Strain-rate-dependent compressive behaviour, impact performance, and thermal analysis of multi-slag–reinforced epoxy composites with data-driven impact–compression correlations

Epoxy-based composites reinforced with Fe-rich, Cu-rich, and Al-rich slags were fabricated at slag volume fractions of 33–58 vol% to assess industrial slag as a partial substitute for epoxy resin and conventional fillers. Compression tests at three strain rates (0.8 × 10 −3 , 3.9 × 10 −3 , and 8.5 × 10 −3 s −1 ) showed that neat epoxy reached maximum compressive strengths ( σ max ) of 80.56, 88.02, and 76.48 MPa, whereas the composite containing 33 vol% slag achieved 104.84, 111.60, and 121.03 MPa. The corresponding energy absorbed up to σ max increased from 2.76, 3.05, and 1.71 MJ/m 3 for epoxy to 3.10, 3.61, and 4.14 MJ/m 3 for epoxy-33 vol% slag composite. Impact testing indicated reduced impact strength; however, the composites remain suitable for applications where impact is not the governing design requirement. Bidirectional, data-driven impact–compression correlations were established using ridge regression, linking impact strength and compressive energy absorption through porosity and slag volume fraction. Differential scanning calorimetry (DSC) showed that slag incorporation can enhance thermal performance in selected slag-filled composites, with an improved glass-transition response, stronger post-cure potential, and delayed thermal degradation at elevated temperatures compared with neat epoxy. Slag-filled epoxies provide improved compressive performance at lower cost while valorising an industrial by-product, supporting their use in cost-sensitive, compression-dominated components requiring high energy absorption.