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.
The introduction of fine and ultra‐fine mineral admixtures in cementing materials generates a granular effect, a physico‐chemical and micro‐structural effect and, possibly, chemical effect. To analyse the specific share of each effect, we propose to use a methodology based on a volume substitution of cement by admixtures in mixes whose absolute volume of the solid phases and workability are preserved constant and then only the effects of the particles of admixtures in the cementing phase are taken into account. We expressed the compressive strength of mortars with admixtures according to a factor of compactness by applying to the mortars the principles of the predictive model of Feret. This factor makes possible to distinguish the modifications generated by the granular effect of the admixtures on the compactness of the cementing matrix independently of the physico‐chemical and chemical effects. The specific chemical action of each admixture has been evaluated by analysing the variation of the coefficient of activity K according to the rate of the cement substitution, by applying to the studied mortars the concept of the equivalent binder and by deducing the coefficient of activity from the predictive model of Feret.
Abstract:This study is an extension of previous researches on mortars with mineral admixtures and superplasticizers. In this way, the same methodology was applied to concretes and the use of mineral admixture was limited to low cost materials available in Algeria as limestone, pozzolan and blast furnace slag, with current cement and super-plasticizer. The experimental methodology used was based on the volume substitution of the cement by admixtures in mixtures with the absolute volume of the solid phases and workability preserved constant. The main results achieved showed that the super-plasticizer demand of concretes depends on the nature and the quantity of the incorporated admixture. The combined use of admixtures and super-plasticizer has generally a favourable effect on compressive strengths at 07 and 28 days at low rates of cement substitution, which vary significantly with the nature, fineness and quantity of the used admixtures. At 07 days, limestone admixtures give better improvements and reach more than 20 % of gain to the compressive strength of the reference concrete with no admixtures or super-plasticizer, at 10 % of the cement substitution and still better until 30 %. At 28 days, blast furnace slag admixtures give better improvements at 28 days and reach more than 20 % of gain to the compressive strength of the reference concrete at 20 % of the cement substitution and still better until 30 %. This contribution to the compressive strength is explained on the one hand by the reduction of the quantity of water in the mixtures at the same consistency, by the use of the super-plasticizer and on another hand by the activity of Limestone admixtures at early ages and to the latent hydraulic properties of blast furnace slag at 28 days.
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