Summary
Differential thermal analysis curves were obtained for a Portland cement paste and a fl-dicalcium silicate paste (β-C2S paste), by heating in vacuo and in air up to 1,200°C. The vacuum thermograms of both pastes showed a first single endotherm at 140 to 143°C which is mainly characteristic for the decomposition of tobermorite gel. For the Portland cement paste, this includes also the decomposition of the smaller amounts of aluminate hydrated salts. A second peak which appeared at 360 to 385°C is due to the decomposition of calcium hydroxide. Thermograms obtained by heating in air differ considerably from those obtained in vacuo. The first endothermal effect which mainly characterizes the decomposition of the tobermorite gel showed more than one peak; and the temperature, shape and area of the peaks are all affected by heating in air. These effects may be due to the reversibility of the reaction and to the liability of the dehydrated products to be rehydrated at an accelerated rate in the presence of air. At high temperatures, DTA in vacuo seems to be a useful technique for resolving some peaks which apparently are fused into a single effect in the air thermogram.
