Pozzolanic Potential of Calcined Clays at Medium Temperature as Supplementary Cementitious Material

Global warming is intensified by substantial greenhouse gas emissions, with the cement industry contributing significantly by releasing around 0.8 tons of CO2 per ton of cement produced. To mitigate these impacts, in this study, we investigated the pozzolanic potential of calcined clays, assessing their influence on the properties of Portland cement as sustainable alternatives for partial replacement. Three clays from Campos dos Goytacazes, RJ, were analyzed. After drying and calcining at 600 °C, they underwent physical and chemical analysis. The samples were characterized in terms of grain size, moisture content, grain density and plasticity limit. Chemical analysis by X-ray fluorescence identified the elemental composition of the clays, while X-ray diffraction determined the presence of crystalline and amorphous phases. A mineralogical characterization confirmed the amorphization process and classified the clay as kaolinitic. Scanning electron microscopy provided detailed images of the morphology of the particles. The surface area was measured using the Blaine method, which is essential for understanding the reactivity of calcined clays. A preliminary analysis showed that the calcination at 600 °C led to greater pozzolanic reactivity in the clay samples. A thermal analysis showed a loss of mass, suggesting the dihydroxylation of the kaolinite. The pozzolanic reactivity was extensively evaluated by isothermal calorimetry, which monitored the release of heat during hydration reactions through compressive strength tests on the mortars that showed higher strength than the reference. In addition, modified Chapelle and R3 tests were carried out, which showed a direct correlation with the compressive strength, also indicating significant pozzolanic reactivity in the material. The results showed that the clays, when calcined, had a highly reactive amorphous structure, resulting from their transformation through the process of dihydroxylation and amorphization. Calorimetry identified the acceleration of the cement hydration reactions, stimulating the formation of calcium silicate hydrates and aluminum compounds, which are essential for mechanical strength. The partial replacement of Portland cement with calcined clays helps to reduce CO2 emissions without compromising strength and durability, representing a promising strategy for reducing greenhouse gas emissions, with a view to greater environmental sustainability and the efficiency of building materials.