Changes to hardsetting properties of soil by addition of metal hydroxides

Hardsetting of soil comprises two contrasting, unfavourable physical conditions at different water contents, namely extreme hardness when dry and complete slaking when wet. This behaviour was changed by adding small amounts of ferrihydrite and aluminium hydroxide to the soil. The aggregation and its stability in the wet state increased, whereas the tensile strength of the dry soils decreased. Both resulted mainly by making the fine particles less mobile by aggregation. In some cases adding large amounts of Al hydroxide caused a resuspension of soil particles by reversing the charge, resulting in instability when wet and high stability when dry. We postulate that aggregation results from the interaction between negatively charged siliceous surfaces and positively charged oxides. In addition we think that newly formed Si–O–Fe bonds play a role in binding particles together as a result of a positive relation between the amount of ferrihydrite added and the oxalate-soluble Si. We conclude that hardsetting resulting from weak structure is caused by lack of aggregating agents such as metal hydroxides.