## 1 Introduction  
The Intergovernmental Panel on Climate Change (IPCC) climate report, 2018 [1] concluded that reduction of CO\({}_{2}\) emissions was necessary, because the current CO\({}_{2}\) concentrations are still considerably rising and currently amount to 408 ppm [2]. Therefore, it is a mandatory challenge to reduce CO\({}_{2}\) emissions. One possible option being currently investigated and discussed [3] is carbon capture and storage (CCS). Using CCS, CO\({}_{2}\) emissions from different sources (energy production, steel, and cement industries) could be reduced by finding and exploring suitable deep geologic formations (e.g., sandstones) for CO\({}_{2}\) storage. The formations considered for injection could be on- or offshore which could be in part already exploited former gas fields [3]. During and after injection, the casing and sealing of boreholes at storage sites must feature high reliability, because CO\({}_{2}\) has an impact on the stability of conventional hydrated cements [4, 5], i.e., it leads to degradation due to carbonation. Therefore, the right choice of suitable sealing systems is challenging. The material has to resist the harsh conditions at the point of injection considering pressure, temperature, acidic environment, and the composition of aggressive fluids (chloride-rich brines, CO\({}_{2}\) accompanied by NO\({}_{2}\) and SO\({}_{2}\)). The high amount of CO\({}_{2}\) and fluids will impose massive impact on cement properties. The commonly used ordinary Portland cements (OPC) are often limited due to their instability under these conditions for sealing wellbores. The current study is based on the investigation of carbonation as a hardening method to obtain carbonates as reaction products by using wollastonite. Although this reaction has been investigated [6; 7; 8], its use for CCS is still not widely evaluated. Wollastonite has a Ca-Si ratio of 1:1 and, therefore, during industrial production, a better CO\({}_{2}\) balance than conventional cements with Ca-Si ratio 2:1 and 3:1 [6; 9]. However, resources are limited. To overcome this bottleneck of wollastonite supply, it can be produced by adjusting a conventional cement rotary kiln production process. Additionally, CO\({}_{2}\) will be redeemed during this process and improve the CO\({}_{2}\) balance. The present study considers the back reaction by carbonation of wollastonite forming CaCO\({}_{3}\) and SiO\({}_{2}\) as a base [10] within the joint BMWi (German Federal Ministry for Economic Affairs and Energy) research ([6; 7; 8; 11; 12] CLUSTER). The conditions, especially non-ambient ones, for the reaction of CaSiO\({}_{3}\) (CS) with CO\({}_{2}\) () were investigated. The carbonation reaction of wollastonite is described as extremely slow in the absence of water at low temperatures [13]. However, at 333 K and ambient pressure, the \(\overline{\mathrm{C}}\)\(CS\) (CO\({}_{2}\) + CaSiO\({}_{3}\)) reaction is accelerated (several hours up to one day) in the presence of H\({}_{2}\)O [6; 7; 13]. Microstructure development of the carbonation was investigated by [PERSON] et al. [14] and [PERSON] and [PERSON] [15]. Both studies showed a carbonation starting from the surface to the center of the wollastonite grains, analogous to the model of the shrinking core [16]. In the presence of CO\({}_{2}\) in aqueous conditions, CaSiO\({}_{3}\) is consumed and calcium carbonate is formed. Adding an internal standard of known weight to the mixture, it is possible to determine the amorphous silicon oxide formation through X-ray diffraction [6]:  
\[\mathrm{CaSiO_{3}+\ CO_{2}\xrightarrow{H_{2}O}\ CaCO_{3}+\ SiO_{2}( amorphous)} \tag{1}\]  
The chosen temperature (333 K) and brine composition (230 g/L NaCl, 15 g/L CaCl\({}_{2}\), 5 g/L MgCl\({}_{2}\)) defined within this joint project [11] simulated conditions in deep geological formations envisaged for CCS. According to [PERSON] and [PERSON] [13] and [PERSON] et al. [17], the presence of water is crucial for the carbonation reaction.