Effect of crystalline admixture and polypropylene microfiber on the internal sulfate attack in Portland cement composites due to pyrite oxidation

Abstract

Internal sulfate attack (ISA) occurs in cement composites due to the presence of mineral sulfide in aggregates, such as pyrite (FeS2). The oxidation of the pyrite releases sulfate ions, which react with hydrated phases of Portland cement and produce expansive phases such as ettringite and gypsum. The objective of this article was to study the influence of crystalline admixture and polypropylene microfiber on the internal sulfate attack in Portland cement composites due to pyrite oxidation. Mortars with 10% pyrite content (by sand weight) were evaluated and compared with reference mortars (without pyrite). Pyrite oxidation was stimulated through wetting and drying cycles at 40 °C. Transport properties (water absorption by immersion, sorptivity, ultrasonic pulse velocity, and electrical resistivity), length change, flexural tensile strength, and microstructural properties were monitored for up to 24 weeks. Pyrite promoted degradation in all mortars over time. Mortars with microfibers had the lowest expansion due to the availability of voids for the consolidation of expansive phases. However, the high porosity and permeability of the material facilitated the entry of water and diffusion of oxygen. A large number of expansive phases and cracks were found in these mortars. The presence of soluble alkaline phases promoted the interaction between crystalline admixture and pyrite, accelerating the ISA.