Anisotropic mechanical and durability behavior of concrete with full ferrochrome slag coarse aggregate replacement through mechanistic assessment

This study provides a mechanistic assessment of the anisotropic mechanical and durability behavior of concrete incorporating up to 100% ferrochrome slag coarse aggregate (FCSCA). Despite extensive research on industrial by-products as aggregate replacements, the comprehensive performance characterization of ferrochrome slag coarse aggregates (FCSCA) across mechanical, durability, and microstructural domains remains insufficiently documented, particularly at replacement levels approaching 100%. Accordingly, this study investigates FCSCA as a sustainable coarse aggregate replacement through systematic evaluation of eight concrete mixtures with replacement levels ranging from 15% to 100% by weight. The experimental program encompassed mechanical testing (compressive, split tensile, and flexural strength at 7, 28, and 56 days), durability assessments including the rapid chloride penetration test (RCPT), water permeability, and absorption, as well as microstructural characterization via scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Results demonstrated a distinct anisotropic response, with compressive strength reductions of 6–11% offset by substantial improvements in tensile properties. Concrete with 60% FCSCA replacement achieved 36% enhancement in split tensile strength (3.20 MPa), while 90% replacement yielded 28% improvement in flexural capacity (5.48 MPa). Durability performance exceeded that of conventional concrete at replacement levels above 60%, with 30–32% reductions in chloride ion penetrability (2221–2282 coulombs) and classification transition from moderate to low permeability per ASTM C1202. Microstructural analysis revealed that enhanced interfacial transition zones (ITZ) govern the observed performance, where angular FCSCA particles promote mechanical interlocking and tortuous ion diffusion pathways. X-ray diffraction confirmed crystalline phase stability with progressive calcium hydroxide reduction (40–45%) at higher replacement levels, indicating microstructural densification rather than pozzolanic reaction. These findings establish an ITZ-based mechanistic framework linking microstructural evolution to anisotropic behavior, while confirming the feasibility of 100% FCSCA replacement for structural applications in aggressive environments.