Cement made with volcanic ash can make cities stronger and greener
It is one of literal building blocks of modern civilization, and somewhat ironically plays a significant role in one of its largest looming threats. Producing cement is a hugely energy intensive process that accounts for around five percent of global CO2 emissions, so scientists are continually searching for greener ways of doing things. Now a team from MIT has found that pulverizing volcanic ash and adding it to the mix can not only make the process more environmentally friendly, but the resulting structures stronger as well.
Incorporating irradiated plastic bottles and tweaking calcium levels are just a couple of ways MIT researchers have looked to improve on cement production in the past. This time around they have turned to volcanic ash, which they say has a number of advantages as an ingredient in the cement manufacturing recipe. It is widely abundant around both active and inactive volcanoes, we don't currently have any real use for it, and when reduced to powder it has handy natural properties when it comes to binding with water and other materials to create cement-like pastes.
"Cement production takes a lot of energy because there are high temperatures involved, and it's a multistage process," says Stephanie Chin of MIT's Department of Civil and Environmental Engineering (CEE). "That's the main motivation for trying to find an alternative. Volcanic ash forms under high heat and high pressure, and nature kind of does all those chemical reactions for us."
In exploring the potential of this new additive, a team led by Chin and study co-author Kunal Kupwade-Patil began by poring over existing data on what's called the "embodied energy" of different industrial processes associated with the production of cement. This refers to the total energy used to do things like crush rock, cure cement or make concrete.
Using this data, the team calculated the total embodied energy for cement recipes containing different proportions of volcanic ash, ranging from 10 percent to 50. Then they took to the lab to produce some samples.
The researchers found that substituting 50 percent of the traditional cement with volcanic ash ground down to a particle size of 17 micrometers reduced the embodied energy of the resulting concrete by 16 percent, although this hampered the overall strength of the material. Reducing the ash down to a particle size of around six micrometers greatly boosted its strength, by creating more surface area for the water and cement to bind together.
They then looked to apply this knowledge to the real world, turning to a neighborhood in Kuwait consisting of 13 residential and 14 commercial buildings made from traditional Portland cement. They used drones to fly over the structures, collecting images and measurements, and combined that with data from the local authorities to calculate the neighborhood's embodied energy.
Building on their earlier work, the scientists then calculated how that embodied energy could be altered were the structures crafted with different proportions of volcanic ash, something that is widely abundant in the Middle East. Thirty percent was the number they settled on as the optimal amount, which they say would greatly reduce the embodied energy but still provide the necessary strength.
"What we've found out is that concrete can be manufactured with natural additives with desired properties, and reduced embodied energy, which can be translated into significant energy savings when you are creating a neighborhood or a city," says Oral Buyukozturk, a professor in MIT's Department of Civil and Environmental Engineering (CEE).
The beauty of this approach lies in its flexibility. Engineers can fine-tune the recipe, altering the amount of volcanic ash used depending on the task at hand.
"You can customize this," Buyukozturk explains. "If it is for a traffic block, for example, where you may not need as much strength as, say, for a high-rise building. So you could produce those things with much less energy. That is huge if you think of the amount of concrete that's used over the world."
The team's research was published in the Journal of Cleaner Production.