Influence of Fine and Coarse Waste Tyre Rubber Aggregates on the Workability, Mechanical Properties and Rubber–Paste Interface of Concrete: An SEM/EDX Study

Waste tyre rubber serves as a cost-effective aggregate substitute. The elemental-level interfacial mechanisms underlying the associated strength reduction remain insufficiently characterised. In this study, eleven concrete mixes were evaluated, with fine-rubber (2.5–5 mm) replacing river sand and coarse-rubber (5–20 mm) replacing crushed limestone, each at 10, 20, 30, 40 and 50% by volume. Cement content was maintained at 400 kg/m3 and the water-to-cement ratio at 0.45. Workability, hardened density, and 28-day compressive and flexural strengths were measured in accordance with Moroccan Standards (NM), which are aligned with the European Standards (EN). Scanning electron microscopy with energy-dispersive X-ray (SEM/EDX) microanalysis was conducted on six mixes to characterise the rubber–paste interfacial transition zone (ITZ). Compressive strength decreased by 8.5% with 10 vol. % fine rubber replacement and by 81.8% with 50 vol. % coarse rubber replacement, with coarse rubber consistently resulting in greater strength loss at all levels. Every qualifying paste-side field in the rubberised mixes returned a Ca/Si ratio above the reference bulk-paste value of 1.24, reaching 8.09, indicating portlandite accumulation at the hydrophobic rubber surface. Sulphur and zinc, introduced during vulcanisation, confirmed these regions as rubber rather than cement paste. The apparent debonding-zone width at the fracture surface increased from 10–25 µm to 100–200 µm and scaled with particle size, consistent with a flaw-size effect rather than with total interfacial area. Preliminary guidance on mix selection based on 28-day mechanical performance is proposed.