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September 5, 2000 10:30 AM CDT

Choosing the Right Mortar

A guide to selecting the right mortar for the materials, process, product

By

Mortar selection impacts both the construction process and the quality of the finished masonry product. Unfortunately, mortar selection is not always given proper consideration in planning for a masonry construction project. Selecting the right mortar requires an understanding of the materials (mortar and units), the process (masonry construction), and the product (masonry).

Underlying Principles and Issues
Mortar is used to lay brick, block, or stone units. Within that context, it must:
  • facilitate the placement of units,

  • contribute to the serviceability of masonry,

  • provide required structural performance, and

  • exhibit the desired appearance.
Consideration to each of these areas of performance needs to be given in the selection of mortar type and mortar materials.

Different members of the design and construction team have different perspectives on the relative importance of these areas of performance. The engineer focuses on the structural implications of mortar selection, the architect looks at appearance, and the mason contractor expects a workable product to facilitate productivity. The owner/builder wants a completed project, delivered on time and within budget, which will serve his needs. The relative influence different team members have on mortar selection varies from project to project. However, a balanced perspective is needed in the selection process. Basic principles to remember are:
  • No one mortar type is best for all applications

  • No one mortar type will rate the highest in all areas of performance

  • No single mortar property defines mortar quality
Mortar Types
ASTM Standard Specifications provide a means for specifiers to identify acceptable materials and products without limiting those items to specific brands or manufacturers. Project specifications should reference ASTM C 270, the Standard Specification for Mortar for Unit Masonry. Mason contractors and specifiers need to understand the provisions of that specification and available options.

ASTM C 270 defines four different types of mortars intended to address the variety of needs stemming from different masonry applications. Type N mortar is a general-purpose mortar that provides good workability and serviceability. It is commonly used in interior walls, above-grade exterior walls under normal loading conditions, and in veneers. Type S mortar is used in structural load-bearing applications and for exterior applications at or below grade. It also provides increased resistance to freeze-thaw deterioration. Type M is a high-strength mortar that may be considered for load bearing or demanding freeze-thaw applications. Type O is a low-strength mortar that is sometimes used for interior masonry or repointing.

ASTM C 270 provides two alternatives for specifying mortars: proportion specifications and property specifications. Under the proportion specifications, mortar ingredients must meet indicated product standards and be volumetrically proportioned within limits. No physical requirements are placed on the mortar itself. Under the property specifications, in addition to meeting the individual product standards, mortar materials mixed in the laboratory using job site proportions must meet certain property requirements. Either the proportion specifications or the property specifications should be selected, not both. If project specifications do not indicate which has been selected, the proportion specifications govern, unless data qualifying the mortar under the property specification are presented to and accepted by the specifier.

It should be understood that the property requirements of ASTM C 270 are for laboratory specimens and are not for field quality control. The ASTM standard presumes that the proportions developed in the laboratory to meet the property requirements will result in satisfactory performance in the field. Field testing is not required, and if field testing of compressive strength is conducted, results are not required or expected to meet the minimum compressive strength requirements of the property specifications.

Mortar Materials
ASTM C 270 places requirements on component mortar materials (water, sand, cementitious materials, and possibly admixtures). For example, sand must meet the requirements of ASTM C 144, masonry cement must conform to ASTM C 91, mortar cement to ASTM C 1329, portland cement to ASTM C 150, and hydrated lime to ASTM C 207. It is important to understand the contribution of each component material to the performance of mortar. Water acts as a lubricant in the plastic mortar and is required for hydration of the cement. Strength gain of mortar is not related to evaporation of water but to the chemical combination of water with cement compounds in the mortar. Since some mixing water is lost to absorptive units and evaporation, the maximum amount of water consistent with optimum workability should be added to mortar.

Masonry sand provides the basic "framework" for mortar. Sand particles are coated and lubricated by the mortar paste to provide body and flowability required in the plastic mortar and are bonded together as the paste hardens to provide required structural properties. Sand quality affects both workability characteristics of plastic mortar and properties of hardened mortar such as compressive strength, bond strength, and drying shrinkage. Masonry cement is a hydraulic cement, that is, it hardens by chemically reacting with water and will do so under water. It consists of portland or blended cement and inorganic plasticizing materials (such as pulverized limestone, hydrated or hydraulic lime) together with other agents introduced to optimize workability, board life and water retention, contribute to improved durability; and reduce drying shrinkage and water absorption of mortar. Mortar cement is a relatively new product designed for use in demanding structural masonry applications. Mortar cement, like masonry cement, is a hydraulic cement, primarily used to produce masonry mortar. However, mortar cement must meet lower maximum air content limits than masonry cement and is the only mortar material or system that has minimum bond strength requirements.

Whether present as an integral part of masonry cement or as a separate ingredient added at the mixer with hydrated lime, portland cement acts as the glue, which holds the mortar and, ultimately, the masonry, together. Compressive strength and bond strength are related to the portland cement content of mortar. Hydrated lime may be batched with portland cement, sand, and water at the job site. In this system, lime functions as a plasticizer contributing to workability, board life, and water retention of the mortar.

Either air-entrained portland cement or air-entrained hydrated lime may be used to improve the workability and durability of the portland cement-hydrated lime mortar. However, air-entrained cement and air-entrained lime should not be combined in the same mix.

Selection
Once the design loads, type of structure, and masonry units have been determined, the mortar type can be selected. It should be remembered that stronger is not necessarily better when selecting mortar for unit masonry. For example, it is not typically necessary to use Type M mortar for high-strength masonry. Type S will provide comparable strength of masonry, and in fact the Masonry Standards Joint Committee's design standard ACI 530/ASCE 5/TMS 402 does not distinguish between the structural properties of masonry constructed using Type S mortar from that constructed using Type M mortar. Moreover, Type S and Type N generally have better workability, board life, and water retention. As a rule of thumb, the specifier should not require use of a mortar of higher compressive strength than necessary to meet structural design criteria.

Unless otherwise indicated, either a masonry cement or cement-lime mortar may be used. Masonry cement mortars, generally offering improved convenience, workability, durability, and uniformity, are used in a majority of masonry construction. For structural masonry, the designer may require Type S cement- lime mortar or mortar cement mortar if the allowable tensile flexural stress values associated with these mortars have been used in the design of the masonry. Codes may also require the use of Type S cement-lime or mortar cement mortars for structural masonry in high seismic performance categories.

In addition to structural requirements, the properties of units and expected environmental conditions should be considered. For example, more workable, water retentive mortars are better suited for use with high absorption units than a high-strength, low water retentive mortar. Conversely, with a low absorption unit under cold weather construction conditions, a less water retentive, faster setting mortar is desirable.

Special attention to mortar selection should be given when severe exposure conditions are expected. Type O mortar should not be used in saturated freezing conditions. For severe frost action such as horizontal paving applications, Type M mortar should be considered. Air-entrainment should be used to improve freeze-thaw durability.

Summary
As can be seen from this discussion, selection of mortar though appearing to be a simple matter, involves some rather complex issues related to materials, design, and construction. Ideally, input from the engineer, architect, mason contractor,and owner would be considered along with influencing factors such as weather conditions to optimize the selection of mortar on a project. In reality, the design-bid-build process used for most masonry construction precludes that level of interaction. However, as specifiers and contractors better understand the influence of mortar selection on the construction process and the masonry product, they can become more confident of making the right choices.


About the Author

John Melander is the Director of Product Standards and Technology of the Portland Cement Association.

 

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