The significant increase in large high-rise concrete structures has dictated diverse requirements for concrete. Concrete technology has improved, widening the range of applications of this material. In this regard, the technology of polycarboxylate-based superplasticizers has made remarkable progress; a number of new superplasticizers have been developed and applied to big construction projects. To elucidate the fluidizing mechanism of superplasticizers in cement, we focused on the fact that the chemical structure of these superplasticizers changes in an alkali environment. We determined the absolute molecular weight of the superplasticizers using the light scattering method, measured the amount adsorbed, zeta potential, and nuclear magnetic resonance. Based on our finding pertaining to polycarboxylate-based superplasticizers, we discuss the fluidizing mechanism of cement compositions with relevance to dispersibility, dispersibility retention, and flowability. We refer to DLVO theory, steric repulsion effect theory, depletion effect theory, tribology effect, as well as the results of mortar and concrete tests. We also report on the applications at big projects, such as the anchorage of the Akashi Channel Bridge and the deep ground continuous wall of the Tokyo Trans-Bay Highway.

This paper describes the development and applications of several lithium-based chemical admixtures for set control of cementitious-based construction and building materials. Comparative evaluations show the effect of these admixtures on the set time of (1) calcium aluminate cement; (2) portland cement; and (3) blended systems of calcium aluminate cement and portland cement. General information is provided to address the principles of material selection, dosage rate and application of cementitious systems.

This paper describes the effect of a lithium nitrate-based admixture on the hydroxide ion concentration of the pore solution of hydrating pastes made from portland cement and water. No significant increase in the hydroxide ion concentration results from using this admixture in the mix, which is thus different than any published study with any other lithium compound. It has been reported that underdosing with lithium salts can increase the expansion due to ASR. The authors propose that this is mainly due to increases in hydroxide concentration observed with other lithium salts and therefore this admixture will not show such an effect. Mortar bar tests with the new admixture verify the hypothesis that the lithium nitrate-based admixture does not increase expansion at any dose. This is then a much safer admixture to use in the field with respect to risk from damaging ASR expansions. It is also much safer to handle than lithium hydroxide-based admixtures since solutions of lithium nitrate are much closer to neutral pH than lithium hydroxide solutions.

It is broadly recognized that the adsorption of super-plasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of super-plasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH)2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH)2 and CaO. The surface charge of such model suspensions was studied as a function of added superplasticizer. These were either commercially available or currently under development, ranging from strongly to very weakly ionic. Adsorption isotherms for two polymeric super-plasticizers, with similar structures but with different ionic group spacing, have been measured for both MgO and Mg(OH)2 at pH 12 and 11.3 respectively and between 10 and 40°C. Results showed a strong temperature dependence for the adsorption of the less ionic polymer on MgO.

Betanaphthalene sulfonate condensate (NS) and melamine sulfonate condensate (MS) polymers based superplasticizers have been extensively used in the Precast Industry. Continuos need for improving the performance of concrete has led to the development of products with a higher water reduction and thus higher early strength, typical of melamine based super-plasticizers, but without the drawback of workability loss sometimes encountered with this kind of super-plasticizers. The chemistry of the new product described in this article has been specifically designed to achieve high early and long term strength maintaining the workability typical of NS products at dosages in the range commonly used with melamines. The superior performance obtained with this new product makes it a good alternative to melamine based super-plasticizers when used in precast applications. The paper reports the effect of the new p-naphthalene sulfonate based super-plasticizer on water reduction, air content and compressive strength in concrete prepared with different types and brands of cement conforming to the new EN 197-1 standards. The results show that significant improvements in terms of water reduction and strength development can be achieved with this new NS based product, especially when used with CEM I type cements.