Because bulk density and specific gravity represent different things. Bulk density considers air voids for calculating volume, which is 1.44 g/cc for cement (as voids increase the volume, not the mass). But true specific gravity is calculated without considering voids, that’s why it is 3.15 .
Let’s take the example of sugar.
Now, as you can see, the volume considered is not entirely filled with sugar. It also contains air in the voids in between the sugar crystals. Hence the density thus measured is called bulk density. The bulk density of sugar is 0.7 g/cc.
This value does not represent the actual density of a single sugar crystal, which is taken care by specific gravity.
Imagine a single sugar crystal as above. If you measured its mass and volume, you will be able to find the actual density of sugar. Naturally, this value will be higher than its bulk density (as there are no air voids). The value of the actual density of sugar is around 1.2 g/cc.
Now specific gravity is nothing but the ratio of density of the substance to density of water. Since density of water is considered as 1 g/cc, the specific gravity of sugar has the same value of 1.2 without the units.
Coming to the case of cement, the concept is similar. There are countless air voids in cement. If you measure the mass of the cement and its bulk volume, you get bulk density which is around 1.5 g/cc. If somehow you were to measure the mass of a single cement particle and its volume, you get the specific gravity which is around 3.1.
1- 1.5/3.1 = 0.52, meaning, more than 50% of a cement bag is just air! Like a chips packet! When I first found out about it, I was like – MY WHOLE LIFE WAS A LIE!
Both BD and SG have their own significance. If you have truck load of sand, and you want to know the weight of the load, you can simply calculate the volume of the compartment and multiply it with the bulk density. But bulk density doesn’t give all the information about the material. The sugar’s BD is only 0.7 g/cc. But water is 1 g/cc, so does sugar float on water? Of course not. The value you should be looking for is specific gravity i.e., 1.2, which is greater than water.
The size of single cement particle can be as small as Obviously the actual procedure of measuring the bulk density and specific gravity is more sophisticated than what I’ve described. You can read the for the Indian Standards of measuring the same.