Fine-grained amorphous calcium silicate CaSiO3 from vacuum dried sol–gel – Production, characterisation and thermal behaviour

The production of amorphous calcium silicate, CaSiO3, by vacuum drying a sol–gel is described. The material exhibits a highly porous, irregular extended aggregate morphology. Its amorphous nature is confirmed by X-ray powder diffractometry and its silicate nature by vibrational spectroscopy. The latter is used to characterise the bridging/non-bridging oxygen distribution and hydration state, which comprises SiO3 bonded hydroxyl and interstitial water. No indicators of silicon hydride defects were observed. From Raman/FTIR spectroscopy, the Si-O tetrahedral species are predominantly peaked around SiO32−, as predicted by the stoichiometry of the starting materials. Variable temperature X-ray diffraction was used to measure the silicateʹs thermal expansion behaviour up to the point of crystallisation, where the mineral phase wollastonite was observed. Refined lattice parameters were in good agreement with standard values for wollastonite. The procedure as implemented does result in the formation of a small amount of poorly ordered carbonate, which can be removed by post-drying annealing. The carbonation in the presence of CO2 of the dried amorphous silicate was also investigated, showing the conversion to CaCO3 calcite and SiO2.