The neutralization of tricalcium aluminate hexahydrate and its spontaneous transformation into Friedel's salt, a layered double hydroxide

Kása, Eszter and Szabó, Yvette and Szabados, Márton and Kukovecz, Ákos and Kónya, Zoltán and Sipos, Pál Miklós and Kutus, Bence (2024) The neutralization of tricalcium aluminate hexahydrate and its spontaneous transformation into Friedel's salt, a layered double hydroxide. CEMENT AND CONCRETE RESEARCH, 177. No.-107414. ISSN 0008-8846

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Abstract

Tricalcium aluminate hexahydrate, C3AH6, forms during the hydration of the oxide form, C3A. Modelling the resulting phase composition over time has been the subject of intensive research; yet, understanding the effect of pH, a crucial parameter of cementitious mixtures, has remained elusive. To this end, we studied the stability of C3AH6 in a wide pH range via the addition of HCl. We compared the behavior of conventional ‘cement’ C3AH6 and ‘Bayer’ C3AH6, synthesized via alkaline digestion. Upon addition of HCl to mixtures in H2O or NaCl, both phases slowly transform in an equilibrium reaction into Friedel's salt, C4ACl2H10, a layered double hydroxide (LDH) with chloride ions being intercalated between the layers. Consequently, C3AH6, C4ACl2H10, and Al(OH)3 are likely to coexist in the pH range of 11–12. Furthermore, we find the reverse process, i.e. C4ACl2H10 → C3AH6 to be kinetically hindered, consistent with the high stability of C4ACl2H10 compared to hydroxide-intercalated LDHs.

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