Special Techniques For Improving The Quality of Recycled Materials

By Julien Hubert, Zengfeng Zhao, Michel, Luc Courard, University of Liège

One of the goals of the SeRaMCo project is to improve the sustainability of the concrete industry. One possible avenue to reach this goal is the use of recycled aggregates obtained from Construction and Demolition Waste (CDW). They are obtained by crushing suitable CDW such as concrete, bricks, tiles, ceramics, natural stones. However, the crushed material still contains unwanted elements such as rebars coming from reinforced concrete, plastics, woods, and paper. The rebars can be easily sorted using electromagnets. The other elements, however, require manual sorting on a low-speed conveyor belt. Another possible solution to get rid of these unwanted elements is to wash the recycled aggregates. For example, as part of this project, Tradecowall has upgraded their Saint-Ghislain (Belgium) recycling plant to be able to wash recycled aggregates for the production of washed aggregates (4-80 mm in diameter) with the desired fractions and washed sands (0-4 mm in diameter). The installation is visible in Figure 1.

Figure 1: Tradecowall washing plant in St-Ghislain

Washing

The process consists of washing and immersing crushed aggregates into water in order to clean the coarse fraction from the sandy fraction and from the clayey fraction. The washing process is summarized in the following diagram:

The goal of this research is to explore the effect washing has on the aggregates’ properties and more specifically whether it allows the recycled aggregates to respect the national regulations on aggregates quality in the North West Europe region.

Experimental campaign

Washed and unwashed aggregates come from the same source. Two types of aggregates have been studied: concrete aggregates and mixed aggregates. The properties that were reviewed are: grain size distribution, fine content, constituent, bulk density, particle density, water absorption, and resistance to fragmentation. These correspond to the properties submitted to regulations in standards related to concrete.

Results

Grain size distribution

The grain size distributions of the different batches studied are presented in Figure 2:

Figure 2: grain size distribution

Figure 2 clearly illustrates that washing tends to significantly decrease the sandy fraction (0-4 mm in diameter). Washed concrete and mixed aggregates are respectively composed of 4.4 % and 12.0 % of sands, while unwashed aggregates are characterized by a sandy fraction much more abundant, between 45 and 50 % for both types of recycled aggregates. This means we get a much more constrained grain size distribution which does not respect the minimum values of cumulated passing (EN 480-1- Min on Figure 2). This implies that the use of recycled aggregates in concrete will require the addition of a sandy fraction. It also means that we obtain better sorted aggregates. Most of the investigated commercialized natural aggregates have a percentage of aggregates included in the declared grain size range, usually around 80 %. The washed recycled aggregates produced have higher rates of sorting since the proportion reaches 85 – 90 %. It means that more than 85 – 90 % of the produced washed aggregates are included between 4 and 20 mm in diameter.

Fine content

Another important improvement of the recycled aggregates quality is the significant decrease in fine content. Fine content is one of the most restrictive parameters for aggregates used in concrete, and as can be seen in Figure 3, washing allows the aggregates to respect the regulations pertaining fine content in all the partner countries.

Figure 3: Fine content

Constituents

Another advantage of washing the recycled aggregates is that it gets rid of part of the unwanted elements. Indeed, some of them such as wood, paper, and plastic are floatable and can easily be separated from the rest of the recycled aggregates by submerging them into water. Figure 4 exhibits the expected decrease.

Figure 4: Floating content in washed and unwashed recycled aggregates

Bulk and particle densities

The bulk density of both washed and unwashed aggregates respect the regulations. In fact, the bulk density of washed concrete and washed mixed aggregates 4/20 corresponds to 1.22 and 1.19 g/cm³ respectively, whereas the bulk density of washed concrete and washed mixed sands corresponds to 1.42 and 1.34 g/cm³ respectively.

Recycled concrete aggregates’ particle density ranges between 2.3 (unwashed) and 2.5 g/cm³ (washed) and recycled coarse mixed aggregates particles density varies between 2.1 (unwashed) and 2.4 g/cm³ (washed). In every case, the measured values of particle density are above the standard requirements in every investigated NWE countries. Indeed, the minimum particle density of concrete aggregates is defined at 2.0 or 2.2 g/cm³. These values move at 1.7 or 2.0 for mixed aggregates.

Washing is not required for the recycled aggregates to respect the regulations in term of bulk and particle densities. Note, however, that the measured values are lower than those usually characterizing natural aggregates. For reference, the particle density of sandstone is 2.6-2.65 g/cm³ and of limestone is 2.65-2.7 g/cm³.

Water absorption

The water absorption was measured on both sands and coarse aggregates. The general trends show that mixed fractions have higher water absorption than concrete samples. In fact, water absorption of concrete aggregates ranges between 6.0 and 6.3%. This parameter reaches 9.2% in mixed aggregates. Sandy fractions have lower water absorption for both concrete and mixed samples, with respectively 4.9 and 4.8%. Overall, washing has no influence on the water absorption of recycled aggregates. Nonetheless, the measured values were lower than the maximum values required in standards from Germany and Belgium. The maximum values of water absorption in concrete and mixed aggregates are defined at respectively 10% and 15% in standards from both countries.

Note, however, that measured values are much higher than those usually characterizing natural aggregates. Water absorption usually ranges between 1 and 1.5% in limestone and between 1 and 2% in sandstone.

Resistance to fragmentation

Resistance to fragmentation was measured through the Los Angeles test. The results show that no significant difference exists between washed and unwashed materials. Concrete aggregates have a Los Angeles coefficient of around 34 and 36, and mixed aggregates have values between 42 and 43. Thus, concrete aggregates have a higher resistance to fragmentation than mixed aggregates. The most restrictive standards in all of the partner countries set a maximum value of 35 for concrete aggregates and 50 for mixed aggregates. This means that the measured results are in agreement with the national standards. Note that Luxembourg does not define a value for mixed aggregates. Consequently, recycled mixed aggregates in Luxembourg are submitted to concrete recycled aggregates standard for which the maximum value is 40. The mixed aggregates’ resistance to fragmentation is therefore too low for Luxembourgish standards.

Conclusion

Washing significantly improves some properties of recycled aggregates. It also allows for the production of clean coarse aggregates with the smallest grain diameter higher than 0 mm (d>0mm). Moreover, Washing also decreases fine content and the amount of floating elements. This matters because fine content is the most constraining parameter for aggregates used in concrete, and washing brings fine content levels below the maximum values defined in the investigated national standards.

However, washing has less influence on the following parameters: particle density, water absorption, resistance to fragmentation, and the proportion of non-floating elements.