General Classification of Aggregates

Aggregates, as defined in the field of construction and civil engineering, are granular materials of mineral composition, used either in combination with a binding medium to form concrete, mortar, and plaster. It is also used for various infrastructure projects and even as a material for decorative pieces.

Generally, aggregates may be broadly termed as either natural or artificial. 

Natural aggregates – Those derived from natural deposits in the earth’s crust include mountain/river rocks or rock fragments, like stone and sand. These are used in their natural state and may or may not have been subjected to screening, washing, beneficiation, or crushing. 

From left to right: Crushed Aggregates, Uncrushed Aggregates.

Artificial or synthetic aggregates – Those which result from the modification of materials, which may involve both physical and chemical changes. They are made out of various by-products such as fly ash, power plant solid waste, rice husk ash, blast furnace slag, granulated slag, granite powder, iron ore slag, a variety of lightweight aggregates such as expanded clay or shale, and a few specially processed materials of mineral nature. These particular aggregates are manufactured by subjecting to high heat through the rotary kiln or sintering process, resulting in the bloating or expansion of the raw materials. 

Types of Natural Aggregates:

A. According to origin:

Igneous – rocks formed originally by cooling from a molten mass and are commonly found in areas of former volcanic activity. Examples of materials of igneous origin are granite, basalt, diorite, gabbro, peridotite and diabase.

From left to right: Basalt, Granite, Diorite

Sedimentary – stratified rocks formed from sediments deposited for the most part underwater but occasionally as the result of wind action. Limestone, dolomite, shale, sandstone, chert, conglomerate, and breccia are rocks of sedimentary origin.

From left to right: Limestone-Chalk, Dolomite, Sandstone

Metamorphic – rocks that may have igneous or sedimentary origins but, as a result of prolonged intense pressure over geologic times, have been altered or metamorphosed into rocks with significantly different properties. Examples of these materials are gneiss, schist, amphibolite, slate, quartzite, marble, and anthracite.

  From left to right: Marble, Anthracite-Coal, Quartzite

B. According to density or weight:

Lightweight aggregate – These are aggregates used to make concrete with a density range from 160 kg/cu.m. to 1920 kg/cu.m, and a compressive strength range from 2,500 psi to 6,000 psi. Concrete produced from this type of aggregates are used as:

• walls, dividers, or in any non-load-bearing structural parts

• heat insulation              

• toppings or masonry

• aquifers for rooftop gardens, and

• nailing concrete.

Lightweight natural aggregates include volcanic cinders, scoria, pumice, diatomite, coquina, and perlite.

From left to right: Pumice, Scoria, Perlite 

Normal weight aggregates – The density of concrete using normal-weight aggregates ranges from 2.2 mt/cu.m. to 2.9 mt/cu.m., and these aggregates are commonly used in all types of general construction. When not particularly specified, the term aggregates usually refer to this type of material. Gravel-sized materials derived from igneous, sedimentary, and metamorphic rocks are generally normal weight aggregates.

Normal Weight Aggregates from Basalt Rocks. Photo courtesy of Lafarge Holcim

Heavyweight aggregates – These aggregates are characterized by a concrete density ranging from 2.9 mt/cu.m. to 3.6 mt/cu.m. Concrete produced with this type of aggregates are used as nuclear or radiation shielding, as counterweights in movable bridges, and submerged pipelines for gas, air, liquids, and as wear-resistant flooring in heavy-industrial factories. Heavy-weight aggregates are usually derived from magnetite, barite, and scrap iron.

From left to right: Magnite, Barite, Iron Ore 

C. According to size:

Coarse Aggregates – Materials bigger than 4.75 mm (No. 4 sieve) are considered coarse. They are classified further into their more specific dimensions such as:

1) Boulders – rock fragments with an average dimension of 12 inches or more. These are used for massive backfill, filter beds, soil stabilization, massive foundations, dams, dikes, embankments, etc.

From left to right: Mountain Quarry Boulders, River Boulders 

2) Cobbles – aggregates with average dimensions from 3 inches to 12 inches. Used for embankments, backfills, foundations, dikes, soil stabilization of smaller magnitudes, backfill, ripraps, canal linings, decorative units, etc.

From left to right: River Cobbles, Mountain Quarry Cobbles 

3) Gravel – includes sizes from 3 in. down to that retained in a No. 4 (4.75 mm.) sieve. These are used as ingredients for concrete and asphalt, pre-cast concrete, base and sub-base courses for roads, granular bedding, filler, railroad ballast, ornaments, etc.  

Crushed Gravel. Photo courtesy of  LafargeHolcim Aggregates Inc.

Fine Aggregates or Sand – These are particles that pass the No. 4 (4.75 mm) sieve and be retained on a No. 200 (75 micron) sieve and are used as a vital ingredient for concrete and asphalt mixes, pavement seal coat, sand bedding, fairways sand cap, masonry, etc.  These are commercially known as S-1, S-3, manufactured sand, natural sand, or white sand.

From left to right: Crushed Mountain Quarry Sand and River Sand

Silts and clays – Materials that pass the No. 200 (75 micron) sieve size and below and are distinguished from one another more by their ability to exhibit plasticity when wet. Some coarser silt are tolerated in aggregates, but clays are undesirable.

Presently, in the construction industry, aggregates are prescribed by their sizes and whether they are “manufactured” or natural.

Manufactured Aggregates – Aggregates from substantial deposits (such as mountains), which are extracted through quarrying and processed to be of commercial value.

Different Sizes of Basalt “Manufactured” Aggregates. Photo courtesy of LafargeHolcim Aggregates Inc. – Angono, Rizal

Natural aggregates – Those which need only be removed from their natural deposits as unconsolidated sediments (gravel, sand, stone from the river). 

From left to right: River Sand Quarry and Gravel Quarry

D. Other Classifications

Aggregates are also classified according to their shape (angular, cubical, rounded, flat, flaky elongated), their surface texture (smooth, rough), and their applications. However, these are usually included in the aggregate quality considerations; hence, not taken in this article.