Microstructural and compaction characteristics of tropical black clay soil subgrade modified with lead-zinc mine tailings

Lead–Zinc Mine tailings (LZMT) are wastes generated after the extraction of lead and zinc from mined mineral ore, whose disposal mechanism is gradually becoming environmentally unfriendly. For effective recycling of LZMT, this present study utilized a combination of LZMT and Portland limestone cement (PLC) to improve the compaction and microstructural characteristics of tropical black clay soil (TBCS) for use in pavement design and construction. The LZMT and PLC were added to the expansive soil in varying proportions with mix ratios generated from Taguchi orthogonal array. The result obtained for the compaction characteristics showed that the maximum dry density (MDD) increased significantly when a combination of 20% LMZT and 4% PLC were blended with the expansive soil. The increase in the MDD was attributed to the formation of cementitious compounds. In addition, the optimum mix ratio obtained from the unconfined compressive strength of the TBCS, was used for the evaluation of the pore structure characteristics which included porosity, tortuosity and permeability. The result obtained from the analysis that was implemented with a combination of fractal geometry and Bradley and Roth adaptive thresholding image segmentation technique, indicates the possibility of a slight reduction in the strength properties of the modified soil due to its high level of porosity. Also, the permeability and tortuosity values obtained from the present study suggest a slight increase in the permeability of the modified soil-additive mixtures which may not be unconnected to the occurrence of pozzolanic reaction that resulted in the agglomeration and flocculation of the LZMT-PLC modified TBCS. Furthermore, microstructural analysis was executed on the modified TBCS and LZMT using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The result from the FTIR analysis indicates the pozzolanic character of LZMT in the presence of Si–O and Al–O tension bond with the bonds around 1000 cm⁻¹ wavenumber, while the SEM analysis reveals the formation of a cementitious compound in the modified expansive soil-LZMT-PLC mixture.