Orthogonal design optimization of low-grade limestone calcined clay cement: Workability, strength, and microstructural insights

To explore the potential application of low-grade limestone (LGL) and illite clay (IC) in limestone calcined clay cement (LC 3 ) systems, this study investigates the compressive strength, flowability and microstructure of mortar incorporating LGL and IC. An orthogonal experimental design (OED) was used to optimize the mix design by evaluating OPC replacement levels (30 %, 40 %, 50 %, 60 %) and the influence of IC and LGL calcination temperatures (300, 500, 800 °C). The highest 28-day strength (39.05 MPa) was achieved with both IC and LGL calcined at 800 °C. The optimized LC 3 -50 mix reduced CO₂ emissions by 33 % and energy use by 15 %. Response surface methodology (RSM) was applied to refine mix proportions. From an engineering perspective, LGL with incomplete calcination should be strictly avoided, as it negatively affects both flowability and compressive strength. Microstructural analyses (XRD, SEM/EDS, FTIR) revealed carboaluminate formation, denser C-A-S-H gels, and portlandite consumption. Pearson correlation analyses showed delayed hydration. The findings support optimizing locally available IC and LGL in sustainable LC 3 formulations.