Gypsum vs Quicklime
Gypsum and Quicklime are genuine alternatives to one another — a choice somebody actually makes — and differ on 1 measured property, density most clearly.
Gypsum vs Quicklime
These are different kinds of thing (mineral, compound). Fewer properties mean the same thing across both, so a like-for-like reading may not be available.
Gypsum
CaSO4·2H2O
A soft sulfate mineral that gives up its water when gently heated and takes it back on mixing — the basis of plaster.
Quicklime
CaO
Calcium oxide, produced by burning limestone — the reactive intermediate behind mortar, plaster and cement.
| Property | Gypsum | Quicklime |
|---|---|---|
| Cleavage Only one of the two has a figure we can stand behind, so there is nothing to compare. | perfect in one direction | Not known |
| Crystal system Only one of the two has a figure we can stand behind, so there is nothing to compare. | monoclinic | Not known |
| Density | 2.31–2.33 g/cm³ | 3.34 g/cm³ higher |
| Lustre Only one of the two has a figure we can stand behind, so there is nothing to compare. | vitreous to pearly, silky in fibrous form | Not known |
| Melting point Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | 2613 °C |
| Mohs hardness Only one of the two has a figure we can stand behind, so there is nothing to compare. | 2 | Not known |
| Molar mass Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | 56.08 g/mol |
| Streak Only one of the two has a figure we can stand behind, so there is nothing to compare. | white | Not known |
Choosing between them
Gypsum is recorded as a substitute for Quicklime — easier to work.
What it gives up: Sets in an hour and stays impermeable, so it cannot be used where the wall must breathe.
In this application, on the evidence cited above.