International Concrete Abstracts Portal

Superplasticizers are considered to be the most important chemical admixtures in cement mixtures due to their influence on the hardened concrete properties (related to the water-cement ratio) as well as on those of the concrete in the fresh state (workability and workabilty loss). During the last decade a new family of polymers based on polycarboxylate as the main polymer chain and polyether as side graft chains have been developed. This new family of products appear to be in general more effective in terms of higher water reduction, lower slump loss and lower retarding effect at very early ages. More recently these performances have been enhanced in view of some specific applications: a) in precast concrete structures, the very early strength (such as at 12-16 hours) can be increased even in cold climates and in the absence of steam curing; this effect depends on the number and length of the polyether graft chain which are responsible for the dispersion effect related to the steric hindrance; b) in ready-mixed concrete mixtures, the slump maintenance behavior can be designed as a function of the transport time and placing temperatures, so that fresh superplasticized concrete mixtures can be transported from the batching plant to the job site without any slump loss even in summer times; this effect depends on the gradual liberation of special molecular groups which are responsible for the adsoption of these polymers on the surface cement grains; c) a new series of polycarboxylate polymers has been synthetized in which functional chemical groups, acting as shrinkage-reducing admixtures, again are gradually liberated by the superplasticizer polymer as a function of the pH of the aqueous phase related to the cement hydration.

SP-239 This Symposium Publication includes 36 papers selected from a conference that took place in Sorrento, Italy, in October 2006. Topics include cementitious systems, ultra-high-strength concrete, artificial superplasticized aggregate, mortars, and self-consolidating concrete.

The adsorption of sodium lignosulphonate onto cement was investigated. The cement has either been pre-hydrated or hydrated with the lignosulphonate present. The pre-hydrated cement particles grow with time, thus a larger surface is created and the plateaus in the adsorption isotherms increase. With lignosulphonate present there is hardly any growth of new surface. It is found that, a lignosulphonate that adsorbs strongly to the surface causes, less retardation than weakly adsorbed lignosulphonates. This can be explained by different interactions with the different minerals present.

Extreme performances are required in superplasticizers technology nowadays; the new generation of polymers makes it possible to reach a high water reduction together with a good workability retention, ensuring excellent properties to the hardened concrete. With most of the cements, outstanding performances are achieved with the right combination between cement and superplasticizer; on the other hand, high performing superplasticizer are inevitably highly sensitive to the materials used, and in particular to the cement. Among the parameters that characterize the chemical composition of cement, the sulfate amount and type are recognized as key factors for understanding the reasons of possible problems of incompatibility between cement and superplasticizer; however, little was done on the concrete scale. In this article, five samples of cement containing different amounts of sulfates were selected. The influence of the sulfate content on the performances of PNS- and polycarboxylate-type superplasticizers was evaluated by means of adsorption tests, minislump tests and mortar tests. In the case of addition of sulfates to cement, concrete tests were also made in order to check the results on the concrete scale. The results of this study highlight the importance of an adequate content and balance between the different types of sulfates in cement, in order to guarantee the right interaction with superplasticizers.

The concrete industry is heading towards more sustainable development and aims to make concrete an even more cost-effective building material. In this effort, the cement industry uses increasing amounts of secondary fuels to improve efficiency. Cement kiln dust from the clinker cooling and grinding processes are returned to the cement. Also, the use of supplementary cementitious materials, marginal and recycled aggregate in concrete production is increasing. These practices introduce materials, not foreseen, in the past, in the concrete composition. They are not always inert and often negatively affect the performance of the superplasticiser. In this paper the parameters that cause incompatibility between the cement, other components of the concrete mixture and the superplasticiser are examined. Tailor made superplasticisers are required to overcome these incompatibilities. Some case studies are presented, where the tailor made approach made a positive difference.