Influence of SiO2 /Na2O molar ratio on mechanical properties and durability of metakaolin-fly ash blend alkali-activated sustainable mortar incorporating manufactured sand

Manufactured sand (MS) is an optimal alternative for river sand used in sustainable construction materials to decrease the detrimental effects caused by excessive sand mining. In the current study, the mechanical performance and durability of alkali-activated sustainable mortar incorporating 100% MS and using various SiO2/Na2O molar ratios was investigated. The metakaolin-fly ash (MK-FA) blend activated by sodium-based alkaline liquid was used as the alkali-activated binder, and MS from crushed limestone was used as fine aggregate for the preparation of alkali-activated mortar (AAM) to ensure green and sustainable development. The properties of MK-FA based AAM using MS (MKF3A2MS) were measured including cube compressive, splitting tensile, flexural strengths, and modulus of elasticity, water permeability, and cracking resistance. The influence of molar ratio on mechanical performance and durability of MKF3A2MS were analyzed. An exponential fitting model for the compressive and tensile strengths of MKF3A2MS was proposed. In addition, analysis of variance (ANOVA) was adopted to discuss the statistic effect for the factors. The results demonstrate that the molar ratio of SiO2/Na2O has remarkable influence on mechanical performance and durability of MKF3A2MS. The compressive, splitting tensile and flexural strengths, and elastic modulus of MKF3A2MS first increased and then decreased, while the depth of water penetration and cracking index first decreased and then increased as the molar ratio was within the scope of 1.1–1.6. The mechanical properties and durability of MKF3A2MS using a molar ratio of 1.2 were optimal.