Hydrothermal synthesis of hydrogarnet and tobermorite at 175 °C from kaolinite and metakaolinite in the CaO–Al2O3–SiO2–H2O system: A comparative study

The hydrothermal synthesis of hydrogarnet and tobermorite from kaolinite and metakaolinite was examined at 175 °C for 24 h in the CaO–Al2O3–SiO2–H2O (CASH) system, which is important in cement science and especially in cement chemistry and is closely related to the pozzolanic reaction, the CaO-aggregate reaction and the glass fibre reinforcement of hardened cement. Starting mixtures were prepared with molar ratios Al/(Si + Al) = 0.10–0.13 and Ca/(Si + Al) = 1.00–7.00. The hydration products were characterized by X-ray diffraction, scanning electron microscopy, Fourier transformed infrared spectroscopy, Magic Angle Spinning Nuclear Magnetic Resonance and thermogravimetric analysis in order to elucidate their mineral chemistry and microstructure. Results reveal that several poorly crystalline materials were formed, with un-reacted Ca(OH)2 appearing at shorter reaction times. Hydrogarnet always tended to form more rapidly than tobermorite. It was transformed into aluminium-substituted tobermorite with curing time. CaO is present in the further reaction with SiO2 forming calcium silicate hydrates, and released Al3+ ions were inserted into tobermorite.