This paper analyzes the effect of fly ash chemical character on early Portland cement hydration and the possible adverse effects
generated by the addition of gypsum. Behaviour was analyzed for pure Portland cements with varying mineralogical compositions
and two types of fly ash, likewise differing in chemical composition, which were previously characterized under sulphate attack
as: silicic-ferric-aluminic or aluminic-silicic ash in chemical character, irrespective if they are in nature, siliceous or
siliceous and aluminous materials according to the ASTM C 618-94a.
The experimental results showed that water demand for paste with a normal consistency increased with the replacement ratio
in fly ash with a more aluminic than silicic chemical character, whereas it declined when silicic-ferric-aluminic ash was
used. On the other hand, the differences between the total heat of hydration released at the first valley and the second peak
also clearly differentiated the two types of ash. While the relative differences increased in the more aluminic than silicic
ash, they declined in the more silicic than aluminic. In another vein, the findings indicate that within a comparable Blaine
fineness range, the reactive alumina (Al2O3r−) content in pozzolanic additions has a greater effect on mortar strength than the reactive silica (SiO2r−) content, at least in early ages up to 28 days. Finally, the adverse effect generated in the presence of excess gypsum is
due primarily to the chemical interaction between the gypsum and the C3A in the Portland cement and the reactive alumina (Al2O3r−) in the fly ash.