Masonry Magazine April 1997 Page. 36
The mortar and, ultimately, the masonry, together. Portland cement is defined as a hydraulic cement produced by grinding clinker consisting of hydraulic calcium silicates, usually containing one or more of the calcium sulfates as an interground addition. The term hydraulic indicates that the material sets and hardens in a reaction with water and will do so under water. Compressive strength and bond strength are related to the portland cement content of mortar.
ASTM C150, the Standard Specification for Portland Cement, defines eight different types of portland cements. However, only types I, IA, II, IIA, III, and IIIA are used in conventional masonry construction.
Blended hydraulic cements are produced from portland cement or portland cement clinker and pozzolans or slags. ASTM C595 blended cements Types IS, IS-A, IP, IP-A, I(PM), I(PM)-A may be used as an alternative to portland cement in producing mortar. Types S and SA may also be used, but only under the property specification requirements of ASTM C 270.
HYDRATED LIME
Hydrated lime is produced by burning limestone to produce quicklime that is then treated with sufficient water to satisfy its chemical affinity for water. It consists essentially of calcium hydroxide or a mixture of calcium hydroxide and magnesium oxide or magnesium hydroxide, or both. 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.
Hydrated lime for masonry purposes is covered by specification ASTM C207. This specification defines requirements for Type N, NA, S and SA limes. These designations should not be confused with the ASTM C 270 mortar Types N and S. Most mortar specifications require the use of Type S lime to assure the soundness 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.
ADMIXTURES
Admixtures may be used to modify specific motar properties. Perhaps the most common example is the use of accelerators to shorten the setting time of mortar during cold weather. The manufacturer of an admixture should provide data substantiating performance of the product, referencing applicable ASTM standards. ASTM C 270 indicates that admixtures are not to be added to mortar unless specified. Do not use chloride based admixtures in mortar. Chlorides contribute to corrosion of steel reinforcement, ties, and accessories.
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, cement-lime mortar or mortar cement mortar may be required by the designer if the allowable tensile flexural stress values associated with these mortars have been used in the design of the masonry. Codes may require the use of Type S cement-lime or mortar cement mortars for structural masonry in high seismic performance categories.
Have Questions? - Ask.
The mason contractor may not be able to determine the owner's or specifier's expectations with respect to mortar by simply reading the project specifications and knowing the provisions of the referenced standards. Sometimes project specification requirements for mortar are not clearly stated or may even conflict with the intent of the specifier to assure the best quality of masonry. Specifiers, not familiar with mortar specifications or mortar materials, may include requirements for mortars in the specifications which compromise the quality of the masonry or unnecessarily complicate the construction process. If the mason contractor has questions about project specification requirements for mortar, he should ask them and ask them as early as possible. Verbal communication followed by written correspondence is generally the best way to make certain that there are no misunderstandings about mortar requirements and that a record of communication is maintained. Circumstances that should trigger such communication include specification of a mortar type or composition that is not compatible with the units or apparent structural application, or inconsistencies between provisions of the referenced ASTM Standards and the project specification requirements. An example of the latter would be a statement in the project specifications that mortar tests performed on field sampled mortar are required to meet the compressive strengths given under the property specifications of ASTM C-270.
The contractor can obtain technical assistance on issues such as these from materials suppliers, materials manufacturers, and masonry institutes. The Brick Institute of America (BIA), the National Concrete Masonry Association (NCMA), and the Portland Cement Association (PCA) have technical publications on these topics and have technical staff available by phone to discuss such issues.
Meet Project Requirements for Mortar
Knowing what is needed is only the first step. Success depends on consistently meeting those needs. To assure that he is providing a quality mortar, the mason contractor needs to make the proper submittals and establish effective quality control of mortar production. The mason contractor should also know what mortar testing will be conducted.
Submittals. Submittals and approvals of submittals assure that the procedures and materials used in construction meet the requirements of the specifier. Submittals should confirm that mortar used to construct the masonry will contain specified mortar materials, be of the proper mortar type, and conform to additional project specific mortar requirements.