Adsorption of lignosulfonates and polycarboxylates - depletion and electroacoustic methods

The adsorption of superplasticizers onto cement particles is a key factor in the rheology of cement and concrete. The adsorbed amount is generally measured as the difference between the amount of polymer present in the aqueous phase before and after contact with cement (depletion method). Adsorption isotherms of well characterized lignosulfonate and polycarboxylate admixtures have been measured on a model powder (MgO) using the depletion method and on an ultrafine cement by using an electroacoustic method. This method allowed us to follow the variation of zeta potential of the same suspensions as a function of increasing amounts of superplasticizers. It was found that the influence of the adsorption of superplasticizers on zeta potential measured by electroacoustic technique are significantly lower than those measured earlier with preceding instruments. These differences are due in part to the models used for the zeta potential computation. Apart from improvements in the instrumentation, errors were due to the particle size distribution of cement particles not fully sampled by electroacoustic techniques. It is shown that the electroacoustic method is also both useful and complementary for studying the adsorption of superplasticizers. It measures zeta potential values necessary for interparticle force calculations. Differences in polymer structures and the effect of different ions on polymer adsorption can be observed.