A number of years ago, Edison Coatings was approached to become a distributor of pre-packaged lime putty for use in masonry mortars. Naturally, we wanted to know more about the material's technical advantages before we could endorse lime putty in place of hydrated lime at 10 times the price. We just wanted to know: "Why?" The response, "Because Architects like it and specify it" wasn't quite what we had in mind. But the manufacturer simply couldn't point out any specific technical advantages. Still, increasing use of this traditional material in Historic repointing work had not escaped our notice, so we decided to do some further research into lime putty: Is there something about this traditional material that remains just as valuable today as it was 150 years ago? Why are some architects specifying lime putty mortar to the exclusion of mixes using hydrated lime? To gain a clearer understanding of the differences between hydrated lime and lime putty, we started by looking into the manufacturing processes by which they are made. Both begin as limestone, which is composed primarily of calcium carbonate and contains varying proportions of magnesium carbonate with minor amounts of other minerals. The rock is calcined at temperatures near 10000C (18000F), which drives off chemically combined carbon dioxide. Calcined limestone (CaO/MgO, calcium oxide/magnesium oxide) is called quicklime, which Edison Coatings uses as a minor ingredient in one of its specialty cement products. It is characterized by fast reaction with water, during which it releases significant amounts of heat. It is this material which traditionally required extended "slaking" in water before it could be used in masonry work. We spoke with a commercial lime producer about the process by which this material is converted to hydrated lime today. The process takes place in a hydrator, which combines the dry, pebbly quicklime with a carefully controlled amount of water. In their process, they add 23% water to quicklime", they told us. This produces Type S hydrated lime, Ca(OH)2/Mg(OH)2, calcium hydroxide/magnesium hydroxide. All of the water is chemically combined with the quicklime, so the product remains a "dry", free-flowing powder. If more water is added, the result is lime putty. Lime putty was the traditional pre-industrial era material used, because excess water was needed to assure complete hydration when using relatively inefficient manual slaking methods. The only difference between hydrated lime and lime putty is the amount of water that has been added to them. Both lime and lime putty contain variable amounts of calcium hydroxide and magnesium hydroxide, based on the composition of the limestone used to make them. Dolomitic limestone, with a magnesium content of 35 to 46%, is favored for use in producing lime for masonry applications due to superior water retention. Lime based on high calcium limestone (less than 5% magnesium carbonate), is often rated as a Type N hydrate, considered less reliable than Type S dolomitic lime. Type N hydrates must be proven not to be detrimental to mortar soundness before they can be used, according to the ASTM C270 masonry mortar standard. (Type N and Type S Lime should not be confused with Type N and Type S mortars, as all of the standard ASTM C270 mortars (Types O, N, S, M and even K) are produced with Type S hydrated lime.) Edison Coatings utilizes ASTM C207 Type S hydrated dolomitic lime in our Spec Joint 46 Custom Masonry Mortar formulations. All required mortar performance properties can be met by lime or lime-cement based mortars utilizing hydrated dolomitic lime. There is no technical justification for requiring the use of lime putty for this application. Though some try to make a case for its use in restoration work, no imported Scottish or French lime was used to build buildings in the United States. Traditional materials were selected on the basis of available technology, economy and convenience. If Type S hydrated lime were available 150 years ago, they would have undoubtedly used it then, too.

FROM LIMESTONE TO LIME, AND BACK...
CaCO3, MgCO3 LIMESTONE	+ HEAT	= CaO, MgO    QUICKLIME (ASTM C5)
CaO, MgO QUICKLIME	+ H2O    WATER	= Ca(OH)2, Mg(OH)2    HYDRATED LIME (Stoichiometric Water) ASTM C207    or LIME PUTTY (Excess Water)
HYDRATED LIME or LIME PUTTY	+ GRADED SAND    (ASTM C144)    PORTLAND CEMENT (ASTM C150)	= MASONARY MORTAR    (ASTM C270)
Ca(OH)2, Mg(OH)2 LIME IN MASONRY MORTAR	+ CO2    CARBON DIOXIDE	= CaCO3, MgCO3    LIMESTONE

 

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Other Lime Mortar Links:

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deGruchy Masonry Restoration, Inc., Quakertown PA, Historic Masonry Restoration Consultant.

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U. S. Heritage Group, Inc., Chicago IL, Custom Matched Lime Mortars.

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Edison Coatings, Inc., Plainville CT, Masonry Restoration Materials.

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Abstract Masonry Restoration Inc., Salt Lake City UT, Historic Masonry Restoration Specialists.