Thermal Treatment and Pozzolanic Activity of Calcined Clay and Shale

Clay minerals are the most abundant raw materials for calcined pozzolans. Thermal transformation includes dehydration, dehydroxylation, mineral collapse, and neo-formation. Depending on the type and amount of clay mineral, and the nature and amount of associated minerals, the thermal treatment determines the pozzolanic activity. In this paper, 10 natural shales or clays (six illite, two kaolin, and two loess) from Buenos Aires Province (Argentina) were identified using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetry (TG). The thermal transformation was checked by XRD or FTIR and the electrical conductivity (EC) at the calcination window. The pozzolanic activity was evaluated using the Frattini test and strength activity index (SAI). The Frattini test gives positive results after 7 days and SAI is greater than 0.90 at 28 days, but the compressive strength development depends on main clay mineral. The EC was effective to evaluate the Ca(OH)(2) fixation during the first time and it is related to the specific surface obtained.