It is expected that a bridging Ca coordinates to one oxygen less than a pairing Ca. For the bridging site a Ca-O coordination number of 6 or higher was assumed. Simultaneously, the bridging Ca needs to be coordinated to two silicate oxygens. Our DFT database predicts for such a Ca site a Ca chemical shift of 46.84 ppm. For the pairing Ca the database was screened for pairing sites with a Ca-O coordination of 7 or higher from which at least 2 need to be to silicate oxygens. Such a pairing site is expected to have a Ca chemical shift of 27.50 ppm.