Valorization of Low-kaolinitic Calcined Brown Clay in Carbon-Fiber-Reinforced Mortar: Mechanical Compensation, RSM Optimization, and Embodied Carbon Assessment

Brown clay is an abundant yet underutilized supplementary cementitious material (SCM) due to its low kaolinite content and mixed-clay mineralogy. This study demonstrates that calcined brown clay (CBC) functions as an effective cement replacement when combined with carbon fiber (CF) reinforcement in mortar. CBC (0-30%) and CF (0-0.5%) were evaluated using a 13-run RSM central composite design covering fresh, mechanical, and microstructural responses. XRD indicated CBC's quartz-rich mineralogy, while weaker portlandite reflections at 30% replacement were consistent with dilution and possible pozzolanic reaction. Both CBC levels (15 and 30%) met ASTM C618 strength activity index requirements. The M0.25/15 mix achieved 48.6 MPa compressive strength, with CF reducing the brittleness index from 10.6 to 7.0-7.4, indicating pseudo-ductile behavior. RSM models yielded R2 of 0.94-0.99, with optimal formulation at 0.30% CF and 29.5% CBC. CBC alone reduces carbon intensity 13% below control, positioning strength-normalized carbon intensity as a more meaningful sustainability metric.