Masonry spalling is a type of deterioration in brick or stone where the surface material breaks away, often in layers or chunks, leaving behind a rough, uneven surface. It’s most common in brickwork and is usually caused by water damage and the freeze-thaw cycle.
When replacing brick and repointing mortar joints, the new mortar should have similar physical properties to the existing mortar to ensure proper performance of both the new and existing brickwork. Using the wrong mortar may cause spalling and cracking in the existing brick masonry. Matching the existing mortar color is also important to blend the repaired area with the surrounding masonry.
Testing (IMPORTANT): Few will find the need to have a laboratory test the composition of the mortar. However, for a “layman’s method” to determine the correct mortar type, use the following tips.
The Scratch Test (or Fingernail Test): Try to scratch the existing mortar with your fingernail or a small, non-marring tool (like a copper penny or a house key).
- If it’s very soft and crumbles easily, it’s likely a high-lime mortar (Type O or K), common in older homes. These mortars were designed to be softer than the brick to allow for movement and prevent damage to the brick units.
- If it’s somewhat soft but still offers resistance, it might be a Type N mortar, a general-purpose mortar.
- If it’s very hard and difficult to scratch, it’s likely a high-cement mortar (Type M or S), typically found in newer construction or where high strength is required. This is often the wrong type of mortar for older, softer bricks.
The Acetic Acid (Vinegar) Test: Take a small, loose piece of the old mortar (if available, otherwise carefully chip a tiny, inconspicuous amount). Place it in a small dish and add a few drops of white vinegar (acetic acid).
- If it fizzes or bubbles significantly, it indicates a high lime content (calcium carbonate reacts with the acid). The more vigorous the fizz, the more lime is present. This points towards a softer, historic lime mortar.
- If there’s little to no reaction, it suggests a higher Portland cement content.
Generally speaking, the mortar should ideally be softer than the brick. Feel both the brick and the mortar. If the mortar feels much harder than the brick, it’s likely too strong and could be contributing to spalling. While not a direct test, understanding the age of the structure provides a strong clue.
- Pre-1930s (roughly): Most homes built before this period used primarily soft lime-based mortars. Portland cement started becoming more common later.
- 1930s-1950s: Blends of lime and early Portland cement became more common and harder.
- Post-1950s: Portland cement became dominant, often with a smaller lime content for workability.
Aim for softer, not stronger mortar, and when in doubt, especially with older brick, it’s generally safer to err on the side of slightly softer than the original, rather than harder.
A too-hard mortar can cause more spalling of the softer brick units. Learn more about brick maintenance and repair HERE.