Calcined clay plays a critical role in controlling the rheology of limestone-calcined clay cement (LC³). This study shows that the yield stress of LC³ mixtures with ultra-high volumes of calcined clay and limestone evolves significantly faster within the first 90 min than that of plain cement pastes at the same solid volume fraction. Two key mechanisms were identified: (1) calcined clay alters particle packing through its layered structure and micro-cavities, promoting physical water absorption; and (2) it enhances interparticle attractive forces via “bridge” formation (C-(A)-S-H bridges and colloidal attractive forces) between cement and calcined clay particles in the fresh state, accelerating rigidification. However, this bridging effect can be hindered by the addition of PCE-based superplasticizers.