The effects of partial and complete drying on the internal structure of hydrated cement paste were investigated using small angle neutron scattering (SANS). Each specimen was analyzed in the initial saturated state, in the dried state, and then again after resaturation, allowing reversible and irreversible effects to be separated. Quantitative changes in the nanometer-to-micrometer scale microstructure determined from the SANS data were irreversible, from which it was inferred that the reversible component of the bulk drying shrinkage arises from small strains distributed evenly throughout the specimen. Partial drying causes significantly more changes in the local microstructure than does complete drying. The observed changes, which were quantified by applying a fractal model to the SANS data, are interpreted as the result of two related effects. the capillary tension developed by partial drying compresses the nanometer-level pore structure of the calcium-silicate-hydrate (C-S-H) gel phase, significantly increasing the gel density by a mechanism related to that governing the classical constant rate period for pure gels. Due to the restraining effects of the other solid phases in cement paste, this decrease in the volume of the C-S-H gel also causes an increase in the volume of capillary pores. This process is manifested in the SASNS data as an increase in the intensity of surface fractal scattering arising from the interface between nanoscale hydration product and the capillary pore system. The latter effect is not observed in pure gels due to the lack of restraining phases. All of the structural changes on drying were more significant in younger pastes. indicating that chemical aging of the C-S-H gel increases its ability to resist deformation.
Citation: Journal of the American Ceramic Society
Pub Type: Journals
calcium silicate hydrate gel, cementitious materials, microstructure characterization, nanoscale materials, small-angle neutron scattering