Salt precipitation and evaporation in porous media is an important research topic. However, several aspects remain controversial; notably, whether efflorescence inhibits evaporation in wet soil, the influence of soil texture on salt crust formation and the effect of sulphate precipitation on evaporation under changing hydraulic conditions. Therefore, this study investigates the influence of salt type (NaCl and Na2SO4) and soil texture (sandy soil, sandy loam and silt loam) on salt precipitation and evaporation under different hydraulic conditions (with and without fixed groundwater). Our results demonstrated that a NaCl salt crust strongly inhibited evaporation, even from wet soil (with groundwater), owing to its denser salt structure, which inhibited fluid flow. Moreover, the salt crust pattern differed based on soil texture. Relatively thin, flat and uniform salt crusts were developed in finer soils (silt loam), with greater evaporation resistance, whereas thick and rough salt crusts were developed in coarser soil (sandy soil), with less evaporation resistance. Na2SO4 was precipitated as subflorescence in drying soil, whereas wet soils exhibited a mixed salt precipitation pattern including both efflorescence and subflorescence. Notably, evaporation was primarily inhibited by efflorescence, rather than subflorescence. Our research provides novel insights into the dynamics of salt precipitation and evaporation in natural soils.

Salt precipitation in porous media is widespread, which has garnered great research attention. However, the mechanisms governing the salt precipitation, water flux, and surface temperature changes in homogeneous and heterogeneous porous media remain unclear. This study investigated the dynamics of salt precipitation, evaporative loss, and surface temperature in homogeneous fine sand (0.1–0.25 mm), coarse sand (1-2 mm), and a heterogeneous column with fine and coarse sand. All sand columns were initially saturated with NaCl solution. The experimental results showed that the salt was precipitated as efflorescence above the surface of the fine sand, whereas it was mainly precipitated as subflorescence below the surface of the coarse sand, causing the unconsolidated sand to form a strong stone-like mass. The evaporated loss was significantly higher in heterogeneous than in homogeneous sand, but this difference in evaporation was insignificant in the stage where vapor diffuses through the precipitated salt to the external air. The salt crust formed not only decreased the surface temperature due to increased albedo by salt precipitation, but also resulted in a more discrete temperature distribution in the porous media. Our research results can promote further understanding of salt precipitation and evaporation in porous media.