Dry process for recycled concrete aggregate

In the UK, a formalised quality control procedure for the production of aggregates from recovered inert waste has been produced by the Quarry Products Association, the Highways Agency and the Waste and Resources Action Programme, and there are similar schemes in force elsewhere. These cover minimum requirements for such characteristics as particle density, resistance to fragmentation, bulk density, water absorption, magnesium sulphate, abrasion resistance, drying shrinkage, chemical and organic impurities, alkali silica reaction, and so on.

In Japan, the wide imbalance between the supply of demolition concrete and suitable applications for this material when conventionally crushed and screened, has led to the development of a highly effective process for enhancement.

Aggregate equipment manufacturers Kotobuki Engineering & Manufacturing Co Ltd (Kemco) of Hiroshima, renowned for its innovative dry sand manufacturing process, set the target of recovering coarse and fine aggregate from recycled concrete, with quality as close as possible to the original constituents. The approach was to analyse the difference between fresh aggregate and traditionally produced recycled aggregate, and then to set about improving it.

Comparing bulk density and water absorption of material in a wide range of samples gives figures for coarse aggregate. Fine aggregate displayed similar characteristics, but with different values; the bulk density of fine aggregate was much lower, and the range of water absorption between samples was more variable.

Clearly, the difference lay in the presence of the adhering cement paste, amounting to between 25 per cent and 54 per cent of the mass of material, and a process was needed to strip it away, exposing the original surfaces.

Kemco has long experience with Barmac VSI production and application (after 32 years, it is a long-standing licensee of the original technology), and has developed these machines in a number of job-specific ways. This led Kemco to experiment with the preferential crushing ability of an autogenous VSI to allow the softer, lighter cement layer to be peeled away from the original aggregate. Kemco found that multiple passes through a specially designed machine could easily remove the bulk of the cement paste, and research was carried out to find the optimum practical, and economically viable, processing needed for the product to be reused in concrete manufacture.

The target was the H grade of the Japanese Industrial Standard (JIS), which specifies 2.5g/cm3 density and less than three per cent water absorption. Although it was entirely possible to improve upon this, the standard system provides for this level of quality by using three passes of the material through the crusher.

Five passes resulted in a return, in any practical sense, to the original properties of the constituent aggregates. However, it was found thatlittle benefit accrued from the extra work done, since the density of the remaining cement paste increases as more is removed, and once reduced to less than 10 per cent, its negative effect on the quality of concrete is considered insignificant.
Testing in concrete mixes was carried out on many samples of processed and unprocessed recycled concrete, comparing it with fresh aggregates, with some quite impressive results2, as illustrated in the following examples:

1. Drying shrinkage of concrete using three aggregate samples.
?  The drying shrinkage of three passes sample is equivalent to that of natural sand.
?  Unprocessed aggregate has 2.2 times greater drying shrinkage.
?  Aggregate description refers to both coarseand fine aggregate.

2. W/C ratio versus compressive strength for four coarse samples.
C/W ratio and compressive strength for each sample are related as shown.
? The strength of the three passes sample is equivalent to that of natural aggregate.
? The strength of unprocessed aggregate is much lower than that of the other three, especially at higher strengths.

3. C/W ratio versus compressive strength for four fine aggregate samples.
? Concrete with processed sample plus 10 per cent fly ash can attain compressive strength equivalent to or better than that of natural sand, regardless of W/C ratio and curing time.
? Compressive strength of unprocessed sample is inferior, especially at higher W/C ratio.

4. Dynamic modulus of elasticity of concrete for five coarse and fine aggregate samples.
? Dynamic modulus of elasticity of concrete using three passes coarse aggregate, with either processed or natural sand, is equivalent to that of natural aggregates even after repeating freezing and thawing tests 300 times.
? The concrete made with unprocessed coarse and natural fine aggregate dropped approximately 50 per cent after 300 freeze/thaw cycles compared with natural and processed samples.
? The concrete using unprocessed coarse and fine material dropped to 40 per cent after 120 freeze/thaw cycles. Thereafter, no data was obtained.

Having clearly proven the benefits of removing cement paste from the crushed concrete, Kemco could now proceed with development of a suitable process, incorporating features of the V7 Dry Sand-Making System (Figure 1). A four-deck version of the air screen was incorporated to give a full range of product sizes while taking care of disposal of the unwanted cement in the form of micro fines. The other features of the V7 that were equally desirable in this process can be summarised as follows:
1. Low capital cost achieved by using a dry process.
2. Lowest possible operating and maintenance costs through the use of autogenous VSI crushing technology.
3. Small footprint, allowing the system to be sited in a limited space near the source of recyclable concrete material.
4. Minimal noise level.
5. Full dust encapsulation and elimination of emissions.
6. Optional add-on process to pelletise micro fine material for use in soil stabilisation, etc.

The RC7, as the unit is called, is available in three capacities: RC7-1 (35 tonnes per hour), RC7-2 (55tph), and RC7-3 (75tph). Feed size is restricted to 40mm, and the recovery ratio will depend upon a number of factors such as feed gradation, original cement content of the concrete, etc. However, as a guide, around 45 per cent is delivered as fine aggregate, 23 per cent coarse aggregate, and 32 per cent filler, which consists of 50 per cent cement paste and 50 per cent rock fines.

The system can be supplied in modular form for easy erection, and to facilitate re-siting when necessary.

As with their V7 System, Kemco is set up to accept bulk samples – generally three tonnes per test – of customers? materials for process evaluation through a pilot scale plant, and useful quantities of the resulting finished material can be returned for testing in concrete and for other uses.

Work is proceeding to establish means of utilising the potential cementitious content of the filler, but even without doing so, it seems that Kemco has produced a system capable of adding quality and value to a material which we can expect to be produced in ever increasing quantities from end of life demolitions, and which will otherwise have only limited potential for re-use.

Hugo Pettingell is a building materials consultant and manages his own consultancy, Hugo Petingell Mineral Services, based in London, UK.

1. The quality protocol for the production of aggregates from inert waste. Published by the Waste and Resources Action Programme.
2. Kemco product development research.
3. Sakurai K, Chikamatsu R, Kaya T, Itaya A. Quality of concrete using recycled aggregates manufactured by rotary centrifugal crusher and applicability of recycled micro fines to admixtures. Concrete Journal (46:8), August 2008.

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