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Apr 28, 2020

Grain sizes – the meaning and effect on production

Oldemar Meneses
Oldemar Meneses
Manager, Impactor and Sand solutions
Different grain sizes are familiar to those working in the aggregate industry, but we rarely stop to think how much effect these grains have on the design of the production process. It turns out that the desired grain size is one of the defining factors, not only in what the aggregate is used for, but also for how it is produced in the first place.
Four piles of example grain sizes.
Aggregates of different grain sizes for testing. From left to right: manufactured sand of 1-1.25 mm and 4-5 mm, fine aggregate of 10-12.5 mm and 20-25 mm

The general idea is that the smaller the aggregate size, the more we need equipment to produce it. Production of aggregate starts typically from 800 mm pieces of blasted material. One crusher can only reduce the size by a certain amount. For example, manufactured sand is a very fine product with high quality requirements.

Even though all crushing processes produce fine products as a result of nature of the crushing process, those fines will not necessarily achieve the specifications for manufactured sand, or any desired specification. So they may need an additional crushing and screening process to meet those requirements. Today, manufactured sand is a lucrative business opportunity for many plants in this age of rapid urbanization. This is why more and more aggregates producers are looking into adding it to their repertoire.

Base materials

The base materials, in turn, are the easiest aggregates to produce. There are some demands for their grading curve, but typically, making base materials only requires two crushing stages. In that sense, producing them is relatively simple and cost-efficient.

Nowadays, the trend still seems to be that there are more and more quality parameters to be met, even for the base materials. This is especially true in the Nordic countries. The industry has nonetheless welcomed this change, because it means that the quality of all grain sizes has improved, or at least has been studied. High-quality material can now be found in all classes of aggregates, not just in those containing the finest products.

Sand equivalent parameters in road base material is nowadays a specification that needs special attention when the base material has to be produced from clean blasted rock. Therefore, the proper selection of the crushing and screening equipment – as well as the process design – plays a very important role in achieving this parameter.

Large grain sizes

Aggregates with larger grain sizes require less equipment, so they are more commonly produced than the finer products. However, production does not happen in an economic void. The demand for different aggregates varies based on local needs and the level of development. Some countries, like South Korea and India, have growing populations, and their cities are expanding substantially. These countries are building a lot of new infrastructure, for example roads and skyscrapers. The top layer of an asphalt road consists mostly of fine aggregate, but that layer is only about 15 centimeters thick. Underneath it, there is a huge amount of base material.

The selection of crushing and screening equipment plays a very important role; once the material source characteristics are known, the process design will define the best arrangement to meet the requirements.

The situation is different in Europe, where most of the infrastructure is already built. In countries like Finland, aggregates are mainly used for maintenance and resurfacing the existing road network. In this case, only high-quality fine and coarse aggregates are needed. Some countries, like Japan and South Korea, have introduced strict regulation on the use of natural sand, which in turn creates a need for a certain aggregate of a finer grain size: manufactured sand that has to meet very strict specifications.

All in all, there are multiple factors to consider when starting aggregates production. Deciding what grain sizes are the best option for your business requires thorough consideration. No matter what end product needs to be produced, there are two parameters that have to be taken into consideration to achieve the market quality demands:

  • Specifications related to meeting the gradation curve grain sizes
  • Product shape along all the gradation curve grain sizes

That being said, the selection of crushing and screening equipment plays a very important role in achieving those parameters; once the material source characteristics are known, the process design will define the best arrangement to meet the requirements.

You can compare different grain sized aggregates using the tables below. The first one displays general sizes and end uses around the globe, whereas the second one gives us a peek at the aggregates offering of the Gimcheon quarry in South Korea.

General grain sizes

Example: Gimcheon quarry grain sizes

Quick facts on grain sizes

The most common classes of aggregate are coarse aggregate, fine aggregate and manufactured sand. Mixed aggregates, also called base material, consist of different mixtures of these three. The term “grain size” is used to classify these aggregates based on the size of the rock grains in the material.

Coarse aggregate is material with a larger grain size, usually varying from 10 mm to 32 mm. One of the most well-known types of coarse aggregate is ballast stone. Whereas coarse aggregate is generally used in ready-mix concrete, the most common application for ballast stone is train track beds. For that reason, ballast stone is also referred to as “railway ballast”.

Fine aggregate is small in grain size, usually smaller than 8 mm, and it is used in making asphalt concrete and ready-mix concrete. Fine aggregate has different sub-classes that depend on the end use of the aggregate.

Manufactured sand is produced in an industrial process that has specific parameters for the product. Its grain size is usually smaller than 5 mm. The production process of manufactured sand needs to be designed carefully to ensure the high and consistent quality of the product. The consistency is important, as the properties of the sand have a large impact on the properties of the product for which it is used. Manufactured sand is most commonly utilized in making premium-quality concrete used in buildings, or asphalt concrete.

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