Volcanic ash makes concrete "greener"

Replacing cement with ash reduces energy consumption

Ash from volcanic eruptions is suitable as a climate-friendly building material - if it is added to the concrete as a replacement for cement. © USGS
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According to the Romans' recipe: Concrete could become more stable and environmentally friendly by adding volcanic ash. Replacing parts of the cement with ground volcanic rock reduces energy requirements and CO2 emissions for the production of building materials by almost 20 percent, researchers have determined. Another advantage: deposits of volcanic ash are abundant in many areas of the world.

Concrete and its main component cement are one of the most important building materials of humanity. Hardly any other material is used so often. But the cement has a downside: When burning the limestone enormous amounts of carbon dioxide (CO2) free, it also requires large amounts of energy. It is estimated that around five percent of global CO2 emissions are attributable to cement production alone.

Researchers around the world are therefore looking for ways to make concrete more environmentally friendly. Especially promising is the replacement of at least part of the cement with alternative building materials. For example, the admixture of carbon nanotubes can make concrete more stable and even shredded plastic waste comes as an alternative in question.

Romans recipe in the test

By contrast, researchers around Kunal Kupwade-Patil of the Massachusetts Institute of Technology (MIT) have copied their alternative from the Romans. Already 2, 000 years ago, the ancient master builders mixed their cement and concrete with volcanic ash to increase the durability and water resistance of their buildings. Whether the Roman concrete but also brings in terms of its CO2 emissions and energy consumption benefits, was previously unknown.

The cement content in concrete requires large amounts of energy for its production. © tortoon / thinkstock

To find out, the researchers have tested various concrete recipes with volcanic ash. They crushed the ashes to different fine powders and replaced between 30 and 50 percent of the cement in their concrete. In stress tests they subsequently determined the stability of the building material and calculated how much energy and CO2 "costs" the production. display

Smaller energy footprint

The result: as with the Romans, the volcanic ash had a positive effect on the stability of the concrete: the finer the ashes were milled, the stronger and more durable the concrete was. However, with the finer degree of grinding, the energy consumption during production also increases. But even then, replacing cement with volcanic ash improves the overall energy balance of the concrete.

In the test, the energy footprint dropped by 16 percent when 40 percent of the cement was replaced by finely ground volcanic ash. "Cement production consumes a lot of energy because it requires high temperatures and it is a multi-step process, " explains Stephanie Chin from MIT. "Volcanic ash, however, is formed even in high heat and high pressure. Nature, so to speak, takes the necessary chemical reactions away from us."

Volcanic ash in the electron microscope USGS

Works well with whole blocks of flats

What these savings mean at the level of entire buildings and blocks of flats, the researchers have examined using the example of a neighborhood in Kuwait. For the 13 residential buildings and 13 commercial buildings, they determined the amount of concrete used and calculated the energy balance for it. Then they used model calculations to test how the energy needed for the building material would change if up to 50 percent of the cement were replaced by the ground volcanic ash.

The result: The energy savings determined in the laboratory can also be transferred to real conditions and the size of entire buildings and neighborhoods. According to researchers, around 16 percent less energy would have been needed for the 26 buildings if up to half of the cement had been used against volcanic ash. According to this, the R mer recipe saves a lot of energy and thus CO2 emissions during construction.

In addition, rock formations from ancient volcanic ash are found in many parts of the world - both volcanoes that are still active and places of primeval volcanic activity. Because this material is hardly used so far, it would be abundant and comparatively cheap. (Journal of Cleaner Production, 2018; doi: 10.1016 / j.jclepro.2017.12.234)

(Massachusetts Institute of Technology, 12.02.2018 - NPO)