Early-age pore-structure evolution in internally cured concrete with super-absorbent polymers: A multiscale study

Superabsorbent polymers (SAPs) have been widely applied as internal curing agents to mitigate autogenous shrinkage and enhance hydration, yet their influence on the pore structure of concrete remains insufficiently understood. In particular, the regulation of early-age pore development has received limited attention, despite its decisive role in determining long-term durability, transport properties, and volumetric stability. To address this gap, this study employed a comprehensive multiscale experimental approach to investigate the pore structure evolution of SAP-modified concrete. Specifically, hardened air void analysis, mercury intrusion porosimetry (MIP), and nitrogen adsorption–desorption (BET) methods was performed to characterize the pore structure from the macroscopic to the microscopic scale. In addition, X-ray computed tomography (X-CT) was employed to visualize the three-dimensional pore morphology and quantify the evolution of pore number and spatial distribution. Results showed that pore refinement and stability (characterized by a reduced fraction of pores >100 μm and minimal variation of pore size distribution between 1 d∼7 d) were jointly governed by water-to-cement ratio, SAP dosage, and particle size. The mixture showing the most balanced pore-structure evolution was achieved at the w/c ratio of 0.40, the SAP dosage of 0.20 %, and the particle size of 60–90 mesh. BET and MIP analyses revealed that the peak water-release effect occurred between 1 d and 3 d, generating temporary porosity due to the partial contraction of SAP, whereas subsequent hydration progressively densified the matrix. X-CT further confirmed that the optimized mixture developed a more uniform pore network compared with the control group, defined by a narrower pore size distribution, lower spatial variability, and a higher proportion of spheroidal pores. These findings offer multiscale evidence clarifying the regulatory role of SAPs in early-age pore evolution and establish a foundation for performance-controlled mix design of internally cured concrete. • A multiscale experimental approach was used to study SAP-modified concrete. • SAP internal curing regulates pore evolution across macro, micro, and nanoscale levels. • SAP induced a temporary increase in pore volume within the 1–3 days interval, followed by progressive matrix densification. • The optimal mix with a water-to-cement ratio of 0.40, an SAP dosage of 0.20 %, and a particle size of 60–90 mesh produced a uniform and stable pore network.