I alluded to researching the right mortar mix for the masonry repair work in the last post. Knowing a thing or two about concrete, I went into the research with great confidence, which eroded very quickly.
I was ready to run into the nearest home improvement store and get bags of a pre-blended mortar mix and start work. Well, those mixes are meant for contemporary construction and materials.
Our house is over 100 years old and so is the material used in the masonry assembly. While evaluating the state of our masonry structures with Martin Bazula, a restoration mason, I learned of the importance to match the mortar type for any repairs to the original mortar mix.
Mortar is used to bind the masonry units (relatively soft common brick in our case) into a single element. The flexural bond strength of the mortar must correspond to the strength of the masonry unit.
“Mortar should be flexible enough to absorb movement due to temperature change, settlement or vibration. Lime mortars provide this flexibility. In contrast, when cement is the sole binder used in a mortar, the masonry is forced to absorb any movement and becomes subject to erosion, spalling or cracking.”
Most pre-blended mortar mixes from the home improvement store would have been cement based with high flexural bond strength and too hard for our soft common brick.
Mortar types are organized into four categories (type M, S, N and O) to respond to the variety of different masonry units available. The compressive strength of the mortar decreases from type M to O and is determined by the ratios of materials mixed together.
The key ingredients are Portland cement (Type I), hydrated lime (Type S), sand (coarse, medium or fine) and water. The greater the ratio of Portland cement, the greater the compressive strength (type M and S). The greater the ratio of lime and sand, the softer the mortar (type N and O).
That is all very good, but what type of mortar do I need to match the original? There are a couple of crude ways to investigate.
Take a piece of the original mortar and drop it onto a concrete or stone floor. If it makes a “ping” noise upon impact, it is likely cement based mortar. If the noise is more like a muffled thump, it is probably the softer lime based mortar.
Taking a close look at the original mortar also helps. A gray colored mortar is indicative of more modern, cementous based mixes. Small white speckles or a white dust that is picked up on touch is typical for lime based mortars.
Because these methods are rather crude, they still don’t help me determining what type of mortar I need for our repairs. This is where the expertise of a restoration mason (Martin Bazula) becomes handy.
His opinion was that the original mortar is close to the type O (one part Portland cement, two parts lime and nine parts medium sand). Further research and reading corroborated his judgment as well as the mixing of a test batch.
- Restoring mortar joints in historic buildings
- Mortars for Brickwork – Selection and Quality Assurance
You can contact Martin Bazula at [email protected]