Helium gas is used as a nanostructural probe to investigate the structural changes in C-S-H (I) due to the removal of interlayer water. Changes in the 002 basal spacing are correlated with helium inflow characteristics. Similarities to helium inflow experiments conducted on hydrated Portland cement and C3S pastes are discussed. Conclusions are drawn with respect to the viability of considering C-S-H (I) as a physical model for the drying of Portland cement and C3S pastes.