This is a perfect example of heading off a distastrous and expensive restoration project by making sure you have done a correct mortar analysis with the existing mortar so you can match it.
Various Sands for Comparision / Matching Historic Mortar
The traditional volume mix design of 1 part lime putty to 3 parts sand may be insidious to follow straight up without more details. First, mix designs historically used quicklime as the 1 part of lime mixed to the 3 parts of sand by volume. Quicklime when it is slaked with water will increase volumetrically 70-100 percent – or basically double its size. This fact would reduce the sand content closer to that of a 1 part lime to 1.5 parts sand – a much sticker richer mix design.
Secondly, sand must be measured in a damp loose condition according to ASTM C270 when mixing mortar. Dried sand will bulk up to 30 percent and grow volumetrically by the addition of a small amount of water. This can send your mortar mix designs at the construction site off the specified requirements.