The Effects of Inorganic Additives on the Nucleation and Growth Kinetics of Calcium Sulfate Dihydrate Crystals

The effects that 50–500 mM aqueous Li +, Na +, K +, and Mg 2+ have on the crystallization kinetics of calcium sulfate dihydrate (gypsum; CaSO 4 ·2H 2 O) were determined by in situ and time-resolved UV–vis spectrophotometry. The mechanisms of surface or structural associations between these additives and the end-product gypsum crystals were evaluated through a combination of inductively coupled plasma mass and/or optical emission spectrometric analyses of digested end-products and X-ray photoelectron spectroscopy of the surface of the solids. Furthermore, X-ray diffraction and scanning electron microscopy were utilized for determining any changes in phase composition and growth morphologies of the formed crystals. Our results revealed that Mg 2+, even at low concentrations, decreased the nucleation and growth kinetics 5–10 fold more than Li +, Na +, and K + . In all cases, the additives also changed the shapes and sizes of the formed crystals, with Mg 2+ and Li + resulting in longer and narrower crystals compared to the additive-free system. In addition, we show that, regardless of concentration, Mg 2+, Li +, and K + only adsorb to the newly forming surfaces of the growing gypsum crystals, while ∼25% of Na + becomes incorporated into the synthesized crystals.