Effect of Admixtures on Time-Dependent Gas Permeability of Concrete and Correlation with Its Microstructure Parameters

The long-term durability of concrete exposed to marine environments is governed by progressive changes in transport behavior and pore network characteristics. Concrete mixtures incorporating fly ash (FA), slag (SG), silica fume (SF), and basalt fiber (BF) were subjected to natural tidal exposure, after which gas permeability and microstructural evolution were systematically characterized using nuclear magnetic resonance (NMR). Temporal trends in gas permeability were assessed in parallel with porosity and pore size distribution (PSD) measurements. Correlation analysis was subsequently performed to identify the pore characteristics that predominantly control gas transport. All investigated admixtures reduced gas permeability, although their effectiveness varied with exposure duration. Silica fume exhibited the greatest reduction during the early exposure stage, whereas fly ash became increasingly effective over prolonged exposure. Basalt fiber also contributed to improved gas transport resistance, while slag showed a comparatively weaker influence under the investigated conditions. The incorporation of admixtures strengthened the relationships between gas permeability and pore structure parameters. Among the investigated pore characteristics, the fraction of pores within the range of 100–500 nm exhibited the strongest correlation with permeability evolution. Furthermore, critical pore diameter and median pore diameter proved to be more representative indicators of gas transport resistance than mean pore diameter.