Influence of Al/Fe ratio on the hydration of brownmillerite in the presence of calcium sulfate

The brownmillerite, as an indispensable mineral phase in Portland cement, is actually a series of solid solution of Ca2(AlyFe(2–y))O5 (0 ≥ y ≤ 1.33). The varying values of y would significantly influence the hydration characteristic of brownmillerite. The hydration performance of brownmillerite where y = 0.9, 1.1 and 1.3 in Ca2(AlyFe(2–y))O5 in the presence of calcium sulfate, is deeply researched in this work. The brownmillerite Ca2(Al1.1Fe0.9)O5 and Ca2(Al1.3Fe0.7)O5 show a doublet of heat flow curve corresponding to formation of different amounts of ettringite while a single exothermic peak appears in Ca2(Al0.9Fe1.1)O5 due to generation of a layer of gel-like phases. Combining with density functional theory (DFT) calculations, it is concluded that the hydration reactivity of brownmillerite increases with reducing Al/Fe ratio. Fe is inclined to occupy octahedral sites as augment of Fe content in brownmillerite. Due to highly hydraulic activity of these sites, the lowest Al/Fe ratio of brownmillerite corresponds to highest hydration rate in first few hours. However, more iron hydroxide radicals aggregate in the surface of unhydrated brownmillerite and form a layer of iron-rich hydration products which would dramatically slow down the following hydration progress. In addition, this postponement to hydration will gradually recede with reduction of Fe content in brownmillerite. This finding provides a novel perspective on the understanding of Al/Fe ratio to the hydration performance of brownmillerite.