Sustainable Stabilization of Kaolin Clay Using Lime and Ignimbritic Volcanic Tuff Waste: Mechanical and Microstructural Performance

Lime-based stabilization of clayey soils remains a cornerstone of ground improvement, yet the high carbon footprint of lime production drives the search for sustainable supplementary binders derived from industrial and quarrying wastes. Volcanic tuff waste (VTW), a fine powder by-product of wet cutting of ignimbritic tuff blocks, is an underutilized quarrying residue, already fine enough to use directly without grinding or thermal processing, yet its use as a supplementary binder in lime-stabilized clays has not been systematically investigated. This study evaluates VTW sourced from Ahlat (Bitlis, Türkiye) in kaolin clay stabilized with 6% lime, with VTW added at 0%, 10%, 15%, and 20% by dry weight. Mixtures were characterized through Atterberg limits, compaction, unconfined compressive strength (UCS) at 1–28 days, California Bearing Ratio (CBR), XRD, SEM, and FTIR. VTW reduced plasticity index, increased maximum dry density, and lowered optimum moisture content. The 15% VTW mixture achieved the highest 28-day UCS of 4296 kPa, a 17.2% improvement over the lime-only control, and the highest CBR of 80%. XRD revealed Tobermorite 9 Å formation, while SEM and FTIR confirmed cementitious gel phases consistent with pozzolanic reactions. The findings demonstrate that ignimbritic VTW, used directly without processing, is an effective supplementary binder that partially replaces carbon-intensive lime, supporting low-carbon, cost-effective stabilization and the valorization of quarrying waste within a circular economy framework.