This study aims to investigate cementitious compound behaviour at early stages (up to 168 days), primarily when dosed with different levels of pyrite. To accomplish this, a study was conducted with cement pastes and mortars contaminated by sulphide. For the cement pastes, four contaminant levels were adopted: with one as a reference value (0%), two as cited by the standards, and one value greater than the set standards. In the mortars, two series were studied, one without contamination and the other with use of an aggregate contaminated naturally by pyrite. Pozzolanic cement was used (65% Portland cement CPV ? 35% fly ash) and the materials were submerged in curing compounds for 28 days.Subsequently, the samples were subjected to two exposure conditions: 1-a wet-dry cycle and 2-immersion in aqueous solution with aeration. To identify the products formed by pyrite oxidation, and the subsequent by-product-based reactions, and cement hydrates, scanning electron microscopy with elemental analysis was used. For the analysis of the mechanical properties, strength tests were performed to test axial compression and tensile strength in bending. Furthermore, monitoring of the dimensional variation in prisms (2.5 9 2.5 9 28.5) cm 3 was performed. In conclusion, we determined that at the exposure times tested, there were signs of sulphide oxidation in the contaminated compounds. It was proven that the higher the degree of pyrite contamination, the higher the relative expansion of the series, which was confirmed by the formation of expansive products identified in the scanning electron microscopy.