Use of treated municipal waste incinerator ashes as concrete aggregate after fragmentation and sorting

Annually about 5.7 million tons of waste incinerator ash accumulate in Germany through the incineration of municipal waste. The new method of electrodynamic fragmentation allows waste incinerator ashes with complex material compositions to be broken at material interfaces and then sorted into materially pure components. The aim of current ASHCON research project is to substitute natural aggregate of the grain fraction 2/8 mm in conventional formulations for ready‐mixed concrete and concrete blocks by these secondary raw materials in order to keep them within the material cycle and reduce the use of primary raw materials. Since the utilization of fragmented waste incinerator ashes in concrete must be below the limit values with regard to possible pollutant concentrations (e.g. heavy metal concentrations), the suitability of a so‐called non‐destructive neutronic analysis method on real samples is also being investigated within the ASHCON project. The first results of sample collection, preparation and analysis as well as material properties of the produced concretes with the implementation of mineral residues from waste incinerator ashes are presented in this paper.


Introduction
The incineration of municipal waste in Germany produces about 5.7 million tons of incinerator ash per year.After the burning process the ash is abruptly cooled down in a water tank and continuously extracted by conveying systems [1].As an inhomogeneous, particulate mixture of materials municipal waste incinerator ashes (MWIA) are composed of about 45 % ash, 40 % melting products, 10 % waste-specific ingredients such as glass, ceramics and rocks, up to 3 % metals and up to 2 % of unburned organic material [2], whereby the composition varies depending on the incineration method and temperature as well as the waste composition.While the ashes contain particles of glass debris, inorganic and organic residues, as well as soot and dust in the range of < 0,002 to 2 mm, the melting products are highly porous, irregularly shaped mineral grains of a particle size > 2 mm consisting of a silicate matrix (glass) with crystalline new formations (silicates and oxides) [3,4].Especially in the fine fraction metals such as antimony, tin, molybdenum, tungsten, cobalt and rare earth elements, for example lanthanum, niobium, cerium and yttrium, which are classified as "critical raw materials" by the EU, may be present.However, the demand for suitable landfills (class DK I) by 2030 is in various regions of Germany so high that the security of waste disposal is at risk [5].
At the same time, ways are currently being explored to substitute natural raw materials for the production of concrete, in particular gravel and sand, with secondary raw materials e. g. construction waste.From the economic and ecological point of view the pressure to reduce or stop further mining of natural aggregates for concrete is increasing.The availability of river sand is already limited in various regions of Germany.
In order to conserve valuable landfill volume and natural resources, ways for recycling of waste incinerator ashes are being developed with the focus on the principles of a closed-loop economy.The current research project ASHCON aims to fragment and sort the coarse fraction of waste incinerator ashes using new technologies and to use the produced mineral fraction for the production of concrete products.The use of fragmented and sorted MWIA concentrates on the fraction of particle size 2/8 mm.This paper deals with the current state of the research project.

Abstract
Annually about 5.7 million tons of waste incinerator ash accumulate in Germany through the incineration of municipal waste.The new method of electrodynamic fragmentation allows waste incinerator ashes with complex material compositions to be broken at material interfaces and then sorted into materially pure components.The aim of current ASHCON research project is to substitute natural aggregate of the grain fraction 2/8 mm in conventional formulations for ready-mixed concrete and concrete blocks by these secondary raw materials in order to keep them within the material cycle and reduce the use of primary raw materials.Since the utilization of fragmented waste incinerator ashes in concrete must be below the limit values with regard to possible pollutant concentrations (e.g.heavy metal concentrations), the suitability of a so-called non-destructive neutronic analysis method on real samples is also being investigated within the ASHCON project.The first results of sample collection, preparation and analysis as well as material properties of the produced concretes with the implementation of mineral residues from waste incinerator ashes are presented in this paper.

2.1
Sampling at the disposal site The investigations of the research project ASHCON on processing, analysis and use of secondary (mineral) raw material for concrete production focus on municipal waste incinerator ashes (MWIA).Samples of the MWIA were obtained at the location of :metabolon situated at the landfill Leppe of the Bergischer Abfallwirtschaftsverband in Lindlar.Samples were taken from two selected landfill sections (Da and Db) as well as straight after delivery from waste incineration plants in Leverkusen, Bonn and Cologne and after removal of coarse scrap and organics.The variation of sampling locations considered a possible influence of different sample ages or storage periods in landfills on the properties of the material.A total of 8 samples of MWIA have been collected so far (Table 1).For the subsequent processing method of electrodynamic fragmentation, the fraction 2/8 mm was obtained from the individual samples of the landfill by sieving, which reduced the further treated sample quantity to about 30 % to 40 % of the original sampling size.The samples taken straight after delivery were collected as fraction 3/9 mm due to the treatment process.

Fragmentation and sorting of waste incinerator residues
In the following, the material of the 8 samples was separated into its constituent materials at Fraunhofer IBP using the innovative processing method of electrodynamic fragmentation.
The electrodynamic fragmentation process is based on a principle according to which composite materials are selectively separated under water by means of ultrashort (< 500 ns) lightning discharges.The discharges run along phase boundaries in the solid, providing very effective fragmentation of the material.Each discharge generates a pressure wave (p = 10 GPa) as it exits the material, which further breaks down the composite material into its components.Currently, the technology is used only sporadically and on a small scale for special applications.As for an industrial application in the recycling sector or mining, the technology must first be adapted to larger volume flows and designed for continuous operation.
This technology opens up many new areas of application in the recycling and use of dwindling resources and in the development of new raw material sources.For example, the large quantities of waste incinerator ash produced worldwide could be efficiently processed to produce raw materials for high-quality construction materials.
After electrodynamic fragmentation, sieving and drying of the MWIA samples, the components of the original ash were subsequently present as pure substances (ferrous metals, non-ferrous metals, glass, minerals, organic components) without adherence of other substances in a mixture.These coarse substances with a minimum particle size of 2 mm can be separated from each other relatively easy and with a high degree of purity by using sorting technologies.Finer fractions, on the other hand, can only be separated from each other with great effort and relatively low degrees of purity using current sorting technologies.Table 2 provides an overview of the samples that have been processed to date.

Process water analyses
The use of water as a process medium in the electrodynamic fragmentation process is a prerequisite for the separation along the grain boundaries of the solids in the composite.When the waste incinerator ashes come into contact with the water in the process vessel, a dissolution processes occurs which changes the chemical (e.g.pH) and physical (e.g.turbidity) properties of the water.The turbidity of the water immediately after the application of a series of high voltage pulses to MWIA is visible in Figure 2. A very similar result could be observed for all previous samples.The measurement of the pH value of the process waters produced during fragmentation resulted in a measured value in the alkaline range for all samples obtained so far.The values were measured with pH measuring strips and ranged between pH 7 and pH 8. Table 3 shows the chemical compositions of all the process waters measured so far after examination by means of the ICP-MS method.At the same time, the conductivity of the process water samples in the state after the fragmentation process was determined.Typical conductivity values for drinking/piped water in Germany range between 50 -500 µS/cm.

Sorting
After processing the ashes by electrodynamic fragmentation, the samples with grain sizes > 2 mm were largely liberated from ferrous and non-ferrous metals as well as glass fragments using sorting technology currently available on the market.However, the effort involved should not be unrealistically high, especially in terms of time and cost.It was possible to reduce the content of these potential interfering or harmful substances to a minimum in the subsequent tests for use in concrete by applying magnets, eddy current separation and optical measuring technology.Table 4 shows the various weighed fractions of the electrodynamically fragmented ashes after sorting.The mass balance of the fractions after sorting show that the sorting techniques used were able to remove slightly higher ferrous and non-ferrous contents from the ashes originating from landfill section Da and Db during sorting than from the fresh grate ashes.There was also a tendency for slightly higher amounts of glass to be present in the grate ashes than in the samples from the landfill.

Chemical composition of the fragmented and sorted fractions
Figure 3 provides an overview of the chemical composition of the fragmented and sorted mineral fractions of the waste incinerator residues with a grain size > 2 mm, subsequently referred to as WIR, which were dry after sorting.Despite the generally very heterogeneous composition of MWIA, the chemical composition of WIR (after sorting) was remarkably uniform across all samples.Thus, the contents of the main elements in the different samples were in a similar order of magnitude and the fluctuation of the measured values was in a range that is usual for MWIA.
The samples were also analyzed for their heavy metal contents using aqua regia digestion (DIN EN 13657: 2003-01).Representative samples of the 8 treated ashes (Table 4) were ground to a particle size < 125 µm.Remaining metallic pieces were sorted out and a small fraction (< 1 wt%) of a rubbery substance remained after grinding.The powder was digested and the elements listed in Table 5 were determined.

Note: n. c. = not calculated
The limits for copper and zinc are exceeded and in one ash sample also for lead.Since numerous mineral wastes are currently tested for their suitability as concrete components, the limit values will have to be discussed and updated in order to save resources and reduce land filling.However, the indispensable condition for the use of wastes in building products like concrete is the environmental compatibility during use and second life.Therefore, in the next step, the release of heavy metals will be tested for different application scenarios.Results will be reported at the conference.

New approach to determine the chemical composition
For material characterization, it is state of the art to take individual representative samples for subsequent analyses.Mostly, laboratory analytical tests, e.g.mass spectrometry, and/or surface analytical measuring methods, e.g.X-ray fluorescence analysis (XRF), are utilized.Prompt-gamma neutron activation analysis (PGNAA) is a measurement technique that is used in other industrial sectors, such as oil and gas exploration.Within the project ASHCON PGNAA is used to analyze ashes from waste incineration.Unlike other measurement techniques, PGNAA provides a nondestructive and integral analysis of largevolume samples.Neutrons penetrate the whole sample material and excite the sample´s atomic nuclei, which emit characteristic gamma radiation as a result of their energetic deexcitation.These nuclear reactions are independent of the chemical compound in which the atom is present.Elemental analysis by means of PGNAA is in principle calibration-free.Necessary measurement parameters are determined simulatively on the basis of the known sample properties in an iterative procedure.

Grain size distribution
The particle size distribution of the WIR samples was determined by sieving analysis according to DIN EN 933-1.
As a result, the WIR samples gained from fresh grate ashes corresponded to the average particle size distribution of the two samples from different landfill sections (Figure 4).

Bulk density and water absorption
The water absorption and bulk density of the samples were determined according to DIN EN 1097-6.For this purpose, the water absorption of oven-dried sample material was determined after 10 minutes and after 24 hours.The results are shown Table 6.Rhine gravel from the laboratory stock was used as reference material.In principle, the (apparent) bulk densities ρa of the WIR samples are in the order of magnitude or tend to be slightly lower than those of natural aggregates, although characteristic differences can be observed with respect to the sampling locations.For example, the subsamples obtained from fresh grate ash and from landfill section Db showed higher porosities and lower bulk densities, respectively, than the subsamples obtained from landfill section Da.The significantly higher water absorption of WIR samples (factor 3 to 5) compared to Rhine gravel indicates an increased porosity, which is reflected in the differences of the bulk densities.

Concrete technological properties
On basis of a concrete formulation for a conventional C20/25 with natural aggregates (reference concrete) modified formulations were developed in which 50 % by volume of the 2/8 mm aggregate fraction were substituted by the different samples of WIR in a way that the particle size distribution remains almost constant (Table 7).Due to the increased water absorption of WIR, supplementary water (SW) was added to the mixture in the amount of the respective water absorption of the WIR sample after 10 minutes (WA10).For the purpose of comparability, additional water corresponding to the water absorption WA10 was also provided for the natural aggregate of fraction 2/8.This principle of counteracting the absorption behavior of porous aggregates by adding water according to its water absorption WA10 (without considering in the watercement ratio) has already proven useful for the production of concrete with recycled aggregates [7].The other concrete components remained constant for the different investigated concretes.
After mixing the concretes were characterized with respect to rheological parameters using a rotational viscometer (Viskomat XL, Schleibinger) after intensive homogenization.The yield strength and viscosity parameters determined on the basis of the Bingham model are shown normalized in relation to the respective parameters of the reference concrete in Figure 5.In addition, the spread according to DIN EN 12350-5 was determined.The investigated concretes with WIR tended to reveal a lower relative viscosity and yield strength and a similar consistency compared to the reference concrete.With regard to the fresh concrete properties, the 50% substitution of the natural aggregate 2/8 mm by WIR 2/8 mm did not result in any significant or critical changes in terms of the relevant concrete technological properties.
After the fresh concrete tests, three cubes (a = 100 mm) of each investigated concrete were produced as test specimens for the compressive strength test according to DIN EN 12390-3.At the age of 7 days and storage under water, compressive strengths were determined as exemplarily shown in Figure 6.The compressive strengths of the concretes with WIR at the age of 7 days are more or less at the level of the reference concrete.Indications of a significant reduction of the compressive strength, which could be attributed to a lower grain strength of the WIR material, for example, are not evident from the present results.
Whether and to what extent the use of different WIR batches affects the uniformity of the concrete strength is currently being investigated in ongoing tests.

Summary and outlook
In the current research project ASHCON, samples of municipal waste incinerator ashes are taken from the :metabolon site of the TH Köln and separated by the electrodynamic fragmentation process.By a subsequent sorting process a residual mineral fraction of 2/8 mm is gained, which is composed mainly of SiO2 and to a lesser extent of CaO, Al2O3 and Fe2O3.Due to the exceedance of heavy metal limits for copper and zinc, the longevity of replacement concrete and the release of these metals into the environment needs to be assessed for various applications and suitability.Its high porosity leads to water absorption rates between 3 and 8 wt%.In initial material technological investigations this fraction was used as a partial substitute for natural aggregate in a conventional concrete mix.Neither fresh nor hardened concrete properties changed significantly by the substitution with respect to the reference concrete with 100 % natural aggregate.
Currently ongoing investigations focus in particular on durability aspects, environmental requirements and largescale implementation of the concrete production.

Figure 1 Table 2
Figure1Fragmented and sieved ash with a grain size > 2 mm (Source: Leiss/Fraunhofer IBP) Table2Overview of the MWIA processed so far in the ASHCON project and the sample quantities with particle diameters > 2 mm available after treatment

Figure 2
Figure 2 Top view of the process container at the moment after treatment by means of high-voltage pulses (Source: Leiss/Fraunhofer IBP)

Figure 3
Figure 3 Chemical composition of fragmented and sorted waste incinerator residues (WIR) with diameters > 2 mm by XRF

Figure 4
Figure 4 Particle size distributions of the WIR samples

Figure 5
Figure 5 Standardized rheological parameters (normalized to reference concrete) and spread of the investigated concretes

Figure 6
Figure 6 Mean compressive strengths (and min/max values) of the investigated concretes at the age of 7 days

Table 1
Collection of samples of MWIA

Table 3
Chemical composition and conductivity of all process waters examined to date

Table 4
Weights of different fractions after sorting

Table 5
Contents of environmentally relevant elements in the treated waste incinerator residues (aqua regia digestion) and limit values according to [6], Table3, in mg/kg

Table 6
Water absorption of the WIR samples after 10 minutes and 24 hours and bulk densities