*6.1. Mechanical Properties of Cement-Based Materials Containing Waste Glass as Natural Aggregate Replacement*

#### 6.1.1. Workability

WG's effect on the workability of fresh concrete has been reported to be inconsistent in previous studies. Partial substitution of sand with WG improved workability. For instance, Elaqra et al. [79] used a soda-lime glass (size: <20 μm) and found that increasing the replacement percentage from 0% to 30% increased the slump from 130 mm to 190 mm. However, the utilization of WG decreased the workability of concrete. For example, an electric glass (size: <150 μm) reduced the slump from 200 mm to 45 mm as the amount of WG increased from 0% to 40% [80]. The above-mentioned inconsistencies in WG's effect on the workability of composites are the result of two competing effects. Firstly, glass improves workability by reducing water adsorption and friction due to its dense microstructure and smooth surface. Indeed, WG has been used to create SCC that is vibration-free during construction [81–85]. Secondly, glass can reduce workability due to extremely fine glass particles; the surface-to-volume ratio increases, increasing water adsorption. Thus, the size of the glass particles is a critical parameter in determining their effect on workability. However, if coarser glass particles are utilized, the irregular shape of the glass particles improves mechanical interlocking among adjacent particles and ultimately reducing the workability [86]. This is consistent with Topcu and Canbaz's [39] findings, i.e., the addition of coarse glass particles (size: 16 mm) decreased the workability of the fresh mix. With increasing glass particle size, the surface-to-volume ratio decreases, resulting in a small quantity of paste or mortar adsorbed on the surface of glass particles for lubricating nearby glass particles. This effect of under-lubrication may have been facilitated by the glass particle's smooth and dense surface.

#### 6.1.2. Compressive Strength

According to the literature, there is a detrimental effect on CS of composites containing WG as a partial or complete substitute of NA, as shown in Figure 10. Mostly, a decreasing trend is observed with the increasing replacement ratio of WG. However, using a smaller size WG at a lower replacement ratio can enhance the CS by filling the voids in the matrix. Liu et al. [87] found a decrease in CS by 2.9%, 10%, and 15.7% when coarser WG (5–10 mm) was used at 10%, 20%, and 30% content, respectively. Though, using finer WG (size: <4.75 mm), the reduction in CS was minimal. They reported two primary explanations for this decline. Firstly, the WG aggregate has a lower strength than the NA aggregate, and secondly, the smooth surface of the WG aggregate affects its binding with cement paste, reducing the strength of the concrete. A similar pattern of reduction in CS with the increasing amount of WG in composites as an aggregate replacement was also noted by other researchers [88–90]. Conversely, Ismail and Hashmi [91] and Abdallah and Fan [92] found a 4.3% and 4.9% increase in CS, respectively, when fine aggregate was replaced at 20% by WG. Similarly, Malik et al. [93] investigated the CS of composites containing finer WG particles (size: <1.18 mm) at various replacement ratios. The results revealed that composites containing 10%, 20%, and 30% WG enhanced the CS by 20.0%, 25.1%, and 9.8%, respectively, compared to the reference mix. However, at 40% replacement ratio, the CS decreased by 8.5% than the reference mix, as shown in the figure. Thus, the use of finer WG at a lower replacement ratio could enhance the CS by filling voids in the matrix, while coarser WG reduces the CS because of weak ITZ between WG aggregate and cement matrix.

**Figure 10.** Effect of waste glass as a natural aggregate replacement on 28-days compressive strength.

#### 6.1.3. Split-Tensile Strength

The WG usage as an NA replacement also has an unfavorable effect on the STS of composites, as depicted in Figure 11. The STS decreases with the increasing replacement ratio of WG, as reported by most researchers [87,90,93,94]. Liu et al. [87] reported a decrease in STS than that of reference mix by 8.5%, 14.1%, 21.1%, and 25.4% when WG replaced natural fine aggregate at 25%, 50%, 75%, and 100% ratios, respectively. Song et al. [90] found a decrease in STS compared to control mix by 2.3%, 2.3%, 5.2%, 6.8%, and 10% when NA was replaced by 20%, 40%, 60%, 80%, and 100% WG, respectively. However, Abdallah and Fan [92] and Malek et al. [95] found an increase in STS of WG composites than NA composites using lower content (up to 20%) of WG. It can be concluded that WG with smaller particle size and used in lower proportions positively influences the STS of composites.

**Figure 11.** Effect of waste glass as a natural aggregate replacement on 28-days split-tensile strength.

#### 6.1.4. Flexural Strength

Figure 12 depicts the influence on FS of composites with increasing WG content as the NA replacement. It shows that at lower WG content, the FS of composites can be improved, while higher WG content results in decreasing FS. However, the reduction in FS is minimal compared to CS and STS. Wang and Huang [88] found improvement in FS of 14.7% at a 10% replacement ratio, while a further increase in WG decreased FS compared to the reference sample without WG. The study of Ismail and Hashmi [91] and Abdallah and Fan [92] reported improvement in FS of WG aggregate composites in comparison with the NA composites. The improvement in FS with WG addition may be attributed to the pozzolanic properties of glass, which helped to improve the microstructure of the matrix [91]. Kim et al. [96] and Sikora et al. [97] observed a drop in FS of WG composites in comparison to the NA composites. The reduction in FS was more at higher WG content as aggregate replacement. This could be because the smooth surface of WG relative to the NA having lower adhesion to the surrounding matrix than that of NA. However, Malek et al. [95] reported increasing FS with increasing WG percentage up to 20% with an increment of 5%. At a 20% replacement ratio, the FS increased by 14.3%. The reason is the use of fine WG (size: <2 mm), which helped improve the microstructure by filling pores in the matrix. Thus, the use of WG can improve the properties of composites if used in smaller sizes and lower replacement ratios. This is because the finer WG can improve the microstructure by filling voids in the matrix [93]. In addition, a higher replacement ratio and a larger size of WG used as NA replacement reduced the properties of composites. This may be attributed to the more glass-matrix interfaces, and the smooth surface of WG may reduce the interfacial bond strength, resulting in decreased strength of composites [90].

**Figure 12.** Effect of waste glass as a natural aggregate replacement on 28-days flexural strength.
