The lime in the merchants is usually hydrated non hydrauilic lime, which is calcium hydroxide powder. This is normally used as an additive to cement mortar. It is basically the same as lime putty, although it is not considered as good a quality as lime putty which has been left for months, or even years to mature. Hydraulic lime is generally considered to be much better for re-pointing as non hydraulic has been found to fail many times in the past. Recent analysis of old lime mortars has shown that the mix was often 2/1 or 1 1/2 /1 sand/ lime rather than the 3/1 that many builders thought was used. It is thought that 3/1 was done by weight and not volume. If using lime putty you also have to consider the amount of water already in the lime in its volume and allow for that.