## 2 Materials and Methods  
### Binder Material Characteristics and Mix Proportioning  
In a first research stage, natural and accelerated carbonation experiments were performed on a potentially 'green', yet carbonation sensitive, HVFA mortar with a 50% replacement of traditional Portland cement CEM I 52.5 N from Holcim by pozzolanic class F fly ash originating from a coal fired power plant in Germany. The chemical and mineralogical composition and basic physical properties (Blaine fineness, 45 \(\upmu\)m fineness, density, 2-day and 28-day compressive strength requirements) of this cement are shown in Table 1. Same goes for the fly ash (except for the strength requirements). Key properties in relation to the CO\({}_{2}\) sequestration potential of the HVFA mortar made with this cement and fly ash are its CaO and CO\({}_{2}\) content as will be discussed further on in Sections 2.4 and 2.5. A batch of the studied HVFA mortar consisted of 1350 g of sand, 225 g of CEM I 52.5 N, 225 g of fly ash and 247.5 g of water. Sand and total binder content were in agreement with the specifications for a standard mortar as mentioned in NBN EN 196-1 [19]. The water content was higher than the normally proposed 225 g per batch in order to achieve a water-to-binder ratio of 0.55 which is common for exposure class XC3 (exposure to carbonation with moderate humidity, e.g., external environment, sheltered from rain) as defined in NBN EN 206 [20]. Note that the 50% fly ash content of the binder fraction implies a low carbonatable material content for the HVFA mortar and thus a low buffering capacity for penetrating CO\({}_{2}\) and a fast inward movement of the carbonation front cf. [21]. This means that the binder proportioning of this potentially 'green' HVFA mortar may be less suitable for steel reinforced applications and also less ideal for maximizing its CO\({}_{2}\) sequestration potential. Obviously, this issue of a mixture merely being 'green' from a production point of view without necessarily being it performance-wise will be the subject of further discussion in Section 3. Table 2 gives the mix design of this HVFA mortar per m\({}^{3}\) which is a more useful format for further CO\({}_{2}\) sequestration and production related CO\({}_{2}\) emission calculations. A typical air content of 2% was assumed for each mix design. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **CEM I 52.5 N** & **Fly Ash** & **CEM II/B-V 42.5 N** & **CEM I 42.5 N** \\ \hline CaO & 62.88 & 3.15 & 49.28 & 63.12 \\ SiO\({}_{2}\) & 18.24 & 56.05 & 28.26 & 20.32 \\ Al\({}_{2}\)O\({}_{3}\) & 5.31 & 22.62 & 8.95 & 4.60 \\ Fe\({}_{2}\)O\({}_{3}\) & 4.14 & 6.92 & 4.32 & 3.30 \\ MgO & 1.28 & 1.86 & 1.90 & 1.92 \\ K\({}_{2}\)O & 0.38 & 1.86 & 0.91 & 0.61 \\ Na\({}_{2}\)O & 0.44 & 0.71 & 0.37 & 0.26 \\ SO\({}_{3}\) & 3.15 & 0.32 & 2.64 & 3.20 \\ CO\({}_{2}\) & 2.15 & 0.90 & 1.20 & 0.98 \\ LOI & 2.61 & 2.33 & 1.86 & 1.69 \\ Blaine fineness & 408 & - & 413 & 264 \\
45 \(\upmu\)m fineness & - & 22.5 & - & - \\ Density & 3100 & 2200 & 2890 & 3160 \\
2-day strength & \(\geq\)20 & - & \(\geq\)10 & \(\geq\)10 \\
28-day strength & \(\geq\)52.5 & - & \(\geq\)42.5 & \(\geq\)42.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical and mineralogical composition (% m/m), loss on ignition (% m/m), Blaine fineness (m\({}^{2}\)/kg), 45 \(\upmu\)m fineness (% m/m) and density (kg/m\({}^{3}\)) of the binder materials used, and 2-day and 28-day compressive strength requirements (MPa) for the cements used. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **High-Volume Fly Ash (HVFA) Mix \({}^{*}\)** & **CEM II/B-V 42.5 N Mix** & **CEM I 42.5 N Mix** \\ \hline Sand & 1419.85 & 1449.64 & 1471.08 \\ Cement & 236.64 & 483.21 & 490.36 \\ Fly ash & 236.64 & - & - \\ Water & 260.31 & 265.77 & 269.70 \\ \hline \hline \end{tabular} \({}^{*}\)consisting of 50% fly ash and 50% DEM I 52.5 N by mass.  
\end{table} 
Table 2: Mix designs of the studied mortar mixes.