Influence of Thixotropy on Stability Characteristics of Cement Grout and Concrete

Abstract

The use of viscosity-enhancing admixtures increases the homogeneity of cement-based materials and leads to greater uniformity of hardened properties. These admixtures increase the yield value and viscosity rheological parameters; however, their impacts on enhancing the thixotropic behavior of the cement paste and concrete are unknown. The effect of both classes of admixtures on the rheological parameters can be quite different. Common viscosity-enhancing admixtures function by imparting structure to the liquid phase, whereas thixotropy admixtures function by imparting structure to the solid phase. The effect of welan gum and cellulose-based viscosity-enhancing admixtures and propylene carbonate (PPC) thixotropic admixture on the performance of cement grout made with a water-cement ratio (w/c) of 0.40 was investigated. The mixtures had equal initial flow-time consistencies and were tested to determine their rheological parameters, thixotropy, bleeding, setting time, heat flux, and strength development. The effect of combined additions of the thixotropic admixture with a low dosage of viscosity-enhancing admixture on slump retention, stability, setting time, and strength development was also evaluated for flowable concrete with a w/c of 0.41. Grout and concrete mixtures made with the thixotropic admixture exhibited a greater demand for high-range water-reducing admixtures and losses in fluidity than those made with a viscosity-enhancing admixture. The use of PPC led to considerable improvement in time-dependent stability characteristics, including static bleeding and surface settlement. On the other hand, the increase in thixotropy was not effective in improving stability characteristics determined shortly after mixing and prior to the development of thixotropy. This included resistance to pressure filtration and washout. The combined additions of PPC and low concentrations of a cellulosebased viscosity-enhancing admixture were very efficient, however, in enhancing stability compared with similar mixtures made with either admixture. No adverse effects on setting or strength development were observed with these combinations.