## 4 Conclusions  
The experimental investigation focused on assessing the impact of cyclic hydrogen exposure on the mechanical and microstructural properties of different formulations of cement samples under simulated reservoir conditions. Significant insights were gained through a comprehensive analysis encompassing computed tomography, scanning electron microscopy coupled with energy dispersive spectroscopy, porosity and permeability evaluations, and measurement of ultrasonic velocities.  
The results indicate that the cement samples exhibited minimal cracking or fracture generation after exposure to hydrogen gas, as confirmed by CT scan imaging. The SEM-EDS analysis revealed changes in elemental composition, indicating hydrogen-induced reactions leading to the dissolution of cement compounds. Notably, a reduction in at% of Oxygen and Calcium alongside an increase in Carbon content suggests a complex geochemical interaction involving hydrogen. Porosity measurements demonstrated increased sample porosity after cyclic hydrogen exposure, with significant increments observed particularly after the first exposure cycle. This suggests ongoing geochemical reactions within the cement matrix, albeit approaching equilibrium after subsequent cycles. Permeability results showed a notable increase in post-exposure cycles, indicating alterations in the pore network due to dissolution phenomena. Additionally, the dynamic Young's modulus decreased after exposure cycles, making the cement less brittle and susceptible to cracking under cyclic loading conditions.  
It can be concluded that cement with different additives performed differently when exposed to hydrogen. This study can be improved by including kinetics for the dissolution and precipitation of minerals and microbial kinetic equations. Furthermore, using reactive transport simulations, the diffusive movement of hydrogen inside the cement might be considered. Further investigations are warranted to assess the long-term effects of hydrogen exposure on cement properties. Extended exposure durations beyond the scope of this study would provide valuable insights into the sustainability and durability of cement structures under continuous hydrogen exposure. As discussed in [43,44] in the context of CCS, the occurrence of a self-sealing mechanism that can fill fractures and open spaces in the cement through the growth of secondary minerals can be relevant in UHS, and future studies can be conducted to investigate its effects. Advanced techniques such as micro-CT scanning can enhance understanding of microstructural changes induced by hydrogen exposure. This would facilitate a more detailed examination of crack initiation, propagation, and their implications on cement integrity.  
Conceptualization, A.H.; methodology, A.H. and S.R.T.; software, D.M.F.; validation, A.H., S.R.T. and H.E.; formal analysis, A.H.; investigation, A.H. and S.R.T.; resources, I.I., H.E. and M.W.; data curation, A.H.; writing--original draft preparation, A.H.; writing--review and editing, A.H., S.R.T., D.M.F. and H.E.; visualization, A.H., S.R.T. and D.M.F.; supervision, H.E. and I.I.; project administration, M.W.; funding acquisition, H.E. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author/s.  
The authors acknowledge [PERSON] and [PERSON] for their support throughout the research.