I have a house which was built in 1890 in western michigan. The house is a fieldstone foundation. It appears at some point it was plastered or covered.
On the west side of my basement there also seems to be a bump out that appears to reinforce the interior of the foundation. This bump out seems to be full of sand and then plastered over the sand. Some of the plaster is cracking and falling off and I would like to repair/replace the damaged sections. I cannot find any information on what it might actually be or even if it is plaster. Does anyone have any insight on this?
Best Answer
The material underneath the plaster looks remarkably like the clay lump that my farmhouse in Suffolk, England was made from. Clay lump is sandy-gravelly-clay+straw+lime, formed into blocks which are dried, and then built into walls with clay+lime mortar.
My guess is that your bump is not actually clay lump, but it is very likely to be an earth-based construction material (cob for example).
With luck, the plaster is lime plaster rather than a gypsum plaster (lime plaster is much more flexible). Mixing lime plaster from lime putty (hydrated calcium hydroxide) is quite easy (particularly if you have a cement mixer). Use a ratio of 2 parts sharp sand to one part lime putty, reinforce with finely teased hair, and you should be good to go.
You may well find that the aggregate in the plaster is your local subsoil (quite possibly from the basement you are standing in!) If you have some subsoil exposed, it would be entirely reasonable to use it.