Hydrothermal formation of the calcium silicate hydrates, tobermorite (Ca5Si6O16(OH)2·4H2O) and xonotlite (Ca6Si6O17(OH)2): an in situ synchrotron study

In situ energy-dispersive X-ray diffraction (XRD) techniques have been employed to study the hydrothermal formation of crystalline tobermorite and xonotlite. Alkoxide gels of tobermorite composition with varying aluminium contents (Al/(Al+Si)=0 to 0.15) were reacted with a saturated calcium hydroxide solution at temperatures varying from 190°C to 310°C on the saturated vapour pressure curve. Reaction products consisted of tobermorite, xonotlite or a mixture of both. Tobermorite is stabilised by increasing aluminium content and decreasing temperature, whereas xonotlite forms at higher temperature and lower aluminium contents. Reaction times ranged from 3 to 5 h, with the first Bragg peaks forming within the first 10 min. The formation mechanism involves a two-stage process. Firstly, a poorly crystalline C–S–H gel phase forms, which has good periodicity parallel to the ab plane but is poorly ordered parallel to the c direction. The second stage involves the ordering of the C–S–H gel along the (001) direction to form ordered crystalline tobermorite or xonotlite. Kinetic analysis indicates that the reaction rate is increased with increasing aluminium content and increasing temperature. Activation energies for these reactions at different aluminium contents were calculated from two datasets; firstly, from the change in 2θ position of the tobermorite(220)/xonotlite(320) peak during the reaction (19–30 kJ/mol) and secondly, from the rate at which the background hump intensity decreases for each experiment (26–33 kJ/mol).