Why Mortar Composition Is the Starting Point
The mortar in a nineteenth-century brick building was not an afterthought. It was selected—or mixed from available materials—to balance adhesion, workability, and long-term compatibility with the specific brick being used. In most Canadian commercial construction before roughly 1910, that mortar contained little or no Portland cement. The dominant binder was air-lime putty or natural hydraulic lime, combined with local sand.
When a repair mortar is harder, denser, or less permeable than the original, stress that would otherwise be absorbed by the joint is transferred into the brick face. The result, over several freeze-thaw cycles, is spalling—the characteristic sheet-like delamination of the outer brick surface that indicates an incompatible repair. Identifying the original mortar formulation is therefore not merely an academic exercise; it is the prerequisite for specifying a repair that will not cause progressive damage.
Field Assessment: What to Look For
Before sending samples to a laboratory, a walk-around assessment of the façade provides useful context. Key observations:
- Joint colour and texture: Original lime mortars tend toward cream, buff, or pale grey tones with a slightly rough or pitted surface. Portland cement repointing from the mid-twentieth century is typically darker, more uniform in colour, and harder in appearance.
- Joint profile: Historic joints in commercial buildings were commonly flush-finished or slightly raked. A pronounced concave (rodded) profile often indicates later work.
- Condition of existing repointing: Areas where Portland cement repointing has been applied can often be identified by adjacent brick spalling, hairline cracks along the mortar-brick interface, or staining.
- Efflorescence: White salt deposits on the surface can indicate moisture pathways and may reveal areas where the original mortar system has been disrupted.
Sample Collection
Laboratory analysis requires intact mortar samples drawn from joints that have not been repointed. The preferred sampling approach is to use a carbide-tipped drill to remove a sample from the centre of a horizontal bed joint in an inconspicuous location—typically below a window sill or behind a downspout bracket. Sample depth should reach at least 25 mm (1 inch) to avoid surface contamination from weathering or prior application.
Collect samples from at least three locations on the façade, noting orientation (north, south, etc.) and height, as later additions or repairs may be present on certain elevations.
Parks Canada's Standards and Guidelines recommend petrographic analysis by a qualified petrographer as the most reliable method for characterising historic mortars, particularly when the original binder type is uncertain.
Laboratory Analysis Methods
Two principal methods are used to characterise historic mortars:
| Method | What It Reveals | Typical Application |
|---|---|---|
| Petrographic analysis (thin section) | Binder type, aggregate grading, porosity, weathering profile | Definitive characterisation of historic mortars |
| Acid dissolution (ASTM C1324) | Approximate binder-to-aggregate ratio | Screening for mix proportions when petrography is unavailable |
| X-ray diffraction (XRD) | Mineralogical composition of binder and aggregate | Distinguishing natural hydraulic lime from Portland cement |
| Scanning electron microscopy (SEM) | Microstructure and interface bond | Research-level characterisation; rarely required for routine repair |
Selecting a Compatible Repair Mortar
Once the original mortar composition is known, a compatible repair mix can be specified. The general principle, drawn from Parks Canada's guidance and widely followed by heritage practitioners, is that the repair mortar should be:
- Weaker in compressive strength than the brick unit (not the original mortar)
- Equal to or weaker than the original mortar in compressive strength
- Comparable in water absorption and vapour permeability
- Matched in colour and texture to the original joints
For many pre-1910 buildings with soft handmade or wire-cut brick, ASTM C270 Type O mortar (1 part Portland cement : 2 parts lime : 9 parts sand, by volume) is cited as a starting point. For buildings predating the widespread adoption of Portland cement—roughly pre-1895 in most Canadian urban centres—a natural hydraulic lime or lime putty mortar without Portland cement may be more appropriate. Laboratory-tested proprietary lime mortars are available from several Canadian and imported suppliers.
Sand Selection
Aggregate matching is frequently underestimated. The colour and texture of a repaired joint is determined primarily by the sand, not the binder. Original mortars in Ontario often used local glaciofluvial sand with a characteristic yellowish tone; Quebec buildings of the same era may show grey or white aggregate derived from carbonate sources. Matching the original sand type requires either sourcing from the same geological formation or blending aggregates to approximate the original gradation and colour.
References
- Parks Canada. Standards and Guidelines for the Conservation of Historic Places in Canada, 2nd ed. Ottawa, 2010.
- ASTM International. C270 Standard Specification for Mortar for Unit Masonry.
- ASTM International. C1324 Standard Test Method for Examination and Analysis of Hardened Masonry Mortar.
- National Park Service (US). Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings. (Referenced for comparative methodology.)
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