International Concrete Abstracts Portal

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.

Self-leveling concrete presents a remarkable fluidity: the static flow measured by means of a truncated cone (0inf = 225 mm; O sup = 170 mm ; h = 12Omm) reaches 600 mm within 10 seconds. Such concrete is particularlysuitable for casting horizontal structural elements and allows a considerable workmanship reduction. Sometimes, this concrete has to be transported and the flow must be maintained for one hour or more. Therefore, a study was conducted to investigate the influence of the superplasticizer on the loss of workability of self-leveling concrete. The viscosity agent used in this study was a solution of modified starch (10 % solids); its content was kept constant over all the study. Two types of inorganic materials were tested; fly ash and powdered limestone. The total cement + mineral admixture was kept constant at 400 kg/m3, and the amount of water was adjusted for each composition. Five types of commercially available superplasticizers were analyzed: a melamine sulfonate, a naphthalene sulfonate, a polyacryalate, and two polycarboxylates. Their dosage was adjusted to a get an initial flow of 600 mm within 10 seconds. The following tests were carried out: loss of workability within one hour, segregation, bleeding, plastic shrinkage, strength at 18 hours, and 28 days. The main finding of this study is that only one type of polycarboxylate associated with modified starch allows the concrete to keep its initial flow for more than one hour.

A newly developed ‘polyether-based super-plasticizer’ was found to be much more suitable for self-compacting concrete than the conventional ones because of several advantages, such as high dispersability with minimal setting retardation while maintaining segregation resistance. Polyether-based superplasticizers’ features long ethylene oxide (EO) graft chains which give high dispersability by their steric repulsion, and short main chain with smaller amount of carboxyl group which leads to high early-strength. Furthermore, segregation resistance is improved by using this new superplasticizer with higher content of EO. This new superplasticizer has made possible the self-compacting concrete applications resulting in manpower savings, and improvement of quality and productivity.