Old mat well

That would probably work providing the underlying cement is solid. Sometimes, some wire mesh such as chicken wire is embedded in a screed layer less than 2" thick, although that's probably not necessary in this case.

Why not just buy a mat for it? It makes an hell of a difference to wipe your feet before stepping on a vinyl floor.

Strong 'playpit' sand and cement with a plasticiser would be OK.

Or use floor tile cement, or floor levelling compound, which are formulated to be strong in thinner layers

thin layers break up due to tensile forces. The inclusion of 1-2% of chopped plastic fibres gives a large increase to tensile strength. In bridge construction these fibres may be anything from 1/4" to 1/2" long. Scrap synthetic fibre clothing strikes me as a readily to hand source of such fibre, or you can buy the bundles used in bridges etc for about =A32 a bundle + postage.

NT

If its in a well and therefore bounded, there shouldn't be tensile forces..especially if its slightly flexible.

That encouraging because it going to be done tomorrow, one way or another. ;-)

Andy

If I understand correctly you are covering everything with hardboard, so this should spread any load.

If this is the case then almost anything should do to fill in the well for the mat.

If the base of the well is cement then as long as you bond the new layer of cement to the underlying layer then it should be stable - a 3cm layer is quite thick and the underlying cement should support it.

You could always chip at the underlying layer to provide a rougher surface to bond with.

A free floating 3cm layer would not be particularly stable, however.

I have a similar problem - all my floors are solid and I plan to put down laminate flooring including covering a mat well in the hall.

HTH

Dave R