Transmission electron optical study of ettringite and thaumasite

Selected area electron diffraction studies of ettringite, carbonate ettringite and thaumasite have been made. Ettringite suffers severe dehydration effects in the electron microscope whereas carbonate ettringite and thaumasite are relatively stable and their a dimensions are close to those obtained by XRD. The c dimension of thaumasite is also preserved, but that of carbonate ettringite is reduced to less than half its XRD value by a symmetry change and slight longitudinal shrinkage of the calcium aluminate hydrate columns. This is believed to be the result of the stronger hydrogen bonding of apical water molecules in the columns by carbonate ions, which are not volatile in the conditions experienced in the electron microscope and which stabilises the structure. (C) 2003 Elsevier Ltd. All rights reserved.