Study on the mechanism of Ca2+ and Na+ interaction during the hydration of multi-source solid waste geopolymers

Abstract

As a green and new environmentally friendly material, geopolymer has been rapidly developed and applied in engineering construction and other fields. However, the interaction mechanism in the hydration process of geopolymer prepared by alkali activators containing Ca2+ and Na + respectively is not clear. Therefore, in this study, on the basis of determining the optimal amount of NaOH as an alkali activator, different contents of calcium carbide slag (CCS) were incorporated and the effect of Ca2+and Na + interaction on macroscopic properties was evaluated by compressive strength test. The mechanism of their interaction was also analyzed by XRD, FTIR, SEM-EDS and ICP to determine the microscopic morphology. The results show that when (Na + Ca)/(Si + Al) ≤ 0.6, Na+ and Ca2+ have a synergistic effect, which balances the excess negative charge generated by Al3+ and Si4+ in the four-coordinated state, which promotes the growth of crystals in the hydration products. The hydration products are mainly pin-like C–S–H and N-A-S-H gels. When 0.6 < (Na + Ca)/(Si + Al) ≤ 0.95, with the increase of Ca2+ content, the polymerization degree of Si–O-T bond and O–Si–O bond increases, Ca2+ replaces Na+ in petal-like C(N)-A-S-H gel to form large flake like C-A-S-H gel, and part of the replaced Na+ penetrates into the liquid phase through the pore structure, Al/Si ≈ 0.6, (Na + Ca)/(Si + Al) ≈ 0.95, the concentration of Na+ in the liquid phase is about 49.465 μg/ml to the equilibrium state. The hydration products were mainly C–S–H and C-A-S-H gels, and the compressive strength could reach 43.05 MPa at 28d.The results provide an experimental basis and reference for the study of the hydration mechanism of geopolymer prepared from multi-source solid waste.