## 2 Materials and Material Production
### Reactive Coating  
Following the reaction shown in Equation (1), calcium carbonate forms the sacrificial reactive component. To support sustainability, rather than using synthetic precipitated calcium carbonate, which relies on the production of burnt lime (a major emitter of CO2) and, thus, contributes to the greenhouse gas load, ground calcium carbonate (GCC) was chosen as the preferred resource. Additionally, GCC can be produced from waste limestone and marble arising from architectural cladding production as a result of sawing and blasting. Another environmentally-friendly source of marble can be found in underground deposits by avoiding open-pit scarring of the landscape, and by being ground to the required fine particle size using carbon-free hydroelectric power. This latter route is exemplified by production on the west coast of Norway, which has excellent shipping access for effective long-range transport. A further alternative is using calcium carbonate or dolomite sources located near their end use, and dry milling plants exist globally for the sustainable production of low-cost GCC. In this study, GCC was made from Norwegian marble, which was wet ground and chemical-free (Omya Hustadmarmor AS, Molde, Norway). Field emission scanning electron microscope (FESEM) images (JEM-2200 FS; JEOL Ltd., Akashima, Tokyo, Japan) of the fine ground material are shown in Figure 6. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Area, \(A\)** & **Mass, \(m\)** & **Basis Weight** & **Thickness, \(t\)** & **Volume, \(V\)** & **Density, \(D\)** & **Porosity** \\  & **(m\({}^{2}\))** & **(g)** & **(g\(\cdot\)m\({}^{-2}\))** & **(\(\mu\)m)** & **(cm\({}^{3}\))** & **(g\(\cdot\)cm\({}^{-3}\))** & **(\%)** \\ \hline \hline \multicolumn{1}{l}{* UK} & 0.027 & 2.18 & 80 & 218 & 5.94 & 0.37 & 75.6 \\ \hline \multicolumn{1}{l}{** NP} & 0.027 & 2.18 & 80 & 183 & 4.98 & 0.44 & 70.9 \\ \hline \hline \end{tabular}  
* UK = unbleached Kraft sheet. ** NP = recycled newsprint sheet.  
\end{table} 
Table 3: Hand sheet properties. Figure 6: The field emission scanning electron microscope (FESEM) images of ground calcium carbonate (GCC) particles displayed at increasing magnification (see scale bars): (**a**) through (**d**). Marble and limestone exhibit the calcitic crystal form, having a rhombohedral micro habit. The product, Covercarb 60 ME (nominally individual dispersed particle size distribution being 60 w/w% \(<\) 1 \(\upmu\)m and 95 w/w% \(<\) 2 \(\upmu\)m with a Zeta potential of \(\zeta\) = -27.13 mV, at pH 9.2) was supplied by Omya International AG (Oftringen, Switzerland) in a dispersant-free crumble-like form. The material was produced by wet grinding and dewatered by filtration (Omya Hustadmarmor AS, Molde, Norway). Though less important in this context, the product was extremely pure and had high brightness, as it had an ISO brightness \(\geq\) 96%. The particle size in the undispersed (as delivered) state was measured using static laser light diffraction as a function of the scattering angle. The GCC particle size values were taken as the equivalent light scattering diameter, \(d\), cumulatively distributed over volume% (v/v%) of the particles smaller than or equal to \(d\) and are shown in Table 4.  
The GCC was dispersant-free to maintain a chemical-free surface on the particles. This means that the particle size values in Table 4 were not expected to match the distribution of single particles in aqueous suspension, as the particles were likely flocculated due to the missing stabiliser. Evidence for this is seen by the floc size reduction after ultrasonication in the aqueous suspension. This was an advantage in the coating due to enhanced gas permeation and associated gas sorption. The surface characteristics of the GCC particles are summarised in Table 5. A specific surface area (SSA) was calculated from the linear part of the BET nitrogen adsorption isotherms [12] with total pore volume (\(V_{\text{tot}}\)) given at relative pressure \(p/p_{0}\) = 0.998.