Upcycling excavated soils into compressed earth blocks using low-dosage gelatinized starch and hemp fibers: comparative study between France and Côte d’Ivoire

Large infrastructure and urban projects generate massive excavated-soil streams that are commonly downcycled, despite their potential to supply local, low-carbon masonry units. In this work, compressed earth blocks were manufactured from two contrasted excavation-derived soils (a silty soil and a sandy-clayey soil) and mineral stabilization (5% cement) was compared with low-dosage bio-based stabilization (gelatinized starch 1.5–2.5%) with and without hemp fibers (0.5%). All formulations were compacted at 150 bar and tested at 28 days. The direct France / Côte d’Ivoire comparison was based on properties measured with comparable procedures in both contexts, namely bulk density, compressive strength, volumetric shrinkage and thermal conductivity. Open porosity, specific heat capacity and XRD–Rietveld analyses were conducted on the French soil series (Soil A) to further support the mechanistic interpretation, while capillary water absorption was assessed on the Ivorian soil series. Cement stabilization delivered the highest compressive strength, up to 9.92 MPa, whereas starch-based formulations achieved multi-MPa performance at much lower stabilizer contents. The starch–fiber formulation reached comparable strengths in both soils (5.07 ± 0.87–5.34 ± 0.10 MPa), indicating the contribution of fibers to crack bridging and performance stabilization. Thermal conductivity remained moderate (0.280 ± 0.006–0.343 ± 0.006 W·m⁻¹·K⁻¹). During 24 h capillary absorption/partial immersion, unstabilized blocks collapsed, whereas starch-stabilized and starch–fiber blocks maintained integrity, with sorptivity values of 0.070 ± 0.005 and 0.068 ± 0.004 g·cm⁻²·min⁻⁰·⁵. XRD–Rietveld results distinguished cement hydration from starch-gel-mediated bonding and densification, confirming gelatinized starch, especially with hemp fibers, as a credible low-dosage CEB stabilization route.