Belite cements: Modifications of calcium silicate hydrate (C-S-H) gel by alkaline hydrothermal activation

The C-S-H gel of two types of fly ash belite cements (FABC) are studied at a nanwscale (1 to 100 nm [39.4 to 3940 nin.]) by means of the specific surface area and pore-size distribution, which were measured by the sorption isotherms of nitrogen gas and the BET method. The two belite cements were fabricated by the hydrothermal-calcination route of Class C fly ash in NaOH 1M solution (FABC-2-N) and demineralized water (FABC-2-W). Two differentiated zones in the pore-size distribution of the C-S-H gel at ∼12 and ∼3 nm (∼473 and ∼118 nin.) with different influences in the mechanical strength were produced. The proportion of both zones depends on the age of hydration and type of belite cement. At early ages, the C-S-H gel of FABC-2-N develops both zones in a greater proportion than those of FABC-2-W. At later ages, the behavior is opposite: the C-S-H gel ofFABC-2-W develops high proportion of pores of 3 nm (118 nin.) of diameter. Important quantitative correlations were found among these nanostructure characteristics of the C-S-H gel and macrostructural engineering property. copyright