Porous concrete floor - what could have happened?

[Bjorny]

Hello,
I am renovating a house from the 1940s. In the basement, in the former boiler room, it sounded hollow when I knocked with a hammer. About 1.5 sqm where the oil boiler and tank had been. I chiseled up this area where the surface coating and partially the rough casting were porous. In some places, it turned to gravel directly when I drove the chisel tool down (a light tool). The concrete is also black, not gray. Some areas feel smooth.
It seems that oil has leaked down, which then dissolved the binding agent in the concrete and made it porous.

Can heating oil dissolve concrete, or what might have happened? I can't find any information on this. Does anyone know anything?

Everything will be recast, I am just interested in what caused the problem.

 

[Autodidak1]

To save money, many basement floors from that time are nonexistent. Often just a thin layer of poor-quality concrete on gravel/stone, sand. Over time it can crumble for natural reasons (load, vibrations, and movements). The basement was not made for "usage" as it is today.