Density and Specific Gravity of Cement are different
Density is not the same as specific gravity. Specific gravity is a unitless number. It is basically the ratio of the density of one material compared to the density of a standard material at a standard temperature. Because it is a ratio, there are no units. In this case the material is usually water at a standard temperature. Water has a density of 1 kg/liter at 4 degrees celcius or 39.2 degrees fahrenheit. We say the specific gravity of water is 1.0. If something is higher than 1.0, it will sink in water. If it is lower than 1, it will float. In your example, a specific gravity (SG) of 3.15 would sink.
Let’s take something like fuel, or petrol. You would expect this to float in water because it has a SG of about 0.72. Notice, that it doesn’t matter if I use English or metric units. Lead on the other hand has an SG of 11.35. You would expect it to sink in water, of course.