Reaction mechanism of the hydrothermally treated CaO-SiO2-Al2O3 and CaO-SiO2-Al2O3-CaSO4 systems

Mixtures of CaO, amorphous SiO₂ and kaolinite in the presence or absence of SO ₄ ^2– ions (added as CaSO₄·2H₂O) were treated in suspension under hydrothermal conditions at temperatures of 80–200 °C. Kaolinite was added to replace 3 and 10% of the total weight of the dry mix with the overall CaO/SiO₂ mole ratio being 0.83. The hydration products were investigated by XRD, IR spectroscopy and DTA techniques in order to elucidate their phase compositions. The results indicate that the presence of SO ₄ ^2– ions leads to a marked increase in the reaction rate at all temperatures investigated. In the C-S-A system, the detected hydration products are calcium silicate hydrate which is then transformed into 1.13 nm aluminium-substituted tobermorite and -C₂SH by increasing the autoclaving temperature. In the C-S-A-C¯s system the hydrated products are calcium silicate hydrate, ettringite and monosulpho-alumirtate. On increasing the hydrothermal temperature they decompose, recrystallize and 1.13 nm aluminium-substituted tobermorite, -C₂SH and anhydrite II are formed. In both systems the excess Al₂O₃ appeared as a hydrogarnet phase, C₃ASH₄.Cement Notations used are C CaO - S SiO₂ - A Al₂O₃ - H H₂O - CH Ca(OH)₂ - C¯s CaSO₄