Acid Corrosion Resistance of Hybrid Alkaline Mortar Exposed to Hydrochloric Acid and Sulfuric Acid Solutions

The objective of the present research work is to investigate the durability performance of hybrid alkaline mortar (HAM), made with two different ordinary portland cement (OPC)/fly ash proportions (15%/85% and 25%/75%), exposed to 5% hydrochloric acid (HCl) and 5% sulfuric acid (H2SO4) solutions. The performance was evaluated through visual observation, weight change, compressive strength, surface alkalinity, pH, ionic concentration (ICP-MS), and microstructure analyses. The results revealed that the extent of degradation was greater for the HAM mix containing 25% OPC than 15% OPC and under 5% HCl exposure than 5% H2SO4 exposure. The degradation of HAM mixes during acid exposure is attributed to the dealumination and decalcification of gel products. During 5% HCl exposure, the higher solubility of calcium chloride (CaCl2) left behind porous silicon-rich gel, facilitating easier acid ingress and higher degradation. Conversely, gypsum formed during 5% H2SO4 blocked the porous silicon-rich gel, which reduced acid ingress and degradation. The degradation of aluminosilicate gels converts the chemically bound water to free or physically bound water, and it was more during 5% HCl exposure than 5% H2SO4 exposure. Moreover, the degraded gel had a higher silicon–aluminum (Si─ Al) ratio, leading to shifting of the main band of Fourier-transform infrared (FTIR) spectra to a higher wavenumber.