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To analyze the chemical activity of admixture in mortars, we used a methodology which guarantees that the obtained mixtures still have a volume of the flexible phase and an initial porosity constant. Then, only the clean chemical effect of the admixture is taken into account in the variation of the mechanical strengths. The chemical contribution of admixtures on the strengths of mortars had been evaluated in the basis of the factors of activity K deduced from the concept of the equivalent binder and using the predictive model of strength of Feret. To study the influence of the parameters of formulation on the activity of admixtures in mortars, we used seven admixtures different by their mineralogical natures, their fineness, and their morphological characteristics, two types of cements and two types of superplasticizers. The main results achieved showed that the factors of activity are very sensitive to the nature fineness and proportion of admixtures substituted to cement, but the influence of the nature of cement or superplasticizer was less important. Consequently the activity of admixtures differs from the concept of the equivalent binder recommended by the standard of the ready-mixed concretes which are based on a coefficient of reactivity with contractual values.

The use of mineral admixtures in cementing materials can generate significant modifications in the fresh and hardened mixes, which can depend on the parameters of formulation. To study the influence of the parameters of formulation on the rheological properties of cementing materials with admixtures, we used seven admixtures different by their mineralogical natures, their fineness, and their morphological characteristics, two types of cements and two types of superplasticizers. The experimental methodology used is based on the volume substitution of the cement by admixtures in mixtures whose the absolute volume of the solid phases and workability are preserved constant. The main results achieved showed that the rheological properties of the fresh mortars with admixtures are very influenced by the amount and the properties of admixtures, and the nature of the superplasticizer, but depended rather on the nature of the cement used.