On the relevance of pyrite oxidation: A thermodynamic and experimental evaluation of concrete degradation

The oxidation of pyrite in cement-based materials induces complex chemical and mechanical degradation. This study combines thermodynamic modelling with experimental analysis to understand the mechanisms behind this deterioration. The results show that pyrite oxidation leads to iron hydroxide precipitation, generating localised crystallisation pressures that promote intra-granular fracturing. The release of sulphate raises the formation of ettringite and gypsum, further increasing expansive stress. The inclusion of air-entraining admixtures alters this degradation process by redistributing internal stresses, thus mitigating crack propagation. Thermodynamic simulations predict phase stability and precipitation kinetics, highlighting the role of local supersaturation in driving microstructural disruption. The primary contribution of this study is the development of a model for the progression of internal sulphate reaction. The model suggests that the initial damage phase is driven by secondary ettringite formation, resulting in a buildup of crystallisation pressure. As oxidation progresses, iron hydroxide precipitation dominates, exacerbating expansion and accelerating degradation.