Masonry Magazine August 1993 Page. 18
Coarse Sand
Ideal Sand
Fine Sand
Fig. 1 The level of liquid in the graduated cylinders, representing voids, is less for a sand having the ideal gradation compared to a coarse or fine sand indicating minimum vold-content for the ideal sand. The illustration is not to scale.
mix. We have seen that the volume of voids or air spaces is influenced by the range and distribution of the different-sized sand particles. A good distribution of particle sizes helps reduce the void content, and also the separation of materials in a plastic mortar mix. This in turn reduces bleeding and improves water retention. Thus mortar made with well-graded sand is generally more workable and retains its workability over a longer period of time than mortar made with poorly graded sand.
Sands not Meeting Gradation Requirements
While the use of sand that meets the gradation limits of ASTM C 144 is preferred, use of sand that does not conform to these limits is allowed provided a mortar can be prepared that meets the aggregate ratio, water retention, and compressive strength requirements of the property specifications of ASTM C270, the Standard Specification for Mortar for Unit Masonry. In many regions, most masonry sands do not meet ASTM C144 gradation limits and must be qualified under this procedure. With such sands, masonry mortar cannot be qualified under the proportion specification requirements of ASTM C270. Rather, laboratory tests must be conducted in accordance with the test methods outlined in ASTM C270 using the sand and cementitious materials intended for use in construction. The volume ratio of aggregate to cementitious materials may be selected and tested at various values within the range of 2 1/4 to 3 1/2 times the sum of the volume of cementitious materials. Test results that meet the property specification requirements of ASTM C270 then qualify that combination of sand and cementitious materials for use in construction at the aggregate ratio tested.
An aggregate ratio of 3 parts sand to 1 part cementitious materials is typically favored for simplicity of batching. However, with some of these sands lower aggregate ratios are required to produce a mortar meeting the property specifications of ASTM C270. Guidance on the selection of aggregate ratios for qualification tests with specific sands may be available from local manufacturers of masonry mortar materials.
Deleterious Substances
Deleterious substances such as clay and lightweight particles with a specific gravity of less than 2.0 must not be present in harmful quantities. These materials usually have a detrimental effect on mortar's strength and durability. Table 3 shows potentially harmful materials that may be contained in a mortar sand and includes references to ASTM tests that can be used to determine the presence of injurious amounts of such materials.
Table 3. Deleterious Substances
| Material | Effect on Mortar | ASTM Test |
|---|---|---|
| clay | affects workability, air content, absorption, shrinkage, durability. strength, and may cause popouts | C142 |
| lightweight materials (coal, lignite, and others) | same as above | C123 |
| organic impurities | affects air content, setting and hardening, strength, may cause deterioration and discoloration | C40 |
| silt and powdered clay | affects workability, air content, absorption, shrinkage, durability, and strength | C117 |
ASTM C144 limits the friable particle (usually clay) content of sand as determined by ASTM C142 to a maxi-