Brick walls, stone patios, and concrete block foundations all depend on the same binding material to hold everything together. Mortar cement is the paste-like mixture of cement, sand, and water that fills the joints between masonry units, bonding them into a unified structure while cushioning expansion and contraction from temperature changes. Choosing the wrong type or mixing it incorrectly leads to cracking, water infiltration, and premature failure, so understanding the basics before you start is essential.
What Mortar Cement Is Made Of
At its simplest, mortar is a combination of Portland cement, hydrated lime, sand, and water. The cement provides compressive strength, lime adds workability and flexibility, sand acts as an aggregate filler, and water activates the chemical reaction that hardens the mix. Pre-blended mortar cement bags from Quikrete, Sakrete, and Lehigh combine the cement and lime in factory-controlled proportions, so you only need to add sand and water on site.
Mortar cement is not the same as concrete, though the two are often confused. Concrete includes coarse gravel aggregate and is used for foundations, slabs, and structural pours. Mortar omits the gravel, resulting in a smoother, more adhesive mix specifically designed for joining masonry units.
Mortar Types and When to Use Them
ASTM C270 classifies mortar into five types, each with a different balance of strength and flexibility. The types are designated by letters from the word “MASON WORK,” using every other letter: M, S, N, O, and K.
- Type M: Highest compressive strength at 2,500+ PSI. Use for below-grade applications like foundations, retaining walls, and any masonry in direct contact with soil.
- Type S: 1,800 PSI minimum. The most versatile type for structural applications including load-bearing walls, outdoor paving, and areas subject to lateral pressure from soil or wind.
- Type N: 750 PSI minimum. The standard choice for above-grade exterior walls, chimneys, and general-purpose masonry. It offers the best balance of strength, flexibility, and workability.
- Type O: 350 PSI minimum. Used for interior non-load-bearing walls, repointing historic brick where a softer mortar is needed, and low-stress applications.
- Type K: Very low strength, rarely used today. Historically applied in the restoration of very old, soft-brick structures.
When in doubt, Type N handles the majority of residential projects. Type S is the go-to for any structural or below-grade work.
How to Mix Mortar Properly
Proper mixing is the difference between mortar that lasts 50 years and mortar that crumbles in 5. Follow this process for consistent results:
- Dry mix first. Combine the mortar cement and sand in a wheelbarrow or mixing tub at the ratio printed on the bag, typically 1 part mortar cement to 2.5-3 parts sand by volume.
- Add water gradually. Pour water into a well in the center of the dry mix and fold the dry material into the water with a hoe or margin trowel. Add water slowly until the mortar reaches a thick, peanut-butter consistency.
- Test the consistency. Slice a trowel through the mix. The sides of the cut should hold their shape without slumping. If mortar slides off the trowel, it is too wet. If it crumbles, add small amounts of water.
- Let it slake. Allow the mixed mortar to rest for 5 minutes, then remix briefly before use. This slaking period lets the cement fully absorb the water and improves workability.
Mixed mortar stays workable for about 90 minutes in moderate temperatures. On hot days above 90 degrees F, that window shrinks to 60 minutes. Never add water to re-temper mortar that has started to set, as this weakens the final bond.
Application Techniques
Load your trowel by scooping mortar from the board and snapping the trowel downward to shape it into a log along the blade. Butter the bed joint (horizontal joint) with a 3/8-inch to 1/2-inch thick layer, then press each masonry unit into the mortar with a slight twisting motion. Butter the head joint (vertical joint) on the end of each unit before setting it.
Check alignment with a level every 3-4 courses and stretch a mason’s line to keep rows straight over long runs. After the mortar firms up enough to hold a thumbprint without sticking (usually 20-30 minutes), tool the joints with a concave jointer or a V-jointer for a finished profile that sheds water.
Common Mistakes to Avoid
- Adding too much water: Soupy mortar is easy to spread but dries weak and porous. Always err on the thick side.
- Using the wrong type: Type M mortar on a historic brick wall can be stronger than the brick itself, causing the bricks to crack instead of the mortar absorbing movement. Match mortar strength to the masonry material.
- Skipping the dampening step: Dry bricks and blocks absorb water from the mortar too quickly, preventing proper curing. Mist bricks with water 15-20 minutes before laying them, but do not soak them.
- Working in extreme temperatures: Below 40 degrees F, mortar freezes before curing. Above 100 degrees F, it dries too fast. Use anti-freeze admixtures for cold weather and shade the work area in extreme heat.
Repointing Existing Mortar Joints
Over decades, mortar joints deteriorate from weather, settling, and freeze-thaw cycles. Repointing involves grinding out the old mortar to a depth of 3/4 inch minimum and packing fresh mortar into the cleaned joints. Use an angle grinder with a diamond blade or a cold chisel and hammer for small areas.
Match the new mortar color and type to the existing joints for a seamless repair. Pre-colored mortar or mortar tint additives help achieve a visual match. Type N or Type O mortar is generally appropriate for repointing residential brick above grade. Allow 24-48 hours of curing before exposing repointed joints to rain or direct sun.
Buying Guide and Costs
A standard 60-lb bag of mortar cement costs $6-$10 at home improvement stores. Mixed with sand, one bag yields enough mortar for approximately 30-40 standard bricks or 10-12 concrete blocks. Masonry sand costs $30-$50 per cubic yard or $4-$6 per 50-lb bag. For a typical backyard project like a brick mailbox surround or a small garden wall, total mortar material costs run $30-$80. The real expense in masonry work is labor, which averages $40-$80 per hour for a skilled mason.
