Fine aggregate
The sand fraction of concrete. Fine aggregate fills the spaces between stones and largely determines how a mix finishes under a float and trowel.
What does fine aggregate mean?
Too little sand and a mix is harsh, rocky, and difficult to close; too much and it becomes sticky, requires more water, and shrinks more. Sand also carries moisture into the batch, which is why plants adjust batch water based on aggregate moisture readings. Manufactured sand from crushed rock behaves differently than natural river sand and often needs mix adjustments.
In practice
Finishers feel sand content immediately: a sandy mix creams up easily, a rocky one fights the bull float.