Sulphate optimisation of cements

For optimum cement performance to be achieved, it is indispensable to fine-tune the sulphate supply in the cement with the reactivity of the Portland cement clinker. Insufficient or excessive sulphate supply leads to the formation of reaction products which bridge the space between the cement particles. This in turn results in “premature” or “false” setting. When the sulphate supply is optimised, an ettringite layer that retards setting is formed on the aluminate surface. This process is called optimised setting as it ensures the long workability of the cement paste without undesired, spurious reactions occurring. In this way, optimum consolidation of the microstructure is achieved at an early stage of hydration already.

A mix of gypsum and anhydrite has gained acceptance as a sulphate agent in cement production. Both natural and artificial materials are utilised. Depending on the mill temperature prevailing during grinding, gypsum may dehydrate to varying degrees, forming calcium sulphate hemi-hydrate or anhydrite. The various sulphate agents differ in terms of their solubility and thus in the sulphate supply that is available for ettringite formation during the first minutes and hours of hydration, respectively.

The sulphate supply that is optimum for a clinker can be determined by measuring the setting times of laboratory-made cements. To that end, different portions of gypsum, anhydrite or hemi-hydrate are added to the clinker while its SO3 content is kept constant.

The setting behaviour of the clinkers is monitored by penetrometer measurement. Spurious reactions, such as the formation of secondary gypsum, can be monitored by thermal analyses (DSC). Moreover, the hydration of the cement paste can be observed with the ESEM.