Dr Volker Thome investigates the use of electrodynamic fragmentation to reduce emissions generated by the concrete industry.
Whether in Rome’s Pantheon or at the German concrete canoe regatta, whether ultra light or decorative, concrete is unbelievably versatile and is the world’s most widely used material – next to water.
Concrete is made of cement, water and a mixture of stone particles such as gravel or limestone grit in various sizes, commonly known as aggregates. The production of one cubed metre of concrete requires approximately 270 kg cement, 135 litres of water and 1,400 kg of aggregates. As seen in Figure 1, aggregates form 75% to 80% of the volume of typical concrete. These aggregates are embedded in a binder matrix, or ‘cement stone’. Cement stone is the product of a hydraulic reaction between cement powder and water. This reaction generates various calcium silicate hydrates, which are responsible for the strength development in concrete.
Approximately 1.4 billion tonnes of cement are produced every year worldwide; however, the resulting CO2 emissions are problematic. According to estimations, the production in 2030 may increase to five billion tonnes, meaning that unless action is taken promptly, the issue of CO2 emissions is only set to escalate.
The production of one tonne of cement commonly results in the release of 0.65 to 0.95 tonnes of CO2 depending on the efficiency of the process, the fuels used and the specific type of cement produced. This means that every year approximately eight percent of global CO2 production is attributable to concrete manufacturing.
According to estimates there are two billion tonnes of waste concrete every year worldwide and 900 million tonnes of accrued waste concrete every year in America, Europe and Japan alone. Recovering of concrete would reduce environmental costs and the unnecessary landfill of valuable materials. The disposal of construction waste is the least favoured option, because customers have to pay transport costs to the landfill sites as well as costs for the disposal of waste concrete, which is approximately $USD 100 per tonne in America, or €20 per cubed metre in Germany.
Currently there is no suitable processing technique available to recover raw materials for cement production from waste concrete, and as a result the consumption of natural resources such as limestone quarries and gravel pits for cement production is still increasing.
Common recycling methods for waste concrete based on mechanical processing methods, such as crushing, lead to products that consist of gravel with adherent cement stone. The re-use of these ‘recycled aggregates’ in new concrete causes problems, such as reduced mechanical strength and a higher demand on water and organic additives to overcome the disadvantages.
One of the major problems when recycling construction waste is that the aggregates cannot be detached from the cement stone and separated from each other. If concrete is treated by common mechanical methods such as grinding or crushing, then the obtained products consist of aggregates with adherent cement stone.
These recycled aggregates are of a lower grade than the original concrete and as such any re-use is limited to specific applications. They are mainly used as a sub-base for roads or as a filling material. Moreover, the current method for shredding waste concrete produces huge amounts of dust. This demonstrates that the current process of recycling construction waste is actually a down-cycling process.
The return of construction waste into material cycles is currently not very efficient and the cement industry still requires a large number of non-renewable natural resources, such as gravel pits and lime quarries, for the production of cement and concrete. So far it is possible to regain low grade secondary aggregates from waste concrete to a certain extent, but it is not possible to regain raw materials for cement production.
This means that the separation of gravel from the cement stone is crucial to recycling waste concrete in an efficient way. Beyond this, if it were possible to separate the stone particles from the cement stone, the gravel could easily be reused as an aggregate in new cement. This could be the first decisive step in the direction of recycling waste concrete.
The recovery of valuable aggregates from waste concrete would multiply the recycling rate by a factor of around ten and thereby increase it to 80%. If it were possible to obtain a cement substitute from waste concrete, the cement industry’s CO2 emissions would be considerably reduced.
To achieve these goals, researchers revived a method originally developed by Russian scientists in the 1940s: the electrodynamic fragmentation technique. The principle of this approach is based on a simple physical effect. Usually, lightning prefers to travel through air or water, not through solids; however, more than 70 years ago it was discovered that the dielectric strength, i.e. the resistance of every fluid or solid to an electrical discharge, is not a physical constant, but in fact changes with the duration of the lightning. With an extremely short flash of lightning, of less than 500 nanoseconds, water suddenly attains a greater dielectric strength than most solids.
In simple terms, this means that if the concrete is under water and a 150 nanosecond bolt of lightning is generated, the discharge prefers to run through the solid and not through the water, as seen in Figure 2.
To achieve this lightning bolt effect, a high voltage of between 90 to 200 kV is generated by a Marx generator. A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high voltage pulse. Marx generators are often used to simulate the effects of lightning on power line gear and aviation equipment.
Pulsed power processing
A number of capacitors are charged in parallel to a given voltage and then connected in series by spark gap switches, ideally producing a voltage (V) multiplied by the number (n) of capacitors or stages. This kind of pulsed power processing is a dust free and contactless method. There’s no contamination of the grist due to abrasion of the grinding tools.
In the first step, an inhomogeneous electric field between the electrodes is generated, which depends on the configuration of the electrodes. This electric field induces a polarisation on the particles’ surfaces, depending on the dielectric strength of the material, which furthermore leads to electrostriction.
Electrostriction is a property of all dielectric materials, and is caused by the presence of randomly aligned electrical domains within the material. When an electric field is applied to the dielectric, the opposite sides of the domains become differently charged and attract each other. Just as in Poisson’s ratio, this reduces material thickness in the direction of the applied field and increases thickness in the orthogonal directions. The resulting strain, or ratio of deformation to the original dimension, is proportional to the square of the polarisation. Reversal of the electric field does not reverse the direction of the deformation.
In a second step, streamers start to grow and follow the way of the strongest polarisation, which acts as a marker for the streamers. The first pre-discharge streamers to reach the counter-electrode in the fragmentation plant then cause an electrical breakdown. At this instant a plasma channel is formed in the concrete, which grows within a thousandth of a second from the inside outwards, much like a pressure wave. The force of this pressure wave is comparable with a small TNT explosion, in the range of approximately 10 Gigapascals (GPa).
In the concrete the lightning runs along the path of the least resistance or along the way of the strongest polarisation, which marks the boundaries between the components, such as between the gravel and the cement stone. The initially generated impulses weaken the material mechanically, which enhances the effects of the shock waves generated by the expanding plasma channel. The concrete is torn apart and broken down into its basic components, aggregate and cement stone.
It was shown in the mid Nineties that even reinforced waste concrete can be selectively fragmented into aggregate and a fine fraction consisting mainly of limestone and cement stone, see Figure 3. The limestone is generated by a subsequent carbonisation of the alkaline process water. In the laboratory fragmentation plant, the researchers can currently process one tonne of concrete waste per hour.
Currently, the obtained products are analysed in terms of their reusability potential in new concrete, as well as for cement production’s raw materials. Even reinforcements and steel fibres enhance the fragmentation process, because the pre-discharges will preferably run along these metallic inclusions and blast away the surrounding cement stone.
One of the main reasons why this technique has not yet reached a market ready state, however, is that concrete poses a serious problem during the fragmentation process. In comparison to other materials such as granite or glass, concrete changes the physical properties of the process water in a negative way.
During the electrodynamic fragmentation of concrete, water soluble ions are released from the concrete matrix, leading to an increase of the electrical conductivity in the process water. The process water becomes turbid and highly alkaline which should be avoided, as when combined, these effects lead to a phenomenon whereby at a specific point, the streamers prefer to run through the more conductive water and not through the solid.
When this phenomenon occurs, the electrodynamic fragmentation process is disrupted and ceases to work. It must therefore be ensured that the electric conductivity of the water does not reach a specific threshold limit value. The problem can be solved by combining a fragmentation plant with a wastewater processing plant.
Fortunately, there are already industrial wastewater plants available to clear the wastewater from fresh concrete. This type of wastewater accumulates, for example, at tunnel construction sites, where sprayed concrete is collected from the ground and recycled onsite.
The highly alkaline wastewater possesses both an elemental and physical composition similar to that of the wastewater obtained from the electrodynamic fragmentation process. This means there will be a similar or identical solution: the alkaline water can be neutralised by CO2, leading to the precipitation of calcium carbonate. On the one hand this has the effect that the turbidity of the water is reduced, while on the other hand the pH value, and most importantly the conductivity, can be controlled and kept at specific values. The cleaned water can then be lead back into the process.
To ensure that during the process no harmful gases evolve from the process water, we use gas chromatographic analyses to identify volatile components. This method works as follows: in the process chamber the volatile components are adsorbed either on small capillary tubes or on molecular sieves. The used capillary tubes can adsorb carbon-hydrogen’s from molecules with a carbon number between C6 – C16, whereas molecular sieves can adsorb molecules with a carbon number from C2 – C24. Then a thermodesorber releases the volatile molecules from the traps by heating. The thermodesorber is connected with a gas chromatograph where the volatiles can be analysed quantitatively with an emission flame photometer and quantitatively with a mass spectrometer.
Applying this testing method on the fragmentation of waste concrete showed an interesting effect. Surprisingly, we could identify the gas propane. The only possible sources for this gas are the organic additives in concrete like plasticisers or retarders. How this gas is generated during the fragmentation process is not known yet and needs further testing.
There is already an exploitation plan in place for the obtained fragmentation products. Demolition companies, road construction agencies and individuals could bring their waste concrete directly to pre-cast concrete plants, where the concrete could be processed. This would offer benefits for all those involved, as customers would avoid fees for depositing waste concrete on landfills, pre-cast concrete plants could produce their aggregate on site and save transport costs, limestone quarries and gravel pits would be preserved, and importantly, emissions would be reduced.
Emissions from the concrete industry must be controlled, and electrodynamical fragmentation technology could be crucial in this process. If this method can be applied on a large industrial scale, the CO2 emission from the cement production process can be largely reduced. The main objective is to advance the required technology and the design of a continuously operating plant with a high throughput for waste concrete. To work efficiently, the goal is to develop a throughput rate of at least 20 tonnes per hour. In as little as two years’ time, an appropriate installation consisting of a fragmentation plant combined with a wastewater plant could be operational.
Published: 31st May 2013 in AWE International