Compressibility, structure and leaching assessments of an alluvium stabilised with a sewage treatment sludge biochar-slag binder

Deep dry soil mixing is a ground improvement technique commonly used for treating soft soils. Portland cement is the most commonly used binder, but its long-term use is unsustainable due to the high CO2 emissions associated with its manufacture. Alkali-activated cements are a low carbon alternative, involving the use of pozzolanic industrial by-products and wastes. This study provides insights into the one-dimensional compressibility, internal cemented ‘structure’ and leaching characteristics of an alluvial soil stabilised with a new 100% waste-based cementitious binder, comprising biochar as the alkali activator and blast furnace slag as the pozzolan. The binder has recently been demonstrated by the authors to satisfy European soil stabilisation 28-day compressive strength requirements when using dosages of 7.5 and 10% by dry weight. The biochar successfully activated the pozzolanic properties of the slag; whereby the stabilised soil mixtures developed a cemented microstructure which resulted in improvements in compressibility and stiffness. Oedometer datasets for untreated, biochar-slag- and CEM-II-stabilised alluvium were successfully processed through a framework developed by the authors to quantify their artificially cemented internal structure, for use as an input parameter in advanced constitutive soil models. Leaching results indicated that the heavy and trace metal content of 1- and 28-day cured biochar-slag stabilised samples complied with UK and European waste acceptance criteria, and mean baseline heavy metal concentrations for groundwater resources in England and Wales. This study advocates the new biochar-slag binder as a suitable replacement for Portland cements in soil stabilisation, contributing to the path towards net zero carbon emissions for the ground engineering sector and improving the circular economy.