Clay stabilization using coal waste and lime — Technical and environmental impacts

• Coal waste powder and its ash were used as stabilizer additive for intermediate plasticity clay.
• The addition of coal waste powder and its ash slightly reduced the soil plasticity.
• Stabilized soil had higher CBR and compressive strength than the natural soil.
• The combination of hydrated lime and coal waste is recommended especially at saturated condition.
• Micro-structure analysis proved the existence of pozzolanic reaction products in soil matrix.

This study evaluates the technical and environmental effects of coal waste materials on the stabilization of a medium plastic clay. The coal waste was used in its natural state and after igniting at 750 °C to produce the coal waste ash. In addition, hydrated lime powder was applied as a traditional stabilizer. Atterberg limits, California bearing ratio (CBR) in dry and saturated condition along with swelling and unconfined compressive strength tests were carried out. Soil structural changes during the curing period were analyzed using the X-ray diffraction and scanning electron microscope (SEM) tests. About the environmental issue, toxicity characteristic leaching procedure (TCLP) test was carried out to analyze the leachate obtained from different studied samples for heavy metal concentrations. Based on the CBR and compressive strength tests, the addition of coal waste powder and its ash to some extent enhanced the soil bearing capacity. However, the combination of these additives with lime resulted in considerably higher compressive strength and CBR especially in saturated condition. The results of X-ray diffraction and SEM analyses indicated substantial changes in the soil structure after adding the additives. The initial structure of the soil was porous and flaky which transformed to a solid and coherent structure after treating with the combination of coal waste and lime. Concentration of heavy metals in stabilized samples containing the coal waste and its ash was less than the regulatory levels determined for hazardous materials and properly satisfied the minimum requirements.