slow reaction rate during the geopolymerization process. Zhang et al. [34] demonstrated that higher Na2O content resulted in higher drying shrinkage of alkali-activated binder, which might result in microcracks and thus strength loss. Note that the use of RG in AAM might induce the alkali-silica reaction (ASR) within the matrix [34]. However, the ASR expansion could be controlled by incorporating additional aluminium sources and adjusting the RG to binder ratio in the mixtures. Consideration should be given to this ASR for the usage of AAFA with RG in construction works.Figure 3
Compressive strength of AAFA mortar containing RG under different curing times and L/B ratios.At 28 days, the samples with 25% RG replacement provided the highest strengths of 32.5 and 29.5 MPa for L/B ratios of 0.6 and 0.7, respectively. However, when the amount of RG replacement exceeded 25%, the compressive strength of AAFA mortars obviously decreased. This is agreed with the SEM images of a 28-day-cured AAFA, as shown in Figure4. According to Figure 4, increasing the RG replacement content had a detrimental effect on the strength development of the composites owing to the formation of poor pore structures. Note that the 28-day and 60-day compressive strengths of specimens were higher than the 7-day compressive strength due to the increase in geopolymerization degree, as confirmed by several publications [20, 26, 35–37]. Additionally, the reaction between SiO2 and/or Al2O3 and CaO in the high-calcium FA system facilitated the formation of calcium aluminosilicate hydrate (C-A-S-H) in the matrix [20, 38, 39], resulting in later strength development.Figure 4
SEM images of a 28-day-cured AAFA: (a) FA-0RG, (b) FA-25RG, (c) FA-50RG, (d) FA-75RG, and (e) FA-100RG.
(a)(b)(c)(d)(e)Figure5 shows the relationship between normalized compressive strength and curing time of AAFA mortar containing RG at the 28-day compressive strength. As seen in Figure 5, the strength development of mortars with RG increased gradually as the curing period increased. The correlation between normalized compressive strength and curing time of RG mortars was shown using the 28-day compressive strength as the logarithm function. This empirical equation has been effectively used to materials based on cement materials and alkali-activated binder [16, 40, 41]. The compressive strengths of AAFA mortars can be described empirically in terms of curing durations, as seen in equation (1) and displayed in Figure 5. The constants in equation (1) were derived by curve-fitting the findings of RG contents at 0%, 25%, 50%, 75%, and 100%. The curve fitting produced an R2 value of 0.892, which is good for engineering applications(1)fc,d′fc,28′=0.27453lnd+0.00258,where fc,d′ is the compressive strength at d days of curing time and fc,28′ is the compressive strength at 28 days of curing time.Figure 5
Relationship between normalized compressive strength and curing time of AAFA mortar containing RG by the 28-day compressive strength.


### 3.2. Flexural Strength

The flexural strength of AAFA mortars at different L/B ratios cured for 7, 28, and 60 days is illustrated in Figure6. The flexural strength of mortars exhibited a similar trend to the compressive strength results. The trend of flexural strength seemed to decline with increasing RS replacement with RG. For example, at an L/B ratio of 0.60 and a curing time of 28 days, the flexural strengths of FA-25RG, FA-50RG, FA-75RG, and FA-100RG mortars were 0.51, 0.22, 0.12, and 0.10 MPa, respectively. While the highest 28-day flexural strength of FA-0RG mortar was 1.05 MPa for L/B ratios of 0.60. As seen in Figure 6, the flexural strength of mortars with 25% RG replacement was slightly reduced. And also, they declined significantly as RG replacement increased. The decline in flexural strength may be related to the replacement of RG, which increases the Na2O content in the matrix due to its high Na2O composition (see Table 1). As explained previously, a greater Na2O molar ratio increased its drying shrinkage [34], resulting in the formation of many microcracks. In addition, increasing of Na2O content induced the alkali-silica reaction (ASR) in the alkali-activated binder system, as reported by Long et al. [42] and Idir et al. [43]. Previous publications [42, 44, 45] reported that the ASR growth was first inhibited owing to the consumption of OH- ions in the pore solution. And, when the SiO2 was gradually consumed, the Na2O in the pore solution reacted with the RG. This probably led to the occurrence of the ASR. This observation can be confirmed by using the XRD patterns of a 28-day-cured AAFA, as illustrated in Figure 7. It is found that all AAFA mortars mainly consist of crystalline phases of quartz (SiO2). Moreover, Tho-In et al. [46] demonstrated that the friction between RG and the paste within the matrix had a negative effect on its strength development.Figure 6
Flexural strength of AAFA mortar containing RG under different curing times and L/B ratios.Figure 7
XRD patterns of a 28-day-cured AAFA: (a) FA-100RG, (b) FA-75RG, (c) FA-50RG, (d) FA-25RG, and (e) FA-0RG (Q = quartz (SiO2)).Figure8 shows the relationship between normalized flexural strength and curing time of AAFA mortar containing RG based on consideing the 28-day flexural strength equation (2) illustrates an empirical relationship between the flexural strength of mortars containing RG and the curing time at various L/B ratios, with a high R2 value of 0.752. The constants in equation (2) were determined by curve-fitting the data for RG replacement levels of 0%, 25%, 50%, 75%, and 100%:(2)ft,dft,28=0.57720lnd−0.73670,where ft,d is the flexural strength at d days of curing time and ft,28 is the flexural strength at 28 days of curing time.Figure 8
Relationship between normalized flexural strength and curing time of AAFA mortar containing RG by the 28-day flexural strength.


### 3.3. Life-Cycle Assessment

The life-cycle assessment (LCA) of AAFA mortars incorporating SG was evaluated. According to previous publications [27, 47], the measurement unit of emission in the LCA analysis is CO2-e emitted (t CO2-e/m3). Table 3 shows the emission factors obtained from the literature [13, 48–50]. However, the emission factors derived from various studies, especially those conducted in different regions or countries, may vary significantly. The further use of results should be done with care. According to Table 3, FA emitted the least CO2, while SS solution emitted the highest values. The LCA analyses for AAFA mortars incorporating SG and Portland cement mortar (PCM) are shown in Tables 4 and 5. It is revealed when the RG replacement level increased, the CO2-e emitted values tended to marginal increase. According to Tables 4 and 5, AAFA mortars emitted lower CO2 than PCMs. For example, the CO2-e emitted values of FA-25RG and PCM were 0.28161 and 0.5053 ton CO2-e/ton, respectively, for L/B ratios of 0.60 and w/c ratios of 0.60. In addition, all AAFA mortars generated lower CO2 than Portland cement concrete (PCC). Note that the 28-day compressive strengths of PCMs were 46.2 and 37.1 MPa for w/c ratios of 0.60 and 0.70, respectively. While the 28-day compressive strength of PCC was 35.5 MPa. As shown in Figure 3, the strengths of AAFA mortars were slightly lower than those of PCM and PCC. As shown in Tables 4 and 5, the FA content of the AAFA mortar is around 800 kg/m3, while the PC content of the PCM mortar is approximately 600 kg/m3. However, PC is approximately 2 times more expensive than FA, according to Thailand market prices (2022) and Phoo-Ngernkham et al. [27]. Therefore, the relatively large amount of FA used in the production of AAFA has no significant effect on the total price of AAFA. According to the basis of empirical evidence, it is recommended that 25% RG be used in replacement of RS as fine aggregate in order to have a strength index greater than 75% and a low carbon footprint. For further studies, it is recommended to apply the findings in this study to construction practice [51, 52] and also compare them to other materials [53, 54].Table 3
The emission factors of tested materials [13, 48–50].
Materials (kg)Emission factor (t CO2-e/m3)Portland cement, PC0.8200High-calcium fly ash, FA0.0070River sand, RS0.0139Recycled glass, RG0.0200Coase aggregate, CA0.4300Sodium hydroxide, SH0.7000Sodium silicate, SS1.5140Table 4
LCA analysis of AAFA mortar incorporating RG from this article.
L/B ratiosMix symbolFA (kg)CO2-e/m3from FARS (kg)CO2-e/m3from RSRG (kg)CO2-e/m3from RGSH (kg)CO2-e/m3from SHSS (kg)CO2-e/m3from SSTotal CO2-e/m30.60FA-0RG862.00.0060862.00.0120—0.0000172.40.0362344.80.22450.27869FA-25RG859.00.0060644.30.0090343.60.0069171.80.0361343.60.22370.28161FA-50RG856.00.0060428.00.0059428.00.0086171.20.0360342.40.22290.27936FA-75RG853.00.0060213.30.0030639.80.0128170.60.0358341.20.22210.27969FA-100RG850.00.0060——850.00.0170170.00.0357340.00.22130.280000.70FA-0RG815.00.0057815.00.0113—0.0000190.20.0399380.30.24760.30456FA-25RG813.00.0057609.80.0085203.30.0041189.70.0398379.40.24700.30507FA-50RG810.00.0057405.00.0056405.00.0081189.00.0397378.00.24610.30518FA-75RG807.00.0056201.80.0028605.30.0121188.30.0395376.60.24520.30528FA-100RG804.00.0056——804.00.0161187.60.0394375.20.24430.30537Table 5
LCA analysis of PCC and PCM.
Mix symbolPC (kg)CO2-e/m3 from PCRS (kg)CO2-e/m3 from RSCA (kg)CO2-e/m3 from CATotal CO2-e/m3PCC495.00.4059510.00.0071938.00.40330.8163PCM (w/c = 0.60)596.00.48871192.00.0166——0.5053PCM (w/c = 0.70)562.00.46081123.00.0156——0.4765


## 3.1. Compressive Strength

The compressive strength of AAFA mortars under different L/B ratios at various amount of RS replacement with RG is summarized in Figure3. As seen in Figure 3, the compressive strength of mortar specimens depends on the L/B ratio and percentage of RG content in the mixtures. At 7 days, it is evident that compressive strength of specimens slightly decreased with increasing the amount of RG replacement for both L/B ratios of 0.6 and 0.7. For example, the compressive strengths of FA-0RG, FA-25RG, FA-50RG, FA-75RG, and FA-100RG mortars at the L/B ratio of 0.60 are 16.9, 16.6, 16.4, and 14.5 MPa, respectively. The highest compressive strength of mortar was obtained at the reference mortar (0% RG replacement), which was 17.8 and 13.2 MPa for L/B ratios of 0.6 and 0.7, respectively. The significant decrease in strength development occurred due to a weaker interfacial transition zone in the mortar with RG. Ling and Poon [32] also reported that the reduction in bonding between aggregates and paste occurred due to the smoother surface of RG compared to that of RS. Also, the effect of the Na2O concentration (see Table 1) played a significant role in the development of its strength. Detphan et al. [33] claimed that the excess Na2O content in the matrix induced a slow reaction rate during the geopolymerization process. Zhang et al. [34] demonstrated that higher Na2O content resulted in higher drying shrinkage of alkali-activated binder, which might result in microcracks and thus strength loss. Note that the use of RG in AAM might induce the alkali-silica reaction (ASR) within the matrix [34]. However, the ASR expansion could be controlled by incorporating additional aluminium sources and adjusting the RG to binder ratio in the mixtures. Consideration should be given to this ASR for the usage of AAFA with RG in construction works.Figure 3
Compressive strength of AAFA mortar containing RG under different curing times and L/B ratios.At 28 days, the samples with 25% RG replacement provided the highest strengths of 32.5 and 29.5 MPa for L/B ratios of 0.6 and 0.7, respectively. However, when the amount of RG replacement exceeded 25%, the compressive strength of AAFA mortars obviously decreased. This is agreed with the SEM images of a 28-day-cured AAFA, as shown in Figure4. According to Figure 4, increasing the RG replacement content had a detrimental effect on the strength development of the composites owing to the formation of poor pore structures. Note that the 28-day and 60-day compressive strengths of specimens were higher than the 7-day compressive strength due to the increase in geopolymerization degree, as confirmed by several publications [20, 26, 35–37]. Additionally, the reaction between SiO2 and/or Al2O3 and CaO in the high-calcium FA system facilitated the formation of calcium aluminosilicate hydrate (C-A-S-H) in the matrix [20, 38, 39], resulting in later strength development.Figure 4
SEM images of a 28-day-cured AAFA: (a) FA-0RG, (b) FA-25RG, (c) FA-50RG, (d) FA-75RG, and (e) FA-100RG.
(a)(b)(c)(d)(e)Figure5 shows the relationship between normalized compressive strength and curing time of AAFA mortar containing RG at the 28-day compressive strength. As seen in Figure 5, the strength development of mortars with RG increased gradually as the curing period increased. The correlation between normalized compressive strength and curing time of RG mortars was shown using the 28-day compressive strength as the logarithm function. This empirical equation has been effectively used to materials based on cement materials and alkali-activated binder [16, 40, 41]. The compressive strengths of AAFA mortars can be described empirically in terms of curing durations, as seen in equation (1) and displayed in Figure 5. The constants in equation (1) were derived by curve-fitting the findings of RG contents at 0%, 25%, 50%, 75%, and 100%. The curve fitting produced an R2 value of 0.892, which is good for engineering applications(1)fc,d′fc,28′=0.27453lnd+0.00258,where fc,d′ is the compressive strength at d days of curing time and fc,28′ is the compressive strength at 28 days of curing time.Figure 5
Relationship between normalized compressive strength and curing time of AAFA mortar containing RG by the 28-day compressive strength.


## 3.2. Flexural Strength

The flexural strength of AAFA mortars at different L/B ratios cured for 7, 28, and 60 days is illustrated in Figure6. The flexural strength of mortars exhibited a similar trend to the compressive strength results. The trend of flexural strength seemed to decline with increasing RS replacement with RG. For example, at an L/B ratio of 0.60 and a curing time of 28 days, the flexural strengths of FA-25RG, FA-50RG, FA-75RG, and FA-100RG mortars were 0.51, 0.22, 0.12, and 0.10 MPa, respectively. While the highest 28-day flexural strength of FA-0RG mortar was 1.05 MPa for L/B ratios of 0.60. As seen in Figure 6, the flexural strength of mortars with 25% RG replacement was slightly reduced. And also, they declined significantly as RG replacement increased. The decline in flexural strength may be related to the replacement of RG, which increases the Na2O content in the matrix due to its high Na2O composition (see Table 1). As explained previously, a greater Na2O molar ratio increased its drying shrinkage [34], resulting in the formation of many microcracks. In addition, increasing of Na2O content induced the alkali-silica reaction (ASR) in the alkali-activated binder system, as reported by Long et al. [42] and Idir et al. [43]. Previous publications [42, 44, 45] reported that the ASR growth was first inhibited owing to the consumption of OH- ions in the pore solution. And, when the SiO2 was gradually consumed, the Na2O in the pore solution reacted with the RG. This probably led to the occurrence of the ASR. This observation can be confirmed by using the XRD patterns of a 28-day-cured AAFA, as illustrated in Figure 7. It is found that all AAFA mortars mainly consist of crystalline phases of quartz (SiO2). Moreover, Tho-In et al. [46] demonstrated that the friction between RG and the paste within the matrix had a negative effect on its strength development.Figure 6
Flexural strength of AAFA mortar containing RG under different curing times and L/B ratios.Figure 7
XRD patterns of a 28-day-cured AAFA: (a) FA-100RG, (b) FA-75RG, (c) FA-50RG, (d) FA-25RG, and (e) FA-0RG (Q = quartz (SiO2)).Figure8 shows the relationship between normalized flexural strength and curing time of AAFA mortar containing RG based on consideing the 28-day flexural strength equation (2) illustrates an empirical relationship between the flexural strength of mortars containing RG and the curing time at various L/B ratios, with a high R2 value of 0.752. The constants in equation (2) were determined by curve-fitting the data for RG replacement levels of 0%, 25%, 50%, 75%, and 100%:(2)ft,dft,28=0.57720lnd−0.73670,where ft,d is the flexural strength at d days of curing time and ft,28 is the flexural strength at 28 days of curing time.Figure 8
Relationship between normalized flexural strength and curing time of AAFA mortar containing RG by the 28-day flexural strength.


## 3.3. Life-Cycle Assessment

The life-cycle assessment (LCA) of AAFA mortars incorporating SG was evaluated. According to previous publications [27, 47], the measurement unit of emission in the LCA analysis is CO2-e emitted (t CO2-e/m3). Table 3 shows the emission factors obtained from the literature [13, 48–50]. However, the emission factors derived from various studies, especially those conducted in different regions or countries, may vary significantly. The further use of results should be done with care. According to Table 3, FA emitted the least CO2, while SS solution emitted the highest values. The LCA analyses for AAFA mortars incorporating SG and Portland cement mortar (PCM) are shown in Tables 4 and 5. It is revealed when the RG replacement level increased, the CO2-e emitted values tended to marginal increase. According to Tables 4 and 5, AAFA mortars emitted lower CO2 than PCMs. For example, the CO2-e emitted values of FA-25RG and PCM were 0.28161 and 0.5053 ton CO2-e/ton, respectively, for L/B ratios of 0.60 and w/c ratios of 0.60. In addition, all AAFA mortars generated lower CO2 than Portland cement concrete (PCC). Note that the 28-day compressive strengths of PCMs were 46.2 and 37.1 MPa for w/c ratios of 0.60 and 0.70, respectively. While the 28-day compressive strength of PCC was 35.5 MPa. As shown in Figure 3, the strengths of AAFA mortars were slightly lower than those of PCM and PCC. As shown in Tables 4 and 5, the FA content of the AAFA mortar is around 800 kg/m3, while the PC content of the PCM mortar is approximately 600 kg/m3. However, PC is approximately 2 times more expensive than FA, according to Thailand market prices (2022) and Phoo-Ngernkham et al. [27]. Therefore, the relatively large amount of FA used in the production of AAFA has no significant effect on the total price of AAFA. According to the basis of empirical evidence, it is recommended that 25% RG be used in replacement of RS as fine aggregate in order to have a strength index greater than 75% and a low carbon footprint. For further studies, it is recommended to apply the findings in this study to construction practice [51, 52] and also compare them to other materials [53, 54].Table 3
The emission factors of tested materials [13, 48–50].
Materials (kg)Emission factor (t CO2-e/m3)Portland cement, PC0.8200High-calcium fly ash, FA0.0070River sand, RS0.0139Recycled glass, RG0.0200Coase aggregate, CA0.4300Sodium hydroxide, SH0.7000Sodium silicate, SS1.5140Table 4
LCA analysis of AAFA mortar incorporating RG from this article.
L/B ratiosMix symbolFA (kg)CO2-e/m3from FARS (kg)CO2-e/m3from RSRG (kg)CO2-e/m3from RGSH (kg)CO2-e/m3from SHSS (kg)CO2-e/m3from SSTotal CO2-e/m30.60FA-0RG862.00.0060862.00.0120—0.0000172.40.0362344.80.22450.27869FA-25RG859.00.0060644.30.0090343.60.0069171.80.0361343.60.22370.28161FA-50RG856.00.0060428.00.0059428.00.0086171.20.0360342.40.22290.27936FA-75RG853.00.0060213.30.0030639.80.0128170.60.0358341.20.22210.27969FA-100RG850.00.0060——850.00.0170170.00.0357340.00.22130.280000.70FA-0RG815.00.0057815.00.0113—0.0000190.20.0399380.30.24760.30456FA-25RG813.00.0057609.80.0085203.30.0041189.70.0398379.40.24700.30507FA-50RG810.00.0057405.00.0056405.00.0081189.00.0397378.00.24610.30518FA-75RG807.00.0056201.80.0028605.30.0121188.30.0395376.60.24520.30528FA-100RG804.00.0056——804.00.0161187.60.0394375.20.24430.30537Table 5
LCA analysis of PCC and PCM.
Mix symbolPC (kg)CO2-e/m3 from PCRS (kg)CO2-e/m3 from RSCA (kg)CO2-e/m3 from CATotal CO2-e/m3PCC495.00.4059510.00.0071938.00.40330.8163PCM (w/c = 0.60)596.00.48871192.00.0166——0.5053PCM (w/c = 0.70)562.00.46081123.00.0156——0.4765


## 4. Conclusion

This article investigated the compressive and flexural strengths of FA-based alkali-activated mortar mixed with RG. Here, RG was used to partially replace fine aggregate in the amounts of 0%, 25%, 50%, 75%, and 100% by weight. The results showed that the compressive and flexural strengths decreased with the increased amount of RG replacement. The reduction in strength development occurred because of the weaker interfacial transition zone of RG in mortars. Additionally, high Na2O content resulted in greater drying shrinkage of the alkali-activated binder, which might result in microcracks and subsequent strength loss. The increasing Na2O concentration could induce the alkali-silica reaction (ASR) in the alkali-activated binder system because all AAFA mortars consist mostly of quartz crystalline phases (SiO2). The use of 25% RG in AAFA mortar at a L/B ratio of 0.60 is a feasible choice for sustainable concrete because the 28-day compressive and flexural strengths of AAFA incorporating 25% RG were 32.5 and 1.08 MPa for L/B ratios of 0.60, while those of AAFA without RG were 35.8 and 1.59 MPa. Hence, it provides a strength index greater than 75% while emitting little CO2.



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*Source: 9418619-2023-02-20.xml*