The second possible Ca site is the pairing Ca, which is positioned on top of the silicate dimer (Q . While there is yet no experimental evidence for this Ca site, it is a reasonable assumption since: 1) the typically reported C-S-H interlayer distance (below 1.3 nm) is too small for a Ca being solely fully hydrated by water and hydroxide ions, and 2) Ca adsorbs, in contrast to the solid-liquid surface, strongly in confined spaces . The main difference between the bridging and pairing Ca is in the coordination, which is known to influence the measured chemical shift . As the Ca-O coordination number increases the Ca chemical shift decreases . Since, in this case, both sites are exposed to an identical interlayer environment, a similar coordination to water and hydroxide ion oxygens can be expected. Therefore the difference arises from the silicate oxygens. Since the bridging Ca connects two Q 1 silicates it can only coordinate to two silicate oxygens, while the pairing Ca sitting on top of the dimer therefore can coordinate to 3 silicate oxygens. Depending on the interlayer distance, the pairing site can potentially coordinate to a fourth oxygen of the Q 2b silicate of across the interlayer silicate chain. This across interlayer connectivity of the Ca pairing site is only possible due to the silicate chains of the tobermoritelike mainlayer being shifted by b/2 .