Why use lime mortar
When concrete was developed and widely used by the Romans in their larger projects, lime mortar had been the preferred binding material in construction for centuries. In fact lime mortars have been in continuous use in domestic building since before 400BC, superceding mud brick as the method of choice and lime has remained the most effective of materials until the present day. If your home or outbuilding was constructed before 1900, it will almost certainly be built of stone or brick with lime mortar holding it all together. The flexibility of lime, its ability to self heal and its incredible durability have given it the edge over all the alternatives until less than a century ago.
By the 1930's industrial production of the modern 'Portland' cement had been much refined. The grey powder that we know today became cheaper to make and and more widely available and it soon replaced lime mortar for construction of new builds. The 'new' cement was easier to prepare, with a fast set and drying time and it could be left open to the elements after just a few hours. In an increasingly hurried world, this gave developers shorter build times and larger profits and after thousands of years, lime mortar almost became a thing of the past.
But cement has its downsides. It is not porous and so, any moisture trapped behind cement pointing or render is likely to stay within the walls and as water always finds a way, so this retained moisture can and does settle and pool to produce pockets of damp that may never wholly disappear. A second cause for concern when building or repairing a stone built house with cement is in fact the very hardness of it. If your pointing material is harder than the stone of the construction, then erosion will begin to etch away at the stone itself. This leads to voids between the stone and the pointing which in turn allow the ingress of rain, which will seep down into the walls and only show itself later as damp patches on the inside walls of your home. Thirdly, from the the environmental point of view, cement produces 4 times more CO2 in its production cycle, while lime actually absorbs CO2 from the atmosphere as part of the curing process.
Lime, however, though self healing through its reaction to the atmosphere, will erode marginally faster over long periods of time. This may sound like a bad thing, but if you consider the cost of repointing with lime against the cost of removing, matching and replacing damaged stones or bricks, drying out the damp and fixing any interior damage such as unhealthy mould, rotten ends to joists, skirting boards and lintels, even the corrosion of electric boxes, then the advantage of a repairable mortar becomes obvious. The thousands of lime built properties constructed and still standing, some after nearly half a century, bear testament to lime's durability and effectiveness and no lime built house should be repaired with anything else.
Cement was of course a great invention and is here to stay. Its architectural qualities made it the perfect material for the fast paced twentieth century construction industry and when reinforced with steel, it has allowed the profusion of skyscrapers and larger buildings that populate our modern cities, however, almost all traditional buildings are held together with lime. Lime, however, is a material with a purpose. What suits one building may not suit another and the construction or repair of your home requires a considered choice. But essentially, a house built with lime should be repaired with lime.
Lime delivers its hardness and its waterproofing qualities through a process called carbonation. The raw lime mortar sits wet on the wall for several days while it absorbs carbon from the abundant atmospheric CO2 and the curing process begins from the outside inwards. In three or four days the outer surface will be hard to the touch and in ten you will need a chisel to scratch it. Over the next weeks and months the lime will gradually continue this bonding and binding process to form a tough water repellent surface, converting this atmospheric carbon to seal to a fine, flexible finish of ninety-five percent pure calcium carbonate and in the process it will have removed a good portion of CO2 from the surroundings. The longer this process continues, the harder and deeper the set becomes, reaching a micro-porous finish that permits the outflow of moisture by evaporation from within the walls, while preventing the larger molecules of liquid rain from penetrating the structure. This finish is known as the 'breathing' quality of lime mortar.
In time, of course, all mortars show wear and tear. As the lime cures and maybe in later years also, you may find, under close inspection, that tiny cracks have appeared in your lime mortar or render, but these cracks belong to the life cycle of a lime build and though you may not immediately see it, the carbonation process is continuing, for wherever 'new' lime is exposed, ie; in weather cracks or other damage, the lime simply takes in more CO2 from the air around and seals the wound by exactly the same process through which it set hard in the first place.
Cement will of course waterproof your home, but when cement cracks it has to be repaired by hand. It has no carbonation process by which to heal itself and where lime will flex and stretch with the passing of time and the elements, cement's rigidity will not allow it to do so and it will crack and spoil and ultimately need professional attention, while lime will quietly continue to mend itself. This self healing quality of carbonation is the main factor that has kept lime in the building industry for so many centuries and now that the world has woken up to the value of natural, traditional, tried and tested materials, it is very much here to stay; a time honoured and proven technology.
Your lime built house will move from season to season and happily do so, due to the soft cushioning qualities of the lime mortar it was first built with, so if your house was built with lime it must be finished with lime. Lime pointing and render will move in harmony with your home, as will interior lime plaster. Hydraulic lime mortars will cure into a durable but flexible, breathable, self-healing, time tested finish for your building, giving a soft and natural look.
As far as we are concerned, lime mortar is a must in the upkeep of old lime buildings. Cement presents only a short term saving in cost and time while having several critical failings. I hope I have convinced you that lime is the sound long term economical and environmental choice for the traditional home and that the lessons learned by builders down the centuries still hold true.