Hydration kinetics of calcium sulphate hemihydrate modified by water-soluble polymers
Abstract
Pastes based on gypsum (calcium sulphate hemihydrate) and two chosen water-soluble polymer admixtures, i.e. hydroxyethylmethyl cellulose (HEMC) and poly(vinyl acetate) (PVAC) were prepared. The impacts of the polymers on gypsum setting, as well as thermal and mechanical properties were studied. Heat effect and the rate of setting depending on water to gypsum ratio and the presence of admixtures were measured. The results have shown a strong effect of the polymer admixture (1%) on the prolongation of gypsum setting time. The presence of polymer molecules in water solution hinders the crystallization process of gypsum dihydrate. HEMC and PVAC have also a significant influence on gypsum mechanical properties especially at low water content. Kinetics of the setting process of gypsum hemihydrate is discussed.
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Introduction
Dramatically altering setting rates as well as microstructures and properties of cement or gypsum products are often accompanied by the use of admixtures. They include accelerators or retarders of setting. Better understanding of the chemistry and physics of product hydration and setting is important. More progress can be made by systematically examining how the setting rate is influenced by chemical admixtures.
Also gypsum industry turns to sustainable development and production of greener concrete [1-6]. Thus, instead of toxic materials (e.g. KNO3, NH4Cl, resins such as sulfonated melanin or sulfonated naphthalene [7]) water-soluble (environment friendly) polymers are used.
Gypsum products are known for their fire resistance, thermal insulation and acoustic properties. However, due to low water resistance and mechanical strength, gypsum is not a suitable material for external construction works. The protection of gypsum requires prevention against penetration of moisture to avoid any damage [8]. Polymer plasticizers are often used as additives in construction materials (gypsum, concrete) in order to reduce water content which generally leads to higher strength of the product, extended or reduced setting time and lowering the possibility of mixture separation. A longer duration of the mixture adequate consistency improves workability and flowability of gypsum and concrete.
Appropriate mixing of hemihydrate sulphate (gypsum) with 1% of polymer (e.g. cellulose derivatives) substantially modifies the mechanical properties, rate of setting (crystallization) and water retention of the material obtained. This has an important impact on the application.
Conclusion
In this paper, the influence of selected polymers on gypsum properties, was studied by DSC, setting process and mechanical tests. The results indicate clear influence of such admixtures as HEMC or PVAC on gypsum properties, i.e. time and rate of setting, mechanical results (flexural strength and modulus), and crystallization kinetics. The setting and crystallization processes can be prolonged with reduced diffusion of water and calcium ions at the formed gypsum crystalline surface because polymer molecules in water solution hinder the process. It is predicted that the polymer film inside material pores is formed changing mechanical properties of the resulting gypsum material. Future work will be focused on the properties changing with the amount of admixtures (0.25-1.5%) and on morphological aspects of the prepared gypsum plasters.The role of calcium sulphate anhydrate in hydration process is not discussed here.