Grades 6–8 · Materials & Matter
Why does wet plaster become solid before all its water has dried?
A freshly mixed gypsum plaster is a workable paste, but it soon becomes firm. This setting is not simply water evaporating. A chemical change creates gypsum crystals that grow together into a connected solid structure.
In the diagramInterlocking crystals set a still-wet pasteFresh mixtureSetting structurewater remains between growing gypsum crystals
The starting plaster powder contains calcium sulfate with less water built into its crystal structure than gypsum has. In the mixture, material dissolves and reacts with water. Gypsum then forms from the solution.
As more crystals grow, they overlap and interlock. The growing solid network resists flow, so the paste loses its ability to move freely. Setting can release thermal energy because the overall hydration process is exothermic.
Some mixing water remains between the crystals and later evaporates. Drying therefore continues after a firm structure already exists. Distinguishing chemical setting from later water loss explains why a newly set piece can be solid yet still contain substantial moisture.
About this explanation
Starting knowledge: Distinguish evaporation from a chemical reaction and recognize crystals as ordered solid structures.
Sources
- James R. Clifton, National Bureau of Standards — Some Aspects of the Setting and Hardening of Gypsum Plaster