Effect of citric acid and the hemihydrate amount on the properties of a calcium sulphoaluminate cement

The influence of citric acid on the hydration and strength development of a calcium sulphoaluminate cement was investigated. Cement pastes were prepared by mixing calcium sulphoaluminate (C(4)A(3)(S) over cap $$) with 15, 20 and 25wt% of hemihydrate (C (S) over cap $$H-0.5). Citric acid was added as a retarder at 0 and 0.5wt%. The samples were cured at 20 degrees C for periods of time from 1 to 28 days to evaluate their compressive strength and to characterize the hydration products by scanning electron microscopy and X-ray diffraction. Calorimetric curves showed that the retarding agent considerably decreases the heat release rate and the quantity of total heat released. The main product after the curing was ettringite (C(6)A (S) over cap $$ H-3(32)). The morphology of this phase consisted of long and thin needles growing radially on the cement grains. Samples with 15wt% of hemihydrate and 0.5wt% of citric acid developed the highest compressive strength (70 MPa) at 28 days of curing.