*3.1. Liquid–Plastic Limit and Moisture Retention Tests Results*

Figure 4 presents the plasticity chart of the clay amended by additives with different dosages and ratios. The dosage of dual-additives-amended compacted clay was 5%. As the dosage of attapulgite increased from 0% to 10%, the plastic limit of attapulgite-amended specimens increased from 28% to 41%, and the liquid limit increased from 54% to 74%. Its liquid limit increased by 36% compared with unamended clay. With the dosage of diatomite increasing from 0% to 10%, the plastic limit of clay increased from 28% to 29%, and the liquid limit increased from 54% to 57%. Its liquid limit increased by about 5% compared with the unamended clay. The effect of diatomite on the liquid limit of amended

compacted clay was not significant. When the mass ratio of attapulgite to diatomite was 0.25, the plastic limit of the dual-additives-amended compacted clay decreased from 32% to 30%, and the liquid limit decreased from 64% to 60% compared with the mass ratio of 4. The decreasing ranges of plastic limit and liquid limit were 9% and 6% respectively. When the dosage is 5%, the liquid limit of the compacted clay amended by attapulgite and dual-additives (ratio of 4) were 62% and 64% respectively.

**Figure 4.** The plasticity chart of the clay amended by additives with different dosages and ratios.

Figure 5 shows how moisture retention percent varies with the dosage of additives. In general, the moisture retention percent increases with the dosage of the attapulgite increasing, while the dosage of diatomite has little effect on its moisture retention percent. The moisture retention percent with the 10% dosage of attapulgite was similar to 5%. When the attapulgite dosage was 5%, the moisture retention percent was about 66% higher than that of unamended clay. Attapulgite could effectively increase the liquid limit of clay and enhance its moisture retention capacity. When the diatomite dosage was 5%, the moisture retention percent was the largest, which was 7% higher than that of unamended clay. Therefore, the dosage of attapulgite and diatomite was found to be 5% considering the cost.

**Figure 5.** The moisture retention percent with different dosages of attapulgite and diatomite.

Figure 6 presents the variation of moisture retention percent of the dual-additivesamended clay with the ratio of attapulgite to diatomite. The moisture retention capacity of dual-additives-amended compacted clay was the best when the dry weight of attapulgite to the dry weight of diatomite was 4, which was 82% higher than that of unamended clay. It shows that the dual-additives have a great effect on enhancing the moisture retention capacity of amended compacted clay. The liquid limit of compacted clay amended by dualadditives was 3% higher than attapulgite with the dosage of 5%. The moisture retention percent of compacted clay amended by dual-additives (ratio of 4) was 9% higher than attapulgite with the dosage of 5%. Therefore, the increase in the liquid limit of compacted clay amended by additives is one of the reasons for the improvement of its moisture retention capacity.

**Figure 6.** The moisture retention percent with different ratios of attapulgite to diatomite.

The results showed that the attapulgite could greatly increase the moisture retention percent, whereas the diatomite has a limited effect. It is attributed to the molecule's structure [46–48]. There are a lot of pores in the crystal of attapulgite, which could adsorb most cations, water molecules, and organic molecules of a certain size with the Van der Waals Forces, similar to a "zeolite molecular sieve". At the same time, the crystalline water molecules combined with Mg2+ at the edge could form a hydrogen bond, which belonged to the synergistic effect of physical adsorption and chemical adsorption. Whereas, the adsorption mechanism of diatomite is mainly chemical adsorption [49–51]. It could adsorb water molecules due to the hydrogen bonds which are combined with the water molecules and the hydroxyl belonged to the Silanol group and silanediol group. However, it has a limited effect on enhancing the moisture retention percent of clay due to the weak chemical adsorption. Therefore, attapulgite could be screened as an additive to enhance the moisture retention capacity.
