Evolution of a natural pozzolan-based geopolymer alkalized in the presence of sodium or potassium silicate/hydroxide solution

The present work studied the effect of the type of alkali on the structural evolution of a natural pozzolan from Hidalgo, México, alkalinized with sodium or potassium solution to obtain geopolymers. The Pozzolan is composed of clinoptilolite-type zeolite, calcite, and quartz. The alkali solution was a mixture of silicate/hydroxide of sodium or potassium with a volume ratio of 12. Alkali solution/pozzolan weight ratio was 0.6, and curing was at 25 °C. The geopolymerization reaction from the natural pozzolan alkalized to geopolymer was followed at different stages by isothermal calorimetry showing that the sodium solution promotes higher aluminosilicates dissolution from the pozzolan than the potassium one, and the condensation step is favored to allow the generation of interconnected structures. This step is delayed at intermediates curing times at the presence of potassium solution due to the less dissolution of aluminosilicates and the loss of water observed by FT-IR analysis, consequently slowing down the evolution of compressive strength at this curing time. The alkalized pozzolan produces a gel identified by the amorphous halo at 2θ < 10° and between 20°-40° 2θ in the XRD patterns. This gel is characteristic of the presence of geopolymer. The final geopolymer microstructure shows the morphology of thick plates. The compressive strength of the geopolymers at 28 days shows similar values (±10.8 MPa) independent of the type of alkali solution used, although this evolved differently.