Effect of curing age and water-cement ratio on the mechanical behavior of concrete under high water pressure

The increasing demand for natural resources has driven exploration of the deep sea. However, knowledge of cementitious material behavior under deep-sea environments remains limited. This research aims to investigate the mechanical behavior and pore structure evolution of mortar and concrete subjected to high water pressure of up to 50 MPa, focusing on the effect of curing age, water-cement (W/C) ratio, and coarse aggregate. The study revealed that early age concrete of 7 days experienced strength losses of up to 30% due to high permeability and increased pore pressure. This suggests that water movement within concrete can damage the cement matrix skeleton, compromising mechanical integrity. The W/C ratio significantly impacted water penetration: specimens with W/C ratios of 70% exhibited up to tenfold deeper penetration and a 6-10% increase in total mass gain compared to W/C 30%. While coarse aggregate had minimal effect on penetration depth, it reduced water absorption by nearly half, potentially reducing changes to the pore size distribution. After applying high water pressure, pores with diameters of 200–300 nm coarsened, while those with diameters of 20–150 nm became denser. A mechanism of pore structure change is proposed: larger pores are filled with water and expand, while unfilled smaller pores are compressed and densified by external and internal pressures. In concrete, low W/C ratio and the presence of coarse aggregate reduce pore size and hinders pore continuity, limiting water penetration and pore structure change compared to mortar. These findings provide new insights into the mechanism of pore structure change, contributing to the strength of concrete in deep-sea environments. • Mechanical behavior and pore structure of mortar and concrete under high water pressure were investigated. • Increasing the W/C ratio from 30% to 70% in the specimens resulted in a tenfold increase in water penetration depth and a 6–10% increase in mass gain. • Coarse aggregate reduced water absorption by nearly 50%, significantly limiting water penetration. • High water pressure alters pore structure, causing larger pores to become coarser and smaller pores to densify. • Pore structure change is driven by water filling larger pores and compressing smaller ones, with coarse aggregate mitigating water penetration and change of pore structure.