Structural evolution of synthetic alkali-activated CaO-MgO-Na₂O-Al₂O₃-SiO₂ materials is influenced by Mg content

Stoichiometrically controlled alkali-activated materials within the system CaO-MgO-Na₂O-Al₂O₃-SiO₂ are produced by alkali-activation of high-purity synthetic powders chemically comparable to the glass in ground granulated blast furnace slag, but without additional minor constituents. Mg content controls the formation of hydrotalcite-group and AFm-type phases, which in turn strongly affects C-(N)-A-S-H gel chemistry and nanostructure. Bulk Mg content and the Mg/Al ratio of hydrotalcite-group phases are strongly correlated. With sufficient Ca, increased bulk Mg promotes formation of low-Al C-(A)-S-H and portlandite, due to formation of hydrotalcite-group phases and a reduction in available Al. Hydrotalcite-group phase formation is linked to increased C-(N)-A-S-H gel polymerisation, decreased gel Al uptake and increased formation of the ‘third aluminate hydrate’. These findings highlight the importance of considering available chemical constituents rather than simply bulk composition, so that the desired binder structure for a particular application can be achieved.