Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil

Shear strength is the key index to determine the stability of a soil slope, and cementation between iron oxide and clay minerals is one of the internal factors affecting soil shear strength; however, the effects of the form of iron oxide on the shear strength of granite-weathered red soil are still unclear. Kaolinite, which is the main clay mineral of granite red soil, was selected as the research object, and the effects of three different forms of iron oxide (hematite: HT, goethite: GT, and amorphous iron oxide: AIO) on the soil microstructure, microscopic quantitative parameters, cohesion, internal friction angle, and shear strength were analyzed by scanning electron microscopy, X-ray diffraction, and the shear strength test. The results revealed that the iron oxide promoted the cementation of soil particles, and the cementation characteristics differed with the different forms of iron oxide. Hematite mainly showed flocculent cementation, poor cementation, and simple soil microstructures. Goethite mainly exhibited acicular cementation and the best cementation effect. The degree of aggregation of the soil particles was increased by the coatings, thus forming larger aggregate particles. The cementation effect of amorphous iron oxide was between those of hematite and goethite but included both the flocculation cementation of hematite and acicular cementation of goethite. Amorphous iron oxide and goethite effectively increased the contact area and friction degree between soil particles, while hematite had the opposite effect. The addition of three kinds of ferric oxide reduced the fractal dimension of soil, increased the apparent porosity, and promoted the irregularity of particles to a certain extent, among which hematite had the most significant growth on the long and short axes of the particles. At a content of 10 g kg−1, the addition of AIO and GT increased the soil cohesion and internal friction angle, and therefore increased the soil shear strength, and it was mainly determined by the soil microstructure: the contact area, apparent porosity, and particle short axis. These results indicated that GT and AIO are the main cementing materials affecting soil mechanical properties, and the transformation of iron oxide should be paid attention to when predicting soil slope stability.