In　this　paper,　the　hydration　process　and　mechanical　properties　of　UHPC　matrix　incorporated　with　limestone　(LS)　and　different　substitution　rates　of　metakaolin　(MK)　were　investigated.　MK　was　added　with　five　levels　of　substitution　(0,　5%,　10%,　15%　and　20%)　and　LS　was　kept　at　30%　of　the　total　binder　mass.　Mechanical　properties,　hydration　heat,　hydration　products　and　pore　structure　measurements　were　carried　out　for　every　mixture.　The　results　show　that　the　hydration　rate　as　well　as　the　hydration　heat　peak　of　UHPC　pastes　decrease　with　MK　level　in　the　first　24　h　possibly　due　to　the　higher　superplasticizer　usage,　but　the　hydration　process　is　accelerated　with　the　incorporation　of　MK　after　24　h.　The　pozzolanic　reaction　of　MK　and　synergistic　effect　with　LS　consumes　portlandite　and　induces　the　formation　of　more　CSH　gels　with　the　improved　microstructure　of　UHPC　matrix.　The　trace　of　calcium　monocarboaluminate　(Mc)　phase　can　be　observed　in　UHPC　paste　samples　with　a　low　MK　substitution　(5%),　and　becomes　weaker　with　the　higher　MK　dosage.　The　pozzolanic　reaction　of　MK　with　replacement　level　of　15%　results　in　the　best　long-term　compressive　strength　of　UHPC　mortars.　Therefore,　the　incorporation　of　both　MK　and　LS　at　an　optimal　level　is　favorable　to　manufacture　UHPC　with　improved　mechanical　properties.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.