*3.4. Compressive Strength*

The experimental test for compressive strength was carried out at different curing ages of 7, 14, and 28 days. Figure 9 shows the impact of EGA and TiO2 inclusion on the compressive strength of the mortar composites at different ages. It is observed that the inclusion of the EGA significantly decreased the compressive strength of cement mortar. The results of 28-day compressive strength demonstrated that 50% replacement of NA with EGA reduced the strength about 65.8% in compare to the control sample (C.S). In addition, it was observed that as the EGA content increased from 50% to 100%, the compressive strength dropped dramatically from 26.25 to 8.20 MPa at the age of 28 days. It is noteworthy that the compressive strength was still in the acceptable range and similar or higher than reported results in the literature [21,25]. Namsone et al. [25] reported the 28-day compressive of 5.7 MPa for a foamed matrix using EGA and obtained the compressive strengths of 6.68-12.49 MPa for the EGA cement mortar. Indeed, the samples containing 100% EGA without nTiO2 had the lowest compressive strength out of all the mixes.

Furthermore, the results indicated a normal increasing trend for the compressive strength for CS, E50, and E100 mixes as the curing process progresses. However, the mixes containing nTiO2 revealed a relatively different strength development tend. It was revealed that CT, E50T, and E100T mixes reached to 84.6%, 87.2%, and 77.2% of maximum strength within 7 days of curing while for samples without nTiO2 (CS, E50, and E100 mixes) it happened at 14 days of curing. This behavior was attributed to the addition of nTiO2 into the cementitious materials, which resulted in an accelerated rate of hydration process. A similar attribute has been reported in previous studies that when nTiO2 is uniformly distributed throughout the matrix, the hydration process and formation of C-S-H gel is accelerated, which results in early strength [32,47,48]. In the other set of mixes, the effect of nTiO2 inclusion on the compressive strength of mixes was investigated after a different curing time. The compressive strength results of E50T and E100T mixes at 28 days showed the similar trend. It was observed that the addition of EGA significantly decreased the compressive strength and the strength significantly dropped as the EGA content increased however, inclusion of nTiO2 compensated some part of the compressive

strength. The average compressive strength at 28 days of CT, E50T, and E100T mixes were 76.72, 29.70, and 11.4 MPa respectively, which shows 1.7%, 13.1%, and 39.0% enhancement in comparison to CS, E50, and E100 mixes respectively. It can be concluded that nTiO2 acts as nanofillers in specimens and recovers their pore structure by decreasing voids and pores in the composite matrix [46].

**Figure 9.** Compressive strength of samples at different ages.

In summary it can be concluded that the compressive strength and water absorption of concrete are highly influenced by the density of the mix. The results revealed an interrelationship between density and compressive strength. It was observed that the compressive strength dropped by decrement of the sample's density (CS, E50, and E100 mixes). Similarly, an increase in density for CT, E50T, and E100T mixes resulted in an increase in compressive strength compared to CS, E50, and E100 mixes respectively. Moreover, the results demonstrated an inverse relation between density and water absorption. It was found that water absorption increased by decreasing the density of the mixes in case of CS, E50, and E100. However, the water absorption decreased by integration of TiO2 into the mixes (CT, E50T, and E100T) and increment of density due to a lower porosity of the matrix.
