Optimize the Filler Fineness to Improve the Rheological Properties of Calcined Clay Limestone Cement and Its Revelation on Hydration

Calcined clay (CC) with metakaolin (MK) can be used to produce low-clinker cement (50% clinker reduction) when combined with limestone (LS). However, the porous structure of CC challenges the workability of calcined clay limestone cement. Previous studies have demonstrated that considerable amount of MK and LS remains unreacted in the system, suggesting potential substitution with inert coarser fillers. To balance flowability and reactivity, this study introduces coarser quartz filler (5.2 μm to 141.5 μm, D50) to improve fresh properties and preserve later strength in blended cement. Given the microstructural variations of MK sourced from different origins, soft metakaolin (SMK) and coal-associated metakaolin (CMK) were evaluated in systems replacing 45% clinker with MK, LS, and quartz to inform future research on diverse MK types. The results show SMK has a greater impact on workability and rheological properties due to its layered kaolinite structure, while coarser quartz (>77 μm) improves workability and reduces 1-hour workability loss by 35%. Meanwhile, the strength loss can be limited to within 5 MPa at 28 days, which is markedly lower than other cement types reported previously. Hydration characterizations revealed that coarser quartz initially dilutes the mix, but strength loss is mitigated by an increased formation of hemi-/mono-carboalumination and ettringite phases, indicating a synergistic reaction between MK and LS.