Mechanism of Settling in Cement Paste

Abstract

WHEN a freshly prepared cement paste is left at rest, the cement particles tend to settle downwards. Water displaced in the motion permeates up through the paste and accumulates as a layer on the top. This process is described as segregation or “bleeding” when it occurs in concrete. A study of the bleeding of Portland cement paste, mortar and concrete, treated as a special case of sedimentation, has been carried out by Powers¹ and Steinour^2,3. In their experiments they measured the settlement of the upper surfaces of paste samples in cylindrical containers. At the concentrations normally used in practice, with water–cement ratios of 0.30–0.65 by weight, a linear rate of settlement was observed for the first 30 min or so of a test, and the rate then decreased until settlement stopped¹. Similar tests made on the same kind of paste, but on samples of different heights, showed that the initial rate of bleeding remained the same regardless of height. Powers inferred that these results “illustrated the whole process of bleeding, showing not only the movement visible at the top surface but also simultaneous changes that go on below the surface”¹. He deduced that during the settling period there are four zones within the paste. At the top is a layer of clear water, followed by a zone of constant water content (initial water–cement ratio) and constant rate of settling. Below this is a transition zone of decreasing water content and decreasing rate of settlement, which extends to the bottom zone of maximum compaction and zero rate of settlement.