The effect of the substitution by 5%, 15%, 30% and 45% of cement in mortars by milled kaolinite has been studied. Milled Kaolinite clay is obtained by using planetary ball mill. The influence on the mechanical performance and on the pozzolanic reactions are discussed. For both CEM I and CEM II formulations, after only 7 days of curing, the compression strength of mortars with 5% of milled kaolinite is comparable to the one of reference mortars for milling times of 3 h and above. For CEM I mortar, the results of the compressive strength show that 15% of kaolinite milled for 9 h allows the best compressive gain for 28 curing days and 90 curing days with 18.5% and 25.6%, respectively. For CEM II mortar, 15% of kaolinite milled for 9 h exhibits the best values of compressive gain for 28 curing days and 90 curing days with 9.8% and 9.7% respectively.