In the 1930s, as Graymont details, the mortar used in most brickwork in the United States changed from lime-based mortars that had been used for centuries to newer formulations that contained portland cement. Masons favored the newer mortars, because they dried much faster than lime-based ones, letting masons finish more work in a single day. Portland cement-based mortars, which are still used today, aren't perfect for every mason job, though. When used to repair pre-1930s homes, they can damage old bricks. If you own a pre-1930s brick home, here's why you should only have it restored with lime-based mortars.
Brick Houses Need to Expand and Contract
To the naked eye, there isn't any sign that brick-and-mortar houses move. As temperatures and humidity levels change, though, bricks expand and contract. The change in size may seem miniscule, but bricks must have the freedom to grow and shrink. If they don't, they will crumble.
Portland Cement-Based Mortars Don't Let Bricks Flex
Mortars made with portland cement are much stronger than bricks that were made before 1930 are. As bricks expand, portland cement mortars don't give way.
Thus, if a portland cement mortar is used to restore an old brick house, the bricks will have nowhere to expand. Instead, they'll just push against the mortar, and the edges of the bricks will grind against the mortar. Every time the bricks try to expand, they'll be in a battle against the stronger mortar -- a battle they'll lose.
The bricks' attempted expansion won't be visible, but the damaged caused by the portland cement mortar will be. Over time, pieces of brick will begin flaking off and the edges will appear worn.
Lime-Based Mortars Let Bricks Flex
Lime-based mortars, in contrast, are softer than bricks -- even old bricks. Stones may not typically be referred to as "soft," but lime-based mortars, which are made with limestone, are relatively soft. Pieces of limestone, after all, can be broken apart by hand.
When bricks expand, lime-based mortars give way to the bricks. They don't push back against the bricks like portland cement-based mortars do, but they work with the bricks -- contracting when bricks expand and expanding when bricks contract.
Water Trapped in Brick Walls Can Damage Bricks
Water that seeps into brick walls will cause damage if the water's trapped in the wall and freezes. Because ice expands when it freezes, pockets of water will push against bricks and break the bricks apart, just as water can break apart sidewalks, roads and other structures if it's inside them and freezes.
Portland Cement-Based Mortars Are Impervious
Portland cement-based mortars are watertight. Counterintuitively, this actually isn't a good thing. While watertight mortars prevent a lot of water from seeping into a brick wall, all brick walls have joints and some water will get in through them. If a wall is restored with a mortar that's impervious to water, the water that comes in through joints won't have any way to leak back out of the wall. It will be trapped -- and in winter it may freeze.
Lime-Based Mortars Are Porous
Lime-based mortars are porous. When a wall is restored with lime-based mortar, any water that sneaks in through joints will be able to seep back out through the mortar. Water won't get trapped in the wall for extended periods of time, so it won't freeze and break apart the bricks in winter.
If you have a pre-1930s brick house that needs to be repaired, ask the masonry restoration company you're hiring to use a lime-based mortar. They may charge you a little more, because the job will take longer than it would to do with a portland cement-based mortar. The additional cost is an investment in your house's future, though, and it will keep you from needing to make more repairs in a few years. If you're looking for a masonry restoration company, check out a site like http://www.mararestoration.com.