The Lime Cycle
Now we can get technical here but lets keep it simple. Lime is derived from limestone. The limestone is quarried and burnt in a kiln where it undergoes extreme temperatures (a minimum of 880 degrees Celsius) and turns from calcium carbonate to calcium oxide. This is now called Quicklime. The Quicklime is put in to lime pits before it cools down too much and water is added causing another chemical reaction.Now calcium oxide becomes calcium hydroxide. This process is called slaking and the product is slaked lime. This is lime putty in its purest form.
So from this slaking process we now have lime in its raw form, ready to use or to add to other ingredients to do other building applications with. So which lime mortar to choose?
With so many options to choose from how do you know which lime is best for the job and the easiest to work with?These are all questions that can be answered when you work with lime mortars consistently and become familiar with their function but here are some quick answers to the most common questions.
Lime putty is a non-hydraulic lime, which means that it will not set when wet. When you are working with lime putty and you want it to set, it must be able to dry out. In the ideal application it is an interior lime product as it can dry out in a controlled fashion.
Lime putty can have an aggregate added to it, making it a type of lime mortar. This can include sands, natural fibres like horse hair, brick dusts and stone dusts. All of these additives help make the lime putty set quicker, which is commonly known as a pozzolan. It can however be used on its own for very fine work of minimal depth and coverage.
Lime putty is mainly used for internal repairs and internal plastering finishes due to the long workability and its softer nature. This lends its self well to flexible backgrounds like timber lathe work.
Hydrated lime is available from most building merchants and is essentially lime putty left to dry after slaking and then ground and powdered and finally bagged. Hydrated lime can be added to cement mortars to increase workability, much like a plasticizer, but please do not think that it is a lime mortar as it is an inferior product.
All of the lime putty mortars take a while to set. Unlike our modern alternative cement, it does not go hard by a chemical reaction. The setting process takes time and the lime should be protected from wet weather and cold temperatures (below 5?C) and persistent heat or direct sun light. Lime can be a fickle mortar to work with but by far the best for the building when used correctly and looked after properly. As the saying goes….all good things come to those who wait.
There is an alternative to non-hydraulic lime which can set underwater and is more resistant to weathering conditions before the final setting takes place. This is known as hydraulic mortar.
Hydraulic lime goes through the same burning process but is a carefully selected limestone with various impurities in the stone naturally, like clay and other materials. This does not mean that it is inferior to a ‘cleaner’ limestone, much the opposite. These impurities when burnt at specific temperatures give the slaked lime various different qualities, like setting under water and an initial setting ability. These powdered bagged products need to be added to aggregates to make mortars and should rarely be used on their own.
There are three main types of natural hydraulic lime which can be used in various applications. These mortars are graded as 2, 3.5 and 5, from softest to hardest initial setting ability consecutively.
NHL 2 is the most common lowest form of natural hydraulic lime mortar. So as the grading stipulates, the initial set is on the softer side and therefore this mortar is best used for internal work and some protected external work. Internal repairs or external rendering on hard backgrounds can be achieved using the NHL 2 mortars.
NHL 3 is the middle ground mortar and is specified more often than not. This mortar has a good initial set and is used mainly on external works like pointing and building works on stone and brickwork. External renders can be made with this product on harder backgrounds like brickwork or stone work.
NHL 5 is the hardest of the standard natural hydraulic lime mortars but can sometimes be wrongly deemed too hard. It is best suited for weathering details like chimneys, copings and other exposed areas. Harder stones like granite can be used alongside NHL 5 due to the relative strengths. Natural hydraulic lime in this strength is normally specified for works close to the ground or underground works, like drainage and footings.