Lime Mortar - Non-hydraulic Lime

Non-hydraulic Lime

Non-hydraulic lime is primarily composed of calcium hydroxide (generally greater than 95%). Non-hydraulic lime is produced by first heating sufficiently pure limestone (calcium carbonate) to between 954° and 1066°C, driving off carbon dioxide to produce quicklime (or calcium oxide). This is done in a lime kiln. The quicklime is then slaked – thoroughly mixed with water to produce liquid slurry—the lime putty—or with less water to produce dry powder—a hydrated lime(or calcium hydroxide).

The slaking process involved in creating a lime putty is an exothermic reaction which initially creates a liquid of a cream consistency. This is then matured for 2 to 3 months - depending upon environmental conditions - to allow time for it to condense and mature into a lime putty.

A matured lime putty is "thixotropic", meaning that when a lime putty is agitated it changes from a putty into a more liquid state. This aids its use for mortars as it makes a mortar easier to work with. If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state. It is advised that a lime mortar should be "knocked up" prior to its use.

As well as calcium based limestone, dolomitic limes can be produced which are based on calcium magnesium carbonate.

A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated.

• Hydrated lime is any lime other than quicklime, and can refer to either hydraulic (hardens underwater) or non-hydraulic (doesn't harden underwater) lime.
• Stored lime putty is always non-hydraulic (since hydraulic putty sets quickly after mixing) and, as the name suggests, lime putty is in the form of a putty made from just lime and water.

If the quicklime is slaked with an excess of water then putty or slurry is produced. If less water is used, then the result is a dry material (any excess water escaping as steam during heating). This is ground to make hydrated lime.

Hydrated non-hydraulic lime can be mixed with water to form lime putty. Before use it is usually left in the absence of carbon dioxide (usually under water) to mature. Putty can be matured for 24 hours to many years, with an increased maturation time improving the quality of the putty. There is an argument that a lime putty which has been matured for an extended period (over 12 months) becomes so stiff that it is less workable.

There is some dispute as to the comparative quality of putty formed from hydrated lime compared to that produced as putty at the time of slaking. It is generally agreed that the latter is preferable. A hydrated lime will produce a material which is not as "fatty" and often due to lengthy and poor storage, the resulting lime produced by hydrated lime will exhibit longer carbonatation periods as well as lower compressive strengths.