Tuesday, November 4, 2014

Is it wrong to be using Portland cement in an historic mix?

We tend to hear that it is very important to use a lime-only mortar on old brick homes.
But so many properties get repaired with a masonry cement mix that I’d say the message has definitely not been getting across.
And as the result in these cases is an incongruent mortar repair, the loss of heritage character — in other words, the aesthetics — in my opinion, is an even bigger issue than the potential impairment of the structural integrity.
First of all let’s be clear that masonry cement is not the same thing as Portland cement.
Portland cement” is a man-made product which is manufactured from limestone and clay and subjected to high temperatures in the firing process which causes it to harden under water and become very dense and hard once set, unlike lime, which remains soft and porous. For our purposes, it is used in addition to lime in a given mix, and not by itself. Sand has to be added.
Masonry cement” is a preblended mortar mix commonly used in modern masonry construction. It contains a large proportion of portland cement, as well as ground limestone and other chemicals to make the mix easier to work with. It is not required to contain lime, and for this very reason seldom does. It is used by itself, with only the addition of sand. This product should not, in my best professional opinion be used on an old structure, typically pre-1940.

To go back to my contention about purism, it is very difficult to be working with a lime mortar in the same, exact way it used to be done back in the day, here in Canada — assuming you'd be doing it largely for sentimental reasons. If you do it for structural reasons, it is fine to use a lime-only mortar, but understand that it requires much more care (which translates to added costs) in order to ensure that the replacement mortar will not turn to powder after the first winter.
If you’re in doubt, particularly if the building has historic import, by all means, be on the safe side and use a lime-only mix (which doesn't have to be a pure lime mix, as will be explained below).
But if the budget is simply not there, I would rather see a lime-cement-sand mix used that matches the original mix exactly in appearance, rather than a masonry cement mix which makes the repair look like an abortion.
In a lime-cement-sand mix (which is also referred to as a “gauged” mortar), lime is used, typically in a ratio of two parts lime to one part Portland cement (not masonry cement) — although there are other possible variations.
This type of mix does not require so much preparation and care in the application, and has been recognized as appropriate for use in historical buildings.
For example, the Toronto Preservation Services website provides the following link:
“U.S. Department of the Interior, National Park Service, Technical Preservation Services Preservation Briefs, http://www.nps.gov/tps/".
This document endorses the use of a lime-cement-sand mix to restore old brick structures.
They show the link on this page of their site.
(Furthermore, the picture on the right side of this page on their site features one of the projects that we have performed through their Grant Program.)
Much of Cabbagetown and other enclaves of Victorian homes in Toronto would benefit from a lime-only mix, as that constitutes what has been used in their construction. But a gauged mix is still much preferable to the all-too-often used masonry cement mix.
Again, I’d argue that the quality of the visual match of the replacement mortar is the more important issue to weight in the balance. And that element can be achieved by a skilled applicator with a lime-cement-sand mix, which is not the case, from what I’ve seen, with a masonry cement mix. And keep in mind that many old neighborhoods in Toronto were built using a gauged mix in the first place, such as most of the Annex, for example.
When you see an Edwardian, or Post-Edwardian building in Toronto, it is most likely a fact that the mortar that was used in its construction contains a portion of Portland cement.
Back in 2004, I have spent much time in the archives section of the Toronto Public library, reading masonry trade publications of the 1880’s, up to about 1930, and plotting the dates of various articles and commercial ads found inside against a timeline, which conclusively corroborates the fact that Portland cement was already in use at the time of the construction of many of Toronto’s neighborhoods, such as the Annex, Parkdale, High Park, parts of Rosedale, etc., as mentioned above. The ads in one of the publications even list the addresses of some of the locations in Toronto which were selling the product at that very time.
Regarding the issue with the use of lime, there are two types of lime essentially, a pure calcium lime (sometimes with magnesium too), and an hydraulic lime, which contains clay as an impurity imparting it with a quick set, similar to a cement. The pure lime makes a relatively soft mortar and requires a huge amount of preparation. Both types of lime risk to fail unless a rigorous regimen is followed, for which, at my company, Invisible Tuckpointing Ltd., we charge extra, when a particular building requires their use (for example, we repaired one of the last few Georgian house to remain in existence in the city and we used a lime-only mortar in its restoration).
It happens that a lot of the information that is currently in circulation in Canada about heritage work comes from England. A prominent British expert of the restoration trade is advocating the use of hydraulic lime in restoration, even where cement + lime had been used originally, as he explains that modern Portland cement is much harder that it was over 100 years ago. But hydraulic lime wasn’t used in North America at the time when England switched from a pure calcium lime to hydraulic lime. It was never used here. Instead, the United States and Canada used Portland cement with lime (and even without!) at that time, which was not only manufactured locally, but also imported, as the German version was known to be superior. As well, a natural form of Portland cement was in widespread use, called Rosendale Cement, due to the presence of a large deposit in upstate New York.
After having weighed the pros and the cons, and after having debated many options, such as using Rosendale cement (which is rare and expensive), I do not feel that hydraulic lime always is a better choice, for the reasons given above, as it still requires much added work to ensure the work will last, and many residential homeowners will not want to pay the added cost, instead resorting to hiring lower-priced contractors who will have no qualms about using a masonry cement mix.
The properties of lime make it quite tricky to use in as the only binder, in addition to sand.
The following was taken from an article I wrote close to 10 years ago that was featured in Masonry Magazine:
Portland cement, the chief ingredient in today’s masonry mortars, sets by adding water, which is called a hydraulic action. Using a quite different process, lime sets over time through a reaction with air, called the carbonation process.
Limestone, chalk and seashells are essentially calcium — calcium carbonate to be precise. Lime is derived from heating limestone (or chalk or sea shells) at high temperatures, which drives off the carbon dioxide that is part of its chemical composition and turns the limestone into quicklime, or calcium oxide.
When water is added to the quicklime, it generates heat, called an exothermic reaction. In the trade, we call this process “slaking” the lime. If the lime is made into a powder at this stage, it will become the lime you may have seen sold at masonry supply stores, called hydrated lime. However, if you add excess water during the slaking process, the lime will turn into putty. (To some confusion, hydrated lime and lime putty are chemically almost the same, and both are referred to as “calcium hydroxide.”)
The longer lime is left as putty, the more workable it will be in the end. Over time, the lime putty undergoes a process whereby the lime crystals reduce in size upon aging and transform into small plates that slide when rubbing against each other. This action allows more water to be absorbed, giving lime the property of being highly workable, or having “plasticity.”
While Portland cement hardens due to the chemical reaction with the water, if stored in a sealed container, lime putty will remain workable and soft indefinitely. Lime will harden over time only when it comes in contact with carbon dioxide and moisture.
Once a lime-sand mortar is mixed and placed in the wall, the water will begin to absorb or evaporate and the mortar will appear to set after several hours. However, the lime then begins to react with the carbon dioxide present in the air and harden, a process that actually will take years to complete. A simplified explanation is that, chemically, the lime is returning to its former state of being a stone.
A more complex explanation is that the carbon dioxide and moisture convert the lime on the surface of the joint back to calcium carbonate. The open pore structure of the lime-based mortar allows the carbonation process to take place deeper in the joint over time.
When tackling a job that requires a straight lime-sand mixture, some experience — at the very least, some caution — is preferred. If the correct application method is not followed, the mortar could soon fail, literally turning to powder.
First, the wall must be kept damp for several days after the lime-sand mortar is applied so that the carbonation process has the necessary time to start before the wall is allowed to dry out. As mentioned earlier, lime mortars can appear to have set. If you are tricked by this and allow the mortar to dry too quickly, it will not have fully started its reaction with carbon dioxide present in the air. With a lack of moisture causing an abbreviated carbonation process, the lime mortar will be so weak that harsh winter conditions could be sufficient enough to make it fall out as early as within the first year.”
And the same caveat applies to hydraulic lime as well.
In summary:
  • -the more historic significance is assigned to a particular heritage building, whether designated or not, the more care should be taken to ensure that it be properly looked after. And by corollary, this means a higher investment.
  • -the last thing it should be repaired with is masonry cement (which typically says Type “N” or Type “S” on the bag).
  • -beware if someone offers to use a lime-only mortar on a low budget, as chances are they will not be willing to put up burlaps and keep the wall moist every few hours for 5 days, for example, and without such care the work will be in real danger of failing after the first winter.
  • -you’d be better off, in my opinion and experience, if there is no real budget for an historic restoration-type of specification, to opt for a lime-Portland-sand mix which is soft enough to not damage the bricks (usually Type “O”, or even Type “K”), and which has been applied so skillfully that it matches invisibly with the original.
  • Below are two relevant ads, as well as one article taken from two different 19th Century trade publications from 1892 and 1895:

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