## 1 Introduction
### Interaction of CO\({}_{2}\) and Conventional Hydrated Cements
[20] showed, that conventional Portland cement does not fulfill the requirements for the well integrity. [PERSON] et al. [21] performed flow-through experiments which simulated the diffusion of brine (1.65 M) and scCO\({}_{2}\) from the interface between wellbore cement and caprock into a fracture-bearing Portland cement. The experiments were performed at 327 K with a pore pressure of 19.9 MPa and a confining pressure of 26.2 MPa for 174 h. They showed that the carbonation of the wellbore cement at first caused closing and blocking of pores and fractures but opened other fractures in the following process, which increased the permeability. The chemical interaction along the cement-sandstone interface with scCO\({}_{2}\) was investigated by [PERSON] et al. [22]. They prepared a well composite sample consisting of casing, Portland cement and sandstone. The experiments were performed under 10 MPa and 323 K with a 0.5 M NaCl-brine. Their results indicate that the formation of carbonation layers act as a barrier to further CO\({}_{2}\)-attack. [PERSON] et al. [23; 24] investigated the reaction of two different pozzolan-amended (Class F fly ash, with ~70% silica, alumina and Fe\({}_{2}\)O\({}_{3}\), ~5% sulfate, and less than 20% CaO) wellbore cements (API RP 10B; 35 vol % pozzolan and 65 vol % pozzolan) being exposed to CO\({}_{2}\) and H\({}_{2}\)S gas mixtures and brine (one percent NaCl) at 323 K and 15.1 MPa for 2.5, 9, 28 and 90 days. The samples of the pozzolan-amended wellbore cement with 35 vol % pozzolan showed higher resistance against H\({}_{2}\)S. [PERSON] et al. [25] and [PERSON] et al. [26] investigated the influence of CO\({}_{2}\)-induced reactions on the mechanical behavior of fractured wellbore cement (class-G Portland cement). They found that the reaction with CO\({}_{2}\) did not produce further geomechanical weakening. In fact, samples with six weeks reaction time recovered peak-strength (83%) and increased frictional strength (15-40%). [PERSON] et al. [27] investigated the reaction of hydrated well cement (class-H) with H\({}_{2}\)S-CO\({}_{2}\) (21 mol % H\({}_{2}\)S) acid-gas for 28 days at 323 K and 15 MPa. The H\({}_{2}\)S added to the experimental setup resulted in significant formation of ettringite (interior region of the cement) and pyrite (carbonated rim of the cement). Although secondarily formed ettringite is known to cause cracking and strength loss, a mechanical damage caused by formation of ettringite was not found. Even the long-termcement corrosion in chloride-rich solutions was investigated by [PERSON] et al. [27]. They performed leaching experiments and thermodynamic simulations on cement (OPC; CEM I 32,5 R; w/c ratio of 0.4) with brine (4.3 mol Mg\({}^{2+}\), 0.41 mol Na\({}^{+}\), 0.54 mol K\({}^{+}\), 8.9 mol Cl\({}^{-}\), and 0.23 mol SO\({}_{4}\)\({}^{2-}\)) for 200 days. Comparing the results of laboratory and full-scale experiments, they were able to establish a link between the results of short-term laboratory experiments to long-term full-scale experiments, using thermodynamic equilibrium calculations. The above research shows that carbonation can negatively impact the sealing qualities of OPC wellbore cements. Therefore, alternative materials are required to ensure wellbore integrity during the subsurface storage of CO\({}_{2}\). One such alternative is a cement based on wollastonite (CaSiO\({}_{3}\)), which will create a seal through a carbonation reaction. In this study, experimental work addressing wollastonite carbonation kinetics under in-situ conditions relevant for subsurface CO\({}_{2}\) storage in aquifers or depleted reservoirs is presented.