Calcium carbide residue modification of natural volcanic ash: Strength enhancement mechanisms and life cycle assessment

In volcanic regions, volcanic ash cannot be directly used as an engineering material in construction, which necessitates stabilization. This study proposes a method for stabilizing volcanic ash using calcium carbide residue. The influence of calcium carbide residue content and curing time on the strength of the specimens, as well as changes in their material composition and microstructure, was evaluated using unconfined compressive strength tests, direct shear tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analyses. The overall environmental performance of the volcanic ash-calcium carbide residue mixtures was assessed using the life cycle assessment (LCA) methodology. The results demonstrate that incorporating calcium carbide residue significantly enhances the unconfined compressive strength and shear strength parameters of the volcanic ash. Compared to the control group of pure volcanic ash, specimens with calcium carbide residue contents of 3%, 6%, 9%, and 12% exhibited significant increases in unconfined compressive strength after 28 days of curing: 414%, 472%, 544%, and 690%, respectively. The calcium carbide residue improved the shear strength of the soil, with the internal friction angle peaking at a 6% calcium carbide residue content. Microstructural analysis revealed that during the initial curing stage, the calcium carbide residue bonded the volcanic ash particles together. With prolonged curing, a hydration gel formed between the calcium carbide residue and the volcanic ash, leading to particle cementation, which enhanced the strength of the volcanic ash. The LCA results indicate the mixture still exhibits lower environmental impacts than that of existing materials such as red mud-recycled polyester fiber.