wonderful lime mortars

[Will Shotton <shottonmasonry@zzzzzzzzz>]

Hi All,

I'm still a newcomer to this motley crew of stoneheads and am enjoying the spirited discourse immensely!

I'll always defer to a highly experienced mason although authors such as Ian Cramb are invaluable resources as well. Having said this  (i'm not going out on a limb here)  I feel that most of the tradition and knowledge of masonry has been lost or eroded over the last century. A common occurence in N.America is to call oneself a master mason when only a decade or two of experience is gained. It is simply not enough time to have experienced enough of stone behavior or their characteristics under different sets of conditions to be calling oneself a master. I could only imagine the guffaws emanating from the master masons of old listening to our discussions on stones moving and lime mortar characteristics when all of this was common sense to them, learned and drilled into them through decades (and centuries) of trials and failures.
This repetitive diatribe was sparked by the common belief that the harder the mortar, the better the wall.
In my region of eastern Ontario, Canada we experience seasonal temperature swings from minus 35C to +40C in the summer. You can imagine the havoc this wreaks on masonry structures. Frost penetrates the groud to four or so feet and since ice crystals expand with a force of approx. 22,000 PSI this can move any building that doesn't have a foundation below that level.
Up until fifteen years ago or so it was common practice to use a relatively high Portland content mortar to repoint heritage buildings, a generally accepted practice over the last sixty years. But thankfully no longer! The damage that Portland cement has done to heritage buildings is severe in my climate. I've repointed a number of valuable buildings whose joints were repointed with Portland mortars (at great expense) only a decade or so ago and were still sealed well but all of the hard mortar had to be removed to save the building from the inevitable damage that results from the unforgiving nature of hard cements.
As soon as a small crack appears in Portland mortars it accumulates moisture as was mentioned before by Pat? and starts to deteriorate. The difference here is that the whole process is accelerated by the winter frost damage. One winter will blow out sections of jointing or cause spalling/cracking in the masonry unit because of the force of ice crystals forming. (In Canada we also use a lot of salt to keep the sidewalks clear of ice/snow--the salt mixes with water and through capillary action works its way into the building wall, dries when the summer comes and the salt crystals cause effloresence as well as severe damage to the mortar, stone and bricks)
The brilliance of using a high lime content mortar for heritage applications are its plastic properties previously outlined in this group. One important term that hasn't been mentioned is 'autogenous healing'. This is the process of lime mortars re-sealing hairline cracks by rain or moisture softening the lime to a point where these cracks fill themselves in and reform a bond with the stone. This is all on a small scale but is integral to the longevity of structures that are still used today such as old bridges in the Old World, aquaducts and such.
Just a note--we do use white Portland cement to accelerate the setting of lime mortars and to increase its compression strength but only just enough. It does reduce some of the great lime mortar properties but there isn't enough to cause damage. Unfortunately we don't have the time window masons of old had to work with.

Hope this was interesting for some, non-offensive to others! Sorry for the length and repetition of previously mentioned items. Like a lot of masons I don't like half measures and once I start I find it hard to stop!

Keep on layin' (stones that is),

Will