International Concrete Abstracts Portal

Triethanolamine (TEA) and Triisopropanolamine (TIPA) are used in small amounts as grinding aids in the cement grinding process. TIPA is particularly known to enhance mechanical strength of mortars at 7 and 28 days while TEA does not. A mechanism based on the formation of a soluble TIPA-iron hydroxide complex which could increase the degree of hydration of the cement and so could improve the mechanical properties, has been proposed for the TIPA. The aim of this work is to explain why addition of TIPA or TEA which have close molecular structure lead to different results on the mechanical properties of mortars. The physico-chemical evolution of a cement's hydration was first followed by coupling isothermal calorimetry and ionic concentrations measurements. Then, mechanical compressive tests were carried out on mortars (limestone aggregate). Tetracalcium aluminoferrite (C4AF) hydration is modified in presence of both additive because of the formation of a soluble complex between trialkanolamine and iron III. An adsorption of TEA on the Portlandite surface is significant during the silicate phase hydration, while TIPA does not adsorb. In the case of TEA, the molecule affinity for the Portlandite surface is stronger than that of the formation of the soluble complex. These results could explain differences obtained on mechanical compressive tests of mortars characterising by an increase of the mechanical strength in presence of TIPA.

A method for measuring air entrainment in cement pastes was developed and validated against mortars and concrete air entrainment protocols with commercially available air-entraining admixtures (AEA). The paste method was found to adequately reflect variations in air content as function of AEA dosage observed in mortars and concrete; it thus provides a useful means for extensive studies of the relative performance of AEA's, or of the air entrainment mechanism using series of reference surfactants. The paste method was employed to investigate air entrainment by several homologous surfactants in the series: n-alkylcarboxylates (n-RCOO-), n-alky}sulfonates (n-RS03-) and n-alkylsulfates (n-RSOa-). Sodium salts of the latter, with hydrocarbon chain lengths between C7 and C 12, were used to investigate the influence of anionic head group and hydrocarbon chain length on the performance of these surfactants as AEA. Parallel measurements were also performed on the solubility of these surfactants in various systems, namely lime solutions, dilute cement slurries, cement pastes and solution pre-extracted from cement slurries. The study included three different cements. The combined results provide a basis for understanding the mode of action of surfactants in terms of a balance between the inherent surface activity (ability to reduce the surface tension of water) and the solubility of the surfactant homologs in alkaline solutions containing Ca ions.

New air-entraining admixtures (AEA), suitable for use with concrete containing fly ashes having levels of unburned carbon higher than the typical 2-4% allowed in the concrete industry, have been tested in this study. A number of new AEA were selected using a mortar test. Results obtained on mortar were validated on concrete. The criteria for the evaluation of the admixtures in concrete were the dosage required for a specific air content, the sensitivity of the admixture to the carbon content of the fly ash, and the stability of the air in the fresh concrete during the first hour after the contact between the binder and the water. The airvoid system of the hardened concrete was determined for the product found to be the most promising. A full testing program in concrete of the selected air-entraining admixture was then undertaken including durability testing. Concrete having three different water-tobinder ratios were used (0.5, 0.42 and 0.32). The fly ash content of the concrete were 30% for the two higher water-to-binder ratios and 55% for the lower water-to-binder ratio concrete which was a typical high volume fly ash concrete. The new air-entraining admixture's family is suitable for fly ash having high carbon content, but it showed a poor compatibility with sulfonated water reducing admixtures. When used in combination with polycarboxylate-based water reducing admixtures, no special problem was experienced with the air entrainment. Generally, the new air-entraining admixture's family does not affect negatively the properties of the fresh concrete. The measured durability parameters and all other properties of the hardened concrete are very satisfying.

Concrete technology has evolved considerably lately with the use of New Generation Superplasticizers (NGS). Both laboratory and job site results show some interesting results in combining NGS and Plasticizers. One example is the combination of polycarboxylate derivatives and plasticizers-with set retardation secondary effectseveral examples have been reported where these combinations are more efficient for keeping the fluidity level as a function of time than the NGS alone. Another example of particular interest is the combination of a PolyOxyEthylene DiPhosphonate (POEDP) with a Plasticizer. Some results have shown that the Plasticizer improves somehow the reactivity of the POEDP Considering that Trisulfoaluminate forms at the early stage of the hydration of cement and that this reaction has a significant influence on the rheological properties of the paste the influence of POEPD and plasticizer combinations on the synthesis of Ettringite was analyzed. Rheological and adsorption measurements have been performed on paste of ettringite produced by mixing calcium hydroxide and aluminium sulphate (i) alone, (ii) in the presence of POEDP, and (iii) in the presence of POEDP and Gluconate.

Three superplasticizers (SP) have been studied in this research: the first is based on modified polycarboxylic ether and is used to improve the workability of concrete and to obtain high mechanical characteristics at early age; the second, which contains naphthalene sulphonate, is used to reduce drastically mixing water in concrete and improve mechanical strength at early age; the third SP investigated is melamine-based and is used to improve the workability of concrete creating electrostatic repulsion between cement grains. The intention of the present investigation was to provide more information about the role of these SP in concrete at early age. The apparent activation, initial and final set times by Vicat needle, chemical and autogenous shrinkage were measured for cement pastes having a water/cement ratio of 0.25. The apparent activation energy has been determined by the "setting times method" at different temperatures: 10, 20, 30 and 40 °C. The volumetric autogenous shrinkage was measured at the same temperatures immediately after setting. The experimental results show that the apparent energy activation is slightly modified by the presence of SP. Also, the evolution of chemical shrinkage shows clearly that the SP acts on the hydration kinetic of cement. The effect of a particular SP on autogenous shrinkage at different temperatures can be correctly predicted by means of the maturity concept.