Multi-scale pore optimization of ultra-low water binder ratio cementitious materials (ULWC) for low-temperature performance improvement: Buffering mechanism of pore networks

Although the pore network is critical for the stable low-temperature performance of concrete, research on the mechanisms linking multi-scale pore structures to low-temperature performance remains limited. This study employed pumice to modulate the pore network of ultra-low water binder ratio cementitious materials (ULWC). LT-DSC, 1H NMR, and CT were used to investigated the relationships between pore characteristics (including pore size, uniformity, fractal dimension, and pore shape) and low-temperature performance from micro-, meso-, and macroscopic perspectives. Additionally, nanoindentation and finite element simulations were employed to reveal the control mechanisms of the pore network on the low-temperature performance of ULWC. The result showed that the addition of pumice increased the compressive strength growth rate at -80 degrees C from 44.38 % to 97.05 %. Pore structure analysis indicated that pumice promoted the aggregation of the fractal space of the gel pores and induced the matrix to form more prolate spheroid-shaped pores. Among them, the fractal dimension and porosity were strongly correlated with low-temperature compressive strength, and the low-temperature strength prediction model based on these factors achieved an accuracy of 0.95. Furthermore, microstructural analysis and simulation results suggested that the stress dissipation characteristics of prolate spheroid-shaped pores, in conjunction with low-temperature pre-stress, collaboratively enhanced the low-temperature performance of ULWC. And the low modulus of pumice and its improvement of micropore network uniformity can reduce the low temperatures damage rate of the UHD C-S-H modulus from 22.62 % to 0.82 %. These findings provide theoretical guidance for the targeted optimization of low-temperature performance of concrete in the future.