Abstract

The physical properties of the calcium silicate hydrate phase (C-S-H) greatly impact material strength and durability of cement-based materials. However, despite its widespread use and influence on material stability, there is still debate concerning its nanostructure. In this paper, the pore structure of C-S-H was characterized by analyzing focused ion beam prepared lamellas from a model tricalcium silicate clinker cement paste using scanning transmission electron microscopy (STEM) performed with a scanning electron microscope (SEM) for measurements at 30 keV and a transmission electron microscope (TEM) operating at low primary electrons energy (80 keV). The composition was mapped in the SEM using energy-dispersive X-ray spectroscopy (EDX). These investigations were accompanied by argon adsorption measurements using non-local density functional theory (NLDFT) models to calculate the pore size distribution. Mercury intrusion porosimetry (MIP) was used to investigate macropores. The STEM investigations performed at 30 and 80 keV both reveal a sponge-like pore structure built up by 0.7–4.0 nm thin C-S-H foils. The pore sizes range between 2 and 10 nm with a dominance of 5 nm wide pores. These results are in line with the pore size distribution determined with argon adsorption, which predominantly found 4.6 nm wide pores. The similar pore width distributions in STEM and gas adsorption validates the gas adsorption measurements and points towards minimal electron beam damage occurring during observation and analysis.

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