Abstract
Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO4·2H2O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg2+ and citric acid) on the precipitation mechanism and rate of gypsum. Detailed analysis of the SAXS data showed that for all tested supersaturations and temperatures the same nucleation pathway was maintained, i.e., formation of primary particles that aggregate and transform/re-organize into gypsum. In the presence of Mg2+ more primary particle are formed compared to the pure experiment, but the onset of their transformation/reorganization was slowed down. Citrate reduces the formation of primary particles resulting in a longer induction time of gypsum formation. Based on the WAXS data we determined that the precipitation rate of gypsum increased 5-fold from 4 to 40 °C, which results in an effective activation energy of ~30 kJ·mol−1. Mg2+ reduces the precipitation rate of gypsum by more than half, most likely by blocking the attachment sites of the growth units, while citric acid only weakly hampers the growth of gypsum by lowering the effective supersaturation. In short, our results show that the nucleation mechanism is independent of the solution conditions and that Mg2+ and citric acid influence differently the nucleation pathway and growth kinetics of gypsum. These insights are key for further improving our ability to control the crystallization process of calcium sulphate.
Highlights
Three crystalline phases are known in the CaSO4 –H2 O system, distinguishable by their degree of hydration: gypsum (CaSO4 ·2H2 O), bassanite (CaSO4 ·0.5H2 O), and anhydrite (CaSO4 ).Natural calcium sulphate deposits, predominantly composed of gypsum and anhydrite, are found throughout the geological record [1], and these minerals are an important source material for the building industry [2]
SAXS provided us with information on the precursor stage of the reaction, while reaction rates and the activation energy for calcium sulphate precipitation were derived from the WAXS data
Three different calcium sulphate precipitation scenarios were designed to obtain a representative set of data: (I) The role of supersaturation was tested by carrying out experiments at four initial calcium sulphate concentrations (50, 75, 100 and 150 mmol·L–1 CaSO4, solutions were obtained by mixing equimolar stock solutions of Na2 SO4 and CaCl2 ). (II) The influence of temperature on the precipitation behavior was analyzed by conducting experiments at five temperatures (4, 12, 21, 30, 40 ◦ C) for a 50 mmol·L−1 CaSO4 solution. (III) the effect of additives on the precipitation of calcium sulphate was studied for 50, 75, 100 and 150 mmol·L−1 CaSO4 solutions at 21 ◦ C
Summary
Three crystalline phases are known in the CaSO4 –H2 O system, distinguishable by their degree of hydration: gypsum (CaSO4 ·2H2 O), bassanite (CaSO4 ·0.5H2 O), and anhydrite (CaSO4 ). A variety of multi-step pathways have been proposed including amorphous calcium sulphate and bassanite as possible precursor phases [7,8,9,10] All these studies are mainly based on ex situ characterization of the precipitation reaction (e.g., electron microscopic imaging combined with quenching techniques to isolate the precipitates from solution for their subsequent characterization). In a previous study, using in situ SAXS/WAXS, we observed the initial stages of calcium sulphate nucleation and found that nanosized primary particles are formed immediately after a supersaturated solution is created. These particles aggregate and transform/re-organize into gypsum [12]. SAXS provided us with information on the precursor stage of the reaction, while reaction rates and the activation energy for calcium sulphate precipitation were derived from the WAXS data
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