Influence of temperature on the hydration products of low pH cements

The chemical evolution of two hydrated “low pH” binders prepared from binary (60% Portland cement + 40% silica fume) or ternary (37.5% Portland cement + 32.5% silica fume + 30% fly-ash) mixtures was characterized over one year at 20 °C, 50 °C, and 80 °C. The main hydrates were Al-substituted C–S–H. Raising the temperature from 20 to 80 °C caused a lengthening and cross-linking of their silicate chains. Ettringite that formed in pastes stored at 20 °C was destabilized. Only traces of calcium sulfate (gypsum and/or anhydrite) reprecipitated after one year in some materials cured at 50 °C and 80 °C. The sulfates released were therefore partially adsorbed on the C–A–S–H and dissolved in the pore solution. The pore solution pH dropped by about 2 units as the temperature increased. Conversely, the soluble alkali fractions did not change significantly. Only the ternary binder resulted in a pore solution pH below 11 at the three temperatures studied.