Mechanical property and microstructure development in alkali activated fly ash slag blends due to efflorescence

Efflorescence of alkali activated materials (AAMs) is caused by alkali leaching and precipitation of carbonated salts, which occurs concurrently with leaching and natural carbonation. Efflorescence is specifically driven by precursor and activator variability in AAMs vis-à-vis phase changes, microstructure, and mechanical properties are not well understood. To that end, this study analyses the effects of long-term (90 days) efflorescence on AAMs with eight varied calcium and activator contents and correlated with compressive and splitting tensile strengths. Microstructural features including N-A-S-H/C, N-A-S-H gel change are analysed using Fourier-transform infrared spectroscopy (FTIR), magic-angle spinning nuclear magnetic resonance (MAS-NMR) and thermogravimetric analysis (TGA). AAMs with 9 wt% Na2O and Ca/(Si + Al) ratio of 0.0 to 0.25 exhibit enhanced efflorescence and dealumination of Al[IV] in N-A-S-H/C,N-A-S-H gels, higher mobility of Na+ ions, and natural carbonation. AAMs with 5 wt% Na2O and Ca/(Si + Al) ratio of 0.0 to 0.25 exhibit lower efflorescence and higher stability, in contrast. Under efflorescence, the binder with 5 wt% Na2O and Ca/(Si + Al) ratio of 0.25 exhibited dealumination and carbonation, and the binders with 5 wt% Na2O and Ca/(Si + Al) of 0.0 to 0.10 showed less dealumination and higher concentration of Al[IV] in aluminosilicate gel, indicative of greater phase stability. This study highlights the criticality of calcium and activator doses in controlling the stability of phase composition and mechanical property, which is essential for the industrial application of AAMs.