Characterization of Geopolymer Masonry Mortars Incorporating Recycled Fine Aggregates

Abstract

This study evaluates the characteristics of geopolymer masonry mortars (GMMs) made with slag–fly ash binder and up to 100% recycled fine aggregates (RFAs). For each RFA replacement rate, two types of GMMs, namely N and S types based on ASTM C91, were proportioned and tested for mechanical, physical, and durability properties. Results revealed that using geopolymeric binder enhanced the flow, water retention, compressive strength, sorptivity, and abrasion resistance of GMMs compared to cementitious counterparts but reduced the initial setting time by up to 75%. Subsequent RFA additions negatively affected the flow, setting time, density, water absorption, porosity, and bulk resistivity but enhanced the water retention, sorptivity, and abrasion resistance of GMM. It also reduced the compressive, pull-off, and flexural strengths by 36, 44, and 27%, respectively. Furthermore, S-type mortars exhibited improved bulk resistivity, sorptivity, and abrasion resistance compared to N-type counterparts. A multifunctional performance index deduced that the GMM mixes incorporating 100% RFAs were superior to geopolymeric or cementitious masonry mortars made with natural fine aggregates (NFAs). Such findings emphasize the sustainability of GMMs made with RFAs in masonry construction, eliminating the need for water curing while maintaining comparable or even superior properties compared to cement-based mortars made with NFAs.

1. Introduction

The ongoing construction, renovation, and rehabilitation activities to meet the demands of the increasing population entailed the significant consumption of construction materials, namely cement and aggregates. In 2022, China was ranked as the world’s largest cement producer, with a production of 2.1 billion metric tons. This production share is equivalent to half of the world’s cement. India and Vietnam come in second and third places with cement production of 370 and 120 million metric tons, respectively [1]. Such a substantial production of cement led to substantial energy consumption, the depletion of non-renewable natural resources, and a significant surge of greenhouse gas emissions [2]. In fact, cement production is accountable for 5–7% of the world’s total carbon dioxide emissions, leading to an uprise in global warming and its severe natural repercussions [3].

The second construction material, natural aggregates, has been excessively consumed, as 60–75% of the weight of a standard concrete mixture comprises aggregates [4,5]. The quarrying and transportation processes of natural aggregates utilize energy and natural capital, contributing to environmental degradation. Moreover, the construction industry generates massive amounts of construction and demolition waste (CDW) that ends up in landfills, inducing economic leakages [6]. In 2018, 600 million tons of CDW were generated by the United States, which was estimated to be more than twice the generated municipal solid wastes [7]. In addition, landfilling the CDW was found to be a less favorable waste management option due to its negative economic, environmental, and public health impacts. For instance, disposing of CDW in landfills may cause serious health problems; negative effects on the fauna and bird migration, soil, air, and groundwater contamination; and a reduction in nearby property prices [8]. This underscores the urgent need for researchers, environmental activists, and governments to actively seek sustainable alternatives for both cement and natural aggregates.

Geopolymers have shown a potentially promising substitute for Ordinary Portland Cement in the construction industry. They are formed through the activation of an aluminosilicate source by an alkaline activator solution. In comparison with cement, recycling industrial by-products, such as fly ash and ground granulated blast furnace slag (referred to hereafter as slag), and using them in the formation of geopolymers resulted in an 80% reduction in carbon dioxide emissions [9,10]. In addition, geopolymer composites demonstrated enhanced mechanical and durability properties compared to traditional cement-based composites [11,12,13,14]. As such, several experimental studies looked into using geopolymers as a substitute for cement in masonry mortar, i.e., GMM.

Masonry mortars are typically developed using masonry cement. ASTM C91 [15] encompasses three types of masonry cement for the use in masonry mortar, namely N, S, and M depending on the 28-day compressive strength (f_c) ranges of 6.2–14.5 MPa, 14.5–20.0 MPa, and 20.0 MPa and over, respectively. Komljenovic et al. [16] reported that utilizing sodium silicate as an alkaline activator solution in fly ash-based GMM resulted in the greatest activation potential among the different alkali activators. According to Oz et al. [17], the bulk density, water absorption, and porosity of fly ash-based GMM were improved with the increase of liquid-to-solid (L/S) and sodium silicate-to-sodium hydroxide (SS/SH) ratios. Therefore, maximum compressive and flexural strengths of 60.1 and 9.9 MPa, respectively, were obtained using an L/S ratio of 0.2, an SS/SH ratio of 2, and a curing temperature of 100 °C. Likewise, several studies [18,19] recorded lower water absorption and porosity as well as greater bulk density and compressive strength at higher curing temperatures and periods. Ranjbar et al. [20] noticed that the crumbled shape of palm oil fuel ash (POFA) increased the water demand to maintain a workable mix, leading to porous and lower-density fly ash-POFA blended GMM. Moreover, adding POFA increased the SiO₂/Al₂O₃ ratio, delaying the ultimate compressive strength whilst improving the elastic deformation behavior. In another study conducted by Shang et al. [21], increasing the slag content of slag–fly ash-blended GMM resulted in faster setting time, higher compressive strength, and lower cost compared to conventional cement-based or magnesium potassium phosphate cement masonry mortar. Sathonsaowaphak et al. [22] showed that increasing the fineness, alkaline activator solution-to-ash ratio, SS/SH ratio, and SH concentration improved the compressive strength of lignite bottom ash GMM. Furthermore, Kwasny et al. [23] used low-purity kaolin called lithomarge and slag as an aluminosilicate precursor activated using a potassium silicate solution. Their results revealed that GMM exhibited faster setting times and higher compressive strength development, where they approached 55–66% of the 28-day compressive strength in the first 24 h corresponding to only 18–28% of the 28-day compressive strength for the cement-based mortars. Furthermore, increasing the L/S ratio enhanced the workability and setting time of GMM but decreased the compressive strength at different ages.

At the same time, several studies examined the use of recycled fine aggregates (RFAs) to replace natural fine aggregates (NFAs) in masonry mortar production. Such replacements had a leading role in reducing pollutants’ emissions, mitigating the consumption of NFAs, and promoting the development of sustainable masonry. Indeed, Jiménez et al. [24] demonstrated that replacing natural sand with RFAs sourced from ceramic wastes up to 40% had insignificant effects on the fresh and hardened properties of masonry mortars except for the density and workability. Meanwhile, Silva et al. [25] claimed that mortars made with 20% and 50% recycled red-clay ceramic wastes had 12% and 13% higher compressive strengths as well as 16% and 9% lower water permeability coefficients under capillary action, respectively, compared to reference mortars made with NFAs. Martínez et al. [26] reported that mortars made with 100% ceramic-based RFAs achieved the minimum required water retention and compressive strength and even higher flexural and bond strengths and electrical resistivity compared to mortars made with NFAs. However, they exhibited higher absorption capacity, porosity, sorptivity, and shrinkage due to the higher water absorption of the aggregates themselves. In another attempt to improve the performance of masonry mortars made with RFAs, Mora-Ortiz et al. [27] emphasized that prewetting RFAs enhanced the mechanical properties of masonry mortars, due to the lower w/c ratio required for normal consistency.

The combined effect of utilizing geopolymers and RFAs in masonry mortars has been seldom investigated. Saba and Assaad [28] examined three classes of metakaolin-based GMM made with RFAs up to 60%. According to their results, GMM exhibited higher sorptivity and water retentivity compared to counterpart cement-based mortars, while utilizing 20% RFAs slightly improved the bond strength. Conversely, De Rossi et al. [29] noticed better mechanical properties of metakaolin-fly ash-blended GMM made with RFAs of wide particle range, due to the higher packing density and the absence of microcracks in the wider size range. Gholampour et al. [30] reported comparable compressive strength, lower flowability, and higher water absorption and drying shrinkage of slag–fly ash-blended GMM made with lead smelter slag and glass sand, compared to GMM made with natural sand. Deng and Zheng [31] specified that the optimum preparation process of lead–zinc tailings–slag-blended GMM made with manufactured sand in terms of calcination temperature, alkali consumption, dwell time, and slag content was at 568 °C, 20%, 83 min, and 60%, respectively. Furthermore, Islam et al. [32] concluded that the optimum slag and POFA percentages for the maximum 7-day compressive strength of GMM made with manufactured sand were 70% and 30%, respectively.

Summarizing the literature, the combined effect of slag–fly ash geopolymer binder and RFAs in the different masonry mortar classes (based on ASTM C91 [15]) has not been investigated yet. Accordingly, this research aims to fill this knowledge gap by evaluating the synergic effect of geopolymer binder, RFAs, and masonry mortar type on the fresh, mechanical, physical, and durability properties of masonry mortars. A blend of slag and fly ash was used as the precursor binder, while NFAs was substituted with RFAs sourced from CDW up to 100%. The mixture proportions of GMM mixes were continuously modified to attain types N and S (ASTM C91 [15]) for each RFA replacement rate. The fresh properties were characterized by flow, air content, water retention, and setting time. The mechanical properties included compressive, pull-off, and flexural strengths, while the physical and durability properties constituted the bulk density, water absorption, porosity, bulk electrical resistivity, sorptivity, and abrasion resistance. Furthermore, the environmental and economic impacts of the developed GMM mixes were assessed and integrated into the multifunctional performance indexing process to determine the optimal mortar mixes for various applications. This work offers new knowledge and provides evidence supporting the feasibility of incorporating sustainable building materials into masonry construction.

2. Materials and Methods

2.1. Materials

For the GMM mixes, a combination of slag and class F fly ash (FA) was used as the binding material. Ordinary Portland Cement (ASTM Type I [33]) served as the binder for control-cement masonry mortar mixes.

Table 1. Chemical composition of cement, slag, and FA.

Oxide	Cement	Slag	FA
CaO, %	66.3	59.7	8.3
SiO2, %	21.5	27.0	52.7
Al2O3, %	4.0	7.5	18.6
SO3, %	2.7	2.5	1.1
Fe2O3, %	2.7	1.2	14.3
Loss on ignition, %	0.9	1.1	1.1
Others, %	1.9	1.0	3.9

presents the chemical composition using X-ray fluorescence (XRF) of the as-received materials. Cement and slag were primarily made of calcium oxide (CaO) and silica (SiO₂), while fly ash consisted of SiO₂ and alumina (Al₂O₃). The specific gravity of cement, slag, and fly ash was recorded as 3.16, 2.50, and 2.32, respectively. The particle size distribution of the binding materials is shown in Figure 1. The particle size of cement, slag, and fly ash ranged between 0.0036–0.3 µm, 3.6–84.9 µm, and 0.2–43.7 µm, respectively.

NFAss consisted of crushed dolomitic limestone sand with a nominal maximum size (NMS) of 2.36 mm. In contrast, RFA were acquired from a CDW local recycling facility and were graded to attain a particle size distribution similar to that of NFAs, as illustrated in Figure 2. Moreover, the physical properties of the NFAs and RFAs are summarized in

Table 2. Physical properties of NFAs and RFAs.

Property	ASTM Test	NFAs	RFAs
Dry-Rodded Density (kg/m3)	C29 [34]	1898	1510
Absorption (%)	C128 [35]	0.22	6.63
Median particle size d50 (µm)	C136 [36]	880	745
Surface Area (cm2/g)	C136 [36]	66.8	113.5
Fineness Modulus	C136 [36]	2.7	2.5
Specific Gravity	C128 [35]	2.69	2.63

. In comparison to NFAs, RFAs had lower density, median particle size, and specific gravity, but higher water absorption and surface area. Therefore, RFAs were prewetted to achieve a saturated surface dry (SSD) state before being introduced into the mixture. Comparable fineness modulus values were registered by NFAs and RFAs, owing to their similar gradation curves.

The alkaline activator solution (AAS) utilized in this study consisted of a blend of Grade N sodium silicate and sodium hydroxide solutions. SS solution was comprised of 63.4% H₂O, 26.3% SiO₂, and 10.3% Na₂O. On the other hand, SH solution was formulated in the laboratory by dissolving 98% pure sodium hydroxide flakes in a specified quantity of tap water to achieve a targeted molarity of 8 M, in accordance with recommendations of prior research [37,38,39].

Sodium dodecyl benzene sulphonate-based air-entraining agent (AEA) was used during air content testing of masonry mortar mixes to supply the minimum air content required as per the ASTM C91 standard for masonry cement [15]. The AEA works on reducing the surface tension among water molecules, thereby stabilizing the air bubbles formed during mixing and vibration processes. The corresponding specific gravity and pH of the AEA were 1.01 and 8.5.

2.2. Mixture Proportioning

A total of 12 mortar mixes were cast and examined using standardized test methods.

Table 3. Mix design of masonry mortars.

Mix No.	Mix Designation	Binder-to-Aggregates	FA-to-Slag	AAS-to-Binder	SS/SH	Water-to-Binder
1	C-N-RFA0	1:3	-	-	-	0.60
2	C-S-RFA0	1:2.75	-	-	-	0.60
3	G-N-RFA0	1:3	5:1	0.5	1.5	0.20
4	G-S-RFA0	1:3	4:1	0.5	1.5	0.20
5	G-N-RFA25	1:3	4:1	0.6	1.5	0.15
6	G-S-RFA25	1:3	3:1	0.6	1.5	0.15
7	G-N-RFA50	1:3	3:1	0.6	1.5	0.10
8	G-S-RFA50	1:3	3:1	0.7	1.5	0.00
9	G-N-RFA75	1:3	3:1	0.7	1.5	0.00
10	G-S-RFA75	1:2	3:1	0.7	1.5	0.00
11	G-N-RFA100	1:3	3:1	0.7	1.5	0.00
12	G-S-RFA100	1:2	3:1	0.7	1.5	0.00

shows the mixture proportions of cement and geopolymer masonry mortars. Mixes were designated as X-Y-RFAZ, where X denotes the type of binder (i.e., C for cement and G for geopolymer), Y indicates the mortar type (N or S), and Z illustrates the RFA replacement percentage by mass. For example, C-S-RFA0 was the S-type cement-based control masonry mortar mix made with NFAs, whereas G-N-RFA25 represents an N-type GMM containing 25% RFAs.

Control cement mixes were designed with binder-to-aggregates ratios of 1:2.75 and 1:3 and a water-to-cement (w/c) ratio of 0.6. GMM mixes were formulated using binder-to-aggregates ratios of either 1:2 or 1:3. The binder was a combination of slag and fly ash mixed at FA/slag ratios ranging from 3:1–5:1. The AAS was made of sodium silicate and sodium hydroxide at an SS/SH ratio of 1.5. The AAS-to-binder ratios varied between 0.5–0.7. Tap water was added to some mixes to achieve a minimal flow of 15.0 ± 0.2 cm. NFAs were replaced with RFAs at replacement rates of 0, 25, 50, 75, and 100%.

For each RFA replacement rate, the mix design was modified to achieve the requirements of the two types of mortars (N and S) based on ASTM C91 [15]. For instance, decreasing the FA/slag ratio from 5:1 to 4:1 and from 4:1 to 3:1 was required to attain an S-type mortar for the GMM mixes made with 0 and 25% RFAs, respectively. In this case, the RFA replacement caused a reduction in strength, which was countered by increasing the slag content. In fact, the higher calcium content, resulting from the inclusion of more slag in the mix, accelerated the geopolymerization reaction and the formation of an amorphously structured Ca-Al-Si gel and increased the strength [40]. At 50% RFAs, the AAS-to-binder ratio was increased to 0.7 to attain the S-type GMM mix, as recommended in previous work on geopolymer concrete [41]. At higher RFA replacement rates, i.e., 75 and 100%, more binder content (binder-to-aggregates ratio of 1:2) was necessary to meet the S-type strength requirements. Indeed, the higher binder content supplied sufficient paste volume for the reaction, leading to a higher compressive strength of fly ash-based geopolymer mortar [42].

2.3. Sample Preparation

Mixes were prepared and cast in the laboratory under ambient conditions, where the temperature and relative humidity were approximately 23 ± 2 °C and 50 ± 5%, respectively. Figure 3 presents a sample of the demolded masonry mortar specimens. For the GMM mixes, the alkaline activator solution was formulated by dissolving SH flakes in water to form an 8 M solution and then adding SS at an SS/SH ratio of 1.5. The AAS was prepared one day prior to mixing, allowing time for the dissipation of heat from the exothermic reactions between SH flakes and water, as well as between the SH solution and the SS solution. Firstly, the dry ingredients, including binders and aggregates, were mixed for 2–3 min. Subsequently, the AAS and additional water (or mixing water in cement mixes) were added and mixed for another 2 min. After determining the flow, the fresh mortars were cast into 50 mm cubes, 150 mm cubes, 100 mm × 200 mm (diameter × height) cylinders, and 160 mm × 40 mm × 40 mm (length × width × height) and covered for 24 h with a plastic sheet at ambient conditions. Subsequently, the samples were demolded and left in the same conditions to cure until testing. For the cement-based control masonry mortar samples, the cement and aggregates were first mixed, after which the mixing water was added in two rounds, 10 s apart. The mixture was blended for 3 min to ensure a homogeneous mix, compact-vibrated for 10 s on a vibrating table, covered, and stored in ambient conditions for 24 h. The samples were then demolded and placed in water tanks for seven days, as per local industry practice.

2.4. Test Methods

The flow of masonry mortar mixes was measured using a conical mold and a flow table in accordance with ASTM C1437 [43]. The air content of freshly mixed mortars was determined using an air meter consisting of a 0.75 L vessel and cover assembly, as per the procedure of EN 1015-7 [44]. Furthermore, the water retention evaluates the ability of masonry mortar to retain water under controlled vacuum suction for a period of 60 s. It was obtained from the ratio of the flow after suction to the initial flow based on the procedure outlined in ASTM C1506 [45]. The setting time by penetration resistance was assessed using 150 mm cubes based on ASTM C403 [46], where the times required for the mortar to reach 500 psi (3.5 MPa) and 4000 psi (27.6 MPa) were used to define the initial and final setting times, respectively.

The compressive strength (f_c) of masonry mortars was evaluated using 50 mm cubes at 1, 7, and 28 days of age according to ASTM C109 [47]. The 28-day pull-off strength (f_p) was measured using the PosiTest AT m and a 50 mm loading fixture (dolly) in accordance with ASTM D7234 [48]. A 5 mm thick layer of fresh mortar was cast over a mechanically hatched 100 × 100 × 50 mm concrete substrate having a 28-day compressive strength of 66 MPa. After the fresh mortar had hardened, it was scored down to the concrete substrate surface, and the dolly was placed over the scored section using a two-component epoxy. The pull-off test was performed by attaching the testing apparatus to the dolly and aligning it so it applies a tensile force normal to the surface. The 28-day flexural strength (f_r) of 160 × 40 × 40 mm (length × width × height) masonry mortar prisms was assessed using the center-point bending test as per ASTM C348 [49]. For each test, triplicate samples were tested per mix to obtain an average.

The 28-day bulk density, water absorption, and porosity of masonry mortars were determined using three cubic specimens of 50 mm size, as per the procedure outlined in ASTM C642 [50]. The bulk electrical resistivity of 28-day cylindrical specimens of 100 mm diameter and 200 mm height was determined as per the specifications of ASTM C1876 [51]. Triplicate samples were tested per mix to record the average. The rate of absorption or sorptivity (I) was determined in accordance with ASTM C1585 [52] by measuring the mass increase of masonry mortar disc samples of 100 mm diameter and 50 mm height over time when one of its surfaces is exposed to water. It is noteworthy that these disc specimens were obtained by cutting the cylindrical specimens using a diamond saw blade. The sorptivity was computed from the slope of the best-fit line to I against the square root of the time curve. For each mix, two samples were tested to obtain an average. The abrasion resistance test was conducted on 28-day disc samples, similar to those used in the sorptivity test, using a drill press device capable of rotating the abrading cutter at a speed of 200 rpm and applying a force of 98 ± 1 N in accordance with ASTM C944 [53]. The test schedule involved three 2 min abrasion periods for each specimen, and the abrasion resistance was computed as the percent mass loss of the samples. Three samples per mix were tested to report the average. Figure 4 portrays the different tests conducted in this work.

3. Results and Discussion

3.1. Fresh Properties

The fresh properties of GMM made with RFA mixes were assessed in terms of flow, air content, water retention, and setting time.

Table 4. Fresh properties of masonry mortar mixes.

Mix ID	Flow (cm)	Air Content (%)	Water Retention (%)	Setting Time (min)
				Initial	Final
C-N-RFA0	13.7	10.5	83	195	700
C-S-RFA0	18.2	8.5	80	290	799
G-N-RFA0	19.5	15.0	89	48	171
G-S-RFA0	20.1	7.3	89	39	168
G-N-RFA25	18.5	13.8	92	43	175
G-S-RFA25	16.8	8.5	90	34	134
G-N-RFA50	18.3	9.2	96	24	86
G-S-RFA50	17.6	8.8	96	25	176
G-N-RFA75	15.7	10.3	98	36	149
G-S-RFA75	19.5	7.8	98	39	226
G-N-RFA100	15.7	11.5	98	21	127
G-S-RFA100	19.0	6.5	98	27	189

illustrates the fresh properties results of masonry mortar mixes.

3.1.1. Flow

As highlighted in Table 4, geopolymer masonry mortar mixes exhibited flow values ranging between 15.7 and 20.1 cm. Meanwhile, the cement-based control masonry mortar recorded an initial flow of 13.7 and 18.2 cm for types N and S, respectively. Generally, the flow varied when transitioning from N to S-type mortar for each mix based on the respective changes in the mixtures’ proportions. The increase in flow when moving from N to S-type mortar for mixes C-N-RFA0, G-N-RFA75, and G-N-RFA100 is due to the higher binder contents supplied in S-type mortars, reducing the shear friction between the aggregates and increasing the flowability [54]. Moreover, it seems that the higher slag content supplied in G-S-RFA0 (FA/slag ratio of 4:1 instead of 5:1 in G-N-RFA0) did not cause a tangible variation in the flowability (~3%). This can be attributed to the relatively higher fly ash content in G-S-RFA0, which mitigates the negative impact of slag on the flow. In this context, other work [40,55,56,57] reported higher flowability for geopolymer concrete made with higher fly ash contents due to the enhanced mobility of spherical-shaped fly ash compared to irregularly shaped slag particles. In contrast, a 9% reduction in the flow was observed in mix G-S-RFA25 (having a FA/slag ratio of 3:1) compared to mix G-N-RFA25 (having a FA/slag ratio of 4:1). Apparently, the use of 25% RFA in G-S-RFA25 compared to 0% RFA in G-S-RFA0 and more slag content in the latter seemed to have led to a compounded reduction in the flow. Although both G-N-RFA50 and G-S-RFA50 mixes had a liquid (i.e., AAS and water)-to-binder ratio of 0.7, using a higher AAS-to-binder ratio (0.7 instead of 0.6) in the G-S-RFA50 mix decreased the flow by 4%. This can be mainly attributed to the increased sodium silicate content and lower water-to-binder ratio, leading to a higher viscosity and reduced workability of the mix [58].

Compared to the control C-N-RFA0 mix, utilizing a geopolymeric binder (G-N-RFA0) in making an N-type GMM improved the workability by 42%. Meanwhile, the flow increased from 18.2 to 20.1 cm, representing a 10% increase, when geopolymers were used as a substitute for cement to produce an S-type mortar. The larger increase in the flowability shown in the G-N-RFA0 mix is mainly due to the presence of high fly ash content that reacts at a slower rate with the alkaline activator than the slag. The higher slag content in the G-S-RFA0 mix resulted in a less prominent increase in flowability, as slag reacts with the alkaline activator faster than fly ash [56,57]. This is in agreement with previous studies on slag–fly ash-blended geopolymer concrete [40,55].

The N-type GMM mixes incorporating RFAs at 25, 50, 75, and 100% registered flow values of 18.5, 18.3, 15.7, and 15.7 cm, respectively. Similarly, S-type GMM mixes made with RFAs maintained a flow of 18.0 ± 2.0 cm. The RFA inclusion caused a drop in the flow, yet it was not as intense as that reported in previous research findings [24,30]. This is primarily owed to the continuous modification of the design mix to sustain comparable flowability, which diminished the negative effect of RFAs on the flow. Nevertheless, the G-N-RFA75 and G-N-RFA100 mixes exhibited the lowest workability, with a flow value of 15.7 cm, owing to the higher specific surface area and porous nature of RFAs compared to NFAs. In this context, Tiwari et al. [59] claimed that the enhanced flowability of mixes containing NFAs is owed to the lower angularity and larger particle size of NFAs compared to RFAs, leading to lower interparticle friction and the creation of thicker water layers in NFA mixes.

3.1.2. Air Content

The air content of cement and geopolymer masonry mortars varied between 6.5 and 15.0%, owing to the incorporation of the air entraining agent (AEA) molecules into the mix. It is noteworthy that the majority of the mixes had air content within 8–19%, as per ASTM C91 [15] requirements for masonry cement, except for G-S-RFA0, G-S-RFA75, and G-S-RFA100, registering air content values of 7.3, 7.8, and 6.5%, respectively, which is marginally less than the standard and can still be accepted. In this context, preliminary trials batched without AEA showed air content values between 2.6% and 6.8%. On a similar note, Saba and Assaad [28] reported that the air content for metakaolin-based geopolymer masonry mortar made with RFAs ranged between 10–14% and 4–6.5% for mixes batched using AEA and AEA-free liquid solutions, respectively.

The results also revealed that the air content was influenced by the mortar category. The air content decreased when transitioning from N- to S-type mortar, owing to the denser mortar matrix created in the stronger mortar category. Similar results were noted elsewhere [60], where stronger concrete mixes had lower entrapped air contents. Furthermore, the entrained air results shown in Table 4 highlighted that the binder type (cementitious or geopolymeric) and the RFA replacement rate had limited effects on the air content of S-type mortar. This finding is consistent with a previous study [28], which showed comparable air contents of cement and metakaolin-based geopolymer masonry mortars made with RFAs. Vegas et al. [61] also noted no difference in the air content of mortars with 25% replacement of natural aggregates with RFAs. Jiménez et al. [24] reported no statistically significant difference between the average air content of masonry mortar manufactured using recycled aggregates from ceramic partition wall rubble at a 95.0% confidence level. Nevertheless, some fluctuations were reported with RFA replacement in N-type mortar but were within the ASTM C91 limits, and a 43% increase was noted upon changing the binder from cementitious to geopolymeric. This could be specifically related to the higher fly ash content in the G-N-RFA0 mix that reacts at a slower rate, leaving pores unfilled with the geopolymerization products in the fresh state. However, this increased air content did not impact the hardened density, water absorption, or porosity properties of the mixture, as will be shown in the following sections.

3.1.3. Water Retention

Water retention evaluates the mortar’s ability to retain water over time under the influence of masonry unit suction [62]. Mortars with very low water retention are susceptible to high evaporation rates, affecting the cement hydration and strength development [63]. As illustrated in Table 4, the water retention of cementitious and geopolymeric mortars mixes decreased from 83 to 80% and 92 to 90% when transitioning from N- to S-type mortars, respectively, owing to the increased water content in the mix. Similar results are reported in the literature [28], where water retention of M-type mortars improved by 16 and 19% upon using S- and N-type mortars, respectively.

The use of geopolymers as a replacement for cement in masonry mortars enhanced water retention. For instance, the G-S-RFA0 mix attained 89% water retention compared to 80% exhibited by the control cement-based mix (C-S-RFA0), indicating its potential to retain the alkaline activator solution under the influence of suction. Several studies linked such behavior to the viscosity of the alkaline activator solution relative to mixing water, which increases the adhesive and cohesive properties of geopolymer mortar [64].

Incorporating RFAs in GMM has further improved water retention. In fact, water retention reached 92, 96, 98, and 98% for N-type GMM made with 25, 50, 75, and 100% RFAs, respectively. A similar trend can be observed in the S-type GMM as well. Such results are in line with the literature [63,65], which are primarily due to the higher specific surface area and surface roughness of RFAs to absorb free mixing water. In another study, Silva et al. [66] attributed the enhancement in water retention to the hydrogen bonds occurring by the electrostatic attraction between the pores of RFAs and the mixing water.

ASTM C91 [15] specifies a minimum water retention of 70% with no upper limit, while ASTM C270 [67] recommends using high water retention mortars in summer or for masonry units with high suction levels. The results presented herein show that all GMM mixes satisfy the ASTM C91 [15] requirements. In this context, some studies reported reduced performance of masonry mortars possessing water retention larger than 95%. For example, mortars with excessive water retention exhibited lower interfacial bonding, limiting moisture absorption by the masonry unit [66]. However, the higher water retention exhibited by the GMM made with RFAs in the current study did not have a negative effect on the pull-off strength, as will be discussed later.

3.1.4. Setting Time

The initial and final setting times of GMM mixes are shown in Figure 5. Their respective values ranged from 21 to 48 min and 86 to 226 min. Conversely, the cement-based control mortar mixes reached initial and final setting times ranges of 195–290 and 700–799 min, respectively. Since the two properties follow a similar trend, the focus herein was on the initial setting time. The effect of mortar classification (N or S) on the setting time was evaluated against the mixture proportions. Increasing the slag content when GMM mixes made with 0 and 25% RFAs moved from N-type to S-type caused 19 and 21% acceleration in the initial setting times, respectively. This is owed to the faster rate of reaction of slag particles with the alkaline activator solution compared to fly ash counterparts [21,40,68]. Conversely, transitioning from N-type to S-type GMM mortar made with 75 and 100% RFAs required and increase in the binder content. Such adjustment to the mix design retarded the initial setting times of these two mixes by 8 and 29%, respectively, compared to the N-type counterparts. This delay in setting is ascribed to the availability of binder, which accelerates the geopolymerization reaction [69].

The use of geopolymeric binders as a substitute for cement resulted in faster setting times. For N- and S-type mortar mixes, replacing cement with geopolymers reduced the initial setting time by 75 and 87%, respectively. The shorter setting times of geopolymer mixes were primarily due to the presence of slag, a major source of calcium, that forms an amorphously structured calcium–aluminate–silicate hydrate (Ca-Al-Si-H) gel and promotes the reactivity of the geopolymer binder [21,40,68]. Furthermore, the effect of replacing NFAs with RFAs on the initial setting time of GMM mixes was investigated. Using RFAs in N-type GMM mixes at replacement rates of 25, 50, 75, and 100% reduced the initial setting time by 10, 50, 25, and 56%, respectively. Similar acceleration in setting time is noted for S-type GMM. Such a loss in performance is primarily owed to the higher angularity and specific surface area of the RFAs [70].

3.2. Mechanical Properties

The mechanical properties of GMM made with RFA mixes were evaluated in terms of compressive, pull-off, and flexural strengths.

Table 5. Mechanical properties of masonry mortar mixes.

Mix ID	Compressive Strength (MPa)			Pull-Off Strength (MPa)		Flexural Strength (MPa)
	1-Day	7-Day	28-Day	28-Day	Failure Mode	28-Day
C-N-RFA0	1.9 ± 0.1	9.1 ± 0.5	11.4 ± 0.8	0.5 ± 0.0	Cohesive	1.95 ± 0.13
C-S-RFA0	6.5 ± 0.4	20.6 ± 1.0	20.7 ± 1.7	0.8 ± 0.1	Adhesive	4.06 ± 0.34
G-N-RFA0	1.9 ± 0.1	9.6 ± 0.1	13.8 ± 0.4	0.9 ± 0.1	Cohesive	2.33 ± 0.01
G-S-RFA0	5.8 ± 0.2	14.5 ± 0.3	16.0 ± 0.6	1.1 ± 0.1	Cohesive	3.51 ± 0.09
G-N-RFA25	2.3 ± 0.2	11.8 ± 0.1	14.3 ± 0.3	0.7 ± 0.0	Cohesive	2.14 ± 0.01
G-S-RFA25	6.1 ± 0.3	14.8 ± 0.4	15.0 ± 0.7	0.8 ± 0.2	Adhesive	3.02 ± 0.25
G-N-RFA50	2.1 ± 0.2	10.8 ± 1.4	11.4 ± 0.2	0.6 ± 0.1	Cohesive	2.12 ± 0.06
G-S-RFA50	6.2 ± 0.2	13.9 ± 1.5	14.7 ± 0.6	0.8 ± 0.2	Adhesive	2.84 ± 0.12
G-N-RFA75	1.3 ± 0.1	9.0 ± 1.2	9.5 ± 0.9	0.6 ± 0.0	Cohesive	2.09 ± 0.15
G-S-RFA75	2.7 ± 0.1	13.4 ± 0.8	14.7 ± 0.9	0.7 ± 0.1	Adhesive	2.49 ± 0.34
G-N-RFA100	1.1 ± 0.1	8.2 ± 0.7	8.8 ± 0.1	0.5 ± 0.1	Cohesive	1.69 ± 0.11
G-S-RFA100	2.3 ± 0.1	13.0 ± 0.5	14.5 ± 0.0	0.7 ± 0.0	Adhesive	2.20 ± 0.26

summarizes the mechanical properties results of masonry mortar mixes.

3.2.1. Compressive Strength

The compressive strength (f_c) of slag–fly ash-blended GMM made with RFAs was evaluated at 1, 7, and 28 days, as shown in Figure 6. ASTM C91 [15] specified the minimum compressive strength required for each mortar type at the age of 1, 7, and 28 days. Yet, the latter age (i.e., 28 days) was further analyzed as the behavior at the different ages was similar. Each mix was classified as N or S based on the minimum average 28-day compressive strength of 6.2 and 14.5 MPa, respectively, in accordance with ASTM C91 [15].

The 28-day f_c varied from 8.8 to 14.3 MPa and from 14.5 to 20.7 MPa for N- and S-type GMM mixes, respectively. The utilization of geopolymers as a substitute for cement in the C-N-RFA0 mix improved the 28-day compressive strength by 21%, owing to the geopolymerization reaction. Conversely, the G-S-RFA0 mix attained a strength of 16.0 MPa, compared to 20.7 MPa for the counterpart cement mix (C-S-RFA0). The higher strength of the control cement-based mix may be attributed to the effect of the water curing regime in enhancing the strength compared to the ambient curing regime adopted for the geopolymer mixes [71,72,73,74]. Still, both mixes suffice to be used in applications where an S-type masonry mortar is needed.

The effect of RFA inclusion on the 28-day compressive strength of GMM was investigated. The compressive strength of N-type GMM significantly dropped when RFA replacement exceeded 50%. In fact, the compressive strength varied from 13.8 MPa for G-N-RFA0 to 11.4, 9.5, and 8.8 MPa with the incorporation of 50, 75, and 100% RFAs, respectively. Such strength reduction can be related to the weak physical properties of RFAs [75,76]. Furthermore, the use of RFAs demanded more liquid, which increased the porosity of the mix and weakened the interfacial zone between the binder and RFAs [28]. The slight strength improvement depicted in the GMM made with 25% RFAs (G-N-RFA25) may be attributed to the improved interlocking between the geopolymeric matrix and the RFA particles or to the presence of non-hydrated cement on the RFAs that completed its hydraulic reaction, resulting in higher cohesion and strength [77]. As for S-type GMM, the effect of subsequent RFA additions was analogous to that observed in the N-type GMM mixes.

The strength development profile of GMM and cement-based mortars is shown in Figure 7. About 10–20% of the 28-day f_c of mortar mixes was achieved at 1-day for N-type GMM, while up to 40% of the 28-day f_c was reached within 1 day by S-type GMM, except for G-S-RFA75 and G-S-RFA100 mixes. The elevated strength attained at this early age is primarily owed to the formation of C-A-S-H and C-S-H gels during the geopolymerization process [39,78,79]. It is noteworthy that such superior early age strength of GMM mixes offers the advantage of expediting the construction process and simultaneously eliminating the need for water curing associated with cement-based masonry mortars [28]. The lowest 1-d f_c/28-d f_c ratios registered by N-type GMM made with 75 and 100% RFAs are attributable to the inherent weak physical properties of RFAs, hindering the geopolymerization reaction. Furthermore, the G-N-RFA0 mix registered a 1-d f_c/28-d f_c ratio as low as 0.14. This finding can be linked to the high fly ash content, leading to a slower geopolymerization process and the formation of Ca-Al-Si gel [40].

3.2.2. Pull-Off Strength

The pull-off strength of geopolymer and cement-based masonry mortars, presented in Figure 8, varied from 0.5 to 1.1 MPa and 0.5 to 0.8 MPa, respectively. Although the ASTM C91 [15] does not specify a minimum pull-off strength, the literature recommends a pull-off strength in the range of 0.25–1.20 MPa [61]. For all RFA replacement percentages, transitioning from an N-type to an S-type GMM improved the pull-off strength, mainly due to the corresponding higher compressive strengths. For instance, the pull-off strength increased from 0.6 to 0.8 MPa for N and S-type GMM made with 50% RFAs.

Utilizing geopolymers as a substitute for cement improved the pull-off strength. The results showed that the strength remarkably increased from 0.5 to 0.9 MPa (80%) when the cementitious binder was replaced by a geopolymeric one in mixes made without RFAs. Similarly, compared to the C-S-RFA0 mix, the pull-off improved by 38% upon using a geopolymeric binder (G-S-RFA0). This increase in strength is consistent with the previous literature [61,66] and can be related to the higher water retention exhibited by the geopolymer mixes, preventing rapid drying that could potentially affect the geopolymerization reaction and the adhesion properties of the mix.

The effect of RFA additions on the pull-off strength of GMM mixes was examined. In comparison with the G-N-RFA0 mix, RFA replacements of 25, 50, 75, and 100% reduced the pull-off strength by 22, 33, 33, and 44%, respectively. Likewise, incorporating RFAs in S-type GMM mixes led to a strength loss range of 27–36%. This drop in bond strength can be related to the weaker aggregate skeleton, thereby reducing the mechanical properties [66,76]. Additionally, the higher alkaline activator content caused by RFA replacement acted as a lubricant and caused a dilution effect [80]. In this context, it is worth noting that most masonry mortar samples displayed a cohesive mode of failure, i.e., a failure within the mortar itself. Such a failure pattern aligns with the lower bond strength values obtained, indicating that the mortar layer was the weakest part of the system at which the failure occurred [28]. For higher-bond strength (S-type) masonry mortars, an adhesive failure between the mortar system and the substrate was noticed, implying a weak bond between the mortar and the concrete substrate.

The experimental results revealed a relationship between the compressive (f_c) and pull-off (f_p) strengths of GMM made with RFAs. Figure 9 illustrates a scatter plot that relates the two properties. Equation (1) was developed to estimate f_p from f_c with good accuracy. Yet, the application of the developed model is limited to GMM made with RFA mixes designed in this investigation.

$$f_{\mathrm{p}}=0.19 \sqrt{f_{\mathrm{c}}}$$

(1)

3.2.3. Flexural Strength

The flexural strength of masonry mortar mixes ranged between 1.69 to 4.06 MPa. The literature has reported a flexural strength range of 2.25–4.26 MPa for N and S-type geopolymer mortars made with 60% RFAs [28]. Concurrent with f_p response, mortars classified as an S-type exhibited higher flexural strengths than N-type mortars. For example, the G-S-RFA0 mix attained 51% higher flexural strength compared to the counterpart N-type GMM mix (G-N-RFA0), owing to the higher compressive strength of the former compared to the latter.

In comparison with the C-N-RFA0 mix (N-type), utilizing a geopolymeric binder in the G-N-RFA0 mix improved the flexural strength by up to 19%. This result is in agreement with the literature [81] and can be directly attributed to the formation of C-A-S-H and N-A-S-H gels alongside the C-S-H gel during the geopolymerization reaction compared to only C-S-H gel being created in the conventional hydration reaction occurring in the cement-based mortar mix. This phenomenon was especially impactful in the weaker N-type mortar. Meanwhile, for the S-type mortars, the highest flexural strength (4.06 MPa) was achieved by the C-S-RFA0 mix. Such performance is mainly due to the 7-day water curing regime that ensured sufficient moisture for continuous hydration and strength development compared to the less water-consuming open air curing for geopolymer mixes [71,72,73,74].

As shown in Figure 8, subsequent RFA additions decreased the flexural strength of GMM mixes. For instance, the flexural strength decreased by 14, 19, 29, and 37% upon utilizing 25, 50, 75, and 100% RFAs in the G-S-RFA0 mix, respectively. Moreover, the lowest flexural strengths in N- and S-type mortars corresponded to mixes made with 100% RFAs and were 1.69 and 2.20 MPa. Such poor performance was due to the weak physical properties in terms of porosity and absorption of these fine aggregates [75,76]. Conversely, some studies reported a flexural strength increase in mixes made with RFAs, owing to the filler effect and the pozzolanic activity of the waste material [82,83]. Neno et al. [77] also linked this increase in the strength to the higher specific surface and porosity of recycled aggregates compared to sand, which results in a greater bond with the binder.

The flexural strength (f_r) was correlated with the compressive strength (f_c), as demonstrated in Figure 10a. As such, Equation (2) was developed to predict f_r from f_c with good accuracy. However, the developed analytical model is applicable to GMM made with RFA mixes in this study. The developed equation was compared to the codified equations in AS3600 [84], ACI 318-19 [85], ACI 363 [86], CEB-FIP [87], and IS-456 [88] used to predict the flexural strength. The comparison of experimental and predicted results showed that the model developed by ACI 363 and CEB-FIP overestimated flexural strength. Yet, the codified equations of AS3600, ACI 318-19, and IS-456 were able to closely predict the flexural strength, as their values converged to the 45°-line, as shown in Figure 10b. Accordingly, correction factors were developed to modify the codified equations so that they can be used for GMM made with RFA mixes in this study, as summarized in

Table 6. Equations relating flexural strength and compressive strength.

Reference	Flexural Strength	Correction Factor	Modified Equation
AS3600 [84]	0.6 fc0.5	1.08	0.65 fc0.5
ACI 318-19 [85]	0.62 fc0.5	1.05	0.65 fc0.5
ACI 363 [86]	0.94 fc0.5	0.69	0.65 fc0.5
CEB-FIP [87]	0.81 fc0.5	0.80	0.65 fc0.5
IS-456 [88]	0.626 fc0.5	1.04	0.65 fc0.5

. It is important to mention that these codified equations were originally developed for conventional cement-based concrete and were used in this context for comparison purposes.

$$f_{\mathrm{r}}=0.65 \sqrt{f_{\mathrm{c}}}$$

(2)

3.3. Physical and Durability Properties

This section presents the results of the physical and durability properties of GMM made with RFAs characterized by the bulk density, water absorption, volume of permeable voids, sorptivity, bulk electrical resistivity, and abrasion resistance.

Table 7. Physical and durability properties of masonry mortar mixes.

Mix ID	Dry Bulk Density (g/cm3)	Absorption (%)	Volume of Permeable Pore Space (%)	Bulk Electrical Resistivity (Ω∙m)	Sorptivity (mm/s0.5)	Abrasion Resistance (%)
C-N-RFA0	2.01 ± 0.01	13.1 ± 0.03	26.4 ± 0.1	28.9 ± 1.4	0.102	9.9
C-S-RFA0	2.08 ± 0.01	12.0 ± 0.27	24.2 ± 0.4	37.3 ± 0.7	0.026	2.1
G-N-RFA0	2.03 ± 0.01	11.8 ± 0.26	23.9 ± 0.5	6.5 ± 0.0	0.164	5.5
G-S-RFA0	2.02 ± 0.02	10.8 ± 0.20	22.4 ± 0.3	7.0 ± 0.0	0.138	3.1
G-N-RFA25	1.96 ± 0.01	12.0 ± 0.67	22.1 ± 1.2	5.5 ± 0.0	0.099	4.5
G-S-RFA25	1.96 ± 0.02	13.0 ± 0.10	23.9 ± 0.3	5.9 ± 0.2	0.096	2.2
G-N-RFA50	1.90 ± 0.01	12.9 ± 0.17	25.2 ± 0.2	5.0 ± 0.0	0.067	3.7
G-S-RFA50	1.95 ± 0.01	13.2 ± 0.17	25.3 ± 0.1	5.4 ± 0.2	0.066	2.4
G-N-RFA75	1.84 ± 0.00	13.8 ± 0.14	26.2 ± 0.2	4.9 ± 0.1	0.066	4.8
G-S-RFA75	1.81 ± 0.01	13.2 ± 0.26	25.8 ± 0.2	5.6 ± 0.1	0.067	3.0
G-N-RFA100	1.78 ± 0.01	16.7 ± 0.09	29.6 ± 0.1	4.3 ± 0.0	0.104	5.2
G-S-RFA100	1.74 ± 0.01	18.0 ± 0.09	31.3 ± 0.3	4.3 ± 0.2	0.089	2.3

summarizes the obtained experimental results.

3.3.1. Bulk Density, Absorption, and Volume of Permeable Voids

The values of dry bulk density, water absorption, and volume of permeable void space of masonry mortars varied between 1.74–2.08 g/cm³, 10.8–18.0%, and 22.1–31.3%, respectively. Generally, N- and S-type masonry mortars demonstrated comparable physical properties for each RFA replacement rate. Similarly, Yankwa Djobo et al. [19] reported a comparable bulk density range of 2.05–2.17 g/cm³ for volcanic ash-based geopolymer masonry mortar. Meanwhile, the lowest and highest water absorption observed by Öz et al. [17] for fly ash-based geopolymer masonry mortars were 10.07 and 17.98%, respectively. Görhan and Kürklü [89] noticed similar porosity values of 25–30% for fly ash geopolymer masonry mortar. This shows that the type of masonry mortar had a limited impact on the dry bulk density, water absorption, and permeable voids.

The effect of utilizing geopolymers as a substitute for cement on the physical properties was examined. According to the results, it seems that utilizing a geopolymeric binder did not cause a tangible variation in the bulk density (<3%). Moreover, geopolymer mixes exhibited lower absorption and porosity compared to control cement mixes. In fact, the G-N-RFA0 mix attained 10 and 9% lower absorption and volume of permeable voids compared to the C-N-RFA0 mix, respectively, as the pores were filled with the geopolymerization reaction products [17]. Similar findings were reported in another work [19], in which water absorption and apparent porosity were linked to the presence of open and closed pores determined by the extent of the geopolymerization.

The incorporation of RFAs in GMM resulted in lower bulk densities. In fact, using 25, 50, 75, and 100% RFAs reduced the bulk density of the G-N-RFA0 mix by 3, 6, 9, and 12%, respectively. Concurrent with the N-type mortar response, the bulk density of the S-type GMM decreased similarly with RFA replacement, with the lowest bulk density of 1.74 g/cm³ attained by the GMM incorporating 100% RFAs (G-S-RFA100). Such a reduction can be attributed to the agglomerated and crushed shape of RFAs, requiring additional liquid to maintain a workable mix. However, this free liquid did not take part in the reaction and evaporated, leaving more pores and reducing the density [20]. Other works [24,25] justified this reduction by the lower density of recycled aggregates compared to natural aggregates. Meanwhile, geopolymer mortars made with RFAs obtained higher absorption and porosity than those made with NFAs. Using 100% RFAs resulted in 42 and 67% higher absorption capacities as well as 24 and 40% higher permeable voids for N- and S-mortar types, respectively. The higher absorption capacity and porosity can be directly linked to the lower density noted earlier. Furthermore, the loss of excess mixing water due to evaporation increased the pore size, thereby increasing the water absorption of the mix [17,19].

According to the results, the three physical properties can be correlated, as illustrated in Figure 11. As such, a strong relationship was developed between the bulk density–water absorption and porosity–water absorption with correlation coefficients, R², of 0.72 and 0.94, respectively, which are given in Equations (3) and (4). These equations can be used to predict the hardened bulk density (ρ_h) and the permeable voids (PV) from the measured water absorption (WA). However, their application is limited to the GMM made with RFAs developed in the current study.

$${\rho }_{\mathrm{h}}=-0.03 \mathrm{WA}+2.36$$

(3)

$$\mathrm{PV}=1.38 \mathrm{WA}+6.76$$

(4)

3.3.2. Bulk Electrical Resistivity

The electrical resistivity of concrete measures its opposition to ion movement under an applied electric field [51]. Table 7 presents the bulk resistivity test results for the 28-day GMM made with RFAs. It can be observed that the bulk resistivity improved when transitioning from N-type to S-type mortar. For example, the bulk resistivity varied between 4.9 and 5.6 Ω∙m for G-N-RFA75 and G-S-RFA75 mixes, respectively. Such an increase is primarily related to the corresponding lower permeable voids and higher compressive strengths of S-type mortars. Indeed, an earlier study [90] reported a positive relationship between compressive strength and electrical resistivity of mortars prepared with three strength grades of cement.

The bulk resistivity results ranged between 4.3 and 7.0 Ω∙m for geopolymer mixes and were within the range of 28.9–37.3 Ω∙m for cementitious control mixes. The significant difference between the bulk resistivity of these two mortars is owed to the free ions in the geopolymeric pore structure, leading to a reduced resistivity [91]. Meanwhile, subsequent RFA replacements decreased the bulk electrical resistivity of geopolymer masonry mortars. In comparison with the G-N-RFA0 mix, the bulk electrical resistivity dropped by 15, 23, 25, and 34% upon utilizing RFAs at replacement rates of 25, 50, 75, and 100%, respectively. This reduction was also noted by Ameen and Al-Numan [92], where the electrical resistivity of reinforced concrete made with recycled coarse aggregates decreased with increasing RCA percentage and exposure time. Horsakulthai [93] reasoned that such a reduction would lead to the increased porosity and degree of interconnection caused by RFA incorporation, leading to a higher electric current flow.

As aforementioned, the ability of ion movement or the electrical resistivity is dependent upon the pore size and connectivity. In other words, more pores with lower interconnectivity lead to higher bulk electrical resistivity [94]. The results of electrical resistivity and permeable voids of GMM made with RFAs are plotted in Figure 12. Clearly, the electrical resistivity decreased with the increase in permeable voids. Therefore, a linear relationship can be fitted in Equation (5) with a good accuracy (R² = 0.74).

$$\rho =-0.18 \mathrm{PV}+9.76$$

(5)

3.3.3. Sorptivity

The rate of absorption or sorptivity is a durability measure that shows the ability of mortar to absorb and transport water by capillary action. It is defined in two stages: initial and secondary. The initial rate of absorption is dependent upon the capillary pores, whereas the secondary rate of absorption is influenced by the gel pores [95]. In this study, only the initial rate of absorption was considered, as it holds greater significance compared to the secondary rate of absorption.

The variations in water absorption as a function of the square root of time are plotted in Figure 13. Generally, the sorptivity remarkably increased during the first 60 min, followed by a more gradual increase until the end of the exposure period. It is clear that the sorptivity decreased from an N- to an S-type of mortar, which can be attributed to the increase of hydration/geopolymerization products, filling the capillary pores and limiting the interconnectivity inside the mortar matrix [96]. For instance, the sorptivity was reduced from 0.104 to 0.089 mm/s^0.5 for N and S-type GMM made with 100% RFAs, respectively. Although GMM had lower water absorption, they exhibited higher sorptivity than the control cement-based mortars. For instance, the G-N-RFA0 mix exhibited a 61% higher initial rate of absorption compared to the cement counterpart (C-N-RFA0). This rise in sorptivity is probably due to the higher connectivity of pores within the geopolymeric composite [97].

The effect of RFA replacement on the sorptivity of GMM followed a slightly different trend than that of water absorption. As illustrated in Figure 13, incorporating RFAs at replacement rates of 25, 50, 75, and 100% resulted in 40, 59, 60, and 37% lower sorptivity compared to the G-N-RFA0 mix made with NFAs. The decrease in sorptivity with the different RFA replacement rates is consistent with previous studies [24,25], and is primarily attributed to the filler effect of RFAs, enhancing the pore size distribution and preventing water percolation. In contrast, some studies [26,27,28] reported higher sorptivity of mixes made with RFAs due to the higher absorption capacity and porosity of recycled aggregates and because most voids were filled with RFAs.

3.3.4. Abrasion Resistance

Abrasion resistance reflects the relative wear resistance of masonry mortar. This property is usually influenced by the mortar’s mechanical properties and the aggregates’ toughness. Figure 14 denotes the mass loss profiles of N- and S-type cement-based masonry mortars and GMM made with RFAs every 2 min up to 6 min. Evidently, the mortars of S-type demonstrated better abrasion resistance, i.e., they exhibited lower mass loss at the end of the 6 min period, compared to N-type mortars. For example, the abrasion resistance decreased from 5.2 to 2.3% upon transitioning from N-type to S-type GMM made with 100% RFAs, respectively. This result can be directly related to the higher compressive strength of S-type mortars, providing higher resistance to surface abrasion.

The effect of utilizing a geopolymeric binder on the abrasion resistance was also examined. The results show that the mass loss of cement-based mortars was greater than that of geopolymer masonry mortars. For instance, replacing cement with geopolymer caused the abrasion mass loss to decrease from 9.9 to 5.5% for mixes made without RFAs. Concurrently, the G-N-RFA0 mix exhibited a 21% higher compressive strength compared to the C-N-RFA0 mix. Alternatively, Ramujee and Potharaju [98] linked such improvement in abrasion resistance to the density of the specimen, as the G-N-RFA0 mix was denser than the C-N-RFA0 mix. Furthermore, the effect of incorporating RFAs in GMM on abrasion resistance was investigated. The data highlight a limited impact of RFA replacement on the abrasion resistance, with values for N- and S-type GMMs hovering around 3.7–5.2% and 2.2–3.0%, respectively. This finding can be explained by the greater roughness and porosity of the RFAs, leading to better adhesion between the binder and the RFAs [99]. Conversely, other studies [100,101] reported lower abrasion resistance upon utilizing RFAs, owing to the increase of porosity that weakened the wear-resistance strength.

3.4. Environmental Impact Assessment

The environmental impact assessment entailed the evaluation of the global warming potential (GWP) associated with each mixture. To compute the associated GWP, measured in kg CO₂/m³, of each mix, the GWP value of each mix component (CO₂/kg) is multiplied by its corresponding mixture proportion (kg/m³), and the products are summed. The GWP values of the cement, slag, fly ash, NFAs, RFAs, SS, SH, and water were adopted from previous studies [8], and corresponded to 0.898, 0.042, 0.027, 0.005, 0.001, 0.424, 0.829, and 0.013 CO₂/kg.

Table 8. Contribution of the masonry mortar mixture components to GWP (kg CO₂/m³).

Mix Designation	Component								Total
	Cement	Slag	Fly Ash	NFAs	RFAs	SS	SH(s)	Water 1
C-N-RFA0	412.18	0.00	0.00	7.16	0.00	0.00	0.00	3.47	422.81
C-S-RFA0	431.94	0.00	0.00	6.87	0.00	0.00	0.00	3.63	442.44
G-N-RFA0	0.00	3.06	9.92	6.88	0.00	56.05	18.99	1.93	96.83
G-S-RFA0	0.00	3.67	9.53	6.88	0.00	56.10	18.99	1.93	97.09
G-N-RFA25	0.00	3.49	9.07	4.91	0.19	64.07	21.73	2.05	105.51
G-S-RFA25	0.00	4.37	8.51	4.92	0.19	64.11	21.73	2.05	105.87
G-N-RFA50	0.00	4.31	8.39	3.23	0.37	63.26	21.48	2.08	103.13
G-S-RFA50	0.00	4.36	8.49	3.27	0.38	74.67	25.29	1.73	118.20
G-N-RFA75	0.00	4.21	8.19	1.58	0.55	72.04	24.46	1.97	113.01
G-S-RFA75	0.00	5.22	10.15	1.30	0.45	89.29	30.27	2.07	138.76
G-N-RFA100	0.00	4.06	7.91	0.00	0.70	69.62	23.64	2.20	108.14
G-S-RFA100	0.00	5.07	9.87	0.00	0.58	86.75	29.44	2.25	133.96

¹ Includes the water needed to obtain the aggregates in SSD condition, the water to formulate the sodium hydroxide solution, and any other water added to the mix.

summarizes the GWP for each masonry mortar mix. Clearly, mixes made with cement were found to have the highest GWP. Indeed, the use of geopolymers as a binder substitute for cement in masonry mortars led to a significant reduction in the GWP. In fact, G-N-RFA0 and G-S-RFA0 mixes exhibited 77 and 78% lower GWP compared to counterpart cement-based masonry mortar mixes, respectively. Such a result reinforces the viability of geopolymer composites as an alternative to cement composites in the construction industry.

Meanwhile, subsequent replacement of NFAs with RFAs in GMM mixes led to gradual increases in GWP. Yet, they were below cement-based masonry mortar mixes. For instance, G-N-RFA50 and G-S-RFA50 mixes had 7 and 22% higher GWP compared to GMM counterparts made with NFAs, respectively. Such an increase was owed to higher solution content resulting from increasing the AAS-to-binder or binder-to-aggregates ratios in GMM mixes made with RFAs. Similar conclusions were noted for S-type GMM with RFAs.

3.5. Economic Impact Assessment

Table 9. Economic impact assessment of masonry mortar mixes (USD/m³).

Mix Designation	Component								Total
	Cement	Slag	Fly Ash	NFAs	RFAs	SS	SH(s)	Water 1
C-N-RFA0	34.7	0.0	0.0	9.3	0.0	0.0	0.0	0.6	44.6
C-S-RFA0	36.4	0.0	0.0	8.9	0.0	0.0	0.0	0.6	45.9
G-N-RFA0	0.0	5.6	32.7	8.9	0.0	38.5	15.0	0.3	101.0
G-S-RFA0	0.0	6.7	31.4	8.9	0.0	38.5	15.0	0.3	100.8
G-N-RFA25	0.0	6.4	29.9	6.4	1.7	44.0	17.1	0.3	105.8
G-S-RFA25	0.0	7.9	28.1	6.4	1.7	44.0	17.1	0.3	105.5
G-N-RFA50	0.0	7.8	27.7	4.2	3.4	43.4	16.9	0.3	103.8
G-S-RFA50	0.0	7.9	28.0	4.2	3.4	51.2	19.9	0.3	115.1
G-N-RFA75	0.0	7.7	27.0	2.0	4.9	49.4	19.3	0.3	110.7
G-S-RFA75	0.0	9.5	33.5	1.7	4.1	61.3	23.8	0.3	134.2
G-N-RFA100	0.0	7.4	26.1	0.0	6.3	47.8	18.6	0.4	106.6
G-S-RFA100	0.0	9.2	32.6	0.0	5.3	59.5	23.2	0.4	130.1

¹ Includes the water needed to obtain the aggregates in SSD condition, the water to formulate the sodium hydroxide solution, and any other water added to the mix.

summarizes the results of the economic impact assessment. Similarly, the total cost of each mix (USD/m³) is computed by summing the products of the unit cost (USD/kg) and the proportion of each mix component (kg/m³). The unit costs of the cement, slag, fly ash, NFAs, RFAs, SS, SH, and water were 0.076, 0.076, 0.089, 0.007, 0.005, 0.291, 0.653, and 0.002 USD/kg, respectively [8]. The comparison between C-N-RFA0 and G-N-RFA0 mixes showed that utilizing geopolymers as a substitute for cement resulted in at least two times higher cost. The higher cost of GMM mixes is primarily attributed to the higher cost of the alkaline activator solution. Additionally, incorporating RFAs in N-type GMM at replacement rates of 25, 50, 75, and 100% increased the cost by 5, 3, 10, and 6%, respectively. The gradual increase in cost was a result of the changes made to the mix design upon RFA replacement, including smore alkaline activator solution and more binder content. Likewise, S-type GMM mixes made with higher RFA replacement rates (75 and 100%) were 21 and 22% more expensive compared to counterparts N-type GMM mixes, respectively, due to the use of higher binder contents.

3.6. Performance Index

This work investigated the effect of the geopolymeric binder and RFA additions on the performance of masonry mortars of types N and S. Yet, in order to comprehend these effects on the different performance criteria, the performance index (PI) tool was used herein to select the optimum masonry mortar mix for efficient performance. In this approach, the individual measures included the environmental impact, cost, waste valorization, and compressive strength. Each of these measures is equally important to the end user. As such, they were assigned equal weights in the analysis. The waste valorization criterion was introduced in the current study to account for the waste material (RFAs) that could be incorporated into the mix rather than being stockpiled at the recycling facility. It is computed by summing the quantity of NFAs (kg/m³) for each mixture and 10% of the processed construction waste needed to produce the quantity of RFAs used in the mix (kg/m³). The latter calculation was based on the recycling plant’s RFA recovery efficiency of 90% [8]. It is noteworthy that only compressive strength was considered in this analysis, as the results of the remaining properties were correlated to the compressive strength. The PI approach is thoroughly discussed in a previous work [8]. In summary, each individual measure was assigned a weighted ranking (W_i), which was multiplied by a factor of 10 to attain a numeric index (N_i). The score (S_n) was calculated by multiplying the numeric indices from other measures, and the PI was finally determined as the ratio of S_n to S_n,max.

Table 10. Multifunctional performance index for N-type masonry mortar mixes.

Mix Designation	Weight (W)				Score (S)	PI
	Environment	Cost	Waste	fc
C-N-RFA0	0.23	1.00	0.09	0.80	172.73	7.50
G-N-RFA0	1.00	0.44	0.10	0.97	419.82	18.22
G-N-RFA25	0.92	0.42	0.13	1.00	514.35	22.32
G-N-RFA50	0.94	0.43	0.19	0.80	606.34	26.31
G-N-RFA75	0.86	0.40	0.32	0.66	738.02	32.03
G-N-RFA100	0.90	0.42	1.00	0.62	2304.30	100.00

and

Table 11. Multifunctional performance index for S-type masonry mortar mixes.

Mix Designation	Weight (W)				Score (S)	PI
	Environment	Cost	Waste	fc
C-S-RFA0	0.22	1.00	0.08	1.00	179.74	10.04
G-S-RFA0	1.00	0.46	0.08	0.77	288.04	16.08
G-S-RFA25	0.92	0.43	0.11	0.72	319.20	17.83
G-S-RFA50	0.82	0.40	0.15	0.71	360.48	20.13
G-S-RFA75	0.70	0.34	0.32	0.71	550.44	30.74
G-S-RFA100	0.72	0.35	1.00	0.70	1790.74	100.00

present the weighted ranking, score, and performance index for types N and S masonry mortar mixes, respectively. C-RFA0, G-RFA0, and G-RFA100 mixes were identified as optimal from cost, environmental impact, and waste valorization perspectives, respectively. Regarding compressive strength, G-N-RFA25 and C-S-RFA0 mixes demonstrated the highest weights among N- and S-type mortar mixes, respectively. The combined effect of the four individual criteria was assessed using a multifunctional performance index. According to the results, the GMM mix made with 100% RFAs was deemed optimal for both N- and S-types of mortars. Despite having inferior mechanical performance to the other mixes, it satisfied the requirements of ASTM C91 [15] for masonry mortar applications. Meanwhile, the remaining GMM mixes made with 0, 25, 50, and 75% RFAs were less favorable, resulting in lower PI scores. On the other hand, control cement-based masonry mortar mixes were least suitable, recording PI values of 8 and 10% for both N and S mortar types, respectively. This is primarily owed to its extensive consumption of natural resources and minimal valorization of waste materials.

4. Conclusions

The fresh, mechanical, physical, and durability properties of slag–fly ash-blended geopolymer masonry mortars made with recycled fine aggregates were examined using mixes of types N and S.

Table 12. Effects of the different parameters on masonry mortar properties.

Property	Parameter
	N-Type to S-Type	N-Type		S-Type
		Cement to Geopolymer	RFA Replacement	Cement to Geopolymer	RFA Replacement
Flow	↑ *	↑	↓ **	↑	↓
Air Content	↓	- ***	-	-	-
Water Retention	↓	↑	↑	↑	↑
Setting time	-	↓	↓	↓	↓
Compressive Strength	↑	↑	↓	↓	↓
Pull-off Strength	↑	↑	↓	↑	↓
Flexural Strength	↑	↑	↓	↓	↓
Bulk Density	-	-	↓	-	↓
Water Absorption	-	↓	↑	↓	↑
Permeable voids	-	↓	↑	↓	↑
Bulk Resistivity	↑	↓	↓	↓	↓
Sorptivity	↓	↑	↓	↑	↓
Abrasion Resistance	↑	↑	↑	↓	↑

* Increase in property. ** Decrease in property. *** No tangible impact on property.

summarizes the conclusive remarks pertaining to the effect of the various mix design parameters, including mortar type, binder type, and RFA replacement, on the properties of masonry mortars. Arrows pointing upward indicate an increase in performance and vice versa. Meanwhile, a “-“ is indicative of an insignificant impact. For instance, transitioning from N-type to S-type mortar resulted in enhanced workability; compressive, pull-off, and flexural strengths; bulk electrical resistivity; and abrasion resistance (represented by percent mass loss). Conversely, the air content, water retention, and sorptivity decreased in S-type mortars compared to their N-type counterparts.

Furthermore, GMM showcased higher 1-d f_c/28-d f_c ratios compared to cement-based mortars. This can be practically advantageous for masonry work, expediting construction operations and eliminating the hassle of water curing demanded for cement-based plasters and renders. The environmental impact assessment of the masonry mortar mixes showed that using geopolymers as a replacement for cement reduced the GWP. However, GMM mixes made with RFAs experienced up to 43% increase in GWP compared to control GMM made with NFAs. Meanwhile, the economic impact assessment revealed that using a geopolymeric binder or replacing NFAs with RFAs increased the cost by up to 100%.

The integration of the different performance response criteria in a multifunctional performance index resulted in the identification of geopolymeric mixes made with 100% RFAs (i.e., G-N-RFA100 and G-S-RFA100) as optimal for masonry mortar applications considering compressive strength, environmental and economic impacts, and waste valorization.