*4.2. Bulk Density*

Past studies on the impact of WG aggregates on the bulk density, which are summarized in Figure 2, revealed that the majority of studies showed that incorporating glass waste into mixtures reduces density. This decrease can be ascribed to the lesser density of WG compared to natural aggregate [42,65,93,94], as well as the lower specific gravity [43,66,87,93,95]. For example, Taha and Nounu [65] substitute fine aggregate in waste-glass concrete (WGC) mixes with recycled soda-lime glass at levels of 50% to 100% by volume. They stated that the fresh density of WG concrete mixes reduced by 1% and 2% incorporating 50% and 100% of WG, respectively. This density drop might be realized as one benefit of using this material in concrete for engineering purposes [96–99].

On the other hand, Liu, Wei, Zou, Zhou and Jian [56] stated that concrete of 10 to 50% WG had a fresh density greater than reference. The authors substitute F.A in UHPC mixes with recycling CRT glass at levels of 25% to 100% by volume. They stated that the fresh density of waste-glass concrete mixtures increased by 1% 2.5%, 3.5%, and 6%, incorporating 25%, 50%, 75%, and 100% of WG, respectively. The authors attributed the reason to the fact that the density of CRT glass (2916 kg/m3) was larger than that of fine aggregate (2574 kg/m3) [100–104].

**Figure 2.** Bulk density of concrete with various content of WG. Adapted from references [56,65,80,83,84,87,105–107].

#### **5. Mechanical Properties**

#### *5.1. Compressive Strength*

By reviewing past studies on the impact of WG aggregates on the compressive strength of waste-glass concrete, summarized in Table 6, it can be noticed that most studies shown that incorporating glass waste into concrete reduces compressive strength. The researchers ascribed this behavior to (i) the sharp edges and smooth particle surfaces, leading to a poorer bond between cement mortar and glass particles at the interfacial transition zone (ITZ) [14,40,42,43,55,66,87,90,108,109]; (ii) increased water content of the glass aggregate mixes due to the weak ability of WG to absorb water [43,110]; and (iii) the cracks caused by expanding stress formed by the alkali-silica reaction produced from the silica in WG [40]. For example, Park, Lee and Kim [89] substitute fine aggregate in WGC with recycled green WG at levels of 30% to 70% by weight. They stated that the compressive strength of concrete decreased by 3%, 13%, and 18%, incorporating 30%, 50%, and 70% of WG, respectively. In addition, Terro [48] noted that concrete, which contains up to 25% of WG, showed compressive strength values greater than the reference, whereas concrete with a substitution level of over 25% declined in compressive strength.

In order to better understand the impact of glass waste on the properties of the wasteglass concrete [111–114]. Omoding, Cunningham and Lane-Serff [115] investigated the concrete microstructure via SEM by replacing between 12.5–100% of the coarse aggregate with green waste glass with a size of 10–20 mm. The authors stated (i) that there is a weak connection between the waste glass and the cement matrix. This is because of a reduction in bonding strength between the waste glass and the cement paste because of the high smoothness of waste glass, consequently resulting in cracks and poor adherence between waste glass and cement paste; and (ii) as the content of waste glass increases, the proportion of cracks and voids increases in the concrete's matrix.

However, some studies have stated that waste glass increases mechanical strength. This increase is primarily realized because of the surface texture and strength of the waste

glass particles compared to natural sand [116–118] and the pozzolanic reaction of waste glass aggregate [119–121]. For example, Jiao, Zhang, Guo, Zhang, Ning and Liu [81] substitute fine aggregate in UHPC with recovered WG at levels of 25% to 100% by weight. They stated that the compressive strength of concrete increased by 2%, 17%, 34%, and 20%, incorporating 25%, 50%, 75%, and 100% WG, respectively.

Regarding the influence of WG color on properties, some studies have stated that the color of WG did not produce any noticeable variation in strength [89,122]. On the contrary, Tan and Du [66] claimed that clear waste glass showed less strength.

**Table 6.** Summary of the results of past studies on the compressive strength of waste-glass concrete.



**Table 6.** *Cont.*

Where: SCGC is self-compacting glass concrete; SCC is self-compacting concrete; HPGC is high performance recycled liquid crystal glasses concrete; HSPC is high-strength pervious concrete; UHPC is ultra-high performance concrete; LCDGC is liquid crystal display glass concrete; LCD is liquid crystal display; CRT is cathode ray tube; WG is waste glass; PVC is polyvinyl chloride; SP is superplasticizer; HRWRA is a high-range water-reducing agent; WR is water-reducing; AE is air-entraining; SF is silica fume; F.A. is fly ash; GBFS is granulated blast furnace slag; MK is metakaolin; SH is sodium hydroxide solution; SS is sodium silicate solution; F.A is fine aggregate; C.A is coarse aggregate; vol. is replacing by volume; wt. is replacing by weight.

#### *5.2. Splitting Tensile Strength*

Past studies on the impact of WG aggregates on the splitting tensile strength of wasteglass concrete, which are summarized in Table 7, revealed that incorporating glass waste into concrete reduces tensile strength. Similarly, as in compressive strength, studies have attributed the main reason for this behavior to the poor bond between cement paste and glass particles at the ITZ. For example, Wang [72] substitutes fine aggregate in liquid crystal display glass concrete (LCDGC) with recycled LCD glass at levels of 20% to 80% by volume. The author stated that splitting tensile strength of concrete decreased by 1%, 7%, 8%, and 9%, incorporating 20%, 40%, 60%, and 80% of WG, respectively, for w/c of 0.44. Moreover, Ali and Al-Tersawy [55] substitute fine aggregate in self-compacting concrete (SCC) with recycled WG at levels of 10% to 50% by volume. They stated that tensile strength of wasteglass concrete decreased by 9%, 15%, 16%, 24%, and 28% incorporating 10%, 20%, 30%, 40%, and 50% of WG, respectively [129–132].

In contrast, Jiao, Zhang, Guo, Zhang, Ning and Liu [81] indicated that concrete of 25% to 100% WG had a tensile strength greater than reference. The authors substitute fine aggregate in ultra-high-performance concrete (UHPC) with recycled WG at levels of 25% to 100% by weight. They stated that the splitting tensile strength of concrete increased by 1%, 3%, 11%, and 7%, incorporating 25%, 50%, 75%, and 100% of WG, respectively. The author attributed the reason to the effect of using steel fibers.

**Table 7.** Summary of the results of past studies on the splitting tensile strength of waste-glass concrete.



**Table 7.** *Cont.*

Where: UHPC is ultra-high-performance concrete; LCDGC is liquid crystal display glass concrete; LCD is liquid crystal display; CRT is cathode ray tube; WG is waste glass; SP is superplasticizer; HRWRA is a high-range water-reducing agent; SF is silica fume; F.A. is fly ash; F.A is fine aggregate; C.A is coarse aggregate; vol. is replacing by volume; wt. is replacing by weight.

#### *5.3. Flexural Strength*

The flexural strength of waste-glass concrete shows comparable tendencies to its compressive strength and tensile strength. Most of the research revealed that introducing WG aggregates reduced flexural strength. However, other research showed that flexural strength increased when WG was included [134–136]. For instance, Kim, Choi and Yang [79] substitute fine aggregate in WGC with recycled CRT glass at levels of 50% to 100% by volume. They stated that flexural strength of concrete decreased by 9% and 14%, incorporating 50% and 100% of WG, respectively, for w/c of 0.45. On the contrary, Jiao, Zhang, Guo, Zhang, Ning and Liu [81] substitute fine aggregate in UHPC with recovered WG at levels of 25% to 100% by weight. They stated that flexural strength of concrete increased by 2%, 1%, 5%, and 1%, incorporating 25%, 50%, 75%, and 100% of WG, respectively.

Moreover, it can be concluded that the discrepancy between studies may be related to the type, size, and source of WG used in the mixtures. The mineral composition varies as the type of glass changes. Therefore, changing the mechanisms of interaction with binders in concrete, in turn, affects the properties. Table 8 presents the outcomes of various studies on the flexural strength of waste-glass concrete.




**Table 8.** *Cont.*

Where: SCGC is self-compacting glass concrete; SCC is self-compacting concrete; UHPC is ultra-high-performance concrete; LCDGC is liquid crystal display glass concrete; LCD is liquid crystal display; CRT is cathode ray tube; WG is waste glass; SP is superplasticizer; HRWRA is a high-range water-reducing agent; WR is water-reducing; AE is air-entraining; SF is silica fume; F.A is fine aggregate; C.A is coarse aggregate; vol. is replacing by volume; wt. is replacing by weight.

#### *5.4. Modulus of Elasticity (MOE)*

The modulus of elasticity of concrete (MOE) depends on the normal and lightweight aggregates elasticity modulus, cement matrix, and their relative ratios in the mixes [39]. In general, the incorporation of WG aggregates into concrete increases the modulus of elasticity [72,84]. For instance, Steyn, Babafemi, Fataar and Combrinck [82] substitute fine aggregate in WGC with recovered WG at levels of 15% to 30% by volume. They stated that MOE of concrete increased by 1%, and 7%, incorporating 15% and 30% of WG, respectively. In addition, Omoding, Cunningham and Lane-Serff [115] substitute coarse aggregate in glass aggregate concretes with recycled WG at levels of 12.5% to 100% by volume. They stated that MOE of concrete increased by 2% to 4% for a replacement rate of 12.5% to 50%, then decreased by 3% to 9% for replacement ratios above 50% [137,138].

However, some studies have stated that including WG decreases the MOE of concrete. For instance, Ali and Al-Tersawy [55] substitute fine aggregate in SCC with recovered WG at levels of 10% to 50% by volume. They stated that MOE of concrete decreases by 2%, 8%, 9%, 12%, and 13%, incorporating 10%, 20%, 30%, 40% and 50% of WG, respectively. Figure 3 presents the outcomes of various studies on the MOE of WG concrete.

**Figure 3.** Modulus of elasticity of concrete with various contents of the waste glass. Adapted from references [55,65,72,82,84,88,107,115].
