Sustainable reuse of excavated soils stabilized with fly ash and slag based geopolymers for backfill applications

A clayey soil excavated from the Paris region was investigated for reuse as a backfill material. It was stabilized with alkali-activated fly ash and ground-granulated blast-furnace slag with lime as a reference material. Geopolymer activation was performed using a 14 M sodium hydroxide solution combined with sodium silicate. Cylindrical specimens (33 mm diameter × 71 mm height) were tested for unconfined compressive strength. Untreated soil showed low compressive strength of 0.15 MPa and was unsuitable for direct backfill applications. Lime-treated soil reached compressive strengths of 0.52 and 0.75 MPa at 7 and 28 days, respectively. Alkali-activated fly ash achieved the strength of 0.65 and 0.89 MPa at 7 days and 28 days, satisfying the strength requirements for controlled low-strength backfill. Activated slag achieved strengths of 0.46 and 0.69 MPa at 7 days and 28 days, respectively. The modulus of elasticity reached 87 MPa, 84 MPa, and 85 MPa for lime-treated, activated fly ash, and activated slag at 28 days. Mineralogical analyses confirmed geopolymerization through reduced quartz intensity and the formation of calcium silicate hydrate and calcium aluminosilicate hydrate. These findings showed that geopolymer binders effectively improved the mechanical properties of excavated soil for its reuse as backfill material.