The effects that 50–500 mM aqueous Li+, Na+, K+, and Mg2+ have on the crystallization kinetics of calcium sulfate dihydrate (gypsum; CaSO4·2H2O) 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 Mg2+, 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 Mg2+ and Li+ resulting in longer and narrower crystals compared to the additive-free system. In addition, we show that, regardless of concentration, Mg2+, 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.
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