Imparting water repellency in completely decomposed granite with Tung oil

Engineered water repellent soils have great potential as construction materials for seepage barriers. By adjusting the water repellency of the soil, an impermeable or semi-permeable seepage barrier can be constructed to suit various engineering applications, such as impervious materials to contain water or semi-permeable materials to allow vegetation growth. Industrial clays, geosynthetics and grouting are proven solutions frequently used for similar applications. To provide a sustainable and low-cost solution to create an engineered water repellent soil, this study explored how Tung oil, a Chinese traditional vegetation oil, affected a Hong Kong completely decomposed granite (CDG). Tung oil has already been used in historical structures and suggested as a self-healing agent for concrete. A natural wettable soil from Hong Kong, CDG, was mixed with Tung oil and heated at different temperatures for different durations. The results reveal that the presence of Tung oil developed severe and persistent water repellency with heating, enhancing water repellency until a critical temperature threshold and heating duration was reached. However, a change to soil physical structure due to aggregation was also observed due to the mixing of the Tung oil with the soil. The newly formed Tung-oil impregnated aggregates were found to be (1) stable when immersed in water, regardless of the temperature and heating duration, (2) and to have increasing tensile strength with aggregate size up to a temperature threshold. Based on these findings, not only can Tung oil induce high and persistent water repellency in a natural soil through simple procedures (by mixing of soil with oil), but also aggregation may offer the opportunity to improve soils for other ground applications including to ameliorate erosion on slopes.