Pure salt expansion behavior in sulfate saline soil under negative temperature conditions

Despite substantial research on stabilizing sulfate saline soil, the behavior of post-stabilization pure salt expansion remains unclear. Understanding this behavior is essential because it can lead to soil degradation, potentially undermining the stability and lifespan of constructed infrastructure. To investigate the pure salt expansion and process of solidified and non-solidified sulfate saline soil under negative temperature, single-cycle cooling test, freeze-thaw cycles test were carried out. Various Na₂SO₄ content (i.e., 0.3%, 1%, 2%, 3.5%, and 5%) and c(NaCl) / c(Na₂SO₄) ratio (i.e., 0, 0.6, 1.2, and 2.0) were considered. Scanning electron microscope (SEM) observations and mercury intrusion porosimetry (MIP) were employ to analyze the pore structures and quantitatively characterize the multi-scale micropores. Results indicated that during a single-cycle cooling, the salt expansion of solidified saline soil was significantly smaller than that of non-solidified saline soil, with the difference reaching up to 36.4 times. After three freeze-thaw cycles at a Na₂SO₄ content of 2%, the final average salt expansion of solidified saline soil was only 41.8% of that in non-solidified saline soil. Under the same salt content, solidified saline soil had smaller and more evenly distributed pores. The proportion of small pores below 1 μm was 37.1%, 4.5 times higher than in non-solidified saline soil. Solidified saline soils with higher Na₂SO₄ content or chlorine-sulfate ion ratio had a greater proportion of large and small pores. Solidified saline soils with 3.5% Na₂SO₄ content had a proportion of pores above 100 μm as high as 23%. The research findings will serve as a reference for controlling salt expansion disease and facilitating engineering construction in sulfate saline soil areas.