## 1 Introduction  
To reduce the amount of CO2 in the atmosphere (currently 403 ppm [1]) several possibilities need to be considered. One possible technology being investigated and discussed is carbon capture and storage (CCS). The goal of CCS is reducing CO2 emissions from burning of fossil fuel by storing considerable amounts in suitable deep geologic reservoirs. The reservoirs considered for injection could be on- or offshore formations which could be in part already former exploited gas fields [2]. After injection, the sealing of wells in deep geologic formations being considered for CCS must be sustainable. Therefore, the choice of a tight and durable cement, used for sealing, is critical. The material has to resist the extreme conditions at the point of injection considering pressure, temperature, pH, and the composition of aggressive fluids (chloride rich brines, supercritical (sc) CO2 accompanied with NO2 and SO2). The high amount of CO2 and the fluids will have a considerable impact on cement properties. Hence, the term carbonation is occupied in the literature as a post hydration reaction of cementitious materials [3; 4]. Ordinary Portland cements (OPC) cannot be used under these conditions of well boring due to their instability. The current study describes carbonation as a curing method to obtain carbonates as reaction products. In the present study, a literature review describing the influence of CO2 on conventional hydrated cements as an alteration reaction is presented. Studies on the reaction of natural wollastonite with CO2 as curing method under different pressures and temperatures were investigated.  
Although natural deposits are limited, wollastonite can be fabricated using a conventional cement rotary kiln production process. Wollastonite has a Ca:Si ratio of 1:1 and therefore during production a better CO\({}_{2}\) balance than conventional cements with Ca:Si ratio 2:1 and 3:1 (Table 1, [5]).  
Additionally, the CO\({}_{2}\) will be consumed during the carbonation reaction thereby improving the CO\({}_{2}\) balance by forming carbonates.  
The interaction of wollastonite and CO\({}_{2}\)[6] was investigated within the joint BMWi (German Federal Ministry for Economic Affairs and Energy) research project CLUSTER [7]. The project CLUSTER focusses on two main topics:  
1. Potential impacts of temporal variations in CO\({}_{2}\) stream composition and mass flow that occur in larger pipeline networks on transport, injection, and geological storage of CO\({}_{2}\).
2. Flexibility of individual steps and the whole CCS process chain in response to this temporal variability [8].  
In nature, wollastonite is formed during metamorphism. The present study considers the back reaction by carbonation of wollastonite forming CaCO\({}_{3}\) and SiO\({}_{2}\) as a base [6]. The carbonation reaction of wollastonite is described as extremely slow in absence of water at low temperatures [9]. Yet at 333 K and ambient pressure the \(\overline{C}CS\) reaction is fast (several hours up to one day) in presence of water [9]. Microstructure aspects of the carbonation were investigated by [PERSON] et al. [10] and [PERSON] and [PERSON] [11]. Both studies showed a carbonation from the surface to the center of the wollastonite grains, similar to the shrinking core model [12]. Wollastonite reacts in the presence of CO\({}_{2}\) in aqueous conditions to calcite and amorphous silicon oxide:  
\[\mathrm{CaSiO_{3}+CO_{2}\xrightarrow{H_{2}O}CaCO_{3}+SiO_{2}(amorphous)} \tag{1}\]  
The chosen temperature (333 K) defined by the project CLUSTER [8] equates to the conditions in deep geological formations envisaged for CCS. [PERSON] and [PERSON] [9] and [PERSON] et al. [13] found that the carbonation reaction strongly depends on the presence of water. Our experiments were performed under aqueous conditions, because aqueous solutions are expected to be present at the point of injection in wellbores.