Why use a mortar that is weak, flexible and vapour permeable? Because that was, and still should be, the method of construction in traditional buildings.
A single, solid, thick wall standing on very shallow, often rudimentary foundations is bound to move as it and the ground it stands on expand and contract in response to variations in humidity and temperature throughout the year.

Thus, the mortar and render must not only allow movement but also allow the moisture, which inevitably will enter through the minute cracks generated, to exit the structure. Lime mortars and renders have been rediscovered as perfect for this purpose. The modern obsession with cube tests and crushing strengths is a product of a totally different mode of building. While strong mortars are absolutely essential in post-war construction with their deep concrete foundations, damp courses and double skin walls, extra strength in old buildings, except in very exposed positions (copings, chimneys etc.) is actually detrimental.

Whether the alumino-silicates in cement that cause it to set are more harmful than those in hydraulic lime is a separate question. Both are harder and and have a lower lime content than pure lime putty. They thus allow less movement and less vapour transfer.

Traditionally, in Britain, the first job on a building site, especially an important one like a church or cathedral, was to dig a pit in which the lime was

slaked so the resultant putty would have time to mature. This makes it very unlikely that the limes used were hydraulic, since by definition, they set under water. Today it is impossible with a dry powdered product, either hydrate or hydraulic, to attain the same plasticity which gives a proper lime putty its fantastic bonding ability. Even if the original mortar contained added strength, the structure has, by now, been exposed to the process of decay and can be seriously weakened. If a replacement mortar or render is stronger than the actual fabric of the building then vapour transfer and its attendant damage will eventually start to break down the brick or stone.

Specialist restoration companies understand these problems and have trained craftspeople, skilled in the use of most appropriate solutions. However, there is a danger that less well qualified people will use hydraulic limes in more places than actually needed, merely because they are more like what they are familiar with, ie. cement.

The preservation of our historic buildings should not be put at risk for the ease of unskilled builders; or engineers not versed in this area who try to impose completely inappropriate parameters; or gullible specifiers who believe it possible to have all the advantages of lime and of cement too. It is a paradox, but a real lime mortars strength is its relative weakness - that and it being cheaper of course.