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.
Construction materials
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.
Choosing between natural and crushed sand is one of the most common questions in mix design. Neither is better in absolute terms; each has properties that become an advantage or a limit in a given application.
Differences in origin and particle shape
Natural sand results from the erosion of rock over time and is taken from alluvial deposits. Its particles are rounded with fairly smooth surfaces. Crushed sand comes from crushing rock in a plant and has angular particles with rough surfaces.
| Property | Natural sand | Crushed sand |
|---|---|---|
| Particle shape | Rounded | Angular |
| Surface texture | Fairly smooth | Rough |
| Fresh concrete workability | Usually better | Needs mix adjustment |
| Bond with cement paste | Good | Usually stronger |
| Fines | Mainly silt and clay | Mainly stone dust |
Effect on concrete
In fresh concrete, rounded particles slide past each other more easily and reach the target workability with less water. Crushed particles have more specific surface and may raise the water demand of the mix.
On the other hand, the rough surface of crushed sand strengthens the mechanical bond with cement paste. With a suitable mix design and a plasticizing admixture, this sand can serve structural concrete well.
The role of fines
The nature of the fines differs between the two. Fines in natural sand may be clayey and harm water demand and admixtures. Fines in crushed sand are often stone dust and, within limits, can help fill voids. This is why one number is not enough to control fines, and the sand equivalent test gives useful information about their nature.
Selection by application
- Ready-mix and structural concrete: both types can be used within grading limits. Some projects combine them to balance workability and strength.
- Mortar and plaster: a sand with finer grading and better workability suits best. See the specifications of mortar sand.
- Asphalt mixes: angular particles help mix stability, and crushed sand is the usual choice.
Summary
The final measure is how the material tests in the actual mix design. Before use, compare a sample with the project specification and make a trial mix to see how it really behaves.
Related products
