Masonry Magazine October 1987 Page. 22
permitted for use in masonry mortars containing air-entrained cement. Chemically, quicklime (CaO MgO) hydrates to form hydrated lime [CaO (OH)2 Mg (OH)2]. Slaking is not required, thus hydrated lime is immediately useable and much more convenient than quicklime. However, slaking of hydrated lime is occasionally practiced, yielding a very workable lime putty.
Aggregates (ASTM C144) for masonry mortars may consist of natural or manufactured sand. Manufactured sand is the product obtained by crushing stone, gravel, or air cooled blast furnace slag. It is characterized by sharp, angular shaped particles. Gradation limits are established in ASTM C144 for both natural and manufactured sands. The specification allows exceptions to the recommended gradation provided compressive strength and water retention limits are exceeded when mortar is prepared using the proper combination of cementitious materials and aggregate to yield an aggregate to cement ratio within the limits as indicated in Table 1.
Water for masonry mortar is required by ASTM C270 to be clean and free of deleterious amounts of acids, alkalis, or organic materials. Whether the water is potable is not in itself a consideration, but the water obtained from drinking supply sources, such as city mains, is generally suitable for use.
Admixtures for masonry mortars are available in a wide variety. Since they are commercially prepared products, their compositions are not generally disclosed in full. Many substances have been employed as admixtures such as tallows, salts of wood resins, and various other chemicals. Admixtures are functionally classified as agents promoting air-entrainment, water retentivity, workability, and accelerated set among other things. Calcium chloride in an amount not to exceed 2% by weight of the portland cement content may be used as an accelerator as provided by ASTM C270. The use of calcium chloride or chloride containing admixtures when metal reinforcing steel, ties, and anchors are present may promote corrosion. Little data have been published regarding the effect of proprietary admixtures on mortar bond or compressive strength, but field experience indicates that detrimental results have occurred in some cases. For this reason, admixtures of unidentified composition should be used only after it has been established by test or experience that they do not materially impair the mortar performance.
Effect of Materials on Mortar
With the diversity of materials available, masonry mortars can be formulated to produce the desired properties for most specific job requirements.
Each of the individual ingredients (cement, lime, sand, and water) contributes to the performance of the mortar. Portland cement provides the primary requisites: strength and durability. Lime imparts workability, water retention, and elasticity as well as contributing along with the cement to the bond strength. Sand acts as a filler which also enhances the strength of the mortar while helping to reduce drying shrinkage, which is a major cause of cracking. In addition, sand enables the semi-plastic mortar to retain its shape and thickness under several courses of masonry, thereby controlling dimensional stability. Water is the mixing agent which also adds to workability, and without which cement hydration and subsequent setting and hardening of the mortar would not be possible.
The various material options alter the characteristics of the mortar in a predictable manner. Changes in cement type promote slight changes in setting characteristics, workability, and strength development. Use of air-entrained cements results in decreased water demand, improved workability, and increased freeze-thaw resistance. A change in lime type creates slight changes in workability; the use of air-entrained lime produces results similar to those obtained from air-entrained cements. Masonry cements, used singly or in combination with portland cement, provide mortars with excellent workability and freeze-thaw durability. Slightly reduced compressive and bond strengths may result. Changes in sand type and gradation affect the workability of the mortar. Natural sand gives improved workability at a lower water demand because of particle shape, while manufactured sands require additional water due to their greater surface area. In general, well graded aggregates reduce segregation in a plastic mortar mix, which in turn inhibits bleeding and improves workability. Sands deficient in fines generally produce harsh mortars, while sands with excessive fines result in weak mortars.
Mortar Types
Building codes generally specify mortar types as referenced in ASTM Standard C270, "Mortar for Unit Masonry." Four mortar types: M, S, N and O, are included in this standard. Each consists of one or more of the cementitious ingredients listed in the previous Materials section and all are governed by either of two alternative specifications. The proportion specification (Table 1) prescribes the parts by volume of each ingredient required to provide a specified mortar. More than one option is available for each separate type. A combination of portland cement and lime will suffice as the cementing agent in each type of mortar, but masonry cement alone is suitable for types "O" and "N" mortar only. For mortar types "M" and "S", masonry cement can be used only when combined with portland cement. This is necessary because a higher portland cement content is required to produce the high strength mortars.
The choice of masonry cement in place of a portland cement and lime combination is largely a matter of economics and convenience. Either will produce mortar of acceptable properties for concrete masonry construction as long as the ASTM specifications are observed. Masonry cements are marketed to provide a general purpose mortar containing everything but sand and water in one bag. The quality and appearance of mortars made from masonry cement are relatively consistent because the materials are inter-ground and thoroughly mixed before packaging. Masonry cement mortars are consequently less subject to variations from batch to batch than mortar produced from cementing ingredients mixed on the
TABLE 1. Proportion Specification Requirements
Proportions by Volume
(Cementitious Materials)
Portland
Cement
Mortar
Type
or
Blended Cement
Cement-lime
M
1
S
1
N
1
0
1
Masonry cement
M
1
1
S
1
N
1
0
1
Masonry Hydrated Lime
Cement or Lime Putty
%
over % to ½
over ½ to 1%
over 1% to 2%
Aggregate Ratio
(Measured in
Damp, Loose Conditions)
Not less than 2% and not
more than 3 times the sum of
the separate volumes of
cementitious materials.