DRY MIXTURES

[:ru]Файфферон А.[:en]Phyfferoen A.[:] [:ru]CP Kelco UK Ltd, Лезерхед, Великобритания[:en]CP Kelco UK Ltd, Leatherhead, Great Britain[:]

[:ru]Донская И.[:en]Donskaya I.[:] [:ru]CP Kelco ApS (московское представительство), Москва, Россия[:en]CP Kelco ApS (Moscow Representative Office), Moscow, Russia[:]

Alitinform №1 (18) 2011 г. 103-109 p.

Abstract

Because a fresh cement or gypsum-based mix is composed of materials with different densities, it will have a tendency to segregate. This is a potentially serious problem in highly fluid mixes such as self-levelling mortars, grouts, self-compacting concrete (SCC), etc. However, segregation can be controlled by the careful selection and proportioning of materials, and also by the use of viscosity-enhancing admixtures (VEAs). The chemistry and rheology of a VEA can significantly influence the properties and performance of a mix when used to control segregation, bleed water formation and stability. There are numerous potential VEAs that can be used to improve the stability of cementitious and gypsum-based formulations, many based upon water-soluble polysaccharides with various rheological profiles. The rheology of the microbial polysaccharide, diutan gum, is unique amongst them, as it exhibits a highly shear-thinning, pseudo-plastic rheology which enables it to create a high viscosity at low concentrations in the mix water and so contribute to stability. But its rheology will rapidly respond to applied forces (e. g. mixing or pumping) in such a way that its viscosity reduces significantly and so have minimal impact on fluidity and workability. When these forces are removed, the initial high viscosity is rapidly recovered. Therefore with careful design, a mix can be stable and homogeneous and yet can still exhibit good workability and fluidity allowing it to be easily pumped, flow, level, compact, and give a high quality finish.

It has also been documented that highly fluidised mixes, and especially concrete, formulated without a VEA, can be much more sensitive to variability in their formulation and raw materials. Also other factors such as aggregate moisture content, cement fineness, temperature, etc can be an issue. It has been demonstrated that mixes designed with diutan gum as the VEA are more robust to these variations which allows consistent quality to be reliably achieved, maintaining performance and final hardened quality.
This article will introduce diutan gum, discussing briefly its properties and use in workable systems.

Key words:

diutan gum; admixture; cement-based mix; viscosity modifying agent; pseudo-plastic rheology; microbial polysaccharide.