Strength gain and calcium hydroxide depletion in hardened cement pastes containing fly ash

Abstract

Synopsis Hardened cement paste specimens, with and without partial replacement of the cement by fly ash, were cured f o r various ages up to one year at temperatures 20-80°C. Compressive strength, calcium hydroxide content and porosity, by mercury intrusion porosimetry, were measured as curing proceeded. Results indicate that elevated curing temperature accelerates the pozzolanic reaction to a greater extent than the normal hydration reactions of Portland cement. When elevated temperatures are used, the strength of fly ash pastes may surpass that of plain pastes at early ages. Strength degradation was observed in pastes cured at the higher temperatures; this degradation was more pronounced in the plain pastes. Consumption of calcium hydroxide with no corresponding gain in strength, or with a reduction in strength, was observed for ash-pastes cured at 50°C and above. Changes in mercury porosity, on the other hand, correlate well with changes in strength during the early strength-gain periods and also during the later strength-degradation periods.