Thaumasite formation in Portland-limestone cement pastes

Abstract

Small cylinders (10-mm diameter × 10-mm height) made at a water:solids ratio of 0.5 from Portland cement with 0, 5, 15, and 30% limestone additions were cured in water at room temperature for 28 days. They were subsequently stored in various solutions at 5°C for periods of up to 420 days. The pastes were inspected visually and examined by X-ray diffraction every 28 days. Selected samples were also examined by thermal analysis and scanning electron microscopy. Pastes containing fine limestone additions were susceptible to formation of thaumasite after only a few months of exposure to sulfate solutions. The extent of thaumasite formation was greater with increasing limestone additions and when magnesium sulfate was present in the solution. Thaumasite formation was then accompanied by formation of brucite and secondary gypsum. Calcium hydroxide was a reactant rather than a reaction product and C-S-H gel was also consumed.