Densification and Migration of Ions in Blast Furnace Slag-Portland Cement Pastes
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DENSIFICATION AND MIGRATION OF IONS IN BLAST FURNACE SLAG-PORTLAND CEMENT PASTES Q.L. FENG, E.E. LACHOWSKI and F.P. GLASSER Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB9 2UE, U.K. Received 15 November, 1988: refereed ABSTRACT Well-cured slag cement blends are known to give rise to paste matrices of low permeability. Electron microscopy has been used to follow microstructural development in slag cement pastes. An initially dense layer of hydrated product develops around the slag grains. Its chemistry and microstructure are not constant but evolve with time. These variations have been explored by analytical electron microscopy, X-ray diffraction, etc. Mg is concentrated in this zone and serves as a useful chemical marker. However an outer matrix of mixed hydration products also develops, to which slag and cement both contribute. The additional contribution of Ca, Al and Si derived from slag to this matrix is considered to explain its gradual densification and resultant low permeability. INTRODUCTION Low permeability of cement matrices can improve their performance, for example by increasing their resistance to attack by diffusional processes. Well-cured slag cement pastes are known to have low permeability [1]. Their low permeability correlates with differences in pore size distribution, compared to Portland cement pastes. Various techniques, e.g. gas adsorption, mercury intrusion porosimetry [1-41, have shown that the pore size distribution of slag cement pastes is shifted to a smaller regime relative to Portland pastes, so fine pores constitute a higher proportion of the total porosity. Furthermore, as total porosity decreases the pores tend to become discontinuous, thus decreasing permeability. The size of pores in cement pastes ranges widely. Daimon et al. [5] classified the pores into different categories according to pore size, although those in the range 16 to 1000 A (1.6100 nm) probably constitute the bulk of the total intrinsic porosity. Electron microscopy becomes an important technique in observing these directly. It has been shown [6,71 that in the hydration of ordinary Portland cement (OPC) or of C 3S, two distinct morphologies of hydration products develop; these are referred to as inner and outer hydrates. Outer hydrate has a low density, and consists of a porous mixture of C-S-H gel with other crystalline phases such as Ca(OH) 2 , AFm etc., partly filling space originally occupied by water. Its C-S-H is often in the form of interlocking fibres, crumpled foils, etc. The inner hydrate, which tends to form radially around the hydrating silicate phases or their relicts, often appears to be relatively dense. This hydrate is composed of C-S-H with a composition not necessarily different from that of the C-S-H in the outer hydrates [7], but with a characteristic microstructure which, once established, persists. This paper describes the microstructural development of slag cement pastes, which is explored by electron microscopy and analytical electron microscopy (AEM). The res
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