Concrete Carbon Footprint: How the 8% Rule is Harming the Planet

Cement is basically glue made out of rocks and is rarely used without something to bind to. Mixed with aggregates (small stones) like sand and gravel, it creates a substance that adheres to other materials under certain conditions.

There are two types of cement that are widely produced, hydraulic cement and non-hydraulic cement.

Non-Hydraulic Cement

Non-hydraulic cement does not harden in wet conditions but in dry ones. Interestingly, this was the first kind of cement created.

Although it can’t be produced at open construction sites, it is still being used for things like brick and stonework. It needs special conditions to set and begins to harden as it comes in contact with CO₂ in the air.

For those reasons, construction crews prefer the alternative, which requires water.³

Hydraulic Cement

Hydraulic cement is the stuff you’re likely familiar with. It’s the grey mud that can be mixed in buckets or wheelbarrows and used to lay bricks and other stonework.

It is mixed with water and hardens as it dries. The only problem is that the production of this product means that the materials in the mix have all of the carbon burned out. Where does it go?

The most common type of hydraulic cement is called Portland Cement. The reason that anyone can use this product lies in the chemical reaction with water that happens. After fresh, clean water has been added to the mix, hydrates are formed and are unaffected by more water so this product will set under almost any conditions.

The carbon that was burned out during the processing leaves behind elements that are ready to form a tight bond.⁴

Portland Cement Processes

From start to finish, cement is relatively harmless to the environment aside from the burning of carbon in the kiln.

But, Portland Cement generates almost a pound-for-pound ratio of CO₂ emissions to concrete during the manufacturing process!

Here’s how it’s made:

Step 1. Ingredients for the cement are acquired.

• The lime shale and slate are mined
• Sometimes companies recycle old buildings or roads

Step 2. Heating a mixture of concrete and clay in a kiln.

• The ingredients vary, but they are natural. The heating process causes unnecessary materials to melt or burn away
• The materials are heated to 1300-1500 degrees Fahrenheit
• Approximately 30% becomes molten and the rest undergoes a slow chemical reaction

Step 3. Clinker is formed from the unmelted materials.

Step 4. Clinker is cooled.

• Brought down to handling temperature with different cooling methods
• The air is recycled back to the kiln

Step 5. Clinker is mixed with gypsum and limestone.

• During the mixing process, this is crushed to a fine powder

Step 6. This is shipped to various companies to make concrete for different types of construction.⁵

For each stage of the cement process energy is required, which also produces a carbon footprint.

Moreover, each part of the process generates specific amounts of emissions…some more than others.

Mining Limestone Slate and Shale

When mining takes place, large amounts of acreage are cleared away and dug down into. All of the trees and natural habitats above the desired mine are destroyed in this process.

Mining of any type is responsible for the destruction of habitats so limestone cannot be collected in this way without harming the environment.

Removing water from underground mines can contaminate the area and change the flow of groundwater.

Underground there are large reservoirs of water and oftentimes when mining for lime they are emptied out for convenience. Lime is very sensitive to moisture and degrades quickly once exposed so sinkholes can happen quickly resulting in large areas that are susceptible to collapse.

Sediment, gas, and oil pollutants seem like low-hanging fruit but are always an issue for the environment when large equipment is being used and when the trucks drive in and out of a mine they kick up a lot of dust and make the air quality decline. Sometimes fine dust flies up and settles miles away from the initial sites.⁶

Cement in the Kiln

Most kilns for cement production have been engineered for the fastest and most energy-efficient way to create clinker.

Clinker is the byproduct of burning Carbon out of different minerals to create a material that is ready to form a tight bond when water is added.

A cement kiln is a very long cylindrical tube, inside the tube is where 1300-1500 ℉ temperatures heat cement.

The kiln is tilted down and slowly rotates to move the product through the center and out the other side. Cement mix is poured into the top and clinker comes through the kiln and out the bottom. During this process, roughly 1960 lbs of CO₂ is made for 1 ton of clinker.

There are many variables to carbon waste when making cement this way. Clinker production generates most of the harmful 8% of the world’s carbon emissions.⁷

Cooling Down and Crushing

In these steps, air is blown over the very hot clinker with fans to bring the temperature down so it can be handled sooner. This air can be recycled back into the kiln so it would take less energy to keep the kiln hot.

Once the clinker is cool enough, gypsum and limestone are added and the cement is almost ready for bags, and then transport.

The materials are crushed into a fine powder that is evenly mixed. To finish up, the product is purchased and water is thoroughly mixed with the fine powder and then poured like cake batter.⁴

CO2 and Cement

There are companies and individuals working hard to create different types of concrete and cement because this material is extremely useful.

Fortunately, concrete and cement love CO₂, and using this relationship could turn the concrete carbon footprint from 8% to, potentially, a negative carbon footprint.

Recycling Cement and Concrete (Reducing Concrete Carbon Footprint)

There are many places to find recycled materials from old concrete and cement structures.

That’s because after a certain amount of time… heavy usage, natural disasters, and sometimes weather-related wear can weaken cement or concrete to where it needs to be replaced.

The recycled concrete is broken up and large magnets remove rebar or anything else that could have been used to make a structure stronger. The steel reinforcements are recycled and the leftover concrete can be used in a couple of different ways.

Most of the time, this broken-down material is used as a sub-base for other building projects. This rubble is called an aggregate. It can also be ground down and mixed with new materials, then put back through the kiln for new concrete.⁸

Ashcrete is made of recycled materials; waste from coal burning… poses an issue if coal is being burned to create fly ash for the ashcrete, it’s no longer recycled. The burning of coal creates a lot of CO₂ so it would be counterproductive to rely on this material as an alternative to Portland cement, but currently, it is a very strong and reliable substitute.¹⁵

Blast furnace slag cement is another recycled cement that uses waste from a kiln and is ground as aggregate for concrete and cement.

What is blast furnace slag? A byproduct from the kiln, the material that melts and does not turn into clinker.

Is Limestone Bad for the Environment?

Limestone represents about 10% of the land surface. Limestone at its core is about 44% carbon dioxide and 56% calcium oxide.

Wait, what? So deep down… limestone, a key ingredient in cement and concrete holds a lot of CO₂.

About 30% to 60% of the emissions created from making cement are absorbed back into the concrete because when it goes through the kiln the elements separate.

Unfortunately, the re-absorption of carbon takes a while.

Amazing Concrete

Concrete is strong, especially the material used today, which is usually reinforced with steel “rebar” rods of various sizes. It does not falter very often and because it is easy to manipulate the possibilities are endless.

Long after a home has fallen to the elements, concrete foundations and support are left as a reminder of what once stood.

And although concrete is a natural product, finding ways to reduce the emissions generated by cement production is becoming crucial.

Keeping People Safe

Natural disasters not only ruin lives, but they can also take them. One family in Mississippi hid in a safe room that doubled as a closet when a tornado approached their home.

The entire house was gone after the twister passed save one, lone, concrete closet. Nothing in the room even moved! The clothes were still hanging and the family of four was saved.⁹

In the southeastern United States, storm-safe shelters are becoming more important. The number of large and powerful storms is increasing due to climate change and families living in these areas are more vulnerable to damage, or even death if they don’t have a safe place to go.

Developers have shared how to design Community Safe Rooms and storm safe shelters in hopes that some people will take initiative for their communities.²⁰

Some of these projects include government funding to assist because tornados come and go so quickly and leave a lot of destruction behind.

Another project designed pod-like shelters called STATIM;¹⁹ Storm, Tornado, and Tsunami Interconnected Module.

The beauty of this concrete shelter lies in its design, it’s easy and as affordable as possible to make. They are buoyant and can seat up to 50 people and the STATIM have precast sections that allow communities to decide what works best for their environment. This means that they are customizable!

If a person or community is being threatened by an attack from another, concrete would most likely be the first available shelter. Currently, most concrete structures with reinforced walls have 2D fibers placed before it hardens.

A simple but effective engineering technique developed by Dr. Alan Richardson could reduce the number of people injured by pieces of concrete flying away from a structure being blown apart. The reinforcement has a 3D shape, and this design really makes a big difference.

Roads, Streets, and Sidewalks

Some of the most populated places have a lot of cement around because it’s so durable and withstands a lot of traffic. Urban areas benefit the most from concrete and it would be harmful to the planet to remove this sturdy building material from large cities.

A big debate between asphalt and concrete is common, because asphalt absorbs heat, and retains it, raising temperatures in the area.

This is an advantage, however, when roads are impacted by icy and snowy conditions frequently during hte year.

Pros to Concrete Roads

• Concrete roads have an average life of 25 years while asphalt has roughly 13.6 years before it needs major repairs.
• Cars have better gas mileage while driving on concrete.
• Concrete is a lighter color so it reflects the sun’s heat instead of absorbing it.¹¹

A lot of concrete in an area creates dirty urban runoff, it can cause flooding and dangerous conditions for people traveling.

There is a sustainable construction material called pervious concrete that allows water to pass through it and into the soil. This material is not something that could cover our interstates but it does very well in areas for parking, sidewalks, or cul de sacs.

Of course, all concrete is porous, allowing water to pass through.

Not just anyone can use it though, it’s a precise science and needs extra testing to make sure the desired strength and porosity are correct.

The measurements of ingredients and the process to strengthen it but keep it permeable enough to allow water to pass through means it is more costly to manufacture.¹²

Concrete in Science: Biocrete, Solida, Hempcrete, and More

Cement and concrete actually absorb CO₂! Depending on how the product is made, it could potentially absorb 30 to 60% of the carbon it creates.

The reabsorption takes a while but it means that each piece of cement or concrete has the potential to soak up harmful carbon emissions.

The ingredients available to make concrete and cement are everywhere. Many different applications for these elements create a broad spectrum of materials. It is reminiscent of baking and how different amounts of specific ingredients can completely change the product.

With a harmful 8% of the world’s carbon emissions coming from concrete, it would make sense to try new theories.

Check out some of the alternatives being developed:

Biocrete

Biocrete is a working theory of ‘living concrete’ that lies dormant until a crack forms and once it is exposed to water it springs into action.

And although biocrete is still in the experimental phase, it would be interesting to see this type of concrete at work.¹³

Solida

Solida, green cement looks very familiar to Portland cement in that it can be run through pre-existing kilns. To cure the product, they actually use CO₂ from the kiln and recycle all materials to reduce their waste as much as possible.

This green cement reduces carbon emissions by 70% and could possibly be the future of cement because it is so cost-effective and works with the systems that are already in place.¹⁴

Carbon Positive Concrete

Sulzer and Blue Planet have been working together to create carbon-neutral or even, carbon-positive concrete.

This project is very attention-grabbing for industries looking to improve the way that they manufacture. Carbon Capture Utilization and Storage unit, or CCUS. These units cache carbon out of the air and use it as an ingredient to create carbonate rocks; these carbonate rocks are used in place of limestone.

The truth is, the elements that make up the key ingredients for concrete are incredibly easy to manipulate. The only problem with carbon sequestration is that it can be expensive and smaller companies most likely, wouldn’t be able to afford this option.¹⁴

Hempcrete

Hempcrete is a biocomposite substance, which can sequester emissions. However, it cannot be used for load-bearing walls, so its only practical use is in places where loads are much less than traditional concrete.

Hempcrete does have other benefits, including acting as great insulation and being resistant to mold, fire, rot, and pests.

How Are Countries Consuming Concrete?

China, India, Europe, and America are the largest consumers of cement and concrete.

Why do some countries have a larger concrete carbon footprint than others?

When a country undergoes urbanization, a large amount of concrete is used to support the growth of cities. So, when people are in need of shelter and move to a more populated area the choice material is concrete to make these structures affordable and strong.

This is one reason that China has such a large usage of concrete between 2010 and 2015.

India has been going through changes as well with urbanization so the demand for concrete skyrocketed. India is currently the second largest manufacturer of concrete only second to China.

All of the urbanization that occurred in the 90s in America, when concrete was at an all-time high is now happening there.

Europe has the most advanced systems for making concrete, but China and India have newer units so they are more energy efficient. Although the US is fourth on this list, they still fall behind in energy efficiency due to older facilities making clinker.¹⁶

Concrete and Its Hall Pass

Humanity’s approach to a crisis within a country can vary a lot, but the one thing that always presents a major issue is sheltering the people who have been affected.

It is devastating enough to see people homeless with their family due to any type of crisis and the quickest, cheapest way to house people and rebuild is with concrete.

Concrete remains a very effective medium for structures and safety (like sea walls).

New discoveries are making concrete more sustainable, but there’s still a long way to go.

One issue that seems to glare at anyone researching this topic is how cost-effective solutions are, for the most part, people have to be funded to research and explore new processes.

Once a new idea has proven to be effective and tested in multiple scenarios it’s up to manufacturers to ‘pony up’ and pay for whatever these changes require. This money continues down the line to consumers and if the product is no longer affordable, a step back is taken.¹⁷

What Is the Future of Cement?

With green innovations and carbon capture technology making strides, the future of concrete as it stands may see some upcoming changes. But, this building material will continue to be one of the most versatile and useful mediums.

Moreover, with recent discoveries about the Roman method of mixing concrete that actually repairs itself over time, the future of cement may be about to undergo a massive shift…back to ancient practices.

Meanwhile, construction companies and individuals can offset the concrete carbon ecological footprint by working with carbon offsetting companies that offer tree planting carbon offsets. These credits represent restored forests and carbon sinks, allowing the planet to recover and remove the emissions generated by concrete.

You can use the concrete carbon footprint calculator above to see exactly how many pounds of emissions your concrete pour will produce.

Can the concrete carbon footprint of 8% really undo all the benefits derived from concrete? At this point, no. But by finding new methods and formulas, that emissions amount can be lowered.

For now, it’s a problem that needs to be addressed, so that the world can lower the concrete carbon footprint by gigatons each year.