Composition design and microstructural characterization of calcined kaolin-based geopolymer cement

A three key parameters (SiO2/Al2O3, M2O/Al2O3 and H2O/M2O, M denotes Na or K) composition design method was firstly proposed based on chemical characteristics analysis of calcined kaolin-based geopolymer cement. A total of 9 Na-poly-sialate-disiloxo (Na-PSDS) geopolymer cement pastes with different mole ratio of SiO2/Al2O3, Na2O/Al2O3, H2O/Na2O were designated to investigate the influence of the three key ratios on mechanical properties and microstructure in accordance with orthogonal design. The results showed that the ratios Na2O/Al2O3 and H2O/Na2O had significant effect on compressive strength. The highest compressive strength (34.9 MPa) was achieved at SiO2/Al2O3 = 5.5, Na2O/Al2O3 = 1.0 and H2O/Na2O = 7.0. Comparing the IR spectra of the 9 pastes also indicated that cement with the highest strength was the most fully reacted one and possessed the largest amount of geopolymers. The microscopic analysis revealed that the fully reacted Na-PSDS cement possessed structural characteristics similar to gel substances in having a wide range of Si endowments, but the framework of SiO4 tetrahedra partially replaced by 4-coordinated Al.