Influence of some admixtures on the formation of primary and secondary ettringite

Different types of admixtures (2% each of superplasticiser, barium nitrate and stearic acid) were added to examine their effect on the hydration reactions of pure tricalcium aluminate (C₃A) in presence of gypsum (G). The effect of 5% magnesium sulfate solution on the formation of secondary ettringite was examined for different time intervals, 3, 7, 28, 60 and 90 days. The hydration products were investigated using X-ray diffraction, Fourier transform infrared and scanning electron microscopy. The time taken for ettringite formation was determined without admixture and in the presence of admixtures. The results showed that superplasticiser, barium nitrate and stearic acid retard ettringite formation whereas they accelerate the formation of calcium mono-sulfoaluminate hydrate phase and tetracalcium aluminate.