THE STABILITY OF ETTRINGITE

摘要

Ettringite has been known as a natural mineral for more than 100 years and is a normal constituent of hydrated Portland cement pastes. Its nominal composition in cement pastes, Ca_6[Al_2(OH)_6]_2(SO_4)_3·24-26H_2O, may be modified by single or double substitution, of carbonate for sulfate and of silicon for aluminium. The stability of ettringite in the CaO-Al_2O_3-SO_3-H_2O system is relatively well established; it decomposes at 114°C. However, 'stability' is frequently used as shorthand for 'compatibility with other phases' and indeed, it is also appropriate to include within the scope of this assessment ettringite stability at water vapour pressures less than saturation. The relevant data are reviewed. Some aspects of ettringite persistence (as distinct from stability) are discussed, for example formation of 'metaettringite' a phase which preserves the columnar structure of ettringite but has lower water content than ettringite itself. A distinction is made between ettringite stability and amount of substance: within the stability range of ettringite, the presence of additional soluble components, e.g., salts, can alter the amounts of ettringite present but this is not conclusive proof that ettringite is intrinsically unstable.