Brick mortar is made of four basic ingredients: portland cement, hydrated lime, sand, and water. These components are mixed in specific ratios depending on how strong and flexible the mortar needs to be, with the exact proportions varying by the type of mortar specified for a project. While the recipe sounds simple, each ingredient plays a distinct role in creating a material that bonds bricks together, resists weather, and lasts for decades.
The Four Core Ingredients
Portland cement is the primary binding agent. When mixed with water, it undergoes a chemical reaction called hydration, forming crystite structures that harden and lock everything together. This is what gives mortar its compressive strength, its ability to bear weight without crumbling. Portland cement was first introduced to the construction industry in the nineteenth century, and it remains the backbone of virtually all modern mortar mixes.
Hydrated lime is the second binder, and it serves several purposes that cement alone can’t. Lime improves workability, making the wet mortar easier to spread and shape with a trowel. It also dramatically improves water retention, keeping the mix from drying out too quickly when it contacts absorbent bricks or blocks. Perhaps most importantly, lime gives cured mortar a degree of flexibility. Mortar joints need to absorb slight movements in a wall from settling, temperature changes, or wind without cracking. Lime provides that give. It also promotes a self-healing property: small cracks in lime-rich mortar can reseal over time as the lime reacts with moisture and carbon dioxide in the air.
Sand makes up the bulk of any mortar mix, typically two to three times the volume of the cement and lime combined. It acts as a filler and provides structure, preventing the mortar from shrinking and cracking as it dries. Not just any sand works. Masonry sand must meet specific grading standards (governed by ASTM C144) that control particle size and limit contaminants. Well-graded sand contains a range of particle sizes so the grains pack tightly together, producing a denser, stronger joint.
Water activates the cement’s hardening reaction and makes the mix workable. Too little water produces a stiff, crumbly mortar that won’t bond properly. Too much weakens the final product and causes excessive shrinkage. Masons adjust water content on site based on temperature, humidity, and how absorbent the bricks are.
How Mortar Types Differ
Not all mortar is mixed the same way. The construction industry classifies mortar into four standard types under ASTM C270, each identified by a letter: M, S, N, and O. The differences come down to the ratio of cement to lime, which shifts the balance between strength and flexibility.
- Type N is the most common general-purpose mortar. It contains 1 part cement with lime ranging from ½ to 1¼ parts by volume, plus sand. It offers a good balance of bonding strength and workability, making it the standard choice for most above-grade brickwork: veneer walls, chimneys, parapets, and interior load-bearing walls.
- Type S provides higher compressive and flexural bond strength than Type N. It’s recommended for reinforced or load-bearing exterior walls and is typically specified in areas prone to high winds or seismic activity.
- Type M has the highest compressive strength of the four but is the least workable. It’s reserved for below-grade applications like foundation walls, retaining walls, sewers, and manholes, where the mortar faces sustained ground pressure and moisture.
- Type O is a low-strength mortar used primarily for interior partitions and restoration work on older buildings where a softer, more flexible joint is needed.
A common misconception is that stronger mortar is always better. In reality, mortar that’s too hard relative to the bricks can cause the bricks themselves to crack. The mortar should always be slightly softer than the masonry units it joins, so that stress and movement are absorbed by the replaceable mortar joints rather than the bricks.
Common Additives
Beyond the four core ingredients, modern mortar sometimes includes chemical admixtures to fine-tune performance. Accelerators speed up setting time, which is useful in cold weather or when a project needs to move quickly. Retarders do the opposite, keeping mortar workable longer during hot weather when it would otherwise set too fast. Air-entraining agents introduce tiny, evenly distributed air bubbles throughout the mix, improving freeze-thaw resistance in climates with harsh winters. Pigments can be added to change the mortar’s color, matching it to specific brick tones or achieving a particular aesthetic.
These additives are supplementary. The vast majority of residential and commercial brickwork uses a straightforward cement-lime-sand-water mix without any special admixtures.
Historical Mortar vs. Modern Mortar
Before portland cement became widely available in the 1800s, mortar was made with lime, sand, and water only. This pure lime mortar hardened through carbonation, a slower process in which the lime gradually absorbs carbon dioxide from the air and converts back into a calcium carbite compound. The result was softer, more breathable, and more flexible than modern cement-based mortar.
That difference matters enormously for old buildings. Historic lime mortar allows moisture to pass through the joints and evaporate, keeping the wall dry from the inside out. Modern cement mortar is much stiffer and far less permeable. When cement mortar is used to repoint or repair a lime-mortar building, it traps moisture inside the masonry, leading to salt staining, flaking, cracking, and long-term structural damage. Restoration specialists consider lime the only material capable of meeting the requirements of historic mortar repair, and Type O mortar or dedicated lime mortars are typically specified for that work.
How Mortar Hardens
Mortar doesn’t dry into hardness the way mud does. The cement component hardens through hydration, a chemical reaction between cement particles and water that produces rigid crystal structures. This reaction generates heat and progresses rapidly in the first 24 to 48 hours, though mortar continues gaining strength over weeks. Full cure typically takes about 28 days.
The lime component hardens through a combination of mechanisms. Some of the lime participates in the cement’s hydration reactions, boosting long-term strength. The rest undergoes carbonation, slowly reacting with atmospheric carbon dioxide. This is why lime-rich mortar can continue to strengthen and self-repair for years after placement. The two hardening processes complement each other: cement provides early strength while lime contributes long-term durability and crack resistance.
Proper curing conditions matter. Mortar needs to stay moist for several days after placement so the hydration reaction can proceed fully. In hot, dry, or windy conditions, masons may mist newly laid walls with water to prevent the mortar from drying out prematurely, which would leave it weak and prone to crumbling.
Typical Mix Ratios
Mortar proportions are specified by volume. For the most commonly used Type N mortar, a standard proportion mix calls for 1 part portland cement, 1 part hydrated lime, and 6 parts sand. Type S typically uses 1 part cement, ½ part lime, and 4½ parts sand. Type M uses 1 part cement, ¼ part lime, and 3 to 3¾ parts sand. Type O shifts the balance toward lime, with 1 part cement, 2 parts lime, and 9 parts sand.
Notice the pattern: as the lime proportion increases relative to cement, the mortar becomes softer and more flexible but lower in compressive strength. As cement dominates, the mortar gets harder and stronger but also more rigid and less forgiving of movement. Choosing the right type is about matching the mortar to the job, not simply maximizing strength.