Comparing the effects of a retarder and accelerator on properties of gypsum building plaster

Gypsum (CaSO4·2H2O) as a widely used building material needs to have good mechanical strength and low water absorption. This paper investigated the effect of sodium tripolyphosphate (STPP) and sodium sulphate (SS) additives on properties of gypsum building plaster. For this purpose, different samples with variable amounts of STPP and SS were prepared. It was found that STPP, a retarder additive, partially inhibited plaster hydration from hemihydrate to dihydrate, as observed by X-ray diffraction analysis and the morphology had changed significantly from needle-like to column or plate ones which led to the lower mechanical strength and higher water absorption. The results showed that SS additive accelerated the rate of hydration and led to the formation of a larger number of short needle-like interlocked crystals, thereby imparting, to gypsum matrix, higher mechanical strength and lower water absorption than the reference material. According to the results, there is a reverse relationship between the strengths of the gypsum samples and their apparent porosity.