Hardsetting of soil comprises two contrasting, unfavourable physical condit
ions at different water contents, namely extreme hardness when dry and comp
lete slaking when wet. This behaviour was changed by adding small amounts o
f 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 aggr
egation results from the interaction between negatively charged siliceous s
urfaces and positively charged oxides. In addition we think that newly form
ed 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.