Portland cements are currently used as Oil‐Well‐Cements for the sheath between the metal casing
and the borehole. At high pressures and temperatures, the resulting C-S-H gel(s) is partly unstable
and it can react to give different crystalline phases and changing microstructure which increases
porosity. It is known that the addition of silica-rich materials to Portland cements, used to decrease
the overall Ca/Si molar ratio, avoids/minimises the early strength retrogression experienced by neat
cement slurries. Alternatively, belite-rich cement could be used but belite reactivity in these
conditions is largely unexplored. Here, belite-rich mixtures have been quantitatively studied at 180
bars and of 160 ºC by in situ synchrotron powder diffraction using a tailored-designed spinning
capillary cell. For model alite/belite mixtures, alite strongly accelerates belite hydration in these
conditions. At 10 h of hydration, a-C2S shows a reaction degree of 30 and 80% for 20wt%C3S-
80wt%C2S and 50wt%C3S-50wt%C2S mixtures, respectively. Furthermore, the 50wt%C3S mixture
hydrates yielding significant amounts of Jaffeite and a-C2SH and the 20wt%C3S mixture gave no
Jaffeite and much lower amount of a-C2SH. Finally, a belite cement has also been hydrated in these
conditions and the reaction degree of belite was 50% at 10 h. The hydration of this cement did not
yield Jaffeite neither a-C2SH in the studied experimental conditions.
