Detection of nanostructural anomalies in hydrated cement systems

Monitoring the flow of helium gas into the structure of hydrated cement systems has proven to be a useful method for following nanostructural changes in the C-S-H phase of hydrated cement systems. The method is sensitive to changes that occur on removal of structural water from the layered silicates. The helium-inflow method was applied, in this study, to normally hydrated low-water-cement ratio (w/c) Portland cement pastes (w/c < 0.38) and to low w/c autoclaved cement systems containing fly ash and elemental sulfur. Unusually, high amounts and rates of inflow were observed for these pastes. It was postulated that inflow occurred into both interlayer and other spaces in the latter. The inflow into the other or 'trapped' space was unexpected and considered anomalous in absence of a widely accepted explanation. The structural differences which were observed at the nanoscale for the low w/c preparations were consistent with behavioral aspects for published structural models of layered C-S-H. These include the models of Richardson and Jennings and concepts involving the existence of two types of C-S-H. Arguments for the existence of 'trapped' space between aggregates of C-S-H layers are advanced. Evidence for the preservation of C-S-H structures (similar to those formed during normal hydration) for the autoclaved systems containing fly ash and sulfur is presented. The evidence is compatible with the existence of 'trapped' space within layered agglomerates and the collapse of C-S-H structure on removal of water from interlayer space, typical of normally hydrated pastes.