These are non natural composites. Concrete with bits and bobs mixed in.
They can be found on the pavement underneath the covered area on the left front corner of the building. These did not seem to have a direct relation with a specific spot in the wall. But since its brutal materiallity corresponds perfectly well with the general construction style of the building, I assume that they have started their existence as a part of it. The bigger chunk is about 8 cm across.
This building is very non descript. It does not have a name indication, nor door bell. The only characteristic element is that always many police vans are parked in the fenced area behind it, it has 'police' written on its parking places and traffic signs indicating 'police' are pointing to it. Its entrance seems to be at 1 Rue Gilbert.