*3.2. Gas Diffusion Test Results*

The results of the gas diffusion test of the compacted clay amended by attapulgite or diatomite are shown in Figure 7. The oxygen concentration in the diffusion chamber increased with the time of all dosages. At which time the oxygen concentrations inside and outside the chamber were the same, it reached equilibrium. Figure 7 presents that the *D<sup>θ</sup>* decreased with the increase of attapulgite dosage and its gas barrier performance gradually improved. With the increase in dosage of diatomite, the diffusion rate of oxygen decreased. It shows that diatomite has a good effect on gas barrier performance. It also indicates that diatomite could reduce the *D<sup>θ</sup>* of clay better.

**Figure 7.** The gas diffusion coefficient with different dosages of attapulgite and diatomite.

The results of the gas diffusion test with different ratios of dual-additives are shown in Figure 8. The initial moisture content was 30% and the dosage of dual-additives was 5% of all the specimens. Among that, the *D<sup>θ</sup>* of the amended clay was the lowest at 4.4 × <sup>10</sup>−<sup>7</sup> <sup>m</sup>2/s when the ratio was 1. After considering the economic cost, the dosage of diatomite should be reduced as much as possible. Therefore, it is suggested that the mass ratio of attapulgite to diatomite be 4, the dosage 5%, and the *D<sup>θ</sup>* of the amended compacted clay 8.5 × <sup>10</sup>−<sup>7</sup> <sup>m</sup>2/s, reduced by about 42% compared with the unamended clay.

**Figure 8.** The effect of ratios of additives on the gas diffusion coefficient.

Comparing the gas diffusion test results of the clay amended by attapulgite and diatomite, it can be obtained that the effect of diatomite on the gas barrier performance of the clay is more obvious compared with that of attapulgite, which can be attributed to those crystal structures. Nevertheless, the attapulgite crystals are chain structures or fibrous structures [48], meaning the gas could still migrate through the gap of fibrous or chain structure between the crystals after mixing and compaction. While diatomite, as a porous layered siliceous rock of biological genesis, is composed of diatom wall shells, which are distributed with microporous structure [49–51]. There are few and complex channels for gas migration owing to its wall shells. Therefore, diatomite could be screened as one of the additives to improve the gas barrier performance of amended compacted clay.

In conclusion, it is apparent that the optimal dosage of dual-additives was determined to be 5% and the mass ratio of attapulgite to diatomite 4, based on the experimental results of moisture retention percent and gas diffusion coefficient. In addition, the economic cost is also a factor to be considered.
