Modifications of the C-S-H Gel by Hydration at 40°C of Belite Cements from Coal Fly Ash Class C

Abstract

The influence of the temperature on two types of hydrated fly ash belite cement (FABC) pastes were investigated at a nanoscale (1–100 nm) by measuring the specific surface area and pore-size distribution by the sorption isotherms of nitrogen gas and the BET method, and at a microscale from the pore-size distribution measured by mercury intrusion porosimetry. The two belite cements were fabricated by the hydrothermal–calcination route of fly ash class C in NaOH 1M solution (FABC-2-N) and demineralized water (FABC-2-W). In the case of FABC-2-W, a densification of the C–S–H gel was produced at the temperature of 40°C, which favored the formation of pores ∼3 nm in diameter leading to higher surface area values, compared with the C–S–H gel formed at 20°C. At a microscale, the temperature led to an increase of capillary porosity (>0.05 μm) at a later age of hydration and, consequently, a decrease of compressive mechanical strength. In the case of FABC-2-N, the densification of the gel was less evident, but the increase of capillary porosity (pores of diameter >0.05 μm) was higher. Significant direct linear quantitative correlations were found among these nanostructure characteristics of the C–S–H gel and macrostructural engineering property such as the compressive mechanical strength, for the two FABC-2-W and FABC-2-N cements under normal conditions. At 40°C, the correlations were not so clear probably due to another microstructural factor such as the increase of the larger capillary porosity (>0.05 μm).