Calcium Hydroxide — Frequently Asked Questions

Because it is only sparingly soluble, around 1.7 grams per litre. Dosed as a slurry, only as much lime dissolves as the acidity present can consume, and the remainder stays as suspended solid that dissolves progressively as more acid arrives. Caustic soda dissolves completely and will overshoot into dangerous alkalinity with a moment's inattention.

Quicklime is calcium oxide, produced by calcining limestone, and it reacts violently with water, generating enough heat to boil it. Hydrated or slaked lime is calcium hydroxide, made by adding water to quicklime under control. Hydrated lime is far safer to handle and is what is normally supplied for water treatment and construction use.

Lime mortar sets by reabsorbing carbon dioxide from the air, slowly converting back to calcium carbonate, whereas cement sets by hydration. That slow carbonation gives lime mortar flexibility, breathability and a degree of self-healing, since fine cracks recarbonate and close. Those properties are why lime is specified in historic building conservation where cement would be too rigid.

Because it is the cheapest bulk source of alkalinity, and sulphur dioxide is an acid gas. Lime slurry sprayed into the flue gas neutralises the sulphur dioxide and forms calcium sulphite and sulphate, which are collected as solids. Cost per tonne of alkalinity is decisive at the scale a power station operates, which is why lime rather than caustic is used.