International Concrete Abstracts Portal

Macromonomers for MPEG type of PCEs are produced through esterification of methacrylic acid (MAA) with methoxypoly(ethylene glycol) (MPEG) yielding the MPEG-MAA ester. However, PEG impurities present in MPEG may lead to MAA diester (PEG-di-MAA) formation. Such diester can cause crosslinking of the PCE polymer which might reduce its dispersing power. To investigate this effect, MPEG-MAA macromonomers containing 0 – 20 wt. % of PEG-di-MAA diester were used in PCE synthesis. It was found that when the PEG-di-MAA content in the macromonomer exceeds 2 wt. %, then dispersing effectiveness starts to decrease and the solution viscosity of the PCE increases. Surprisingly, incorporation of the diester into the PCE polymer does not occur randomly. Instead, two distinct species of crosslinked PCE molecules (Mw ~ 300.000 and ~ 3 mio g/mol) are formed within the first minutes of copolymerization. Apparently, the crosslinked PCE species counteract the dispersing effect of the main product.

The measure of wettability of cement particles and the evaluation of the type and the strength of active surface sites of clinker particles were carried out to score the efficacy of organic grinding aids. The first analysis, based on the Washburn method, allows measuring the contact angle of cement with different solvents and is directly related to the surface tension. The second analysis, based on the Hammett method, helps to classify the sites that may be responsible of the surface tension. Milled clinker (portland clinker CEM I 52, 5R; 0,025% w/w DEG and TEA) was investigated. Firstly the wetting rates of powder with 4 different solvents (ethanol, n-hexane, toluene, and formamide) were detected using a tensiometer. Then the powder dispersions in a solvent were titrated by acid solutions to get information on the acid/basic character of the surface sites. Techniques and results are shown.

Polycarboxylate superplasticizers are known to be most powerful admixtures which exhibit superior dispersing force even at extremely low water-to-cement ratio. In this study, a simplified one-pot synthesis method for a PCE using only maleic anhydride and methoxy polyethylene glycol as sole raw materials was developed. Compared to conventional synthetic routes, the new method constitutes a much simpler process which performs esterification and grafting in one reactor. Macromonomers are no longer needed for the synthesis of this PCE. The resulting copolymer was characterized by size exclusion chromatography and anionic charge density measurement. Performance of the polymer in cement was probed via ‘mini slump test’. To detect a potential retarding effect of the copolymer, time-dependent heat evolution was monitored. Finally, a model for the formation of this PCE is proposed. According to this, maleic anhydride and MPEG maleate monoester are grafted onto MPEG macroradicals which present the backbone of this PCE.

In portland slag cements (PSC), different slag compositions can produce variations in workability due to the disparity in the surface chemistry of the slags. Here, the surface chemistry of different PSCs dispersed in water was studied in the absence and presence of polycarboxylate (PCE) superplasticizers. Six PSCs were prepared by mixing portland cement with 30 or 70 wt.% of three slags. As PCEs, two copolymers based on methacrylic acid–co–?–methoxy poly(ethylene glycol) methacrylate ester were employed. It was found that the slags sequester ions from the pore solution, namely Ca2+ and SO42- ions forming an electrical double layer on the slag surface. Zeta potential measurements confirmed that different slags can exhibit different surface charges which can strongly affect PCE adsorption. The differences in the amounts of PCEs adsorbed result in different dosages required to achieve comparable dispersion. Generally, all slag cements tested required less PCE to achieve the same fluidity as with neat cement.

A polycarboxylate superplasticizer (PCE) with a novel star-shaped structure was prepared through copolymerization of acrylic acid (AA), isobutenyl polyethylene glycol (IPEG), and star-shaped polymerizable active center by an esterification between polyol and AA. In the first esterification step, the esterification rate reached more than 95% with the catalyst/polyol ratio of 0.07:1, inhibitor/AA ratio of 0.04:1 (or 0.011:1), water-carrying agent dosage of 70g and esterification time of 7 hours. In the second polymerization step, the highest fluidity of cement paste was achieved at the initiator/AA/IPEG ratio of 0.28: 3.3: 1. Infrared spectroscopy (IR) and 1H Nuclear magnetic resonance (1H NMR) measurements were used for structural characterization, and the spectral results confirmed the product’s star-shaped structure. Furthermore, this star-shaped PCE exhibited higher energy efficiency than the conventional comb-shaped PCE, indicated by its excellent paste fluidity and adsorption behavior in cement paste.