Mechanical, microstructural and environmental performance of 100% recycled-aggregate concrete with low-carbon binders

• Concrete uses 100 % recycled fine and coarse aggregates from Grand Paris Express. • Three binders tested: CEM I, a slag-rich eco-binder, and a clinker-free alkali-activated binder. • Slag-rich binder hit 40 MPa at 90 d, meeting C30/37 grade with S3 workability. • Porosity dropped from 23 to % 12 %; C-A-S-H gel densified ITZ around recycled aggregates. • All concretes non-hazardous (EN 12457-2) and cut CO₂ by more than 50 % vs CEM I. Ordinary Portland Cement (OPC) concrete contributes significantly to resource depletion and climate change, primarily through the extensive use of natural aggregates and high CO₂ emissions, which accounts for 8% of global output. This study investigates a circular economy approach by completely substituting both fine and coarse aggregates with recycled materials sourced from the Grand Paris Express, combined with low carbon binders. Three concrete mixes were formulated using 100% recycled aggregates and different binders: (i) OPC (CEM I), (ii) a slag-rich eco-binder (CBC_1) with CO₂ impact reduced by more than 50% in comparison to CEM I, and (iii) a clinker-free alkali-activated binder (CBC_2) which also achieves over 50% CO₂ reduction relative to CEM I. The physical and rheological properties of the mixes were evaluated in fresh condition through the measurements of the density, the slump, and the segregation. In hardened condition, the properties of the mixes were evaluated through the measurements of compressive and splitting-tensile strengths (at 7, 28, 90 days) and water-accessible porosity. In addition, X-ray diffraction, thermogravimetric analysis, and SEM characterized hydration products and interfacial transition zones (ITZ). Finally, to assess the environmental properties, leaching tests (EN 12457-2) were performed on all the mixes. The obtained results showed that all mixes achieved satisfactory workability (S3) without segregation. At 28 days, the CEM I-based mix reached 47 MPa, while CBC_1 achieved 32 MPa and continued to strengthen to 40 MPa by 90 days, fulfilling structural grade criteria (C30/37). The CBC_2 mix reached 16 MPa at 28 days and 21 MPa at 90 days. Porosity decreased significantly from 23% to 12% in the calcium-rich systems (CEM I and CBC_1) but remained higher (18%) in CBC_2. Leaching results confirmed that all concretes and aggregates classify as non-hazardous, although recycled aggregates exceeded inert thresholds for sulfate and soluble fractions.