Dehydration/recrystallization mechanisms, energetics, and kinetics of hydrated calcium silicate minerals: an in situ TGA/DSC and synchrotron radiation SAXS/WAXS study

The dehydration and recrystallization of the natural hydrated calcium silicates xonotlite [Ca6Si6O17(OH)2], 11Å anomalous tobermorite [Ca5Si6O16(OH)2·4H2O], and hillebrandite [Ca2SiO3(OH)2] were studied in situ by dynamic heating (5°C min−1) differential scanning calorimetry/thermogravimetric analysis (TGA/DSC) and synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS). All have structures based on silicate chains with a repeat unit of three tetrahedra (‘dreierketten’). Room-T infrared data for the tobermorite sample suggest that it is essentially (OH)-free and that it is likely to contain double chains in addition to the normal single chain units. The in situ techniques allow the mechanisms and kinetics of dehydration and recrystallization processes to be studied directly or deduced. Xonotlite and tobermorite recrystallise to low-temperature (T)-wollastonite (CaSiO3) with two-phase regions existing over the temperature ranges 780–835°C and 835–855°C, respectively. Hillebrandite breaks down via a two-phase region between 520°C and 590°C to form larnite (β-Ca2SiO4) which appears to retain some (OH) in its structure. This hydroxylated larnite in turn recrystallises to αL′-Ca2SiO4 between 775°C and 900°C. The presence of (OH) in the hydroxylated larnite appears to extend its stability limit to higher Ts than those shown for anhydrous larnite. In each case, the widths of the two-phase regions are dependent on the extents of structural changes involved. DSC data for xonotlite, hillebrandite and hydroxylated larnite give a mean energy for dehydroxylation/recrystallization of 4000±1200 J/g (OH as H2O) lost. Radius of gyration values for hillebrandite provide estimates of particle sizes up to about 640 Å; this is believed to represent the lengthscale of domains between zones of defects formed by displacements of structural units along the dreierketten chains.