Amorphous and crystalline aluminum and iron oxide minerals play a major role in stabilizing soil structure as measured by aggregate stability and clay dispersion. Aluminum and iron oxide interactions with clays are pH dependent. At low pH, where the oxides carry sufficient positive charge, they precipitate on clay surfaces. These coatings, once formed, are stable at higher pHs. Precipitation of oxides at high pH occurs as phases separate from the clays. Aluminum and iron oxides stabilize clay minerals by decreasing critical coagulation concentration, clay dispersion, water uptake, and clay swelling and by increasing microaggregation. The presence of aluminum and iron oxide minerals in soils has a favorable effect on soil physical properties, increasing aggregate stability, permeability, friability, porosity, and hydraulic conductivity, and reducing swelling, clay dispersion, bulk density, and modulus of rupture.

Interaction of aluminum and iron oxides and clay minerals and their effect on soil physical properties: A review

Following the partial collapse of a shale embankment in Derbysllire, a detailed mineralogical, chemical and electron microscopical investigation was carried out to establish whether the crushed mudstone fill had been significantly affected by chemical weathering after emplacement. Samples of unweathered and weathered Namurian mudrocks from the area were also analysed for comparison and laboratory experiments carried out to simulate abiotic and biochemical shale weathering. It is concluded that chemical alteration of the shale is a rapid process and that significant post-emplacement chemical and mineralogical changes occurred in the embankment fill. The main reactions involve oxidation of framboidal pyrite, leaching of carbonates by sulphuric acid, formation of authigenic sulphate and iron hydroxide minerals, and leaching of cations from clay minerals. In parts of the fill forming the outer shoulders of the embankment, up to 10lume loss may have occurred due to acid leaching.

Chemical and biochemical weathering of pyritic mudrocks in a shale embankment

Interparticle action and rheology of kaolinite-amorphous iron hydroxide (ferrihydrite) complexes

The Bingham yield stress for suspensions of illite-iron oxide complexes is examined as a function of pH and iron oxide content. Addition of iron oxides to illite increased the yield stress over a pH range of 3 to 10. With increasing pH the yield stress decreased for the complexes with iron oxides of 2 and 5% while the complexes with iron oxides of 7, 10, and 20% exhibited the maximum yield stresses at pH values from 6 to 8. The iron oxides which are not associated with the illite enhanced the yield stress more than the iron oxides precipitated on the illite surfaces. The yield stress for the complexes was correlated with their zeta potential, and the higher yield stress is considered due to smaller interparticle repulsion.

Particle Interaction and Rheology of Illite-Iron Oxide Complexes

As part of a long-term study of factors contributing to soil sensitivity, investigations have been carried out on some volcanic ash soils from the North Island of New Zealand. We report here that some soils have sensitivities of up to 140 and clay mineral contents of >80%. They apparently owe their high sensitivity to the fact that the clay is halloysite of the spherical variety, and that interparticle interactions with this particular soil mineral are remarkably slight. Although the clay soil has a high liquid limit (∼65–80), it has a low plasticity index (∼15–30) and its behaviour supports the inactive particle theory of soil sensitivity. When plotted on the Casagrande Plasticity Chart all samples plotted well below the ‘A’ line.

A.J. Sethi

Papers

Basic characterization and effect of some chemicals on a clay from outardes 2