Most people think their houses were built from just stone and lime mortar. However, in many locations, lime wasn't actually used to build with until the late 1800s. Therefore, if your house was constructed before this time then it's highly likely it will have been built from what's called common mortar otherwise known as earth-lime mortar. This is simply sub-soil and sand mixed with quicklime (10%). As this is easily eroded by the elements it has to be protected in some way. This is where lime mortar comes in. Traditionally it was simply mixed on site from quicklime and sand and then used to point between the stones to keep everyting watertight and breathable.
During the mixing process, the resultant exothermic reaction is mortar which gets very hot. The process of adding water is called slaking and is an essential part of the process. Here, the action of the steam build-up results in a very sticky, porous and flexible mortar which is perfect for using to repoint old stone and brick.
So, when you look at an old stone house, with all its aged lime, what you're actually seeing is a property built with earth and stone but which was pointed many years ago with a hot-mixed lime mortar.
We're currently seeing a revival concerning hot-mixed quicklime mortar because of extensive research into how this traditional mortar works. Basically, when water is added to quicklime it will at least double in volume. Powdered quicklime will expand to 2.2 times its volume whereas kibbled (grains) quicklime will expand to 2.7 times its volume. What this means is a very lime rich mortar which is extremely adhesive and also highly vapour permeable (breathable).
If you would like to know more about how important historic mortars are to your house then follow the SPAB link below:
In sum, if you have an old house then hot-mixed mortar is the best treatment as it is a true like-for-like-repair which allows for maximum pore size which will actually pull moisture out from property allowing it to expel damp which comes up from the ground. It will also pull away rainwater from the surface of the stone which means harmful salts aren't left behind to react with the stone.
Because of its extremely flexible nature it will cope very easily with structural movements. It will also expand with the stone as the sun's rays heat it up in the summer which means it won't crack. If it ever does then it will regrow and bond back together because of it's self healing properties.
If you would like to know more then I would recommend reading 'Hot Mixed Lime and Traditional Mortars. A practical guide to their use in conservation and repair.
This is written by a colleague of mine who is very prolific in heritage conservation work and also involved in research at York University.