Dehydration of Ca 3Al 2(SiO 4) y(OH) 4(3− y) (0 < y < 0.176) studied by neutron thermodiffractometry

Abstract

Hydrogarnet (Ca 12Al 2(OH) 12) and katoite of composition Ca 3Al 2(SiO 4) 0.176(OH) 11.3 were obtained by hydration of tricalcium aluminium oxide, and for katoite synthesis, by addition of amorphous silica. The thermal dehydration was monitored in situ, by neutron thermodiffractometry, from room temperature to 700 °C at atmospheric pressure and at a heating rate of 2 °C/min. On heating, powder neutron diffraction patterns were collected every 300 s. Cell parameters were fitted by the Pawley method. Hydrogarnet decomposed to yield Ca 12Al 14O 32(OH)· mH 2O and Ca(OH) 2 that eventually transformed to CaO. For katoite, phases of formula Ca 12Al 14− y Si y O 32(O,OH) 1+ y/2 related to mayenite were formed. Complementary annealing experiments, for 10 h, at higher temperatures and subsequent quenching lead to the formation of mayenite and traces of Ca 3SiO 5. Thermogravimetric curves were in agreement with the thermodiffractometry experiments. Also, neutron diffraction data allowed to measure thermal expansion coefficients, at atmospheric pressure, between 25 and 250 °C for hydrogarnet and katoite: 1.89 × 10 −5 ± 0.09 and 1.63 × 10 −5 ± 0.07 °C −1, respectively.