International Concrete Abstracts Portal

This paper deals with the consistency of High-Performance Concrete. Three water-reducing admixtures and two cements were tested by casting sixteen series of silica fume concretes. The study of the consistency was investigated as a function of time by using slump test and its comparison with heat flow measurement. The W/C are 0.25 and 0.35, and the amount of silica fume varies between 0 and 30% as a partial replacement of cement mass Two cements with different C3A, contents are used. The influence of the admixture chemistry was studied using polynaphthalene, polymelamine and polyvinyl superplasticizers. The results indicate that the polynaphthalene admixture is the most efficient one to control the consistency of concrete. Nevertheless polymelamine shows better fluidification properties than polyvinyl. Concerning the effect of silica fume, the results show that a partial replacement up to 10% silica fume does not reduce concrete workability. The increase in silica fume content corresponds to a quicker slump loss with time. Correlation between slump test results and heat flow measurements indicates that silica fume has an accelerating effect on the first hydration reactions and leads to a quick modification of the consistency of concrete. About the cement type, it has been noticed that the total amount of admixture is linked to the C3A content.

Methacrylic graft copolymers (hereafter referred to as graft copolymers) were synthesized to investigate the effects of chemical structures on the fluidizing mechanism of concrete superplasticizer containing polyethylene oxide graft chains. The cement-dispersing performance of graft copolymers is strongly affected by the length of the polyoxyethylene graft chains and is governed more by steric repulsion than electrostatic repulsion. Graft copolymers having longer graft chains and a relatively short backbone required a shorter mixing time and exhibited excellent fluidity immediately after mixing. Conversely, the fluidity-retaining capability was lower. The authors therefore synthesized a graft copolymer having different graft chain lengths to attain the high fluidity and fluidity-retaining capability, and investigated its properties. As a result, the graft copolymer having graft chains with different lengths was found to satisfy both performance requirements for fluidity and fluidity retention. Moreover, this copolymer was found to be adsorbed less onto cement particles, resulting in a low set-retarding effect. It was also confirmed that the cement-dispersing capability of graft copolymers varies depending on the cement type. Belite-rich low-heat portland cement with a low C3A content leads to high fluidity with a much lower dosage of graft copolymers than normal portland cement. These properties of graft copolymers and cement suggest their promising applicability to production of high performance concrete.

The variation in the fluidity of normal portland cement pastes containing Synopsis : comb-like graft polymers was investigated as a function of temperature ranging from 10 to 30 ‘C, in order to understand the mechanism of change in the fluidity of concrete with ambient temperatures. The polymers were methacrylic graft copolymers with polyethylene oxide graft chain produced by using the molecule design technique. In this way we prepared three different types of copolymers. Hydration of cements and adsorption of graft copolymers on cements were studied for analysis. The fluidity study indicated that the flow value of cement paste immediately after mixing, depended remarkably on temperature and the paste flow value at 20~ tended to the lowest. On the other hand, the flow loss became larger with increasing temperature. Furthermore, the influence of temperature on paste flow and flow loss of cement paste decreased with increasing length of polyethylene oxide graft chains. The interpretation of results is discussed in terms of influence of temperature on steric repulsion between cement particles using the data of the amount of both adsorbed polymer and deposited hydration products.

Two softwood lignosulfonates of different molecular weights have been investigated for their adsorption behavior in cement pastes and in an alkaline model suspension. For the model suspension we also report the effect of the lignosulfonates on zeta-potential and suspension stability. In most investigations the results of the lignosulfonates are compared to that of a sulfonated naphtalene polymer. The adsorption of lignosulpfonates in cement pastes is found to depend strongly on molecular weight, and it is argued that this suggest a physical adsorption governed by the solvency of the lignosulfonates. With the model suspension, Mg(OH)2 particles in alkaline solution, an adsorption behavior similar to that in cement pastes is found, and the two lignosulfonates are shown to be equally effective dispersants of the Mg(OH)2 particles when no extra electrolyte is present in solution. However, when suspensions are made in aqueous solutions of high ionic strengths the high molecular weight lignosulfonate show the better dispersing properties.

Polycarboxylate-based superplasticizers (air-entraining and high-range water-reducing agents) contain a polycarboxylate-based dispersant (PAS and a dispersion retainer. The PA adsorbs to the surface of the binding material particles. Due to steric effects, the particles are well dispersed, resulting in a high level of water reduction and high fluidity (l),(Z). It is considered that in the study of the dispersion effects of superplasticizers, it is important to understand the adsorption properties of the PA. We focused on the molecular size and adsorbed number of PA molecules. Based on the chemical structure and molecular weight of the PA, we calculated its molecular size and determined that the maximum length of the extended trunk polymer was 20 nm and that of the side chain was 7 nm. Considering the effective volume based on thermodynamics, one PA molecule is adsorbed to every 400 nm2 of particle surface. However, according to calculations based on the actual measurement of specific surface area and adsorption amounts, one PA molecule is adsorbed to every 100 nm2 of the particle surface. It is suggested that PA molecules shrink and are adsorbed on the surface of binding materials more densely than expected.