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). © 2007 The American Ceramic Society.