Y. F. Houst

A simplified view on chemical effects perturbing the action of superplasticizers

Abstract Understanding and quantifying effects of superplasticizers in concrete is a complex task. Even for nonreactive systems, such as ceramic suspensions, the stabilizing effects of dispersants are a subject of ongoing research. In cementitious systems, hydration reactions can perturb the behavior of suspensions. In this article, we propose three categories to describe the interactions and state of the superplasticizers with the cement suspensions. The first part would be consumed by intercalation, coprecipitation or micellization, i.e., by the formation of an organo-mineral phase (OMP). A second part of the polymer could be adsorbed onto the surface of particles and help disperse cement agglomerates. The third part consists of the excess superplasticizer neither consumed nor adsorbed and which remains dissolved in the aqueous phase. Thus, at equal dosage, a cement with a larger degree of consumption (Part 1) could have a lower surface coverage and consequently poorer workability (for cements otherwise equal: specific surface, composition, etc.), unless an excess of superplasticizer is added to ensure saturation. Differentiating consumption from adsorption is essential for correct interpretation of experimental data, a fact that is not yet appreciated often enough and to which this paper attempts to draw larger attention.

Influence of porosity and water content on the diffusivity of CO2 and O2 through hydrated cement paste

Knowledge of the diffusivity of CO2 and 02 is of considerable importance for a quantitative assessment of the carbonation and corrosion of the reinforcement in concrete. A special measuring cell which allows the simultaneous determination of the effective diffusion coefficients of the two gases as a function of the relative humidity has been developed. Measurements have been carded out on carbonated discs of hydrated cement paste prepared with water/cement ratios between 0.4 and 0.8. The results show that, if the water/cement ratio is increased from 0.4 to 0.8, diffusion coefficients increase more than ten times. On the other hand, only little influence of the relative humidity between 50-90% the diffusivity has been observed on samples in adsorption equilibrium. The variation of diffusivity as a function of water content and porosity is explained by the characteristic microstructure, which has been characterized by water adsorption isotherms and mercury intrusion porosimetry measurements. Finally, a model with two levels in the microstructure is proposed to describe CO2 diffusion in a carbonating material.

Depth Profiles of Carbonates Formed During Natural Carbonation

Abstract The depth profiling of carbonates formed during a natural carbonation of mortars with one face exposed directly to rain and the opposite face sheltered have been measured. The amount of carbonates formed on mortars sheltered from rain is generally higher. The depth of carbonation evaluated by the phenolphthalein test cannot be directly correlated with the carbonate profile. The CO 2 diffusion coefficients calculated on the basis of the distribution of carbonates are of the same order of magnitude as those measured by a direct method. A threshold value of diffusivity appears when the sand concentration increases. This point is discussed and compared with other characteristics of mortars and concretes that exhibit similar behavior.

INTERACTION OF SUPERPLASTICIZERS WITH MODEL POWDERS IN A HIGHLY ALKALINE MEDIUM.

Synopsis: It is broadly recognized that the adsorption of superplasticizers 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 superplasticizers 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 superplasticizers, 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.

ELECTROSTERIC REPULSION INDUCED BY SUPERPLASTICIZERS BETWEEN CEMENT PARTICLES: -AN OVERLOOKED MECHANISM?

Synopsis: Dispersion mechanisms of superplasticizers have received much attention over the past years. Recent developments have brought very efficient superplasticizers where the dominant stabilizing mechanism is thought to be via steric repulsion. These new superplasticizers contain an adsorbing backbone onto which non adsorbing side chains are grafted with the objective of getting them to stretch out into the solution from the cement particle surface and induce the steric repulsion upon approach of other particles. Another feature of these polymers is that they induce only very small zeta potentials. Calculations of interaction energies indicate that these polymers act predominantly through steric repulsion. However, the same calculations could lead to the conclusion that all polymers can only act through steric repulsion. The calculation of the steric and electrostatic contributions are greatly dependent on the polymer adsorption conformation and the distribution of charge at the particle surface associated with these adsorbed polyelectrolytes. Many of the assumptions made in calculating interparticle forces are not necessarily good approximations for polyelectrolytes. This paper discusses the limits of the approximations currently used in such calculations and presents a more accurate model for the calculation of these forces. The main result, applicable for a wide range of superplasticizers, is that both electrostatic and steric repulsions should be taken into account, provided the electrostatic charge can be assumed to lie at the outer-bound of the adsorbed layer of superplasticizers. Such information is of primary importance for understanding and solving cement and superplasticizer incompatibilities, as well as for developing novel products.

Compatibility of PC Superplasticizers with Slag-Blended Cements

The aim of this paper 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 slag blended 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.