Cement is a significant source of anthropogenic release of carbon dioxide. The CO2 derives mainly from kiln fuel combustion, transport and distribution and decarbonating of limestone. The latter source is fairly constant. Thus one procedure to lower the release of carbon dioxide is reducing the clinker content of the cement by shifting the production from CEM I to CEM II or CEM III cements. Another approach is replacing cement partially in concrete mix design by Type II additions like fly ash, granulated blast furnace slag or silica fume. An alternative to these afore mentioned options provides the use of calcined clay either as reactive part of the cement  or as Type II addition in concrete . Metakaolin is known as a very reactive calcined clay and has been in focus of many investigations [e.g. 3, 4, 5, 6, 11]. Its widespread use in concrete is prohibited mostly by its high price compared to other Type II additions. Suitable and less expensive clay qualities consist rather of a mixture of clay minerals, which range between the clays used in the ceramic industry and those required for the cement production than of single type clay minerals. Thus it is worth taking a closer look at mixed clays. The reactivity of any calcined clay depends on both its mineral composition and the calcination temperature [e.g. 1, 3 - 11]. In most cases these investigations used homogenous clay samples that were calcined at constant temperature and for a period of several hours. Furthermore these clays were ground prior to calcination ensuring a complete reaction to take place. If coarse crushed clay is fed into a rotary kiln it is exposed to varying temperatures on its journey through the kiln combined with temperature gradients due to the size of the chunks after crushing and in addition a varying degree of oxidation. This paper focuses on the impact of such calcined clay on various mortar and concrete properties and its inherent ecological potential.
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