Happy Birthday Randy Ruth, and FAQ Friday!

Send Randy some love and give him a birthday shout out when you repost his latest blog entry on some of the differences between repointing with Natural Hydraulic Lime (NHL) mortars vs. cement based mortars.

Q: Is pointing with lime different from pointing with cement? I’ve done work with cement but I want to know what the differences are for application and general use for repointing my old house?

A: When you refer to pointing, I am thinking you are referring to repointing mortar joints. As with any masonry project, proper attention to preparation of the substrate is critical. Always follow proper preparation guidelines regardless of the mortar being used.

If you compare the application process of repointing mortar joints with lime-based mortars to cement based mortars, the only real difference is workability.

Lime mortars tend to have better workability than their cement counterparts do. This is because lime is used to add plasticity to modern cement mortars and thus, when you omit the cement you have the greatest workability. However not all lime mortars can be treated equally when it comes to their aftercare during the initial curing process. Like cement-based mortars, there are different grades of lime for different applications, with different characteristics.

Although there are many different types of limes used around the world, I will only address the four most common from softest and slowest setting to hardest and quickest setting. starting with High calcium lime putty, (Natural Hydraulic Limes)NHL 2, NHL 3.5 and NHL 5.

High calcium lime putty is the softest and slowest setting of the lime choices. Proper attention to curing procedures must be adhered to allow it to set properly, this may take six weeks. Lime putty based mortar has its place in the world, where trained professionals should be the applicators to ensure work is executed in way were the margin for error is limited.

Different grades of Natural Hydraulic Lime are followed by a number designation that indicates the minimum compressive strength at 28 days with a particular amount of sand in Newton’s per millimeter squared. The reason for this classification is that there are no Natural Hydraulic Limes currently produced in the United States and must be imported from where the metric system is used and there have been established standard for a number of years on NHL’s, primarily in Europe.

As said before, the lower the number designation for NHL’s the slower setting and softer that type is. So, what does all this mean to you the mason or adventurous DIY homeowner?

Well if you have a conservator mindset then matching the new mortar as closely as possible to the old mortar in color, texture and physical attributes is the end goal. You might want to consult a professional for general advice on what type of mortar to use if you are unsure, after conducting your own research.

In general, for most of the United States NHL 3.5 mixed with local sharp well graded sand, which should meet ASTM C-144 or a pre-blended NHL 3.5 and sand mix can be used for general repointing work of older brick and stone structures. This is because NHL 3.5 has an acceptable initial setting time and more importantly provides good vapor transfer in a wall. This allows repointing work to move along at an acceptable speed while knowing that in most cases moisture is not being trapped in the wall cavity.

Lastly, the difference between cement mortar and lime mortars for repointing is aftercare. Even though cement based mortar should be damp cured it is not always practiced and is not typically the same length of time when dealing with lime.

While working with NHL mortars it is important to allow the mortar to slow cure with high humidity or by misting with water, keeping the recently completed work damp. Although the length of time for aftercare curing of mortar will vary in direct proportion to the particular grade of lime used. Generally, 2-4 days of slow damp curing with either damp burlap misting for repointing work is acceptable. In some cases however, this curing period should be extended.

All about Lime

This is an excerpt from The Stonemason’s Gospel according to Ian Cramb written by Andy deGruchy, an explanation of lime.

Lime

Limestone, the material calcium carbonate, has never changed from its beginning up to and including now. There have always been two classifications, ‘pure’ and ‘impure.’ Today it is classified as pure (high calcium) and two levels of impure lime based on the magnesium content, Dolomitic and Magnesian.

What has changed over the course of time, especially in more recent years, is how the calcium carbonate stones have been prepared by firing them in the kiln to produce quicklime. There is a most simple way of burning limestone in a vertical kiln using wood for fuel and keeping the temperature between 1650F and 2000F and then cooking it slowly over a few days. This has been done for centuries.  The proof that this method of cooking the stone has extreme merit is evidenced by the very old buildings throughout the world which still stand that utilized this method of preparing lime. ‘Lime’ is what limestone is called when it is cooked and slaked to make a putty that is incorporated into making building mortars, plasters and paints. The technical chemistry was unknown to old lime burners and masons. They just knew what worked and kept using the time-honored methods of preparing the lime.

When burned limestone has water reintroduced to it, called slaking, it then blooms into a beautiful white putty-like material. The volume of putty produced is double that of what was once the condensed rock. This ‘lime putty’ will draw carbon dioxide out of the air for a very very long time and slowly convert back closely to a limestone again. Lime putty has its initial set over a six week period by exposure to air. However it will attract carbon dioxide almost to a point of being completely ‘carbon neutral’ over time in regard to the embodied energy first required to produce the lime.  Through lime’s interconnected pores it even knits minor fissures together by moving about some of the not fully burned ‘free lime’ which creates more surface area to draw in the carbon dioxide.

Early masons knew that some limestone deposits produced limes that set quicker and became harder sooner. So, unlike simple air-setting lime putty, hydraulic limes were used throughout the world and in the United States to build with when the impure raw material had reactive silica or certain clays naturally found in the stone. These impurities were cooked along with the calcium carbonate stone. The term ‘hydraulic’ means to set with water and under water. Portland cement is hydraulic lime. The reason it is overall strongly suggested not to be used for masonry building conservation is that the synthetically added materials used to make Portland cement become intensely hydraulic also make the whole lot detrimental by various degrees of incompatibility with original porous building components. Two of those detrimental characteristics are that Portland cement is brittle and does not accommodate movement and secondly it reacts with sulfates. But a great incompatibility and detriment to historic masonry buildings is the increased densification of mortar that consequently occurs with every increment of additional Portland cement added to make the mortar become very hard. Densification does not allow the building to remain ‘breathable’ through the mortar joints but instead allows water to become held back and sometimes trapped into absorptive inner bedding joints. This phenomenon forces the wetting and drying cycles of the building to occur through the porous historic units and this is what greatly contributes to accelerated deterioration of the irreplaceable bricks and stone used to originally build a building.

In Ian’s first book he used and suggested mortar mixes that I and every other mason has typically used. These mixes gauge-in some Portland cement into high-lime (Type S lime) containing mortars. The reason we all did this is because readily available Type S Hydrated Builder’s Lime and cement were what we had to work with prior to the commercial availability of natural hydraulic limes now sold in the US. If Type S lime was blended with sand alone we discovered it would not hold up to the freeze-thaw cycles in northern climates. Why this occurs when nothing has changed about the limestone itself puts the spotlight on the cooking procedures. Too hot and too fast of a burn can cause the limestone to become ‘dead-burned’ and loose its ‘reactive’ nature which allows it to closely convert back to a hard and durable limestone again. A durable mortar made from reactive lime which maintains vapor permeable pores and has a desired malleable nature to accommodate minor building movement is the best for vertical, above grade work. Pure air- setting limes that remain reactive because they are burned at a low temperature can be obtained in the US too. However, due to the six week set time the cost for building with these limes goes up exponentially. So in this book the mortar mixes are more clearly defined from Ian’s first book as being mixes that use a binder of hydraulic lime but not the hydraulic lime that is Portland cement. I hope this helps you in designing appropriate mortar mixes for certain corresponding applications. It is a labor of love and worth understanding in order to realize the greatest long-term service life which can be obtained for repairing a vintage building and its components. I hope my contribution of this knowledge into what makes one lime better than another brings about a higher degree of excellence in the historic building conservation work you endeavor to do.

Sincerely,

Andrew deGruchy