Ninety-day compressive strength of particle-packing-designed self-compacting concrete with metakaolin and recycled coarse aggregate: A central composite design approach

Self-compacting concrete produced with recycled coarse aggregate is still limited by concerns related to the old adhered mortar, the presence of inherited interfacial defects, and the difficulty of maintaining flowability at low nominal water-to-powder ratios. This study examines the 90-day compressive strength of particle-packing-designed self-compacting concrete incorporating metakaolin and recycled coarse aggregate. Eighteen mixtures were produced using a central composite design (CCD), with metakaolin replacing Portland cement from 0% to 20%, recycled coarse aggregate replacing natural coarse aggregate from 0% to 40%, and the nominal water-to-powder ratio varying from 0.26 to 0.30. In addition, a reference mixture without MK or RCA, based on the same baseline particle-packing formulation, was tested as an external benchmark; however, it was not included in the statistical model because it was not part of the CCD matrix and had a nominal w/p ratio slightly outside the modeled design domain. Slump-flow was used to define the fresh-state feasibility of the mixtures, while the 90-day compressive strength was interpreted together with response modeling and selected microstructural observations. The mean 90-day compressive strengths of the CCD mixtures ranged from 78.29 to 92.66 MPa, while the external reference mixture reached 76.69 ± 5.65 MPa. This reference comparison was considered descriptive only and was not used to infer the effects of the experimental factors. Metakaolin improved strength up to moderate-to-high replacement levels, whereas an excessive water-to-powder ratio reduced the mechanical response. More importantly, mixtures with high recycled coarse aggregate replacement did not show a systematic strength penalty; several of them remained within the upper strength range, particularly when high RCA content was combined with an effective metakaolin dosage and controlled water availability. The strongest stable response was obtained for a mixture combining high metakaolin content, high recycled aggregate replacement, and low water-to-powder ratio, reaching 91.94 MPa with very low dispersion. X-ray diffraction and microscopy of selected samples were used as complementary evidence to examine whether the mechanical response was consistent with changes in the crystalline phase assemblage and local matrix–aggregate contact. Overall, the results show that recycled aggregate can be incorporated into high-strength self-compacting concrete when binder composition, water availability, and particle packing are treated as a single mixture-design problem.