Coal mining waste utilisation for soil improvement

Large amounts of residues such as fly ash and carbide lime are produced in southern Brazil due to coal mining for thermal power plants and acetylene gas manufacturing industries. This study looks at the benefits of utilising this sort of lime alone or adding fly ash for improving the engineering behaviour of a sandstone residual soil, comparing both performances with untreated material. Mechanical behaviour of the mixtures are analysed based on triaxial tests using local strain sensors, under drained loading conditions, unconfined compressive and tensile tests, which were performed to evaluate shear strength and deformability characteristics. An analysis by electron microscopy was also carried out to investigate microscopically the influence of the industrial by-products on physico-chemical stabilisation process. The influence of curing time (7, 28, 90 and 180 days), fly ash presence (0 and 25%) and carbide lime content (0, 4, 7 and 10%) were investigated. Results have demonstrated that the carbide lime is a cementitious agent as good as commercial lime and that the presence of fly ash is fundamental for speeding of pozzolanic reactions, causing a sensible increase in stiffness and strength.