Masonry Magazine August 1993 Page. 17
Portland Cement Association
pca
TROWEL TIPS
an aid to the masonry industry
Mortar Sand
Sand is a key constituent of mortar. Sand particles are coated and lubricated by the mortar paste to provide the flow and body needed in a plastic mortar and are cemented together as the paste hardens, providing required structural properties. Since the water and cement occupy voids between sand particles, almost 1 cu ft of damp loose sand is needed to make 1 cu ft of mortar. Any material constituting such a large proportion of the mortar will naturally be a major determinant in the quality of the mortar. Sand properties affect the workability, board-life, compressive strength, bond strength, drying shrinkage, and appearance of mortar. Clearly, the sand selection process should consider these important mortar properties in addition to evaluating a sand's cost and availability.
Either natural or manufactured sand may be used in preparing masonry mortars. Manufactured sand is obtained by crushing stone, gravel, or air-cooled blast-furnace slag. Natural sand usually has rounder, smoother particles than manufactured sand. Manufactured sand, with its characteristic sharp and angular particle shapes, will generally produce less workable mortars than obtained with natural sand.
Gradation
ASTM C144, the Standard Specification for Aggregate for Masonry Mortar, specifies sieve analysis limits for mortar sand. These requirements are given in Table 1. Note that a range of acceptable gradations are allowed by the specification.
Table 1. Recommended Sand Gradation
| Sieve Size | Natural Sand | Manufactured Sand |
| ----------- | ----------- | ----------- |
| No.4 | 100 | 100 |
| No.8 | 95 to100 | 95 to100 |
| No.16 | 70 to 100 | 70 to100 |
| No.30 | 40 to 75 | 40 to 75 |
| No.50 | 10 to 35 | 20 to 40 |
| No.100 | 2 to 15 | 10 to 25 |
| No.200 | | O to 10 |
"Additional requirements: Not more than 50 percent shall be retained between any two sieve sizes nor more than 25 percent between No. 50 and No. 100.
To illustrate the variety in gradations that will pass the requirements, the coarsest allowable and finest allowable sands are shown in Table 2. The middle column is the average between the two extremes and is the ideal gradation.
Table 2. Limits of Allowable
Sand Gradation
| Sieve Size | Coarsest | Percentage Retained (natural sand) | Ideal | Finest |
| ----------- | ----------- | ----------- | ----------- | ----------- |
| | Each Sieve | Cumulative | Each Sieve | Cumulative | Each Sieve | Cumulative |
| No. 4 | 0 | 0 | 0.0 | 0.0 | 0 | 0 |
| No. 8 | 5 | 5 | 2.5 | 2.5 | 0 | 0 |
| No. 16 | 25 | 30 | 12.5 | 15.0 | 0 | 0 |
| No. 30 | 30 | 60 | 27.5 | 42.5 | 25 | 25 |
| No. 50 | 30 | 90 | 35.0 | 77.5 | 40 | 65 |
| No. 100 | 8 | 98 | 14.0 | 91.5 | 20 | 85 |
| Pan | 2 | 100 | 8.5 | 100.0 | 15 | 100 |
| Fineness Modulus | 2.83 | | 2.29 | | 1.75 | |
Results of gradation tests are sometimes reported as the fineness modulus of the sand. An arbitrary method of defining gradation, fineness modulus is the sum of the cumulative percentages retained on the six standard screens, from the No. 4 to the No. 100 inclusive, divided by 100. ASTM C144 limits the variation in fineness modulus to no more than 0.20 from the value assumed in selecting proportions for the mortar unless appropriate adjustments are made to the proportions to compensate for the change in grading.
The values for the coarsest and the finest gradations are the limits. The best sands fall well within these limits. Sands that are close to the limits are not as desirable as those close to the ideal gradation.
Illustrated in Fig. 1 are physical depictions of the gradations in Table 2. The beakers shown in Fig. 1 hold the coarsest allowable sand gradation, ideal gradation, and finest allowable gradation. In each case the graduated cylinder below the beaker contains a volume of liquid representing the void-content of the constant volume of dry sand in the beaker. A relatively lower volume of liquid under the beaker containing the ideal sand gradation illustrates that that minimum void-content is obtained with this gradation. Note that this illustration is for sands that fall within the limits of gradation requirements. Sands that do not meet the gradation requirements can produce more drastic differences in void-content from that obtained with the ideal gradation.
As previously mentioned, water and cementitious materials form a paste that fills the void spaces between the sand particles and lubricates them to produce a workable