# Utilization of Thermal-Activated Coal Gangue to Enhance the Properties of Sandy Soil Composites
## 1 Introduction  
China is facing the serious problem of land desertification, in which soil on the surface layer loses its fine grains (silt and clay) due to soil erosion and becomes gradually sandy. Sandy soil has the obvious defect of poor water retention due to its large soil particles and lack of clay. At the same time, nutrients are lost in sandy soil more quickly compared to fine-textured soils, making it difficult to maintain long-term fertility, which seriously restricts the sustainable development of arable land in China. Although the implementation of a series of ecological protection and restoration projects in recent years has led to some control of land sanding, sandy soil areas continue to increase. Methods and technologies are required to improve the quality of sandy soil in China and maintain the sustainable development of land resources. Previous sandy soil control and prevention methods were mainly based on vegetation, engineering, and chemical measures. The latest research confirms that the content of soil particles and agglomerates has a direct effect on their resistance to wind erosion [1], and the difficulty of intergranular adhesion between the silt and clay components of sandy soils makes them more susceptible to erosion [2]. Therefore,increasing fine-grained components such as silt and clay can promote the formation of aggregates in weathered sandy soils, which is the most direct way to control and improve the quality of sandy soils. Therefore, the application of soil conditioners to improve the physical and chemical properties and optimize the grain size structure of sandy soil has become an efficient method to achieve sandy soil improvements. There is an increasing amount of research on improving the quality of sandy soils, and many amendment methods and amendment substrates have been proposed (fly ash [3], desulfurized gypsum [4], water retention agents [5], etc.). However, the durability of these improvements is not adequate and there is the risk of pollutant leaching. Therefore, some researchers proposed using coal gangue material as an amendment because it is porous, permeable, retains water, and has a low cost, making it an ideal material for sandy soil improvement. Coal gangue, a kind of solid waste produced in the process of coal mining and washing, accounts for about 10-15% of the total coal production [6]. According to incomplete statistics, the accumulated amount of coal gangue in China has exceeded 6 billion tons, covering an area of more than 13,000 hectares [7]. The annual growth rate of coal gangue is about 500-800 million tons, making it one of the largest components of industrial solid waste disposal in China. The accumulation of coal gangue not only occupies a large area of land but also causes serious environmental problems (the destruction of vegetation, soil erosion, groundwater pollution, and even the spontaneous combustion of waste piles and landslides [8]). However, coal gangue is also a useful secondary resource, and its high-end utilization is of great significance to environmental protection and resource conservation. At present, the utilization of coal gangue is relatively limited, mainly focusing on open-air disposal, road construction [9], land reclamation [10], and the backfilling of collapsed areas [11]. Since coal gangue contains a large number of clay minerals, including kaolinite, montmorillonite, quartz, and illite (the kaolinite content is relatively high, which could represent the compositional characteristics of the coal gangue to a certain extent [12; 13]), and kaolinite clay minerals have a layered silicate structure, they are often used as adsorbents due to their excellent stability, high specific surface area, and adsorption ability. However, its limited cation exchange capacity and specific surface area limit the adsorption efficiency of pure kaolinite [14]. The poor surface hydroxyl activity and low cation exchange of pure kaolinite, when used as an adsorbent, lead to poor adsorption selectivity and easy desorption.