**1. Introduction**

In recent years, China has widely used the comprehensive mechanized coal mining method, which has the characteristics of fast advancement speed and long workface. However, this method has a high degree of disturbance to the coal and rock mass, which can make the gas rapidly accumulate in the goaf and pose a serious threat to the safe production in a gas-rich coal mine [1–3]. Filling and sealing, as one of the key technologies to control the goaf gas, is a necessity to isolate the gassy areas and achieve permanent barriers between the workface and gas sources [4]. However, the existing inorganic gas-sealing materials are not efficient enough to seal the leaking gas, and the existing organic foam materials have negative impacts on the underground environment. Therefore, it is necessary to develop a low-cost, effective, and environmentally friendly material for roadway filling and sealing, which has the properties of compression, expansion, filling, sealing, and gastight, so as to ensure the safety, reliability, simplicity, and rapidity of coal mine underground work. Due to its low density, low thermal conductivity, low permeability, high expansibility, and high strength, foamed concrete has interested many experts [5,6]. Wang et al. [7] studied the effects of foam content, crumb rubber content, fly ash content, and water/binder ratio on the physical, mechanical, and waterproof properties of a lightweight foamed concrete by the single-factor and orthogonal tests. Ben et al. [8] investigated the mechanical characterization of non-autoclaved foam concrete according to its macro porosity. Khan et al. [9] investigated the effects of cement and recycled glass powder contents, water to cement ratio, and volume of foam on the plastic density, dry density, and compressive

strength of foam concrete. Bing et al. [10] developed structural foamed concretes by using silica fume, fly ash, and polypropylene fiber and presented the use of fly ash for fully replacing sand to produce foamed concrete. Huang et al. [11] investigated the proportioning and properties of Portland cement-based ultra-lightweight foam concrete which was recommended to achieve energy efficiency in buildings. Zhao et al. [12] developed a new type of foamed concrete used as a seismic isolation material by orthogonal experiment. Hu et al. [13] developed a new type of foamed concrete and low-alkalinity sulphoaluminate cement, to control air leakage. However, the foamed concrete from the above studies is rarely applied in the coal mine goaf to control the gas. Some studies have studied the gas permeability of goaf or concrete: Dziurzynski et al. [14] developed a mathematical model for sealing mine goal based on the balance of the volume of a mixture supplied and contained in the body formed in the goaf. Based on the standard specimen test, Zhang et al. [15] studied the relationship between the gas permeability of slag high performance concrete and the carbonation depth, and the law of influence on carbonation depth of slag high performance concrete by the gas permeability is concluded. Md et al. [16] adopted two different procedures to carry out permeability tests on concrete specimens either unloaded or preloaded during the heating process, which showed that concrete permeability strongly depends on crack width and orientation.

Hence, this paper aims to determine the effect of mix proportions on the properties of foamed concrete and then produce a new type of foamed concrete that could be used as gas isolation material in the coal mine goaf. Firstly, the orthogonal experiment was developed to study the optimum mix, in which the dry density, gas permeability, and compressive strength were studied as material indicators. Then two application modes of the foamed concrete for gas isolation in a mining area were investigated. At last, taking a large-space goaf with high-strength and rapid advance as the engineering background, the convection-diffusion model of gas was built to reveal the mechanism of different application modes using the parameters of the new foamed concrete.
