International Concrete Abstracts Portal

Polybasic organic phosphate and lignosulphonate were combined in appropriate proportions to be a kind of high-range retarder-plasticizer(KM) for use of higher temperatures. Various retarders have been used in ordinary concrete and roller-compacted concrete(RCC)for testing under 45 “C and 25% relative humidity. Rheology and time of setting of fresh concrete and mechanical properties of hardened concrete, allowance of exposure time of bond between two construction layers of RCC,etc. were investigated. Simulated testing for Three Gorges Project was also carried out by testing a t the Development General Co. and the Testing Center of Gezhouba group.

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.

In the present paper a new analytical method for measuring the adsorption of naphthalene sulfonate formaldehyde condensate superplasticizers (NSFC) onto cement is described. By this method, based on the application of Gel Permeation Chromatography (GPC), it is possible to estimate the selective adsorption of the different components of these admixtures (P-naphthalene sulfonate, naphthalene disulfonates, polymer fraction). The results indicate that P-naphthalene sulfonate and naphthalene disulfonates are not substantially adsorbed onto the cement particles, independently on the cement used and the dosage of admixture. The polymeric fractions are adsorbed at different extent according to the dosage of superplasticizer and the cement used. Adsorption tests with two NSFC samples of different molecular weight indicate that the high molecular weight polymer fractions are preferably adsorbed over the low molecular weight fractions. The adsorption isotherms of the polymeric fraction of NSFC superplasticizers with different cements indicate that the saturation dosage depends on the chemical composition, the particle size and the specific surface area of the cements. Moreover, these results indicate that the NSFC polymer fraction is totally adsorbed on different cements up to a certain dosage of superplasticizer. This critical value seems to be related with the minimum dosage of superplasticizer to reach the optimum fluidity of cement mixtures. This observation could be put in relation with the preferred adsorption of the higher molecular weight fractions on NSFC, which exhibit a better superplasticizing effect.

Original Reactive Powder Concrete (RPC) - in form of a superplasticized cement mixture with silica fume, steel fibers and ground fine quartz (150-400 pm) - was studied in comparison with a modified RPC where a graded natural aggregate (max size 8 mm) was used to replace the fine sand and/or part of the cementitious binder. Original and modified RPC were manufactured at a plastic-fluid consistency, cast by vibration and cured at three different conditions: a) room temperature; b) steam-curing at 90°C; c) high pressure steam-curing at 160°C. The addition of the graded aggregate does not reduce the compressive strength provided that the quality of the cement matrix, in terms of its water-cement ratio, is not changed. This result is in contrast with the model proposed to relate the high compressive strength level of RPC (200 MPa) to the absence of coarse aggregate. Both the original and modified RPC (with the coarse aggregate addition) perform better - in terms of higher strength and lower driying shrinkage or creep strain - when they are steam cured rather than cured at room temperature. This improvement was related to a more dense microstructure of the cement matrix, particularly in the RPC specimens steam cured at 16OOC.

The relationships among the amounts of adsorption admixtures to cement particles, the interactive force between cement particles and the zeta potential of cement particle and the fluidity of cement paste prepared at the W/C ratio of 0.3 with lignin sulfonic acid-based admixture (LS), aminobenzene sulfonic acid-based admixture (AS) or melamine sulfonic acid-based admixture (MS) was studied in this paper, aiming at obtaining the basic data for elucidating the effects of the characters of sulfonic acid-based organic admixtures on the fluidity of concrete. The amount of adsorption of the sulfonic acid-based admixture by cement particles are logarithmically related to the molecular weight of the admixtures. The lower the molecular weight, the more the amount adsorbed are. The zeta potential of cement particle in cement paste depends upon the content of sulfonate group in the admixture and the adsorption state of admixture. The interactive force between cement particles are linearly related to the fluidity of cement paste. The steric repulsive force considerably contributes to the dispersion of particles even in the addition of sulfonic acid-based admixture which has been considered so far to disperse the particles mainly by the electrostatic repulsive force. The steric repulsive force is related to the molecular weight of the sulfonic acid-based admixture except AS. The rate of contribution of electrostatic repulsive force to the total repulsive force accounting for the interactive force correlates with the content of sulfonate group in the admixture. The reason why the action of AS is different from those of the other admixtures is probably because the adsorption state of AS is different from those of the others.