Influence of Thermal Treatment on the Chemical and Structural Properties of Geopolymer Gels Doped with Nd 2 O 3 and Sm 2 O 3 .

In this research, the influence of the thermal treatment of geopolymer gels at 300 °C, 600 °C and 900 °C when incorporated with 5% rare earth elements (REEs) in the form of (GP-Sm) Sm 2 O 3 and (GP-Nd) Nd 2 O 3 was investigated. Changes in the chemical and structural properties of the geopolymer gels during thermal treatment for 1 h were monitored. Physico-chemical characterization was performed using the following methods: diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). Besides the characterization of the fundamental properties, some practical macroscopic properties were analyzed as well: sorptivity, open porosity, and Archimedean density. The stretching vibrations of Nd-O-Si and Sm-O-Si were confirmed at a value of around 680 cm -1 and an Nd-O-Si absorption band at a higher value, together with the most dominant band of Si-O stretching vibration similar for all the samples. No significant chemical changes occurred. Structural analysis showed that for GP-Nd, the largest pore diameter was obtained at 900 °C, while for GP-Sm, the largest pore diameter was obtained at 600 °C. EDS confirmed the amount of dopant to be about 5%. X-ray photoelectron spectroscopy showed that for GP-Nd, the ratio of Si and Al changed the most, while for GP-Sm, the ratio of Si and Al decreased with increasing temperature. The contributions of both dopants in the GP-gel structure remained almost unchanged and stable at high temperatures. The atomic percentages obtained by XPS analysis were in accordance with the expected trend; the amount of Si increased with the temperature, while the amount of Al decreased with increasing temperature. The sorptivity and open porosity showed the highest values at 600 °C, while the density of both geopolymers decreased linearly with increasing temperature.