Method for inerting impurities

A method for treating a mixture of clays in sands that are used in the preparation of a hydraulic composition, wherein the clays are present in an amount of from 0.5-5% by weight of the sands present in the mixture, said method comprising, determining the amount of clays that is present in the mixture of clays in sands, treating said clays in sands mixture by adding to the mixture a controlled amount of a cationic polymer having a density of cationic charges which is greater than 0.5 meq/g and an intrinsic viscosity which is less than 1 dl/g, said cationic polymer being added to the mixture in an amount of from 2-20% by weight, based on the amount of clays present in the clays in sands mixture, wherein said amount of cationic polymer added is proportional to the amount of clays determined to be present.