LIME MORTAR, LIME PUTTY, MASON FURNISHES TECHNICAL NOTES (5/13/02)

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. Using St. Astier Natural Hydraulic lime, which will have up to 50-60% free lime content. He is so happy to have found this lime because of its performance characteristics that he now sells it from PA to Maine and west for a total of 16 states. When using a prepared lime putty 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 "Magnesium" lime. Dolomitic lime may be more readily available for construction but high calcium lime putty is easily attainable for your repointing projects if you request it from www.valimeworks.com. St. Astier Natural Hydraulic Lime costs approximately $26 per 55 lb bag in 2002, of which 2-3 parts of sand are added yielding a goodly amount of mortar, which is relatively cheap. Prepared high calcium lime putty may cost around $100 per five gallon pail. (but Andy vouches that VA Limeworks' lime putty is great for getting that glass smooth surface for interior plaster work.) Ask the manufacturer you are planning to buy lime from for use in a mortar to tell you what limestone they use and why.

2. If you decide you are going to use high calcium lime putty for plastering or repointing, request from the manufacturer some written documentation that their lime was burned at low temperatures hovering around 1650 degrees Fahrenheit. If calcium carbonate is burned at 2000+ degrees Fahrenheit or higher there just may be a loss in the reactive properties of the lime. This may effect the performance of the material intended to be used as a lime putty for a durable and long lasting repair when plastering and repointing. P.S. Andy adds here that limes 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, so knowing the final chemical composition of the lime is very important at all times. A producer of lime putty and Natural Hydraulic Lime should publish the chemical composition and burning temperatures of the stone they use up front.

3. If you need a cheaper alternative in repointing a historic building just be sure to know the limitations of mortars containing any Portland cement in the mix. If a building is subject to movement, a mortar containing one part St. Astier Natural Hydraulic Lime, which is low temperature fired, when combined with 2-3 parts sharp, well graded sand will, as time goes by, yield with the movement of the building. Through crystalline bridging, using the free lime content, the mortar will autogenously heal those fissures that occur more so than mortars that become rigid due to the increase of Portland cement in their mix. However, even in mixes which have weaker concentrations of Portland cement, (mostly meant to give hydrated lime mortars a 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 byproduct results in 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 that doesn't autogenously heal, and then lets water in, a breakdown of the pointing mortar may begin when this reaction starts. This breakdown may take as little as three years to start. Repointing with mortars containing portland cement gauged into a the high (hydrated) lime content mortar may simply be considered a more "temporary" fix, (temporary meaning 10-35+? years as opposed to the original lime/sand mortars still in service much over 100 years). Once the reaction process starts in mortars containing any portland cement, they continue and the sulfate attack, or sulphation of cement, leads to accelerated failure of the mortar and often creates irreversible damage to once sound masonry. 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 pointing mortars containing portland cement with St. Astier Natural Hydraulic Lime and make an informed choice of which binder to use.

4. Remember that Hydraulic limes are not the same as what is often referred to as "builder's Hydrated lime" used in mortars made to lay brick and concrete blocks, etc. You can not substitute one for the other in repointing and think you will get the same performance results. You may like to know that even the way limes are made to become hydraulic can vary so ask the manufacturer of a hydraulic lime what they are doing to make the lime hydraulic, (so that it sets with the addition of water). and what final performance characteristics you can expect from their 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.