Hot Lime mortars

The use of hot lime has widespread approval throughout the conservation industry. Non-hydraulic lime, not to be confused with natural hydraulic lime (NHL), is far more traditional while also being more breathable, flexible, and suitable for softer masonry.

Hot lime mortars are made by mixing quicklime with aggregate and water. There are many methods to this, and it should only be attempted by a trained professional. Calcium hydroxide is the chemical name for quicklime; when slaked with water, a chemical reaction occurs, and it very quickly reaches temperatures above 250 degrees Celsius. Correct PPE must be used when slaking, mixing, and using lime mortars, as you could receive serious chemical burns.

When working on listed buildings, it's important to match the original mortar in strength, structure, and colour. The best way to achieve a perfect match is to send a sample off for analysis. The choice of aggregate massively impacts how a mortar performs and looks; it might be that the original mortar had impurities in it, giving it a more hydraulic set. The use of pozzolans like brick dust and ash will show up when the mortar is analysed.

If your property has soft bricks and is of traditional solid wall construction, you should probably be using a non-hydraulic mortar. A lot of so-called professionals will only recommend a lime mortar using NHL3.5 as the binder; this is usually completely unsuitable.

Most modern-day bricklayers and builders will only recommend a cement-based mortar with hydrated lime in it. This is in no way a lime mortar; the hydrated lime purely acts as a plasticiser, making the mortar more fluffy and workable. Cement-based mortars should always be avoided when it comes to rebuilding and repointing traditional lime-built masonry.

Hot-lime mortar