Volume 3, Issue 3 (March 2006)
Using Dynamic Rheology to Measure Cement-Admixture Interactions
The objective of this study was to explore the effects of water-reducing admixtures on the flow behavior of cement pastes using small-amplitude oscillatory rheology. In this technique, a small shear force is applied to the specimen, the strain response is measured, and rheological parameters are calculated. Gelled suspensions exhibit high elastic modulus and yield stress values and dispersed suspensions exhibit low values. The high-range water-reducing admixture based on carboxylic acrylate ester (CAE) fully dispersed the four portland cements used in this study, while the admixture based on sulfonated naphthalene formaldehyde (SNF) dispersed only one of the cements and caused the other three to gel. Such gelation indicates cement-admixture incompatibility. At the same dosage, the CAE gave greater dispersion and more retardation of stiffening than the SNF when mixed with the same cement, but stiffening behaviors were more similar when the admixture dosages were adjusted to provide a similar initial elastic modulus. The incompatible cement-admixture combinations were dispersed when the admixture addition was delayed by a few minutes. Dispersion was also enhanced by increasing the shearing when mixing the paste.