Damage process of gravel-sand under freeze-thaw-dry-wet cycle

It is of great significant to study the weathering process of gravel-sand and its influential factors in Ningxia. The gravels are important for crop production such as watermelon because the sandy fields covered with gravels can prevent evaporation and keep soil moisture. However, the weather process of the gravels on the sandy fields is little understood. This study aimed to clarify the damage process of gravel-sand under the conditions of freeze-thaw-dry-wet cycle. The gravels were sampled from sandy fields (gray-green slate)in Xiangshan area of Zhongwei, Ningxia of China (36°56′24″N,105°13′44″E). An indoor simulation experiment was carried out. According to the international soil classification standard and local gravel-sand particle sizes distributions, four levels of gravel sizes (2, 10, 20 and 30 mm) were designed. Based on the soil salinization classification standard in Ningxia, four salinity treatments were considered. The concentration of NaCl were 0, 3, 6 and 10 g/L, respectively, used for simulation of salinization of no, light, moderate, severe levels. A total of 16 treatments including four particle sizes and four salinity levels were designed. Each treatment replicated three times. Each freeze-thaw-dry-wet cycle started from freezing at ?20 °C for 17 hours, increasing temperature to 25 °C for seven hours, drying at 30 °C in a dry oven for 22 hours and then soaking for two hours. Before starting the test, the water content and water absorption of the gravel-sand were measured. During the freeze-thaw-dry-wet cycles, the mass loss (particle size <0.15 mm) of the gravel-sand was measured every 10 cycles. When the cycles were 0, 20, and 80, the microstructure and mineral composition of gravel-sand were determined by a scanning electron microscopy and a X-ray powder diffraction instrument, respectively. The effects of particle size, salinity and the numbers of freeze-thaw-dry-wet cycles on the weathering of gravel-sand were explored. The results showed that the mass loss of gravel-sand with small particle size (2 and 10 mm) was higher relatively. The mass loss and the cumulative mass loss rate of gravel-sand were negatively correlated with particle size, and the cumulative mass loss rate of gravel-sand was positively correlated with the number of freeze-thaw-dry-wet cycle. In general, the relationship between the cumulative mass loss rate of gravel-sand and the numbers of freeze-thaw-dry-wet cycles followed an increasing linear function (R2≥0.928). The factors affecting the cumulative mass loss rate were the numbers of freeze-thaw-dry-wet cycles and particle size. The porosity was high when the particle size was 10 mm and the salinity was 6 g/L, and the porosity was negatively correlated with particle size but positively correlated with the numbers of freeze-thaw-dry-wet cycles. The factors affecting the porosity of gravel were particle size and freeze-thaw-dry-wet cycle. Five kinds of minerals was found in the gravel-sand and they were muscovite, albite, quartz, cholorite and calcite. The content of muscovite and albite were positively correlated with particle size, and quartz content was negatively correlated with particle size. Particle size was the main factor affecting the contents of quartz, muscovite and albite in gravel-sand. The damage of gravel-sand was caused internally by structural characteristics, fissure development and hydrophilic mineral content, and caused externally by freeze-thaw weathering, water-rock interaction and salt leaching. The results provide valuable information for the healthy development and sustainable utilization of soil in gravel-sand land.