Belite hydration at high temperature and pressure by in situ synchrotron powder diffraction

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.This work has been supported by BIA2017-82391-R research grant, which is co-funded by FEDER. We thank ALBA synchrotron for providing beam time at NCD-Sweet beamline and Marc Malfois for his assistance during the experiment. We also thank Buzzi Unicem SpA for providing us with more than 30 kg of a large-scale production belite cement