Effect of curing temperature on the synthesis, structure and mechanical properties of phosphate-based geopolymers

The purpose of this work was to study the effect of curing temperature on the synthesis, structure and mechanical properties of Phosphate-based geopolymers, using two curing temperatures for geopolymers preparation: room temperature and slightly elevated one (60 degrees C). The main techniques used to achieve this purpose are quasi-isothermal Differential Scanning Calorimetry measurements (DSC), atomic absorption measurements, compressive strength measurements, Fourier Transform Infrared spectroscopy (FTIR), X-ray powder Diffraction (XRD) and Si-29, Al-27 and P-31 magic angle spinning nuclear magnetic resonance (MAS-NMR). Experimental results showed that the increase of curing temperature activates the dealumination of metakaolin and accelerates the different geopolymeric steps. Moreover, for both considered curing temperatures, the obtained geopolymer presents an amorphous composite material composed of two geopolymeric networks: a first geopolymeric network based on Al-O-P units and another one based on Si-O-T units (with T = Si, Al and P). The only structural difference generated by the variation of curing temperature is in the quantity of the formed phases. In fact, the temperature elevation yields to a geopolymer structure richer in aluminum phosphate phases which enhances the material compressive strength (29.9 MPa).