A variety of types of mortar exist and possess a range of different qualities. From traditional cement-based mortars, the most commonly used mortar, to specialized varieties with enhanced properties, the diverse range of mortars allow builders to tailor them to the unique requirements of their projects. A review of the types of mortar will help illuminate their properties and the projects for which they work best.
Types of Mortar
There are several types of mortar that builders use throughout the construction industry, each with its own properties and specifications.
Cement mortar, also called ordinary Portland cement mortar, is the most widely used type of mortar. The ingredients in these products are ordinary Portland cement (OPC), hydrated lime, sand, and water. The most convenient way to use this product is by buying premixed bags. These bags are pre-proportioned to be mixed with water. This ensures the best quality control and the most efficient use. There are five types of cement mortar, each with different levels of compressive strength: Types M, S, N, O, and K.
- Type M – Type M mortar is made from 3 parts Portland cement, 1 part lime, and 12 parts sand. With the highest ratio of Portland cement, it has the highest compressive strength of all cement mortar types, at 2,500 psi. This type of mortar is only used when builders need the highest strength level. They use it specifically for below-level applications that carry heavy loads including foundations and retaining walls. It is also useful for driveways and structures that are close to water. Builders prefer this type of mortar for natural stone, but it has poor adhesion and sealing properties, so it is less useful for exposed applications.
- Type S – Type S mortar is made from 2 parts Portland cement, 1 part lime, and 9 parts sand. It has a compressive strength of a minimum of 1,800 psi, though builders can mix it for higher strength levels, between 2,300-3,000. Type S mortar works well for below-grade and exterior applications. This mortar works well in withstanding pressure because of its high tensile strength. Builders use it for retaining walls, foundations, manholes, brick patios, and sewers.
- Type N – Type N mortar, like Type S, is a medium-strength mortar, but it has a lower compressive strength of 750 psi. Type N mortar contains 1 part Portland cement, 1 part lime, and 6 parts sand. This type of mortar withstands heat, low temperatures, and severe weather, so it is ideal for above-grade applications. Many builders consider this a good all-purpose mix, which is optimal for most DIY projects. It works well for load-bearing applications and for soft stone projects because it is more flexible than other mortars and will not crack the stones.
- Type O – Type O mortar contains 1 part Portland cement, 2 parts lime, and 9 parts sand. Experts consider this a low-strength mortar because it has a minimum strength of 350 psi. This mortar is best used for non-load-bearing projects. It cannot withstand harsh temperatures. Builders use this mortar to repair mortar joints. This mortar is flexible, so it can also be used on softer stone projects.
- Type K – Type K mortar has the weakest compressive strength, at 75 psi. It contains 1 part Portland cement, 3 parts lime, and 12 parts sand. This type of mortar is not useful in modern construction projects, but it is ideal for historic restoration work because it has a strength level comparable to the historical masonry units.
Thin-set mortar is a specifically formulated mortar for tile installations. The term “thin-set” refers to the thin layer of mortar that builders apply to the tiles to adhere them to the subfloor surface. This mortar binds the tile securely, but it also helps to level out the subfloor to ensure an even surface.
Builders use thin-set mortar for ceramic, porcelain, stone, and glass tiles onto various substrates. You can find thin-set mortar in both modified and unmodified forms. Unmodified thin-set mortar is a basic mixture of cement and sand. This requires the addition of water. Modified thin-set has additional additives which give it properties of better adhesion, water resistance, and flexibility.
Epoxy mortar is a specialized type of mortar that contains epoxy resins, hardeners, aggregates, and fillers. Traditional mortar hardens when water and cement connect and begin the hydration process. Epoxy mortar relies on the chemical reaction between the resin and the hardener to form a strong bond. This results in a stronger mortar than other types.
Builders use epoxy mortar for industrial flooring applications, the anchoring of steel reinforcements in concrete, and structural repair projects. Because of its resistance to chemical exposure, they also use it in chemical laboratories and other environments where chemicals degrade the surface. Epoxy mortar is also resistant to water, so it is common in sealing underwater applications like swimming pools, water tanks, and bathrooms.
Polymer Cement Mortar
Rather than utilizing standard cement to bind the mortar, polymer cement relies on polymeric admixtures for adhesion. The polymers are latex or acrylic compounds, which mix with cement and sand to enhance the qualities of the mortar. The polymers improve the workability and overall performance of the mortar.
Polymers give the mortar added adhesion and better crack, flexibility, and water resistance. Builders use polymer cement mortars for tile installation, skim coating on interior and exterior walls, and repairs.
Lime mortar is a historic type of mortar. It contains lime, sand, and water, where lime is the primary binder rather than cement. Builders today primarily use lime mortar in historic preservations and recreation projects, though there are also specific modern applications for which lime mortar is useful.
Lime mortar is breathable, so it is useful in moisture-prone areas for allowing it to evaporate and reduce the damage caused by the build-up of moisture. Lime mortar is flexible, so it can accommodate movement within masonry structures. It is compatible with soft masonry projects where architects have specified the use of certain soft mason units like historic brick, terracotta, or limestone. Lime mortar also has self-healing properties and can repair small damage to itself over time.
Pozzolanic mortar contains pozzolanic materials, either natural or artificial, like volcanic ash, fly ash, silica fume, and calcined clay. These materials react with the lime when water is added to the mix. They form cementitious compounds that give the mortar strength and durability. This type of water performs very well underwater, so it is ideal for water applications like marinas, seawalls, and harbor structures.
Pozzolanic mortar has some benefits over cement mortar in some applications. It exhibits reduced cracking and shrinkage. It also reduces the need for cement production which reduces the carbon-based emissions that cement production produces.
Fire-resistant mortar, also called refractory mortar, is a specialized mortar product that builders use in high-temperature environments. Manufacturers make it fire-resistant with the addition of refractory materials like silica, alumina, clay, and calcium aluminates. These materials have a higher melting point than standard mortar materials, higher heat insulation, and lower thermal conductivity.
Builders typically use refractory mortar in chimneys, fireplaces, and industrial kilns in combination with refractory bricks. These materials withstand high temperatures, but they are also useful in withstanding thermal shock resistance and are non-combustible.