Indeed from the DFT relaxed C-S-H bulk structures in this study, the mean Ca-O coordination number of bridging Ca equals 6.04 and 6.42 for the pairing Ca. This finding aligns well with experimentally determined Ca-O coordination numbers of C-S-H. Typical reported values for Ca-O coordination number in C-S-H are between 5.9 and 6.6 . Assuming 7-fold mainlayer Ca, the decrease in the overall Ca-O coordination number can then only be due to lower coordinated Ca in the interlayer.