Dry Classification of Fine Aggregates for Concrete and Asphalt
Aggregate is defined as particles of rock which in combination are used as a civil engineering material. It is used in bounded form in asphalt and concrete. Historically natural sand has been used as fine aggregate for concrete production in Sweden. Due to environmental reasons the Swedish government is limiting the extraction of natural sand resources. Manufactured sand is therefore being investigated in Sweden as a replacement product.
Properties of the fine aggregates such as shape and particle size distribution are affecting the properties of both the uncured and finished concrete. It is possible to produce manufactured sand with a good shape, however the crushing process results in a relative large amount of fines (particles smaller than 63 µm). Research shows that the amount of fines in manufactured sand affects concrete properties such as, workability, water demand, drying shrinkage, compressive and flexural strength. To achieve a good concrete the manufactured sand should have somewhere between 5-10 mass percent of fines depending on rock type. It is common that manufactured sand has around 15-25 mass percent of fines after the crushing stage. In this thesis, dry air classification has been investigated to reduce the amount of fines in manufactured sand.
There exists a number of air classifiers produced for different industrial use. This work has investigated two air classifiers using different design approaches. The investigated air classifiers were a circulating air classifier and a two-stage air classifier.
The circulating air classifier investigated is based around a centrifugal-crossflow separation zone and an internal aerodynamic cycle. The classification process is controlled by two fans, the circulating fan and the separator fan. The circulating fan creates the internal circulating air flow. The air flow created by the separator fan changes the cut size. Experiments performed on the circulating air classifier showed a diffuse cut size. It was possible to reduce the amount of fines from 15 mass percent to 5 mass percent.
The two-stage classifier is based on two different air classifiers serial mounted using the same external air fan. The first stage consists of a gravitational-crossflow separation zone modified by allowing for recirculating of particles towards the separation zone. The second stage is based around a vertical standing centrifugal-counterflow zone modified to allow for recirculation of material. The two-stage classifier produces three different products, and both performed experiments and models have shown that recirculation is important to get a sharp cut size. The amount of fines was at best reduced in the coarse product from 15 mass percent to 1 mass percent.
In conclusion, air classification has been technically proven to be able to reduce the amount of fines in the finished product. The classification result will depend on the chosen technology. The economic feasibility of air classification for manufactured sand production will depend upon the local supply of natural sand and the quarry situation.
Key words: Air classification, Manufactured sand, Concrete, CFD