Eco-efficient cement is an exciting development in the construction industry with the potential to make a significant contribution to a greener and more sustainable future. Cement is one of the most widely used materials on the planet, and its manufacture contributes significantly to greenhouse gas emissions, particularly carbon dioxide (CO2). Greener concrete is needed to pave the way to a net-zero future, and Rice University scientists know how to make it.
Cement production, an ingredient in concrete, accounts for approximately 8% of global annual CO2 emissions, making it a significant target of greenhouse gas emission reduction goals. To that end, chemist James Tour’s Rice lab used flash Joule heating to remove toxic heavy metals from fly ash, a powdery byproduct of coal-fired power plants that is commonly used in concrete mixtures. Using purified coal fly ash reduces the amount of cement required and improves the quality of the concrete.
In the lab’s study, replacing 30% of the cement used to make a batch of concrete with purified coal fly ash improved the concrete’s strength and elasticity by 51% and 28%, respectively, while reducing greenhouse gas and heavy metal emissions by 30% and 41%, respectively, according to the paper published in the Nature journal Communications Engineering.
Reducing emissions from cement production is very important to mitigate global greenhouse emissions. This is the big picture of this study.Bing Deng
“Reducing emissions from cement production is very important to mitigate global greenhouse emissions,” said lead author Bing Deng, a postdoctoral research associate in the Tour lab. “This is the big picture of this study.”
Rice engineer Satish Nagarajaiah pointed out that “cement production is a significant source of carbon dioxide emissions. Reducing cement content in concrete will help reduce emissions.”
“Using coal fly ash allows you to use less concrete. Fly ash, on the other hand, contains heavy metals,” Tour explained. “We frequently try to fix one thing and end up breaking something else.” We were polluting our environment by attempting to do something with this waste, specifically coal fly ash, because heavy metals were leaching out. Water carried it into our environment, contaminating our soil along highways, and so on.”
Every year, approximately 750 million tons of coal fly ash are produced worldwide. Rice researchers developed a fast and water-free process based on flash Joule heating that can remove up to 90% of heavy metals, making it more suitable for infrastructure use.
“Basically, we mix the fly ash with carbon black, because fly ash does not conduct electricity, and the carbon black makes the mixture conductive,” Deng said. “Next, we place the mixture between two graphite or copper electrodes and use a capacitor to supply a short current pulse to the sample. This current input brings the sample temperature up to about 3,000 degrees Celsius (5,432 Fahrenheit). The high temperature makes the heavy metals evaporate into a volatile stream which is then captured.
“By using this method, we can eliminate the heavy metals from coal fly ash with very high efficiency,” he continued. “For different heavy metals like arsenic, cadmium, cobalt, nickel, and lead, the removal efficiency is up to 70% to 90% in just one second. This is a very rapid discharging process.”
Flash Joule heating was shown to work on different coal fly ash compositions resulting from the combustion of coal extracted from various geographical locations.
“There are two main classes of fly ash with different inorganic compositions, Class C and Class F,” Deng said. “We found that our method works for both kinds of coal fly ash. It also works for other hazardous wastes like red mud or bauxite residue. This shows that the process can become a generalized approach for large-scale industrial solid waste decontamination.”
“The purified coal fly ash is not only better for the environment, but it also increases concrete strength and quality,” said Wei Meng, a Rice civil and environmental engineering postdoctoral research associate and co-lead author on the study.
“We have found that by replacing 30% 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.”
The Tour lab’s process allows for the evaporated heavy metals to be collected in a vacuum chamber rather than released into the environment. Furthermore, the energy consumed during the process is minimal.
“We calculated that the energy consumption is about 532 kilowatts per ton,” Meng explained. “When we convert this to Texas electricity prices, we get about $21 per ton.” The life cycle analysis demonstrates that we can extract value from these waste materials.”