Calcium Sulfate Crystallization Techniques and Properties

Calcium sulfate exhibits a rich array of crystalline forms—most notably the dihydrate, hemihydrate and anhydrite polymorphs—each with distinct structural and physical characteristics. The crystallisation pathways of calcium sulfate are governed by factors such as temperature, supersaturation, pH, additives and solvent composition. Controlled hydrothermal and solution-based methods enable selective formation of desirable phases; for instance, hydrothermal synthesis at elevated temperatures favours hemihydrate whiskers with high aspect ratios, while evaporative crystallisation under ambient conditions typically yields plate-like or granular gypsum. Organic and inorganic crystal modifiers—including surfactants, salts and carboxylic acids—adsorb on specific crystal faces, modulating growth rates and habit to tailor morphology for industrial applications. Advanced spectroscopic techniques and kinetic modelling have elucidated the mechanisms of nucleation and growth, revealing activation energies and transition sequences between dihydrate, hemihydrate and anhydrite phases. These insights underpin practical applications spanning construction materials, bioceramics and chemical recovery processes, where crystal size, shape and purity directly influence mechanical strength, setting behaviour and reactivity.

Research from Nature Portfolio

Recent studies have demonstrated a template-free phase transformation of flue gas desulfurization gypsum to α-calcium sulfate hemihydrate whiskers using cetyltrimethylammonium bromide and potassium chloride in a glycerol–water medium. The combined additives enhance dissolution of the dihydrate phase and selectively adsorb on growing facets to promote one-dimensional growth along the c axis, yielding whiskers with aspect ratios exceeding 180. This foundational work highlights the synergy between surfactant and salt co-modifiers in directing crystallisation pathways for high-performance gypsum materials.

Research from all publishers

Optimised hydrothermal synthesis protocols have been developed to convert flue gas desulfurization gypsum directly into calcium sulfate hemihydrate whiskers by adjusting reaction temperature, stirring rate, material–water ratio and glycerol concentration. Surfactant stabilisation proved essential for achieving uniform diameters and smooth surfaces, resulting in whiskers with aspect ratios surpassing 200. In parallel, crystallisation kinetics of the dihydrate–hemihydrate transformation in electrolytic solutions have been characterised using Avrami–Erofeev modelling, revealing apparent activation energies of 124 kJ mol⁻¹ for nucleation and 810 kJ mol⁻¹ for growth, which facilitates precise control of phase conversion rates. Complementary research has elucidated the role of organic acid modifiers bearing multiple carboxyl groups separated by methylene bridges: these species adsorb on specific hemihydrate facets, altering crystal habit, reducing aspect ratio and enhancing mechanical strength in gypsum-based composites.

Calcium Sulfate Crystallization Techniques and Properties publication trend

The graph below shows the total number of articles in calcium sulfate crystallization techniques and properties across all publications each year (not limited to Nature Index journals).

Technical terms

Calcium sulfate dihydrate (gypsum): Hydrated calcium sulfate phase, CaSO₄·2H₂O, commonly found in nature and industrially produced.

Calcium sulfate hemihydrate: Intermediate polymorph CaSO₄·0.5H₂O, existing as α- and β-phases, valued for cementitious properties.

Hydrothermal synthesis: Crystal growth technique in aqueous solution at elevated temperature and pressure to direct phase formation.

Supersaturation: Condition where solute concentration exceeds equilibrium solubility, driving nucleation and crystal growth.

Aspect ratio: Measure of crystal elongation defined as the ratio of length to diameter in needle- or whisker-like morphologies.

Avrami–Erofeev model: Mathematical description of crystallisation kinetics based on nucleation and growth mechanisms.

Crystal habit: Characteristic external shape of a crystal determined by relative growth rates of different crystallographic faces.

References

1. A facile method of transforming FGD gypsum to α-CaSO4·0.5H2O whiskers with cetyltrimethylammonium bromide (CTAB) and KCl in glycerol-water solution. Scientific Reports (2017).
2. Crystallization Kinetics of α-Hemihydrate Gypsum Prepared by Hydrothermal Method in Atmospheric Salt Solution Medium. Crystals (2021).
3. Effect of Molecular Structure of Organic Acids on the Crystal Habit of α-CaSO4·0.5H2O from Phosphogypsum. Crystals (2020).
4. Insights into the CaSO4–H2O System: A Raman-Spectroscopic Study. Minerals (2020).
5. Preparation of CSHW with Flue Gas Desulfurization Gypsum. Materials (2022).