Effects of initial water content and Na2SO4 on pore structure and radon exhalation of dried remolded loess.

The pore structure of loess directly governs the storage and migration of radon. In this study, remolded loess specimens were prepared at four initial water contents (8%, 10%, 12%, and 14%) and five Na2SO4 contents (0%, 0.4%, 0.8%, 1.2%, and 1.6%). Low-temperature nitrogen adsorption, Frenkel-Halsey-Hill (FHH) fractal analysis, and radon exhalation tests were conducted to investigate the pore-structure evolution and radon-release characteristics of loess under coupled water-salt effects. The results showed that the pore system was dominated by micropores and mesopores, and all specimens exhibited typical type IV adsorption isotherms with H3 hysteresis loops. With increasing initial water content, the pore structure evolved nonlinearly with increasing water content, with enhanced micropore-mesopore development at intermediate water contents, while the fractal dimension D2 first increased and then decreased, reaching its maximum at approximately 12% water content. The radon exhalation rate followed a similar trend, increasing initially and subsequently decreasing, with relatively high values observed near 12% water content. By comparison, Na2SO4 had only a limited influence on the fundamental pore type but likely modified pore connectivity through salt migration and crystallization. These findings indicate that initial water content is the dominant factor controlling pore-structure evolution and radon release in loess, whereas salt primarily modifies the connectivity of the pore network. Radon release from loess is jointly governed by pore-size composition, pore connectivity, and pore-structure complexity. This study elucidates the intrinsic relationship between pore-structure evolution and radon release under coupled water-salt conditions and provides a theoretical basis for evaluating radon migration and environmental radiation risk in saline loess regions.