Masonry Magazine August 1976 Page. 12
the calcium oxide into calcium hydroxide,
Ca(OH). Hydrated lime can be mixed and used without delay. For this reason, it is more convenient to use than quicklime (see Note 1).
Note 1. Quicklime is essentially calcium oxide (CaO). It must be carefully mixed with water (slaked) and stored for as long as two weeks before use (see Standard Specifications for Quicklime for Structural Purposes, ASTM Designation C 5).
Hydrated lime is available in two types, S and N. Because unhydrated oxides and plasticity are not controlled in Type N, only Type S hydrated lime is recommended for masonry mortar.
Aggregates. Either natural or manufactured sands may be used. Although gradation limits are given in the current ASTM C 144, the gradation limits for both natural and manufactured sands in Table 1 are recommended.
TABLE 1
Recommended Sand Gradation Limits
Percent Passing
Sieve
Size
Natural Sand
Manufactured Sand
No. 4
100
100
No. 8
95-100
95-100
No. 16
60-100
60-100
No. 30
35-70
35-70
No. 50
15-35
20-40
No. 100
2-15
10-25
No. 200
0-10
Often gradation can be easily and inexpensively altered by adding fine or coarse sands. Sometimes the most feasible method requires proportioning the mortar mix to suit the available sand, rather than requiring sand to meet a particular gradation.
Water. If water is clean and free of deleterious acids, alkalies or organic materials, it is suitable for masonry mortars.
Proportions. Table 2 gives recommended proportions by volume for four portland cement-lime mortars (Types M, S, N, and O). Each type has specific properties and uses (see Recommended Mortar Uses).
TABLE 2
Mortar Proportions by Volume
Mortar
Parts by Volume
Type Portland Hydrated Lime
Cement or Lime Putty
Aggregate. Measured in
a Damp, Loose Condition
MSNO
1/4
1/2
1
2
Not less than 2 1/4 and not
more than 3 times the sum
of the volumes of the cement
and lime used.
2
It should be noted that no ranges in the lime proportions are recommended, as some proportion specifications for mortar permit. This is done in the interest of reproducibility and predictability of the properties of the resulting mortars.
The assumed weights per cubic foot for the materials in Table 2 are:
Portland cement
94 lb
Hydrated lime
40 lb
Lime putty (quicklime)
80 lb
Sand, damp and loose
80 lb of dry sand
PROPERTIES
Mortars have two distinct, important sets of properties; those of plastic mortars and those of hardened mortars. Plastic properties determine a mortar's construction suitability, helping the designer determine properties of the hardened mortar and, hence, of finished structural elements. Properties of plastic mortars include workability, water retentivity, initial flow and flow after suction, all of which help determine their construction suitability. Most of these are related to water content. Properties of hardened mortars that help determine the performance of the finished masonry include bond strength, durability, extensibility, compressive strength and others.
For many years, the basic constituents of mortars have been portland cement, lime, sand and water. Unfortunately, in recent years, it has become somewhat commonplace to use additives in mortars without regard to their effects on the hardened end product. Although some additives are harmless, some are definitely harmful. Since the properties of both plastic and hardened mortars depend so largely upon mortar ingredients, these ingredients should be required to conform to ASTM specifications and should be properly proportioned.
WATER CONTENT
Water content is possibly the most misunderstood aspect of masonry mortar, probably due to the confusion between mortar and concrete requirements. For this reason, the subject merits special attention.
Many writers have mistakenly based mortar specifications on the assumption that mortar requirements are similar to concrete requirements, especially with regard to the water-cement ratio. Many specifications incorrectly require mortar to be mixed with the minimum amount of water consistent with workability. Often, they also prohibit retempering the mortar. These provisions result in mortars which have maximum compressive strengths but which also have much less than maximum tensile bond strengths.