Organic admixtures and cement particles: Competitive adsorption and its macroscopic rheological consequences
Introduction
Admixtures are fundamental components of cementitious materials in the fresh state. In the construction materials industry, these admixtures can be chosen among an impressive variety of molecules with very different chemical structures and physical properties. Some of them, called plasticizers or superplasticizers, allow, for instance, for a decrease in the yield stress [1] and viscosity [2] of the material, in which they are introduced. Some others, called viscosity agents or stabilizers, allow for a decrease in bleeding and segregation [3] or an enhancement of water retention capability [4], [5], [6]. Retarders can also be used to postpone cement hydration [7], [8] and therefore increase flowability retention and what is often called the “open time” of the product [9], [10]. When targeting specific rheology requirements, mix design of cementitious materials is generally achieved through the blending of these different admixtures.
A major source of difficulty in the simultaneous integration of various admixtures in a cement paste lies in the competitive adsorption between these molecules at the surface of cement particles [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. From a more general point of view, this issue relates to the adsorption competitions between all adsorbing species (i.e. including ions) in the system. As the outcome of this competition dictates the rheological behavior of the cement paste by dictating the nature and intensity of cement particles interactions, the understanding and prediction of this competition has become of utter importance for the mix design engineer [21]. An example that has been extensively studied in the literature of cementitious materials is the competition between sulfate ions and superplasticizers [22], [23]. By adsorbing on the surface of cement particles, these ions restrict the adsorption of polycarboxylate-type superplasticizer and therefore affect their dispersion efficiency from a macroscopic point of view.
In this work, we focus on the competitive adsorption between a superplasticizer molecule and a so-called viscosity agent along with the competitive adsorption between the same superplasticizer and a retarder. We develop a new protocol using dynamic light scattering, which allows for distinguishing the relative adsorption of different admixtures added to a cement paste. We identify from our results, similarly to other authors, three main competitive adsorption regimes. We finally measure the macroscopic changes in rheology in terms of yield stress and residual viscosity and correlate them to the outcome of the competitive adsorption observed at the microscopic scale.
Section snippets
Cement
The cement used in this study is an ordinary Portland cement equivalent to ASTM Type I cement. Its chemical composition obtained through ICP-AES and ATD–ATG is given in Table 1. Its Bogue composition is also given in Table 1. The free lime content was determined by extraction with hot ethylene glycol [24]. The cement powder maximum packing fraction was estimated to be around 60% in [25] and its Blaine specific surface is 3650 ± 100 cm2/g.
Polymers and molecules
We study here a comb co-polymer (PCE) in the flexible
Mixing protocol
All cement pastes were prepared with the same mass ratio W/C = 0.4. Only dosage and nature of organic admixtures varied from one sample to another. All sample preparations were carried out at 20 °C. We used, in this work, a mixing protocol, which was shown to limit the consequences of any early chemical interactions such as polymer intercalation or co-precipitation [30]. This is crucial, for instance, in the case of polymers such as PCE, for which the measured polymer depletion in the solution can
Experimental results
We first study the competition between PCE and NG in cement paste. The PCE dosage is kept constant and equal to 0.4% of the cement mass whereas the dosage of NG is gradually increased from 0.1% to 5% of the cement mass. We remind here that PCE is first introduced in the system while NG is added 15 min later. We plot in Fig. 7 the respective amounts of adsorbed PCE as a function of NG dosage. For comparison, we also plot the adsorption isotherms of NG and the expected adsorbed amount of PCE when
Competitive adsorption between NG and PCE
We plot in Fig. 9 the relative yield stress (i.e. the ratio between the yield stress of the cement paste with PCE and the yield stress of the reference cement paste) and residual viscosity (i.e. the ratio between the residual viscosity at the plateau of the cement paste with PCE and the residual viscosity of the reference cement paste) respectively as a function of NG dosage. We first focus here on the effect of NG and PCE when introduced alone in the cement paste. At a given dosage of polymer
Conclusion
In this work, we studied the competitive adsorption of organic admixtures in a cement paste. We focused successively on the competition between a superplasticizer and a retarder and between the same superplasticizer and a so-called viscosity agent.
We developed a new protocol using dynamic light scattering, which allows for distinguishing the relative adsorption of different admixtures present in a given cement paste. Our results suggest that the result of competitive adsorption is not only a
Acknowledgment
The authors wish to thank Dow Chemical Company for providing the polymers as well as for the financial support of this project.
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