The decision of public authorities to recycle industrial wastes leads to the quasi-systematic use of mineral admixtures in concrete. Predicting the strength of these concretes induces the engineer to characterise the mineral, admixtures: ordinarily, a single coefficient is used, calculated with an empirical law like Bolomey's or Abrams' formulas. Many results, reported in bibliography and completed by the authors, show that the determination of this single coefficient is not sufficient to characterise the activity of the admixture. Many factors like the nature of cement, the substitution rate, the fineness of the admixture and the age of concrete induce a great variability of the efficiency coefficient of the admixtures. Up to now, there is no general law explaining this variability. The authors warn against the excessive use of a single equivalence coefficient and recommend to take into, account not only the physicochemical parameters but also the nature of the works themselves.