Investigation into the effect of calcium on the existence form of geopolymerized gel product of fly ash based geopolymers

Abstract Fly ash-based geopolymer, regarded as a eco-friendly cementitious material instead of ordinary Portland cement, has been rapidly developed and applied in practical engineering practice. Previous researches demonstrated that calcium component produces distinct effect on the formation of polymer gel products, and therefore influencing the macroscopic mechanical behaviors and microstructure of geopolymers. However, the influence mechanism of calcium component on the formation of gels product is still not clear. In this study, different content of Ca(OH)2 were adopted to prepare calcium containing geopolymers. The compressive strength and hydrochloric-acid attack tests were conducted to evaluate the effect of calcium content on the macro-performances. Then, scanning electron microscopy-energy spectrum test (SEM-EDS) was carried out to accesses the morphology and elemental components of the prepared composites. Thereafter, the microstructure of gels product was probed through Fourier transform infrared (FTIR) spectroscopy and 29Si nuclear magnetic resonance (NMR) spectroscopy. The critical value of elemental ratio (Na + K + Ca)/Al to characterize the gel product was specified. Two kinds of fly ash and two alkali-activated solutions were adopted to verify the results. The geopolymerization products will be calcium-containing geopolymer gels (C,N-A-S-H) when (Na + K + Ca)/Al≤0.95, while be coexist form of C-S-H and N-A-S-H gels when (Na + K + Ca)/Al>0.95. The results provide experimental basis and references for the application of calcium-containing solid wastes in geopolymer materials.