A Study on the Cement Compatibility of PCE Superplasticizers

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Title: A Study on the Cement Compatibility of PCE Superplasticizers

Author(s): A. Lange and J. Plank

Publication: Special Publication

Volume: 302

Issue:

Appears on pages(s): 401-414

Keywords: admixture; adsorption; cement dispersion; ettringite; high range water reducer; morphology; nano-size; polycarboxylate

Date: 6/1/2015

Abstract:
It is well established among concrete producers that specific cements seem to be incompatible with most PCE products, thus causing excessive PCE dosages or even a total failure of the PCE. This effect is commonly referred to as “cement incompatibility” of PCE. The study here investigates the reasons for such incompatibility. First, it was found that only cements which upon contact with water instantaneously form large amounts of ettringite exhibit such incompatibility phenomenon. Their characteristics are elevated C3A content (> 7 wt.-%) and high initial heat of hydration. Second, it was observed that PCEs strongly influence early ettringite crystallization by acting as morphology modifying agent. Most PCEs transform common micro meter-sized ettringite into nano-sized crystals which bring about a huge surface area and thus require abnormal dosages of PCE to achieve dispersion. Such nano-sized particles can be separated from the cement paste by centrifugation where it appears as a viscous, gel-like top layer. From five chemically different PCE polymers tested, one (a modified APEG type) was identified as extremely compatible with all cement samples, whereas three other ones (two conventional MPEG and one APEG type) exhibited pronounced incompatibility with C3A rich cements. An IPEG PCE showed moderate cement compatibility. The phenomenon of cement incompatibility occurs only when the PCE is present in the mixing water, and disappears when PCE is added in delayed mode. Finally, a simple and quick test to identify cement–PCE incompatibility is proposed.