Microstructural changes in white Portland cement paste under the first drying process evaluated by WAXS, SAXS, and USAXS

Meso- and micro-scale structural changes in matured hardened cement paste dried slowly for 1.5 years at different relative humidities (RHs) are evaluated by small-angle X-ray scattering profiles covering a q range of 0.017 nm(-1)-30 nm(-1) with a fractal disc-shaped particle analysis and Guinier approximation. The fractal disc-shaped particle analysis revealed that calcium-silicate-hydrate (C-S-H) agglomerations are very thin (similar to 2.5 nm) and of relatively large width (similar to 35 nm) under high RH conditions. However, the agglomerations are segmented (in width) into similar to 10 nm below an RH of 40%. The Guinier approximation found the possibility that the scattering profiles in the range of 2 nm(-1)-8 nm(-1) correspond to a spacing between C-S-H monolayers and it showed stepwise changes at 40%-50% RH. The stepwise behavior of the distance between C-S-H monolayers may explain a kink in water vapor desorption at around 40% RH. (C) 2016 Elsevier Ltd. All rights reserved.