In contrast to the confined interlayer surfaces the Ca ions at the external surface have more space and are expected to fully fill their first hydration shell. A full hydration shell results in a bigger polyhedra volume, more oxygens shielding the Ca nuclei, and therefore lower chemical shifts . However, similar to the interlayer Ca position a loss in symmetry is expected, and therefore broader NMR peaks. We define a surface site as an inner-sphere adsorbed Ca which can coordinate to one or two silicate oxygens (Figure ) . As expected, the DFT calculated surface Ca sites show the lowest mean δ DFT (Table ). This can be attributed to a higher mean Ca-O coordination number (7.14) and therefore a larger polyhedra volume, and consequently a larger mean Ca-O distance . The expected Ca chemical shift for Ca ions at the interface (innersphere adsorbed) is 19.72 ppm. It is observed that with an increased coordination to silicate oxygens O(Si) the mean δ DFT shifts to more positive values (16.32 ppm when coordinated to one silicate oxygens and 21.08 ppm when coordinated to two), showing the effect of silicate oxygen shielding onto the Ca nuclei.