Engineering and environmental evaluation of red mud amended volcanic ash as a sustainable subgrade material

With the increasing stockpiles of red mud (RM) and the pollution caused to the environment, there is an urgent requirement to utilize the RM. This research carried out laboratory tests to explore engineering properties and environmental impact of RM-reinforced volcanic ash (VA) as subgrade material. VA is mixed with RM in six proportions (Percentage of RM is 5%, 10%, 15%, 20%, 25%, and 30% by VA weight). According to scanning electron microscope (SEM) images, the RM particles will not only fill the voids among the VA particles, but also fill the pores formed by the broken bubbles in VA particles, which increases the compactness of the specimens. The mixtures contain a small amount of illite, which exhibit the characteristics of non-cohesive soil. The water stability of the specimens is poor, and the strength of specimens diminishes with the increasing water content. When RM content (RMc) does not exceed 15%, the pH values and heavy metal contents of the specimens do not exceed the safety limit values required by the standards. And when RMc is 15%, the specimens obtain the maximum compactness and mechanical strength. Therefore, the optimal amount of RM is about 15%. This study provides a new material selection strategy for road construction in volcanic areas and is the first time to propose the use of RM as an reinforced material for VA. This also enriches the application of RM and solves the problems of resource shortage in volcanic areas, overproduction of RM and environmental pollution caused by storage of RM, which is of great significance for sustainable development and environmental protection.