Isabel… I found a product at my local lumber dealer that may be
exactly what you are seeking - and it was free!
It is a composite siding material for the exterior of homes. It is
part concrete, and part?? It is flat and about a quarter of an inch
thick. The siding is (or was, 3 yrs ago) shipped with 4 x 6 sheets
of the same material between layers of the huge bundles.It is gray
and hard, It can be sawed, drilled, and makes wonderful fire-stop
around solder area,
I put it on my floor beneath my own solder area to keep from burning
my rug. and around my entire soldering area, on the three wall of the
station. It is highly absorbent, so not what you would want to mop.
My local dealer gave me the sheets and was happy to get rid of it,
for otherwise, it was just dunnage and the company would have had to
pay for it to be carted away.
I just love that kind of a deal. Imagine, all that material finding
its way to a landfill! For shame!