# Aggregate (geology)

In geology, particularly in mineralogy and petrology, an aggregate is a mass of mineral crystals, mineraloid particles or rock particles. Examples are dolomite, which is an aggregate of crystals of the mineral dolomite, and rock gypsum, an aggregate of crystals of the mineral gypsum. Lapis lazuli is a type of rock composed of an aggregate of crystals of many minerals including lazurite, pyrite, phlogopite, calcite, potassium feldspar, wollastonite and some sodalite group minerals.

## Definition and composition

Aggregates are defined as a mass or body of rock particles, mineral grains, or a mixture of both. As the building blocks of construction, they encompass various hard, inert materials such as sand, gravel, crushed stone, and slag. These materials can be used alone, such as in railroad ballast or graded fill, or mixed with cement or bituminous substances to create concrete, mortar, or plaster. In geology, aggregates primarily refer to the natural and man-made materials extracted for construction purposes, but the term can also include rock material used as a chemical or metallurgical fluxstone. Aggregates can also be a combination of minerals joined in an irregular fashion to form a material similar to concrete and are further classified by size, such as chippings, coarse aggregate, and fine aggregate, or by weight, such as lightweight aggregate.

## Types

Aggregates can be classified into several categories based on their origin and physical characteristics.

### Natural aggregates

Natural aggregates include materials like sand and gravel that are extracted directly from the earth. They can be formed through geological processes such as erosion, sedimentation, and glaciation.

### Manufactured aggregates

Manufactured aggregates are produced by crushing larger rocks into smaller pieces. Common examples include crushed stone and recycled concrete.

### Coarse aggregates

Typically retained on a 4.75 mm sieve, coarse aggregates include larger stones and gravel.

### Fine aggregates

Passing through a 4.75 mm sieve, fine aggregates consist of sand or smaller particles that fill voids in coarse aggregates.

### Lightweight aggregates

Lightweight aggregates are made from materials such as expanded clay or pumice and are used to reduce the overall weight of concrete structures.

### Heavyweight aggregates

Composed of dense materials like barite or magnetite, heavyweight aggregates are utilized in applications requiring radiation shielding.

## Geological formation

The geological formation of aggregates is influenced by various geological processes. Over time, natural forces such as wind and water erode bedrock, leading to the creation of sand and gravel deposits. These materials sediment in riverbeds, lakes, and coastal areas.
In regions affected by glaciers, aggregates may consist of till—a mixture of various particle sizes deposited by melting ice—or glaciofluvial deposits formed by meltwater streams. Volcanic eruptions can produce aggregates like pumice and ash, which are lightweight and porous.
The distribution of aggregates is often determined by geological investigations that assess the location and nature of potential aggregate deposits in an area.

## In construction

Aggregates play a crucial role in construction due to their mechanical properties. They provide strength and stability to structures such as roads, bridges, and buildings. In concrete production, aggregates account for about 60% to 80% of the total volume. Their presence enhances the durability and workability of concrete mixes. Aggregates also serve as a low-cost extender for more expensive materials like cement or bitumen, making construction more economical. Additionally, recycled aggregates from construction waste contribute to sustainability efforts by reducing landfill use and conserving natural resources.