This study presents the influence of the calcined kaolinite content of calcined clays on the hydration of Limestone Calcined Clay Cements containing 50% of clinker (LC 3 -50). Above a calcined kaolinite content of 65% in calcined clay, further reaction of clinker is inhibited from 3 days onwards. Detail investigation indicates that this slowing down of clinker hydration is related to a significant refinement of pore connectivity. A limiting critical pore entry radius of 3–5 nm is reached, from which the porosity does not get further refined. The higher the calcined kaolinite content, the faster this refinement limit is reached. The formation of carboaluminate hydrates is also limited after reaching this refinement limit. As a consequence, the on-going reaction of metakaolin impacts C-A-S-H, mainly affecting gel porosity, which is not well characterized by MIP .