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 absen ce of SO42- ions (added as CaSO4 .2H2O) were treated in suspension und er hydrothermal conditions at temperatures of 80-200 degrees C. Kaolin ite 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 product s were investigated by XRD, IR spectroscopy and DTA techniques in orde r to elucidate their phase compositions. The results indicate that the presence of SO42- ions leads to a marked increase in the reaction rat e at all temperatures investigated. In the C-S-A system, the detected hydration products are calcium silicate hydrate which is then transfo rmed into 1.13 nm aluminium-substituted tobermorite and a-C2SH by incr easing the autoclaving temperature. In the C-S-A-CS system the hydrate d products are calcium silicate hydrate, ettringite and monosulpho-alu minate. On increasing the hydrothermal temperature they decompose, rec rystallize and 1.13 nm aluminium-substituted tobermorite, alpha-C2SH a nd anhydrite II are formed. In both systems the excess Al2O3 appeared as a hydrogarnet phase, C(3)ASH(4).