Argille e polimeri fluidificanti per i cementi: caratterizzazione degli effetti di interazione

The adsorption of organic polymer on clay minerals contained as impurities in coarse and sand aggregates, poses a severe problem on the workability of concrete. In detail, polycarboxylate ether-based superplasticizers (PCEs) with polyethylene glycol (PEG) side chains have a poor clay tolerance due to their intercalation into swelling clay minerals (e.g. montmorillonite). Once intercalated, the PCE is immobilized by the clay and loses its efficiency in dispersing the other fine particles. This effect strongly deteriorates the fluidity of concrete with PCE superplasticizers. The aim of this study is to investigate the effects of PCE intercalation in between the layers of a Na-smectite (labelled Vincon clay). X-ray diffraction measurements have been performed on Vincon powder samples after contacting the clay with different superplasticizer chemical admixtures. Thereby, it has been possible monitoring the variations of (d001) interplanar distance of smectite at increasing concentrations of PCEs in the contacting solution. Diffraction measurements have also been conducted at high temperature (up to 400°C) in order to evaluate the thermal stability of the PCE-clay intercalated system. The actual presence of organic molecules into smectite has also been confirmed by infrared spectrometry measurements. The clay tolerance of the different PCEs has been evaluated by means of mini-slump tests on cement pastes “poisoned” by clays. Also kaolinite and sandy samples have been tested as terms of comparison to Vincon clay. The combined analysis of the results obtained by XRPD, adsorption measurements and slump tests, indicate that all the investigated PCEs are easily intercalated in between the smectite layers, whereas they exhibit different efficiency in increasing fluidity of cement-clay pastes. This suggests that PCE conformation and/or PCE hydrolysis properties can play an important role in determining the clay tolerance of polymers.