International Concrete Abstracts Portal

On October 28-31, 2018, the Chinese Ceramic Society and the China Academy of Building Research (CABR), Beijing China, in association with ACI, sponsored the Twelfth International Conference on Superplasticizers and other Chemical Admixtures in Concrete in Beijing China. More than 80 papers from all over the world were received and peer reviewed. A total of 36 refereed papers were accepted for publication in the proceedings of the conference. The proceedings were published by the ACI as SP 329. Also, 54 additional papers were presented at the conference, and were published in the Supplementary Papers Volume. The organizers of the conference were the Chinese Ceramic Society, Beijing and the Committee for the Organization of International Conferences (formerly CANMET/ACI Conferences).

In this article we investigate the effect of precipitation of ettringite on the dispersion effect of four PCE with different structures. We first developed a simple method to qualify commercial cements with respect to early ettringite precipitation. The difference in flow between a paste prepared with the PCE in the mixing water (direct mode) and a paste prepared by adding the PCE after 3 minutes of hydration (indirect mode) can be correlated to amount of ettringite forming for a specific cement. A similar test setup was used to assess the effect of the precipitating ettringite on the flow properties of the different PCE. We found that all tested PCE were impacted by ettringite precipitation but to a very different extent. This suggests that, for concrete applications, the selection of a PCE more robust against variations in ettringite precipitation due to fluctuations in the used raw materials is feasible.

The mechanism of superplasticizers (SP) action implies their obligatory adsorption on the products of binder hydration; the magnitude and kinetics of adsorption determine the efficiency of initial plastification and of the effect retention. It is generally recognized that high-molecular components of SP adsorbed more than low-molecular analogs. However, this conclusion is based on comparison of adsorption levels for individual fraction of different molecular mass while SP comprise set fractions of different molecular mass and their adsorption occurs simultaneously as a competitive process. Polymethylene naphthalene sulfonates (PNS) are the very thoroughly studied class of superplasticizers (SP) and they are considered as suitable model to study sulfonate-type SP mechanism with Portland cement hydrates. Using High Performance Liquid Chromatography this paper examines for the first time effect of molecular mass on adsorption kinetic under competition and reveals some trends. The results obtained were compared with adsorption data by UV-spectroscopy. An applicability of identified trends for PCE was considered.

Polycarboxylate-based chemical admixtures are invariably formulated with defoaming agents to mitigate unwanted air produced by adding such polymers during the manufacture and placement of concrete mixtures. Effective defoaming additives are by nature highly hydrophobic, and thus have very limited solubility in aqueous chemical admixture products, which can present challenges for both admixture formulators and concrete producers. Numerous approaches have been used to create stable polycarboxylate-defoamer formulations including continuous mechanical agitation, emulsification, co-solvents, grafting hydrolyzable defoaming agents, and ionic association between the defoamer and carboxylate functional groups in the polymer backbone. Based on a detailed discussion of the concerns that can occur with formulating admixtures with defoamers, a specification which includes criteria for product stability, air control in both air-entrained and non air-entrained concrete, and air-void quality of air-entrained concrete, is proposed.

In this article, we investigate the effect of small differences in type, amount or blend of sulfate carrier on the formation of early ettringite, especially in presence of PCE polymers. We used a commercial cement clinker before addition of the sulfate carrier and added different types, amounts or blends of calcium sulfate. The resulting blends were hydrated without PCE and including three different PCE. We followed the evolution of the amount, the specific surface area and the total ettringite surface per mass of paste over time. We observed very different evolution of the respective parameters depending on the type/blend of sulfate carrier used. Furthermore, presence of PCE had additional individually different impacts on these parameters, depending on the structure of the PCE. These impacts are mainly attributed to interactions of the PCE with ettringite precursors leading to stabilization of such precursors in presence of PCE.