# Why does cement shrink and plaster expand ? **URL:** \ **Category:** Factual Questions\ **Created:** [August 2, 2014, 2:34pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646 "2014-08-02T14:34:07Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Bones\_Daley](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@Bones\_Daley](https://boards.straightdope.com/u/Bones_Daley)\ **Post date:** [August 2, 2014, 2:34pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/1 "2014-08-02T14:34:07Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 2, 2014, 2:37pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/2 "2014-08-02T14:37:56Z")
I don’t have an answer, but an obvious follow-up question: Is there some mixture of the two which would remain the same volume when it set? ---
**Author:** ![Bones\_Daley](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@Bones\_Daley](https://boards.straightdope.com/u/Bones_Daley)\ **Post date:** [August 2, 2014, 2:45pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/3 "2014-08-02T14:45:11Z")
> [@Chronos](#): > > I don’t have an answer, but an obvious follow-up question: Is there some mixture of the two which would remain the same volume when it set? 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. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [August 2, 2014, 2:51pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/4 "2014-08-02T14:51:07Z")
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. ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [August 2, 2014, 2:57pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/5 "2014-08-02T14:57:24Z")
> [@Chronos](#): > > I don’t have an answer, but an obvious follow-up question: Is there some mixture of the two which would remain the same volume when it set? 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 2, 2014, 3:01pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/6 "2014-08-02T15:01:24Z")
> [@](#): > > Most things shrink when they dry, simply because the material shrinks to fill in the space where the water was. Except that the process of cement setting isn’t really a process of drying. It actually absorbs water, rather than releasing it. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [August 2, 2014, 3:06pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/7 "2014-08-02T15:06:41Z")
cement has the water become part of the material structure as it cures. this is a reason that it is so hard, it becomes one big rock. once organized by chemical reaction or crystallization the cement is less and the plaster more. ---
**Author:** ![bizzwire](https://avatars.discourse-cdn.com/v4/letter/b/958977/32.png) [@bizzwire](https://boards.straightdope.com/u/bizzwire)\ **Post date:** [August 3, 2014, 8:40am UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/8 "2014-08-03T08:40:08Z")
Plaster of Paris gets hot when mixed with water. Hot things expand. Could the crack you saw be due to thermal/mechanical stress? ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [August 3, 2014, 8:48am UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/9 "2014-08-03T08:48:29Z")
Cement gets hot as it dries/cures and it shrinks. ---
**Author:** ![Fishtar](https://avatars.discourse-cdn.com/v4/letter/f/ed655f/32.png) [@Fishtar](https://boards.straightdope.com/u/Fishtar)\ **Post date:** [August 3, 2014, 10:15am UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/10 "2014-08-03T10:15:04Z")
As stated upwards, all in the structure. Limestone vs Gypsum. But here are some microscopic images. Gypsum Plaster [http://www.scielo.br/img/revistas/mr/v11n4/02f9.jpg](http://www.scielo.br/img/revistas/mr/v11n4/02f9.jpg) Concrete [http://staffweb.itsligo.ie/staff/gmuir/MaterialsLab/SEM/SEM%20images/Concrete.jpg](http://staffweb.itsligo.ie/staff/gmuir/MaterialsLab/SEM/SEM%20images/Concrete.jpg) So gypsum swells a bit as it drys, as those crystals do their thing,then hardly expands and contracts at all, and is soft. Lime and concrete shrink a bit, as molecules link up, expand and contract a good bit after that, and are hard. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 3, 2014, 6:27pm UTC](https://boards.straightdope.com/t/why-does-cement-shrink-and-plaster-expand/694646/11 "2014-08-03T18:27:47Z")
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.