International Concrete Abstracts Portal

Comb-type superplasticizers are widely used for producing self-compacting or high-strength concrete. Recently, slag cement binders have been developed for their contribution to sustainable development and for their capacity to improve the durability of concrete. In order to enhance the use of slag cements in concrete formulation, it appears important to progress in the knowledge of their interaction with comb-type superplasticizers. In this way, this paper discusses about the polymer adsorption capacity on blast furnace slag and laboratory prepared mixes of slag and cement compared with portland cement. We observed that the substitution of cement with a very slow reactive binder like slag at early age, which induces then a different polymer adsorption capacity, allows to obtain a polymer adsorption less sensitive towards sulphate ions concentration. Besides, we observed that the dispersing effect of polycarboxylate polymer (PCP) was higher on slag cement paste than on portland cement paste. Therefore, slag cements seem to be interesting in concrete formulation because their interactions with superplasticizer seem to be less affected by the chemical composition of cement, and the superplasticizer effect is enhanced.

The interaction between sulfates and polycarboxylate polymers has become a subject of much discussion and occasional concern over the past years. Laboratory studies have shown that sulfates may prevent some polycarboxylate polymers from adsorbing, which causes a reduction in flow properties of cementitious materials. This seems to have led to the belief that this phenomenon is generally true for all polycarboxylates. In fact, it depends very much on the detailed polymer structure. Examples of this will be shown both with flow examples on paste, and experiments by atomic forces microscopy that illustrate in a very convincing way that this competition does not always turn to the advantage of sulfate ions. Furthermore, it is discussed how the outcome of this competition is governed by variations in the adsorption energy of the polymers.

Significant numbers of studies on the working mechanisms of superplasticizers have elucidated many important characteristics of cement. However, because of the lack of general reviews, similar research activities tend to be repeated. Therefore, it is worthy to summarize a research on this issue based on the authors' experiences, including some unpublished miscellaneous but interesting data, irrespective of its bias or imperfection. The first point is the effect of sulfate ions on the adsorption and dispersing performances of poly- naphthalene sulfonates (NS) and polycarboxylate-type (PC) superplasticizers. With the addition of cations that make insoluble salts with sulfate ion, the dispersing performances of both NS and PC are increased. This emphasizes the importance of absorption. The second point is the effect of carboxylic groups on the initial hydration. A PC with too many carboxylic groups accelerates the initial hydration of cement. This acceleration can be controlled by the addition of soluble sulfate salts. In this case, sulfate ions increase the dispersing performance of PC. The last point is the importance of fine particles in ultra low water-cement ratio (w/c) range. There is a suitable range of particle size around 1 pm.

The aim of the present study was to determine the adsorption isotherms of polycarboxylate (PC) superplasticizers with different structures on slag-blended cement pastes (with a slag content between 0-75%). Also, their effect on the rheological properties and hydration process has been evaluated. The results indicate the adsorption of PCs decreases slightly as the slag content in the cement increases; however, their fluidizing properties are significantly higher in the slag-blended cement. This effect is mainly attributed to the content of C3A (mineralogical phase with the highest affinity for the PCs) which decreases in slagblended cement. Consequently, the amount of PCs consumed and adsorbed by this mineralogical phase also decreases. In this way, most of the PC admixtures are absorbed onto the silicate phases of the clinker and onto the slag particles, inducing an electrosteric repulsion and the concomitant reduction in yield stress. The rheological results show that the highest increase of the fluidity is caused by the admixture with highest molecular weight concluded to be due to the higher steric repulsion expected for thicker adsorbed layers. As a consequence of the adsorption of the PCs, a delay of the hydration process of the cement pastes has been observed.

The increased performance of polycarboxylate superplasticizers is generally explained by the steric hindrance they are intended to develop between cement particles. In fact, direct evidence of this is relatively scarce. The only direct measurements to date have been made by atomic force microscopy on model surfaces of magnesium oxide. In this paper, we report very recent measurements using the same technique but on surfaces of calcium silicate hydrate that constitute a more realistic model system. Furthermore, it is shown that the measured interfacial behavior of superplasticizers can be quantified by a scaling law approach borrowed and extended from polymer physics.