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. 2019 Feb 5;12(3):488.
doi: 10.3390/ma12030488.

Microwave Radiation as a Pre-Treatment for Standard and Innovative Fragmentation Techniques in Concrete Recycling

Maarten Everaert  1 , Raphael Stein  2   3 , Simon Michaux  4   5 , Vincent Goovaerts  6 , Carlo Groffils  7 , Simon Delvoie  8 , Zengfeng Zhao  9 , Ruben Snellings  10 , Peter Nielsen  11 , Kris Broos  12
Affiliations

Microwave Radiation as a Pre-Treatment for Standard and Innovative Fragmentation Techniques in Concrete Recycling

Maarten Everaert et al. Materials (Basel). .

Abstract

Recent advances in concrete recycling technology focus on novel fragmentation techniques to obtain aggregate fractions with low cement matrix content. This study assesses the aggregate liberation effectiveness of four different treatment processes including standard and innovative concrete fragmentation techniques. Lab-made concrete samples were subjected to either standard mechanical crushing technique (SMT) or electrodynamic fragmentation (EDF). For both fragmentation processes, the influence of a microwave weakening pre-treatment technique (MWT) was investigated. A detailed analysis of the particle size distribution was carried out on samples after fragmentation. The >5.6 mm fraction was more deeply characterized for aggregate selective liberation (manual classification to separate liberated aggregates) and for cement matrix content (thermogravimetric measurements). Results highlight that EDF treatment is more effective than SMT treatment to selectively liberate aggregates and to decrease the cement matrix content of the >5.6 mm fraction. EDF fully liberates up to 37 wt.% of the >5.6 mm natural aggregates, while SMT only liberates 14⁻16 wt.%. MWT pre-treatment positively affects aggregate liberation and cement matrix removal only if used in combination with SMT; no significant effect in combination with EDF was recorded. These results of this study can provide insights to successfully implement innovative technology in concrete recycling plants.

Keywords: aggregates; cement matrix; concrete; crushing; electrodynamic fragmentation; microwave weakening; recycling.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The lab-made concrete samples.
Figure 2
Figure 2
Experimental testing procedure based on microwave pre-treatment (MWT), standard mechanical treatment (SMT) and electrodynamic fragmentation (EDF) techniques, yielding four different fragmented samples.
Figure 3
Figure 3
Visible cracks in the concrete blocks, a direct result of the exposure to the MWT treatment.
Figure 4
Figure 4
Particle size distribution of the natural aggregates and of the samples obtained after fragmentation using different treatments. Notice the deviation in the distribution between the SMT and the MWT-SMT samples in the 2–8 mm range (inset). The vertical dotted line indicates the >5.6 mm fraction used for further characterization.
Figure 5
Figure 5
(a) The mass of the >5.6 mm fraction relative to the total mass (%), derived from the particle size distributions, presented for each treatment. The dotted line at 16.7% is the aggregate content of the initial concrete; (b) The mass of the liberated aggregates relative to the mass of the >5.6 mm fraction (%); (c) The liberated aggregates in the >5.6 mm fraction relative to the aggregates present in the concrete (%). The different letters in these graphs indicate statistical (P ≤ 0.05) differences between the treatments without MWT (capital letters) and between the treatments with MWT (lowercase). Significant differences of the effect of MWT on SMT and EDF (six replicates) are indicated by asterisks above the pair of bars (* P ≤ 0.05; ** P ≤ 0.01; ns: not significant). Error bars in the figures represent standard deviation.
Figure 6
Figure 6
Pictures from the hand-sorted particles of the >5.6 mm fraction obtained after the SMT and MWT-SMT fragmentation treatments, classified as fully liberated aggregates (FL), aggregates with adhering cement matrix (AC) and chunks of cement matrix (CM). This overview shows visually that the CM class has a relative higher contribution for material recovered after SMT compared to material recovered after MWT-SMT. The relative distribution of the recovered aggregates along these classes is in alignment with the results shown in Figure 5b. The measured total mass of the particle classes from each of the treatments is presented in Table 1.
Figure 7
Figure 7
The cement content of the different particle classes (FL: fully liberated aggregates; AC: aggregates with adhering cement matrix; CM: cement matrix chunks) in the >5.6 mm fraction, presented for the different fragmentation treatments.
Figure 8
Figure 8
The cement content of the >5.6 mm fraction, derived from the particle class distribution (Table 1) and the particle class cement content. The different letters in these graphs indicate statistical (P ≤ 0.05) differences between the treatments without MWT (capital letters) and between the treatments with MWT (lowercase). Significant differences of the effect of MWT on SMT and EDF (three replicates) are indicated by asterisks above the pair of bars (* P ≤ 0.05; ns: not significant). Error bars in the figure represent standard deviation.
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