The Impact of Slag Composition on the Microstructure of Composite Slag Cements Exposed to Sulfate Attack

The work presented here focuses on the impact of slag composition on the resistance of slag composite cements exposed to sulfates. Such composite systems can be an effective way to provide better sulfate resistance of the binder. However, the differing composition and hydration kinetics between slag and cement results in a change in the phases present and their quantities, the effect of which is often overlooked when assessing their sulfate resistance. Rather, the composition of the slag is typically considered.
Compared to cement, slags are richer in Si, Al and Mg, but poorer in Ca. Blended with cement, they hydrate much more slowly to produce comparable hydrates including a C-S-H phase deficient in calcium plus a hydrotalcite-like phase. Furthermore, aluminium from slag can promote the formation of other aluminate hydrates. The changes in microstructure and ultimately performance is then dependent on the composition of the slag.
The aim of the work was to determine how slag composition affected the performance of slag cements in sulfate rich environments by means of the developed microstructure. A first hydration study was carried out to follow how slag composition affected microstructural developments by comparing a neat (control) cement with several slag blends curing in idealised lab conditions. Subsequently, the same samples were exposed to a 3g.L-1 Na2SO4 solution. By comparing hydration in controlled conditions and in aggressive media in parallel, analysing only the degraded zone in the latter case, the role of slag composition on phase assemblage and composition and, ultimately, on sulfate resistance was highlighted
All the slag blended systems showed great resistance to sulfate attack during the testing period. This can be attributed to several changes in the microstructure in slag blended systems including (i) a lowering of the calcium content of the C-S-H phase which led to (ii) an increase in the amount of alumina bound to a possibly water rich silicate hydrate, (iii) a greater amount of a hydrotalcite phase varying in alumina content and (iv) a diluted calcium hydroxide content. Furthermore, monosulfate had formed in the slag blended systems during attack whereas ettringite had only formed in a neat system. This implies an availability of alumina from the slag keeping the $/Al of the pore solution low favouring monosulfate formation. Lastly, the role of calcium and ettringite stability was tentatively probed at.