Fluoride, F¯, takes Al³⁺ ions into the C₃S crystal structure with it in a coupled substitution. That will leave less Al³⁺ ions available to form C₃A and therefore the setting time of the cement is lengthened.

Dear Dr.Clarke,

In the said white cement setting time and flouride content are coming as given below,

Sample-1, IST = 100 minutes, FST = 160 minutes, Flouride = 0.22% ,Blaine= 416 m2/kg , SO3=3.07%

Sample-2, IST = 50 minutes, FST = 105 minutes, Flouride = 0.35%, Blaine =411 m2/kg , SO3 = 3.43%

It is very difficult to understand that in spite of increasing flouride content in sample No-2, setting time is coming around 50 minutes lower than the sample number-1.Could you please explain the possible reasons of lower setting time in cement No.-2 in comparison to sample No-1?