When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa, which decomposes calcium hydroxide into calcium oxide and water.
Ca(OH)2 → CaO + H2O
A suspension of fine calcium hydroxide particles in water is called milk of lime. The solution is called lime water and is a medium strength base that reacts with acids and attacks many metals. Lime water turns milky in the presence of carbon dioxide due to formation of calcium carbonate:
Ca(OH)2 + CO2 → CaCO3 + H2O
Calcium hydroxide crystallizes in the same motif as cadmium iodide. The layers are interconnected by hydrogen bonds.
One significant application of calcium hydroxide is as a flocculant, in water and sewage treatment. It forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and non-toxicity of calcium hydroxide. It is also used in fresh water treatment for raising the pH of the water so that the pipes won't corrode where the base water is acidic. The reason is that it's self regulating and does not raise the pH too much.
Another large application is in the paper industry, where it is used in the production of sodium hydroxide. This conversion is a component of the Kraft process.
Niche uses...
Because it is produced on a large scale, is easily handled, and is cheap, myriad niche and even large scale applications have been described. A partial listing follows:
In life support systems as a carbon dioxide scrubber, particularly in closed-circuit diving re-breathers such as the US Navy LAR V or MK-16, where the more caustic Lithium Hydroxide is deemed too risky due to inhaled dust, combat handling, or generation of caustic "slurry" in flooding events
An ingredient in whitewash, mortar, and plaster
To fill the root canal for the first stage of endodontic therapy (it is then replaced by rubber).
As an additive to sea water to reduce atmospheric CO2 and mitigate the greenhouse effect.
In the production of metals, lime is injected into the waste gas stream to neutralize acids, such as fluorides and chlorides prior to being released to atmosphere.
An alkali used as a lye substitute in no-lye hair relaxers
A chemical depilatory agent found in most hair removal creams (for example Nair)
In Bordeaux mixture to neutralize the solution and form a long lasting fungicide
In lime-sulfur it is mixed with sulfur and boiled in water for an hour. The ratio by weight of Ca(OH)2:S:water is about 1:1.7:8.7. Diluted (1:32) lime-sulfur is sprayed as a fungicide and used as a dip to combat sarcoptic mange.
In the petroleum refining industry for the manufacture of additives to oils (salicatic, sulphatic, fenatic)
In the chemical industry for manufacture of calcium stearate
In the petrochemical industry for manufacturing solid oil of various marks
In the manufacture of brake pads
In manufacturing the trademarked compound "Polikar", an antifungal and antimicrobial preservative for vegetables in storage
For preparation of dry mixes for painting and decorating
In manufacturing mixes for pesticides
In the manufacture of ebonite
As a calcium supplement and pH/carbonate buffer (known as Kalkwasser) for the aquaculture of corals in reef aquaria.
As a natural "alternative" insecticide. Most crawling insects are killed by its touch. Including: ticks, fleas, beetles and grubs.
Food industry...
Because of its low toxicity and the mildness of its basic properties, it is widely used in the food industry and associated hobbies: Examples:
The separation of sugar from sugar cane or sugar beets in the sugar industry, see carbonatation
For processing water for alcoholic beverages, soft drinks.
Used in the processing of Norwegian lutefisk. Dried cod fish is soaked in a mixture of slaked lime and soda to produce a soft-fleshed fish fillet that is steamed or baked and served with potato lefse.
Home food preservation in the making of pickles.
Clearing a brine of carbonates of calcium and magnesium in the manufacture of salt for food and pharmacopoeia
In Chinese cuisine, for making century eggs
Used to fortify fruit drinks such as Tropicana Calcium + Vitamin D orange juice.
A calcium supplement in mineral fortified infant formula
A condiment used mainly in Bengali families, predominantly the men, when they take areca nuts, calcium hydroxide and a variety of other seeds wrapped in betel leaves. They call this "paan"
Use as alternative ingredient to baking soda (raising agent) in papadam. (South Indian flatbread)
For making a fungicide or a dip for treating mange it is boiled with sulfur and wood ash, then diluted. Most recipes show lime and sulfur in equal proportion, but one shows the sulfur as 10 times the lime. The wood ash is used as a surfactant; cooking oil can be used.