Compatibility Between Cement and Superplasticiser in Combination with Fines, Gypsum and Fly Ash

The interaction between superplasticisers and cement in concrete is complex and can result in unpredictable and unwanted concrete behaviour. It is known that the positively charged tricalcium aluminate (C₃A) component of the cement does not only react with the sulphate (gypsum) present in ordinary Portland cement, but also absorbs the superplasticiser. However, the exact interaction is still not fully understood. This study aims to identify compatibility issues between superplasticisers when exposed to gypsum, fine sand dust and fly ash using the Marsh cone test. The results showed that especially gypsum, which is used in the production process of cement, influence the flow time of the concrete and affects the interaction between the C₃A component and the superplasticiser. The more gypsum added, the more superplasticiser is needed to have the same effect. This indicates that the ratio between C₃A, gypsum and superplasticiser can result in compatibility issues. Particle size and shape, as in the case of the fly ash, was found to play a role in the effectiveness of the superplasticiser. When fly ash is used to increase the flowability of a concrete mix, the superplasticiser does not result in the same significant improvement in flowability than without fly ash.