Structural study of geopolymers obtained from alkali-activated natural pozzolan feldspars

The structural evolution of geopolymers obtained by the alkaline activation of a natural pozzolan composed by a mixture of feldspars (albite and anorthite), calcite and quartz has been followed up to 28 days. The geopolymers have been synthesized using a sodium silicate solution as an activator with an activator/pozzolan volume ratio =0.6, and curing at room temperature or at 40 °C. The reaction has been followed by isothermal calorimetry and the structural evolution has been followed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The results show that the phases of albite and anorthite dissolve and that a gel is produced, which is identified by an increment of an amorphous halo observed in the XRD pattern at <10° and between 20 and 40° 2θ, characteristic of geopolymers. The albite and anorthite phases reappear after 14 days of curing. The FT-IR results show a decrease in the intensity of the OH-bands and a shift to the right of the Si-O-Si and Si-O-Al bands as the curing time increases due to the depolymerization and structural reorganization that occur during the curing, in accordance with a geopolymerization mechanism. The microstructure observed by SEM-EDS shows that the final product contains dense plates and has a compressive strength of 13.2 MPa after 28 days of curing.