Recycling excavated soil waste into low-carbon alkali-activated materials

Rapid urbanization generates billions of tons of excavated soil waste annually from underground developments, presenting significant sustainability challenges. The objective of this study was to develop a sustainable recycling method for soil waste. Soil waste was calcined to transform its kaolin content into metakaolin. Then, the calcined soil, supplemented with ground granulated blast-furnace slag and uncalcined soil, was used to produce low-carbon alkali-activated construction materials. An optimal formulation of the alkali-activated materials was identified through strength, efflorescence, and microstructure testing, alongside preliminary carbon footprint and economic evaluations. Results showed that alkali-activated calcined soil alone exhibited middle-class strength and a high efflorescence risk. Adding slag altered the alkali-activated products from sodium aluminosilicate hydrate to a coexistence of sodium aluminosilicate hydrate and calcium aluminosilicate hydrate, reducing porosity and refining pore size due to the increased chemically bound water, thus enhancing strength and efflorescence resistance, as well as reducing CO₂ emissions and cost per MPa. When slag was added, substituting 25 % calcined soil with uncalcined soil increased the calcium aluminosilicate hydrate content, enhancing sustainability without sacrificing performance, despite the inert nature of uncalcined soil. The binder with the optimized formulation, i.e. 52.5 % calcined soil, 30 % slag, and 17.5 % uncalcined soil, may be an excellent alternative to the conventional cement-based binder, concerning both mechanical performance (high early compressive strength of 40 MPa after one day) and carbon footprint (42 % reduction in CO₂ emissions compared to the blended binder combining 70 % ordinary Portland cement with 30 % fly ash). These advantages show that this method appears to be a promising option for recycling excavated soil waste and reducing CO₂ emissions.