Enhanced Sulfate Resistance: The Importance of Iron in Aluminate Hydrates

Abstract

Sulfate erosion is one of the main durability issue of ordinary Portland cement (OPC) while used in a sulfate-rich environment. Two aluminate phases in OPC, tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF), are primarily responsible for sulfate attack but with different sulfate resistant performance. This paper therefore focuses on the internal sulfate invasion to hydration products of these two aluminate minerals and attempts to explain the role of irons in hydrates. Chemical shrinkage coupled with X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectra (EDS) are employed to determine the overall development of internal sulfate erosion of these hydrates. The results suggested that the doped iron in hydrogarnet (C3(A,F)­H6) dramatically changes crystal growth and leads to large size of polyhedral particle to tiny cubic grain of C3AH6, which contributes superior performance to sulfate erosion. As to another hydrate monosulfate, the introduction of Fe in C4(A,F)­S̅H12 causes a slightly worse sulfate resistance to C4AS̅H12. The substitution of iron to aluminum in hydrogarnet is the radical reason for better sulfate resistance of C4AF to C3A. This study greatly enhances the understanding of sulfate resistance to OPC.

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

“…As the reaction time prolongs, the relative intensity of AFm peaks declined while the peaks of AFt intensified, indicating that AFm, the high-temperature stable phase, slowly transformed into AFt at 90°C. Huang et al 4 also found that It is worth noting that the usual formation rates of Al/ Fe-AFt [2] and Fe-AFt are much slower than that of Al-AFt at room temperature (equilibrium is several days for Al-AFt, while 6 months for Al/Fe-AFt and Fe-AFt). 16,17 Since specimens reacted for no more than 48 h at different temperatures in this work, it is unclear whether all Fe ions are incorporated into the products in the form of Al/Fe-AFt or Fe-AFt.…”

“…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.…”

“…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.…”

“…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.…”

Ionic conductive and photocatalytic properties of cementitious materials: calcium silicate hydrate and calcium aluminoferrite

“…14 C 4 AF undergoes hydration to form ettringite (katoite) in the presence (absence) of gypsum (CaSO 4 ). 15,16 It is also reported that C 4 AF exists as Fe-siliceous hydrogarnet during hydration in the coexistence of silicate. 17,18 It is meaningful to investigate the photochemical properties of these products; however, we consider it as future work, because the crystalline structure and solid solution state of the hydration products cannot be precisely controlled in the synthesis, thus making it difficult to obtain pure and reproducible material at this moment.…”