Old building 1956 - What kind of screed? Harmful substances?

Hello,

maybe someone can help me, we are renovating an old building from 1956 and still need to consider how to proceed with the existing screed. Since a fireplace was removed and now about 1m² is missing on 2 floors (now only concrete floor), we have to consider whether and how we can somehow continue to use the existing screed. Basically, we would like to do that, but first we would need to know what kind of floor construction it is.

Description of the construction from top to bottom:
- ~1mm reddish thin layer, looks like epoxy resin (not visible in the photos - lies under the PVC)
- 10mm black solid layer
- 10mm black layer that yields a bit - somewhat softer. It already crumbles at light touch (see first photo)
- sand layer

According to my research, it could be mastic asphalt, but I wonder what this crumbly softer layer could be? Does anyone also know anything about the PAH content of such floors? Does it still have anything to do with tar content? Could this be "sealed" and reused? It does not smell like tar at all and does not produce a "brown soup" in combination with a solvent (methanol) (this tip for identification was given to me by the local disposal company).






I am very grateful for ideas or answers

Hello, unfortunately I can't link with my phone, but take a look in the same forum category at a post/topic below you - the same covering is discussed there

The construction and layer thickness make me think more of a stone wood or magnesia screed. Until the 1950s, screed was usually installed in two layers as a wearing screed. It consisted of a lower filler layer – made of magnesia ("Sorel cement") with organic fillers (sawdust, wood flour, cork flour) – and a wearing layer bonded on top, usually about 1 cm thick, made of magnesia with inorganic (e.g. sand) or only small amounts of organic components. The wearing layer was usually colored and subsequently treated, e.g. with floor wax. This basic structure resembles the one shown in your photos.

Stone wood screed is basically a nice thing: it is very light, warm underfoot, and has only a low build height. However, the decisive disadvantage is that due to its chemical composition in combination with moisture it is highly corrosive and attacks many metals. Because of the high proportion of organic additives, it swells strongly when moisture penetrates.

In a renovation, the penetration of moisture – also through the coating material – must therefore be protected by a preventive coating, e.g. an epoxy resin. As is very likely the case with you.

If it is a stone wood screed, it has a good bond with the substrate and is mostly level, it could remain. If it should be renovated and covered with another floor covering, a preliminary examination is first necessary to find suitable materials and an appropriate approach. Specialist companies that can assess and work with such screeds are quite rare. In any case, I advise against starting any kind of self-renovation with a spontaneous renovation concept using DIY store materials without these preliminary investigations. Otherwise: see consequential damages.