Cement is the most expensive component of the concrete compound.
The workability of the final concrete mix and the amount of cement used are two of the most important parameters in concrete production. These parameters, in turn, are mainly affected by the quality of sand used in the process.
The consistency of the sand can be a challenge when using natural or crushed sand that has not been produced in a well-controlled industrial process. Inconsistent sand quality leads to a higher safety margin, which results in an unnecessary increase in the amount of cement used. Cement is the most expensive component of the concrete compound, representing more than 50 per cent of the cost of raw concrete, but less than 20 per cent of the volume.
Characteristics of fine-sized aggregates in concrete are also crucial. Only a perfectly cubical and rounded shape of all fractions and their consistent quality can form an easily workable concrete mass. This allows the lowest possible amount of cement to be used in the final cement mix.
Metso has developed an industrial process to reduce the amount of cement needed in the concrete mix. An unbiased study done in Norway in 2015 verified that the production cost of the concrete mix could be significantly decreased by as much as 15 per cent when cement was replaced with a high quality manufactured sand. As a raw material, industrially produced sand is less expensive than cement. The price of manufactured sand can also be lower compared with natural sand, which decreases the production cost of the concrete even further.
If we consider the fact that natural sand sources are getting scarcer, especially close to urban areas, transportation costs can be considerable if the sand has to be transported with trucks from dozens of kilometres away. In some cases, you even need to haul it from other countries, like in the United Arab Emirates, where sand is imported from Australia.
Thanks to its superior quality characteristics, manufactured sand produced with Metso process and equipment enables considerable savings in concrete production. The strength of the concrete improves when gaps between the aggregate fractions are smaller and less binder is needed. Aggregates are practically always stronger than cement when it comes to increasing the strength of the concrete.
The quality characteristics of aggregates are mainly based on an optimised shape, continuous grading and the moisture of different fractions in the concrete mix. The optimal shape can be best achieved by relying on cone and vertical shaft impact crushers with different crushing stages after the primary crushing.
Metso’s Nordberg HP cone and Barmac VSI crushers are the most common solutions to make perfectly shaped manufactured sand and aggregates, also with very abrasive rock as feed material. The Nordberg N13 and NP15 impact crushers are commonly used for the finished shape in the tertiary crushing stage when the abrasiveness of the rock is slightly lower.
One of the key benefits of sand produced with a Metso process is the precise control of the continuous grading. This process requires deep integration and automation between different crushing and separation stages. Screens do the work when oversize aggregate fractions exist by returning these fractions back to the crushers. The screening is then followed by the final and most critical stage when controlling the continuous grading. This is when air classifiers take over the material flow and classify different fractions with an accuracy of microns. Air classifiers are the most efficient way to achieve ultra-fine separation in industrial applications. Dry classifying uses no water, which provides a very environmentally friendly and economical classifying solution.
Metso air classifiers provide a full solution and an alternative to wet classifying. The solution is designed to achieve extremely accurate separations even though these air classifiers contain no moving elements in the air stream. This is achieved through the classifiers’ intelligent airflow design and use of a recirculating, secondary airflow. Because the airflows are not affected by wearing, the grading always remains the same with an accuracy of microns. At the same time, particle moisture remains at an optimal level.
An optimal shape, continuous grading and controlled moisture during the production process all make it possible to decrease the safety margin of mixed concrete. This means that even up to 15 per cent of the cement can be replaced by using Metso’s high quality industrial sand in the concrete mix when the quality characteristics of the aggregates are well controlled. Replacing cement with more affordable aggregate materials leads to a better profitability of the concrete mixing plant and better quality concrete.