Irreversible microstructural changes of calcium silicate hydrate during the first drying-resaturation cycle

Abstract

Calcium silicate hydrate (C-S-H) is the most important hydration product of Portland cement. In this study, a brand-new compacting method based on pozzolanic reaction was used to prepare pure C-S-H block samples. Compressive strength and flexural strength of the C-S-H samples were tested. Water desorption-absorption behaviors of the C-S-H during the first drying-resaturation (D-R) cycle were measured by means of thermogravimetric analysis and low field 1H NMR relaxometry (LF-NMR). A significant irreversible reduction of gel pore volume was detected after D-R cycle, while volume of interlayer pores remained unchanged. Based on results, gel pores were classified into collapsible gel pore (CGP, >2.6 nm), deformable gel pore (DGP, 1.4–2.6 nm) and rigid gel pore (RGP, <1.4 nm). RGPs are stable both in size and volume. DGPs are stable in pore volume but irreversibly changeable in pore size and CGPs are vulnerable and largely collapse causing reduction of pore size and volume after the first drying.