Study on freeze-thaw resistance and pore structure deterioration of fly ash reactive powder concrete based on low-field NMR relaxation

To investigate the effect of fly ash (FA) replacements of cement on the freeze-thaw resistance and microscopic pore structure of reactive powder concrete (RPC), a rapid freeze-thaw test on RPC with different fly ash replacement content was conducted. The macroscopic degradation of RPC was assessed by measuring and relative dynamic modulus of elasticity (RDME) and mass loss. Furthermore, the pore structure deterioration of RPC was examined using low-field nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). The models for freeze-thaw damage and RDME attenuation in RPC were developed. The results demonstrate that an appropriate cement replacement with FA can fill the internal pores of the matrix and fully react with Ca(OH) 2 for secondary hydration, significantly reduce the number of harmful pores in RPC and optimize the pore size distribution, thus inhibiting the generation of cracks during the freeze-thaw cycle. This replacement leads to reduced mass loss of RPC during cycles and an increase in RDME. When the FA replacement was 15 %, the mass loss rate and RDME of RPC after 1100 cycles were 0.136 % and 63.31 %, indicating that the matrix maintained high density. However, when the FA replacement was 35 %, the mass loss rate and RDME of RPC after 600 freeze-thaw cycles reached 0.148 % and 51.37 %, respectively. Excessive FA replacement accelerates the deterioration of the pore structure and weakens the freeze-thaw resistance of RPC.