Most properties built prior to 1920 used lime mortars in construction and finishing.
In the early twentieth century it became the norm to use (opc) ordinary portland cement in new construction, but it was and is used to repair the older lime built propertys.
This has had a serious negative effect on the condition of these buildings.
Lime mortared and rendered buildings have a natural ability to both hold and evaporate moisture from its structure thus maintaning a balance with the surrounding atmosphere.
The method by which lime mortars enable the passage of moisture is critical to the well being of older propertys.
In direct contrast the use of cement mixes for pointing/rendering/plastering in older buildings dramatically reduces the ability for the passage of moistures,therefore trapping it in the structure and slowing the drying out time of the building and there for increasing the risk of internal condensation problems or so called DAMP and poor insulation.
Lime mortars also have an inherent flexibilty which cement mortars do not.
Which means that lime mortars can accommadate small scale movement caused by settlement and thermal movement.
Lime mortars are also described as being self-healing as they can chemically seal themselves in the event of hairline cracks.
Cement mortars do not have any of these characteristics and often fracture and fail and encourage rain to enter the fabric of the building thus becoming trapped.Lime mortars are also a much more eco-friendly alternative to cement mortars.
Firstly the longevity of lime mortars, makes it an excellent green building choice because it will not need to be replaced as often as modern cement mortars,also the bricks and mortar can be recycled/reused, cement cannot be and makes the bricks unusable after application.
Cement also requires a higher temperature to produce, so by using lime you can cut down on your carbon footprint.
We would always recommend you do some research of your own if you are planning on having some exterior/interior work undertaken on a older property.
There is a wealth of knowledge available now ie:books internet etc that can help to ensure the correct method,materials are used.