Self-compacting earth-based concretes with complete replacement of natural aggregates by excavated soils and recycled concrete aggregates, stabilised with low-carbon binders: A performance-based case study from the Grand Paris Express

Conventional concrete made with Ordinary Portland Cement (OPC) consumes large quantities of virgin sand and gravel and contributes significantly to global CO₂ emissions. This study presents a case study on the development of Self-Compacting Earth-based Concretes (SCEC) in which natural aggregates are completely replaced by (i) excavated earth alone (MC series) or (ii) a blend of excavated earth and recycled concrete aggregates (RCA; MCGR series) sourced from the Grand Paris Express project. The excavated soil, a marl and stony gravel from the Paris region, was combined with demolition-derived RCA and stabilised using three binders with contrasting carbon intensities: OPC (CEM I), a slag-rich low-clinker cement (CBC1), and a clinker-free alkali-activated binder (CBC2). All six mixtures exhibited self-compacting behaviour (slump-flow 560–730 mm) with no visible segregation and fresh densities below 2000 kg/m 3 . At 28 d, compressive strengths ranged from 3 to 5 MPa, exceeding the targeted strength threshold for low-strength construction materials. Additional durability-related indicators, including drying shrinkage and freeze–thaw resistance, were assessed to evaluate the robustness of the proposed materials. Thermal conductivity was measured under both moist and dry conditions, highlighting the influence of porosity and moisture state on insulating performance. Microstructural investigations (XRD, TGA, MIP, SEM/EDS) indicated that CBC1 and CBC2 limit portlandite formation and promote low-Ca binding phases, resulting in a denser matrix and well-bonded interfacial transition zones around RCA. Overall, this case study demonstrates that binder optimisation, rather than restrictive aggregate grading, is a key parameter for producing self-compacting, low-strength earth-based concretes from excavation material and demolition waste, offering a practical material solution for circular construction in large infrastructure projects.