Effect of Na+ alkaline activation on the geopolymerization of a pure metakaolin at room temperature

The increase in CO₂ emission during Portland cement manufacturing has motivated studies on new binder systems. The production of alternative cements with less environmental impact (zero clinker) has been gaining increasing interest, mainly because green cements emit smaller amounts of CO₂. One of these binders, the geopolymers, are obtained from the alkaline activation of materials rich in silica and alumina, being binders with reduced environmental impact. Therefore, the aim of this work was to study the structural (by DSC and FTIR) and compressive strength evolution of a reference, stoichiometric metakaolin by alkaline activation with Na + ions at 28 and 90 days. The effect of sodium ion concentration at room temperature during geopolymerization was observed. A pure, stoichiometric kaolin and the derived metakaolin calcined in a rotary kiln at 800 ˚C were characterized by XRF, XRD, DSC/TG and PSD. Geopolymeric pastes activated with NaOH and Na₂SiO₃ were made using mixture design. The pastes were characterized by DSC, FTIR in ATR mode and axial compression strength at 28 and 90 days. The DSC analysis showed that the main reactions occur up to 180 min from the beginning of the preparation of the pastes. The main bands identified by FTIR are from the Si-O-T bond and water contained in the pastes. The index of axial compressive strength shows that the geopolymer pastes reached, on average, 90 % of the final strength in up to 14 days. The use of a metakaolin without impurities for SiO₂/Al₂O₃ ratios close to 1.5 resulted in geopolymers with 85 MPa of compressive strength after 28 days of curing.