Influence of pore structure characteristics on the mechanical response of cementitious materials to high water pressure

Abstract

Deep-sea infrastructure requires cementitious materials capable of withstanding high hydrostatic pressure. However, the mechanisms governing pressure-induced damage, particularly the role of pore structure characteristics, remain insufficiently understood. This study investigated the response of OPC-based and geopolymer-based mortars exposed to 45 MPa water pressure for 22 h. Mechanical properties, water absorption characteristics, mercury intrusion porosimetry (MIP), and microstructural observations were used to evaluate pressure-induced changes. Among the OPC-based mortars, OPC, FA20 (20% fly ash replacement), and S60 (60% slag replacement) exhibited only minor changes after pressure exposure, with compressive strength variations within ±3%. In contrast, the air-entrained mixture showed approximately 19% increases in both absorption capacity and accessible porosity, accompanied by modest mechanical degradation. The geopolymer systems exhibited markedly different behavior despite comparable initial compressive strengths. GP1 (fly ash–slag) developed visible cracking and experienced reductions of 23.4% and 17.8% in compressive strength and modulus of elasticity, respectively, whereas GP2 (fly ash) remained mechanically stable. MIP revealed substantial pore structure differences. GP1 exhibited a finer pore structure (∼7 nm dominant pore size), while cross-sectional observations revealed numerous entrapped air voids uncaptured by MIP. The results indicate that conventional parameters such as compressive strength, absorption capacity, and accessible porosity alone were insufficient to explain the observed pressure response. Building on these observations, a conceptual pressure-equalization mechanism is proposed: materials facilitating pressure transmission throughout an interconnected pore network accommodate prolonged hydrostatic loading, whereas hindered pressure equalization—caused by fine pore structures and isolated internal voids—leads to localized stress concentrations and mechanical deterioration.