Heat release during the hydration of calcinated α-C2SH and its mixture with killalaite

Abstract

In this work, the influence of the mineral composition of the synthesized calcium silicate hydrates on their hydraulic activity after thermal treatment was determined. Primary mixture, consisting of quartz sand and burnt limestone (CaO/SiO2 = 2), was treated hydrothermally with NaOH additive at 200 °C. It was determined that α-C2SH prevailed in the product after 4 h of the synthesis. The results of DSC and XRD analysis revealed that α-C2SH was partially decomposed after 12 h of the synthesis and newly formed compounds were identified—killalaite, portlandite and C–S–H. The products of 4 and 12 h synthesis were treated thermally at temperatures between 450 and 1000 °C. It was determined that dellaite and x-C2S formed already at 450 °C, β-C2S at 600 °C and α′L-C2S at 800 °C when the 4 h synthesis product, in which α-C2SH prevailed, was treated thermally. On the other hand, killalaite remained stable up to 600 °C, and the temperature values, at which mentioned calcium silicates formed, increased in case of a 12 h synthesis product. Heat flow values of the main hydration reaction and total heat release exceeded 3.1 mW g−1 and 140 J g−1 accordingly in case of the samples in which α-C2SH prevailed. However, increase in the thermal treatment temperature resulted in a decrease in the mixtures hydraulic activity. It was proved that killalaite formation in the product of the 12 h hydrothermal synthesis vividly decreases its hydraulic activity after the thermal treatment in the temperature range of 450–1000 °C. No increment in the heat flow values that could be attributed to the main hydration reaction (acceleration period) was witnessed in all the curves of the heat flow analysis in this case.