Synergistic effects of acetic acid and sulfate on the degradation of ternary and alkali-activated concretes

Degradation of concrete structures exposed to silage effluent is primarily caused by the organic acids destabilising calcium-bearing phases in the cementitious materials. The performance of a ternary cement (Portland cement, ground granulated blast-furnace slag (GGBS) and silica fume), alkali-activated GGBS (AAS) and alkali-activated fly ash (AAFA) exposed to acetic acid and combined acetic acid–sulfate solutions for 1 year was investigated. The acetic acid concentration was 0.2 mol/l, while the sulfate concentrations were 0.1 mol/l or 0.5 mol/l. Using the different binders, concrete specimens were used to assess mass and compressive strength changes, whereas paste specimens were analysed to characterise mineralogical alterations. The exposure solutions were examined to gain an understanding of metal ion leaching. The AAFA concrete showed the highest resistance to acetic acid attack, with only a 3% reduction in compressive strength loss compared with 39% and 43% for the ternary and AAS concrete, respectively. Under combined acetic acid–sulfate exposure, the presence of sulfate-exacerbated deterioration in the ternary and AAS blends, with damage increasing with an increase in sulfate concentration, rose. This behaviour is attributed to the formation of expansive gypsum, which disrupted the microstructure and facilitated further ingress of the aggressive solution. In contrast, sulfate had only a minor effect on the AAFA, likely due to the low calcium content of the fly ash.