What are aggregate particles smaller than the size of a grain of sand referred to as?

The term used for aggregate particles smaller than the size of a grain of sand is "fines." Fines refer specifically to the small particles, usually defined as those passing through a specified sieve size, typically less than 0.075 mm (75 microns). These particles are critical in construction and materials engineering because they play a significant role in the workability, strength, and durability of concrete and other materials.

Fines can fill the voids in coarser aggregates, which enhances the packing density and reduces water requirements in mixes, contributing to better overall performance. Understanding the composition of aggregates, including the presence of fines, is essential for any contractor involved in concrete and construction materials.

The other terms provided, such as impurities, microaggregates, and granules, do not accurately describe the particles that are specifically smaller than sand. Impurities generally refer to unwanted materials within a mix. Microaggregates might suggest a broader classification of very small aggregates, and granules typically indicate slightly larger particles than fines.