There are all sorts of reasons why you should not use any Portland
cement on an old house. The arguements between using bagged hydrated
lime and wet lime putty are analogous to the difference between a Rolls
Royce and a mini. They will both get you there but...
When the Victorian and Edwardian houses were built, the quicklime
(limestone that has been burnt in a kiln to change the calcium
carbonate to calcium oxide) was slaked on the building site and used
straight away. It was recognised that leaving the lime putty in the
slaking pit for weeks or months improved it but this was not a
practical proposition in the housing boom around the turn of the 19th
century. The lime was often not very well sieved, hence the large
particles of lime found in old mortars. These days we are just too
careful with the ingredients and find the rough and ready methods of
old difficult to reproduce.