Do you know what is the meaning of complete hydration of cement? I am using this image to answering this question.
C2S, C3S, C3A, and C4AF are the bogues compound present in the cement before hydratioat.
At 0% hydration, less than 50% is the cement content rest hydration is water present as pores but as the time increases degree of hydration also increases and we can see that all this compound quantity is decreasing.
CSH and CH is the hydration product of cement.
And when full hydration of cement is achieved, the C2S and C3S constituents will be zero but as we know that C2S heat of hydration is very less there fore it will take huge time even more than years. Therefore full hydration is very hard to say when it is going to attain.
Hope you can understand complete hydration of cement meaning.