Properties and hydration of blended cements with mineral alunite

The present research study investigates the effect of mixing ordinary Portland cement with alunite, a mineral of the jarosite group. For this purpose, a reference sample and three mixtures were produced by substituting cement up to 20%, by alunite. Initial and final setting times, consistency of standard paste, soundness and compressive strengths after 2, 7 and 28 days were measured. It was observed that the alunite blended cements developed higher strength, at 28 days, compared to the reference one. The strength increase was higher, the higher the alunite content. The increase of cement replacement resulted to a slightly increase of water demand and of the initial and final setting times. No expansion was observed in all cases. The hydration products were examined by XRD analysis at 1, 2, 7 and 28 days. The microstructure of the hardened cement pastes and their morphological characteristics were examined by scanning electron microscopy. The increase of compressive strengths was attributed to the ettringite formation, after the reaction of the Ca(OH)2 with the SO42- and Al3+, produced from the alunite dissolution at the high pH of the hydrated cement.