## 5 Conclusions  
Changes in smectite and mixed-layer illite/smectite minerals from clayey rocks used as cap rock were studied under geological CO\({}_{2}\) storage.  
The high CO\({}_{2}\) pressure used in geological storage induces irreversible changes in the structure of clay mineral particles in the clayey cap rock. Under carbonic acid conditions,promoted by the dissolution of CO\({}_{2}\) in water, a combined dissolution and illitization of smectite was confirmed by changes in exchangeable cations determined by ICP-EOS (from 21.3 to 11.1 mg/L for K, typical in illite). Additionally, a proportion of cations released in the dissolution/destruction of the involved mineral phases precipitate in the carbonate phases, calcite, dolomite and siderite, chemically fixing CO\({}_{2}\). Acidic solutions had a significant impact on both the dissolution of mixed-layer illite--smectite and carbonate minerals and the precipitation of secondary minerals.  
The multipoint BET technique was used to obtain geometrical properties of the pores, such as the specific surface area (from 36.1 to 38.1 m\({}^{2}\)/g by N\({}_{2}\) and from 14.5 to 15.4 m\({}^{2}\)/g by CO\({}_{2}\) adsorption test) and the pore volume (slightly decrease from 0.070 to 0.077 cm\({}^{3}\)/g), which were affected by mineral dissolution and precipitation. These changes were indicative that CO\({}_{2}\) interaction can affect rock reactivity over a long storage time.  
Slight porosity in changes (from 7.8 to 7.0 nm, pore size) are related to the alteration, dissolution, and secondary precipitation of the constituent minerals in the clay-sealing rock. Together with the possible heterogeneity of the mineral composition of the cap rock, the integrity of the cap rock is risked due to the massive dissolution of minerals, encouraging the interconnection of undesirable leakage pathways of the stored CO\({}_{2}\).  
Therefore, knowledge of the interaction between CO\({}_{2}\) and sealing rock and its implications for GCS requires extensive studies to ensure long-term safety.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122211477/s1](https://www.mdpi.com/article/10.3390/app122211477/s1)]([https://www.mdpi.com/article/10.3390/app122211477/s1](https://www.mdpi.com/article/10.3390/app122211477/s1)), Figure S1: Rietveld analysis pattern of the powder diffraction data of Original Marl; Figure S2: Rietveld analysis pattern of the powder diffraction data of the marl tested during 120 h; Figure S3: Rietveld analysis pattern of marl powder diffraction data tested during 240 h; Figure S4: Surface area, pore volume, and pore size by N\({}_{2}\)-BET; Figure S5: N\({}_{2}\) adsorption-desorption isotherms of the pre- and post-test samples; Figure S6: BET-surface area, total volume in pores, area, and total in pores by CO\({}_{2}\) adsorption tests; Figure S7: CO\({}_{2}\) adsorption isotherms of the pre- and post-test samples.  
Conceptualization, D.M. and P.A.; methodology, D.M.; formal analysis, D.M., P.A., S.G. and M.M.M.-V.; writing--original draft preparation, D.M. and P.A.; writing--review and editing, D.M., P.A., S.G. and M.M.M.-V.; supervision, S.G. and M.M.M.-V. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Junta de Andalucia (Conserjeria de Economia y Conocimiento) grand number [P12-RNM-568 MO project] and the contract of [PERSON] granted by the V Plan Propio de Investigacion de la Universidad de Sevilla.  
Not applicable.  
I XRD: BET, and ICP-EOS analyses were performed using the facilities of the General Research Center of the University of Seville (CITIUS). [PERSON] acknowledges [PERSON], [PERSON] and the Research Centre for Carbon Solutions (RCCS) group for facilitating my short research stay with them.  
The authors declare no conflict of interest.