Performance evaluation of cement mortar made of different types of recycled sand

Abstract

This work investigates the potential replacement of natural sand within cement mortar via the use of recycled sand derived from excavation waste, construction and demolition waste and concrete waste. To study the mechanical properties such as compressive strength, flexural strength, hardened density, ultrasonic pulse velocity, and durability characteristics, a series of mortar mixtures is prepared by using different percentages of recycled sand (0%, 10%, 20%, 50%, 75%, and 100%). The water to cement ratio is varied to obtain a target consistency of 110% ± 5% in the flow table test, with the sand to cement ratio set at 3.553 to match field mixing conditions. Elemental and microstructural characterization of the recycled sand and mortar using advanced analytical techniques is performed on these materials. The results show a decline in compressive strength, flexural strength, water absorption, and ultrasonic pulse velocity with increasing replacement, especially when over 50%. Mortar made of recycled sand derived from construction and demolition waste achieves high compressive and flexural strengths across all replacement levels. The findings indicate that recycled sand from excavation waste and construction and demolition waste is effective for replacing natural sand in mortar applications up to 75%, while concrete waste is suitable for replacement levels up to 50%.

Graphical abstract

Keywords

recycled sand / construction and demolition waste / excavation waste / concrete waste / cement mortar / performance evaluation

1 Introduction

As a huge global resource consumer, the construction industry is responsible for approximately 40% of all raw materials and energy consumed worldwide and 33% of carbon dioxide (CO₂) emissions [1]. Cement mortars which are the basis in building construction, including masonry and plastering, are prepared by mixing cement, water and fine aggregates such as sand. Sand makes up a significant portion of the aggregate, which contributes by over one-third of the total volume or mass of mortar [2]. Construction-related waste is growing steadily each year, which presents a considerable sustainability concern on a global scale [3]. In developing countries, the absence of specific waste management plans leads to landfill disposal, causing environmental damage, health risks like asthma and skin conditions, and loss of land resources [4]. The use of recycled sand (RS) derived from construction and demolition waste (CDW) as an alternative to traditional sand in mortar reduces sand mining and fosters cleaner production in the construction industry [5]. While extensive research has been conducted on coarse recycled aggregates, there have been relatively few studies in recent decades investigating the potential use of RS from construction industry wastes such as excavation waste (EW), CDW and concrete waste (CW) in mortar [6]. Research presents contrasting findings regarding the use of recycled fine aggregate from EW in cement mortar. Some studies indicate potential negative impacts on mortar properties, such as higher water absorption (WA) and lower bulk density [7]. In contrast, others suggest that replacing natural sand (WS) with RS can improve the mechanical strength of the mortar [8]. Some research highlights improvements in compressive strength (CS) when up to 30% of the waste is used, with the filler effect of sandstone and dense matrix formation playing key roles [9]. However, other studies show that recycled EW can reduce mechanical properties, with limestone-crushed sand lowering rheological performance and CS [10]. Furthermore, untreated excavation soil in mortar has been shown to cause significant reductions in strength, mainly due to clay particles weakening the bond between cement and aggregate [11].

Research highlights that increasing the amount of recycled concrete fine aggregates in place of natural fine aggregates reduces concrete density [12]. Concretes with up to 30% recycled concrete fine aggregate maintain acceptable mechanical and durability properties [13]. Some studies report enhanced strength properties with up to 60% recycled concrete fine aggregate replacement [14]. However, mortars made with fine recycled concrete aggregates (RCAs) generally have reduced strength and durability [15], although some studies indicate no negative impact with up to 30% replacement [16]. The effect of different granular classes of RS on mortar properties requires further investigation [17]. Though research highlights the benefits of recycled aggregates in cement-based materials, many designers and contractors remain reluctant to adopt them [18]. The use of CDW fine sand in different concrete types has also shown positive outcomes, but careful evaluation of the waste original properties is essential.

On the other hand, the water to cement ratio (W/C) is a key factor in mortar strength, initially highlighted by Feret [19] and later formalized by Abrams [20], who showed that higher W/C reduce CS. However, this principle has been critiqued for not considering other factors influencing strength. Both the W/C and sand content impact cement mortar strength, with limited research exploring their combined effects. Studies show the W/C consistently influences CS, regardless of sand type or grading. Reddy and Gupta [21] observed strength reductions with fine sands, though Haach et al. [22] concluded that sand properties primarily affect workability, not strength. Using fine RCA as a substitute for WS in non-structural concrete and masonry mortar has shown mixed results. While studies like those done by Corinaldesi and Moriconi [23], and Dapena et al. [24] reported that the strength reduced with increasing RCA content, others, such as Vegas et al. [25], suggested that up to 25% substitution can maintain workability and strength. Martínez et al. [26] replaced by volume 100% of WS with different types of RS and found that most of the natural aggregate mortars achieved higher mechanical properties than the recycled aggregate mortars. Very fine RCA, with particle sizes less than 0.150 mm, have also been used in mortars. Braga et al. [16] obtained very satisfactory results for most of the mortars prepared with different incorporation levels of fine concrete aggregates. Neno et al. [27] studied the effect of different replacement levels (RLs) of river sand by recycled concrete sand on mortar properties, although these authors did not propose a maximum replacement ratio that meets all the performance requirements of mortars. Ledesma et al. [28] evaluated the short- and long-term properties of masonry mortar and concluded that a replacement ratio of up to 40% is a viable alternative for masonry mortar production.

This study addresses a critical gap by exploring the effects of using RS sourced from CW, CDW, and EW as a sustainable alternative to WS in masonry mortar production. Unlike previous research focused primarily on WS, this study investigates the mechanical performance and microstructure analysis of mortar with varying RS content, evaluating RLs of up to 100% by weight while matching actual site mixing conditions. Significantly, the study matches actual site mixing conditions by using sand to cement ratio (S/C) commonly employed in real-world plastering and masonry applications. This research establishes practical RLs, thus setting a precedent for incorporating RS in construction projects, supporting environmental sustainability and resource conservation. Figure 1 illustrates the research methodology and significance of this research.

2 Experimental programs

2.1 Materials

Cement, water, and sand were used in this study to prepare the mortar mixes. The cement was locally sourced ordinary Portland cement, which was tested to ensure compliance with the standards (BS EN 197-1) [29]. The potable water used in the mortar mix was tested and confirmed to meet all relevant quality standards, ensuring it was free from impurities that could affect the mortar’s performance. The study utilized locally available WS alongside three types of RS derived from EW, CDW, and CW as shown in Fig. 2(a). The WS used in the research were obtained from the Qatar Sand Treatment Plant. Three types of RSs produced from EW, CDW, and CW were provided by Qatar Primary Materials Company (Al-Awalia). Qatar rapid infrastructure growth over the past two decades has generated substantial amounts of EW sand. This waste undergoes extensive processing, including mechanical, manual, and magnetic separation, to remove foreign materials, metals, and lightweight contaminants. Subsequently, it is crushed and sorted into various sizes through three primary stages: jaw crushing and screening, impact crushing and screening, and secondary impact and screening. Fine materials of 0–5 mm are collected separately from these processes, though their use in mortar production is largely untested. CDW sand, generated from the demolition of buildings and infrastructure, includes materials such as concrete, blocks, interlocks, ceramics, plaster, soil, pavement, and other debris, making it the most abundant type of construction waste. CDW sand is a mix of different materials with varying properties and impurities. Currently, CDW is not sorted at the source into clean concrete or more contaminated categories like wood or plasterboard. Instead, it is separated at processing facilities using manual and mechanical dry methods, including magnets and air-blowing techniques. CW offers a cleaner source of RCA, generally consisting of crushed stone aggregate with remnants of mortar or cement paste. Figure 2(b) shows images of WS and RS.

2.2 Sand characterization

The quality assessment of natural and RS samples involved a series of physical and chemical tests done as per the standard shown in Table 1. The microstructural properties of natural and RS samples were studied by using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FTIR). XRF analysis was carried out with JSX 3201M (Jeol) to analyze the element compositions in each type of sand. Such analyses are centered on the determination of the primary components present (Loss of Ignition (LOI), Al₂O₃, SiO₂, CaO, and Fe₂O₃) in each sand sample which is further essential regarding its chemical composition. A PANalytical EMPYREAN (CuKα radiation (λ = 1.54 Å)) was used for the XRD analysis. The measurement range was from 5° to 50° in 2θ, with an angular width of the step was 0.02, the counting time being two seconds per step. These were very useful in recognizing the crystalline phases (e.g., quartz, etc.) of the sands. A TGA 4000 PerkinElmer was used to perform TGA. The samples were then subjected to heat, ranging from 29 up to 800 °C in a nitrogen atmosphere. Thermal analysis helps to understand the thermal stability and decomposition nature of the samples, indicating weight loss due to moisture evaporation, organic decompositions and other thermal events. For the FTIR analyses, samples were made using KBr pellets method and each sand sampling was grinded repeatedly in KBr ratio that was appropriate with transmittance analysis. FTIR spectra were recorded in the 500 to 3300 cm⁻¹ wavenumber range, allowing for the identification of specific functional groups, such as silicates and carbonates, and other chemical bonds in the sands.

The microstructural behavior of the mixes was assessed using XRD and XRF results together with indirect pore-related indicators such as WA and bulk density. The XRD patterns show that mixes containing EW, CDW, and CW develop comparable hydration phases (primarily CSH and Ca(OH)₂), but with slightly different intensities due to variations in CaO and silica content. XRF results confirm that the RSs possess higher CaO content (36%–39%) compared to natural washed sand (29.8%), which contributes to enhanced CSH formation and supports the higher strength observed in some mixes. In contrast, EW exhibits very high WA (17.5%) and lower bulk density, indicating a more open pore structure that reduces CS and ultrasonic pulse velocity (UPV). Mixes with lower absorption (CDW and CW) show denser packing and better mechanical performance. These trends demonstrate a consistent micro–macro relationship linking chemical composition and inferred pore structure to CS and UPV.

2.3 Mortar mixture proportions

This study investigated the partial replacement of WS with RS at RLs of 0%, 10%, 20%, 50%, 75%, and 100%, aiming to assess how various sand types and proportions of RS impact the properties of mortar mixes as shown in Table 2. All materials were selected according to standard protocols to ensure consistency throughout the experiment. The cement used was in accordance with the standards, with each batch containing 1126 g of cement. An incremental adjustment in the W/C was made to reach workability (110 ± 5) according to ASTM C109, ASTM C91, ASTM C1329, and ASTM C270, by taking into consideration that the RS could exhibit higher WA capacity. The S/C was consistently maintained at 3.553 across all mixes to align with actual site practices for plastering and masonry work. No superplasticizer or other admixtures were introduced into the mixes. The mixing process followed the standard procedure outlined in ASTM C305.

2.4 Mortar characterization

The workability of fresh mortar was evaluated through the flow table test, following ASTM C1437 standards. The prepared mortar was placed in a flow mold and compacted in two layers by rodding. After removing the mold, the flow table dropped 25 times within 15 s. The spread of the mortar was measured in four different directions, and the results were recorded as the flow percentage. Mortar density was calculated in 50 mm × 50 mm × 50 mm cubes. On each of the testing days, 3, 7, and 28 d, three specimens were weighed for each mix. Taking the average mass of these specimens and using the litters volume to express the density in kg/m³. A CONTROLS testing machine was used for the CS test, on 3, 7, and 28 d and flexural strength (FS) at 7 and 28 d. In CS, total of three mortar specimens per testing day were loaded until absolute failure at a rate of 1800 N/s. According to ASTM C109, the CS was determined by dividing the maximum load by the cube cross-sectional area. The maximum load was divided by inertia and span length to obtain a FS after loading mortar prisms (40 mm × 40 mm × 160 mm) at failure in accordance with ASTM C348. The FS test was done according to the requirements of ASTM C348, where about (50 ± 10) N/s controlled movements applied load until the sample failed. The WA test was performed to determine the rate of WA at 28 d. This test starts from the preparation of three cube specimens 50 mm × 50 mm × 50 mm that had been cured and then oven-dried at 105 °C for 72 h. Specimens are left to cool for 24 h before weighing them to get the dry weight. They are then submerged in water for half an hour to allow absorption of some moisture. After the specimens are taken out of water, they are dried on the surface and weighed again to give their saturated weight. Water uptake is determined as the weight difference between dry and saturated, and expressed as a percentage of dry weight. UPV testing was conducted at 28 d for each mortar mix using 50 mm × 50 mm cubes, following the standard direct transmission method by ASTM C597. For each mix, three cubes were tested, with 10 UPV measurements taken per cube to ensure the accuracy and reliability of the results. This method involves placing transducers directly on opposite faces of the cube, ensuring that the ultrasonic waves travel through the entire sample length.

Following a curing period of 90 d, various methods (XRD, TGA, and FTIR) were utilized for the microstructural analysis of the mortar samples to investigate its crystalline phases, surface morphology, thermal stability as well as chemical bonding within this material. The mineral identification and lattice types were confirmed by XRD analysis between 5° and 50° in 2θ with a step of 0.02° and a counting time of 2 s per step, using PANalytical EMPYREAN radiation Cu Kα (λ = 1.54 Å). TGA was performed with a TGA 4000 PerkinElmer device between 29 and 800 °C, to determine the thermal stability and decomposition features of the mortar; FTIR, from 500 to 3300 cm⁻¹ was carried out to detect functional groups and bonding characteristics within the samples. Performance of all test procedures and equipment setups remained consistent with sand characterization, providing comparable data. Figure 3 shows the preparation, mixing, and testing steps of mortar samples.

3 Experimental results and discussion

3.1 Physical characteristics of recycled sand

The quality properties result for WS and RS made from EW, CDW, and CW were analyzed based on various ASTM and BS EN standards, as shown in Table 1. The sieve analysis highlights differences in particle size distribution for EW, CW, CDW, and WS, as illustrated in Fig. 4(a). All materials pass 100% through sieve sizes from 37.5 to 8 mm, with a maximum aggregate size of approximately 5 mm and a nominal size between 5 and 2.36 mm. At finer sieve sizes, notable distinctions arise: EW contains the highest fine content, with 18.25% passing through the 0.063 mm sieve, indicating a large portion of very fine particles. CW, on the other hand, has the least fine content, with only 4.5% passing through the same sieve, suggesting its suitability for applications requiring coarser aggregates. CDW falls in between, with 6.45% fines, while WS shows the least fines at 2.4%, which is consistent with its natural state, making it ideal for applications requiring clean sand with minimal impurities. Multi-stage crushing results in a finer particle size distribution compared to single-stage crushing, as shown by Florea and Brouwers [30]. Evangelista et al. [31] studied the effect of jaw aperture sizes in crushers on fine RCA, finding that aperture size affects produced fractions but does not alter the overall distribution. Sosa et al. [32] found similar particle size distributions for fine RCA with varying CSs and natural coarse aggregates. Ulsen et al. [33] reported that rotor speeds of vertical shaft impact crushers did not significantly change particle shape or size distribution. Fan et al. [34] demonstrated that multi-stage crushing generates more fine particles than single-stage crushing, though both remained within the ASTM C33 range. Gomes et al. [35] observed that while fine RCA and natural aggregates share the same maximum particle size (D_max), their overall particle size distributions differ. The sieve analysis for all sand samples was conducted in accordance with ASTM C136.

WA rates varied significantly among the sands, with WS showing the lowest absorption at 2.20%, followed by CW at 4.70%, CDW at 9.55%, and EW at 17.50%. This indicates that WS is less porous and likely more durable, while EW is highly porous and potentially less durable. WA values for fine RCA range between 4.28% and 13.1%, averaging 8.4%, while natural fine aggregates have WA values between 0.15% and 4.1%, with an average of 1.1% [36]. Testing the WA of fine RCA poses challenges due to its variable properties, including adhered cement paste and fine particles (under 250 μm), leading to inconsistent results [37]. RSs, derived from construction waste materials such as concrete, mortar, bricks, and aggregates, typically exhibit higher WA rates than WS due to their porous nature and processing methods [38]. These materials often contain porous elements like crushed concrete, which absorb more water. Additionally, the shape and rough surface texture of RS particles, as well as remnants of binders and cementitious materials, contribute to their higher WA [39]. WA values for fine sand fractions are determined following EN 1097-6:2013 [40], which involves immersing the aggregates in water for 24 h and then drying them. However, this method presents difficulties when applied to RCAs, particularly fine RCAs, because the fine particles tend to clump together, making it hard to uniformly apply drying energy. Tegguer [41] found that coarse RCAs often require more than 24 h for full saturation, a finding corroborated by multiple studies, which suggest extending the saturation period for fine RCAs [42]. The presence of fine particles in RCAs complicates the immersion method, as they can trap air and cause inconsistent weight measurements [43].

Natural and recycled aggregates have large differences in SG values. WS consistently showed the highest SG values. With respect to oven-dry SG value, WS leads with a value of 2.560 followed by EW with a value of 2.450 and CDW with a value of 2.300 and CW has the lowest at 2.115. For the SSD condition, WS has the highest SG value of 2.620 followed by EW, CDW and CW with values of 2.540, 2.500, and 2.480, respectively. Measured SG values mirror the above pattern, with WS at 2.720, EW at 2.700, CDW at 2.690, and CW at 2.610, respectively. Fine RCA density is less than the one of WS, as SSD densities values calculated ranged between 1630 and 2560 kg/m³ with an average value ranging around 2295 kg/m³, compared to a higher SSD density for WS which shows values between 2530–2720 kg/m³ and an average of about 2637 kg/m³. From bulk density, WS possessed the highest bulk density for rodding (1670 kg/m³) and shoveling (1530 kg/m³), which proved to be the heaviest material. On the other hand, CW bulk densities were also lowest: 1480 kg/m³ for rodding and shoveling with 1350 kg/m³.

3.2 Chemical and microstructural characteristics of recycled sand

The acid-soluble sulfate content was found to be lowest in WS (0.28%) and highest in CDW (1.30%), while water-soluble chloride content was minimal in all materials, with both WS and CW measuring at 0.01%. No organic impurities were detected in any of the materials. In the Methylene Blue test, WS showed the lowest value (0.4), indicating a lower presence of clay particles, whereas EW recorded the highest value (1.8), suggesting a higher clay content.

XRF analysis of RS shows key differences in composition as shown in Table 2. EW showed the highest LOI, indicating potential durability concerns, while WS exhibited high SiO₂ content, providing chemical resistance. CW had the highest levels of CaO and Al₂O₃, ideal for cementitious applications. CDW showed balanced elemental content but higher LOI, which may affect stability. Studies highlight that recycled fine aggregates with high SiO₂ and Al₂O₃ content improve concrete strength and durability through pozzolanic reactions and CSH formation, while higher CaO enhances hydration and strength [44].

XRD analysis of WS, CDW, CW, and EW revealed differences in their mineralogical compositions as shown in Fig. 4(b). WS exhibited sharp quartz peaks, indicating high crystallinity and minimal cementitious content, making it suitable as a stable aggregate. CDW showed both quartz and calcite, suggesting residual cementitious activity from calcium hydroxide (CH) and calcium silicate hydrate (CSH), though carbonation has transformed much of the CH into calcite, contributing to its binding potential when reused. CW displayed quartz and calcite, with broad peaks suggesting both crystalline and amorphous phases, typical of recycled concrete. EW, primarily composed of quartz, showed no cementitious materials, reflecting its natural silicate-rich origin [45].

The TGA results for RS reveal distinct thermal decomposition behaviors that reflect each material’s components as shown in Fig. 4(c). In the initial phase below 150 °C, all samples show weight loss due to moisture evaporation, with EW exhibiting the most significant drop, indicating its higher moisture absorption. Between 150 and 400 °C, weight loss corresponds to dehydration of chemically bound water in compounds like CSHs, with CDW and CW losing more weight due to higher cementitious content. The 400–500 °C range shows pronounced weight loss in CDW and CW, indicating a substantial presence of CH, typical of recycled concrete materials, while WS and EW have lower weight loss here. Finally, between 600 and 800 °C, further weight loss from the decomposition of carbonates is most notable in CDW, reflecting its high calcium carbonate content from recycled concrete, followed by CW. WS and EW show minimal weight loss in this stage, consistent with their lower cementitious content.

FTIR was done to assess the chemical components of the RS, as shown in Fig. 4(d). In the 750–1500 cm⁻¹ range, peaks near 875 and 1420 cm⁻¹ in the spectra for CDW, WS, CW, and EW suggest the presence of carbonate groups ({\mathrm{C}\mathrm{O}}_3^{2-}). Additionally, WS and CW display peaks at around 1000 cm⁻¹, indicating silicates due to Si-O stretching vibrations. A broad absorption band around 1640 cm⁻¹, observed in all samples, corresponds to H-O-H bending vibrations, pointing to absorbed water or hydration products like CSH. Another broad band near 3400 cm⁻¹, linked to O-H stretching vibrations, reveals hydroxyl groups and water molecules, reflecting different hydration levels across the materials. CDW and CW show strong peaks for carbonate and silicate components, along with significant hydration, as indicated by the O-H stretching bands, suggesting the presence of typical concrete compounds such as CSH and CH. WS contains similar components but exhibits a stronger presence of silicates, suggesting it may be purer or less processed than CDW and CW. EW, meanwhile, shows the most intense peaks for carbonates, silicates, and hydration bands, indicating a substantial amount of these compounds in the material.

3.3 Effect of sand replacement ratio on mortar-hardened density

The hardened density results for the mortar mixes incorporating EW, CDW, and CW at various RLs were analyzed in terms of 3, 7, and 28 d as shown in Fig. 5(a). The density results for the mortar mixes show that WS, used in the control mix, had the highest densities, reaching 2280 kg/m³ at 28 d. Replacing WS with EW sand led to a decrease in density as the RL increased, with 100% EW resulting in a 28 d density of 2103 kg/m³. mortar with CDW sand performed well, maintaining higher densities even at 100% replacement, with a 28 d density of 2140 kg/m³, indicating good material compatibility. In contrast, mortar with CW sand showed the lowest densities, with 100% CW resulting in a 28 d density of 1970 kg/m³, reflecting higher porosity and less compact structure.

3.4 Effect of sand replacement ratio on compressive strength

The CS results for the mortar mixes incorporating EW, CDW, and CW at various RLs were analyzed in terms of strength development at 3, 7, and 28 d as shown in Fig. 5(b). The data clearly shows the varying impact of each recycled material on early and long-term strength performance. The control mix (M1) made of WS, exhibited the highest CS across all curing periods. The strength values for samples made of WS and cured for 3, 7, and 28 d were equal to 11, 22.1, and 32.1 MPa, respectively. Mortar samples made of EW sand showed a clear decline in CS as the RL increased. At 10% replacement (M2), the 3-d CS was 10 MPa, comparable to the control mix (WS) and CW mix, but higher than that of the CDW mix, which recorded 5.7 MPa at the same level. At 7 and 28 d, the CS of 10% EW increased to 17.2 and 25 MPa, respectively, indicating good strength development at low RLs. However, a sharp strength reduction was observed at higher RLs. At 50% replacement (M5), the 3-d strength dropped to 5.5 MPa, and the 28-d CS declined to 16.2 MPa, significantly lower than that of WS and other materials. At 100% EW (M6), the 3-d and 28-d strengths further decreased to 2.7 and 9.8 MPa, respectively, indicating limited suitability of EW at high RLs.

In contrast, CDW mixes demonstrated superior performance across all RLs, making it the most effective recycled aggregate. At 10% CDW (M7), the 3-d CS was 8.7, slightly lower than WS. By 7 d, it reached 19.9 MPa and peaked at 31.6 MPa at 28 d, surpassing the control mix and achieving the highest CS at any RL. Even at 50% replacement (M9), CDW maintained high performance with a 3-d strength of 8.4 MPa, a 7-d strength of 18.2 MPa, and a 28-d CS of 28 MPa. At 100% replacement (M11), CDW continued to perform well, with 3, 7, and 28 d strengths of 7.8, 17.2, and 22.1 MPa, respectively. These results suggest that CDW can replace WS entirely without compromising strength, making it a reliable material for structural mortar applications.

CW performed better than EW, especially at RLs between 10% and 50%, though it remained slightly below CDW in performance. At 10% CW (M13), the 3 d CS reached 10.9 MPa, similar to EW and the control mix. The strength further developed to 21.9 MPa at 7 d and 29.6 MPa at 28 d. However, at higher RLs, performance declined. At 75% CW (M15), the 3, 7, and 28-d CS dropped to 7.1, 13.7, and 17.6 MPa, respectively. At full replacement (M16), the CS further decreased to 5.4 MPa (3 d), 10.4 MPa (7 d), and 14.7 MPa (28 d), indicating that CW retains reasonable strength at high RLs, but underperforms compared to CDW in long-term strength.

3.5 Effect of sand replacement ratio on mortar flexural strength

The results of the FS of mortar mixes made with different RLs of WS by EW, CDW, and CW show clear trends over 7 and 28 d of curing as shown in Fig. 5(c). The control mix (M1) containing 100% WS revealed the highest value of FS among all mixes, 4.2 MPa at 7 d and 5.9 MPa at 28 d, matching with CS results, indicating superior performance in both early and long-term FS. This mix acts as a reference point for the other mixes containing recycled materials. The FSs measured across materials vary due to their inherent characteristics related to the recycled aggregates themselves and their bonding interaction with the cement matrix. The natural, well-graded, and dense structure of WS, having minimum porosity, results in better particle packing and stronger bond between the particles and the cement paste, hence giving higher FSs. This consistent particle size distribution and strong cohesive matrix explain why the control mix exhibited the highest flexural performance. The nature of the decrease in FS with higher EW values can be explained by its weaker, more permeable nature. EW sand is essentially made of EW consist of mix of soil, clay, and natural rock fragments, all non-cementitious and, thus, not taking part in the hydration reaction. The higher porosity of EW introduces more voids into the mortar matrix, weakening the overall structure. The poor bond between EW particles and the cement paste further contributes to the reduction in FS, especially at higher RLs. At 50% EW (M4), the FS dropped to 2.8 MPa at 7 d and 4.2 MPa at 28 d. It was further reduced to 2.5 MPa at 7 d, and only 2.9 MPa at 28 d when 100% EW was used, M6. This means that the contribution of EW to FS is very limited for higher replacements. Among the different types of recycled aggregates, CDW fared the best. Its composition, including crushed concrete and unhydrated cementitious materials, allows for continued hydration and strength development, even at higher RLs. CDW particles are denser and less porous than EW and CW, promoting better particle packing and bonding with the cement paste, which maintains FS. At 10% CDW (M7), the 7-d FS was 4.0 MPa, and at 28 d, it reached 5.8 MPa, close to the control mix. Even at 100% CDW (M11), the 28-d FS remained high at 5.2 MPa, confirming CDW’s strong performance at maintaining FS across RLs. CW showed moderate performance, with higher porosity and more irregularly shaped particles compared to CDW, which led to a less effective bond with the cement paste. At 10% CW, M12 obtained a 7-d strength of 3.8 MPa and a 28-d strength of 5.8 MPa, similar to the control mix. However, as the RL was increased, the FS dropped-while 50% CW, M14, achieved a value of 5.5 MPa at 28 d, 100% CW, M16, showed an abrupt drop to 3.8 MPa at 28 d. While CW performed better than EW, it showed a clear decline at higher RLs.

3.6 Effect of sand replacement ratio on mortar water absorption

The WA results for the mortar mixes at 28 d reveal significant differences based on the type and level of replacement of WS with EW, CDW, and CW as shown in Fig. 5(d). The control mix (M1) containing 100% WS exhibited the lowest WA value at 1.6%, indicating a highly compact and dense mortar with minimal porosity. This low absorption is due to the well-graded and dense nature of WS, which creates a tight matrix with fewer voids, preventing excess WA. As the RL of EW increased, the WA values also rose significantly. At 10% EW (M2), the WA was 2.0%, only slightly higher than the control mix. However, at 50% EW (M4), WA increased to 3.0%, and at 100% EW (M6), it peaked at 9.1%, the highest among all mixes. This sharp rise in WA at higher EW RLs can be attributed to the porous nature of EW, which is composed of materials like soil, clay, and rock fragments that do not contribute to hydration and introduce voids into the mortar matrix, leading to increased porosity and WA.

CDW performed better than EW in terms of WA, though it still showed an increase compared to the control mix. At 10% CDW (M7), the WA was 2.8%, and at 50% and 75% CDW (M9 and M10), it reached 4.4%. Even at 100% CDW (M11), the WA was 6.8%, which, while higher than WS, was lower than both EW and CW at the same RLs. The better performance of CDW is due to the presence of crushed concrete and unhydrated cementitious particles that reduce void content and contribute to filling in the gaps in the mortar, leading to better compaction and lower WA compared to EW. In contrast, CW exhibited the highest WA values among the recycled materials. At 10% CW (M12), WA was 2.6%, already higher than the equivalent mixes of EW and CDW. As the RL increased, the WA rose sharply, reaching 4.8% at 50% CW (M14) and 9.6% at 100% CW (M16), the highest value recorded. This significant increase in WA is due to the irregular shape and high porosity of CW particles, which create more voids in the mortar and lead to higher WA, particularly at higher RLs.

3.7 Effect of sand replacement ratio on mortar ultrasonic pulse velocity results

The UPV results, when compared to the provided concrete quality classification, give a clear indication of how the RLs of WS with EW, CDW, and CW affect the quality of the mortar as shown in Fig. 5(e). The control mix (M1), made with 100% WS, had a UPV of 4178 m/s, which places it in the Good category (3500–4500 m/s) as per BS 1881-203 [46]. This indicates that the control mix maintains a dense, compact mortar with strong internal bonding, typical of well-graded WS. While this value is close to the upper end of the “Good” range, it does not quite reach the Excellent category, which requires a UPV of over 4500 m/s. For mixes incorporating EW, the UPV decreases progressively with higher RLs. At 10% EW (M2), the UPV is 4043 m/s, still well within the Good range. However, as the RL increases, the mortar’s quality declines. By 50% EW (M4), the UPV is 3906 m/s, remaining in the Good category but showing a clear reduction in internal compactness. At 100% EW (M6), the UPV drops significantly to 3491 m/s, placing the mortar at the border between the Good and Fair categories. This sharp decrease reflects the porous nature of EW, which introduces more voids into the mortar matrix and reduces overall material quality. CDW showed better retention of quality compared to EW. At 10% CDW (M7), the UPV is 4047 m/s, still firmly in the Good range. Even at 50% CDW (M9), the UPV remains 3913 m/s, indicating that CDW can maintain good internal structure at moderate RLs. At 100% CDW (M11), the UPV drops to 3600 m/s, but this still falls within the Good category. CDW’s crushed concrete particles likely contribute to the matrix’s integrity, ensuring better performance compared to EW. For mixes using CW, the UPV results were lower than both EW and CDW. At 10% CW (M12), the UPV is 3947 m/s, still classified as Good, but slightly lower than the comparable EW and CDW mixes. As the RL increases, the UPV continues to decrease, reaching 3766 m/s at 50% CW (M14) and 3510 m/s at 100% CW (M16). The 100% RL is at the very edge of the Good category, approaching the Fair category (3000–3500 m/s). The lower UPV values for CW can be attributed to its more porous and irregular particles, which weaken the internal structure of the mortar and lead to more voids.

3.8 Relationships between replacement level and mortar results

Figure 6(a) shows the relationship between CS and RL at 28 d for three types of recycled materials EW, CDW, and CW. CDW has the mildest slope, indicating a smaller reduction in CS as the RL increases. CW exhibits a steeper decline. EW shows the most significant decrease in CS with increasing replacement. Figure 6(b) illustrates the decline in FS with increasing RL for three types of recycled materials (EW, CDW, and CW) after 28 d. EW shows the steepest decrease, indicating a significant reduction in strength at higher RLs. CW has a moderate decline. CDW shows the least impact, suggesting better retention of FS. Figure 6(c) shows the relationship between FS and CS of mortar specimens at 28 d. There is an upward trend; thus, a positive relation can be seen. The relation showing that there is a great linear relation between the variables. In other words, as the CS goes up, the FS follows up accordingly. Figure 6(d) illustrates the relationship between WA and CS for mortar samples tested at 28 d. The horizontal dashed line represents the ASTM C91 and C1329 minimum CS requirement for Type S mortars (14.5 MPa), while the vertical solid line shows the QCS 2014 (Qatar construction specification) WA threshold for durable paving flags (< 5%). The data points indicate that samples with lower WA, especially those near or below the 5% threshold, tend to exhibit higher CSs. Notably, the WS 100 sample, with minimal WA, shows the highest CS, while samples with higher RLs, particularly those with EW, CDW content above 75%, and CW above 50% display both increased WA and reduced CS.

Figure 6(e) illustrates the relationship between UPV and 28-d CS for the tested mortar specimens. The results show a positive correlation, where higher UPV values generally correspond to higher CS, indicating improved material density and internal integrity. The fitted regression line (coefficient of determination, R² = 0.9564) confirms a strong linear association between the two parameters, demonstrating that UPV can be used as a reliable non-destructive indicator of strength development in mixes incorporating RS.

3.9 Microstructural analysis of mortar

XRD analysis of the mortar samples after 90 d provides insight into the mineral components of the materials, specifically for WS at 100% and for mixes with 50% replacement by EW, CDW, and CW as shown in Fig. 7(a). The primary crystalline phases identified include quartz (q), CH, and CSHs. The WS 100% sample reveals prominent quartz peaks, indicating its high quartz content, which is typical of WS. Peaks corresponding to calcium hydroxide and CSH are also present, signifying the continued presence of hydration products from cement. In the WS/EW 50/50 mix, quartz remains a dominant phase, but the intensity of calcium hydroxide peaks slightly decreases, suggesting a possible reduction in cementitious content compared to pure WS. The presence of CSH indicates continued hydration [47,48], although the substitution with EW may dilute some cementitious properties. For the WS/CDW 50/50 mix, similar peaks of quartz and CSH are observed, along with noticeable calcium hydroxide. CDW’s composition likely includes recycled cementitious material, contributing to the calcium hydroxide and CSH peaks. This mix maintains a relatively strong mineral profile due to the presence of recycled cement-based compounds in CDW. The WS/CW 50/50 mix also shows quartz peaks but with a lower intensity in calcium hydroxide compared to the other mixes. This suggests that the CW component may contain less active cementitious material, leading to fewer hydration products over the 90-d curing period. However, CSH peaks are present, indicating that some hydration continues, albeit at a reduced rate [49].

The TGA of the 90-d cured mortar samples, with various sand replacements, reveals significant differences in thermal decomposition behaviors, providing insights into each mix’s microstructural composition as shown in Fig. 7(b). In the initial weight loss stage (below 150 °C), weight reduction is observed across all samples due to the release of physically bound water. Among the mortars, WS/EW exhibits the highest initial weight loss, indicating a more porous structure that absorbs more moisture, while WS shows the lowest loss, consistent with its dense, less porous structure. Between 150–400 °C, weight loss continues as chemically bound water from hydration products, such as CSH, is released. The WS/CDW and WS/CW mixes show a more gradual, sustained weight reduction in this range, reflecting higher CSH content. In comparison, the WS/EW mix shows a smaller weight loss, suggesting a lower presence of hydration products. In the 400–500 °C range, a more pronounced weight loss occurs, associated with the decomposition of calcium hydroxide (Ca(OH)₂) into calcium oxide (CaO) and water vapor. Mortars containing CDW and CW exhibit considerable weight loss at this stage, indicating higher residual cementitious material content. By contrast, WS and WS/EW samples show relatively minor weight reductions, suggesting lower Ca(OH)₂ content. Finally, between 600–800 °C, further weight loss is attributed to the decomposition of carbonates, particularly calcium carbonate (CaCO₃), into calcium oxide and CO₂. The WS/CDW and WS/CW mortars show prominent weight reductions, indicating a higher carbonate presence due to recycled concrete content [50]. The WS and WS/EW mortars exhibit minimal weight loss, consistent with their lower carbonate content.

The FTIR spectra for the 90-d cured mortar samples, WS 100, WS/EW 50/50, WS/CDW 50/50, and WS/CW 50/50, illustrate key differences in the functional groups present, reflective of the materials used in each mix as shown in Fig. 7(c). In the WS 100 sample, a clear transmittance pattern is observed, showing peaks typically associated with quartz and other silicate minerals. This is consistent with the composition of WS, which primarily consists of silica. The WS/EW 50/50 sample, containing 50% EW, exhibits slight variations in transmittance peaks compared to WS 100, indicating the presence of additional mineral phases from the EW. The broader bands suggest the introduction of clay and organic materials, which tend to absorb at lower wavenumbers. For the WS/CDW 50/50 sample, which includes 50% CDW, the FTIR spectrum shows peaks that are likely representing CSH phases and calcium hydroxide. These are associated with residual cementitious materials in CDW, contributing to the increased transmittance at certain wavenumbers.

3.10 Carbon footprint

The carbon footprint for producing 1 t of mortar from both sands is illustrated in Fig. 8. Cement production remains the largest contributor to the overall carbon footprint, emitting approximately 790 kgCO₂e/t of cement produced [51]. Water, while essential for the mortar mix, generates an average of 10.6 kgCO₂e/m³ consumed. The carbon footprint of sand, however, varies depending on its source. The CO₂ emissions associated with the production of WS amount to 1.7 kgCO₂e/t, whereas RS contributes 3.0 kgCO₂e/t due to its additional processing requirements. These findings were published in the implementation book for using recycled aggregate in the Ministry of environment in Qatar [52]. All other operational processes, including transportation, mixing, and related activities, are considered the same for both WS and RS. This assumption is made to isolate the impact of material production on carbon emissions, ensuring a consistent basis for comparison.

To determine the total CO₂ emissions (kgCO₂e) for 1 t of mortar production, the CO₂ emissions of each individual material are calculated by multiplying its unit CO₂ emission factor (kgCO₂e) by its respective percentage contribution to the mix as shown in Fig. 8(a). Figure 8(b) demonstrates that cement is the primary contributor to CO₂ emissions, accounting for over 98% of the total emissions across all categories. This dominance makes it evident that cement production significantly impacts the overall carbon footprint. Furthermore, the chart highlights that there is no notable difference in total CO₂ emissions between using WS and RS. This finding suggests that switching from WS to RS does not substantially affect carbon emissions. However, the use of RS offers considerable environmental benefits. By incorporating RS, the burden on landfill sites is reduced, minimizing the waste generated by construction and demolition activities. Additionally, RS helps conserve natural resources by replacing the need for WS, which is often overexploited in many regions. This approach supports sustainable construction practices and contributes to a circular economy by reusing materials. Although adopting RS does not significantly reduce CO₂ emissions, it plays a crucial role in preserving the environment by reducing landfill waste and conserving natural resources

4 Conclusions

This study evaluated the mechanical and durability performance of cement mortar incorporating RSs derived from EW, CDW, and CW as partial replacements for WS. Beyond summarizing the results, the findings provide practical guidance for engineers, material producers, and sustainability-focused construction projects.

1) WS delivered the highest performance across all measured parameters (CS, FS, WA, and UPV), confirming its suitability as a benchmark for comparison.

2) CDW sand exhibited consistent and reliable mechanical behavior up to 75% replacement, indicating strong potential for use in general-purpose mortar production without compromising performance.

3) EW sand, due to its high fines content and limited cementitious contribution, showed reduced strength at higher replacement ratios. However, it can be safely used at moderate levels (≤ 50%–75%) in non-structural or secondary applications where sustainability and cost reduction are prioritized.

4) CW sand performed favorably at lower RLs (up to 50%), making it suitable for moderate-strength mortar mixes, repair mortars, and applications with medium performance requirements.

5) From a practical standpoint, the results demonstrate that CDW and EW sands can replace WS by up to 75%, while CW is most effective at RLs up to 50%, enabling significant reductions in WS consumption, cost, and environmental impact.

Overall, the study provides evidence-based recommendations for selecting the appropriate type and proportion of RS depending on project requirements, making it directly applicable to sustainable construction practices and material optimization.

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