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Masonry Mortaring

by admin on July 27th, 2010

Masonry mortar is the mixture of cement, sand, water, and other ingredients, which acts as a bonding force to fill voids and hold together masonry units. Beyond its most apparent use of holding together the masonry units, mortar also acts as a seal against unwelcome outside forces such as moisture and air penetration. Nearly every masonry project will utilize mortar, as it such a crucial and universal component of most projects. As described below, there are numerous types of mortar available to use, dependent on the specifications and designs of the project being considered.

A variety of materials, or ingredients, are used to compose the right mortar mix as specified by its intended use. Though materials used in mortar mixes vary, there are some similarities in the core ingredients – cement, sand and water. Other materials are added when a desired effect or purpose needs to be achieved. To make the selection of the right mix easier, most mixes are available at large hardware or home-improvement stores. These mixes, named by code, include type N, type S, type M, type O, refractory mortar, and glass-block mortar. Each has a slight variation in terms of ingredients, and the most common mixes are listed in this section under “Design/Basics.”

Colored mortar is a popular option as a means of providing an aesthetically appealing effect and as a way to create harmony between masonry units. Colored mortar is achieved either through a premixed batch or through the addition of additives to create the desired color effect. Premixed colored mortars are popular because they remove another step in the process and, as the materials are added in controlled environments, there is usually a good consistency in the color. If the color mixing is to be done on site, this should be achieved through mixing additives or pigments in a large, controlled manner to achieve a uniform effect. The following pigments should be added to achieve the desired color effect:

Red – Red iron oxide

Brown – Brown iron oxide

Yellow – Iron hydroxide

Gray – Manganese dioxide

Blue Slate – Black iron oxide

White – White cement, white sand, and white stone.

Water penetration is the most important concern for masonry mortaring. Freezing and thawing, the direct results of water penetration, are most harmful and most common to mortar. The degree of which the masonry is able to protect against these issues is in large part determined by the workmanship or quality of installation. Because it is nearly impossible to prevent all water from entering, it is recommended to have a flashing to deter water. A flashing is a thin, impervious sheet of material used to absorb or re-direct the flow of water. Additional environmental considerations include the preparation and installation of mortar in extremely cold or warm conditions.

  • Type N mortar is a medium-strength mortar that is recommended for use in above-grade projects including outdoor barbeques, outdoor chimneys, and exterior walls of structures that are susceptible to harsh conditions and forces. Its common composition is as follows: 1 part Portland cement, 1 part hydrated lime and 6 parts sand.
  • Type S mortar is a high-strength mortar that is commonly used in applications such as foundations, brick and block retaining walls, patios, walks, and driveways. As it has great strength, it is a popular choice for outdoor projects that are subject to harsh conditions. Its composition is 1 part Portland cement, ½ part lime and 4 ½ parts sand.
  • Type M mortar is a very high-strength mortar that is used in special applications to support exterior walls and stone retaining walls. Its composition is 1 part Portland cement, ¼ part lime and 3 parts sand.
  • Type O mortar is a low-strength mortar that is recommended for load-bearing walls not exceeding pressure of 100 psi and is not subject to excessive moisture. Its composition is 1 part Portland cement, 2 parts lime and 9 parts sand.
  • Refractory mortar is a calcium aluminate mortar known for withstanding high temperatures. Because of this ability to survive in such conditions, it is usually used in fireplaces or barbeques.
  • Glass-block mortar is most commonly used for projects dealing with glass unit masonry. Its composition allows it to bond better with glass units, and thus it should not be used with concrete or brick units.

Many of the mortar types above are available pre-mixed at hardware or home improvement stores. If the mortar is being made from a scratch mix, however, it is important to note that some special accommodations must be made. The cement used should be Portland cement, the most common type of cement, while the sand should be screened, well cleaned, and free of any organic materials such as alkaline or salts.  Whether the mix is pre-made and bought, or composed from scratch, the water must be added by hand and this step is often more difficult than it may first appear. Adding too much water will make the mortar messy and weak, while not enough water means the mortar is thick and not easy to spread and utilize. Manufacturers will provide an estimate of the amount of water required, but some experimentation is usually always required. Spread small bits of the mortar onto the trowel and onto a test surface to see which composition of water is best.

Mixing of pigments to add color to the mortar is a popular choice. To do so, simply add the pigments into a large mix of the mortar. Color mixes often come with detailed directions, but to test out the color, simply spread a bit of it onto a test surface. If the color streaks, not enough pigment has been added.

Mortar can be mixed in either a mortar box, a large mixer or revolving drum. It should be noted, however, that mixing too much mortar at once is a common mistake. On hot days, mortar mixes can harden and be deemed useless in as little as thirty minutes.

Special preparations should be made for the use of mortar in extremely hot or cold conditions. In cold temperatures, the mortar and work area should be covered and protected. Until its ready use, store mortar in sheltered areas where temperatures do not dip below 70°F. When temperatures exceed 100°F, it is crucial to keep the mortar cool. This can be achieved by a simple squirt of water prior to application, along with covering the mixing area with a shade structure.

Mortar has a nearly limitless range of applications; it is used in nearly every project that falls under the concept of masonry. Because of the sheer number of applications it has, installation does vary from project to project. Generally, however, the mixing and application of mortar has some similarities and important steps that must be followed each time. After careful mixing (described in previous sections), the mortar is ready to be applied to bond the masonry units. This application of the masonry is achieved with a tool called a trowel that allows for quick mixing and placement of the mortar. Using the trowel, the mason can move from a mixing station to the application area to apply the mortar. The mortar should be spread generously and evenly on the surface in order to bond to each masonry unit which will be placed above or beside it.

Mortar and grout are often confused to be the same thing. In fact, each is different and has a different set of applications.  Grout generally has a much higher plasticity and fluidity than mortar. It is a popular option to fill cavities, small joints and floor tiles. Generally, mortar is used for much more complex and demanding structural projects than grout.