
A chemical reaction in ancient Roman concrete makes it stronger over time (2017) - tzhenghao
https://science.howstuffworks.com/why-ancient-roman-concrete-stronger-than-modern.htm
======
dang
Roman concrete threads may not be made of Roman concrete, but they do ok:

[https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...](https://hn.algolia.com/?dateRange=all&page=0&prefix=true&query=roman%20concrete&sort=byDate&type=story)

Last year:
[https://news.ycombinator.com/item?id=20482050](https://news.ycombinator.com/item?id=20482050)

2017:
[https://news.ycombinator.com/item?id=14690329](https://news.ycombinator.com/item?id=14690329)

2013:
[https://news.ycombinator.com/item?id=5883443](https://news.ycombinator.com/item?id=5883443)

2010:
[https://news.ycombinator.com/item?id=1852000](https://news.ycombinator.com/item?id=1852000)

------
pitdicker
It is always a bit annoying to read an article based on the work of someone
specialized in one area that talks down specialists in another area.

Is is definitely interesting to read about some unexpected properties that
ancient concrete with volcanic ash as one of its ingredients has. For modern
cement there is a different set of choices for binding materials, were you can
choose the ingredients that are locally good available.

Also there are definitely mistakes that get made in modern construction. \-
Maybe the structure has enough rebar to handle the forces that act on the
structure, but not enough to prevent the small cracks that hurt durability
(although that is something that should be checked). \- Maybe the detailing of
the rebar is not correct. Lapping lengths are too short. Often I see that the
workers who put in the rebar come up with creative solutions but understanding
why it now doesn't work well is outside their expertise. \- Maybe the concrete
layer that protects the rebar (covering) from rusting is to thin or to porous.
The amount of covering required depends on the exposure classification. And
the concrete mixture influrences how porous the concrete becomes over the
years.

For modern engineering you specify the exposure classes that a structure falls
in. Carbonatation
([https://en.wikipedia.org/wiki/Carbonatation](https://en.wikipedia.org/wiki/Carbonatation)),
a reaction where calcium in the concrete with CO2 in the air, which changes
the acidity in the concrete. Chlorids, from sources such as seawater or de-
icing salts, also impact the acidity of the concrete. Chemical acids,
basically everything biological such as milk, beer, manure, and other acids
dissolve the calcium. And frost can cause parts of the concrete to flak off.
All those effects can get worse if the concrete gets porous. But all are well
understood problems that inform the necessary mixture.

------
jf___
A material science professor at TU Delft [1] and this material is commercially
available [2].

From the abstract of `Bacteria-Based Self-Healing Cementitious Composite for
Application in Low-Temperature Marine Environments` [3]: "The composite
displayed an excellent crack-healing capacity, reducing the permeability of
cracks 0.4 mm wide by 95%, and cracks 0.6 mm wide by 93% following 56 days of
submersion in artificial seawater at 8 ◦C. Healing of the cracks was
attributed to autogenous precipitation, autonomous bead swelling, magnesium-
based mineral precipitation, and bacteria-induced calcium-based mineral
precipitation in and on the surface of the bacteria-based beads. "

[1] [https://www.tudelft.nl/citg/over-
faculteit/afdelingen/materi...](https://www.tudelft.nl/citg/over-
faculteit/afdelingen/materials-mechanics-management-design-3md/sections-
labs/materials-environment/staff/dr-hm-henk-jonkers/)

[2] [https://www.basiliskconcrete.com/hoe-werkt-
het/?lang=en](https://www.basiliskconcrete.com/hoe-werkt-het/?lang=en)

[3] doi:10.3390/biomimetics2030013

------
mysterypie
> _We may think we 're at the height of human knowledge, but the ancients did
> possess precious knowledge that has been lost to time._

I would love to hear why a chemical analysis doesn't solve the riddle here.
Scientists were able to isolate a strain of the coronavirus and publish the
entire genetic sequence in a matter of days, but we can't analyze a sample of
concrete?

~~~
cmroanirgo
Just as carbon ash from the fireplace has the same chemical makeup as a
diamond, there's a bit more to it: namely the changing structure of the stuff
itself. The CSH reaction
([https://en.wikipedia.org/wiki/Calcium_silicate_hydrate](https://en.wikipedia.org/wiki/Calcium_silicate_hydrate))
is something that evolves over time, and even then the modern chemical
reaction is not completely known in all its states (as far as I'm aware).

> _The [balanced chemical mass] of C-S-H in cement paste is variable and the
> state of chemically and physically bound water in its structure is not
> transparent, which is why "-" is used between C, S, and H_

The article actually misses how modern concrete (portland) get stronger over
the first few years as the crystalline structures grow. So, I can well
understand how it's a bit tricky to find out what the original roman mix was,
just using observations 2000 years later: because a lot of things have
happened from the original cake mix to what we see now.

~~~
catalogia
> _Just as carbon ash from the fireplace has the same chemical makeup as a
> diamond_

AFAIK wood ash is predominantly mineral oxides, particularly of calcium. The
carbon in the wood is the fuel that burns up and goes out the chimney as
carbon dioxide. When wood burns it first boils and burns off various volatile
compounds, leaving behind black charcoal coals, made predominately of carbon.
This charcoal also burns, leaving behind wood ash.

~~~
kragen
I think they meant charcoal.

------
trappist
Is this subject to survivorship bias? Will future generations wonder at the
inexplicable durability of a few modern structures still available to wonder
at, for accidental reasons?

~~~
exclusiv
No. We make shit these days. Fast and cheap. 20 yrs is a long time.

~~~
nkrisc
Whether or not the Romans meant for any of their structures to last 2000
years, it was clearly overkill for their purposes.

~~~
conradfr
I mean the Roman Empire spans between 500 and 1500 years depending of what you
count.

------
antiterra
Some prior discussion on this at
[https://news.ycombinator.com/item?id=20482050](https://news.ycombinator.com/item?id=20482050)

The top comment there links to two other discussions.

------
jonplackett
This is amazing - I wonder if some luck was involved. I’m sure they were
deliberately trying to make strong/lasting concrete but they couldn’t have
tested it over hundreds of years, let alone 1000s. Is there a degree of chance
to it being this effective?

~~~
pietroglyph
This seems like it's a great example of survivorship bias: the Romans probably
made lots of concrete that didn't survive the elements, but there was some
that the Romans _did_ get lucky with. It's also possible that we don't see a
similar thing today because our manufacturing processes are so much more
standardized (and critically, they are standardized around a process that is
apparently less robust.)

~~~
londons_explore
There are literally thousands of types of concrete you can buy today, and even
now occasionally you get a 'bad batch' that doesn't test as strong as it's
supposed to be. That indicates there must be quite a bit of per batch
variation too.

Overall, considering we build far more things per day than the Romans did, I
reckon there's a good chance more of our buildings are around in 2000 years
than the Romans have left now

~~~
jaclaz
The point is not much about strength (and measured strength according to
norms) but rather about durability and strength over time.

With "modern" cement (Portland) it is relatively easy to get high strength at
4 weeks/28 days, while (good ol') pozzolanic cement the reaction/hardening is
much slower but over time (and with adequate humidity) it can reach and beat
the "better" portland cement.

Almost nothing we build today will be there in 2000 years, but not because of
the concrete itself, but because of the reinforcing steel we use (and because
of the different use of concrete in much slimmer structures).

Anecdata: in the '80's/'90's I was working in a large tunnel project and we
used in tunnels pozzolanic cement (not reinforced) for the lower part of the
lining, and it was not easy to get the "right" (according to norms) 28 days
strength (250 Kg/cm^2 at the time), but, once the tunnel was finished (roughly
3-5 years laters) and we did further testing of the structures, we found that
it reached strength of the order of magnitude of 500-600 Kg/cm^2 whilst the
corresponding Portland based concrete, targeted at the same 250 Kg/cm^2,
reached "only" 300-350 Kg/cm^2.

~~~
londons_explore
Is anyone building with stainless steel reinforcement bars for buildings they
want to last 2000 years?

EDIT: Looks like they are: [http://www.concrete.org.uk/fingertips-
nuggets.asp?cmd=displa...](http://www.concrete.org.uk/fingertips-
nuggets.asp?cmd=display&id=106)

~~~
jaclaz
Yes, though the other issues remain and stainless is very different from
"ethernal" ( _much_ longer lasting, yes, 2000 years I doubt), besides
stainless steel (which is very expensive, 4 to 6 times common reinforcing
steel as an order of magnitude) also zinc or resin coated steel is used.

Concrete is only a good material to resist compression forces - generally
speaking - it was used by Romans only in structures that were exclusively (or
almost exclusively) subject to compression (arches).

The revolution that reinforced concrete made was about combining two materials
(steel very apt to resist tensile forces and concrete very apt to resist
compression) with very similar other characteristics (thermal expansion) and
compatible between them, if you want it is one of the first examples of
composite material, to obtain something that could be used in structures
subject to tensile and compression (besides shear) forces.

Our recently (last 100 years or so) reinforced concrete structures are very
lean and elastic, which implies that they move and crack.

All the research on new concrete is about making mixtures where cracks are
reduced to the minimum, as before or later through these (micro) cracks air
and water penetrate, oxidizing (or rusting) the steel.

As well in the years the norms about the cover (i.e. the minimal distance from
steel to the outside) has been increased (it depends on countries and types of
structure but 30 years ago 2 or 2.5 cm were common, nowadays 4 or 5 cm are
common).

------
__initbrian__
All concrete gets stronger over time!
[https://www.concrete.org/tools/frequentlyaskedquestions.aspx...](https://www.concrete.org/tools/frequentlyaskedquestions.aspx?faqid=13)
edit: [adding more detail, removing all caps] Contracts will often specify a
7day or 28day strength because the concrete will keep getting stronger over
time. Separate cylinders are poured for independent testing of the strength

Here’s a good textbook on concrete: Concrete: Microstructure, Properties, and
Materials Textbook by Paulo J. M. Monteiro and Povindar Kumar Mehta

------
uniformlyrandom
1\. Only in salt water

2\. Only over time

3\. And actually we still do not know.

Slightly disappointing.

------
dghughes
I'm more amazed at how long of a period ~1,000 years where people forgot about
concrete.

------
29athrowaway
Another interesting piece of lost ancient knowledge:

[https://en.wikipedia.org/wiki/Greek_fire](https://en.wikipedia.org/wiki/Greek_fire)

~~~
eb0la
Another lost product, though you can find modern recreations in amazon:

[https://en.m.wikipedia.org/wiki/Damascus_steel](https://en.m.wikipedia.org/wiki/Damascus_steel)

------
allovernow
Our modern building practices and materials are not optimized for longevity.
What incentive in modern times does a contractor have in building homes that
last hundreds of years?

The average person is too ignorant to know or care anyway.

~~~
loeg
Roman concrete is slow setting and didn't have rebar.

We prefer to build quickly (more efficient use of land multiplied by time) and
prefer our buildings not fall over in earthquakes.

Also much of residential construction is wood frame (edit: US), which also has
pretty bad longevity unless extremely overbuilt.

~~~
saberdancer
Wood frame construction seems to be much more popular in US than in Europe. At
least parts of Europe I am familiar with.

~~~
thesmok
It's becoming more popular in Eastern Europe, because cheap and fast.
Architects say that typical age of their client is 40 to 50, and these people
started to realize that their children wouldn't want this house. So the house
only has to last another 40 years.

------
lazylizard
I had this impression that pozzolans were a thing... Like. Solved problem.

