Effects of Volcanic Slag and Lime on the Engineering Properties of High-Plasticity Clay

This study investigated the effects of the combined use of ground volcanic slag and lime on the engineering properties of a high-plasticity clay (CH) soil. In the experimental program, volcanic slag was added to the soil at contents of 5%, 10%, 15%, and 20% in combination with lime contents of 3%, 6%, and 9%. Atterberg limits, Standard Proctor compaction, and unconfined compressive strength (UCS) tests were conducted on the prepared specimens. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed to evaluate the effects of the additives on the soil microstructure. The results showed that the combined use of lime and volcanic slag significantly reduced the plasticity index, increased the optimum water content, and decreased the maximum dry unit weight of the soil. The UCS of the untreated soil was 391.85 kPa, whereas it increased to 2295.24 kPa in the specimen containing 9% lime and 15% volcanic slag, corresponding to an increase of approximately sixfold. However, specimens containing only volcanic slag exhibited limited strength improvement. The results indicated that a volcanic slag content of approximately 15% resulted in the highest strength among the investigated mixtures, while strength decreased at higher volcanic slag contents. Notably, the specimen containing 6% lime and 10% volcanic slag achieved a UCS of 1892.32 kPa, compared with 1847.56 kPa obtained with 9% lime alone, while reducing the lime content by 33.3%. SEM and EDS analyses revealed the formation of cementitious products and the development of a denser microstructure as a result of pozzolanic reactions. Overall, the findings demonstrate that ground volcanic slag can be considered an effective additive for the stabilization of high-plasticity clay soils when used in combination with appropriate lime contents.