Delphine Marchon

Molecular design of comb-shaped polycarboxylate dispersants for environmentally friendly concrete

Concrete is the most widely used material in the world and, because of the large volume used, the production of cement, the main component of concrete, is responsible for a high CO2 emission. To reduce the quantity of CO2 emitted, one solution is to substitute a part of cement by supplementary cementitious materials, SCMs, such as fly ash. Because fly ash is largely inert in the first days of blended cement hydration, it is necessary to accelerate its dissolution by physical or chemical means to compensate the loss of mechanical strength in the early stage. The solution studied in this project is the alkaline activation by addition of NaOH that prevents the dispersive effect of PCE superplasticizers used in modern concrete with a low content of water. In this work, we investigated the influence of NaOH on hydration, rheology and mechanical strength of superplasticized blended cementitious systems. From the results and theoretical aspects of polymer adsorption, a simple criterion was established that defines which polymer structures are or are not compatible with alkaline activated systems.

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.

Impact of chemical admixtures on cement hydration

Abstract Many chemical admixtures are known to retard cement hydration. This is an intentional effect of retarders. However, for many other admixtures such as superplasticizers, it is mostly an undesired side effect that becomes more and more problematic in modern concrete with reduced clinker content. In this chapter, the general mechanisms that affect the hydration processes and the behavior of most chemical admixtures are described. This includes possible modification of the rates of the dissolution, nucleation, and/or growth of various phases as well as destabilization of the silicate–aluminate–sulfate balance. Because of the importance of polycarboxylate ether (PCE) superplasticizers and the flexibility in their molecular design, a special section presents information concerning their impact on hydration. Finally, sugars are discussed because they are the most used set retarders and that important steps have been made in studying their impact on retardation.