Hydration behaviour of mixtures of cement and fly ash with high sulphate and chloride content

DTA/TG thermoanalytical investigations and X-ray diffractometry were carried out which demonstrate the effect of MSW fly ash on the hydration reactions of pozzolanic cement. The MSW fly ash has high content of calcium sulphate, alkali chlorides and heavy metals. During the first curing period the calcium aluminate reacts with the sulphate to form ettringite. In that period also the presence of syngenite is noted in the pastes. With the growth of the fly ash content of the mixture there is a lengthening of the period in which the hydration reactions of the calcium silicates are inhibited. Subsequently with the progress of hydration in the pastes the CSH phase develops and the formation of calcium chloroaluminate phase is observed.