Volume 40, Issue 5 (September 2012)
Time-Dependent Rheological Changes of Fly Ash Blended Cement Paste
Cement-based materials are known to be time-dependent, especially at early ages. The time-dependent flow-behavior change controls the workability and compactability of fresh concrete. This time-dependent nature of cement-based materials is associated with the cement hydration and the consequent microstructure development. Time-dependent rheological changes of fly ash blended cement paste are investigated in this study with the flow spread, needle penetration, and rheometer tests. Hydration of the fly ash blended cement paste is monitored continuously with the ultrasonic propagation method. The results show that the thixotropy of fresh cement paste is increased by lowering the water-to-binder ratios and is mitigated by replacing Portland cement with fly ash or adding superplasticizers. The yield stress changes of the cement paste are mainly affected by the curing time rather than the water-to-binder ratio and the fly ash proportions. On the other hand, the viscosity changes slightly with time. Viscosity of the fresh paste is the controlling factor on the flow spread, and the yield stress determines the needle penetration depth. Models show that the flow spread of fresh paste is related to its viscosity with a power function and the needle penetration depth is inversely related to the yield stress.