Advoclimate!

Bacterial Self-Healing Concrete

In the modern world, concrete is seen everywhere. Concrete is used in infrastructure across the globe due to its versatility and abundance and has now become the most widely used man-made material in existence.

However, what most people don’t realise is how much it costs the environment in terms of Co2 emissions. In 2022, about 8% of the globe’s total carbon dioxide emissions came from cement production - equivalent of 1.6 billion metric tonnes of Co2. And in 2018 the BBC reported that ‘if the cement industry was a country, it would be ranked 3rd in the world for carbon emissions.’

In the same report, the BBC revealed that it’s the production of a substance called clinker used in cement that causes the carbon dioxide emissions for concrete to be so high. Limestone and clay are crushed and mixed together before being fed into cylindrical rotating kilns where they are heated to 1,400˚C or more. In the kiln, a process called calcination occurs and splits the mix into calcium oxide (clinker) and CO2. About 90% of the industry’s emissions can be attributed to the production of clinker.

So, it’s clear that something has to change in the industry to make for a more sustainable future!

One of the main reasons that the demand for concrete is so high is because of the short lifetime of the material. I’m sure you’ve seen it before – an old concrete building with cracks all over the front and dirt and grime seeped into all of the corners. As seen in the previous article, the lifespan of a concrete is only about 100 years with regular maintenance. These cracks can open up without maintenance and eventually lead to the failure of the structure as a whole.

The more buildings break, the more concrete will need to be produced to replace or fix them. So, one of the main routes that scientists are looking into at the moment is a way to make concrete last longer.

Inspired by Ancient Roman concrete (have a look at our article on this below!) some scientists are trying to find a way to make a new and improved version of self-healing concrete. There are several methods have been proposed but one of the more promising ones is microbial self-healing concrete!

In this method, bacteria spores are mixed into the wet concrete alongside a calcium nutrient such as calcium lactate. These bacteria have to be selected carefully as not many can survive in the high alkaline environment of concrete. After setting, the bacteria will lay dormant inside the concrete until it is needed.

Much like its Roman predecessor, this concrete’s self-healing abilities are activated when cracking occurs and allows moisture to seep inside of the concrete. When the dormant bacterium in the concrete receives oxygen and water, it will come back to life and start eating the calcium nutrients in the concrete. The bacteria will then process this calcium nutrient and produce limestone as it’s waste product. This limestone waste will slowly be built up and fill the crack in!

This would mean that concrete made with this method would have a much longer lifetime and wouldn’t need to be maintained either. So, it is a lot more cost effective after it has been put in place.

Unfortunately, at the minute this microbial concrete costs about double the price of standard concrete so it is unlikely that companies will use this technology any time soon.

But it is incredibly important to keep talking about it!

The more we talk about these new innovations, the more companies will see the demand for it and invest in it. Cutting cement usage is imperative for the climate crisis to be resolved. Keep pushing for more sustainable methods of industry, innovation and infrastructure!

The First Case of Self-Healing Concrete...

The first known case of self-healing concrete can be found in Italy, in what was then known as the Roman Empire. Buildings such as the Pantheon and the Colosseum built over 2000 years ago still stand today - a feat that modern day concrete would not be able to do.

For years, it was a mystery as to why Roman concrete was able to stay standing for so long. Scientists originally thought that it must have been due to pozzolanic material (ie. volcanic ash!) mixed into the concrete before it was set. However, recent research at MIT has proved that this is not the case…

Throughout the lifetime of a concrete structure, hairline fractures and cracks appear in concrete. This can happen for many reasons, but the main cause is the concrete being put in tension. In compression, concrete is very strong, but the smallest bit of tension can cause concrete to fracture! Compared to the lifetime of these ancient buildings, modern day concrete only lasts about 100 years or so. And this is with regular maintenance! So how can these Roman structures still be standing?

The answer lies in the presence of white chunks of material in the mix called ‘lime clasts’. It was originally believed that the presence of these chunks was due to improper mixing of the concrete before setting it. It turns out that these lime clasts were actually giving the concrete a self-healing ability!

When fractures and cracks occurred in the concrete, the lime clasts were activated by the moisture inlet and recrystalise, filling up the cracks with calcium carbonate. The calcium carbonate solution could also react with the pozzolanic materials in the concrete to further strengthen the material. So, whilst Roman concrete is weaker than the Portland mix we use today, it stands the test of time with no issues!

After this discovery, it was clear that the Romans had used a different concrete mixing method than we thought. Initially, it was assumed that the Romans had mixed lime with water to form a paste – a process called slaking. However, this process does not account for the presence of the lime clasts in the concrete. So, it was clear that the Romans must have used a different technique in junction with slaking.

Admir Masic, MIT professor of civil and environmental engineering, wondered whether the Romans had used lime in a more reactive form, known as quicklime. The answer was found after a spectrographic examination of a sample of concrete that proved that the lime clasts had been formed at extreme temperatures. If quicklime was added to the concrete, an exothermic reaction would have occurred, heating up the concrete as it was mixed.

The answer was clear – Romans had hot mixed their concrete.

There are many benefits to hot mixing concrete. One of these benefits being the formation of lime clasts in the concrete, allowing the structures to supersede their builders’ lifetimes. Another benefit is that the setting times of the concrete are radically improved due to the high starting temperature. This means that construction can be completed a lot faster than concrete made with slaking methods.

Unfortunately, it would be incredibly difficult to apply the hot mixing method to the modern world. Using quicklime would mean that concrete would have to be mixed on site due to the faster reaction, which can prove difficult in cramped modern building sites. There are also health risks due to the exothermic reaction involved in making the concrete. Portland cement is also a lot easier to mass produce, which is a necessity in the fast-growing concrete jungles now spreading across the globe.

However, there is a lesson to be learnt in the importance of the durability of the materials we build out of. With cement production currently accounting for about 8% of global greenhouse gas emissions, it is incredibly important that the structures we build last as long as they can to work towards the SDG goal of more sustainable cities.