LIME MORTAR, LIME PUTTY, MASON FURNISHES TECHNICAL NOTES

Here is a summary, with some added comments to the full discourse made in March 2002, where Andy deGruchy, President of deGruchy Masonry of Quakertown, PA, revealed his thoughts to a reporter who writes for the Washington Post, regarding lime for mortar. Andy says that he believes that any person who intends on repointing a masonry building that has been built with lime and sand bedding mortar should consider:

1. A Sustainable Builder and Historic Masonry Restoration professional should consider using Natural Hydraulic lime which sets with water and can have up to 50-60% free lime content that improves the self-healing characteristics of lime mortar. When using a prepared lime putty that sets with air through carbonation, and does not set with water, consider a high calcium lime which hovers around 95% pure calcium carbonate. This height of purity makes high calcium lime closer to just the sediment of bones, shells and plant ash. The more magnesium found in a limestone deems it "Dolomitic" and finally high enough concentrations deem it "Magnesian" lime. Dolomitic lime may be more readily available for construction but high calcium lime putty is available. If you are going to use lime putty that sets with air it should be produced in a gas-fired vertical kiln at a temperature not exceeding 2000 degrees Fahrenheit. Lime's that contain reactive clays burnt at high temperatures can form tricalcium silicates and tricalcium aluminates, depending on the chemistry of the original stone. Low temperature firing does not allow tricalcium silicates to form. The composition of the stone may still lead to the formation of tricalcium aluminates. This is one of the concerns in using Argillaceous, or "clayey" limestone for making Natural Hydraulic Lime and Natural Cement. A Pure Siliceous Limestone is one that has reactive silica caught up in the stone without the impurities of clay. When limes have a more gray or ochre color and coarse feel they reveal that impurities and higher burning temperatures may be the reason for this. Knowing the final chemical composition of the lime is very important because an impurity may lead to pre-mature failures of the mortar and adjacent areas to the mortar work in some applications. A producer of lime putty and Natural Hydraulic Lime should publish the chemical composition and burning temperatures of the stone they use. The Natural Hydraulic Lime we currently sell is St. Astier Natural Hydraulic Lime which is a pure siliceous lime. The cost is approximately $35 per 55 lb bag in 2008. When 2-3 parts of sand are added it will yield a goodly amount of mortar. This is relatively cheap since the expense of a project is labor and scaffold and not the mortar. However, the mortar should give a long service life and while Ordinary Portland cement (OPC) mortars are not characterized as a "Green Building Material", they also are not suitable as a gauged-in binder for making new replacement mortar formulas for mortars originally made from pure lime and sand, if the new mortar is meant to last as long as the original mortar. Prepared high calcium lime putty burned at a low temperature may retail around $125 per five gallon pail in 2007. Due to the height of cost for "Hi-Cal" Lime putty, the Pure Natural Hydraulic Limes we import from St. Astier, France become even more attractive to the buyer at $35/bag which will yield approximately 4-Five gallon pails of mortar when sand is added on site. Ask the manufacturer you are planning to buy lime from to tell you what limestone they use and what the burning temperature is.

2. If you need a cheaper alternative in repointing an historic building or for building a sustainable structure be sure to know the limitations of mortars containing any Portland cement in the mix. If a building is subject to movement, a lime mortar will move with the building. Through crystalline bridging, using the free lime content, the lime mortar will autogenously heal those fissures that occur. Portland cement mortars become brittle and do not heal themselves due to their miniscule amount of free-lime remaining as a result of their high burning temperatures. However, even in mixes which have weaker concentrations of Portland cement, (mostly meant to give hydrated lime mortars a synthetically created hydraulic set), do in fact, and in every case contain tricalcium aluminates. This is important because it is possible that tricalcium aluminates can meet with water and the soluble salts present in old buildings to produce a detrimental reaction. This reaction's product is a sulfate attack upon the mortar. If the movement of a building can not be controlled and due to eventual voids in cap stones or fissures that open up on the side of a repointed mortar joint or a crack in stucco that doesn't autogenously heal, then lets water in, a breakdown of the mortar may begin or trapped water in the substrate may occur. The breakdown of mortar containing Portland cement may take as little as three years to start. Repointing and installing stucco containing Portland cement gauged into the "High (Type S hydrated) lime" content mortar may simply be considered a more "temporary" fix when met over friable-type substrates. Temporary meaning 10-35+? years as opposed to the original lime/sand mortars still in service over 100 years throughout the world. Friable meaning "weak, very sandy" type bedding mortars or other friable and punky substrates like strawbale, mud for adobe structures, etc. Once the reaction process starts in mortars containing any Portland cement, they continue and the sulfate attack, also known as the "sulphation of cement" occurs and leads to accelerated failure of the mortar. It also will often create irreversible damage to once sound masonry that swells in freeze-thaw cycles and moves units out of position. So exactly how long a high lime content repointing mortar that had Portland cement gauged in to create a hydraulic set can last varies due to the above aforementioned mix of conditions. Compare the cost of prepared building and pointing mortars, stuccos and paints containing Portland cement with the cost of our imported Natural Hydraulic Lime and make an informed choice of which binder to use.

3. Remember that Hydraulic limes are not the same as what is often referred to as "Builder's Type-S Hydrated lime" used in mortars made to lay brick and concrete blocks, etc. Type-S Hydrated lime is used to create plasticity in mortar and control the setting time and is not expected to be the primary binder for any extreme weather exterior application or for structural building without a pozzolan-type material such as what Portland cement is meant to do, (to some degree), when it is added to Type-S hydrated lime. You can not substitute Type-S hydrate for Natural Hydraulic Lime and think you will get the same performance results. My observation is that as historic preservation and sustainable building remains lucrative, manufactures are compelled to offer some form of "Hydraulic Lime." Ask the manufacturer to tell you what final performance characteristics you can expect from their lime and what they used to create a "Hydraulic Lime" if it is not a Natural Hydraulic Lime.

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   Other Lime Mortar Links:

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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.

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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.