Air (Non-Hydraulic) Lime Mortar Specifications
General:
Traditional buildings are generally of a solid-wall construction. Unlike their modern-day counterparts, they have a need to ‘breathe’. That is, to allow the passage of moisture through the building fabric in order for the building to cope with the constant wetting and drying cycles of our natural environment. Traditional building masonry is often of a porous nature and as such it will absorb water when it rains, either through the walls or from the ground. When the rain stops, with the help of the sun and the wind, the building will eventually dry out to complete the cycle. Air lime mortars play a key role in aiding and governing the effects of this historic routine by providing the main route of moisture transportation through traditionally constructed walls.
Air lime differs from natural hydraulic lime (NHL) in that is achieves its set mostly through carbonation. As the air lime mortar absorbs CO² from the atmosphere it begins a slow transformation back to its original chemical state – this process is a key part of the ‘lime cycle’. An NHL on the other hand achieves much of its set through a chemical reaction between impurities within the lime (silicates and aluminates) and water. This chemical process results in a harder mortar, with much less capillarity than an air lime. As such, for the majority of historic buildings air lime mortars are most suited as modern hydraulic mortars are simply too hard.
It is important to remember that lime mortar is designed to be the sacrificial element within the wall – approximately 80% of moisture evaporation occurs through the mortar joints themselves. Due to this constant passage of moisture, the salts it contains and the added exposure to freeze-thaw cycles in the winter, the mortar eventually breaks down and has to be replaced through repointing.
Masonry walls around the country are often repointed with modern, cementitious mortars. Cement is relatively impermeable and does not allow moisture to pass through easily. As a result, moisture is forced to exit the wall via the historic masonry rather than through the mortar joints, effectively making the soft stone and brickwork the sacrificial element. It is therefore critical that compatible lime mortars are used when repointing or repairing traditionally constructed walls so we can ensure they function as they were designed to, and that they last well into the future.
Note, on occasion the removal of an incompatible cement mortar will cause more damage than if it were to be left in place. An individual assessment should be made to determine the risks and appropriate action taken.
If repairs are to be carried out in unfavourable conditions or during winter, additional protection and aftercare will be critical. It is important to ensure any application of lime mortar does not dry out too quickly, as this will interfere with the curing process, resulting in a substandard mortar that will be susceptible to premature failure.
Masonry/Brickwork Repointing:
Failed or incompatible mortar should be raked out to a depth of approximately twice the width of the joint. Ensure the joints are cleaned out square and all old mortar is removed from the top and bottom of the exposed masonry. This will allow the new mortar to create a proper bond with the masonry and prevent it from dropping out.
Hand tools, such as a lump hammer and a selection of chisels, should be used to rake out but care should be taken when breaking out very hard cement, as pieces of stone will often detach with the cement. If cement is particularly tough, holes can be drilled through the joints at intervals making it easier to remove. On occasion, relief cuts can be made through a cement mortar with a disc cutter before chipping the remainder of the mortar out with a chisel. However, this should only be undertaken by a skilled operative as there is a risk of irreparably damaging masonry through careless use.
Once the old pointing has been removed to the required depth, all dust and loose material should be removed. An industrial vacuum cleaner or compact blower will help clear any excess from the joints without creating more dust.
Prior to repointing, the wall should be pre-wet to help prevent the newly applied mortar from drying out too quickly – this is known as managing the suction. Depending on the application, the damping down process can begin in the days preceding repointing so to allow the moisture to absorbed into the wall, rather than simply evaporate off the surface. When ready to repoint, the substrate should be damp but not wet. Ensure there is no running water on the masonry as you will create a mess that will require substantial cleaning once the mortar has hardened.
For the repointing process, you will require various width pointing irons, a leaf and square tool and a mortar hawk. The mortar should be of a thick and reasonably sticky consistency so that when you turn your hawk upside down, the mortar remains in place.
The pointing mix should be pushed right to the back of the joint so to ensure no air pockets remain. Fill the joint with mortar so that it is slightly protruding from the face of the masonry. There is no need to achieve a tidy finish at this point of the process. As the mortar dries and shrinks, it should be periodically pushed back to ensure any voids to the rear of the joint remain completely filled. If pointing masonry with excessively wide joints, pinning stones should be pushed into the joints to help pack out the mortar, improve stability and reduce the risk of shrinkage.
The roughly finished joints should now be covered left until ready to tend. A damped down hessian sacking should be draped over the wall to protect the fresh mortar from the sun, wind and rain. While the hot sun will bake the mortar and heavy rain will wash out the joints, the wind will very quickly dry out fresh pointing and careful attention must be paid to prevent this. If it is particularly warm, the hessian and pointing itself can be occasionally misted with a garden sprayed but make sure to avoid soaking the wall
A good way to test whether the mortar is ‘green’ and ready to tend is to stick your thumbnail in it. When your thumbnail will leave an imprint but your knuckle won’t, the pointing is ready to tend. The time between applying the mortar to tending will vary depending on the substrate, the weather, orientation among other factors. The best approach is to ensure you are on hand to inspect the mortar every so often as the ‘green’ stage can sometimes reached just a few hours after application. In other instances, it may take a few days.
Once thumbnail hard, the mortar should be beaten flush with a churn brush to open the surface, improve the bond between the pointing and the substrate, and to expose the aggregate. If timed well, the aggregate will appear clean and crisp – if brushed too early, the lime will tend to smear over the aggregate giving a messy finish. A well-chosen aggregate to match the original will enhance the overall look of the wall, particularly the wider joints of a rubble stone wall.
It is generally good practice to match the original or historic mortar in both form and function, so the wall looks and functions in unison and as it was designed to do so. If in doubt, mortar samples should be sent for analysis and replacement mortars should be based on the results. Historic mortars vary greatly – many were earth based, or lime rich with little in the way of sharp aggregates that we’ve come to know in the modern day.
Note, there are instances where a churn brush finish is not suitable. If documentary evidence of a particular type of finish is available, you might be required to match this.
The longer it takes for the mortar to carbonate, the better. It is advisable to keep the freshly tended pointing covered with hessian for a few weeks while initial carbonation takes place. Remember, depending on the depth of the joints and the conditions, it may take months for the mortar to reach full strength so do keep an eye on the weather and protect the work as required. Aftercare at this early stage will ensure the longevity of the lime and the ongoing health of the wall.
Mortar mix:
Most traditional building and pointing mortars were mixed hot on site and often used soon after production. The process is reasonably straightforward and as a general rule of thumb, a fairly typical mortar would comprise 1 part slaked quicklime to 3 parts aggregate (sand or earth).
Quicklime is the product of quarried limestone burned in a kiln at approximately 900℃. In this process, carbon dioxide is driven from the limestone in a process known as calcining. The end product is generally in lump form but is often processed into a fine powder.
If required, a pozzolan can be added to the mix in small quantities, just prior to use. This will help speed up the set and give the mortar some hydraulic qualities without a significant compromise in capillarity and ‘breathability’. Pozzolans may come in handy if building in the winter time, or repairing an elevation which is particularly exposed to the elements.
The process for slaking and producing a mix on site for building and/or pointing is as follows:
- Deposit 3 parts aggregate in a thick rubber trug (the ones made from recycled car tyres are particularly good) or onto a timber board on the floor. Add one part fresh quicklime into the centre of the aggregate.
- If the aggregate is reasonably wet, bank it over so the lime is completely covered. If dry, a volume of water should be added.
- As the slake begins, the lime should be left to steam for 3-4 minutes. Ensure the aggregate remains well packed to prevent steam and heat escaping.
- Additional water should be added as you begin to turn the mortar over. Little and often is the best approach so to avoid drowning the lime. The optimal temperature for slaking is around 100℃ – if too much water is added before the slake is complete, temperatures will drop and the lime will lose critical binding qualities.
- The mortar should be turned over and worked until it reaches the desired consistency for either building or pointing.
- Do keep an eye on the mix as slaking will often continue for a while and can result in excess shrinkage once the mortar has been applied to the wall.