Adsorption of Naphthalene Sulfonate Superplasticizers by Cement Particles Through Gel Permeation Chromatography

Abstract

In the present paper a new analytical method for measuring the adsorption of naphthalene sulfonate formaldehyde condensate superplasticizers (NSFC) onto cement is described. By this method, based on the application of Gel Permeation Chromatography (GPC), it is possible to estimate the selective adsorption of the different components of these admixtures (B-naphthalene sulfonate, naphthalene disulfonates, polymer fraction). The results indicate that B-naphthalene sulfonate and naphthalene disulfonates are not substantially adsorbed onto the cement particles, independently on the cement used and the dosage of admixture. The polymeric fractions are adsorbed at different extent according to the dosage of superplasticizer and the cement used. Adsorption tests with two NSFC samples of different molecular weight indicate that the high molecular weight polymer fractions are preferably adsorbed over the low molecular weight fractions. The adsorption isotherms of the polymeric fraction of NSFC superplasticizers with different cements indicate that the saturation dosage depends on the chemical composition, the particle size and the specific surface area of the cements. Moreover, these results indicate that the NSFC polymer fraction is totally adsorbed on different cements up to a certain dosage of superplasticizer. This critical value seems to be related with the minimum dosage of superplasticizer to reach the optimum fluidity of cement mixtures. This observation could be put in relation with the preferred adsorption of the higher molecular weight fractions on NSFC, which exhibit a better superplasticizing effect.