Sara Mantellato

Chemistry of chemical admixtures

Chemical admixtures are nowadays very important for concrete design. This chapter presents an overview of the chemical structures of different organic chemical admixtures, ranging from small organic compounds to large polymers having a certain polydispersity, and of both natural and synthetic origin. The choice is guided by the fact that this is where the real added value of molecular structure comes into play in terms of design of new or modified chemical admixtures. Such admixtures offer the greatest possibility to chemists to modify properties and target improved performance by specific exploitation of structure/property relationships. The overview gives a basis for better understanding of the working mechanisms of these admixtures.

Adsorption of chemical admixtures

Abstract The properties of most of admixtures come from their ability to adsorb on the surface of particles (as observed for water reducers, superplasticizers, and retarders) or on the liquid–vapor interface (e.g., for air-entraining or shrinkage-reducing admixtures). Their adsorption behavior depends not only on their chemical composition, molecular structure, and dosage but also on the characteristics of the adsorbent surface and the composition of the liquid phase. This is particularly important during cement hydration, when the formation of new phases and an evolving ionic activity may alter strongly adsorption efficiency of admixtures. In practice, the determination of an adsorption isotherm allows for comparing the adsorption behavior of admixtures. The most common way to quantify the amount of adsorbed molecules is the solution depletion method. This method and its limitations, as well as the modeling of the adsorption isotherm, are deeply discussed to show that adsorption measurements and their interpretation are not trivial.

Superplasticizers in practice

Abstract Superplasticizers are essential components of modern concrete, allowing the production of highly durable structures with reduced environmental impact. Their working mechanisms rely on their molecular structures and physical–chemical interactions taking place between them and the surface of cement particles. In some situations, it is difficult or impossible to find a robust combination giving the desired workability and workability retention without excessive retardation. Such situations are described as cement–superplasticizer incompatibilities. An attempt to better define the origin of these incompatibilities is presented in this chapter, highlighting the role of a soluble alkali, C3A, and a specific surface. The mechanistic insight provided along with recommended testing approaches should help to better control the quality of admixed concrete.

Formulation of commercial products

Most concrete admixtures on the market are not single molecules but formulated products. Therefore, they contain more than one compound, up to many, for various reasons. A commercial product must balance many requirements. However, this reality is not clearly visible to most end users and is very often misperceived as a “dark art” in the academic community. This chapter presents a brief overview on some aspects of formulation and some typical additions that may be found in commercial admixtures. Some of the aspects discussed include slump retention, defoaming, setting and hardening control, and biocides.