Efficiency and influence of calcined phyllosilicates during the early hydration of cement

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Calcined clays represent a promising material with great potential. These materials may compensate the increasing demand for cement substitutes in future and the decreasing availability of established supplementary cementitious materials such as fly ash or blast furnace slag. Calcined clays develop their chemical-pozzolanic reactivity by thermal treatment of the phyllosilicates in the temperature range between 550 and 850 °C. Clay mixtures, which are especially interesting for a broad application in concrete industry, exhibit a very complex mineralogy. They consist of different phyllosilicates (kaolinite, illite, mica, montmorillonite) at varying quantities. The composition of clay mixtures can vary not only from region to region but also within one single clay-pit. Basic knowledge on the mechanism of structurally different calcined phyllosilicates and their interaction with the cement clinker phases during early cement hydration is essential for the intended use of calcined clay mixtures in mortar and concrete. During the first phase of the project, it is necessary to investigate the reactivity of different calcined phyllosilicates depending on their mineralogical structures and the impact of chemical-pozzolanic reaction at early age. The interaction during early hydration (within the first 48 hours after water addition) with a model cement will be investigated and quantitatively analyzed on hydrating systems. For the experimental investigations, the two clinker phases Alit and C3A are synthesized and a reduced cementitious model system is developed with the addition of sulfate carrier. Adjusting the granulometry of clinker phases and the addition of the sulfate carrier it is possible to separate the silicate from the aluminate reaction in the cement hydration. It will be possible for the first time to consider the reaction mechanisms of the two clinker phase separately with the addition of different calcined phyllosilicates. The combination of established methods from binder chemistry (in situ X-ray diffraction, thermogravimetry, calorimetry) enables a model for the reaction kinetics of individual calcined phyllosilicates. The results provide basic information for a fundamental understanding of the reaction mechanisms of calcined clay mixtures. Based on this model, it is possible to evaluate the influence of calcined clay mixtures on the hydration kinetics of cements and to assess the suitability of naturally occurring clay mixtures sufficiently for their use in composite cements or as concrete additives without extensive empirical tests.