The impact of Fe dosage on the ettringite formation during high ferrite cement hydration

Abstract

Illustrating the influence of Fe dosage and temperature on the formation of sulfoaluminate hydrates is of great significance to understand the early hydration process of high ferrite cement under steam curing. Herein, the impact of Fe dosage on the ettringite formation at different temperatures was revealed through the combination of experiments and computational simulations. A Fe dosage no more than 20% conspicuously accelerated the crystal growth of ettringite by increasing the surface energy in the (0 0 1) direction, whereas a higher dosage suppressed the formation of ettringite as the high incorporation of Fe ions into the ettringite crystal was energetically unstable. The chemical environment analysis of Fe in products shows that Fe(OH) 6 3− , compared with Al(OH) 6 3− , prefers to participate in the formation of Al 2 O 3 -Fe 2 O 3 -mono than ettringite, which is also confirmed by the calculated thermodynamic properties' results. The understanding of the impact and mechanism of Fe on the formation of ettringite under steam curing conditions plays a pivotal role in the utilization of high ferrite cement. K E Y W O R D S crystal stability, ettringite, high ferrite cement, molecular dynamics, monosulfate| 3653 WAN et Al.stable phase, AFm, which gradually retransforms into AFt (delayed ettringite) with the internal temperature of concrete decreasing after steam curing. 9-12 Hydrates of C 4 AF possess superior sulfate resistance ability than that of C 3 A owing to the different characteristics between their hydrates. 4 During the C 4 AF hydration, the Fe-bearing AFt and amorphous Fe(OH) 3 formed at the beginning when the solution was rich in gypsum and the Fe-bearing AFm precipitated as the gypsum was consumed. 13 Comparing the reaction of Al-AFm (traditional AFm) with Al/Fe-AFm (Al 3+ is partially replaced by Fe 3+ ) in the presence of gypsum, respectively, Huang et al. 4 found that Al/Fe-AFm rapidly transformed into Al/Fe-AFt after 168 h, while Al-AFm remained, which implies that there was a lower risk of delayed ettringite formation in HFC than OPC for the reason that the proportion of C 4 AF is much higher. When the cement is enriched in the ferrite phase, the hydration product AFt has not only higher but also varied Fe content, 4,6,14,15 making it even more difficult to associate the chemical composition of AFt with its physical properties in the cement system. Therefore, Möschner et al. 16,17 synthesized Fe-ettringite (Ca 6 [Fe(OH) 6 ] 2 (SO 4 ) 3 •26H 2 O) and Fe-bearing ettringite (Ca 6 [Al x Fe 1-x (OH) 6 ] 2 (SO 4 ) 3 •26H 2 O)with different iron-to-aluminum ratios at room temperature, the solubility of Fe-AFt was obtained, and a continuous solid solution of Al/Fe-AFt was observed when the ratio of Al content is between 0.3 and 0.6. To date, the effect of Fe on the formation and conversion of AFt and AFm during steam curing largely remains behind a blindfold, although works have been published on the crystal structure and physical properties of Fe-bearing Aft. 16,17 In this work, we synthesized AFt with...

Microscopic effect of iron dosage on the stability of Fe‐doped ettringite

“…51 Thus, Al and Fe ions tend to occupy individually the whole layer rather than a mix with each other in the same layer. 21,51 Using the most stable Fe-AFt structures, the defect formation energy (ΔE def ) was calculated to evaluate the stability of Fe incorporation. The defect formation energy was calculated as follows:…”

“…This result aligns well with SEM observations, indicating that the stability of the AFt structure decreases with an increase in Fe dosage. 21 This phenomenon results in the formation of new phases, including AFm and other ferrite-containing phases, particularly at higher Fe dosages. 21 One possibility to give rise to such defect formation energy is by the temperature.…”

“…21 This phenomenon results in the formation of new phases, including AFm and other ferrite-containing phases, particularly at higher Fe dosages. 21 One possibility to give rise to such defect formation energy is by the temperature. Accordingly, the required temperature was calculated by the given expression:…”

“…Wan et al explored the impact of Fe content on AFt formation at various temperatures using chemical synthesis. 21 Adjusting Fe content above 50% (F/A + F = 0.5) in the solution inhibits AFt but promotes AFm formation, even at low temperature (25 • C). Conversely, low Fe dosage enhances AFt crystallization, and elevated temperatures accelerate AFt to AFm conversion.…”

From selective dissolution to crystal chemistry of brownmillerite in sulfate resisting cement

“…Most of the key studies on reactivity of brownmillerite were performed on synthetic ferrites, [12][13][14][15][16] which are not fully representative of the industrial phase. Indeed, industrial ferrites are different from synthetic ones, in particular by the impurities (particularly Mg and Si) incorporated from raw materials and fuels (Taylor 1997), the burning conditions (temperature and kiln atmosphere) and liquid quenching.…”