The "Correct" Lime
Masons who built beautiful edifices in the middle ages and before, which still stand today as testament of their success, simply knew what worked and relied on proven materials and techniques.
Masons today also know what seems to work, but the window of time to see if what they are using will stand the test of time is small because some of the processes have changed in how lime is produced which may adversely change the lime's ability to be the sole and primary binder it once was.The problem that can occur if one is considering to use ordinary Type S hydrated lime as the sole binder in creating a durable mortar is that of the possibility of "over-burnt" limestone. An excerpt from "Building with Lime" by Stafford Holmes & Michael Wingate, ISBN # 1853393843 published by Intermediate Technology Publications, 103-105 Southampton Row, London WC1B 4HH, UK pp. 210 states:
“The method of burning the stone, fuel used, type of limestone and weather conditions will all affect the quality of quicklime produced. Over-burning is most unlikely when wood is used as the fuel, but extremely high temperatures (up to 1400°C) can be reached with coal, coke and even charcoal. All limestones will be adversely affected by temperatures above 1100°C as the various impurities they contain start to melt and fuse. In extreme cases considerable quantities of clinker may be formed. Impurities in the fuel may also combine with the clinker.
The result is that in addition to hard fused surfaces the overburnt stone is also likely to be discoloured with fuel contaminants. The quicklime under these conditions will clearly be far less reactive under water and is likely to leave clinker residue. Generally the higher the temperature and the longer this has been maintained the greater the quantity of clinker produced. Pure lime can become deadburned and will lose its reactivity. Hard burnt particles may take weeks or months to slake. In addition to observing these signs, a further check is to test for reactivity to compare with well-burnt lime.”
Much of the Type S hydrated lime in the U.S is burned over 1100 degrees C in a oil-fired Rotary Kiln. Although the lime may be used on its own as a primary binder for some interior plaster applications, it will not stand up alone to exposed weather in extreme freeze-thaw regions for building. Ask your putty lime supplier if they are using Type S or N hydrate to make lime putty and to send documentation of the burning temperatures of the limestone they used. It is possible to get gas-fired Type S hydrated lime in the U.S. which is burned under 1100 degrees C. In order to get this type of lime to set hydraulically one could add a pozzolan such as a Natural Hydraulic Lime to get measured performance criteria for their application without using any Portland cement.