Answer by Mihir Salanke:
Mix proportion for M25 grade of concrete is 1:1:2(cement:fine aggregates:coarse aggregates). So if you need 4cu.m of M25 concrete, we need to have 1cu.m of cement. So, theoretically, for 1cu.m of M25 concrete, we need to have 0.25cu.m of cement.
But in practice, whenever we estimate quantity of cement for concrete or mortar, we consider 50-55% extra quantity. This extra quantity is for voids, wastage, etc.
So, for preparing 1cu.m of M25 concrete, quantity (dry volume) of cement required = 1.55 X 0.25 = 0.3875 cu.m
Now a bag of cement(50 kg) corresponds to 0.035cu.m of cement.
Hence 11 bags of cement are required for preparing 1cu.m of M25 concrete.
Admixtures(like plasticisers, super plasticisers) are usually added to increase workability of the mix at lower water cement ratio. So, I don’t think addition of admixture would change the quantity of cement.