Flow analysis of water-powder mixtures: Application to specific surface area and shape factor

This paper addresses the characterization of powder materials with respect to their application in concrete, Given that powders provide by far highest percentage of specific surface area in a concrete mix, their packing behavior and water demand is of vital interest for the design of concrete. They dominate physical properties like workability or strength and durability in hardened state. Regarding the granular properties of powders, different states of packing are analyzed and compared. In reference to water demands, this paper compares and analyzes four different methods, including the spread-flow test. It is shown that linear relations can be derived in order to correlate the methods. Besides comments and modifications to individual tests, the spread-flow test is analyzed in more detail. In this way new measures are derived which contribute to a deeper understanding of wet granular mixtures at the onset of flowing. Furthermore, the deformation coefficient which is derived by the spread-flow test is confirmed to correlate with the product of Blaine surface and intrinsic density of the individual powders when the mixture is flowing only under its own weight. Similarly, correlations with equal accuracy are found with a computed specific surface, based on measured particle size distributions. Using the flow experiments it is possible to derive an overall factor for assessing the non-spherical shape of the powder particles. it is shown that the computed surface area and the Blaine value have a constant ratio (of about 1.7). Finally, the value of a constant water layer thickness around the powder particles is computed for all powders at the onset of flowing, This implies the possibility to predict flow behavior of paste only based on the knowledge of their specific surface area, either determined by computation or Blaine measurements. (c) 2008 Elsevier Ltd. All rights reserved.