Lime and portland cement are both what we call the “binder” in mortar. The binder is what holds the sand together and makes it last a long time.
This is confusing because most portland cement mortar has lime in it (“mason’s lime”) to make it more workable and sticky, but the lime that’s used for this is totally different than the high-calcium lime that can be used alone as the binder in a mortar. If mason’s lime is used in mortar alone as the binder, it will not get hard and the mortar will wash away after a very short time.
So to clarify: when we say lime mortar, we’re talking about the kind of lime that can be used alone as the binder in a mortar. In other words it gets hard by itself without anything else added. This is what has been used predominately throughout history as mortar: lime and sand.
There are several properties of portland cement that make it an inferior choice for historic homes.
- Portland cement mortar is very brittle, and so it cracks easily with tiny building movements. (Small movements in a building are very common.) These tiny cracks suck water in very rapidly when exposed to rain.
- Portland cement does not let moisture escape, so the mortar and the bricks (or stones) are constantly damp next to the ground.
- Portland cement has compounds within it that turn to salt over time. These salts deteriorate the wall from the inside out. Salt is very damaging to masonry. Salt dissolves into the moisture in a wall, but then re-crystallizes, expanding like ice to break apart the wall from the inside.
Therefore, despite being harder, portland mortar has a shorter life expectancy than a high-calcium lime mortar, such as what we use from Lancaster Lime Works. Worse, it destroys the masonry that it is intended to support!
By contrast, lime mortar is always softer, more flexible, and more moisture-friendly than the masonry around it. It cushions, protects, and releases water from the walls where it is used.
Used for thousands of years, lime mortar is still the best choice for building and maintaining structural masonry homes and buildings!