Through characterization techniques (XRD, TG / DTA, chemical analysis classical, 27Al and 29Si MAS NMR,Blaine, BET, granulometry laser) and tests of reactivity of metakaolin with calcium hydroxide (calorimetry isotermic conduction, Chapelle test at 55℃ and Index pozzolanic activity), an unknown phenomenon that is difficult to be reproduced was noticed, but proved to be attached to contents of Si / Al, degree of disarrangement of the material and quantity aluminum coordination V in the sample.The presence of species hydrogarnets like in reactions of homogeneously calcined samples with calcium hydroxide appear to be crucial to a high reactivity or reactivity lower than expected.A factor that may have potentiated the occurrence of the phenomenon appears to be ramp heating the samples in the test isothermal calorimetry conduction with pozzolanic material and the proportion of calcium oxide in excess of 1:1.NMR spectroscopy was used to help unravel with the peculiarities of this phenomenon.