Concrete is responsible for 8% of global CO2 emissions. Cement is usually made from mined limestone, which is one of the largest natural stores of carbon dioxide. Using that to make cement is a bit like burning oil. The world is addicted to concrete, so this problem is not going away. We make synthetic limestone using atmospheric CO2, such that when it is used to make cement, the process is carbon neutral.
We were both master's students in engineering at Oxford University in the UK. I decided to write my dissertation on direct air capture of CO2. While looking through existing solutions it struck me that none were sufficient. They all operated a circular process that left them with gaseous CO2 that needed to be stored somewhere. A circular process is one that uses a sorbent to trap atmospheric CO2 but then re-releases the trapped CO2 as a pure gas stream to regenerate the sorbent for re-use. We don't have enough high-quality cheap stores of CO2 to justify such an approach. Storage must be permanent and safe. We realized that by taking a linear approach, we both make the process of capturing CO2 profitable and avoid the problem of where to store the CO2. We make sorbents for trapping CO2 in the form of mineral carbonates, these compounds are inert and trap CO2 for millions of years. They can also be commercialized as raw materials for making building materials including glass and concrete. In one step we solve three key problems of carbon capture: 1. How to trap CO2 energy efficiently 2. How to store the CO2 3. How to make money while doing all this.
Specifically, we use renewable electricity to extract dissolved oceanic CO2 as mineral carbonates of calcium and magnesium by contacting seawater with our proprietary alkaline sorbent. These mineral carbonates are important ingredients in cement as well as other building materials. The undersaturated ocean then re-absorbs an amount of atmospheric CO2 equivalent to the amount we removed when reacting with our sorbent. Effectively, the world’s oceans become our air contactor.
There are other companies addressing emissions from concrete production, but they don’t address the unavoidable process emission from the raw materials used in concrete. Start-ups in this space have so far focused on curing concrete with CO2 at the end of the production process. These are great solutions that can create low-carbon cement, however they’ll never get to carbon neutral cement that the world needs. The 70% of emissions from production are not being tackled by anyone on the market today. Until now concrete producers have favoured capturing emissions at the point where they’re released as their “2050-solution”, ie. in the distant future. Point source carbon capture can expensively capture 80-90% of emissions. This solution has the same problem as circular DAC solutions where a method of permanent CO2 storage is needed. There is a trial $3B (!) project in Norway to pump CO2 into empty gas fields at a cost of ~$1000/tCO2. This is expensive and complicated engineering. On the other hand, all we need is renewable electricity and seawater.
We make money from selling synthetic limestone to cement producers and commercializing parallel byproducts including green hydrogen and desalinated water. We also generate carbon credits from our process. We are currently negotiating with concrete producers to decarbonize their limestone supply. Response has so far been very positive with multiple LOIs signed with producers across Europe. We are also working with a construction company to build the world’s first carbon neutral houses this decade. We are currently building a demo plant just outside Oxford. It has the capacity to remove and store 1 tonne of CO2 per year. We will use this plant to make enough product that we can deliver to our commercial partners to confirm compatibility with their manufacturing set-up. Following successful testing, we will scale this up to replace all of global limestone mining; currently >2 billion tonnes of limestone per year.
We're excited to hear any thoughts, insights, questions, encouragement and concerns in the comments below! Erik and I will be monitoring the thread over the course of today to answer any questions. Also feel free to reach out to me by email at Marcus.lima@heimdalccu.com.
https://www.moderndescartes.com/essays/carbon_neutral_concre...
TL;DR: ignoring all sources of overhead/inefficiency and purely on consideration of thermodynamic costs, $70 of electricity can generate $100 of lime, $300 of chlorine gas, and $75 of hydrogen gas.
Much more details on the chemistry and economics in the blog post...
This is probably the most substantive Launch HN thread (I mean of the official YC startup launches at https://news.ycombinator.com/launches) that we've yet seen. And it was very little work to put together.
Rule of thumb for electrolytic water splitting is 50 kWh per kg of hydrogen. That would mean about 0.29 as much H2 as estimated in the blog post. If you scale all salable products by 0.29 (not sure if that's sensible since I haven't identified the root error) it would still yield $142 of products from $70 of electricity. And good news there is that industrial scale electricity can be had for well under $0.13/kWh.
They don't need to do it all alone. If this idea is solid, a lot of different companies and nations could be doing this.
I'm excited for China to have carbon-neutral cement. If they do, it will probably be of their own making and not tied to this company growth.
Rau, G.H., Willauer, H.D. & Ren, Z.J. The global potential for converting renewable electricity to negative-CO2-emissions hydrogen. Nature Clim Change 8, 621–625 (2018).
https://www.nature.com/articles/s41558-018-0203-0?WT.feed_na...
I'd also like to clarify up front that I'm limiting my analysis to the proposed electrochemical mechanism, and have no information on the specifics of Heimdal's proposed implementation - which may vary significantly and materially:
The theoretically minimum electrical energy input (often) winds up being set by one of the reaction intermediates, rather than the overall reaction enthalpy. Typically these analyses are done via determining the corresponding half cell potentials, then counting electrons and computing power via P = IV.[0] So we only need to consider H2O (and its dissociation -0.83[1]) and Cl- to Cl2 at 1.36). That sums to approx 2.19V for the cell, and 2 electrons to do 2H2O + 2Cl- -> H2 + 2OH- + Cl2. That the formed OH-'s pair with Ca2+, and/or CO2 is immaterial to the theoretical electrical efficiency of the cell. Accordingly, the cell energy requirement is on a molar basis identical to the chloralkali process. The only difference is the presence of 2Na+ vs. Ca2+.
Less concretely the lower reactant concentrations have two specific negative effects: Cell potential is adversely affected (per the Nernst equation) and Cell current can be adversely affected if/when depletion occurs. At 0.01M of Ca2+, vs. 6+M Na+[2], current densities could be 1-3 orders of magnitude lower. Cell count (CAPEX) is inversely proportional to current density.
Ultimately making CaCO3 this way (and CaO) winds up substituting a 400$/tonne product in NaOH for an approx 40$/tonne product in CaCO3. It is a very technically feasible approach for turning $$$ into sequestered carbon via non emission from natural limestone.
The analyzed approach reminds me a lot of Calera, who had an apparently similar electrochemical approach to CaCO3.
[0]Not that it can't be done from Gibb's energies, indeed the standard potentials for a reaction can be computed from the delta Gibbs, but the specific species the electrons are being pulled from/pushed into matters. Phrased a different way: electrical energy and overall reaction enthalpy are not necessarily fungible. e.g. (at a simple level) the reaction of CO2 with Ca(OH)2 doesn't affect the electrical energy requirement, nor does CaCO3 --> CaO because neither reaction involves electrons. Any exotherm just winds up 'wasted' as heat, instead of lowering the electrical demand.
[1]Per 2H2O + 2e- --> H2(g) + 2OH-(aq) #6.8.11 https://chem.libretexts.org/Courses/Mount_Royal_University/C...
[2] Saturated near room temp, don't have a better source handy.
You're pointing out the crux of the issue: "Ultimately making CaCO3 this way (and CaO) winds up substituting a 400$/tonne product in NaOH for an approx 40$/tonne product in CaCO3."
I calculated that Heimdal's process has 7x reality factor overhead, whereas chloralkali has 2x reality factor. This is seemingly in opposition to your statement. I think the diff comes from considering all the other energetics - precipitation of CaO from the hydroxide is thermodynamically quite favorable, reducing the overall theoretical cost by 65% relative to the chloralkali process. So to the extent that Heimdal can usefully harness that energy gradient, that's the difference.
I guess the crux of my argument is that it is impossible[0] to capture the energy from CaO precipitation. E.g.[1] one has to spend 2J of electrical to create the hydroxide in Ca(OH)2, but the 1.5J get released on precipitation of CaO comes as heat[2], rather than a reduction in energy input.
So, in summary, Electrochemical CaCO3 would have [approximately] equal theoretical electrical demand to chloralkali and, to borrow your term, a higher reality factor overhead[3], to make a product an order of magnitude cheaper than NaOH.
[0]For a loose definition of impossible, to be fair reality's never quite so black and white.
[1]Made up numbers.
[2]And heat in sub-boiling water is not useful for doing work.
[3]I broadly agree with your position on reality factor overhead, just made less of an emphasis on it trying for 'equivalent but worse and creates less value'
Sorry I don't have the chops to work out if that's economically feasible.
We need zero CO2.
I'm aware of that. There are two main sources of CO2 in cement manufacture: the calcium carbonate raw material, and the fossil fuel required to cook the raw materials into clinker. Heimdal only deals with one. The questions I'm interested in are which source contributes more CO2 and which can be most economically eliminated.
> We need zero CO2.
Agree, but Heimdal won't get us there. We'll either have to find a replacement for cement, or devise a practical method to convert CO2 waste to something benign, like an improved Bosch process.
I immediately associated this to possible disruption in the cement supply chain that's about to hit Sweden. Cementa, which produces about 75% of Sweden's cement, will no longer be allow to mine limestone on Gotland. Their permit was due to be renewed but due to the low quality of their environmental impact study the court was unable to determine if the mining might impact the local residents groundwater. So just a week or so ago a Swedish court decided Cementa muts cease mining operations some time in October. And as I said this mine/factory produces 75% of Sweden's cement. So there are more problems with the current way of producing cement than just CO2.
Most probably there will be a limestone/cement void in Sweden that needs to be filled, and I'd rather have you doing it than some new limestone mine. Best of luck to you!
[1]: https://www.xprize.org/prizes/elonmusk
You should put that (slightly edited) paragraph on your home page. I looked at your home page first and didn't understand what you did until I came back here.
Best of luck!
(Btw: Try to avoid using the passive voice.)
If your process is scaled up massively, will the oceans run out of Calcium? Or will they absorb Calcium from somewhere? What would that Calcium source be?
Might have to spread your plants across multiple regions, or place it around somewhere with low impact to marine life I think.
Edit: Also, it’s great that your de-carbonating the ocean as part of this project. Ecological damage to the ocean is out of sight, and usually out of mind.
Not bad tackling ocean acidifcation to boot
Would your process reduce the local acidification significantly? Could there actually be a win-win situation here deploying around coral reefs? Especially given that such reefs are found in countries with massive solar potential (i.e Australia).
If you pull enough to dissolve more limestone, then they’re just mining limestone hydraulically, and they are mining it outside of the economic exclusion zone of their own country. This product plan is literally “I drink your milkshake.”
On the plus side Florida will fall into the ocean sooner.
The cement industry specifically needs lime, CaO. Lime is most easily obtained by burning CO2 off of limestone, CaCO3. As you point out, this is effectively "burning off" captured carbon dioxide and is bad.
Where does the carbon savings come from when the ultimate destination is to just burn CO2 off and make the actual desired product, CaO? Is this process ultimately just a better way to make CaO?
You do the same with limestone, rapidly creating it by capturing atmospheric CO2 (indirectly, via the ocean) so it can then be burned in cement production instead of the naturally occurring kind. In both cases, CO2 is released into the air at the end when the product is burned, but because the released CO2 had just now been sucked out of the air anyway (instead of having previously been sequestered in natural limestone) you're not adding to the total amount of CO2 in circulation in the system, making the process neutral.
I really hope this is scalable worldwide.
> The world needs CaO for cement. We have a carbon neutral process for making it
vs
> We make synthetic limestone using atmospheric CO2, such that when it is used to make cement, the process is carbon neutral.
Do you make CaO or CaCO3?
You'll be adding more CO2 to the air -increasing oceanic acidification- while increasing the ocean's ability to dissolve more CaCO3. Far from fighting global warming it sounds like this will put exactly as much CO2 into the atmosphere and double the leaching impact on shellfish and coral.
I am shit at chemistry and would really like cement to not release CO2, but I don't understand this.
Alternatively, we do end up extracting Ca from the ocean that is not replenished (there's probably so much we don't care) and rely on the atmospheric CO2 to correct ph balance of the ocean?
to my limited understanding cement production emits CO2 in two ways: by splitting limestone into lime and co2 and by burning carbon based fuels to split the limestone.
your method addresses both sources of co2 from cement production? or just one of them?
The OP says they'll make money by selling synthetic limestone ("We make money from selling synthetic limestone to cement producers..."), so I think the CO2 still needs to be burned off of it before the cement is produced. However they say they already (indirectly) pulled that same CO2 out of the air, instead of the ground, so overall the process is carbon neutral.
I find that difficult to understand. If the output is CaCO3 that still needs to kilned to make CaO, then CO2 gets emitted. Even if that volume of CO2 was obtained from dissolved CO2 in the ocean, one would have had to expend energy to extract that CO2 from the ocean and that energy would have generated emissions as well.
If the output of this is just CaCO3 again, then I fail to see how this is a better solution than carbon capture using a clay geopolymer technique that goes directly to a concrete like structural material. What I mean is, wouldn't it be better to just skip all of this and go focus on rediscovering the technology for "creating" rock like what was possibly achieved at Cuzco (Hatun Rumiyoc) or Pumapunku? Or more realistically in the short term, using fly ash and silica flume or slag to make concrete without requiring CaO?
I suppose they could planning to use renewable energy?
> If the output of this is just CaCO3 again, then I fail to see how this is a better solution than carbon capture using a clay geopolymer technique that goes directly to a concrete like structural material.
I don't have a horse in this race, but one possibility is that concrete is a better understood material than some novel "concrete like structural material," so it's more acceptable in safety critical situations (e.g. people know how it fails, how to detect failures, how to remediate problems, etc.).
> one possibility is that concrete is a better understood material than some novel "concrete like structural material," so it's more acceptable in safety critical situations (e.g. people know how it fails, how to detect failures, how to remediate problems, etc.).
Your explanation and argument looks correct to me. However, I should point out that concrete structures fail regularly and frequently, and sadly with great loss of human life, most recently in Miami. I should also point out that I've seen this argument used as a tactical trick. I watched a Microsoft rep using this argument successfully convince a management team that they should use Embedded Windows instead of Linux because, just as you pointed out, "we know how it fails", "we know how to detect failures", "we know how to remediate problems".
With some new material, they'd probably need to rely significantly on accelerated aging tests (at least for a few decades), and there's always the question with those of how accurately those actually model real aging under real conditions.
That's not to say they shouldn't try new materials, just that the rollout probably should be slower, and maybe not so good for tackling climate change. For that reason, I can see the benefit of a less carbon intensive way to produce an existing material, since that could be rolled out/scaled up immediately without some of the concerns of alternatives.
what bothers me is the 1400°C needed in the kiln. to my understanding the ratio of limestone-to-lime co2 to fuel-for-the-process co2 is 2:3. you need lots of co2 intensive energy for that kiln.
and when and where the hydrogen is replacing some carbon based fuels, somehwere, that counterbalances the carbon based fuel used in the cement furnace.
now I get how it becomes carbon neutral overall.
do you guys have data points at hand how long it takes for an ocean to recapture the co2?
We've also thought about working with some big name companies like Apple, Amazon, big name hotels etc. to build a carbon neutral office/store/hotel. Haven't been able to reach the right people here yet though. Any intros/suggestions are appreciated!
If you’re looking for a novel way to generate excitement, how about the X-Prize?[0] You’re doing a demo of “1 tonne of CO2 per year“, that’s enough to enter, and entering is enough to tell investors. Doing well could provide dilution free capital, technical validation, in addition to free publicity.
[0]: https://www.xprize.org/prizes/elonmusk
X Prize is on our radar, only a shame we mised the cement specific one. Though the dollar value on this one is certainly better
I really like the idea. One thing I'm curious about is what's in it for the contractors (edit: cement producers) buying from you instead of others? I get the environmental impact, but my guess is they only care if touches their bottom line. Will it be cheaper, either in raw price or because of green incentives etc?
Our experience so far has been that the environmental angle has been sufficient to persuade. Cement companies are in a bit of a bind given the attention to their sustainability efforts. However we're pitching ourselves as a cost competitive solution. Depending on geography we'll be able to positively affect their bottom line through the carbon credits system. Under the European ETS for example, they reward companies that reduce emissions (https://carbonmarketwatch.org/wp/wp-content/uploads/2016/11/...)
Another curious question: Do you make an "actual" limestone, or what is the final output? A rock, chalk, mudlike or something?
Good luck, great idea, hope it works out!
I figured you guys just like Marvel movies. :)
A couple questions:
1. How does your synthetic limestone compare to natural limestone? Are there any important performance differences in terms of the material properties of the resulting concrete?
2. What are the biggest bottlenecks/obstacles in terms of scaling this to the point it could replace a significant portion of natural limestone used today?
Edit: do you have any blog posts/more information about what you're doing? I would love to share this around, but unless I'm missing it your site is very light on details.
Unfortunately no blog post or anything like that just yet. Website is very light on details for now, we haven't prioritised updating (+ some patent considerations).
How do you get permission to mine seawater? Its kinda of a weird question and I imagine its very country dependent.
Very intersting project.
If you wish to do it privately, check links at my bio.
Best!
You're talking about ancillary products and regulatory credits, which is a fine business model; but I'm asking about the core industrial process. Trying to get a sense of how much more efficient your scale-up needs to be, before your process is in the black.
It’s an interesting point. The most sunny region might be not the target market. Then you need to factor in the CO2 emissions arising from the transport to consumers.
Of course they are now electrolytically strip mining the entire ocean floor…
also, transport is a minor fraction compared to the net saving
Whether your country has invested in renewable sources like hydro plants and associated high-voltage, long-distance transmission lines is what matters. The geography of whether hydro plant is near the coast or not is of less importance than whether it exists or not!
Can you handle intermittency in your power, or do you need to run continuously? What capacity factor do you assume in your cost model?
Also good read for everyone: https://www.gatesnotes.com/Energy/Introducing-the-Green-Prem...
We are a bit behind in the US in credits or taxes, we are now treating the social cost at $51, but not using that for tax or trade policy.[1]
There's one more number that is useful to get a numerical sense of the costs: $258 / ton, an estimate of the actual cost to society. [2]
[0] https://carbonpricingdashboard.worldbank.org/map_data [1] https://www.scientificamerican.com/article/cost-of-carbon-po... [2] https://www.nature.com/articles/s41467-021-24487-w
Hopefully the US does this: https://news.climate.columbia.edu/2021/05/06/proposed-45q-ta...
What is still to be checked is how more expensive this lime will be, and how it will stack up compared with traditional limestone+carbon tax
If this is (roughly) correct, what price point per ton of removed CO2 are you at today, assuming the cement ingredients can be sold at an optimistic price point? I'm currently with Climeworks but it's prohibitively expensive to remove all unavoidable emissions that I cause by living a normal life today, so my subscription doesn't cover all emissions yet and I would love to.
Finally, I don't see a way to buy anything on your site, or even a waiting list. Is there some call to action, like if I were a cement producer could I buy your product today? Any plans to offer CO2 removal to consumers? Or anything else people or businesses can buy at the moment?
We tend to focus on obvious sources of carbon: fuel and electricity, probably because it's something we all have some familiarity with. Agriculture and industry are hidden from our day-to-day lives, so few people are aware of their massive impacts on climate.
[1] https://arstechnica.com/science/2021/07/quest-for-green-ceme...
Just one thing.
https://www.heimdalccu.com/our-story linked from your front page gives 'Page Not found'
Thanks for pointing out - will sort this out!
Sustainable cement and concrete production is needed now more than ever. Many people do not realize that the [1] cement industry is one of the main producers of carbon dioxide greenhouse gas emissions.
[1] https://en.m.wikipedia.org/wiki/Environmental_impact_of_conc...
How much seawater do you need to get one kg of CaCO3 precipitate?
Assuming that the new process stochiometrically generates CaO according to the amount of CO2 taken up from the ocean you'd produce 1.27 tonnes of CaO per tonne of CO2. (molar masses of CO2 and CaO: 44 g/Mol and 56 g/Mol)
Cementa website states [1] an annual output of 2.7 million tonnes of cement.
So in order to produce what is only a portion of Sweden's (a comparatively small country) annual demand for cement, Heimdal's process would require processing of ~ 23 billion tonnes of seawater.
That's 23 billion cubic metres or 23 cubic kilometres per year or ~ 730 litres of seawater per second.
[1]: https://www.cementa.se/en/about-cementa
But let's not forget that it's not about merely pumping the water around. All this water must be processed. And I figure the process is a little more involved than simple reverse osmosis for seawater desalination.
In the desalination business 1 m diameter pipes with water traveling at 1 m/s and the resulting volumetric flow rate isn't a big thing .. but I'd guess for more complex processes it indicates huge/expensive apparatuses.
you say we are "addicted" to cement/concrete which is correct, but then propose more cement? i get we might need cement here and there for energy transition and might need your tech. but isn't using less cement a better/complementary approach?
im addicted to tobacco, not sure natural tobacco is my best bet.
at a time where half of the world is burning, do we need more 10 ways bridges for huge cars?
how do you tackle the worldwide sand shortage problem?
does your tech solves the issue that comminution (grinding) of clinker is one of the biggest electricity consumers worldwide?
how does that scales up, how long to deploy? what are the consequences (please be honest) to marine life, knowing that most kelp forests/corals are now gone/bleached?
how does your tech tackles the massive corruption problems that come with concrete?
thanks in advance
edit: don't take it badly, your tech looks promising on paper.
Also I like solutions that don't rely on people changing their habits, because they are most likely to work.
The alternative to tobacco is no cigars or cigarettes, the alternative to cement is not simply no buildings, no railways, no roads, no infrastructure.
still not convinced of hydrogen for steel rebar production also.
Can we talk turkey? What's your cost of capture per ton, and how do you scale from one ton to two billion tons?
The average emissions per capita per person in the US is 15 to 20 tons per year. You're building a plant that can capture 1 ton per year.. so 1/15th of a single person. How does this make a dent?
Any tips for what exists out there in the world of “exciting” concrete? Talking to him he didn’t have too much idea what even existed for research, or if civil/physics was an appropriate background for working at least close to research in concrete.
https://www.buildinggreen.com/news-article/autoclaved-aerate...
It’s lighter, less energy intensive, better insulator, basically just by inserting bubbles into the structure. Widely used in Europe but not adopted by the US because lumber is cheap and we don’t have any established manufacturers.
Kinda a ramble as my friends and I are actively talking about this right now in regards to this thread, but I'm curious how other HNers feel about this. Like when I was at Electric Imp, the real magic seemed to happen when the soldering irons got busted out. Here in Taipei I asked my buddy who's driving all the lamborghinis, and he said "just regular entrepreneurs, import/export types." I'm not money-motivated but I have been feeling I guess "left out?" I'm thinking about all those engineering projects I hear about out in Africa where they make super efficient mini stream turbines, or float wifi balloons. Feels like the good shit happens IRL.
I understand that you might not be ready to share numbers, but you mention both hydrogen and desalinated water as byproducts, which both are quite energy intensive to produce. Any chance to give a hint along the lines of:
When at scale, will your synthetic limestone be much more expensive than mined limestone or roughly the same?You hit the nail on the head there. The most important cost factor is energy. We're looking at <3MWh/tonne of CO2 removed. Somewhat higher than some competing DAC companies that claim 1.5-2.5MWh. But these guys have the added cost of finding permanent storage of their gaseous CO2. Cost estimates for this, at scale, are as high as $100/tonne
https://en.m.wikipedia.org/wiki/Biorock
I'm thinking, for example, whether this could sink some of the overabundance of solar energy in places like California.
One question: why can't the electricity be from nuclear power as well as renewables? The UN IPCC says nuclear is as low in carbon emissions as wind, and around 1/4 that of solar.
Do you have more technical details on your process? Did you publish anything? I try to dig more on your website but the content is rather thin (I learn more about what you are doing by reading your post here).
What are the by product? You mention cement and drinkable water, but there should be other byproduct to handle such as brime?
What would the net effect on carbon be (presumably the carbon not emitted + the carbon captured) per ton of concrete?
Regarding this statement:
> The undersaturated ocean then re-absorbs an amount of atmospheric CO2 equivalent to the amount we removed when reacting with our sorbent
How can it be effectively measured/confirmed? Were you able to produce data to prove it, or is it a theoretical assumption?
Just curious if you're interested in an incubator? We went through the Oxford Foundry[1] last year and would highly recommend it.
[1]https://www.oxfordfoundry.ox.ac.uk/
Just curious: at what point does concrete release C02? In a particular part of the manufacturing process? Or throughout the process?
CaSiO3 + NaCO3 >> Na2SiO3 + CaCO3
Is there a use for your synthetic limestone in the agricultural sector? E.g., for soil treatment?
Here's my random question. Assume your process works perfectly, and truly represents a solution to the CO2 problem.
Some very wealthy person shows up at your door, and offers you and your partners serious money to put the entire idea and process into the public domain, and release all IP rights worldwide, immediately.
How much?
I have to imagine global temperatures are affected by the reflection of cement quite a bit. Blacktop certainly returns heat into the air, as do roofs, but I would guess so does most cement -- why does no one talk about it? Soil aborbs, plants absorb, solid cement reflects, no?
I guess something which might help is if you could change the wavelengths that went out from it to be ones that would be more likely to leave earth rather than being absorbed/reflected by the atmosphere? But I don’t know if that can be done efficiently enough to not produce more heat in the process of changing the wavelengths than just reflecting it would, if it is even possible.
My issue with this post is the claim that you will be making the world's first carbon-neutral houses. I understand the need for strong exciting marketing language, but, surely you mean something like, first modern Western-style carbon-neutral houses.
Where are you going to get the seawater?