Long-lasting mortar starts with getting the mix right
Q: It appears I wasted some money five years ago. I had my brick building repointed with new mortar, and it has deteriorated quickly. Do you have any idea what caused that? The building is about 500 yards from the sea and is exposed to salt spray and fog. — Ian L., Brighton, England
A: It’s a shame when a repair that was supposed to last 100 years fails in less than five.
Let’s start by talking about mortar. Most mortars are a mix of sand, a dry powder that contains Portland cement and/or hydrated lime, and water.
When the water is added, small crystals start to grow, a chemical reaction called hydration. Mortar is such a great building material because these crystals interlock with microscopic cracks in bricks, creating a solid wall if mixed and placed correctly.
Here’s a partial list of what might have caused your failure:
■ Too much water was used to mix the mortar.
■ Too little mortar mix or Portland cement was used.
■ The mason re-tempered the mortar by adding more water.
■ Rain pelted the fresh mortar before it had a chance to harden.
■ Hot and windy weather sucked the water from the fresh mortar too quickly.
I’m not at all worried about how close your building is to the sea. Rocky limestone outcroppings get pounded by salt spray and they don’t crumble easily. I suggest we make a mortar mix that resembles tough limestone.
Buy some bags of hydrated lime. It’s a cheap material, and it’s been used for thousands of years to make mortars that have stood the test of time.
You can mix the hydrated lime with clean sand that matches what’s in your existing mortar and some volcanic ash that’s high in silica. I’d mix 1.5 parts sand and 0.5 part volcanic ash to 1 part hydrated lime.
Only mix as much mortar as can be applied in an hour. You never want to add more water if the mortar starts to get hard; it fractures the invisible crystals that have formed.
To get the best results, chisel or grind out some of the old mortar between the bricks. (Wear a mask in case your existing mortar contains silica.) It would be ideal if the new mortar is applied to a depth of at least threeeighths of an inch.
Next, rinse off the brick and mortar joints to remove any and all dust.
You’ll want to start replacing the mortar on a cool, dry day.
Slightly dampen the old mortar joints and brick first. This water will prevent suction from drawing too much water from the fresh mortar into the old mortar and brick. You need the new mortar to cure and harden as slow as possible so it attains full strength.
Use a curved mortar joint tool to create a slightly concave profile to the new mortar. You often need to tool the mortar within 10 minutes, before it gets too hard.