Effect of thermal pre-treatment conditions of common clays on the performance of clay-based geopolymeric binders

Abstract

This study focuses on the influence of the thermal pre-treatment conditions on the reactivity of common clays in alkaline and alkaline earth solutions, in respect to form geopolymeric binders. Illite and illite–smectite clays were thermally activated between 550 and 950°C in oxidizing and reducing atmosphere, respectively. Material and structural changes were characterized by different methods, like thermal analysis, XRD and dissolution tests. Isothermal calorimetric measurements were carried out to characterize the reaction process in alkaline media. The potential of a raw material for the geopolymer reaction by alkaline activation can be derived from the molar ratio of soluble Si/Al in alkaline solutions. Additionally, the standard Chapelle test was executed to determine the pozzolanic reactivity.It could be found that the molar ratio of soluble Si/Al increases with increasing pre-treatment temperature. The presence of 2:1 clay minerals results in a Si/Al ratio between 2 and 3 and indicates a good potential for a geopolymer reaction by alkaline activation. This behavior was remarkably influenced by the atmosphere during the thermal pre-treatment. The maximum pozzolanicity (reactivity) could be obtained at lower temperatures in the case of pre-treatment in reducing atmosphere than in oxidizing atmosphere. Clays calcined in reducing atmosphere result in a more intensive condensation of the aluminosilicates than clays calcined in oxidizing atmosphere at the same temperature. That is possibly caused by the higher ratio of soluble Si/Al after pre-treatment in reducing atmosphere.The performance of the clay-based geopolymer binders was determined by means of bending strength measurements.