Comparison of ternary and dual combined waste-derived alkali activators on the durability of volcanic ash-based geopolymers

In this study, combined waste-derived alkali activators are used to prepare volcanic ash-based geopolymers and to evaluate their durability under freeze–thaw conditions. The comparable alkali activators are Na2SO4–Ca(OH)2 (N–C), Na2SO4-calcium carbide residue (N-CCR), Na2SO4-calcium carbide residue-NaOH (N-CCR-H), Na2SO4-calcium carbide residue-Na2SiO3 (N-CCR-S). Microstructure and strength of the geopolymers are characterized through XRD, SEM, FTIR, NMR and compressive strength measurements. Pore structure of the geopolymers is determined by adsorption/desorption isotherms and BET calculations. It is found that the ternary alkali activators give superior activation synergy than dual alkali activators. The compressive strengths of N-CCR-S and N-CCR geopolymers are 61.0 MPa and 23.1 MPa after 7 d in air. After freeze–thaw cycles for 30 days, the compressive strength of the N-CCR-S geopolymer is three times higher than that of the N-CCR geopolymer. These significant differences originate from their N-A-S-H and C-(A)-S-H gel structures. The N-CCR-S geopolymers possess a more homogeneous and dense microstructure with low pore structure. This study proposes a method for the effective utilization of volcanic ash and alkaline solid wastes.