The brownmillerite, as an indispensable mineral phase in Portland cement, is actually a series of solid solution of Ca(AlFe)O (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 Ca(AlFe)O in the presence of calcium sulfate, is deeply researched in this work. The brownmillerite Ca(AlFe)O and Ca(AlFe)O show a doublet of heat flow curve corresponding to formation of different amounts of ettringite while a single exothermic peak appears in Ca(AlFe)O 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.

中文翻译：

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硫酸钙存在下Al/Fe比对褐针石水化的影响

褐针石作为硅酸盐水泥中不可缺少的矿物相，实际上是Ca(AlFe)O(0≥y≤1.33)的一系列固溶体。 y 的变化会显着影响褐针石的水化特性。本工作深入研究了 y = 0.9、1.1 和 1.3 的褐针石在硫酸钙存在下在 Ca(AlFe)O 中的水化性能。褐针石 Ca(AlFe)O 和 Ca(AlFe)O 显示出对应于不同量钙矾石形成的双峰热流曲线，而 Ca(AlFe)O 中由于生成一层凝胶状物质而出现单个放热峰阶段。结合密度泛函理论（DFT）计算，得出结论：随着Al/Fe比的降低，褐针石的水化反应活性增加。随着褐针石中 Fe 含量的增加，Fe 倾向于占据八面体位点。由于这些位点的高水合活性，褐针矿的最低铝/铁比对应于最初几个小时的最高水合速率。然而，更多的氢氧化铁自由基聚集在未水化的褐针石表面，并形成一层富含​​铁的水化产物，这将大大减慢随后的水化进程。此外，随着褐针石中铁含量的减少，这种水合作用的延迟将逐渐消退。这一发现为理解铝/铁比对褐针石水化性能的影响提供了新的视角。