Soil/aggregate fines identification – particles of about 0.002 mm size In standard soil and fines classification, particles with a nominal size around 0.002 mm belong to which category?

Introduction / Context:
In both soil mechanics and aggregate fines control for concrete, distinguishing silt from clay is important because clay exhibits plasticity, high surface area, and strong water sensitivity. The conventional boundary between silt and clay is based on particle size coupled with plasticity characteristics.

Given Data / Assumptions:

• We refer to standard geotechnical size bands.
• Plasticity is a behavioural attribute but size is the quick index.
• 0.002 mm (2 microns) is the conventional dividing size.

Concept / Approach:
Clay is defined as the finest fraction, typically smaller than about 0.002 mm. Due to their plate-like crystal structures and large specific surface area, clay minerals strongly interact with water, causing plasticity, shrink–swell, and high cohesion—properties that markedly influence workability and water demand in concrete fines as well.

Step-by-Step Solution:
Identify the conventional silt–clay boundary at ~0.002 mm.Classify particles near or below this size as clay.Select “Clay”.

Verification / Alternative check:
Unified Soil Classification System and related standards use 0.002 mm as a reference divider; Atterberg limits further confirm clay behaviour.

Why Other Options Are Wrong:

• Sand: ranges roughly from 0.075 mm to several millimetres—much coarser.
• Gravel: coarser still, above the sand range.
• “None” is incorrect because clay is the recognized category.

Common Pitfalls:
Assuming all fines behave like clay; silt is finer than sand but lacks clay’s plasticity and surface activity.

Final Answer:
Clay