International Concrete Abstracts Portal

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.

Recent interest in energy saving has stimulated the use of by-product materials in portland cement. After the introduction, in 1992, of the new standard specifications for cements, with the European Norm ENV 197/l, limestone blended cements have had a widespread diffusion, especially the CEM II AL portland cement, containing from 6 to 20% of limestone. Now, great attention is being paid to limestone blended cements in the manufacture of concrete, particularly for ready-mix concrete. Also, many evaluations of their performances compared with those of normal Portland cement have been undertaken in recent years. Some controversy has arisen, regarding problems of incompatibility which could eventually emerge between limestone cements and the admixtures traditionally used with normal portland cement. was therefore made to gain an insight into the elect of limestone on hydration and rheological properties of cement pastes. In this paper several types of limestone and clinker are taken into account and their chemical and physical characteristics are reported. Some superplasticizer admixtures have been tested and the behaviour of clinker/limestone blends in different proportions investigated in terms of their rheological properties (mini slump tests) and their capability of adsorbing the different superplasticizers. The absorption characteristics of various types of limestone and the dispersive efficiency of superplasticizers were studied, especialIy concerning limestone’s porosirnetric characteristics.

Welan gum is a viscosity-enhancing admixture used to retain some of the free water in cement-based materials and modify flow properties. The incorporation of welan gum and super-plasticizer can enable the production of highly flowable, yet stable systems with relatively low yield value and moderate viscosity. Such flow characteristics are useful for proportioning underwater concrete, post-tensioning grout, and self-consolidating concrete. Despite recent advances in such technological areas, limited data are available on the effect of welan gum-superplasticizer combinations on physico-chemical characteristics and kinetics of cement hydration. The study reported here was undertaken to evaluate the influence of welan gum and a naphthalene-based superplasticizer on fresh and hardened properties of cement paste with 0.40 W/C. The performance of reference grouts made without any admixture and those made with super-plasticizer were compared to the performance of mixtures made with 0.025, 0.055 and 0.075% welan gum by mass of cement. The effect of the admixtures on consistency, rheological properties, and stability of cement paste as well as setting time and changes in heat flux are discussed in this paper. Test results on hardened cement paste involving mercury porosimetry, scanning electron microscopy, and compressive strength development are also given.