Title:
The Role of Non-Adsorbed PCE Molecules in Cement Dispersion: Experimental Evidence for a New Dispersion Mechanism
Author(s):
Alex Lange, Tsuyoshi Hirata, and Johann Plank
Publication:
Symposium Paper
Volume:
288
Issue:
Appears on pages(s):
1-15
Keywords:
admixture; cement dispersion; high-range water reducers; polycarboxylate.
DOI:
10.14359/51684246
Date:
9/14/2012
Abstract:
The working mechanism for cement dispersion of an isoprenyloxy ether-based polycarboxylate superplasticizer (PCE-A) and of several PCEs based on MPEG-ester (PCE-B0, PCE-B1, PCE-B2) or APEG-ether monomers (PCE-C) was investigated via cement paste flow testing and measurement of hydrodynamic radius of solved PCE mole¬cules, adsorption isotherm, adsorption rate and zeta potential. Using “mini slump” test method it was found that the isoprenyloxy ether – acrylic acid copolymer (PCE-A) exhibits the highest water reduction, followed by the methacrylic acid – MPEG – 1100 – methac¬rylate copolymer (PCE-B2), whereas the methacrylic acid – MPEG – 250 – methacrylate copolymer (PCE-B0) required a very high dosage. Further experiments revealed that in cement pore solution, the conformation of PCE-A is star-like; while PCE-B0, PCE-B1 and PCE-B2 show a worm like conformation with flexible backbone and PCE-C exhibits a rod like structure possessing high rigidity. PCE-B2 disperses cement mainly via strong steric repulsion instigated by the long side chain. Based on adsorption and zeta potential data obtained it is proposed that PCE-A, PCE-B0, PCE-B1 and PCE-C disperse cement not only by steric repulsion originating from adsorbed PCE, but also by electrostatic and/or steric repulsion owed to non-adsorbed PCE molecules present in the pore solution. Especially PCE-B0 was found to require a dosage substantially beyond its saturation adsorption. This suggests that free PCE dissolved in the pore solution may provide the main contribution to cement dispersion. Consequently, a new type of dispersion mechanism which originates from dissolved PCE molecules exercising a “lubricating” effect between cement particles is postulated for PCE-B0.