Table 2 was the effect of mixing amount of fly ash on
cement strength. F1, F2 and F3 showed that mortar strength
of cement decreased obviously with the increasing amount
of fly ash. When fly ash ratio was 40%, compressive
strength decreased 50% at 3 days and 23% at 28 days.
F3±F6 indicated that with the increasing amount of gypsum,
the strength at all stages was also reduced. Therefore,
high- strength cement with large amounts of fly ash could
not be obtained by means of increasing the amount of
gypsum. So, BFS and G were added to improve the strength
of composite cement.