Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum

The tricalcium aluminte phase was prepared from pure chemicals on a laboratory scale. Five mixes were formulated from the prepared C(3)A phase, beta-hemihydate, phosphogypsum, calcium hydroxide and quartz. Different mixes were hydrated at various time intervals, namely, 6, 24, 72 and 168 h. The kinetics of hydration was measured from chemically combined water and combined lime contents. The phase compositions and microstructures of the hydrated products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA)/TG, scanning electron microscopy (SEM) techniques and FT-IR spectroscopy. This work aimed to study the effect of partial to full substitution of phosphogypsum by beta-hemihydate on the hydration characteristics and microstructures of tricalcium aluminte phase. The results showed that the combined lime slightly increases with the increase of amounts of phosphogypsum. The XRD patterns showed the increase in the intensities of monosulphate and different forms of calcium aluminate (C(4)AH(13) and C(4)AH(19)) with phosphogypsum content. Ettringite is less stable than monosulphoaluminate, so it transformed into monosulpho-aluminate after 24 h, which persisted up to 168 h. The mechanism of the hydration process of C3A phase in the presence of phosphogypsum proceeds in a similar path as with beta-hemihydate. Phosphogypsum reacts with C3A in the presence of Ca(OH)(2) forming sulphoaluminate hydrates, which are responsible for setting regulation in cementitious system. (c) 2004 Elsevier Ltd. All rights reserved.