## 1 Introduction  
Fly ash has the advantages of being light weight, of low compressibility, and having a rapid rate of consolidation. Taking fly ash as the material for roadbed paving can not only save energy and reduce emissions, but also reduce the pollution caused thereby to some extent [1; 2; 3; 4; 5]; however, when the sulphur content of fly ash is high, the fly ash reacts with lime and hardens. Volumetric expansion occurs at the later stage of strength development, which leads to the cracking, arching, and uneven settlement of pavement bases [6; 7; 8; 9; 10]. Therefore, it is necessary for geotechnical engineers to find a treatment method using high-sulphur fly ash for roadbed paving. In the present research, high-sulphur fly ash, cement, and a dense foam were mixed to form a light porous material (called lightweight cement-treated material), to overcome the adverse effects encountered in the application of high-sulphur fly ash. Lightweight cement-treated materials (LCMs) are a filling material consisting of cement, low-liquid-limit soil, water, and bubbles. Its density and strength can be freely regulated by changing the bubble content [11; 12; 13; 14; 15]. Due to its adjustable density and workability, LCMs are widely used in foundation treatment, backfill grouting, and slope reinforcement [16; 17; 18]. Especially in soft soil foundation construction, LCMs are often used as a more economical and reasonable treatment method. [PERSON] [19] showed the potential of a foamed concrete layer as the base of a pavement structure. As a typical pozzolanic material, fly ash can improve the hydration and compactness of a material's microstructure when it is used in cement-treated soil systems [20; 21; 22; 23; 24; 25]. Most importantly, the expansive stress caused by the late hydration of high-sulphur fly ash can be buffered by the numerous pores in cement-treated materials. Studying the effect of high-sulphur fly ash on LCMs is not only expected to improve some properties of LCMs but also provide an economical and safe treatment method for high-sulphur fly ash. Studies have shown that the materials with the properties of volcanic ash can be used with cementitious materials, such as cement. [PERSON] [26] showed three trends in mixes of Mount Etna volcanic ash and cement materials; among them, the mechanical and thermal properties of the mixture are good [27] when Etna volcanic ash is applied to insulating mortar and light mortar. [PERSON] et al. [28], [PERSON] et al. [29], [PERSON] et al. [30], and [PERSON] et al. [31] found that the curing performance of mixed mortar can be improved to a certain extent when volcanic ash is used as a partial replacement for Portland cement. [PERSON] [32] showed the effectiveness of volcanic ash in replacing part of the cement. [PERSON] [33] further concluded that the best ratio of pozzolanic ash to cement was 30%, at which the strength and impermeability of concrete products were improved. In recent years, fly ash, used as a pozzolanic material, has attracted much research interest. For example, the durability of fly ash concrete has been studied [34] and it was found [35] that pore distribution directly affects the performance of LCMs, the addition of fly ash can increase the degree of bubble coalescence, and the spherical structure of fly ash can make the bubbles move and coalesce more easily. In addition, the presence of fly ash also improves the workability, mechanical properties, and freeze-thaw resistance of such materials [36; 37; 38; 39; 40; 41]. The partial replacement of cement and sand with fly ash can also reduce material shrinkage [42]. Compared with LCMs without fly ash, samples containing fly ash have better thermal insulation [43; 44]; however, when using fly ash with a high sulphur content as an admixture, there is no relevant research to prove that LCMs can still meet all the engineering performance criteria, especially when LCMs are used in some areas where there is a large amount of sulphate present.