Investigating the sulfate resistance of mortars with multiple mineral admixtures in ammonium–magnesium sulfate solution

Sulfate resistance is evaluated in mortars containing sulfate-resistant cementitious materials (SRCMs) developed by mixing ground granulated blast furnace slag, fly ash, silica fume and desulfurisation gypsum. Compressive strength testing, X-ray diffraction analysis, differential thermal analysis and mercury intrusion porosimetry are carried out. Results show that the sulfate resistance of mortars mixed with SRCMs at a replacement percentage of 74 wt% is superior to that of mortars with 30 wt% fly ash when exposed to drying–wetting cycles in sodium sulfate solution, because adding SRCMs decreases the calcium hydroxide content, causing a reduction in gypsum formation. Moreover, reducing the water-to-binder ratio (W/B) from 0.50 to 0.35 increases the loss of compressive strength ratio (L_f) of mortars with SRCMs immersed in ammonium–magnesium sulfate complex solution. Specifically, the L_f values of mortars with W/B of 0.50 and 0.35 are 42.7% and 36.0% after 100 days of immersion, respectively. Furthermore, the main component of samples subjected to complex solution is identified as gypsum. In addition, both L_f and porosity exhibit a strong linear positive correlation with W/B. Finally, the findings confirm that optimising the composition of cementitious materials and lowering W/B could improve the sulfate resistance of concretes used for a sulfate-rich sewage environment.