Plaster and Masonry Aggregates

Plaster and Masonry Aggregates share requirements with those for Portland Cement Concrete (PCC).  These  properties, which are presented below, are important because the aggregate represents 70% to 85% by weight of PCC mixes.  Plaster and Masonry products are typically smaller than other concrete aggregates, with a maximum size of approximately 3/8″. The grading of the sand, which is explained below, is finer than concrete sand.

Although some variation in aggregate properties is expected, a few characteristics that should be considered when selecting aggregate for Plaster and Masonry include:

  • size and grading
  • cleanliness
  • soundness
  • particle shape and surface texture

Size and grading
The maximum size of an aggregate designates the smallest sieve size through which 100 percent of the material will pass. Grading of an aggregate is determined by sieve analysis. Maximum size and grading are controlled by specifications that prescribe the distribution of particle sizes to be used for a particular aggregate material for mixtures. The distribution of the particle sizes help determine the stability and density of the mixture.

Some aggregates contain foreign or deleterious substances that make them undesirable for concrete mixtures. (Example: clay lumps, shale, organic material, etc.) The sand-equivalent test, is a method of determining the relative proportion of detrimental fine dust or clay-like materials in the portion of aggregate passing the No. 4 (4.75 mm) sieve.

Aggregates for plaster and masonry should be durable. Often, they represent the outer layer of a house and are subject to weathering, and as such, they should not deteriorate or disintegrate under the action of weather. Items for consideration under weathering action are freezing, thawing, variations in moisture content, and temperature changes. The soundness test is an indication of the resistance to weathering of fine and coarse aggregates.