Climate-friendly cement and concrete

 From SciTechDaily

[See also Building's hard problem--making cement green]

Holding half its weight in carbon dioxide, the material could replace sand in concrete and other construction materials while trapping greenhouse gas. Credit: Northwestern University

An innovative process converts CO2 into solid, durable materials that trap carbon.

Using seawater, electricity, and carbon dioxide (CO2), scientists at Northwestern University have developed a new carbon-negative building material.

As the climate continues to warm, researchers around the world are searching for ways to capture CO2 from the atmosphere and store it underground. While this method offers several climate benefits, it does not fully capitalize on the potential value of atmospheric CO2.

Northwestern’s new approach tackles this issue by permanently locking away CO2 and converting it into useful materials that can be used to manufacture concrete, cement, plaster, and paint. The process also produces hydrogen gas, a clean fuel with a range of applications, including transportation.

The study will be published on March 19 in the journal Advanced Sustainable Systems.

“We have developed a new approach that allows us to use seawater to create carbon-negative construction materials,” said Northwestern’s Alessandro Rotta Loria, who led the study. “Cement, concrete, paint, and plasters are customarily composed of or derived from calcium- and magnesium-based minerals, which are often sourced from aggregates — what we call sand. Currently, sand is sourced through mining from mountains, riverbeds, coasts, and the ocean floor. In collaboration with Cemex, we have devised an alternative approach to source sand — not by digging into the Earth but by harnessing electricity and CO2 to grow sand-like materials in seawater.”

The new study builds on previous work from Rotta Loria’s lab to store CO2 long term in concrete and to electrify seawater to cement marine soils. Now, he leverages insights from those two projects by injecting CO2 while applying electricity to seawater in the lab.

“Our research group tries to harness electricity to innovate construction and industrial processes,” Rotta Loria said. “We also like to use seawater because it’s a naturally abundant resource. It’s not scarce like fresh water.”

Rotta Loria is the Louis Berger Assistant Professor of Civil and Environmental Engineering at Northwestern’s McCormick School of Engineering. Jeffrey Lopez, an assistant professor of chemical and biological engineering at McCormick, served as a key coauthor on the study. Co-advised by Rotta Loria and Lopez, other Northwestern contributors include Nishu Devi, a postdoctoral fellow and lead author; Xiaohui Gong and Daiki Shoji, Ph.D. students; and Amy Wagner, former graduate student. The study also benefited from the contributions of key representatives from the Global R&D department of Cemex, a global building materials company dedicated to sustainable construction. This work is part of a broader collaboration between Northwestern and Cemex.

To generate the carbon-negative material, the researchers started by inserting electrodes into seawater and applying an electric current. The low electrical current splits water molecules into hydrogen gas and hydroxide ions. While leaving the electric current on, the researchers bubbled CO2 gas through seawater. This process changed the chemical composition of the water, increasing the concentration of bicarbonate ions.

Finally, the hydroxide ions and bicarbonate ions reacted with other dissolved ions, such as calcium and magnesium, that occur naturally in seawater. The reaction produced solid minerals, including calcium carbonate and magnesium hydroxide. Calcium carbonate directly acts as a carbon sink, while magnesium hydroxide sequesters carbon through further interactions with CO2.

Rotta Loria likens the process to the technique coral and mollusks use to form their shells, which harnesses metabolic energy to convert dissolved ions into calcium carbonate. But, instead of metabolic energy, the researchers applied electrical energy to initiate the process and boosted mineralization with the injection of CO2.

Through experimentation, the researchers made two significant discoveries. Not only could they grow these minerals into sand, but they also were able to change the composition of these materials by controlling experimental factors, including the voltage and current of electricity, the flow rate, timing and duration of CO2injection, and the flow rate, timing, and duration of seawater recirculation in the reactor.

Depending on the conditions, the resulting substances are flakier and more porous or denser and harder — but always primarily composed of calcium carbonate and/or magnesium hydroxide. Researchers can grow the materials around an electrode or directly in solution.

“We showed that when we generate these materials, we can fully control their properties, such as the chemical composition, size, shape, and porosity,” Rotta Loria said. “That gives us some flexibility to develop materials suited to different applications.”

Depending on the ratio of minerals, the material can hold over half its weight in CO2. With a composition of half calcium carbonate and half magnesium hydroxide, for example, 1 metric ton of the material has the capacity to store over one-half a metric ton of CO2. Rotta Loria also says the material — if used to replace sand or powder — would not weaken the strength of concrete or cement.

Rotta Loria envisions industry could apply the technique in highly scalable, modular reactors — not directly into the ocean — to avoid disturbing ecosystems and sea life.

“This approach would enable full control of the chemistry of the water sources and water effluent, which would be reinjected into open seawater only after adequate treatment and environmental verifications,” he said.

Earth-packed tyre wall as good as concrete

From New Atlas

Australian researchers have analyzed the structural properties of walls made from end-of-life car tires packed with dirt, giving engineers some figures to work with and making a well-known upcycling technique available to the construction industry.

Car tires are an environmental nightmare. Ubiquitous, consumable, filthy to burn and they take hundreds of years to decompose in landfill, where an awful lot of them end up. Any project that can recycle them, like grinding them up for use as asphalt filler or insulation, or using them as swings and bumpers in kids' playgrounds, is an environmental boon.

For decades, people have been recycling yesterday's rubber donuts into tomorrow's tire walls, stacking them in staggered formations and packing them with dirt. You'd have seen this kind of thing plenty at motor racing facilities – indeed, some tires have been unusually keen to get involved. The Earthship movement takes the idea further, creating retaining walls and entire buildings out of tire walls, sometimes filling them with concrete rubble and broken bricks to allow for drainage, or even rendering over the top for a smoother appearance.

These kinds of projects, though, are pretty niche. And researchers at the University of South Australia saw an opportunity in the 55 million tires the country disposes of every year. This 450,000-odd tonnes of toxic garbage could become free building materials for use in all sorts of mainstream construction projects if it was studied and documented to the point where engineers can use it.

"The lack of supporting data has prevented wider uptake of tire walls by engineers and architects, and we’re hoping this study will change that and expand the range of projects in which these walls are used," said Dr. Martin Freney, co-author of a new study that submits these tire walls to a full structural analysis. "The wall we tested was the first of its kind to be scientifically tested in this fashion, and all the data indicates tire walls can be extremely strong and safe structures.”

"Not only are the tire walls as structurally sound as concrete or wood sleeper retaining walls, they are also extremely resilient," he continued. "Unlike a concrete wall, we found these walls have the ability to ‘bounce back into shape’ following impact, such as from an earthquake. And if a drainage material such as recycled concrete rubble or crushed bricks is used to fill the tires, they also offer excellent drainage, which can be a major consideration in many retaining wall scenarios. Furthermore, the use of recycled fill materials reduces the environmental impact of the wall."

Building's hard problem--making cement green

 

The Pantheon in Rome - almost 2,000 years old and built from concrete
Source: BBC/Getty Images

From The BBC

A time-travelling Victorian stumbling upon a modern building site could largely get right to work, says Chris Thompson, managing director of Citu, which specialises in building low-carbon homes.  That's because many of the materials and tools would be familiar to him.  The Victorian builder would certainly recognise concrete, which has been around for a long time.

The world's largest unreinforced concrete dome remains the one at Rome's Pantheon, which is almost 2,000 years old. The Colosseum is largely concrete too.

Today we use more concrete than any substance, other than water.  That means it accounts for about 8% of the carbon dioxide (CO2) we emit into the atmosphere. That is substantially more than the aviation industry, which makes up about 2.5% of emissions.

But some companies are developing concrete that has a much lower CO2 impact.

Citu is building its headquarters in Leeds from a new low-carbon concrete that it says cuts CO2 emissions by 50% compared to traditional concrete.  It has used 70 cubic metres of it for the building's foundations.  This concrete, released last year by Mexico's Cemex under the label Vertua, is one of a series of recent developments helping pave the way to greener concrete.

Making cement, which makes up 10-15% of concrete, is a carbon-intensive process. Limestone has to be heated to 1,450C, which normally requires energy from fossil fuels and accounts for 40% of concrete's CO2.  This separates calcium oxide (which you want) from carbon dioxide (which is the problem).  This calcium oxide reacts further to form cement. Grind some into powder, add some sand, gravel and water, and it forms interlocking crystals.

Voilà, concrete.

So how can you do all this without releasing so much CO2?

One way is by replacing much of the conventional cement with heated clay and unburnt limestone, says Karen Scrivener, a British academic and head of the construction materials laboratory at Switzerland's Ecole Polytechnique Fédérale de Lausanne.  For a long time, people (think, Romans) knew you could substitute some of the cement with ash from burning coal (or volcanoes). Or more recently, slag from blast furnaces. This even improved concrete's strength and durability.

Prof Scrivener was approached by Prof Fernando Martirena from Cuba, who thought it might be possible to use clay in the production of concrete.  So together they worked out a way to replace a really big chunk of conventional cement, and produce equally strong concrete.

Not only would that mean 40% less CO2, it also works with existing equipment, according to Prof Scrivener.  And that's crucial for a material that has to be competitively priced.

Two companies last year began commercially cooking up this product, called LC3 (for limestone calcined clay cement).x

"I reckon next year about 10 plants are going into operation, and really we can see an exponential take-off after that," she says.

A further 10-20% savings on CO2 emissions can come from finding new ways of making cement more reactive, she adds.

Often people pour in more cement than they actually need, to get early strength.

But if you put in very tiny amounts of other minerals instead, that seems to increase the reactivity too, she says.

Another approach is just coming up with an utterly different way to clench the sand and stone particles together, without cooking limestone into calcium oxide.

This is what Vertua does, says Davide Zampini, head of research for Cemex, the world's second biggest building materials business.  "It's a binder that's rich in aluminosilicates (minerals made from aluminium and silicon), and we have produced chemicals to activate those, and go through a reaction called geopolymerisation," he explains.  This forms a 3D network of molecules, and a solid binder to grip sand and stone in place.

But it's not as cheap as conventional concrete, admits Dr Zampini.  You have to find a customer who is really keen on significantly reducing the CO2 footprint of their buildings, he says, like Citu in Leeds.

A third approach is using a big steel tube, says Daniel Rennie, co-ordinator of a project called LEILAC (Low Emissions Intensity Lime and Cement).  It's 60m (197ft) tall. You can add it to an existing cement plant.  You "chuck materials down from the top" and it gently floats down the tube, which is heated from the outside.  As CO2 comes off the particles, "we just capture it at the top, the calcium oxide continues to the bottom and continues its journey in the cement-making process," he says.

The project is run by Calix, an Australian company that makes environmentally sustainable technology for industry.

The company had been thinking about how to decarbonise another building material.

"And just, the penny dropped, and we could apply this to cement," Mr Rennie says.

A little pilot tower, built in 2019, is now accounting for 5% of production at Heidelberg Cement's Lixhe plant in Belgium.  This is capturing about 25,000 tonnes a year of CO2.

In Germany, they're building one at another Heidelberg plant in Hanover, where 20% of total production will go through the new process, capturing about 100,000 tonnes of CO2 a year.

Once captured, the CO2 is compressed, shipped in a barge to Norway, and stored in an empty oil reservoir under the North Sea.

Normally "90% of the cost is capturing the carbon", so this just leaves the cost of transport and storage.

"I've been in this industry 20 years, and I really see a big change," says Claude Loréa, cement director from the Global Cement and Concrete Association.  "Stuff we dreamed about 20 years ago is now coming through," she adds.  And cement makers have already reduced their carbon emissions "almost by 20% since 1990", she says, largely by making kilns more energy-efficient.

Still, while we can probably get overall CO2 emissions down by 60-80%, we'll still end up with some we'll need to capture and store, says Prof Scrivener.

Also, there's no point looking for intricate solutions that can just be used in "some very sophisticated factories in the US", she says.

Around 90% of future cement production will take place outside the wealthy OECD countries.

A concrete path to cutting concrete's carbon emissions needs alternatives that will work well and cheaply for the coming building booms in India and Africa.

Concrete may have been born in Rome and Britain.   But China made more concrete between 2011 and 2013 than the US did in the whole 20th Century.

Here are some other pieces I've posted which you might find interesting:

Cement produces more CO2 than trucks

Concrete is tipping us into a climate catastrophe

We can deal with cement

CO2-infused cement

Green concrete

Self-healing concrete draws CO2 from the air

 From ZME Science

Examples of self-healing concrete whose cracks have been filled with calcium carbonate made from CO2 from the air and catalyzed by a red blood cell enzyme. Credit: Worcester Polytechnic Institute.

Using an enzyme normally found in red blood cells, researchers have designed a concrete mixture that can automatically seal cracks in the construction material by absorbing CO2 from the air and converting it to calcium carbonate crystals. The resulting concrete is almost four times more durable than traditional concrete, vastly extending the life of structures and slashing the huge upkeep costs required for repairs or replacements.

Concrete is the most ubiquitous construction material in the world. We use it to build everything from skyscrapers to sidewalks due to its durability and low cost. However, concrete is far from perfect, being prone to cracking due to continuous exposure to the elements. Humidity, sunlight, and stress from use slowly chip away at concrete. Over time, harmless microcracks can expand and lead to a loss of structural integrity. In the case of dams and bridges, concrete cracks could threaten the lives of countless people.

“If tiny cracks could automatically be repaired when they first start, they won’t turn into bigger problems that need repair or replacement. It sounds sci-fi, but it’s a real solution to a significant problem in the construction industry,” said Nima Rahbar, associate professor of civil and environmental engineering at Worcester Polytechnic Institute.

Rahbar is the lead author of a new study that took inspiration from nature to find a solution to this problem. The research centered around carbonic anhydrase (CA), an enzyme found in red blood cells that quickly transfers CO2 from the cells to the bloodstream. 

The researchers simply added the enzyme to a conventional concrete powder before it was mixed with water and poured. These experiments showed that the enzyme acts as a catalyst, triggering a chemical reaction between atmospheric CO2 and molecules in the concrete to create calcium carbonate crystals.

Calcium carbonate is a common substance found in rocks such as the minerals calcite (a major component of limestone) and aragonite. It is also the main component of eggshells, snail shells, seashells, and pearls. Its atomic matrix is very similar to that of concrete, so when the calcium carbonate forms inside gaps in the concrete, the structural integrity of the material is preserved.

“We looked to nature to find what triggers the fastest CO2 transfer, and that’s the CA enzyme,” said Rahbar, who has been researching self-healing concrete for five years. “Since enzymes in our bodies react amazingly quickly, they can be used as an efficient mechanism to repair and strengthen concrete structures.”

According to Rahbar, the patented method described in the journal Applied Materials Today heals millimeter-scale cracks within 24 hours. 

The mixture can also be applied to already-set traditional concrete to mend bigger cracks or holes.

The concrete industry is one of the most environmentally damaging in the world, accounting for 9% of total global CO2 emissions in 2018. Nearly 80% of concrete’s carbon emissions come from cement, which accounts for about 8% of the world’s carbon dioxide (CO2) emissions.

If the cement industry were a country, it would be the third-largest emitter in the world — not far behind China and the US. It contributes more CO2 than aviation fuel (2.5%) and is not far behind the global agriculture business (12%). But, overall, the construction industry, which includes not only the manufacturing of cement but also the transportation of heavy materials across the world, was responsible for a staggering 38% of all carbon emissions in 2019, according to the United Nations Environment Programme.

The enzyme-based mixture developed at the Worcester Polytechnic Institute extracts a negligible quantity of CO2. However, the mixture would offset a sizable amount of CO2 currently associated with the concrete industry by extending its life.

Rahbar makes a bold claim, predicting self-healing concrete could extend the life of a structure from 20 years, for example, to 80 years.

 

Fascinating stuff. But I'm wondering whether this technique can be used to extract CO2 from the air in other contexts.   This might be a lot cheaper than trying to remove it from the air via various other industrial processes. 

Cement produces more CO2 than trucks

From Bloomberg:

The most astonishing thing about cement is how much air pollution it produces.

Manufacturing the stone-like building material is responsible for 7% of global carbon dioxide emissions, more than what comes from all the trucks in the world. And with that in mind, it’s surprising that leading cement makers from LafargeHolcim Ltd. in Switzerland to Votorantim Cimentos SA in Brazil are finding customers slow to embrace a greener alternative.

Their story highlights the difficulties of taking greenhouse gases out of buildings, roads and bridges. After wresting deep cuts from the energy industry, policymakers looking to extend the fight against global warming are increasingly focusing on construction materials and practices as a place to make further reductions. The companies are working on solutions, but buyers are reluctant to pay more.

While architects and developers concentrate on the energy used by their buildings, it’s actually the materials supporting the structure that embody the biggest share of its lifetime carbon footprint. Cement’s contribution to emissions is especially immense because of the chemical process required to make it.

About two-thirds of the polluting gases that come from cement production stem from burning limestone. Kilns are heated to more than 1,400 degrees Celsius (2,600 Fahrenheit), about four times hotter than a home oven set to the self-clean cycle. Inside the kiln, carbon trapped in the limestone combines with oxygen and is released as CO2, the most abundant greenhouse gas.

A ton of cement yields at least half a ton of CO2, according to the European Cement Association. That’s more than the average car would produce on a drive from New York to Miami. And a single mixer truck can carry about 13 tons. Hundreds or even thousands of tons go into ordinary office buildings.

What comes out of the kiln is called clinker, the key raw ingredient of cement. It’s the substance that, when mixed with gypsum and water, binds with gravel to harden and form concrete. Many companies are working to cut the amount of clinker in their cement, which requires new and sometimes untested recipes.

Others are looking at substitutes. Those include fly-ash, which comes from the chimneys of plants that burn coal, or slag from steel-making blast furnaces. They trigger a chemical reaction and form what’s known as a geopolymer binder.

Geopolymer cement has performance advantages and a huge sustainability edge over traditional mixes, according to Cameron Coleman, chief executive officer of Wagners Holding Co., which is based in Toowoomba near Brisbane in Australia.


“This alternative eco-friendly binder technology reduces the carbon emissions associated with normal Portland cement by 80% to 90%, and also has a much lower embodied energy,” Coleman said by email. “We have been working with leading companies in South East Asia, New Zealand, India, Europe and the Middle East who are extremely interested in adopting this technology.”

That strategy won’t work for long in Europe and the U.S., where fly-ash is the main clinker substitute and coal plants are closing. There, the focus is on efficiency and using fossil-fuel alternatives for heat. The European Cement Association says its producers already get 44% of their energy from cleaner sources and wants to raise that proportion to 60% by 2050. Instead of using coal, it’s creating heat with used tires, mineral oil and industrial waste.

[Read more here]

I am very confident that the world will replace fossil fuels in electricity generation within 20 or 25 years, and will convert most land transport to EVs over the same time frame.  This will happen because people are getting frightened by global heating and the climate emergency, and because the costs of these new technologies are plunging. Why not do something about global heating when you'll actually cut costs by doing it?

But that will leave agriculture, iron and steel and cement, which by 2040 or 45 could make up 80% of emissions.  There are alternatives in cement production as this article discusses.  There are others: I talked about green concrete here.  Iron and steel could be produced using methane or hydrogen to reduce iron ore to pure iron.  Unlike electricity from renewables or EVs, these will not be cheaper than their high-carbon equivalents.

Clearly, to encourage the update of low-carbon cement and steel, we need a price on carbon.  In Europe, there is one, currently €24 (US$ 27) per tonne of CO₂.  It's only a question of time before Europe starts applying that price to the carbon content of imports from countries which do not have a carbon price.  The surge of Greens in the recent European elections makes that inevitable.  All the conventional parties are starting to feel how the breath of environmentalism is starting to become a breeze and then a gale.  What's more, the current extreme heatwave in Europe, as bad as or worse than last summer's,  will only harden attitudes.  With a carbon price, cement and steel will start to produce low-carbon products.  And the only remaining sector to de-carbonise will be agriculture.  But it will happen there too.  Because it has to.

Concrete is tipping us into a climate catastrophe

HS2: the new high-speed rail line in the UK to connect London to Birmingham and ultimately Scotland

From The Guardian:

Tucked away in volume three of the technical data for Britain’s £53bn high speed rail project is a table that shows 20m tonnes of concrete will have to be poured to build the requisite 105 miles of track, culverts, bridges and tunnels. It is enough, it has been calculated, to pave over the entire city of Manchester.

A more modest 3 million tonnes of concrete will be needed to construct the Hinckley B nuclear power station in Somerset, and the proposed new runway at Heathrow will require one million tonnes.

Cement, the key component of concrete and one of the most widely used manmade materials, is now the cornerstone of global construction. It has shaped the modern environment, but its production has a massive footprint that neither the industry nor governments have been willing to address.

Because of the heat needed to decompose rock and the natural chemical processes involved in making cement, every tonne made releases one tonne of C02, the main greenhouse warming gas.

Including the new Crossrail line through London, the building of Britain’s four largest current construction projects will, if completed, together emit more than 10m tonnes of CO2 – roughly the same amount as a city the size of Birmingham, or what 19 million Malawians emit in a year.

Nearly 6% of all UK greenhouse gas emissions, and up to 8% of the world’s, are now sourced from cement production. If it were a country, the cement industry would be the third largest in the world, its emissions behind only China and the US.

So great is its carbon footprint that unless it is transformed and made to adopt cleaner practices, the industry could, on its own, jeopardise the whole 2015 Paris agreement which aims to hold worldwide temperatures to a 2C increase. To bring it into line, the UN says its annual emissions need to fall about 16% in the next 10 years, and by far more in the future.

While some of the biggest cement companies have reduced the carbon intensity of their products by investing in more fuel-efficient kilns, most improvements gained have been overshadowed by the massive increase in global cement and concrete production. Population increases, the urban explosion in Asia and Africa, the need to build dams, roads and houses, as well as increases in personal wealth have stoked demand.

Annual cement production has quadrupled from nearly one billion to over 4 billion tonnes a year in 30 years. In the next decade it is expected to increase a further 500m tonnes a year. Unless there is a dramatic change, cement emissions are expected to continue to rise beyond 2050.

Industry leaders are now embarrassed, aware that they are in danger of being financially penalised and tarred as climate laggards who refuse to change in the face of the climate emergency. They well know that not only is it quite possible to build most structures safely without cement, but their own research has shown that green, cement-like products using recycled byproducts which are just as strong can be made from other industries, such as steel slag, fly ash from coal-fired facilities or some types of clay. Instead they trust that nascent technologies like carbon capture and storage which could allow emissions to be buried will come on stream, and that more efficient plant will reduce cement emissions by as much as 20-25%.

This is wishful-thinking. The great bulk of the industry is inherently conservative, ignorant of the alternatives to cement and reluctant to adopt change. Conventional, or Portland, cement is trusted to be safe and strong and developers continue to specify it because it is cheap and the alternatives are not well-known. Without major demonstration projects showing what is possible, and the education of architects and planners, progress will be incremental and possibly too late.

[Read more here]

More and more, I'm coming round to thinking that we have to have a carbon tax.  To reduce political opposition, it will need to be repaid to residents of a country as a "carbon dividend", perhaps by way of a monthly cheque or bank credit.  To reduce economic impact, it would have to start out low, but rise each year.  As a possibility, an initial level of $20/tonne, rising  by $5/tonne every year after that.  And it would have to be levied on imports from countries which do not have a similar carbon tax, to encourage them to also levy a tax, and to stop emissions from being "outsourced".  A carbon tax would also accelerate the transition to green electricity in the grid and to replacing ICEVs with EVs.

Green concrete

Source

To make cement, you "cook" limestone (CaCO3) or dolomite (CaMg(CO3)2 ) , which drives off the CO2.  So cement production is a contributor to carbon emissions, and ultimately we will need to find new ways to make cement and concrete.

An Ozzie firm has plans to produce "green concrete", which doesn't use cement, but instead actually uses CO2 instead of emitting it.

An Australian pilot project capturing carbon emissions and storing them in building materials aims to have a full-scale production plant by 2020. 

Mineral Carbonation International, an Australian company developing carbon-utilisation technology will officially launch its technology and research program at the Newcastle Institute for Energy and Resources on Friday. 

The launch will include a demonstration of the hour-long process bonding CO2 - stored in large cylinders at one end of the warehouse - with crushed serpentinite from the nearby Orica Kooragang Island operation, permanently converting it into solid carbonates. 

“This mimics but greatly speeds up the natural weathering by rainfall which produces common types of rocks over millions of years,” MCI said. “These carbonates and silica by-products have the potential to be used in building products such as concrete and plasterboard to create green construction materials.”

[read more here]

I talked about converting CO2 to stone here, which is a similar idea, except that in the case of green concrete, they have a product they can sell.  Given just how much CO2 will need to be removed from the atmosphere if we are to avoid 3 degrees C of warming, this process by itself isn't going to make a huge difference.  But consider: if concrete production can go from contributing CO2 to the atmosphere even just to zero emissions, let alone negative emissions, that will be a huge step forward.  And obviously (as always) a carbon tax would help.

You might also like:

Extracting CO2 from the atmosphere

Carbon capture and storage