Lime plasters are open and porous materials, as opposed to ordinary cement, allowing vapours in and out of the walls. This helps to stabilise the internal humidity of a building, making for a more comfortable environment, reducing surface condensation and mould growth - crucial in all stone and building restoration but even more so in historic buildings and monuments as they are generally quite delicate due to their age.
Below are some of the questions we frequently get asked. If your question isn't answered below, please email us on firstname.lastname@example.org and we'll be happy to offer you further advice and guidance.
Hydraulic limes are set by a reaction caused by water, known as hydrolysis. This causes a faster and harder set, hence why these Natural Hydraulic Limes are more often used for exterior work, such as repointing and rendering, especially in exposed or damp conditions. Our Natural Hydraulic Limes are available as a bagged powder and in differing degrees of strength.
Non Hydraulic Limes are putties and set by the process of carbonation. This causes a much slower set and the lime remains softer and more breathable - hence the name Lime Putty. Our non-hydraulic lime putties can be used for flexible background plastering, soft brickwork and stone repairs mixed with stone dusts and fine aggregates .
A standard drum mixer can be used to mix hydraulic lime, although for most industrial scale (larger) projects it is advisable to use a roller pan or paddle mixer.
It will depend on the specification for the constituent parts but let’s take a standard 3:1 mix for pointing.
In an empty mixer, add one part sand, followed by one part lime and then two parts sand. Mix it dry for at least five minutes. Then gradually mix in the water until the required consistency is reached. The mortar should be mixed with the minimal amount of water and the maximum amount of mixing. It should look like a paste which will hold its own shape. The substrate will be wet and the mortar drier to avoid any shrinkage and problems during the application process. Continue to mix for a further twenty minutes. The longer you mix the mortar, the more pliable it will be to use and the more it will be ‘chopped’. This will result in a far superior bond when it achieves full carbonation after 28 days. The bond is a physical bond, not a chemical set, so the more the molecules are broken in the mixing process the greater the bond will be upon setting.
We have even specified that mortar is mixed one day, left to rest overnight (protected of course) and then re-mixed again. This produced an amazingly pliable mortar to apply and the setting strength was brilliant. Of course, this is labour intensive and not necessary to achieve a good finish but as the Italian saying goes ‘there should be only sweat in that mortar for it to mix’