The intended increasing use of binder with low clinker content requires effective and sufficient supplementary cementitious materials (SCM) in future. Calcined clays thus represent a promising material with great potential. This concerns mainly calcined mixed layer clays with complex mineralogical composition due to their global availability. Naturally occurring clays exhibit pozzolanic properties after a calcination procedure adapted to their mineralogy. The evaluation of the suitability and performance of calcined clays as SCM in future is possible only with an in-depth understanding of chemical reaction mechanisms. The present thesis provides a fundamental understanding of the pozzolanic efficiency as well as differences in the pozzolanic reaction mechanism depending on the type of clay. The mechanisms of the used calcined mixed layer clay have been compared to those of a high-reactive metakaolin. Chemical-mineralogical investigations (ICP-OES, in situ-XRD and TG/DTG) have been conducted in order to evaluate the reactivity and pozzolanic mechanism in a clinker-free model system. The results demonstrate differences in reactivity depending on mineralogical composition and on the solubility of Si- and Al-ions. Furthermore, calcined clays exhibit an autonomous formation of pozzolanic hydrate phases (C-S-H, AFt, AFm). The calcined mixed layer clay shows a significantly lower ion-solubility with a higher Si/Al-ratio compared to metakaolin. The pozzolanic efficiency of calcined mixed layer clay is considera-bly lower than high-reactive metakaolin. The degree of reaction of calcined mixed layer clays is minor after two days, but until 28 days this clay gains about 80 % in relation to the degree of reaction after 180 days. In the clinker-free model system, the majority of hydrate phases are preferentially formed within the first 28 days. Based on the results obtained for the clinker-free model system, the influence of calcined clays has been investigated in cementitious systems on early hydration, the hydration until an age of 180 days as well as on selected properties of the hardened cement paste. Both calcined clays have impact on the aluminate clinker reaction via a significant formation of AFt-AFm-phases during early hydration. An increasing content of reactive aluminates in the cementitious compound promotes in the course of hydration the formation of AFm-phases as compared to CSH-phases. During the main part of the pozzolanic reaction (between 7 and 28 days), portlandite is preferentially bound in AFm-phases and not - as has been assumed so far - in C-S-H-phases. The traditional approach for Si-rich SCM must be extended for calcined clays. The formation of pozzolanic phases increases the total volume of hydrate phases and modi-fies the properties of the hardened cement paste. The results of porosity measurements show a refinement of pore structure, the increase of gel pore content and the decrease of average pore diameter. The denser microstructure results in an increasing strength for cementitious compounds due to the addition of calcined clays. The present thesis confirms the pozzolanic reactivity of calcined mixed layer clays which results not only from the amount of kaolinite but from the contribution of three- and four-layer silicates.    «

The intended increasing use of binder with low clinker content requires effective and sufficient supplementary cementitious materials (SCM) in future. Calcined clays thus represent a promising material with great potential. This concerns mainly calcined mixed layer clays with complex mineralogical composition due to their global availability. Naturally occurring clays exhibit pozzolanic properties after a calcination procedure adapted to their mineralogy. The evaluation of the suitability and pe...    »