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Mortar Materials
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 December 2006
Feature

Mortar Materials

Cements — Committee C01 on Cement develops standards for hydraulic cements including portland, natural, pozzolanic, masonry and slag cements. Hydraulic cements are cements that set and harden by chemical reaction and are capable of doing so under water. C01 hydraulic cement specifications referenced by Committee C12 standards for mortar and grout include C 91, Specification for Masonry Cement, C 150, Specification for Portland Cement, C 595, Specification for Blended Hydraulic Cements, C 1157 Performance Specification for Hydraulic Cement, and C 1329, Specification for Mortar Cement. With the exception of masonry cement and mortar cement, hydraulic cements need to be mixed with hydrated lime, sand and water to provide a masonry mortar of desired workability in the freshly mixed (plastic) state.

Masonry cements were developed by cement manufacturers shortly after World War I as a patented, packaged cementitious product specifically designed to be mixed with sand and water to produce a masonry mortar. During the 1920s, the use of masonry cements became increasingly popular in the eastern part of the United States as different manufacturers developed products that were generically classified as masonry cements. Reflecting this expanded use, the ASTM Tentative Specification for Masonry Cement (C 91-32T) was issued by ASTM Committee C01 on Cement in 1932. From its inception, ASTM C 91 sought to specify masonry cement by defining requirements for physical performance properties rather than by placing prescriptive limits on ingredients. The original tentative specification had requirements for fineness, setting time, soundness, and compressive strength. The tentative specification also defined test procedures used to obtain values for these parameters. By 1940, ASTM C 91 also included a water retention test and minimum requirement. ASTM C 270-51T used the water retention test method of ASTM C 91 and other Committee C01 laboratory mortar test methods as a basis for property specification requirements. Currently, ASTM C 91 includes provisions for Type N, S, and M masonry cements intended to produce corresponding ASTM C 270 mortars without additional cement or lime.

In 1996, Committee C01 developed ASTM C 1329 for use in demanding structural applications. Mortar cement is similar to masonry cement, except the standard has lower maximum air content limits and has a minimum flexural bond strength criteria as determined using standard concrete masonry testing units. The minimum flexural bond strength criteria were established to ensure comparable bond strength performance to conventional non-air-entrained cement-lime mortars.

Lime — While straight sand-lime mortars had been used in previous centuries, by the 1930s the use of mixtures of portland cement, lime, sand, and water had become the norm. A primary function of lime in cement-lime mortar was and is to provide desired workability in the plastic mortar. Significant advances in lime production technology during the 1930s and 1940s also influenced the development of C12 standards for mortar and C07 standards for lime used in mortar. Traditionally, lime was either delivered to the job site as quicklime (primarily calcium and magnesium oxide), slaked (mixed with water in a process that generates considerable heat and results in formation of calcium hydroxide) to produce lime putty, and stored on site for a period of time (usually more than a day and less than a week), or it was delivered as a dry hydrate and soaked with water on site to produce a lime putty. However, development of efficient pressure hydration technology enabled the use of dry hydrated limes in mortars without prior soaking to form lime putty. In 1946, Committee C07 published ASTM C 207-46T, which included designations for Type N (normal) and Type S (special) hydrated lime in response to the rapid application of the technology and market performance needs. Type S hydrated lime included a maximum limit of 8 percent unhydrated oxides to ensure soundness as well as plasticity and water retention criteria.

The current edition of ASTM C 207 includes designations for Type N (normal), Type S (special), Type NA (normal air-entraining), and Type SA (special air-entraining) limes. It should be noted that ASTM C 207 Type N and S designations for lime precede and do not relate to current ASTM C 270 mortar type designations. Type S lime is generally preferred for use in masonry mortar, although Type N is permitted by ASTM C 270 if shown by test or service record not to be detrimental to soundness of mortar.

Interestingly, there has been some resurgence in the use of lime putty, particularly for historic masonry restoration. However, for convenience and safety, lime putty is produced by a supplier and shipped to the project in sealed plastic containers, rather than slaked on the job site from quicklime. This use prompted the development of ASTM C 1489, Specification for Lime Putty for Structural Purposes, by Committee C07 in 2001. ASTM C 1489 is now a referenced material in ASTM C 270.

Aggregate — As noted, ASTM C 144-39T, Specification for Aggregate for Mortar, was the first standard developed by Committee C12. The provisions of this standard were largely borrowed from ASTM C 33 requirements for fine concrete aggregates although gradation limits differ to provide required workability characteristics for mortars for unit masonry. The current edition of ASTM C 144 includes gradation criteria for natural and manufactured sand, limits on deleterious materials and organic impurities, and provisions for soundness. The standard also indicates that sands not meeting the gradation limits can be used, provided mortar made with the sand meets the property requirements of ASTM C 270.

Water — Water contributes to the workability of mortar and is available for the strength development of hydraulic cement. Given the importance of maintaining a workable mortar mix and the fact that mortar is placed in thin layers between absorptive masonry units, ASTM C 270 indicates that mortar should be mixed with the maximum amount of water consistent with desired workability. Water should be clean and free of amounts of substances that are deleterious to mortar or embedded metal.

Admixtures — ASTM C 270-51T indicated that, “If admixtures or mortar colors are used, they shall be provided for in the contract documents.” This convention of leaving the use of admixtures to the discretion of the specifier has continued. However, in 1998 Committee C12 developed ASTM C 1384, Specification for Modifiers for Masonry Mortars. ASTM C 1384 is now titled Specification for Admixtures for Masonry and includes criteria for materials classified as bond enhancers, workability enhancers, set accelerators, set retarders, and water repellents. The committee is considering a proposal to reference this standard within ASTM C 270.

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