TY - JOUR
T1 - A comprehensive analysis of aluminate phase equilibria in the process of delayed ettringite formation
AU - Danfour, Abubaker
AU - Perko, Janez
AU - Jacques, Diederik
AU - Govaerts, Joan
AU - Phung, Quoc Tri
AU - Cizer, Özlem
N1 - Score=10
Publisher Copyright:
© 2026 Elsevier Ltd
PY - 2026/6
Y1 - 2026/6
N2 - This study investigates the thermodynamic stability of primary and delayed ettringite across a wide compositional and thermal domain, with the aim of establishing compositional boundaries and reaction paths relevant to delayed ettringite formation (DEF) in cementitious systems. Thermodynamic modeling is used to examine the coexistence of ettringite with other aluminate phases in CaO–Al2O3–SiO2 systems at varying temperatures and fixed minor-oxide contents. The results reveal that ettringite is stable at 20 °C across most compositions, except in systems with a low sulfate-to-aluminate (S/Al) ratio. At higher temperatures, ettringite becomes unstable and is replaced by phases such as monosulfoaluminate (S-AFm), katoite (C3AH6), aluminum hydroxide, and CASH-type products. The temperature at which ettringite destabilizes ranges from 65 °C to 78 °C when SO3/Al2O3 is above 0.45, depending on the CaO/SiO2 ratio. In systems with low CaO and low SO3/Al2O3, ettringite can destabilize at even lower temperatures, around 55 °C. The study identifies two main pathways for DEF: (1) the reaction between S-AFm and dissolved sulfate in systems with high CaO and SiO2 (Zones 1–4, where CaO/SiO2 > 2.75 and SO3/Al2O3 > 0.45), and (2) the reaction of dissolved sulfate with calcium and aluminum sources in high-Al2O3 systems (SO3/Al2O3 < 0.45) with lower CaO/SiO2 ratios (Zone 4: CaO/SiO2 < 2.25). Additionally, no coexistence was found between ettringite (S-AFt) and katoite (C3AH6) at high temperatures, as they stabilize in different aluminum concentration ranges. This lack of coexistence may inhibit DEF when large amounts of C3AH6 form.
AB - This study investigates the thermodynamic stability of primary and delayed ettringite across a wide compositional and thermal domain, with the aim of establishing compositional boundaries and reaction paths relevant to delayed ettringite formation (DEF) in cementitious systems. Thermodynamic modeling is used to examine the coexistence of ettringite with other aluminate phases in CaO–Al2O3–SiO2 systems at varying temperatures and fixed minor-oxide contents. The results reveal that ettringite is stable at 20 °C across most compositions, except in systems with a low sulfate-to-aluminate (S/Al) ratio. At higher temperatures, ettringite becomes unstable and is replaced by phases such as monosulfoaluminate (S-AFm), katoite (C3AH6), aluminum hydroxide, and CASH-type products. The temperature at which ettringite destabilizes ranges from 65 °C to 78 °C when SO3/Al2O3 is above 0.45, depending on the CaO/SiO2 ratio. In systems with low CaO and low SO3/Al2O3, ettringite can destabilize at even lower temperatures, around 55 °C. The study identifies two main pathways for DEF: (1) the reaction between S-AFm and dissolved sulfate in systems with high CaO and SiO2 (Zones 1–4, where CaO/SiO2 > 2.75 and SO3/Al2O3 > 0.45), and (2) the reaction of dissolved sulfate with calcium and aluminum sources in high-Al2O3 systems (SO3/Al2O3 < 0.45) with lower CaO/SiO2 ratios (Zone 4: CaO/SiO2 < 2.25). Additionally, no coexistence was found between ettringite (S-AFt) and katoite (C3AH6) at high temperatures, as they stabilize in different aluminum concentration ranges. This lack of coexistence may inhibit DEF when large amounts of C3AH6 form.
KW - Absence of delayed ettringite
KW - Aluminate phase assemblage
KW - Chemical composition
KW - Delayed ettringite
KW - Primary ettringite
UR - https://www.scopus.com/pages/publications/105035121535
U2 - 10.1016/j.apgeochem.2026.106814
DO - 10.1016/j.apgeochem.2026.106814
M3 - Article
AN - SCOPUS:105035121535
SN - 0883-2927
VL - 203
JO - Applied Geochemistry
JF - Applied Geochemistry
M1 - 106814
ER -