Micro/meso scales characterization of alkali-activated fly ash-slag concrete under sustained high-temperatures with X-CT, MIP, and SEM tests

Based on X-CT, MIP, and SEM tests, the micro/meso scales evolutions of alkali-activated fly ash-slag (AAFS) concrete under sustained high temperatures are studied. The results show that the water loss of the C–S–H and C-A-S-H gel phases at 60 °C is continuous (about 60d). The variation law of meso-scale pore volume based on the X-CT test is consistent with that of micro-scale pore volume obtained by the MIP test. The 2D fractal dimension can be used to qualitatively evaluate the internal micro-cracks and microstructure complexity of AAFS concrete after sustained high-temperature action. The mass loss rate can reach stability within 3d-7d under the action of 100 °C, 150 °C, and 200 °C, and there is no cumulative effect of micro-cracks and pores caused by initial water loss. Under sustained high-temperatures, the internal pore structure, micro-cracks, and microstructure changes of AAFS concrete are persistent, and these persistent changes lead to a significant change in the mechanical properties of AAFS concrete.