Geotechnical Properties and Chemical Composition of Black Roof Shale as an Industrial Raw Material

Abstract

Cretaceous pyritic roof shale from Onyeama mine in Enugu colliery was studied for the geotechnical and compositional characteristics relevant to its use in civil engineering construction and other industries. Properties investigated included moisture absorption, particle size distribution, plasticity, linear shrinkage, compaction, California bearing ratio (CBR), permeability, consolidation, the pH the sulphate content and chemical composition. The shale had a high fines content of 85%, a liquid limit of 55 %, a good standard Proctor compaction strength of 1690 kg/m3, preconsolidation pressure of 36 kN/m2, pH of 6.1 and sulphate content of 0.37%. The shale-water extract indicated a possibility of sulphate attack on concrete in contact with weathered pyritic shale. Permeability impairment of between 1x10-7cm/s to1x10-8cm/s was observed on the compacted shale exposed to water and atmosphere. This was attributed to the formation of jarosite and ferric oxide which are the oxidation products of shale weathering. The relatively high silica content of 56% and the low combined potash and sodium content of 1.74% pointed to the potential of the oxidized black shale as a foundation for road bed, as a raw material for brick and a wide range of structural ceramics, and as filler for paints and plastics. The objectionable pyrite in the clay shale can be removed prior to use by treatment with sulphur-and iron-oxidizing bacterium-Thiobacillus ferroxidans or by aging the shale stockpile.