Compressive Yield Stress of Cement Paste
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KELLY T. MILLER*, WEI SHI*, LESLIE J. STRUBLE*, AND CHARLES F. ZUKOSKI* *Department of Chemical Engineering **Department of Civil Engineering University of Illinois at Urbana-Champaign, Urbana, IL 61801
ABSTRACT Compressive yield stresses have been measured for pastes (0.35 < w/c < 0.50) of portland cement, calcium aluminate cement, and weakly and strongly flocculated alumina (00 = 0.20) using the centrifuge sediment height technique. Equilibrium sediment heights are reached quickly, allowing all measurements to be taken during the cement's induction period. The compressive behavior showed little dependence on the compressive history. Compressive yield stress was, however, dependent upon initial volume fraction, decreasing as the initial volume fraction increases. This behavior was observed in both the cements and alumina suspensions, implying that strong dependencies on initial structure may be a general property of the compressive behavior of flocculated suspensions. INTRODUCTION Compressive rheology is used to characterize suspension behavior during consolidation (gravitational settling, pressure filtration, centrifugation). As described by Buscall and White 1, consolidation behavior depends on the balance of the various forces acting on the particles: 1) gravitational or centrifugal force, 2) externally applied loads, 3) interparticle forces (which may be either attractive or repulsive), 4) hydrodynamic forces (the drag acting on the particles and fluid as they move past one another), and 5) Brownian forces (which cause submicron particles to move randomly). To get consolidation, the compressive forces must exceed the other forces acting upon the suspension. In stable suspensions, one must overcome the osmotic pressure. In flocculated suspensions, the exact constitutive properties are debatable; one fruitful approach assumes that compaction occurs when the compressional force acting on the suspension exceeds a volume fraction dependent compressive yield stress, Py(0). For pressures acting on the particles less than P ,, the bed stores energy elastically. For an applied load larger than Py, the bed consolidates irreversibly until it reaches a volume fraction capable of storing the load elastically1. The compressive yield stress, Py(0), can be measured using simple centrifuge techniques 1-3. The approach used here is to compute consolidation pressure, Py, as a function of volume fraction 0 from measurements of equilibrium sediment height at a number of centrifuge speeds. In this experiment, the particle bed is subjected to a body force, which increases as the spinning speed is increased. When P y is exceeded, the particles compact and expel water, reaching an equilibrium height at which the centrifugal load can be accommodated elastically. The Py(0) curve is generated from the sediment heights by calculating the pressure and volume fraction at the bottom of the cake at each spinning speed. In this paper, we show that this technique can be successfully applied to measure the compressive properties of cement paste
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