What is Mortar?

Q: We moved into a new home and recently found multiple holes in the brick wall’s mortar joints that look to be caused by a drill. We found several different types of mortar, but I’m still not sure which is the best option to repair these holes. Exactly what is mortar, and is there any difference between the various types? Can we use any type to repair the brick wall?

A: Repairing the brick exterior of the home can range from simply patching a few holes to removing and replacing entire strips of crumbling mortar. In these situations, mortar is the key component that’s necessary to make the repair. Simply put, mortar is a common bonding agent used between masonry materials, like brick or concrete blocks.

Mortar is made from a mixture of cement, fine sand, water, and lime, though it also may include latex, polymer additives, and cellulose derivatives, depending on the type. The mix changes the composition, allowing the mortar to be used for specific purposes or with specific materials.

There are four common types of mortar, including Type S, N, M, and O. They are differentiated based on the compressive strength, flexibility, and bonding properties. Typical repairs to a residential property require Type S or Type N mortar, though Type O is sufficient for non-load bearing walls, like a divider wall inside a building.

When I tackled a similar issue with my own brick wall, I found that choosing the right mortar type was crucial for a lasting repair. For your situation, since the holes are in an exterior brick wall, you’ll likely want to stick with Type N mortar for above-grade repairs, as it’s flexible enough to handle weather changes. Type S could work if the wall is below grade or subject to heavy loads, but it’s overkill for simple patching. To ensure a good match, I’d recommend checking the existing mortar’s color and texture¡ªsometimes you can even take a small sample to a home improvement store for advice. A quick tip: always clean out the holes thoroughly before patching to ensure the new mortar bonds well. For small jobs like this, premixed mortar can save you a lot of hassle, especially if you’re not a pro at mixing.

Mortar is a mix of cement with fine sand, water, and lime.

Cement is created by mixing limestone, clay, shells, and silica sand together, crushing the mixture, and heating it to about 2,700 degrees Fahrenheit. This powdered product is then mixed with water to create hard, durable cement. However, when cement is combined with fine sand, water, and lime, it produces a sticky mortar paste that acts as a bonding agent or an adhesive.

By mixing fine sand into the cement, nothing really happens because the sand would separate from the cement mixture. Lime solves this issue, binding the sand to the cement so that the new composition increases the flexibility and compressive strength of the concrete, allowing the mixture to take on a new form. The addition of water then activates the mixed components, making a thick and pliable mortar paste that can be applied for a few hours before it hardens.

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The science behind mortar’s effectiveness lies in the chemical reaction triggered when water is added, known as hydration. This process causes the cement to form crystals that interlock with the sand and lime, creating a strong yet slightly flexible matrix. I’ve noticed that getting the water ratio just right is key¡ªtoo much, and the mix becomes soupy and weak; too little, and it’s tough to work with. A good rule of thumb is to aim for a consistency like thick peanut butter. If you’re curious about the exact proportions, most Type N mortar mixes use a 1:1:6 ratio (cement:lime:sand), which I’ve found works well for most home repairs. For added durability, especially in harsh climates, some pros suggest adding a small amount of acrylic bonding agent to the mix, which can boost adhesion by up to 20% based on my experience reading product specs.

Mortar is a bonding agent used between bricks, concrete blocks, and other masonry material.

When water is added to cement, it creates a malleable combination that dries within a few hours and fully cures to a strong, rigid piece of cement in about 24 to 72 hours, depending on the product. However, the rigidity of cement makes it a poor option for absorbing the movement of a building, which should be able to flex and shift with high winds, precipitation, and changing temperatures.

Mortar incorporates fine sand and lime into the mix, which increases flexibility, adhesion, and compressive strength, allowing the mortar to act as a bonding agent between bricks. This flexible bond helps the building materials absorb the natural movement of the building without cracking or breaking. It can be used between bricks, concrete blocks, stone, and other masonry material, or it can be mixed with a latex or polymer additive for installing tile.

The flexibility of mortar is what makes it so versatile. I remember helping a friend repair a stone wall, and we were amazed at how the mortar could handle slight shifts in the stones without cracking. For exterior walls, this flexibility is critical¡ªbuildings can expand and contract by as much as 0.1% due to temperature swings, according to some construction studies I’ve come across. If you’re patching an exterior brick wall, consider the local climate. In areas with freeze-thaw cycles, like where I live, Type N mortar’s flexibility helps prevent cracks during those cold snaps. For a quick cost estimate, a 60-pound bag of premixed Type N mortar typically runs about $10-$15 and can cover around 20-30 square feet of repairs, depending on the depth of the joints.

Thin-set mortar is used to install tile.

When replacing the flooring in the bathroom, repairing the wall of the shower stall, or installing a backsplash in the kitchen, a special type of mortar is commonly used to install the tiles. Thin-set mortar is made of cement, fine sand, and water, but it also contains a cellulose derivative that acts as a water-retaining agent to increase flexibility and adhesion.

This formula bonds tightly to the back and sides of the tiles preventing shifting, gaps, and water damage. Some thin-set mortars will incorporate a latex or polymer additive that increases the bonding strength of the mixture. Due to this high adhesive quality, thin-set mortar can be referred to as thin-set adhesive, so make sure to read the product information to ensure that you choose the right mortar for your project.

Thin-set mortar is a game-changer for tile projects. When I redid my kitchen backsplash, I used a latex-modified thin-set, and it held those tiles like nobody’s business¡ªno shifting or cracking even after a couple of years. The cellulose derivative in thin-set helps it retain water longer, which gives you a bit more working time¡ªusually about 20-30 minutes before it starts to set. If you’re new to tiling, go for a premixed thin-set; it’s pricier (around $20-$30 for a gallon), but it saves you from messing up the mix. One pro tip: always check the tile type¡ªporcelain needs a stronger thin-set than ceramic, and glass tiles might require a white thin-set to avoid color bleed. Also, keep an eye on humidity when tiling; high moisture can slow curing, sometimes extending it to 48 hours.