Non-Hydraulic Lime. What's Involved?
Non-hydraulic lime or as it's also known, fat lime putty is, as the name suggests, a white, putty like material with a fairly solid consistency. It is relatively simple to work with but there's two fundamental things you'll need to know before you get started. That is knocking up & cherishing.
As fat lime putty is not a powdered lime, it can't be mixed with sand and water in the conventional way you'd make mortar, render or plaster. It needs to be knocked up. Knocking up is the process of kneading, beating, ramming and chopping the materials together with a wooden stick or batten.
If you're thinking about making dough right now then you're thinking along the right lines. The great thing about non-hydraulic lime mortar, render and plaster is that it can be knocked up time and again. This gives it a pretty much indefinite shelf life.
Once knocked up, moisture in the material will evaporate, it will harden after a while. You'll be able to knock up again to re-introduce workability, only the smallest amount of water is added. Just remember that the lime or any mortar should be kept sealed, air-tight in a container when not in use. Non-hydraulic lime sets by carbonation, this happens when exposed to the air, it can't be knocked up again once set.
Cherishing is the process of moisture control undertaken during the curing process of mortar, render or plaster. Fat or high calcium limes set by carbonation which make them particularly prone to shrinkage. Controlling moisture is critical within the first few days of mortar placement.
As a first step, before the placement of any mortar, render or plaster, it is essential that the masonry substrate is wet (but not saturated). When placed, slow drying or wet curing will help prevent shrinkage cracks developing and will also help improve the pore structure.
When fat or high calcium limes dry, carbonation will occur at the margin of wet and dry material present. The wet curing effectively slows and stabilises this process. As the mortar dies out it should be sprayed periodically with a fine mist sprayer at intervals determined by the weather conditions on site but generally anywhere from once a day to every 4 hours.
Remember that the work should be screened from the sun and wind to prevent rapid drying or from rain and frost to prevent damage during the drying cycle. This can be achieved using screening materials such as mortar fleece, hessian, plastic sheets or tarpaulins.