Experimental study on pore structure and capillary absorption characteristics of concrete under cyclic sulfate attack

Understanding the erosion of concrete pore structure by sulfate solutions is critical for preventing sulfate-induced damage. This study quantitatively analyzes the evolution of pore structure and capillary absorption characteristics during concrete capillary absorption after cyclic sulfate attack at ambient temperature, employing experimental methods including nuclear magnetic resonance (NMR) and uniaxial compression, alongside conducting mechanical compression tests. The findings demonstrate: (1) The evolution of pore structure under sulfate attack can be divided into three stages: initial stage (0-10 cycles) dominated by erosion product filling leading to porosity reduction; intermediate stage (10-30 cycles) where crack propagation causes a sharp drop in mechanical uniaxial compressive strength ( UCS ); late stage (30-40 cycles) with crack development stabilizing. (2) Concrete capillary absorption intensifies with increasing erosion cycles. Accumulated products reduce average pore size, converting partial pores into effective capillary pores; the dominant pore sizes in mesopores/micropores approach the optimal range (0.1-5 μm); microcrack proliferation increases total capillary pores, collectively enhancing concrete absorption capacity. Quantitative analysis based on the Washburn equation revealed a dramatic increase in the equivalent hydraulic radius, signaling a fundamental shift in the water transport mechanism from capillary absorption to crack-dominated flow. (3) Micropores serve as the core driving force for capillary absorption; mesopores relay water transport; macropores rely on micro-mesopore networks to distribute water; open pores maintain dynamic absorption-diffusion equilibrium. (4) As erosion intensifies, capillary absorption rate in edge zones significantly surpasses that in core regions; furthermore, enhanced overall absorption capacity synchronously amplifies this effect. • This paper comprehensively characterizes the coupling relationship between pore structure evolution and water absorption behavior during cyclic sulfate attack, focusing on revealing dynamic response patterns of water transport mechanisms throughout erosion progression, aiming to provide experimental foundations for refining sulfate attack theory and developing long-term protection technologies for concrete.