## 1 Introduction  
Loess is an eolian precipitate formed by the aggregation of silt, wind-blown clay, and fine sand; it is broken from a larger rock or unit rock, has been deposited by the wind, and is generally found in arid and semi-arid regions [1; 2; 3; 4]. Silt is the paramount grain size portion of loess, which constitutes 50%-70% by weight, whereas sand particles larger than 1 mm constitute less than 5%, and clay particles constitute only a few percent [5]. Loess is characterized by an open structure, in which the primary quartz particles are connected with one another by bonding, thereby resulting in high porosity, low density, and low water content [6; 7; 8; 9]. The bonds will easily be broken down after loading and wetting [10]. Then, the loess can change in degree of saturation, which results in failure of the soil structure; this condition causes geological and geotechnical hazards, such as abrupt collapse, ground subsidence, landslides, surface cracks, and differential settlement [11; 12]. Approximately 10% of loess areas cover the Earth, and they are found in continental dry lands of Asia, Africa, Europe, and South America [13; 14]. In China, loess area covers nearly 4.4% of China's land area and approximately 631,000 km\({}^{2}\) of the total area, and it is mainly distributed in the provinces of Shanxi, Shaanxi, and Gansu located in the central part of the Yellow River basin [4; 15; 16]. One-third of the geohazards occur in the Loess Plateau of China, whereas around 15,000 geohazards occur in the Loess Plateau of Shaanxi Province with an average density of over 6 per km\({}^{2}\)[17]. Geohazards result in damage of infrastructure and urban construction, such as step-down in farmlands [18; 19]. Accordingly, the treatment of the geohazard loess and improvements in its performance by the increase in compaction degree should be the focus of research on the Chinese loess area. Geohazard loess can be also stabilized or reinforced by chemical stabilization and mixing various additives, such as lime, cement, and fly ash (FA) or any mixture of these additives. Many studies have been conducted by engineers and scientists in science and laboratory tests in China [20], New Zealand [21; 22; 23; 24], USA [25], Thailand [26], Bulgaria [27], and the Czech Republic [28]. However, most previous works have focused on loess stabilization, whereas the cost and the environmental pollution have not been paid attention yet. Lime or cement has been widely used in the stabilization of loess [27; 28; 29; 30] in spite of consuming a large quantity of energy, emitting the greenhouse gas CO\({}_{2}\), their negative environmental effects during manufacture, and their cost [31; 32]. Therefore, the recycling of FA is becoming more important than before with the proposal of strict environmental regulations in North and East China [33]. [PERSON] et al. (2012) studied the decrease in CO\({}_{2}\) discharge from normal and saline soils and obtained improved results by the addition of coal FA [34]. FA has usually been used as a soil additive agent with low cost and long-term exploitable resource to alleviate the problem of solid waste worldwide. Massive amounts of FA of approximately 500 million tons are collected every year from coal-fired power plants in China, which accounts for nearly 60% of the total power supply [35]. The particles of FA are generally in the form of spheres of silicon, aluminum, and iron oxides, and the size ranges from 0.01 \(\upmu\)m to 100 \(\upmu\)m [36]. FA is usually used in the fields of building materials, road engineering, backfill engineering, agriculture application, and mineral extraction [37]. However, nearly 70% of FA is estimated to be used in China [38]. Thus, more than 150 million tons of FA are dumped every year, thereby resulting in rapid filling in landfills located in coal-concentrated provinces, such as Shaanxi, Inner Mongolia, and Shanxi [39; 40; 41; 42].