The Significance of Hydrophobicity for the Water Retention Properties of Sand and Coal

For mine wastes such as coal tailings, management of these materials requires complex geotechnical engineering that uses many soil properties, such as water retention. However, coal itself is chemically heterogeneous and often appears to be partially hydrophobic, which affects its water retention properties. This study aims to outline how hydrophobic soil particles and coal alter water retention curves compared to hydrophilic materials. The study involves four materials: sand, hydrophobized sand, crushed rock and crushed coal. Mixtures of sand with different proportions of hydrophobic particles had their water retention curves measured and compared, with the only variable being the particle surface characteristics. The rock and coal were separated into different particle size fractions and had their water retention curves measured and compared, with the only variable being particle hydrophobicity. A clear trend was observed for the sand mixtures: the degree of saturation at any suction was reduced when increasing the hydrophobicity of the material. This trend indicates the fundamental water retention behavior expected for soils more hydrophobic than is typical, which was not clearly demonstrated in previous studies. However, a similar trend was not seen when comparing the rock and otherwise identical hydrophobic coal samples, which actually appeared hydrophilic in terms of water retention. ESEM imaging shows a dual hydrophilic and hydrophobic behavior for coal which may explain the result. However, further research is required to understand the discrepancy, which appears to be caused by an unknown coal–water phenomena.