Effect of sulfuric acid concentration on morphology of calcium sulfate hemihydrate crystals

Abstract Calcium sulfate hemihydrate whiskers were prepared in sulfuric acid-distilled water system by hydrothermal method using calcium sulfate dihydrate as raw materials. The influence mechanism of sulfuric acid concentration on morphology of calcium sulfate hemihydrate crystals was investigated. The results show that the morphology of hydrothermal products is fibrous when the suitable sulfuric acid concentration is from 5.0 × 10 −6 mol l −1 to 5.0 × 10 −4 mol l −1 . The promoting effect of sulfuric acid on the dissolution of calcium sulfate dihydrate increases the amount of Ca 2+ in the solution, which is beneficial to the crystallization of calcium sulfate hemihydrate crystals. Meanwhile, the selective adsorption of H + produced by sulfuric acid ionization on the (310) crystal plane of calcium sulfate hemihydrate crystals can hinder the aggregation of Ca 2+ on the crystal plane, which results in preferential growth of the calcium sulfate hemihydrate crystal along the (001) crystal plane. However, when the sulfuric acid concentration is up to 5.0 × 10 −3 mol l −1 , the HSO 4 − produced by the interaction of H + with SO 4 2 − can reduce the electronegativity of SO 4 2 − and inhibit SO 4 2 − adsorption on the crystal surface. So the growth of calcium sulfate hemihydrate crystals along (001) crystal plane is hindered, which leads to the decrease of the fibrous products.