One of the most common questions we get regarding installation is centered on the type of mortar mixes that we recommend using with our products. Currently, we recommend using either Type N or Type S mortar. Let’s look at some basic information about mortar and the differences between these two mixes.
There are actually quite a few types of mortar mixes. The most common type that is currently used today is a Portland cement and lime based masonry mortar mix. This type of mix includes Portland cement, lime, and an aggregate or sand with water. The ratios of cement, lime and sand used in each mix will affect the characteristics for that particular mix.
Portland cement is the workhorse of a good mortar mix. Portland cement contributes to the compressive strength of the mortar mix. Lime provides “workability” and elasticity to the mortar mix, allowing the mortar to easily be applied to masonry veneer and ensure water retention of the mix. Portland cement and lime together creates the bond strength of the mix, or how well the mortar mix bonds the stone to the wall. Sand or aggregate is used as filler in the mix and allows the mortar to maintain its shape during installation.
The American Society for Testing and Materials (ASTM) recognizes 5 formulations, with each formulation designated by a specific letter: M, S, N, O, and K. Type M mortar mixes have the highest strength and Type K is the weakest.
Generally speaking, a Type S mortar mix will have a higher percentage of Portland cement than a Type N mortar mix. Type N tends to utilize a higher percentage of lime, allowing a Type N mortar mix to have better water retention properties and better workability. However, the lower amount of Portland Cement used in a Type N mix reduces the strength and freeze-thaw resistance of the Type N mortar mix in comparison to a Type S mortar mix.
Another type of mortar mix that is gaining popularity is a polymer cement mortar mix, or polymer-modified mortar mix. Polymer cement mortar mixes utilize polymeric admixtures that help to increase the compressive strength and bond strength of the mortar mix. While polymer-modified mortar mixes may be slightly more expensive than Type N or Type S mortar mixes, they provide many enhanced characteristics that may be valuable for many types of applications.
One type of installation that can benefit from the use of polymer-modified mortar mixes are installations performed in cold weather climates. The issue of freeze/thaw cycles is a common concern among masons, contractors, homeowners, and architects. Polymer-modified mortar mixes provide an added bond strength that will help to prevent detachment of the stone from the wall due to damage of the mortar setting bed from the freezing and thawing of moisture in the mortar.
Another excellent use for polymer-modified mortar is with dry stack (groutless) installations. Unlike Type N or S mortars, stones applied with polymer-modified mortars can be shifted or moved for a period of time after the application. Once a stone if seated with conventional mortar, any subsequent movement of the stone will weaken the bond. This is important when one considers the potential to bump a previously applied stone when installing dry stack in which stones are purposely in contact with each other. Type N mortars provide the least performance when installing stone in a dry stack application. Any installer should elect to avoid Type N for dry stack applications.
Mortar mixes can also include items such as various admixtures, bonding agents, and color additives. Most admixtures or bonding agents are used to increase the bond strength of the mortar mix and are typically used in Type N or Type S mixes where enhanced bond strength may be required for local building codes or for extreme climates. Color additives, such as iron oxide pigments, can be used to adjust and change the color of the mortar mix. Colored mortar mixes provide personalization of any installation and can greatly heighten the appeal of the stone installation. For Eldorado Stone, admixtures, bonding agents, or color additives are not required, but may be used in cases that require certain properties from the mortar mix in use. It is important to note that the addition of pigments has the unintended consequence of reducing the strength of the mortar. The reduction is proportional to the amount of pigment used. Therefore, it might be a wise choice to graduate from a Type N to a Type S mortar or from Type S to polymer modified in order to compensate for the lower bond strength.
When considering the addition of bonding agents, choose one that is recommended by the mortar manufacturer or the bonding agent manufacturer that is appropriate for addition to wet mixes. Many bonding agents (white liquids known as “milk” due to their resemblance to milk) are common in the industry. However, these products are designed to be applied to a cure concrete surface to enhance the bond of wet concrete to that cured concrete surface; they are not designed to be mixed into concrete or mortars.
Mortar mixes have been used for hundreds of years and have been through many formulation changes. Manufactured stone produced from lightweight concrete, such as Eldorado Stone, qualifies as an adhered veneer cladding system under building codes, which requires bond strengths of 50 PSI or greater. For most applications, Type N, Type S, or polymer-modified mortar mixes are more than adequate to get the job done. In some cases, the use of Type S or polymer-modified mortar mixes may be an industry “best practice” that should be considered.
Although mortar can be prepared from cement, lime and sand, it is wise to choose a product that is pre-packaged and has the designation “Meets or Exceeds ASTM C270” or "Meets or Exceeds ANSI A118.1A”.