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Aggregates are classified by size using the ASTM No. 4 sieve as the dividing point. Particles retained on the sieve are classified as coarse aggregates, while those passing through are classified as fine aggregates. This size classification is fundamental to determining aggregate behavior and performance in concrete mixtures.

Natural aggregates are obtained from three primary sources: crushed stones from rock quarries, gravel extracted from gravel pits, and sand mined from riverbeds. These materials are directly sourced from natural deposits and form the basis of most conventional concrete construction applications.

Artificial aggregates are manufactured through industrial processes. Expanded blast furnace slag is created by spraying water on molten slag, expanded clay is produced by heating clay in rotary kilns, and recycled aggregates are made by crushing demolished concrete. These engineered materials offer specific performance characteristics.

Aggregates are classified petrographically based on their mineral content, which depends on the parent rock source. Common minerals found in natural aggregates include quartz, feldspar, mica, and calcite. These mineralogical characteristics influence the aggregate's durability, strength, and suitability for different concrete applications.

Aggregates are classified by weight into three categories: normal-weight aggregates for standard concrete construction, heavyweight aggregates for producing dense concrete used in radiation shielding, and lightweight aggregates for creating concrete with reduced structural load or improved thermal insulation properties and performance.

Aggregate classification determines concrete performance and suitability for specific applications. Size classification affects workability and strength, source classification influences availability and cost, mineral composition affects durability, and weight classification enables engineers to select appropriate materials for standard construction, radiation protection, or thermal insulation requirements.

Aggregates make up nearly three-quarters of concrete's total volume, making their classification and selection critical to concrete quality. Understanding aggregate properties through size, source, composition, and weight classification enables engineers to design concrete mixtures that meet specific performance requirements and environmental conditions effectively.