Superplasticizer Adsorption on Synthetic Ettringite


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Title: Superplasticizer Adsorption on Synthetic Ettringite

Author(s): J. Plank and C. Hirsch

Publication: Special Publication

Volume: 217


Appears on pages(s): 283-298

Keywords: adsorption; ettringite; polycarboxylate; polycondensate; polymer structure; superplasticizer

Date: 9/1/2003

The mode of action of polycondensate and polycarboxylate based superplasticizers is still under investigation. Major unresolved aspects are whether superplasticizers adsorb to a higher extent on specific hydrate phases and whether superplasticizers are not only adsorbed, but also absorbed (e.g. intercalated). Ettringite, portlandite and gypsum are important mineral phases present during early hydration of cement. In this study, the interaction of different superplasticizers with these pure mineral phases is investigated. Naphthalene and melamine based polycondensates and methacrylic acid/methacrylate ester based polycarboxylate superplasticizers were synthesized and used in the experiments. Ettringite was synthesized in suspension as well as from solution. Superplasticizers were added either at the beginning of ettringite crystallization or to a slurry of synthetic ettringite. Superplasticizer adsorption, quantity and morphology of ettringite formed were determined for each case. The influence of superplasticizer chemical structure on adsorption behaviour was found to be similar for cement and ettringite. SPs showing high adsorption on cement also show high adsorption on ettringite, and vice versa. The adsorption of polycarboxylate SPs on cement increases with increasing C3A/C4AF content. Polycondensate adsorption is generally high and increases little with increasing C3A/C4AF content. In the case of ettringite, polycondensate adsorption is significantly higher when the superplasticizer is added early in the ettringite crystallization process whereas polycarboxylate adsorption is not affected by early or delayed addition. Crystal size of ettringite is reduced in the presence of superplasticizer. The effect is particularly strong from polycondensates. Similar experiments carried out with portlandite and gypsum showed no significant adsorption of superplasticizer. It is concluded that ettringite is a sink for superplasticizer in cement. In the XRD patterns, no evidence for absorption (intercalation) of superplasticizers into the ettringite crystal structure was found.