Role of hydroxyaluminium ions in the interparticle bonding of layer-aluminosilicate clays

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Abstract
Montmorillonitic clay and halloysitic clay were treated with hydroxyaluminium chloride solutions, which were prepared by dissolving metallic Al in AlCl3 solutions. Aggregates of the clays adsorbing hydroxyaluminium ions were produced by airdrying of the clay pastes. The stabilities of the aggregates to dispersion treatments with water, 0.02N sodium hexametaphosphate solution, pH 9. 8, 0.1 N-ammonium acetate solution and HCl and NH4OH solutions with different pHs were tested by comparing them with those of other homocationic clays saturated with Na, Ca, and Al. Clays adsorbing hydroxyaluminium ions, once formed into aggregates by airdrying, reduced their dispersibilities to great degrees in every dispersion treatment. When clays adsorbed aluminium ions hydroxylated to higher degrees, and also in higher saturation ratios, the aggregates became more stable. X-ray diffraction and DTA patterns of Al(OH)3 precipitates obtained from the hydroxyaluminium salt solution showed that the components were well arranged in a manner leading to Al-octahedral layer formation. It was also recognized that hydroxyls of Al(OH)3 precipitates prepared from a series of hydroxyaluminium salt solutions were not readily substituted by F- ions, which differed from those prepared from AlCl3. These suggest that the hydroxyaluminium ions were apt to interact with themselves becoming polymerized. From the results it was presumed that irreversible aggregation caused by airdrying for clays adsorbing hydroxyaluminium ions might be due to formation of olation-bridges of the adsorbed ions among separated clay particles. It is highly probable that aggregation in allophanic soils may proceed by similar interactions of hydroxyaluminium groups on allophane. The effect of hydroxyaluminium ions in aggregate stabilization was very remarkable, and an improvement of soil physical properties with basic Al salts is expected.

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