Effect of relative GGBS/fly contents and alkaline solution concentration on compressive strength development of geopolymer mortars subjected to sulfuric acid

Abstract

The effect of submerging geopolymer mortar samples in highly acidic solution for 7-, 28-, and 90-days on stability of mass and the development of compressive strength development was assessed experimentally. The mortar binder consisted of GGBS or blends of GGBS and fly ash activated using combinations of NaOH and Na2SiO3 solutions, and samples were cured in room temperature. It was found that maintaining mortar samples continuously under sulfuric acid doesn’t cause reduction compressive strength or mass from one age to the other, up to 90 days. While decalcification, delaumination, and formation of calcium salts due to sulfate attack may have affected mass and strength, submerging samples under water supported formation of geopolymerization products C-A-S-H and N-A-S-H, and consequently increased the mass and compressive strength of cubic mortar samples with fly ash + GGBS blended binder. The resistance of mortar to sulfuric acid remained consistent when mortars were prepared using GGBS:fly ash ratio of 3:1, equal amounts of GGBS and fly ash, and GGBS as sole binder. When geopolymer mortar samples made with each of the three binders was left exposed to air after casting, compressive strength increased from 7- to 28-days after casting, but at 90-days, all mortar samples experienced decrease in compressive strength relative to the 28-day values. The relatively high content of GGBS (≥ 50%) and absence of curing water in relatively dry conditions caused shrinkage cracking and decrease in compressive strength.