Gypsum plaster (calcium sulphate hemihydrate, CaSO₄·0.5H₂O) is an air hardening binder with a low manufacturing carbon footprint, which is in principle recyclable, and is being increasingly promoted as a sustainable alternative to cement for indoor construction, prefabricated panels, drywall, decorative items and self levelling floor screeds. However, its rapid setting, fragility and low compressive strength and high water absorption restrict its practical application. Chemical admixtures set retarders, accelerators, superplasticizers, cellulose ethers, starch ethers, hydrophobic agents and foaming agents are the key technological tools to solve these problems and to create application specific gypsum binders. This review offers a critical, sustainability oriented overview of the state of the art on chemical admixtures and additives for building gypsum. After a short recap of the hydration process of the bassanite dihydrate in the Avrami model, the main classes of admixtures are discussed in terms of: (i) chemical structure and typical dosages, (ii) physico chemical mechanism (chelation, adsorption, electrostatic and steric repulsion, water retention, pore blocking, hydrophobic film formation), (iii) impact on setting time, fluidity, microstructure, mechanical strength and water resistance, (iv) compatibility with industrial by product gypsums (FGD, phosphor gypsum, Citro gypsum) and (v) environmental sustainability compared to traditional alternatives. Comparative tables and synthetic performance maps are used to compare the advantages and limitations of each admixture with those reported in the recent literature. Special attention is paid to bio and waste derived admixtures (leather/protein hydrolysates, chitosan, starch ethers, vegetal foaming agents, bio carbonate fillers) that are key to the gypsum technology circular economy. The review points out the need for further research into: (i) dosage standardization, (ii) long term water resistance of modified gypsum, (iii) water resistance vs recyclability trade off, (iv) life cycle assessment of modified gypsum and (v) a road map for the next generation of sustainable gypsum binders.
Bouhayek et al. (Wed,) studied this question.