Influence of lime, calcium silicate and portlandite on alkali activation of calcined common clays

The application of calcite rich common clays has marked impact on the formation of alkali-activated binders. Experiments have shown that the carbonate decomposition can be well controlled via the calcination procedure, whereby the majority of the decomposition and the subsequent CO$_{2}$ release occurs above the calcination temperature of 750 ​°C. The decarbonation of the calcite can mostly be separated from the dehydroxylation of the layered silicates. Depending on the composition of the raw clay material, the CaCO$_{3}$ decomposition leads either to the formation of lime or other Ca rich minerals. The mechanical properties of the alkali-activated binders were investigated and despite the very low amounts of layered silicates of the clay raw materials and the high content of unreactive minerals, compressive strengths of above 20 ​MPa of the mortars could be obtained. The presence of lime in calcined clays up to an adequate amount has a positive effect on post-solidification and the carbonation resistance of the mortars.