Why were ancient Roman concrete structures so durable

[traveller]

Thank you traveller.

If I understand this correctly, it is a matter to controll the progress of the reaction of the quicklime with water.
And at some point this reaction should be stopped, maybe by a shortage in moisture, so it can infiltrate and start again in the cracks when they become wet again?
The quicklime should be mixed and dispersed very well in the concrete.

I've first heared about this in self-healing fibre-reinforced polymer composites, which are very prone to cracks.
https://en.wikipedia.org/wiki/Self-healing_material

Thanks for the link!

Apparently the quicklime particles have to be big enough not to hydrate completely.



This is a different process from the Roman undersea concrete:

“Whereas previous evidence supports the formation of Al-tobermorite minerals in lime clasts of maritime structures due to the heat of the pozzolanic reaction (23), the results presented here suggest that, by contrast, when quicklime is introduced via hot mixing in terrestrial structures, the reaction is limited to the outer rim of the lime clast, encapsulating calcium-rich core structures within mortar matrix.”

It would require particle sizes that are “just right” to stay active over a long period but not jeopardize the strength and dimensions of the concrete/mortar, as well as much finer particles or prehydrated lime to achieve initial strength. Actually, using this as mortar to bind stones in a wall massive enough to be stable on its own might make more sense than a big chunk of concrete.

Still, I don’t think the self-healing can last through a longer period of crack formation as at some point all the active lime would be spent.

 

[traveller]

The healing effect may also be unintended and just a consequence of imperfect grinding/hydration/mixing.

 

[bicicleur 2]

The healing effect may also be unintended and just a consequence of imperfect grinding/hydration/mixing.

yes, I would guess so

however the Roman engineers may have been smart enough to notice this effect and have kept the recipee

I guess there are no written sources on this and this kind of knowledge was transferred from master to apprentice in practice

 

[bicicleur 2]

Thanks for the link!

Apparently the quicklime particles have to be big enough not to hydrate completely.

View attachment 13756

This is a different process from the Roman undersea concrete:

“Whereas previous evidence supports the formation of Al-tobermorite minerals in lime clasts of maritime structures due to the heat of the pozzolanic reaction (23), the results presented here suggest that, by contrast, when quicklime is introduced via hot mixing in terrestrial structures, the reaction is limited to the outer rim of the lime clast, encapsulating calcium-rich core structures within mortar matrix.”

It would require particle sizes that are “just right” to stay active over a long period but not jeopardize the strength and dimensions of the concrete/mortar, as well as much finer particles or prehydrated lime to achieve initial strength. Actually, using this as mortar to bind stones in a wall massive enough to be stable on its own might make more sense than a big chunk of concrete.

Still, I don’t think the self-healing can last through a longer period of crack formation as at some point all the active lime would be spent.

what is carbonation, and does it promote the infiltration of water into the encapsulated quicklime particle?

 

[traveller]

what is carbonation, and does it promote the infiltration of water into the encapsulated quicklime particle?

Actually it’s carbonatation - the hardening of lime. See https://en.wikipedia.org/wiki/Carbonatation

They found that the whole particle doesn’t carbonatate and this the center remains active. It’s weaker than the surroundings, which encourages cracks to split the particle.

 

[traveller]

I guess there are no written sources on this and this kind of knowledge was transferred from master to apprentice in practice

The article says there are sources that are quite detailed, but funnily enough not detailed enough. I doubt if the finer details of lime mixing were common knowledge.

From the article again:

”Despite extensive literature related to the composition and applications of ancient Roman concretes, the exact order of operations for Roman mortar production based on historical evidence remains ambiguous. There is even debate as to whether preparation techniques differed between the production of marine and terrestrial cementitious structures (12). Imperial age mortar (according to Vitruvius) was prepared by mixing lime with volcanic sand (materies ex calce et harena mixta). During the Republican period, Cato, in his De Agri Cultura (50), describes the mortar mix as calx harenatus (“lime with sand”). The wet mortar mix would then be mixed with tuff and brick caementa to form a concrete. In general, for frescoes and wall plaster, for example, the ancient scholars would often suggest the aging of lime in water before use and ensure that it was as finely ground as possible (17), because incompletely hydrated lime particles, known as bottaccioli in these applications, could absorb water over time and expand, damaging the paint (fresco) layer. For this reason, both Vitruvius (6) and Pliny (17) describe the preparation of lime for plasterwork to involve a thorough soaking or softening process (macerata). When referring to lime for structural use, however, Vitruvius uses the word extincta (II.5.1) instead of macerata. While extincta and macerata are both frequently interpreted as referring to slaking, Vitruvius’ change in diction points to a potentially different process. On the basis of the results of our chemical characterization of the Privernum mortars, it is thus possible that in contrast to the use of macerata (which specifically refers to the slaking process), extincta could refer to lime hydrated simultaneously with the other mortar components, supporting the hot mixing hypothesis proposed here. Furthermore, it has also been previously proposed that some Roman mortars were prepared through the sequential addition of both slaked lime and quicklime in a two-step process (51).”

 

[bicicleur 2]

I doubt Cato, Vitruvius or Pliny ever mixed mortar themselves.