Setting of Cement and Concrete

Abstract

Abstract The standard test for setting of cement provides little understanding of how the cement behaves in concrete. Setting is measured for both cement and concrete using penetration resistance, but the standard tests for these two materials are quite different. With cement, the penetration is usually measured using the Vicat needle; with concrete, the penetration is measured using a series of Proctor needles, which vary in diameter. There is no theory to relate the tests to one another, although data are presented showing empirical relationships between the Vicat and the Proctor measurements. These tests utilize quite different samples: the Vicat test utilizes cement paste, and the Proctor test utilizes mortar extracted from concrete. The Vicat test utilizes a very stiff paste with a very low water-to-cement ratio, and the water-to-cement ratio is seen to have a substantial effect on the way cement paste stiffens. However, it is shown here that fluid pastes, more representative of paste in ordinary concrete, can be tested using either the Vicat or the Proctor. Setting may be studied using rheology, and a dynamic rheology test has been developed in our laboratory. This test provides similar results as the standard setting tests, but does not allow measurement of initial or final set. Based on these results, modifications in the cement test are proposed to improve the correlation with concrete setting, and advantages of a rheology-based definition of set are discussed.