Not all concrete is hard, or better put, not all concrete is strong. Sometimes the way a concrete floor is finished leaves it "soft" which is not a problem unless you want a decorative concrete floor. We ran into this on a job in Chapel Hill for a bouldering club. After we removed the VCT, we started to wet grind the floor. Once we had started, we noticed that the cement or the paste that held the floor together was soft in the extreme. The concrete in North Carolina is typically medium to hard, however this pour was very old and not finished well.
After exposing the aggregate and before moving on to polishing steps we applied densifiers 4-5 times, and here is why.
Science ....well chemistry to be more specific.
When concrete hardens there is a reaction that results in calcium silicate hydrate (CSH). This compound is what holds the cement and aggregate tightly together. The Mighty CSH is what determines the strength of your slab. Concrete also has what is called "free Lyme," meaning Lyme that is left over from the reaction (after water leaves the concrete). That is when adding a silicate densifier comes into play.
Quick note on silica: Silica is a natural occurring colorless mineral that is found in sandstone and quarts. When silica is compressed you get glass
Adding silica to the concrete bonds it with the Lyme and forms more of the mighty CSH this leaves your floor less and less porous and strong. Think glass with each application of densifier. Do this step after grinding but before polishing the concrete.
By doing this, you can help strengthen soft or weak concrete. You are not going to get glass or mirror results when you start out with such a soft and poorly finished slab; but in this case we were able to polish the concrete and get a decent shine, and give the rock club a sounder slab than they previously had.