Pros and Cons of Polishing Low Carbon Concrete

What is Low Carbon Concrete?

Low-carbon concrete is concrete produced with a lower carbon footprint than traditional Portland cement concrete. Other than a reduced carbon footprint, the goal is to produce a concrete mix design, using cement substitutes, that “will” behave identically to its straight cement counterpart.

To create low carbon concrete, producers can implement a series of low-impact changes to their production processes and mix designs. For example: by switching fuel sources and replacing some of the cement content with mineral compounds like calcined clays, fly ash, or blast-furnace slag.

Type 1L cement, also known as Portland-limestone cement (PLC), is a blended cement, not straight cement, with a higher limestone content (cement replacement), which results in a product that is produced to work the same, measure the same, and perform the same, but with a reduction in its carbon dioxide (CO2) footprint of 10% on average. But when polishing a Type 1L cement mix, some intricacies will impact the polishing process.

Why Traditional Concrete Has a Large Carbon Footprint?

Some form of concrete is on every building project in the world. Not only in buildings but in roads, bridges, pipes, sidewalks, tunnels, and dams, to name just a few.

Cement is the glue that binds together the other materials in concrete, such as sand and crushed rocks. Portland cement is the most common type of cement used around the world.

The main raw material in cement is limestone which is burned at extremely high heat to produce a lump material called clinker. The clinker is then ground and mixed with other materials to produce Portland cement.

The biggest source of emissions is the process of producing the clinker. Clinker is produced by burning limestone in kilns at 2,300° to 3,000° F (1,260° to 1,650° C). The process typically uses coal or natural gas as fuel, consuming a large amount of energy and releasing carbon dioxide (CO2) into the atmosphere from combustion. Typically, 30-40% of direct CO2 emissions come from the combustion of these fuels; the remaining 60-70% comes from the chemical reactions involved in converting limestone to calcium oxide which gives cement its strength.

One ton of Portland cement produces one ton of CO2 emissions during production. Cement accounts for 7% of all global carbon emissions. Concrete is responsible for 50-85% of the carbon produced in any building project. If it were a country, the concrete industry would be the third-highest emitter of CO2 after China and the United States. The demand for concrete worldwide is second to only water.

Advantages of Low Carbon Concrete

Cement replacements will lower the carbon footprint. When cement is produced, carbon is produced. The amount of carbon produced by the cement replacement is much less since it can be a byproduct of another manufacturing process unrelated to the cement industry and is not as refined as cement.

Cement replacements can be lower cost than straight cement. Using them will reduce the cost per yard of concrete. The amount of reduction will depend on the cement replacement used and what the performance criteria need to be for the project.

There have been times when straight Portland cement has been in short supply. Having a replacement available for cement, providing the properties the project requires, can be advantageous. Cement shortages can not only drive up the cost of each yard produced but also cause project delays.

How to Reduce the Carbon Footprint

One way to reduce the carbon footprint is by reducing the dosage of cement in concrete. As we know, much of the carbon is produced during the production of cement. This requires using alternative binders instead of cement with pozzolanic and cementitious properties. Available alternative binders of waste materials include the use of calcined clay, ground granulated base slag (GGBS), pulverized fly ash (PFA), finely crushed limestone, post-consumer glass, and other natural pozzolans.

Challenges in Polishing Low Carbon Concrete

When polishing concrete, you grind the surface like sanding wood. You start with aggressive grit sandpaper and gradually move to finer and finer grits. With polished concrete, you use manufactured diamonds adhered to a metal or composite tool base to perform the grinding process.

Having a hard uniform surface enhances the polishing process. A straight cement mix design is known to be just that. It cures at a consistent rate, the cement paste is durable and predictable. When the cement is replaced by other pozzolanic materials, set times and strength gains change.

Fly ash is quite often used as a cement replacement. Typically, when polishing the cured concrete slab, the amount of fly ash in the mix should be around 10% of the total cement content. The higher the percentage of fly ash, the longer the set times. It takes the fly ash longer to react and gain strength than straight cement.

If you polish a slab with elevated levels of fly ash too early, before the designed strength has been obtained, the concrete surface can display a smearing appearance. This is due to partially reacted fly ash. This can also happen well after the designed strength has developed.

Fly ash-type materials develop strength differently than normal straight cement. During the cement hydration process, calcium hydroxide is produced as a byproduct. This byproduct is the chemical that cement substitutes react with to develop their strength. Calcium hydroxide is the same chemical that the liquid silicate hardener that is used during the polished concrete process reacts with. Cement substitutes lessen the reaction the silicate provides and might render it almost useless.

Some, if not all, cement replacements will have an impact on the color of the cured concrete. Slag, for example, will lighten the concrete. Fly ash might cast a tan hue into the mix.

Overcoming the Challenges

The easiest way to minimize any negative effects of using cement substitutes is to lower the percentage of usage to a reasonable amount. Typically, keeping the replacement percentage below 15%, preferably 10%, will pose a minimal effect on the polished concrete process. Cement replacements can lower the cost of each yard of concrete and lower the carbon footprint. Reducing the amount used might be a difficult sell to the owner or designer if having a low carbon footprint is a key factor in the project.

No matter the amount of cement replacement or the type of replacement used, always require a paid mockup to be installed before the start of the project. Once completed, the owner can approve the finish, color, and aggregate exposure. The final expectations need to be set. When installing a mockup, the same placement methods and mix design as new the floor needs to be utilized.

When setting up for the mockup, the amount of concrete that can be mixed efficiently in a ready-mix truck is 3 cubic yards. Usually, a mockup ranges from 100 sq. ft. to 400 sq. ft. One yard of concrete at 4” thick will cover 80 sq. ft. Three yards will cover 240 sq. ft. A 200 sq. ft. mockup is a very reasonable size to be installed. With this size, the concrete finishing process can also be like the final project. A power trowel can be used to finish the plastic concrete replicating how the floor will be placed. Remember, the cure time will increase, and polishing a 30-day-old mockup slab might not produce the desired results. Allowing the mockup to cure for a minimum of 60 days or more will produce a more realistic sample. That means the mockup needs to be installed well before the polishing begins.

Conclusion

Reducing our carbon footprint is a key factor in providing a healthy environment for now and into the future. With every action, there is a reaction. By developing concrete mix designs to reduce the emitted carbon into the atmosphere, the final product might appear to be the same as a straight cement mix but there are a few subtle differences. These differences will have an impact on a surface that is polished. If you have an understanding of how the surface will react with the diamond tooling and color, these differences can be worked around.

Installing a mockup to determine the diamond tooling that will work the best to produce the finish required is also beneficial. This will minimize the time spent setting up on the job site or not having what you need to polish the floor. The owner will also be able to see a representation of the result they will end up with. This is a particularly effective way to set realistic expectations.

When setting up the mockup, give Niagara Machine a call. They have field representation throughout the U.S. that has the knowledge to help guide you through the tooling, coloring, and chemical selection processes.

Low-carbon concrete can be polished. Having the knowledge and understanding of how the substrate will react will allow the process to develop a positive result.