International Concrete Abstracts Portal

This Symposium Publication includes 37 papers selected from the conference that took place in Nice, France, in October 2000. Topics include effects of superplasticizers, interaction of admixtures with calcium aluminate cements, lignosulfates, admixtures for improving resistance to chemical attack, effects of admixtures on concrete shrinkage, and many more. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP195

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.

This research was aimed at the use of agroresources as admixtures for mortar and concrete. Thirteen modified starches from potato, wheat, corn and waxy corn were investigated. They were compared to methylhydroxyethylcellulose (MHEC) and Welan gum for the following properties: thicknening effect, water retention, setting time and 28-day compressive strength. The study was carried out on standard mortars and the starch dosage was either 0.75 % or 0.25 % of the total dry matter (sand + cement). The results obtained show that the recommended dosage is 0.25 %, which is still economic compared to MHEC and Welan gum. The best performances were obtained with waxy corn starches and, to a less extent, with potato starches. The latter performed well except for water retention.

Certain di-phosphonate terminated monofunctional polyoxyethylene are widely used as concrete superplasticizers. In order to understand its action mode, its effect on hydration of pure tricalcium silicate suspensions has been investigated by conductimetry, isothermal calorimetry and ionic and total organic carbon (TOC) analyse of the liquid phase. The polyoxyethylene di-phosphonate modifies the nucleation and growth process of C-S-H by reducing the number of initial nuclei, decreasing the growth rate in the accelerated period and increasing the rate during the difision limited period comparatively to control samples. The evolution of ionic concentrations in the solution during hydration reveals an apparent increase of the critical supersaturation required to nucleate both C-S-H and portlandite. This is due to the formation of a calcium-diphosphonate polyox complex. The calcium complexation constant and pKa of the phosphonate polyox have been determined from pH and conductivity studies of calcium-hydroxide and sodium-hydroxide polymer solutions and well account for portlandite solubility in presence of phosphonate. The polyox phosphonate does not seem to adsorb on C3S surface but is rather adsorbed on C-S-H. This is probably the origin of the decrease of the growth rate leading to the modification of the texture of C-S-H and subsequently the modification of the rate during the diffusion limited period.

In connection with environmental problems, the utilization of industrial materials waste, municipal refuse, sludge or its incineration ashes as raw materials for cement has recently begun. If their utilization amount increases in the future, it will result in the increase in aluminate phase. It is necessary to clarify the influence of superplasticizers on the hydration of aluminate phase. In addition, the fluidity of the cement paste which the sulfate ion content is high, is decreased by addition of comb-type superplasticizers. This is also closely related to initial hydration of cement. In order to clarify the action mechanisms of comb-type super-plasticizers, this paper discusses the influence of super-plasticizers on the early hydration of C,A-calcium sulfates by quantitative XRD compared with the case of naphthalene type superplasiticizer. Early hydration of C,A is accelerated by adding of comb-type super-plasticizers in C,A-calcium sulfates systems. In the case of comb-type superplasticizers, SO,“- contents in gel formed around C,A is decreased. It is necessary to control the early hydration reaction of aluminate phase in order to control the fluidity of the cement with comb-type superplasticizers.