Since the mainlayer Ca in tobermorite 14 Å is 7-fold coordinated, a highly similar connectivity is expected in the C-S-H structure. In tobermorite 14 Å six Ca coordinating oxygens are from the in-chain Q 2p silicates and the seventh is from the bridging Q 2b silicate or a water molecule. Along the b-axis direction in tobermorite 14 Å the water and Q 2b oxygen interchange . The 7-fold coordination of mainlayer Ca is shown in Figure . Molecular dynamics studies showed that a 7-fold Ca coordination number in C-S-H is achieved through an oxygen of the water molecule or a hydroxide ion in the interlayer space . Since the mainlayer Ca site is located in the well-structured Ca-Si mainlayer, a higher symmetry in comparison to other sites is expected (Figure ). Previous studies on synthetic C-S-H suggest that an increase in the Ca/Si ratio promotes structural ordering , which could result in even higher local symmetry.