Rheological optimization and microstructural enhancement of underwater 3D-printed concrete via solvent-borne epoxy-induced lubrication

Underwater 3D-printed concrete (U3DPC) has emerged as a promising approach for underwater construction and maintenance, but a trade-off persists between material dispersion resistance and printability. This study employs a laboratory-prepared two-component solvent-borne epoxy resin (SEP) system as a modifier to enhance the extrudability and structural compactness of U3DPC through its hydrophobic and lubricating effects. The effects of SEP on the rheological behavior, washout resistance, mechanical properties, and microstructural evolution of U3DPC are evaluated. The results indicate that the dynamic yield stress range for the modified mixtures that print well is 1300–2000 Pa, and the static yield stress range is 1700–2400 Pa, achievable within a suitable SEP dosage range of 2.5%–5% relative to the mass of OPC. X-CT scanning reveals that SEP reduces the average porosity from 4.51% to 1.74%, with the SEP-rich phase partially occupying the original pore space. SEM and low-field NMR analyses suggest that SEP forms protective films on the micro-pore edge wall and avoids interconnected large-scale interfacial defects with volumes exceeding 10 mm 3 . SEP increases the electrostatic repulsion between cement particles by introducing polar groups. XRD and FTIR analyses indicate that SEP restricts the growth of bulk Ca(OH) 2 crystals and contributes to a denser interfacial microstructure between the polymer phase and hydration products. Regarding the critical underwater anti-dispersal property, the SEP-modified U3DPC achieves a 43.1% mass loss rate reduction after standing for 60 min. These findings demonstrate the effectiveness of the proposed SEP-modified mixture design in regulating the rheological behavior and promoting structural densification of U3DPC.