Engineers harness the power of flash joule tech for stronger, greener concrete

Toward those efforts, the Rice lab of chemist James Tour has developed cement made with coal fly ash purified through a flash Joule heating-based process. Recently, a paper on their work was published in Nature's Communications Engineering.

To obtain a deeper understanding of their greener concrete production method, Interesting Engineering (IE) conducted an interview with Bing Deng, one of the paper's co-authors.

Fly ash cement can reduce carbon emissions by 30 percent

"Coal fly ash is the by-product of coal combustion, with an annual production yield of hundreds of millions of tons," Deng told IE. In fact, a recent study shows roughly 750 million tons of coal fly ash are produced worldwide yearly.

Deng discussed how reusing coal fly ash as a substitute for cement can reduce the amount of concrete required – but not without challenges. 

"You can use less concrete if you use coal fly ash. However, fly ash contains heavy metals," clarified James Tour in a previous statement. 

"Often, we try to fix one thing, and we mess something else up. In our effort to do something with this waste, namely coal fly ash, we were polluting our environment because the heavy metals were leaching out. Water carried it into our environment and contaminated our soil along roadways, etc." he added. 

In this context, Deng explained to IE how the team has since used a process called flash Joule heating to rapidly remove up to 90 percent of heavy metals in coal fly ash, making it fitter for infrastructure use.

"The purified fly ash was then used as cement. The fly ash substituted cement reduces heavy metal emission by ~40 percent and carbon emission by ~30 percent while the strength is improved," he stated. 

In one second, flash Joule heating removes up to 90 percent of contaminants

"The technique we used to purify coal fly ash is flash Joule heating. [This process] uses electric pulses to heat materials to 3,000 degrees Celsius within milliseconds, and then rapidly it cools," revealed Deng. 

To explain further, fly ash is mixed with carbon black – a fine powder made of elemental carbon produced through the incomplete combustion of fossil fuels, such as oil or gas. This makes the mixture conductive because fly ash does not conduct electricity, whereas carbon black does. 

Next, the mixture is inserted between two electrodes made of graphite or copper. A capacitor delivers a brief current pulse to the sample, which heats the sample to roughly 5,432 Fahrenheit (3,000 degrees Celsius). This high temperature prompts the heavy metals to vaporize and transform into a volatile stream, which is then captured.

Better yet, the team asserted that their method can achieve removal efficiencies of 70 to 90 percent for various heavy metals such as arsenic, cadmium, cobalt, nickel, and lead, all within a mere second.

"The purified coal fly ash is not only better for the environment, but it also increases concrete strength and quality," said co-author Wei Meng in a previous statement. 

"We have found that by replacing 30 percent of the cement in a concrete mixture with the purified coal fly ash, the compressive strength and the elastic modulus of the composite increased significantly." 

"This is very meaningful for structural engineering and the construction industry because stronger structures can be built with less cement," he continued. "That is why this research is valuable to civil engineers."

Beyond concrete: decontaminating large-scale industrial solid waste

According to the researchers, their process also works for other hazardous wastes like red mud or bauxite residue, demonstrating that it could become a generalized approach for large-scale industrial solid waste decontamination.

"We initially demonstrated the conversion of various carbon sources, including coal, coke, waste food, plastics, and rubbers, into flash graphene using this technique," Deng revealed. 

"The ultrahigh temperatures and ultrafast processing also enable precious metals recovery from electronic waste for urban mining and recycling of rare Earth metals from industrial waste streams."

"The electricity cost of the flash Joule heating process is approximately $30 per ton of treated materials," he said. 

But is this cost high or low? While IE didn't get into the details of this during the interview, the answer likely depends on the context. 

That is, compared to traditional heating methods, such as burning fossil fuels, the electricity cost of the flash Joule heating process may be considered high. However, the price may be justifiable when considering the potential environmental and economic benefits of producing greener concrete. 

Additionally, as renewable energy sources become more prevalent and cost-effective, the cost of electricity may decrease, making the flash Joule heating process even more attractive. 

Ultimately, the cost-effectiveness of the process will depend on factors such as the scale of production, local energy prices, and the availability of alternative production methods (to name a few).

Greener concrete: paving the way for a sustainable future 

"The big picture is that the cement industry has huge carbon emissions," noted Deng. "Construction is responsible for 25 percent of global greenhouse gas emissions, with cement production alone contributing 7-8 percent of total carbon emissions worldwide." 

"The primary type of cement used, ordinary Portland cement, requires a high-temperature calcination and sintering process at temperatures between 900 to 1500 degrees Celsius to produce from limestone," he added.

He emphasized that this process releases carbon dioxide and consumes significant energy. Developing cement materials with a lower carbon footprint is crucial in mitigating greenhouse gas emissions. 

"The coal fly ash cement could be used to partially replace traditional Portland cement in construction, building, etc.," said Deng. Since the strength of the fly ash cement has been demonstrated to be superior to traditional Portland cement, in some cases, it can be used where high-strength construction is required, he argued.  

"The main limitation might be the scaling up. The start-up, Universal Matter, is scaling up the flash Joule heating process; but in that case, it is for graphene production purposes," he revealed. 

Deng concluded by emphasizing his team's need to demonstrate the scalability of the flash Joule heating process for coal fly ash decontamination. With this in mind, future steps will involve scaling up the process and demonstrating the applicability of fly ash cement on a larger scale.