Microstructure and Microchemistry of Slag Cement Pastes
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MICROSTRUCTURE AND MICROCHEMISTRY OF SLAG CEMENT PASTES A.M. HARRISSON*, N.B. WINTER* and H.F.W. TAYLOR** *Blue Circle Industries PLC, Research Division, London Road, Greenhithe, Kent, DA9 9JQ, England **Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB9 2UE, Scotland Received 24 October,
1986; refereed
ABSTRACT Pastes of a portland cement (60%) blended with a granulated blastfurnace slag (40%) were examined, principally by SEM with EDX analysis. Reaction rims around slag particles and relicts of fully reacted slag particles had compositions compatible with mixtures, in varying proportions, of C-S-H having Si/Ca -0.62 and a phase of the hydrotalcite family having Al/Mg -0.38. Calculations taking into account relevant densities and water contents. indicated that replacement of the slag by its in situ hydration products entails little or no change in the numbers of Mg and 0 atoms per unit volume, but that substantial proportions of the Ca, Si and Al are released and an equivalent amount of H gained. In other respects, the microstructures qualitatively resembled those of pure portland cement pastes of similar ages, but less CH was formed and the C-S-H not formed in situ from the slag had a Si/Ca ratio of 0.56, higher than that of 0.50 to 0.53 found in the absence of slag. None of the individual phases in the slag cement pastes showed significant compositional variation with time in the 28 day to 14 month period studied. The relative amounts of Ca, Si and Al expelled from the slag are such that, in order to form C-S-H and AFm phase, more Ca is required. It is obtained partly at the expense of CH formation, and partly through increase in the Si/Ca ratio of the C-S-H formed from the clinker phases. Mass balance, volume composition and bound water content were calculated for the 14 month old paste and compared with corresponding results for the pure portland cement.
INTRODUCTION Previous studies have shown that the principal hydration products of slag cements are essentially the same as those of portland cements, though less calcium hydroxide is formed even after allowing for the dilution of the cement clinker by the slag [1-6]. The C-S-H surrounding the clinker grains has a lower Ca/Si ratio than that formed in portland cement pastes [6-8]. Reaction rims around the slag grains have been observed; they broadly resemble C-S-H in
composition, but are richer in A12 03 and MgO [5,6,9]. Hydrotalcite (Mg6 Al 2 (OH) 1 6 (CO3 )-4H 2 0), or possibly a carbonate-free phase of closely similar structure, has sometimes been reported [10,11]. An attempt was made to estimate the amounts and compositions of all the phases in some cement pastes, and to test the results by making mass balance calculations for each element and by calculating some derived quantities, e.g. bound water, that could be compared with experimental data [8,12]. With slag cements, poor agreements were reported for Mg balance and between calculated and observed
contents of non-evaporable water. We have recently reported preliminary r
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