Initial hydration reactions and mechanisms of delayed ettringite formation in Portland cements

The aluminate hydration reactions in Portland cements were followed by X-ray diffraction. Using a computerized focusing Huber-Guinier diffractometer, the patterns were obtained from the pastes directly, without preliminary drying or grinding. It is shown that the widely accepted theories on the hydration of C(3)A in the presence of calcium sulfate cannot be applied generally to commercial Portland cements. Only in cases of pastes nearly free of CO2 will the ettlingite formed in the course of the induction period transform to monosulfate. In cement pastes containing more than about 0.5% CO2 the conversion is prevented and monosulfate is replaced by hemicarbonate or monocarbonate. The quantity of CO2 sufficient to replace monosulfate by carbonated AFm phases depends on the ratio of SO3 to C(3)A and the amount of available A1(2)O(3). The results clearly show that with cements or concretes containing monosulfate the risk of delayed ettringite formation as a result of carbonation reactions should be taken into account. The interlayer sulfate groups in the crystal structure of monosulfate can be replaced by carbonate. This exchange results in increasing sulfate concentration in the pore solution and thus lends to delayed ettringite formation. (C) 1996 Elsevier Science Limited.