Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C(3)A) in the absence and presence of gypsum (C(S) over barH(2); where A = Al2O3, C= CaO, H = H2O, (S) over bar = SO3). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C(4)AH(19)) and cubic calcium aluminate hydrate (C(3)AH(6)) are observed on the surface of C(3)A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C(6)A (S) over bar H-3(32)) and monosulphate (C(4)A (S) over barH(12)) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport. (C) 2004 Kluwer Academic Publishers.