I am assuming that by liquid cement you mean cement mixed with water, which is called cement slurry.
Specific gravity of cement slurry depends largely on the amount of cement in the mix. But for practical purposes it can be taken as 1440 kg/m3 which is equal to 98 ponds per cubic foot.
Cement mortar, on the other hand, has sand with the cement and water mix. Its specific gravity is about 2160 kg/m3, which is about 135 pounds per cubic foot.
Hope this helps.