To reduce CO₂ emissions in cement production, Limestone Calcined Clay Cement (LC3) emerges as a promising alternative. However, its rheological behavior is influenced by the morphology of the clays used, including their specific surface area, roughness, and flocculation tendency, which often lead to rheological loss. This study evaluates three clays with varying kaolinite contents: a low-kaolinite clay (7% kaolinite), a clay derived from red mud (37% kaolinite), and a high-kaolinite clay (43% kaolinite). Mortars were assessed through mechanical strength tests and workability. The results showed that 37K and 43K clays exhibited significant rheological losses due to their rougher particle morphology and higher specific surface areas. Despite this, both clays demonstrated pozzolanic activity and reactivity after calcination, acquiring the minimum mechanical strength required for structural applications, albeit lower than reference mixtures. In contrast, the 7K clay displayed superior flowability due to its smoother particle morphology and reduced specific surface area but had reduced mechanical performance. Additions of ornamental stone waste (OSW) improved the flowability of high-kaolinite mixtures. This study underscores the critical influence of clay morphology and mineralogy on LC3 performance, balancing workability and mechanical properties.