Natural Sand or Crushed Sand: Which Suits Your Project Better?
Natural and crushed sand compared on particle shape, water demand, workability and strength, with selection notes for concrete, mortar and asphalt.
Construction materials
Choosing Sand for Mortar and Plaster: Cracking, Adhesion and Workability
Unsuitable sand leads to cracked, detached plaster. Learn ASTM C144 limits, how fines and clay act, and how to select sand for masonry mortar and plaster.
Much plaster cracking starts not with the cement but with the sand. Mortar goes on in a thin layer with a large surface exposed to air, so it is sensitive to changes in grading and fines.
How mortar sand differs from concrete sand
Mortar sand is finer and has a smaller maximum size because it must spread in thin layers. Grading limits for masonry mortar sand are defined in ASTM C144, and ASTM C897 is a suitable reference for cement plaster. See also the specifications of mortar sand.
Three deciding factors
Balanced grading
Very fine sand needs more water and shrinks more on drying. Very coarse sand is harsh and does not give a smooth surface. Between the two lies good workability and less cracking.
Cleanliness
Clay raises water demand and reduces mortar adhesion to the substrate. Clay and soluble salts can lead to cracking and efflorescence.
Uniformity
Changing sand mid-job changes the colour and texture of the surface. For large areas, supply material from one source and one stockpile.
A simple site check
Squeeze a handful of damp sand. If your palm comes away muddy or the lump stays too sticky, there is probably a lot of clay. This is only an early sign and does not replace the sand equivalent test.
Placement notes
- Dampen the substrate before plastering so it does not draw water from the mortar.
- Keep the plastered surface damp for several days.
- Do not add more water than the mix calls for.
For the requirements of this field, see the Buildings, mortar and plaster page.
Related products
