**2. Material and Methods**

The foamed concrete in this paper mainly consists of the foam, cement, water, and other admixtures. Foamed concrete has unique properties such as low density, low strength, high impact resistance, and thermal insulation. However, its expansibility and gas permeability are dominant when used as a gas isolation material in the coal mine goaf. The objective of the orthogonal experiment was thus to produce a new type of foamed concrete that had low density, certain strength, and low gas permeability.

#### *2.1. Material*

The materials used in this investigation mainly include the Portland cement (grade 42.5, China United Cement Group Co., Ltd., Xuzhou, China), naphthalene series superplasticizer, sodium silicate solution and concentrated high-efficiency cement foaming agent (Zhengzhou Pengyi Chemical Building Materials Co., Ltd., Zhengzhou, China). The properties of the Portland cement and the sodium silicate solution are shown in Tables 1 and 2.


**Table 1.** Indexes of ordinary Portland cement.


**Table 2.** Parameters of sodium silicate solution.

## *2.2. Design of the Orthogonal Experiment*

The four main factors of foamed concrete that can affect its properties were chosen according to the experimental goal: water cement ratio, foam (by volume), water reducing agent (by weight), and sodium silicate solution (by weight) based on 1 kg mass of solid powder. For each component, there were four levels, and thus, in total, 16 concrete series were investigated. According to the early analysis of the single factor influence of foamed concrete, the most suitable proportioning interval is determined, as shown in Table 3.


**Table 3.** Parameters for orthogonal experiment.

#### *2.3. Specimen Preparation*

As shown in Figure 1, the specimen preparation can be summarized as follows: firstly, the foaming agent was diluted with water at a ratio of 1:20 (by volume) and then added to the foaming machine. Secondly, the Portland cement, water-reducing agent, and sodium silicate solution are mixed evenly according to a certain mix proportion to make a cement slurry of certain water cement ratio, and then the foam produced in the first step was added and mixed until a uniform slurry was produced. Thirdly, the specimens were poured into molds and compacted with an external vibrator, demolded after 24 h, and kept in a box at constant temperature (22 ± 2 ◦C) and humidity (95%) up to the day of testing.

**Figure 1.** Procedure of specimen perpetration.
