Influence of limestone on physico-chemical properties of white portland cement pastes

This paper aims to study physico-chemical properties of white Portland cement blended with 5-25 wt. % limestone filler. It was observed that limestone filler increases whiteness, Blaine and loss on ignition of dry blended cement powders increases with limestone content. Limestone filler increases setting times, free lime content, total porosity, decreases compressive strength, combined water content and bulk density of cement pastes due to the filling effect of limestone that replaces clinker and decreases the amount of hydration products. Portlandite was detected in limestone blended cement pastes hydrated for 7 days by XRD indicating the filler effect of limestone at early ages of hydration. Whereas, C3S was not detected in limestone blended cement pastes hydrated up to 90 days by XRD due to that limestone filler accelerates the rate of hydration of C3S. Calcium monocarboaluminate was detected by FTIR in cement pastes hydrated for 7 days this means that C3A reacts with CaCO3 giving calcium monocarboaluminate hydrate. The absorption band of ettringite disappeared due to that SO42- ions were probably replaced by CO32-. The OH- band of combined water, associates hexagonal hydrates and calcium hydroxide was shifted in FTIR spectra of cement pastes, demonstrating the accelerating effect of CaCO3 on C3S hydration and formation of calcium carboaluminate hydrate. Relative weight losses that correspond to dehydration of C-S-H, ettringite and portlandite (calculated from TGA/DTA thermograms) prove the dilution effect of limestone filler. SEM micrographs of limestone blended cement pastes hydrated up to 90 days show that limestone filler alters the microstructure of hardened cement pastes. The morphology of calcium monocarboaluminate hydrate originally changes with progress of hydration.