Thermodynamic Modeling of Sulfuric and Acetic Acid Attack on Hardened Cement Paste: Effect of Silica Fume

The exposure of concrete structures to acid attack is a growing concern. This study employs thermodynamic modeling to investigate the changes in phase assemblage of powdered cement pastes subjected to a wide range of sulfuric and acetic acid concentrations. A modeling approach utilizing IPHREEQC implemented through Matlab is presented, and the obtained results are compared with pH measurements and compositions of equilibrated calcium and sulfate solutions. The influence of incorporating 11% silica fume (SF) as a replacement for cement predicted a 70% reduction of Portlandite content in the hardened cement paste. Consequently, the acid attack processes and subsequent pH reduction are affected. The modeling approach demonstrates good agreement with experimental data for acetic acid, across a broad range of acid concentrations, for both Portland cement and a blend with SF, without the need for any fitting parameters. However, significant discrepancies between the model and experiments are observed in the case of sulfuric acid. This discrepancy arises due to the formation of lump pieces of material in the experimental setup at higher acid concentrations. These lumps consist of a thin layer of altered hardened cement paste, primarily composed of sulfate‐rich phases, encapsulating unaltered hardened cement paste. Since the reaction was not homogeneous and the powder did not entirely react, the sulfuric attack experimental setup was not representative for validating the thermodynamic model.