Synthesis, structural and thermal behavior study of four Ca-containing silicate gel-glasses: Activation energies of their dehydration and dehydroxylation processes

Calcium silicate amorphous materials of general formula xCaO(1–x)SiO2, being 0.3 B x B 0.6, and pure silica (SiO2) have been prepared by the sol–gel method and characterized by X-ray diffraction. Freshly prepared glassy samples of these materials were reduced to fine powders by gentle grinding in an agate mortar. Subsequently, a simultaneous thermogravimetry (TG) and differential thermal analysis equipment was used to study their thermal behavior under a stream of argon in the temperature range from room temperature to 1200 C. All samples undergo dehydration up to 250 C followed by a second process with a slight mass loss in the range 350–500 C due to the thermal degradation of the residual nitrate, followed by a third endothermic process ascribed to dehydroxylation, due to evolution of water by condensation of the OH– surface groups, took place between 500 and 600 C. At high temperatures, around 900 C, all gel-glasses undergo an exothermic process without mass changes ascribed to crystallization. A reliable isoconversional procedure was applied to investigate the conversion dependency of activation energy for the thermally activated release of water molecules physically or chemically bound to the structure of the materials as well as that to dehydroxylation. The results obtained were discussed in terms of the different water absorption capabilities related to their different molar ratio Ca/Si and, depending on the amount of Ca, to the formation of one or more calcium silicate phases that crystallize at high temperature.