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 endproducts 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.

Results are reported here of an investigation into the effects of three carboxylic acid additives (tartaric, maleic and citric acids) on the precipitation of calcium sulfate phases. Precipitation reactions were followed at pH 7 in the pure CaSO 4 system and in experiments with 0À20 ppm carboxylic acids added using in situ UV-VIS spectrophotometry (turbidity). The solid products were characterized in terms of their mineralogical composition, using X-ray diffraction, during and at the end of each reaction, and in terms of their morphological features, by scanning electron microscopy. All additives increased the time needed for turbidity to develop (induction time, start of precipitation) and the comparison between additive and additive-free experiments showed that, at equivalent concentrations, citric acid performed far better than the other two carboxylic acids. In all cases bassanite precipitated first and with time it transformed to gypsum. The addition of citrate stabilized bassanite and changed the final gypsum habit from typical needle-like crystals in the pure CaSO 4 system to plates in the citrate-additive experiments.

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This is a repository copy of Effectiveness of Green Additives vs Poly(acrylic acid) in Inhibiting Calcium Sulfate Dihydrate Crystallization.