Strengthening effects of finely ground fly ash, granulated blast furnace slag, and their combination

The effect of finely ground fly ash (FGFA), finely ground granulated blast furnace slag (FGGBS), and their combination on the compressive strength of concrete was studied. Test results showed that incorporating 20% FGFA or FGGBS can significantly increase the compressive strength of concrete after 3 days. The compressive strength of concrete incorporating the combination of FGFA and FGGBS is higher than both FGFA concrete and FGGBS concrete, and is quantitatively similar to that of silica fume concrete. Scanning electron microscopic analysis showed that, after the incorporation of the combination of FGFA and FGGBS, a great quantity of a stick-like substance exist in the surface of hardened paste being investigated. The electron probe x-ray microanalyzer analysis showed that the stick-like substance was most probably the sheet C-S-H rolled up during specimen preparation for scanning electron microscopic analysis, which may be the mechanism of the strengthening effect of the combination of FGFA and FGGBS.