DTA-TG Study of the CaO-SiO2-H2O and CaO-Al2O3-SiO2-H2O Systems Under Hydrothermal Conditions

Abstract

Simultaneous DTA-TG is an excellent technique for evaluating phases formed in hydrothermally treated CaO-SiO2-H2O and CaO-Al2O3-SiO2-H2O systems. Thermal analysis in combination with XRD and SEM, revealed that in the CaO-Al2O3-SiO2-H2O system the amount of hydrogarnet formed was the largest when gibbsite was used as the Al source, smallest for kaolin and intermediate for metakaolin. The endotherm peak temperature of the hydrogarnet dehydration endotherm was affected by the amount of hydrogarnet and the Si content of hydrogarnet. The thermal stability and structural order of 11 Å tobermorite were reduced with the incorporation of Al and, as a result, 11 Å tobermorite transformed into 9.3 Å tobermorite at lower temperatures while the transformation of the latter into beta-wollastonite required more energy. There exists a direct relationship between the 9.3 Å tobermorite and beta-wollastonite formation temperatures. Solid-state 29Si and 27Al MAS NMR data support these findings.