Masonry Magazine August 1970 Page. 16
Ca(OH)2 during the slaking process. Because hydrated lime can be mixed and used without delay, it is much more convenient than quicklime.
Aggregates for mortar may consist of natural or manufactured sand. Manufactured sand is obtained by crushing stone, gravel, or air cooled blast furnace slag. It is characterized by sharp, angular shaped particles. Specifications for aggregates are listed in ASTM C-144. If the aggregate falls within the percentage limits specified in table 2, it meets the ASTM gradation standards. Well graded aggregate retards separation of material in a plastic mortar mix. In turn this reduces bleeding and improves workability. Sands deficient in fines generally produce harsh mortars, while sands with excessive fines result in weak mortars. In addition to the gradation requirements for sand, ASTM C-144 requires a fineness modulus of 1.6 to 2.5. Fineness modulus equals 1% of the sum of the cumulative percentages retained on the six standard sieves from No. 4 to No. 100 inclusive. It is an arbitrary method of defining gradation.
Water for masonry mortar is required by ASTM C-270 to be clean and free of deleterious amounts of acids, alkalais, 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 mixing mortar.
Composition
Each of the principal constituents of mortar (portland cement, lime, sand, and water) makes a definite contribution to the performance of the mortar, Portland cement contributes to strength and durability. Lime, which sets only on contact with air, gives the mortar workability, water retentivity, and elasticity. Both lime and portland cement contribute to bond strength. Sand acts as a filler which also contributes to the strength of the mix. It greatly decreases the setting and drying shrinkage of mortar thereby reducing cracking. The presence of sand enables the unset mortar to retain its shape and thickness under several courses of masonry units. Water is the mixing agent which gives workability and begins the cementing action.
| Mortar Type | Parts by Volume of Portland Cement* or Portland Blast Furnace Slag Cement** | Parts by Volume of Masonry Cement | Parts by Volume of Hydrated Lime or Lime Putty | Aggregate, Measured in a Damp, Loose Condition |
|---|---|---|---|---|
| M | 1 | 1 (Type II) | 1/4 | Not less than 2½ and not more than 3 times the sum of the volumes of the cements and lime used. |
| S | 1 | 1 (Type II) | over ¼ to ½ |
| N | 1 | 1 (Type II) | over ½ to 1¼ |
| O | 1 | 1 (Type I or II) | over 1¼ to 1½ |
| K | 1 | ... | over 2½ to 4 |
*Types I, II, III, IΑ, ΠΑ, ΙΙΙΑ
**Types IS, ISA
Table 3. Mortar Proportions by Volume (ASTM C-270)
Of the cementing materials listed in table 1, the most extensively used at present are either masonry cement, or a combination of portland cement and lime. Mortar produced from these cementing agents, meeting the respective ASTM specification, may be given an ASTM mortar designation based on the proportioning of materials as prescribed in table 3. 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 using 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 units as long as the ASTM specifications are met. Masonry cements are marketed with the offer of providing an all 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 in the masonry cement are ground together and mixed before being packaged. Masonry cement mortars are consequently less subject to variations from batch to batch than mortar produced from cementing ingredients mixed on the job. When masonry cement is used, there are fewer materials to handle and mixing is therefore faster. The plasticity and water retaining characteristics of masonry cement contribute to the workability and bonding properties of mortar. If a combination of portland cement and lime is chosen for mortar production, equally acceptable properties can be achieved, especially on large projects where accurate proportioning and mixing present no significant problem. The decision to select the most desirable method should be based on the requirements of the particular project involved.
Instead of using the proportioning method of producing mortar just mentioned, approved materials may be mixed in controlled percentages as long as the resultant mortar meets a minimum compressive strength designated in ASTM C-270 as shown in table 4. Whether an ASTM mortar designation is met through proportioning materials as in table 3 or meeting strength tests in table 4, the mortar must be mixed to an initial flow of 100 to 115% and retain a flow after suction of at least 70%. These values are specified to assure consistent laboratory test results and are not related to the flow values of 130 to 150% common to mortar in actual construction.