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This idea is mainly based around the self-healing concrete idea that has appeared in the past and scientific posts have been made about the self healing concrete using bacteria to self heal cracks. What my question asks is whether or not this could apply to a vertical structure such as a castle wall and whether it would be practical.

Recognizing that the bacteria in order to produce the limestone used to fill the cracks, water is needed in order to activate, I thought perhaps a rain or mist would work. Granted this would mean that a castle with this sort of function would only self heal with applied water or would be dependent upon the weather but still my thought process.

Another requirement for the bacteria is the sugar or food component which the website I provide uses calcium lactate so I suppose part of my question is whether or not this would work as well for castle walls.

Also to clarify, the healing component I'm trying to apply for this question is the outer defensive castle walls of this hypothetical castle that is able to self heal using mainly impressive science, or at least as scientifically based as possible before I resort to magic into this idea specifically.

https://www.cnn.com/2015/05/14/tech/bioconcrete-delft-jonkers/index.html

Thank any and everyone again for providing any sort of answer/input.

(Edit)

The castle I have in mind would be made of concrete at least partially and I do believe this is an example of a castle that is made of concrete and some parts reinforced concrete. Concrete itself wouldn't require steel as I've come to understand it.

https://buckslib.org/the-mercer-museum-castles-made-of-concrete/

Would it still be possible for a concrete castle above ground and not in the water to employ this sort of idea hypothetically?

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Castle walls are not made of concrete. At least not the castles built during the castle golden age: they are made by stones held together by some cement, where the cement act as a binder.

Your bacteria seems at most capable of repairing the cement (if made out of the right material), not the stones.

Also please note that concrete contains a steel element which is untouched by this bacteria and which is a key component for ensuring the overall behavior under load.

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  • $\begingroup$ Concrete does not contain steel. Reinforced concrete does. Reinforced concrete is an exceedlngly modern material, which postdates castles by quite a few centuries. $\endgroup$ – AlexP Dec 2 '20 at 8:59
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    $\begingroup$ @AlexP Roman concrete buildings had at times iron bars, and medieval churches have cast iron in between building rocks as anchors! $\endgroup$ – Trish Dec 2 '20 at 9:24
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it feel similar like Roman Concrete just want to share, dont know the component you bring can work for your purpose or not, but at least this may can be an alternative.

from: https://www.nature.com/news/seawater-is-the-secret-to-long-lasting-roman-concrete-1.22231

Seawater is the secret to long-lasting Roman concrete

Ancient Romans built concrete sea walls that have withstood pounding ocean waves for more than 2,000 years. Now, an international team has discovered a clue to the concrete’s longevity: a rare mineral forms during chemical reactions between the concrete and seawater that strengthen the material.

Structural engineers might be able to use these insights to make stronger, more-sustainable concrete, says team leader Marie Jackson, a geologist at the University of Utah in Salt Lake City. She and her colleagues report their findings on 3 July in American Mineralogist1.

Modern concrete uses a paste of water and Portland cement, a fine powder made mostly of limestone and clay, to hold together small rocks. But it degrades within decades, especially in harsh marine environments. Instead of Portland cement, the Roman concrete used a mix of volcanic ash and lime to bind rock fragments. The Roman scholar Pliny the Elder described underwater concrete structures that become “a single stone mass, impregnable to the waves and every day stronger.” This piqued Jackson’s interest. “For me the question was, how does this material become a rock?” she says.

In earlier work, Jackson and colleagues reported some of the unusual chemistry of Roman concrete, such as the presence of a rare mineral known as aluminium tobermorite2. For the new study, the scientists took samples of Roman harbour concrete to the Advanced Light Source, an X-ray synchrotron at Lawrence Berkeley National Laboratory in Berkeley, California, and mapped out the location of minerals in the samples.

The researchers found a silicate mineral called phillipsite, which is common in volcanic rocks, with crystals of aluminium tobermorite growing from it. Tobermorite seems to have grown from the phillipsite when seawater washed through the concrete, turning it more alkaline. “It's a very rare occurrence in the Earth,” Jackson says. Such crystallization has only been seen in places such as the Surtsey volcano in Iceland. As tobermorite grows, it may strengthen the concrete because its long, plate-like crystals allow the material to flex rather than shatter when stressed.

Marie Jackson

A scanning electron microscope image of minerals within Roman concrete. Applying ancient knowledge

Modern concrete-makers could learn from the ancient Romans’ knowledge, says Nele De Belie, a materials engineer at Ghent University in Belgium. She and her colleagues have used materials such as fly ash, produced during the burning of coal, to give concrete ‘self-healing’ properties, whereby the material closes up cracks after they form3. Fly ash is similar to the volcanic ash that Romans used in their mix.

Jackson has been working to recreate the Romans’ concrete recipe in the lab. She is also a consultant for a cement company in Nevada that is using volcanic ash from the western United States to formulate such concrete.

“I’m not saying this would be the concrete that would be used in everyday infrastucture,” she says. “But for materials like sea walls, we could formulate mixtures with lime and volcanic ash materials in the way that the Romans did.” The Romans may have got their ideas from studying how ash from volcanic eruptions crystallized into durable rock, Jackson says.

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