Effect of Cement on Superplasticizer Adsorption, Yield Stress, Thixotropy and Segregation Resistance

摘要

The successful application of self-compacting concrete depends more and more on the exact knowledge of the rheological properties during mixing, handling and placement. Rheological investigations with mortar compositions of similar flowability reveal that different Portland cements of similar fineness affect static yield stress and thixotropy differently. Higher amounts of superplasticizer added to adjust flowability reduce the static yield stress and thixotropy of the mortar and the segregation resistance of the equivalent concrete. This behaviour can be explained by superplasticizer adsorption and surface coverage by the adsorbed polymer on the cement particles. Surface coverage affects inter-particle forces as well as nucleation probability at the surface. An increase in superplasticizer dosage leads to a higher surface coverage by polymers. At higher surface coverage the effective layer thickness increases and causes a reduction in the maximum attraction between the particles. Furthermore, the number of available nucleation sites decreases and the bridging distance between the particles increases. Less force is needed to disperse the particles, static yield stress and thixotropy become lower. The results illustrate the importance of understanding the inter-particle interactions in concrete if rheological properties, workability and segregation resistance are to be controlled.