Durability and strength enhancement of engineered mortars with and without nanosilica under sulfate and acid attacks

In this study, the influence of nanosilica (NS) on the mechanical properties and chemical resistance against sulfate and sulfuric acid attack of lightweight engineered geopolymer mortars (EGMs) and engineered cementitious mortars (ECMs) is studied. Expanded glass was employed as a lightweight aggregate for producing fly ash–based, slag-based, and hybrid fly ash–slag geopolymer systems, as well as for typical cement-based control specimens for comparison. For uniform dispersion and better matrix densification, Nanosilica was dispersed ultrasonically. The mortars were tested for workability, density, compressive strength, mass loss, and microstructural study. The outcomes indicate that the application of nanosilica as an additive positively influences compressive strength, matrix densification, and durability in all the geopolymer systems, with lower porosity and permeability. Compared to ECMs, strength retention and mass change were significantly greater when NS-modified EGMs were exposed to sulfate and acid, confirming their superior chemical resistance. The slag-based EGM with nanosilica reached the highest strength and durability, but the hybrid fly ash–slag system ensures adequate strength, lower density, and reasonable durability. In conclusion, the results also suggest that the combined effect of nanosilica and geopolymer binder composition provides a pathway to achieve lightweight and low-density, durable, and sustainable alternatives to conventional cementitious materials for severe environments.