Micro-chemomechanical properties of red mud binder and its effect on concrete

Red mud is a hazardous waste generated from alumina production, and its safe disposal is a worldwide issue. Although some previous studies have reused red mud in cement-based composites, there is still a lack of research on the nano-scale chemomechanical properties of red mud binders. In this paper, atomic force microscopy with innovative QI™ mode, quantitative backscattered electron image, and quantitative x-ray diffraction analysis are conducted to uncover that Young's modulus of the C-(A)-S-H phase was decreased due to high content of red mud. The high porosity of corresponding binders allowed C-(A)-S-H gel to grow relatively unimpededly, resulting in lower packing density and lower modulus. The incorporation of red mud promoted the formation of aluminium-bearing phases and increased both Al/Ca, Si/Ca, and Fe/Ca ratio of formed C-(A)-S-H gel, especially for calcined red mud. At a moderate replacement ratio, the unreacted red mud enhanced the skeleton of the binder matrix with a filler effect. However, in concrete block samples, the size of red mud was insufficient to transfer the stress over limestone particles to mitigate the weak zones of blocks, and the reactivity of red mud always played a critical factor. Besides, recycling red mud in concrete blocks with a dry mix method overcomes the issue of workability reduction.