Influence of the nature and amount of carbonate additions on the thermal behaviour of geopolymers: A model for prediction of shrinkage

Abstract In order to develop fire resistant geopolymer materials, it is necessary to understand the parameters controlling their thermal behavior such as the content of alkali or rare earth cations. This study highlights the effect of carbonate nature and amount on the thermal behavior of geopolymers in order to propose a model of thermal behaviour and predict shrinkage. For this, various mixtures of kaolin and calcite and/or dolomite with different percentages were calcined at 600 and 750 °C. The feasibility of consolidated materials based on these mixtures were evaluated and the thermal behavior of the obtained materials was investigated. The characterization of the mixtures revealed the persistence of carbonates after calcination at 600 °C and their partial decomposition after calcination at 750 °C. This fact was explained in the case of high dolomite content by the existence of kaolinite gangue hindering its decomposition. Consolidated materials were obtained from the different mixtures. After thermal treatment at 1000 °C, it was evidenced that the amorphous phase crystallizes to form in a major part leucite. At lower available alkaline earth species (Ca2+ and Mg2+) content, only wollastonite is formed and the shrinkage varies between 15 and 12%. However, at higher content, different calcium and magnesium silicates can be formed such calico-olivine and larnite in the case of calcite based materials and akermanite and merwinite for dolomite based materials. This fact leads to lower shrinkage value (from 6 to 8%). Thus, it is possible to control the thermal behavior and predict the shrinkage of geopolymer materials by controlling the available alkaline earth cations.