*6.2. Mechanical Properties of Cement-Based Materials Containing Waste Glass as Cement Replacement*

6.2.1. Workability

The finer/powder WG is usually used as cement replacement. Islam et al. [98] performed a flow test on mixes of WG powder mortar. Water/binder ratio (w/b) was kept 0.5 for mix preparation. The findings indicated that as the percentage of WG as cement substitute increased, the flow diameter increased. The flow diameter of the reference mix was 132.5 mm, while the flow diameter of mortar samples containing 25% WG as cement replacement was 135 mm. As a result, a slight increase in flow was observed. Aliabdo et al. [51] evaluated the workability of WG powder-modified concrete using a slump test. It was observed that the slump of mix containing WG powder as a substitute for cement improved as the WG powder content increased. The smooth surface and minimal water absorption capacity of WG powder may contribute to the slump increase. Additionally, WG powder contains coarser particles than cement, which might have caused the improvement in a slump. Soliman and Tagnit-Hamou [99] also demonstrated that incorporating WG powder in place of cement increased the workability of concrete, which may be due to the low water absorption and smooth texture of WG powder than the cement particles. Another factor contributing to increase the workability is the dilution of cement. The reasons outlined above account for the reduction in the formation of hydration products of cement during the initial time. As a result, there is an insufficient number of products available for combining disparate particles. As WG powder has a smaller specific surface area than cement, the total surface area of the cement and WG powder mixture is reduced. Therefore, it decreased the water requirement for particle surface lubrication and resulted in an increased slump.

#### 6.2.2. Compressive Strength

Islam et al. [98] performed a CS test on mortar specimens containing recycled WG as cement replacement. Compared to controlled mortar specimens, recycled WG mortar had a lower CS at the age of 7, 14, 28, and 56 days. At 90 days, an increase in CS was observed; the highest CS was obtained with a 10% cement replacement. Similarly, 15% cement replacement at 180 days and 20% cement replacement at 365 days exhibited maximum CS. The reason could be the pozzolanic behavior of glass, which reacted slowly and improved the microstructure of the matrix at later ages and resulted in improved CS. Figure 13 is generated based on the past studies depicting the variation in 28-days CS with increasing WG content as cement replacement. A slight increase in CS can be observed at lower replacement ratios. Rehman et al. [82] used WG powder as cement replacement (20%, 30%, and 40%) and steel slag as a fine aggregate replacement (40%, 60%, and 80%) in SCC and investigated their influence on MPs. They observed an increase in CS when 20% cement is replaced by WG powder, but it decreased as the WG powder content is increased further. When the proportions of all other ingredients were constant, increasing the steel slag content increased the CS of SCC. At constant WG powder content, the CS of concrete improved as the steel slag content increased. The maximum increase in CS was observed by 11% in comparison with the control specimen when 20% WG powder was used in place of cement, and 80% steel slag was used in place of fine aggregate. On the other hand, there was a slight decrease in CS of SCC as the WG powder content increased while the steel slag content was kept constant. The minimum CS was 5.7% lower than the control specimen when WG powder and steel slag were used in place of 40% cement and 40% fine aggregate, respectively. The increase in CS with the addition of steel slag could be attributed to the pozzolanic action of steel slag or the difference in hardness between steel slag and the replaced aggregates. Al-Zubaid et al. [100] studied the effect of brown, green, and neon glass on MPs of concrete used as cement replacement by 11%, 13%, and 15%. The best results of CS were observed with neon glass at 13% content due to the high concentration of SiO2 (68%) in neon glass, combined with the high CaO content (66.11%) in cement, and their combination with water formed a significant amount of CaCO3 during the hydration process. Anwar [101] also observed improvement in CS at lower content of WG powder. At 10% WG powder content, the CS improved by 16.6% than the reference sample. The increase in CS occurred due to the pozzolanic reaction of glass powder. Because the glass powder acts as a pozzolanic material, it reduces the effect of carbonation and increases the strength of concrete. Thus, the smaller particle size of the glass powder interacts more readily with the lime in the cement, resulting in increased CS in the concrete. Aliabdo et al. [51] reported a 5.1% increase in CS for 33 MPa concrete containing 5% WG powder in place of cement when compared to the reference mix. Whereas CS decreased with further addition of WG powder, as shown in the figure. Additionally, the CS of concrete mix grade 45 MPa increased by 2.5% at 5% replacement and 4.8% at 10% replacement compared to the reference mix. When more than 10% of the cement was replaced by WG powder, a decrease in CS was observed; this decrease could be attributed to the increased percentage of cement replacement, which resulted in cement dilution. A similar trend was also observed from various studies with the use of WG as cement replacement [102]. Hence, WG powder as cement replacement is preferable only at lower replacement ratios.

**Figure 13.** Effect of waste glass as cement replacement on 28-days compressive strength.

#### 6.2.3. Split-Tensile Strength

The effect of using WG as cement replacement on STS has been shown in Figure 14. It also indicates that at lower content of WG, the STS can be increased while higher content of WG decreases the STS compared to the reference samples without WG. Similar to the CS, Rehman et al. [82] noted that the maximum improvement in STS was 13.2% when 20% of cement was replaced with WG powder, and 80% of cement was replaced with steel slag. The minimum STS was 5.6% less than that of the control mix when 40% steel slag and 40% glass powder were used as cement and aggregate replacements, respectively. Whereas, Al-Zubaid et al. [100] mostly found a decrease in STS with the addition of different types of WG in concrete. However, using green glass at 13% replacement of cement showed improvement in STS by 16.2% than the control mix. Aliabdo et al. [51] described enhancement in STS by 16.6%, 19.4%, and 5.9% for 33 MPa concrete containing 5%, 10%, and 15% WG powder, respectively, when compared to the control mix. Whereas STS decreased by 10% and 13.8% when 20% and 25% WG powder were substituted for cement in a 33 MPa concrete mix, respectively. Additionally, for 5%, 10%, and 15% replacement in 45 MPa grade concrete, the STS increased by 11.7%, 13.0%, and 18.1%, respectively. Whereas, at 20% and 25% replacement, a slight decrease in STS of 1.0% and 2.3%, respectively, was observed. STS decreases when more than 20% of cement is replaced with WG powder. The reasons for the improvement in STS at lower WG contents and reduction in STS at higher WG contents are the same as described earlier for CS.

#### 6.2.4. Flexural Strength

The influence of WG powder as cement replacement on the FS of composites has been displayed in Figure 15. It also shows an almost similar trend as CS and STS. For instance, the results of Rehman et al. [82] showed enhanced FS at 20% and 30% contents of WG powder while at 40% content of WG powder, the FS reduced compared to the reference sample. The highest value of FS was observed at 20% content of WG powder as cement replacement and 80% steel slag as fine aggregate replacement. Similar to the CS, the maximum FS was observed with neon glass at a 13% replacement ratio [100]. Also, the addition of WG with up to 20% content exhibited improvement in FS while a further increment in WG content reduced the FS than the reference sample [101]. Hama [103] also reported maximum FS at 20% replacement of cement by WG powder. However, several studies reported a reduction in FS with the utilization of WG powder as cement replacement [100,104], as shown in the figure. The reasons for the CS behavior of composites with WG addition also apply to the flexural behavior.

**Figure 14.** Effect of waste glass as cement replacement on 28-days split-tensile strength.

**Figure 15.** Effect of waste glass as cement replacement on 28-days flexural strength.

#### *6.3. Microstructure of Cement-Based Materials Containing Waste Glass*

The microstructure study of various past studies revealed that the size of WG used as aggregate replacement greatly influences the ITZ and porosity of the matrix. Afshinnia and

Rangaraju [105] found weak glass-matrix ITZ and high porosity when coarser WG was used as an aggregate replacement, compared to the NA concrete, as depicted in Figure 16. As illustrated in the figure, WG aggregates have a smooth particle surface that interfaces with the cement paste, resulting in minimal mechanical interlocking between the two phases. The weak bond between WG and cement matrix is also reflected in the mechanical characteristics of composites. Conversely, the use of finer WG as an aggregate replacement can improve the microstructure of composites [105]. Soliman and Hamou [106] performed SEM analysis to study the microstructure of composite containing 50% quartz sand and 50% recycled WG with a mean particle size of 275 μm. They stated that the bond among WG and cement matrix is comparable with that among quartz and cement matrix, as shown in Figure 17. Thus, the microstructure study also supports the use of finer WG as aggregate replacement. Kong et al. [107] examined the effect of WG powder on the microstructure under various curing conditions. The study demonstrates that WG powder exhibits strong pozzolanic reactions when cured in microwave or steam rather than under standard curing conditions. Matos and Sousa-Coutinho [108] conducted SEM analysis to study the microstructure of mortar containing 10% WG powder as cement replacement and compared it with the control mix as depicted in Figure 18. The glass particles appear to have been completely compressed and scattered within the hydration products of a compact, dense, and mature gel containing needle-shaped ettringite crystals (Figure 18c,d). It was seen that the C–S–H gel in samples containing WG powder has more calcium as well as more alkalis compared to the reference sample due to the pozzolanic properties of WG powder. This is the reason for having improved MPs of composites with WG powder.

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## *6.4. Durability of Cement-Based Materials Containing Waste Glass*

The durability of concrete is inextricably linked to its permeability [109]. Water permeability was increased by partially replacing cement with WG. A concrete sample having a 25 mm penetration depth of water when 30% of the cement was substituted with soda-lime WG (size: <120 μm), decreased the penetration depth of water to 9 mm; however, when 60% of the cement was substituted with WG, the penetration depth of water decreased it to 5 mm [110]. By substituting 60% natural sand for flint WG (size: <4 mm) in concrete with a 20 mm penetration depth of water, the penetration depth of water was decreased to 16 mm [111]. The use of finer WG reduced chloride penetration [79,102,112,113], while coarser WG showed less resistance to chloride penetration [114]. The percentage of water absorption reduced as the amount of WG in CBMs increased. When 20% WG (size: 100 μm) was used in place of cement, the percentage of water absorption ratio decreased from 4.6% to 3.2% [115]. Likewise, by substituting 25% cement for WG (size: <80 μm), the percentage

water absorption was decreased from 6.2% to 3.8% [116]. When CRT WG (size: <5 mm) was used as sand substitute at 0%, 50%, and 100% the water absorption ratios were 7.3%, 7.0%, and 6.4%, respectively [96]. To replace coarse aggregate, WG (size: 3–16 mm) was used, and the percentage of water absorption decreased from 6.0% to 2.5% as the proportion of WG increased from 0 to 75%. Hence, consistent results revealed that proper usage of WG contributes to the reduction of CBMs' permeability and the impediment of detrimental elements transport in CBM. This enhancement is a result of the synergy of several effects, including the pozzolanic and filler effects that enhance the hydration process and improve the microstructure, thereby decreasing permeability. Simultaneously, appropriate glass gradation can increase the packing density of particles, thereby lowering the permeability even further.

**Figure 17.** Microstructure of composite containing quartz and glass sand (glass mean particle size = 275 μm) [106].

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**Figure 18.** *Cont.*

**Figure 18.** Microstructure of composites: (**a**) control mix, 10,000 times enlarged; (**b**) control mix, 20,000 times enlarged; (**c**) containing 10% waste glass powder, 10,000 times enlarged; (**d**) containing 10% waste glass powder, 20,000 times enlarged [108].

The sulphate immersion test was used to observe the resistance of CBM to sulphate attack. After five wet/dry cycles, the mass loss of concrete was determined. The mass loss was measured to be 0.8% for concrete with a w/b of 0.68; however, when 40% cement in concrete was replaced by WG (size: <150 μm), the loss in mass was decreased to 0.2% [81]. The anti-sulfate attack test revealed a consistent beneficial phenomenon. A WG (size: <4.75 mm) was used in place of natural sand in concrete, having a w/b ratio of 0.55. With increasing proportion of WG from 0% to 80%, the concrete's five-cycle mass loss decreased from 11% to 4% [117]. This increase in resistance to sulphate attack can be attributed to the concrete's refined microstructure, resulting from the pozzolanic reaction and filler effect of finer WG. The sulphate ion degrades CBMs by reacting with portlandite and forming gypsum, which sequentially forms expansive ettringite, which can crack the matrix of CBM [117]. A compact and improved microstructure prevents the passage of the sulphate ion, thereby increasing the resistance to sulphate attack.

Concrete with a w/b of 0.5 and cement was replaced by WG (size: 0.1–40 μm) at 0%, 10%, and 20%, proportions used to investigate the influence of WG on the depth of carbonation. At four months, the carbonation depth increased from 3 mm to 8 mm as the proportion of WG increased from 0% to 20% [108]. Similar observations were described in [118] when CRT WG (size: <5 mm) replaced the natural sand up to 100% in the production of heavyweight barite concrete with a w/b of 0.48. Depth of carbonation rose from 7.5 mm to 11.5 mm as the proportion of WG increased from 25% to 100%. The available data indicate that the use of WG degrades carbonation resistance. This is primarily due to the fact that glass reacts with Ca(OH)2 contained within concrete. The Ca(OH)2 aids in the delay of CO2 diffusion, and glass consumption of Ca(OH)2 speeds up carbonation. It is important to mention here that the studies cited above were performed on standard concrete. It is reasonable to consider that the detrimental impact of WG for resistance to carbonation can be alleviated or eliminated through microstructure refinement and low porosity, as concrete's diffusivity is decreased. There are numerous ways to decrease concrete's diffusivity, including increasing the packing density, decreasing the w/b, and using fillers.

Concrete's freezing-thawing resistance was increased by partially replacing cement with WG. Cement in concrete was replaced by electric WG (size: 13 μm), and loss in mass of the sample was reduced by 30% after 310 freezing-thawing cycles [119]. Similar findings were published in [120,121]. Cement was replaced by WG (size: 75 μm) at proportions of 5%, 10%, and 15%. It was found that as the proportion of WG increased from 0% to 10%, the loss in mass decreased monotonically, implying that the WG enhances the resistance to freezing-thawing, most possibly because of the pozzolanic reaction and filler effect. However, as the amount of WG increased from 10% to 15%, the loss in mass increased because of the dilution effect [120]. The dilution effect is related to the concrete's w/b. Due to the significant increase in the w/b caused by substituting an extreme quantity of cement with WG, the additional water raises the porosity and results in the dilution effect. Though it is envisioned that in high-performance concrete or ultra-high-performance concrete with a very low w/b, a greater amount of cement can be substituted by WG without a dilution effect.
