Just to clarify, by “cement” I mean the grey powder known as Portland cement, the stuff that bricklayers mix with sand to keep their bricks together (or apart if you prefer)
By “plaster” I mean the grey (or sometimes pink) powder which plasterers use to coat plasterboard (sheetrock) in the better class of building work.
Many many moons ago, an old plasterer showed me a phenomenon which I found interesting at the time. He took two identical glass bottles, filled one with a slurry of neat cement mixed with water to a pourable consistency, and filled the other with a slurry of plaster mixed with water to a similar consistency.
A day or so later , the bottle with the hardened cement rattled when you shook it, but the bottle with the hardened plaster displayed a large crack, which I took as evidence that the plaster had expanded and the cement had shrunk.
I assume that there must be a scientific reason for this difference in behaviour, and would be interested in any explanation thereof.
Interesting question … I have never known anybody to mix plaster and cement together, so I have no idea what would happen … or whether indeed the mixture would set hard in the first place.
Most things shrink when they dry, simply because the material shrinks to fill in the space where the water was.
The reason plaster expands is that it forms crystals as it dries, and the crystal structure takes up more space. It is similar to the reason water expands when it freezes, since the water also crystallizes as it turns into ice.
ETA: I don’t know how useful it would be as a material, but it should be relatively simple to come up with a mixture that would remain roughly the same size. Since the cement parts would still shrink and the plaster parts would still expand, this might have a rather adverse effect on the material’s structural integrity. But if you mixed it fine enough, you could do the bottle trick and the material would neither shrink away from the bottle nor would it crack the bottle due to expansion.
Finding the mixture would be the equivalent to David Letterman’s “Will it Float”. Unless there is a threshold point (like water to ice), where the crystallization of the plaster would be defeated by the presence of a chemical property of the cement.
Not all cement shrinks as it cures. There are hydraulic cement formulations intended to expand as they cure. This is necessary to fill holes and cracks, and can even be used to break rocks by drilling into them and filling with a highly expansive cement.