# Utilization of Thermal-Activated Coal Gangue to Enhance the Properties of Sandy Soil Composites
## 3 Results and Discussion
### Water Vapor Adsorption of Modified Sandy Soils  
The water vapor adsorption curves of sandy soil modified by coal gangue or calcined coal gangue are shown in Figure 8. It is clear to see that the adsorption curves exhibit a character of type II [22]. The adsorption behavior of water molecules on the particles of mixed sandy soil can be divided into three stages: monolayer adsorption, multilayer adsorption, and capillary condensation [23]. It can be seen that the relative pressures corresponding to the two bending regions of the curve are 0.1-0.7 and 0.7-0.9, respectively. The results show that the water vapor adsorption capacity of the coal gangue and calcined coal gangue mixed sandy soil was enhanced compared to CK. For example, when the relative pressure reaches 0.8, CK reaches a water adsorption capacity of 10 cm\({}^{3}\)/g, while CK+ 600 \({}^{\circ}\)C calcined coal gangue shows a capacity of 14 cm\({}^{3}\)/g, and CK + 850 \({}^{\circ}\)C calcined coal gangue reaches 12 cm\({}^{3}\)/g. This indicates that under the high relative humidity, the application of coal gangue and calcined coal gangue all promoted the water adsorption ability of sandy soil. However, the low environmental RH is most often presented in practical situations.  
The relative pressure in the isothermal water adsorption test can be equaled to the relative humidity at the evaluated temperature.  
When P/P\({}_{0}\) < 0.7, the moisture adsorption curve rose slowly with an approximately linear increase, which processed moisture adsorption in the monomolecular layer and multimolecular layer on the surface of the mixed sandy soil. As the relative pressure continues to rise, when P/P\({}_{0}\) > 0.7, the moisture adsorption mechanism shifts from multilayer adsorption gradually to the capillary cohesion of the tiny pores; the moisture content increases sharply, and the adsorption amount is much larger than the previous section of the adsorption amount.  
Under the low humidity environment (P/P\({}_{0}\) < 0.5), the modified sandy soil with 600 \({}^{\circ}\)C calcined coal gangue exhibited the most excellent water adsorption capacity compared to other soil samples. The raw coal gangue-modified sandy soil presented similar water adsorption characteristics to unmodified sandy soil (CK). It is clear to see that the calcination temperature showed a significant influence on the water adsorption of coal gangue-modified sandy soils. After calcination at 850 \({}^{\circ}\)C, the calcined coal gangue contributed an obviously poor water adsorption ability under low RH (P/P\({}_{0}\) < 0.5). On the other hand, under the high humidity environment, the water adsorption capacity of the  
Figure 8: Water vapor isothermal adsorption curve (15% coal gangue mixed sandy soil).  
composites with coal gangue and calcined coal gangue was improved compared with that of CK. This indicates that the application of coal gangue has a positive effect on the water retention of the sand soil under drought conditions, and the water vapor adsorption capacity can be improved by the modulation of the calcination temperature. The water adsorption capacity of TACG can be improved by regulating the calcination temperature, which was induced by the pore densification effect after calcination treatment.