## 3 Discussion of Results
### Effect of Glass on Compressive Strength of Mortar and Concrete  
From Figure 8, the compressive strength of the cement-glass blended mortar can be seen to decrease with glass powder content. Glass powder affected both early and later day strength development. The 3-day strength in cement mortar decreased by 24.2%, 31.4%, 39.71% and 53.1% in the mortars MG5, MG10, MG15 and MG20, respectively. The maximum 28-day compressive strength obtained at zero inclusion of glass in mortar was 40 MPa and reduced by 7.5%, 10%, 15% and 45.5% for glass-cement replacement levels of 5%, 10%, 15% and 20%, respectively. Highest strength development (43.5%) was noticed between 7 to 14 days in MG15 and then followed by 36.7% in MG10 with just 5% in MG15. There was no significant strength development from 14 to 28 days in the glass blended mortar sample except in MG5, where the maximum development (23.3%) was achieved. The optimum glass dosage for cement replacement was between 5 to 10 wt.% (by weight of the total binder).  
Figure 5: (**a**) Variation of flow with glass powder content in mortar. (**b**) Variation of flow with glass powder content in concrete.  
Compressive strength in mortar is modeled as shown in Equation (2) with a correlation coefficient of 0.88, as shown in Figure 9. The compressive strength depends on the cement content (C), glass content (G) and age of the mortar.  
\[f_{c_{mater}}=-1248.3+790.15\mathrm{C}^{0.0106}-5.7\times 10^{-33}\mathrm{G}^{1 2.44}+150.63\mathrm{Age}^{0.04} \tag{2}\]  
Figure 6: Effect of glass on the density of mortar.  
Figure 7: Experimental and predicted density value of glass blended cement mortar.  
Figure 8: Variation of strength with glass powder in mortar.  
Figure 9: Experimental and predicted value of glass blended cement mortar.  
From Figure 10, the compressive strength of concrete increases with days of maturity. Concrete sample C\({}_{80}\)G\({}_{20}\) has the best strength characteristics, and its maximum 3-day early strength was 22 MPa, which was 27.2% of the achievable strength in C\({}_{100}\)G\({}_{0}\) (glass-free). This decreased to 37.2% and 60.3% in C\({}_{90}\)G\({}_{10}\) and C\({}_{70}\)G\({}_{30}\), respectively. The strength development increased linearly with a 90-day strength of 63 MPa in C\({}_{100}\)G\({}_{0}\) and 47 MPa in C\({}_{80}\)G\({}_{20}\) (20% glass). The strength achieved in this sample could be regarded as high-strength concrete. As the glass contents of 10 wt.% (C\({}_{90}\)G\({}_{10}\)) and 30 wt.% (C\({}_{70}\)G\({}_{30}\)) were added, the strength reduced by 22.8% and 34.7%, respectively.  
The rate of strength development in C\({}_{70}\)G\({}_{30}\) is higher (48.3 wt.%) between 21-28 days in comparison with 56-90 days, where 9.2% only was achieved. Similarly, C\({}_{80}\)G\({}_{20}\) developed 3.35% between 21-28 days, whereas the development was 19.75% between 56-90 days. This implies that there were significant pozzolanic reactions in glass blended concrete at a later time beyond 56 days. Therefore, we can conclude that glass present in OPC concrete underwent a delayed pozzolanic reaction, thereby corroborating the hydration of belite (C\({}_{2}\)S).  
The model predicting the concrete strength in concrete is given in Equation (3) with a correlation coefficient of 0.85, as shown in Figure 11. The model is developed to predict compressive strength at 3, 7, 14, 28, 56 and 90 days for varying glass/cement ratios (G/C) and ages at a constant water/binder ratio of 0.42.  
\[f_{c\_concrete}=-0.3-24.22(\mathrm{G/C})^{0.282}+29\mathrm{Age}^{0.153} \tag{3}\]