Adsorption of Superplasticizers on a Model Powder

Adsorption isotherms of carboxylate type superplasticizers with different architectures have been measured on MgO considered as a non-reactive model powder. This system allowed us to study the influence of pH, electrolyte type, concentration and ionic strength on the adsorbed amount. On same MgO powder, we have measured the zeta potential variations by adding increasing amounts of superplasticizers. Most of the isotherms initially exhibit a linear section followed by a hyperbolic behavior which can be fitted by a function identical to the Langmuir equation. The slope of the linear part allowed us to define the affinity of the superplasticizers for MgO surfaces and with the asymptotic value of the hyperbolic function a plateau value for the adsorption. The results are discussed taking into consideration the molecular parameters of the superplasticizers, such as the molar mass, backbone chain and side chain lengths. The mechanisms of fixation of the superplasticizers at the surface are also discussed.