Synergistic effect of silica fume and nano silica on multiscale pore structure regulation and permeability enhancement of full coral aggregate concrete

Coral aggregate concrete (CAC) is a promising material for marine structures but suffers from high porosity and permeability. This paper presents an experimental investigation into the rheological, mechanical, and impermeability characteristics of 28 C50 CAC mixes cured in natural seawater and incorporating silica fume (SF, 0–12%) and nano silica (NS, 0–3%) as partial replacements of cement. Mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to link multiscale pore network refinement with reduced transport. The individual and synergistic effects of SF and NS were quantified, and the role of pore-network refinement in reducing permeability was elucidated. When used individually, both SF or NS improved mechanical strengths and reduced permeability, with NS exhibiting stronger efficacy at comparable dosage; however, it was found that using more than 10% SF or 2% NS significantly reduces workability, mechanical strength and impermeability performance of the concrete. The synergistic combination of 8% SF + 2% NS provided the best overall performance, combining significant strength gains with marked reductions in water penetration and connected capillary porosity, alongside a distinct shift toward finer, less connected pores. Microstructural and phase analyses indicate that the core of the SF–NS synergistic effect originates from nano-bridging and secondary nucleation mechanisms. The SF–NS synergy adequately consumes portlandite (CH) to form calcium silicate hydrate (C–S–H) gel, densifies the interfacial transition zone (ITZ), fills nanopores and micropores, and suppresses pore connectivity—collectively enabling substantially lower permeability suitable for CAC offshore applications.