This　work　studied　the　effect　of　metakaolin　fineness　on　the　performances　of　cement-based　materials　through　measurements　of　calorimetric　response,　mechanical　properties,　and　capillary　water　absorption.　Using　multi-technique　characterization　(X-ray　diffraction,　SEM,　TGA,　MIP,　and　solid　state　29Si　NMR),　the　phase　composition　and　microstructural　characteristics　of　samples　were　analyzed.　Results　indicate　that　the　presence　of　metakaolin　accelerates　the　hydration　of　cement,　and　the　acceleration　effect　is　more　significant　with　the　finer　MK.　Increased　MK　fineness　enhances　its　pozzolanic　reactivity　at　early　age.　Fine　metakaolin　significantly　enhances　the　mechanical　properties　of　cement　mortars　compared　to　coarse　metakaolin.　Finer　metakaolin　reduces　both　initial　and　secondary　water　absorption　rates,　with　optimal　effects　observed　at　20　%　replacement.　The　incorporation　of　metakaolin　refines　the　ITZ　and　the　cement　matrix,　these　effects　are　more　pronounced　with　increasing　fineness.　However,　29Si　NMR　results　suggest　an　optimal　fineness　below　which　further　benefits　do　not　increase.　©　2024　The　Authors