Degradation of Alkali-Activated Slag and Fly Ash Mortars under Different Aggressive Acid Conditions

Acidic environments constitute serious chemical threats to concrete-like cementitious materials. The purpose of this study is to experimentally investigate the degradation of alkali-activated slag/fly ash mortars with different slag/fly ash ratios: 80/20, 60/40, and 40/60 in acidic environments. Mortar samples were exposed to three different types of aggressive acidic solutions: phosphoric acid, sulfuric acid, and a mixture of phosphoric acid and sulfuric acid maintained at a constant pH value of 2.5±0.5 for a period of 150 days. Results showed that, for all mortar samples, the aggressivity of the phosphoric acid is greater compared to the other acids. Moreover, samples with a slag/fly ash ratio of 60/40 demonstrate the highest resistance against the three types of acidic environments. In addition, it shows that the degradation process of alkali-activated mortars can be divided into two degradation stages: an early stage and a subsequent stage. The chemical-reaction dominated early degradation stage is described by using Hill function, whereas a diffusion process-dominated subsequent stage is simulated with Fick's second law. Finally, the results of theoretical analysis predicted that the degradation depth of alkali-activated slag/fly ash mortars exposed to sulfuric acid environment (pH=2.0) for 50 years could be reduced by about 52%-60% compared to that of an ordinary Portland cement (OPC)-based mortar.