Ask the Builder: Why brick mortar repairs fail – and how to do the job right

DEAR TIM: 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 it to happen? The building is close to the sea, about 500 yards, and is exposed to a fair degree of salt spray and fog. What’s the best brick mortar repair mix to use, and how would you apply it? – Ian L., Brighton, England

DEAR IAN: It’s always a sad day when I hear stories like this. Money is a precious resource for most people, and it’s a shame when a repair that was supposed to last 100 or more years fails in less than five.

Let’s discuss what most mortars are made of. Most mortars are a mix of water, fine or medium sand and a dry powder that almost always contains Portland cement and/or hydrated lime. In rare instances a mason may add some sort of liquid bonding agent.

When water is added to Portland cement, some other bagged mortar mix or hydrated lime, it starts an irreversible chemical reaction called hydration. Small crystals start to grow as the dry powder mixes with the water. The crystal growth is what causes the mortar mix to transform from a plastic mixture that resembles applesauce into an artificial rock once it’s fully hardened.

The crystals that grow interlock with microscopic cracks and holes in the brick. This is why the mortar is such a great building material; it cements one brick to another, creating a solid wall if mixed and placed correctly.

Here’s a partial list of errors that might have caused your failure:

Too much water was used to mix the mortar.

Too little mortar mix or Portland cement was used to make the mortar.

The mason re-tempered the mortar by adding additional water to it.

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.

Any one of these errors, or a combination of them, could have created your failure. Without doing expensive laboratory testing, it’s hard to say what the actual cause is. Let’s focus on how to do the job right.

I’m not at all worried about how close your building is to the sea. You’ve got rocky limestone outcroppings that are pounded by the salt spray and they don’t crumble easily. I suggest we make a mortar mix that resembles tough limestone.

There are ancient buildings across Britain that are made using a mortar mix that’s lasted for hundreds of years. Sadly some masons have forgotten about this super-strong mortar because it’s easier to buy bags of mortar mix sold at building supply stores. The masons of old used just hydrated lime and sand. Once mixed with water, the hydrated lime and fine sand create actual limestone. You know how durable this stone is, so your new mortar will be exceedingly strong.

I recommend that you 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 will match your existing mortar. The proportions can vary, but I’d recommend you mix two parts clean sand to one part hydrated lime.

To create the best mortar mix, add some volcanic ash that’s high in silica content to the mix. I’d mix 1.5 parts sand. 0.5 parts 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 to a mortar mix if the mortar starts to get hard. This is called re-tempering the mortar, and it fractures the invisible crystals that have formed. When these crystals are broken, they don’t always grow back.

The fresh mortar should have a consistency that resembles a stiff applesauce, not a runny one where liquid collects on the plate. If you add too much water to fresh mortar you simply dilute the amount of lime or cement that’s required to hold the sand together.

To get the best results, chisel or grind out some of the old mortar in between the brick. It would be ideal if the new mortar is applied to a depth of at least 3/8 inch or about 9 mm in the joint. It’s a mind-numbing task to remove the existing mortar. Wear a mask in case your existing mortar contains silica.

Once you’ve removed the mortar, you want to rinse off the brick and squirt out the mortar joints to remove any and all dust. Dust will interfere with the growth of the lime crystals as the new mortar tries to bond to the existing mortar and the brick.

Try to work on a cool, overcast day with as little wind as possible. I realize these conditions may be hard to come by along the seashore, as wind seems to be a constant. Wind can suck out the water from the new mortar too quickly, preventing it from becoming as strong as it might.

Slightly dampen the old mortar joints and brick before you add the new mortar. 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.