The aimed reduction of carbon dioxide emission resulting from cement production requires an increasing substitution of cement clinker by supplementary cementitious materials (SCMs). The group of materials that holds the greatest potential in serving this demand are calcined clays, which therefore attracted much attention from researchers during the past decade. However, the great diversity of clay resources together with their mineralogical complexity on the one hand and open questions regarding their impact on hydration mechanisms on the other, are still factors preventing a more widespread use. This thesis therefore aims at improving the knowledge concerning the influence of clay mineralogy on the pozzolanic reactivity and the mechanisms regarding the impact of calcined clays on the early cement hydration. The first part of the work presents an evaluation of methods for the characterization and reactivity assessment of common clays that are available as raw materials for calcined clays. While a major part of previous studies focused on kaolinitic clays, this study also includes common clays dominated by 2:1 cay minerals. A multiple-technique approach comprising quantitative XRD analysis implementing structure models that consider different types of disorder, complemented by thermal analysis and infrared spectroscopy, enables a reliable mineralogical characterization of the raw clays. The influence of 2:1 clay minerals on the pozzolanic reactivity is clearly verified while stacking disorder in kaolinite influences the rate of heat development but not the cumulative heat during R³ test. Solubility measurements of aluminum and silicon and R³-test provide consistent results and are proven to be suitable for the assessment of common clays in a wide range of mineralogical compositions. The second part of the work deals with the influence of calcined clays on early cement hydration, with a special focus on the aluminate reaction. Investigations in a C3A model system containing calcium sulfate allow the examination of their exclusive impact on the early aluminate reaction. Calcined kaolinite and illite are shown to drastically accelerate the C3A dissolution and the associated ettringite formation, while limestone does not. This observation is related to a preferable adsorption of calcium and sulfate ions or ion pair complexes onto the surface of the calcined clay minerals, which is strongly related to their surface charge and area. The acceleration of the aluminate reaction in a blended Portland limestone cement is attributed to the same mechanisms so that previously produced theories based on the SCM’s alumina content or accelerated C-S-H precipitation through the filler effect have to be extended by direct ion adsorption onto calcined clays. The filler effect of calcined clays is lower than it would be expected based on their large specific surface area in comparison to limestone. The influence of aluminum ions provided by the metakaolin is considered minor with regard to the acceleration of sulfate depletion but enhances the sulfate requirement for a proper resulfation of blended cement. This effect is most evident in the C3A model system. The new findings can improve the assessment of potential raw materials for the production of calcined clays and increase the knowledge required for an optimal mix design of calcined clay blended cement.    «

The aimed reduction of carbon dioxide emission resulting from cement production requires an increasing substitution of cement clinker by supplementary cementitious materials (SCMs). The group of materials that holds the greatest potential in serving this demand are calcined clays, which therefore attracted much attention from researchers during the past decade. However, the great diversity of clay resources together with their mineralogical complexity on the one hand and open questions regarding...    »