Valorization of pozzolanicity of Algerian clay: Optimization of the heat treatment and mechanical characteristics of the involved cement mortars

The aim of this study is to optimize the pozzolanicity of Algerian clay deposits that are essentially composed by calcite, dolomite, and illite/muscovite. The heat treatment cycles were applied to these raw materials deposits by varying target temperature and the holding time. The evaluation of dehydroxylation degrees is determined by different techniques such as thermal gravimetric (TG) analysis, differential scanning calorimetric (DSC) analysis and Fourier transform infrared spectroscopy (FTIR). Results show that the optimal pozzolanicity was obtained, if the clay is calcined at 700 °C for 5 h. Moreover, adding 10% of the heat-treated clay deposit to 85% of clinker and to 5% of gypsum by weight leads to manufacturing blended cements with the highest mechanical characteristics. Mortars elaborated with the blended cements were studied and their compressive strengths after exposure to high temperature up to 1000 °C were investigated through compressive test up to the age of 90 days. It appears that adding heat-treated clay to clinker enhances strengths of normalized mortars at early age by comparison to those elaborated with CEM I. However, opposite effects were observed for ages up to 28 days except for blended cements with clay treated at 700 °C-5 h. This was related to the presence of new cementitious compounds (NCC) highlighted by TG analysis. The behavior of mortars after exposure to high temperatures is not affected significantly when blended cements are used. The optimum blended cement is elaborated with deposit clay calcined at 700 °C for 5 h. Concretes manufactured with this blended cement exhibit quite similar compressive strengths than the control one.