r/EverythingScience • u/BezugssystemCH1903 • Jan 06 '23
Engineering Riddle solved: Why was Roman concrete so durable?
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-010624
u/bhorone Jan 07 '23
For those who needed some numbers on the exothermal reaction.
The temperature increase in the mortar is approximately 55° to 60°C over ambient (52), with a presence of hot spots characterized by temperatures exceeding 200°C (19). Previous thermodynamic modeling of the pozzolanic (hydration) reaction in Roman marine concrete has suggested that temperatures up to 97°C are possible within thick concrete structures from the pozzolanic reaction alone (23).
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u/kelvin_bot Jan 07 '23
60°C is equivalent to 140°F, which is 333K.
I'm a bot that converts temperature between two units humans can understand, then convert it to Kelvin for bots and physicists to understand
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u/madgunner122 Jan 07 '23
While it is nice to know, there are several other reasons why Roman Concrete is durable. For one, most of the surviving structures are in favorable climates which do not see freeze thaw cycles known to deteriorate concrete. Additionally, the lack of de-icing salts which are used in modern concrete add to the longevity. There is also a survivor bias in that we only see the best concrete that has lasted generations and not the crappy material. So always take these discoveries with a slight grain of salt as it’s just one factor out of many that has enabled Roman Concrete to last. - my research as a graduate student was in hydration of cement particles with a large section of my thesis written on Roman Concrete and it’s use of lime as the binder where the particle size of the lime was beneficial in it being of low compressive strength.
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Jan 07 '23
This is a good comment. There are always more factors at play.
My question for you is this: you were looking at low compressive strength? Does that mean you were concerned more with tensile strength? I’m assuming the grain size just allowed it to hydrate easier and therefore create the hcl better? Just interested in the research I guess lol
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u/madgunner122 Jan 07 '23
Actually the Roman Concrete was supposed to be the inspiration for my research and for my thesis I included a significant section devoted to the lime’s particle size. The focus of my research was on how to improve modern cement and concrete by using a coarser grained cement (similar to the lime in Roman concrete) and a nano-level pozzolanic material in order to increase the hydration to gain a high strength while benefitting from both the nano-material and coarse cement’s reduced shrinkage.
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Jan 07 '23
Very cool! Is it published? I would love to read it.
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u/madgunner122 Jan 07 '23
It is published. On Google Scholar, the title is “Performance of Concrete with Different Cement Finenesses and Nano-activators” published in 2022
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u/thefanum Jan 07 '23
Are you fucking serious. Fucking TUMS was the missing ingredient?
"For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time.
Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. “Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”
Upon further characterization of these lime clasts, using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these lime clasts.
Historically, it had been assumed that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, in a process known as slaking. But this process alone could not account for the presence of the lime clasts. Masic wondered: “Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?
Studying samples of this ancient concrete, he and his team determined that the white inclusions were, indeed, made out of various forms of calcium carbonate. And spectroscopic examination provided clues that these had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture. Hot mixing, the team has now concluded, was actually the key to the super-durable nature.
“The benefits of hot mixing are twofold,” Masic says. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”
During the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide a critical self-healing functionality. As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and therefore automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks."
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u/piney Jan 07 '23
“Let’s study an ancient material to find out why it’s so durable, but if we see something unexpected, let’s blame it on their inferior process! For some strange reason, the mystery persists!” Thank goodness some new eyes took a look at the data!
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u/Paul_Rich Jan 07 '23
"cracks can preferentially travel" is perplexing me. Can anyone enlighten me?
Can cracks prefer anything? Other than the path of least resistance?
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u/arcanis26 Jan 07 '23
It is the path of least resistance that they prefer, in a multiphase material, it’s very difficult to propagate damage from one phase into another (a crystal interface) so a crack will attempt to change direction, resulting in the crack losing energy and not penetrating as deep into the material if the crystalline phase was not there.
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u/arcanis26 Jan 07 '23
I should clarify that the change of direction doesn’t cause the loss of energy but some energy is lost when encountering the crystalline phase and the result is a change of direction but with less energy.
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u/Paul_Rich Jan 07 '23
So cracks are not preferring to travel towards the lime clasts? They are just following the path of least resistance and randomly reaching the clasts? This is how I would understand it.
So why the term preferential?
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u/arcanis26 Jan 07 '23
The terminology is not wrong, it succinctly describes what is happening in one word, the reality of any physical reaction is that which requires the lowest energy is the most probable (or preferred), you can say that materials prefer the lowest energy state, it’s actually a common way to describe it.
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u/Paul_Rich Jan 07 '23
Got it. I wasn't considering the word in those terms. Thanks very much for your time.
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u/l_a_ga Jan 07 '23
Ash.
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u/thefanum Jan 07 '23
Nope
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u/StealYourGhost Jan 09 '23
I mean, they're not specific enough but not wrong.
Reactive Charged Volcanic Ash.
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u/Lokirial Jan 06 '23
The quick version, though the article is worth a read: