Influence of Fly Ash Content on the Durability of Mortar Specimens under Dry/Wet Sulfate Attack

To investigate the durability of cementitious materials under complex environmental conditions in Xinjiang, this study conducted durability tests on mortar specimens with different fly ash contents under dry/wet sulfate attack conditions, with standard curing and steam curing at 70 °C. The appearance loss and flexural and compressive strength variations in the specimens were analyzed, and an evolution model of the mortar strength under a dry/wet sulfate attack was established. Moreover, XRD and SEM techniques were used to characterize the erosion products and microstructure, and to explore the erosion resistance mechanism of fly ash cementitious materials. The results showed that, after 160 cycles of erosion, the flexural strength of the specimens decreased with the increase in the fly ash content. In the context of steam-cured mortar specimens, throughout the entire erosion period, specimens with a fly ash content of 45% exhibited the highest relative compressive strength. The established strength evolution model had a minimum determination coefficient of 0.879, indicating a good agreement between the model and experimental results. Microscopic research showed that fly ash would undergo a pozzolanic reaction under the action of sulfate and calcium hydroxide, which was beneficial to the improvement of the erosion resistance. As the fly ash content increased, the erosion products of the specimens gradually became dominated by gypsum.