Models of cement have been formulated to further understanding. The dominant being Jenning’s layered colloid model. Therein, idealised silicate layers are interspersed with Ca-ions and H2O to form Ca-Si-Hydrate ‘gel’ (C-S-H); comprising ~70% of the matrix and responsible for cohesion of the components and bulk mechanical properties. Jennings models have never been tested nor questioned. NMR work evidences a C-S-H distribution of Q0:Q1:Q2 as ~10:67:23. Hence, the majority of Si-atoms are terminal and bridging O-atoms in the minority, making layered and extended-silicate structures improbable. Entropy would agree.Our work on C-S-H gel showed layered structures to be the least stable in free-energy(+2.27eV). The presence/absence of layered type structures will be signaled by periodic patterns in atomic pair distribution functions. We propose measurements on NIMROD to help resolve this