*3.2. E*ff*ect of Foam Content on Properties of Foamed Concrete*

In order to further verify the effect of foam content on the properties of foamed concrete, the samples of 0.5, 1.0, 1.5, and 2.0 L foam based on 1 kg mass of solid powder with a water cement ratio of 0.40 was prepared, and the change rules of gas permeability, dry density, and compressive strength were studied. As shown in Figure 7, when other conditions remain unchanged, the dry density and compressive strength decrease with the increase of foam content, and the gas permeability increases with the increase of foam content, which is consistent with the above results.

**Figure 7.** (**a**) The effect of foam content on dry density and gas permeability; (**b**) the effect of foam content on compressive strength of 3d and 28d.

#### *3.3. Optimum Mix*

The comprehensive balance method was used to compare the results of each factor. As the effect of foam content on foamed concrete is far greater than the other three factors, we first determined the foam content. Due to the large consumption of foamed concrete in the coal mine goaf, considering the economy, Level 1 and Level 2 were excluded according to the dry density in Figure 5a. Meanwhile, considering the stability of the foamed concrete after consolidation, it can be seen that the compressive strength of Level 4 is far below the average level according to Figure 5c,d, so the foam content was selected at Level 3.

As can be seen from Figure 5, the water cement ratio is the secondary factor. In order to isolate gas better, Level 4 with highest gas permeability was first eliminated according to Figure 5b. Considering the economy, the water cement ratio should be as large as possible, so Level 3 was selected according to Figure 5a.

The above analysis can preliminarily determine the selection of the 11th mix proportion of 16 concrete series. In the 11th mix proportion, water reducing agent is at Level 1 and sodium silicate solution is at Level 2, which is very economical. According to the research results and the actual engineering requirements, the 11th mix proportion was chosen to be the optimum mix which can be seen in Table 4, as well as its engineering properties.



#### **4. Application Modes of the Foamed Concrete**

#### *4.1. Application Modes*

For the gas-rich coal mine, the gas flows into the goaf from the coal wall, the falling coal and the roof, which makes a large number of high concentration gas accumulated in the goaf, causing the gas leakage into the workface. Under normal conditions, the air pressure between the goaf and workface is basically balanced, and the gas in the goaf is mainly brought out from the upper corner by the roadway air from the lower corner, as shown in Figure 8. In order to reduce the gas concentration and ensure the safety and stability during the operation of the workface, two ways of isolating gas by using the foamed concrete are put forward:


**Figure 8.** Schematic diagram of application modes.

#### *4.2. Mechanism of Application Modes*
