A chemical reaction in ancient Roman concrete makes it stronger over time (2017)(science.howstuffworks.com)
science.howstuffworks.com
A chemical reaction in ancient Roman concrete makes it stronger over time (2017)
https://science.howstuffworks.com/why-ancient-roman-concrete-stronger-than-modern.htm
66 comments
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), 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.
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), 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.
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...
[2] https://www.basiliskconcrete.com/hoe-werkt-het/?lang=en
[3] doi:10.3390/biomimetics2030013
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...
[2] https://www.basiliskconcrete.com/hoe-werkt-het/?lang=en
[3] doi:10.3390/biomimetics2030013
> 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?
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?
Available resources for the tasks are probably different.
[deleted]
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) 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.
> 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.
> 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.
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.
I think they meant charcoal.
You can chemically analyze a piece of cake and know exactly what it's made of. You would still not know how to make one with that knowledge alone.
I guess some chemical reactions which happen over time are kind of like hash functions. So it's hard the find out what the input was just by observing the output.
I'm not a chemist, but perhaps a good analogy is making Mona Lisa with Game of Life? Recently posted on HN.
Finding Mona Lisa in the Game of Life
https://news.ycombinator.com/item?id=22552006
From the article: "This looks kinda cool, but what if we want to find a Life state that eventually, after following the rules of Life for a few rounds, reaches a state that looks like Mona Lisa? This requires working backwards instead of forwards from the target picture, which is a much more difficult problem. (...)
We call Life state A the "parent" of state B if A turns into B by following the rules of Life. The reason that it's difficult to find the parent of a state is that the rules of Life are non-reversible. There's no direct way to go from a Life state to its parent, and in fact, it's possible for a state to have multiple parents or even no parents. "
Finding Mona Lisa in the Game of Life
https://news.ycombinator.com/item?id=22552006
From the article: "This looks kinda cool, but what if we want to find a Life state that eventually, after following the rules of Life for a few rounds, reaches a state that looks like Mona Lisa? This requires working backwards instead of forwards from the target picture, which is a much more difficult problem. (...)
We call Life state A the "parent" of state B if A turns into B by following the rules of Life. The reason that it's difficult to find the parent of a state is that the rules of Life are non-reversible. There's no direct way to go from a Life state to its parent, and in fact, it's possible for a state to have multiple parents or even no parents. "
I'm sure they can analyze a sample of 2000 years old concrete, but it's not going to look the same as when it was built and it'll probably not be a simple mix of basic elements but very complex. The Romans might have used a mixture with actual lava or corals e.g. and it'd be hard to trace it back to such ingredients.
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?
We tend to build bridges out of steel nowadays for all kinds of reasons (cost, speed, durability, strength), and steel ain't gonna last 1000 years. There's not a lot of benefit to designing for 1000 year endurance.
I considered that as well. I live in an area of town where a lot of 110+ year old town houses still stand proudly (European cities would laugh at that statement) but I know that only the stuff built well is standing today while whatever had been made to a lower quality crumbled and was built over. Just because old buildings survive to this day doesn't mean everything from that era was built to last.
With regards to the featured article, I'd say multiple factors are at play - Roman concrete has amazing properties and structures that used it combined with good quality-control (i.e. correct material ratios in the concrete) and good engineering survived.
The human elements, quality-control and design, are less interesting than a magical substance.
With regards to the featured article, I'd say multiple factors are at play - Roman concrete has amazing properties and structures that used it combined with good quality-control (i.e. correct material ratios in the concrete) and good engineering survived.
The human elements, quality-control and design, are less interesting than a magical substance.
Some prior discussion on this at https://news.ycombinator.com/item?id=20482050
The top comment there links to two other discussions.
The top comment there links to two other discussions.
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?
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.)
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
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
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.
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.
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...
EDIT: Looks like they are: http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=displa...
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).
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).
Everything involves luck, but until very recently (past 100 years) mortar maker was an expert occupation with thousands of years of accumulated art and science. These days most professional concrete and foundation contractors don't have a clue about mixing or pouring anything other than portland cement and whatever is most popular at Home Depot.
Who knows how much knowledge has been lost, but my guess is that the Romans developed their processes much more deliberately than haphazardly. You don't have to understand the 21st century chemistry of why something works in order to build a systematic and even scientific methodology. It was part and parcel of a mortar makers job to understand how to make use of regional materials and handle local environmental conditions.
Who knows how much knowledge has been lost, but my guess is that the Romans developed their processes much more deliberately than haphazardly. You don't have to understand the 21st century chemistry of why something works in order to build a systematic and even scientific methodology. It was part and parcel of a mortar makers job to understand how to make use of regional materials and handle local environmental conditions.
why not? The romans were around for a long time.
1. Only in salt water
2. Only over time
3. And actually we still do not know.
Slightly disappointing.
2. Only over time
3. And actually we still do not know.
Slightly disappointing.
All concrete gets stronger over time!
https://www.concrete.org/tools/frequentlyaskedquestions.aspx...
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
Here’s a good textbook on concrete: Concrete: Microstructure, Properties, and Materials Textbook by Paulo J. M. Monteiro and Povindar Kumar Mehta
I'm more amazed at how long of a period ~1,000 years where people forgot about concrete.
Another interesting piece of lost ancient knowledge:
https://en.wikipedia.org/wiki/Greek_fire
https://en.wikipedia.org/wiki/Greek_fire
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
[deleted]
We might not use that any more, but unfortunately the concept of lighting one's enemy on fire hasn't gone away. What about flamethrowers? Napalm? Is there any indication that Greek fire is somehow more destructive than the modern shop of incendiary horrors?
Related video that I liked: https://www.youtube.com/watch?v=pW-SOdj4Kkk
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.
The average person is too ignorant to know or care anyway.
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.
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.
Wood frame construction seems to be much more popular in US than in Europe. At least parts of Europe I am familiar with.
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.
When I was last in Italy, concrete seemed to remain popular for house building.
Ah, sorry, your observation is astute -- I am in the US.
I think that is a US thing. In Europe and South America at least we don't use much wood.
> much of residential construction is wood frame
Why is this?
Why is this?
It's cheaper.
Whishing people would realize it is also much more sustainable.
Even if you have to rebuild every 50 years or so? A brick structure will last thousands of years.
If it is even half-ass maintained a wooden house will survive far longer than that. If your roof isn't leaking and your siding is either painted or not wood, there is very little that will do actual structural damage to the house. Dry wood can last hundreds of years if it has any sort of protection from the sun and rain.
Also, wood houses can be built to much higher quality than what people are building new to sell on the open market. Square footage has a much higher profit margin than thicker and more insulating walls or better wood or more careful and accurate framing. Even putting in something highly visibly like way better windows and doors isn't going to get you jack shit of a return if you are building to sell rather than ever paying for heating and cooling costs or what not.
Also, wood houses can be built to much higher quality than what people are building new to sell on the open market. Square footage has a much higher profit margin than thicker and more insulating walls or better wood or more careful and accurate framing. Even putting in something highly visibly like way better windows and doors isn't going to get you jack shit of a return if you are building to sell rather than ever paying for heating and cooling costs or what not.
> If it is even half-ass maintained a wooden house will survive far longer than that.
I mostly agree with the other points you make but not this.
Modern wood-frame and plywood construction with vapor barriers, Air-conditioning and impermeable facades are very sensitive to moisture. Once water enters, it has nowhere to go and this has to be addressed quickly and thoroughly or else there's a serious and relatively quick structural deterioration, mold issues and a downward-spiral of problems.
Mcmansion owners who want to keep their house in excellent condition better be on top of this stuff, or else they or the suckahs they sell to are going to be paying a hefty price. Avoiding this requires decidedly more than half-assed maintenance.
I mostly agree with the other points you make but not this.
Modern wood-frame and plywood construction with vapor barriers, Air-conditioning and impermeable facades are very sensitive to moisture. Once water enters, it has nowhere to go and this has to be addressed quickly and thoroughly or else there's a serious and relatively quick structural deterioration, mold issues and a downward-spiral of problems.
Mcmansion owners who want to keep their house in excellent condition better be on top of this stuff, or else they or the suckahs they sell to are going to be paying a hefty price. Avoiding this requires decidedly more than half-assed maintenance.
Any suggestions for preventive maintanence / monitoring? Do you put moisture sensors in the walls? Low-power dehumidifiers on infrequent cycle?
I think it's enough to stay on top of what's going on in the house and deal with problems as they appear or even before. Old drafty houses are a bit more resilient because they can dry out relatively quickly. New houses with stuff like OSB (chipboard) will just soak up moisture and "tell you" in a variety of ways that something is wrong.
Like mold. If it appears, you got a moisture source that needs to be discovered and dealt with. Crawlspaces need to be encapsulated and have negative pressure. It's amazing how simple stuff like water leaking from a gutter can find it's way into a house with even the smallest gap at a window interface or if the drainage is bad where the house meets the ground.
Like mold. If it appears, you got a moisture source that needs to be discovered and dealt with. Crawlspaces need to be encapsulated and have negative pressure. It's amazing how simple stuff like water leaking from a gutter can find it's way into a house with even the smallest gap at a window interface or if the drainage is bad where the house meets the ground.
The Vindlausloftet in southern norway is a wooden cabin built in 1170 — granted, this is a massive wood construction and not a wood frame construction.
My feeling when I went to the US was that many houses felt incredibly "flimsy" as if you could just punch your fist through the wall.
My feeling when I went to the US was that many houses felt incredibly "flimsy" as if you could just punch your fist through the wall.
You can easily put your fist through drywall. That's just the decorative coating on the inside. The framing is treated 2x4, mostly, and good luck putting your fist through that.
You don't need to put your first through the 2x4, because they're only every half-metre or so - you can walk between them if you've knocked out the drywall.
yes, in the US, 16"-on-center is the standard for stud spacing (12"-on-center used to be used too but not so much anymore).
one of the interesting tidbits about passivhaus construction is that it tries to minimize even more the wood content of a building (in favor of insulation), so studs can even go to 24" apart.
one of the interesting tidbits about passivhaus construction is that it tries to minimize even more the wood content of a building (in favor of insulation), so studs can even go to 24" apart.
I grew up in a New England school house that was over 200 years old when I was there 40 years ago. Wood buildings last for generations with basic maintenance. I'd much rather live in one than a concrete slab.
New England houses that have survived 150+ years are the exception to this rule. They're extremely overbuilt compared to today's wood-frame construction and you get some survivorship bias effect (the ones that didn't make it have been replaced). They're also mostly not on fault lines, and the climate may or may not be beneficial to the longevity.
Do you realise there are other building materials beside concrete and wood?
This is an underestimate - I'm reading your comment from a 170-year-old wooden house.
> I'm reading your comment from a 170-year-old wooden house.
That's great, but that wooden house you live in is vastly better in terms of quality of wood than what is common today in USA construction. If OSB had existed 170 years ago, I assure you would have long ago turned to dust by today. You probably have magnificent wood beams and single-piece shipboards spanning all studs and joists.
It all comes down to material-quality, maintenance, design, expectations and weather. Sadly, all of those factors take a back seat to cost and speed where modern housing developers are concerned. Modern building techniques make it easy to fool unsavvy consumers into an illusion of durability and quality.
That's great, but that wooden house you live in is vastly better in terms of quality of wood than what is common today in USA construction. If OSB had existed 170 years ago, I assure you would have long ago turned to dust by today. You probably have magnificent wood beams and single-piece shipboards spanning all studs and joists.
It all comes down to material-quality, maintenance, design, expectations and weather. Sadly, all of those factors take a back seat to cost and speed where modern housing developers are concerned. Modern building techniques make it easy to fool unsavvy consumers into an illusion of durability and quality.
> This is an underestimate
No it isn't - if you ask the people actually building American wooden houses how long they think they'll last - they'll tell you almost what I did - just 60 years! Check national building standards 7543:1992. They increased it under pressure a few years ago, but that's what they were designing against until recently!
Of course some houses will survive longer (and 170 is a pretty modest age to be boasting about) but even the people building them don't have confidence that they will.
No it isn't - if you ask the people actually building American wooden houses how long they think they'll last - they'll tell you almost what I did - just 60 years! Check national building standards 7543:1992. They increased it under pressure a few years ago, but that's what they were designing against until recently!
Of course some houses will survive longer (and 170 is a pretty modest age to be boasting about) but even the people building them don't have confidence that they will.
Not on a fault line it won't. Also vastly more labor intensive to build or redevelop as needs change.
The average person doesn't build his home in seawater.
I had this impression that pozzolans were a thing... Like. Solved problem.
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
Last year: https://news.ycombinator.com/item?id=20482050
2017: https://news.ycombinator.com/item?id=14690329
2013: https://news.ycombinator.com/item?id=5883443
2010: https://news.ycombinator.com/item?id=1852000