The production of cement used in concrete is a major contributor to greenhouse gas emissions, which is why some people have tried to replace it with fly ash. New technology makes fly ash itself more environmentally friendly. This results in more environmentally friendly and stronger concrete.
Calcium and other ingredients must be heated to very high temperatures to produce the widely used Portland cement. The heat-producing process is estimated to be responsible for 5% to 8% of all anthropogenic CO2 emissions.
With this issue in mind, scientists have tried to replace some of the cement used in concrete mixes with fly ash (the other ingredients being sand and/or gravel, and water).
Fly ash is a waste product from burning coal. It has properties similar to cement, but it also contains toxic heavy metals. When that ash-laden concrete is used for roads and sidewalks, those heavy metals can slowly leach out of the concrete and into the surrounding environment.
In an attempt to address that shortcoming, scientists at Rice University in Texas developed a process that begins by mixing fly ash with conductive carbon black powder.
Using a technique known as flash joule heating, the mixture is placed between two copper or graphite electrodes (connected to a capacitor) and a short electrical pulse is applied to the material. When that current is applied, the mixture is rapidly heated to a temperature of approximately 3,000 ºC (5,432 ºF), causing the heavy metals to evaporate into a vapor that is drawn into the vacuum chamber.
“By using this method, we can remove heavy metals from coal fly ash with very high efficiency,” said postdoctoral researcher Bing Deng, co-lead author of the study. “The removal efficiency of various heavy metals such as arsenic, cadmium, cobalt, nickel and lead is up to 70% to 90% in just 1 second.”
Gustavo Laskoski/Rice University
As an added benefit, concrete made with refined fly ash is stronger than concrete made with pure cement. When replaced, the resulting concrete was found to be 51% stronger and 28% more elastic than the control sample.
“This has huge implications for the structural engineering and construction industry as we can build stronger structures with less cement,” said Wei Meng, another co-lead author and postdoctoral researcher.
A paper on this study was recently published in the journal Communication engineering.
Source: Rice University
