Definitions & History

What are “Slaking” and “Blowing”?

Quick lime is an unstable and slightly hazardous product and therefore is normally ‘hydrated’ or ‘slaked’, by adding water, becoming not only more stable but also easier and safer to handle. 

The addition of water to quicklime results in a great deal of heat being given off and the formation of calcium hydroxide.

CaO         +         H₂O    ----------->            Ca(OH)₂           +      Heat

Quicklime        Water                       Calcium hydroxide

Chemical analysis of historic mortars indicates that many of these contain inclusions of lime that have remained unmixed and unslaked during mortar production.

The ancient Greeks and Romans produced mortar by burning limestone and slaking (mixing with water) the resultant product with water and, then mixing it with sand. Currently the calcination of limestone is generally undertaken in shaft or rotary kilns.

As time progressed the process was refined and after the addition of water the mixture was passed over a screen to remove particles which were reacting slowly. The mixture was then left to mature for about a month prior to use. The inefficiency of this process could lead to the expansion of some of the unslaked particles resulting in popping and pitting of lime based plasters.

Expansion of the mortar could also take place, a phenomenon known as blowing.

The slaking of quicklime has largely been superseded by modern methods of producing hydrated lime.