Concrete is one of the most consumed man-made materials on Earth. The sheer scale of concrete use across the globe is phenomenal.
Whether it’s in the construction of homes, roads, bridges, building foundations, slabs and so much more, concrete is a main material used.
One of the primary components of concrete is cement. And cement has a significant carbon footprint. It’s thought in the past 100 years, cement production has released 50 billion tonnes of carbon dioxide. To put that in perspective, this website has planted 1,300 trees (at the time of writing) which has offset just 35 tonnes of CO2. Not 35 million of 35 thousand, just straight up 35 tonnes.
With the global floor area of buildings set to double by 2060, we really do need a green alternative to concrete.
This article is all about eco-concrete and the green concrete solutions we hope become the normal.
What is eco-concrete or green concrete?
Green concrete or eco-concrete is a construction material that’s less harmful to the environment compared to traditional concrete.
Green concrete can be achieved by implementing at least one of the following:
- Consumption of recyclable and reusable waste
- Conservation of energy in the production process
- Production of a high-performance material with a sustainable life cycle
Using eco-friendly concrete alternatives results in a reduction in natural resource consumption as well as lower carbon dioxide emissions.
Carbon footprint of standard concrete
To understand green concrete we need to understand standard concrete.
Traditional concrete consists of cement, sand, gravel, water and aggregates. Processing these materials leads to the release of carbon dioxide. However, the majority of this greenhouse gas is emitted during cement production.
Cement is the element that binds all the other materials together. The most common type of cement is known as Portland cement (named after the Isle of Portland in England, rather than the US state).
Starting minerals for the cement mixture include:
- Silica sand (a key component of silicone production)
- Shells and other materials
These components are acquired through mining. The clay and limestone are crushed and subjected to high heat that contributes even further to the carbon load.
In 2021, around 4.4 billion tons of cement were produced. This came with carbon emissions almost reaching 2.6 billion metric tons, which is more than 7% of the world’s total emissions.
This figure is twice the data from 20 years ago and so highlights the pressing need for eco friendly and sustainable concrete solutions.
What are the green concrete solutions?
Most eco friendly solutions focus on incorporating recyclable and reusable materials into the concrete mixture. As cement has the highest impact on the environment, emphasis has been placed on cycling this out of the production process for a greener material.
Other solutions are aggregate replacements as well as reusing wash water. What alternatives are used and to what degree in the concrete formulation are all to do with what the building purpose is.
The Pozzolanic Reaction to replace cement
Replacing cement in concrete production requires a supplementary cementitious materials called pozzolans.
Pozzolans are a wide class of silica or aluminium-silica containing materials. The name comes from pozzolanas, which are naturally occuring pozzolans from volcanic origin.
In the pozzolanic reaction, the material is first grounded. It’s then reacted with calcium hydroxide in the presence of water to form a cementitious compound.
In contrast to the regular cement and water hydration reaction, less heat is generated and the result is a slower strength gain.
In eco-concrete, man-made pozzolans are used to replace cement, or at least a percentage of the cement. Ideally, we’d replace all of the cement in concrete, but this currently isn’t possible on a large scale.
Let’s take a look at each of the greener alternatives.
Fly Ash Concrete
Fly ash is a cement alternative that offers great advantages to the environment.
This is a high-volume material, coming as a waste product from the combustion of coal in power stations.
Fly ash is usually combined with lime and water to create a strong and durable material. When combined with concrete, it results in what’s known as Ashcrete.
Ashcrete can be used as a replacement in cement formulation ranging between 15–25% for standard constructions.
However, it can go as high as 40–60% in structures such as dams. This is due to its capacity to control temperature rise that prevents thermal cracking.
Other benefits of ashcrete are:
- Increased durability
- Reduced shrinkage compared to traditional concrete
- Resistant to alkali-silica reactivity
- Higher acid and fire resistance
- Reduced bleeding
- Requires less water to prompt the hardening reaction
Silica Fume or Micro Silica Concrete
Silica fume, also called micro silica, comes from the condensation of silicon dioxide in silicon and ferrosilicon alloy production.
It’s at least 85% silicon dioxide that has an ultra-fine texture.
This fineness makes it highly reactive to compounds like calcium hydroxide. This pozzolanic reaction produces a gel and fills the pores of the concrete.
Using silica fume has many advantages including:
- Increase in compressive strength
- Improved durability
- Decreased permeability
- Freeze and thaw resistance
- High abrasion and corrosion resistance
- Low bleeding
In application, micro silica can replace 7–12% of the cement. This substitution is usually made for concrete that will be exposed to harsh chemicals.
Examples of structures where silica fume is used are nuclear waste facilities, dams and high-rise buildings.
The reason silica fume is considered a green alternative is because it’s a by-production from silicon production. However, in itself, silicon production isn’t good for the environment either. You can read up on silicone vs plastic debate here.
Blast Furnace Slag Concrete
Blast furnace slag is a solid waste by-product from the iron extraction process performed in a blast furnace. The slag is segregated first and then cooled down.
This results in unreactive crystal-type elements composed of silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide. These will then be grounded finely.
Depending on the application, blast furnace slag’s composition in concrete can vary. It can go as high as 80% cement replacement in marine construction or structures with exposure to chloride and sulfates.
Blast furnace slag improves the durability of concrete. The production process also helps the environment by producing less heat during the hydration of the material.
Other advantages offered by blast furnace slag concrete include:
- High workability
- Low bleeding
- Reduced permeability
- Less prone to thermal cracking
- Initial strength is low but final strength is higher or equal to regular cement.
A slight downside is that blast furnace slag cement has a longer setting time than ordinary cement. So, it can’t be used in immediate repairs or emergencies.
Aggregates replacement for eco concrete solutions
Besides cement replacements, eco-concrete solutions also include aggregate substitutions.
In standard concrete, aggregates are made up of coarse and fine materials such as gravel, sand and crushed stone.
Alternatives to aggregates include reusable and recyclable materials like waste paper, plastic, glass and even concrete debris.
Papercrete is a construction material that’s made from a combination of recycled paper, cement, aggregates and water.
It acts as a heat and sound insulator in structures. It also provides high compressive strength and malleability.
However, it has weak tensile strength and low moisture resistance.
Wouldn’t it be brilliant if we could put all that waste plastic to use?
The non-biodegradability nature of plastic makes it an ideal aggregate replacement. It also greatly reduces the non-recyclable plastic waste that goes to landfills.
In application, this material can take the place of traditional aggregate by as much as 20%. This makes the concrete more lightweight too.
Nevertheless, this results in weaker and less fire-resistant concrete.
Waste glass is another substitute for aggregate but can also serve as a cement alternative. It can replace cement by 10–30% and aggregate by 100%.
This cement material makes concrete more durable without compromising its compressive strength. As an aggregate, it provides better abrasion resistance.
Nonetheless, it has certain drawbacks. Aggregate glass can make concrete more prone to alkali-silica reactions. This can be addressed by using glass in combination with silica fume or fly ash.
You can read more on is glass sustainable here.
Recycling debris from crushing old construction materials lowers the cost of processing. This also saves on virgin resources, which is always better for the environment.
Concrete debris can substitute up to 30% of virgin aggregate without compromising the concrete’s compressive strength.
Wrap up on green eco concrete
The concrete industry, especially traditional cement production, produces high levels of carbon dioxide emissions. This greenhouse gas is a major contributor to global warming.
To address this significant issue, green concrete or eco-concrete solutions are being developed.
As you can see however, there’s still a long way to go to develop a fully eco concrete version. None of the solutions are perfect but they’re a step in the right direction.
Most of the green concrete alternatives make use of cement replacements like fly ash, micro silica and blast furnace slag. These are generally waste by-products from other carbon intensive production processes. As a waste product, they help to lower emissions as opposed to using virgin materials.
Alongside eco cement options, we have greener aggregate substitutes like waste paper, glass, waste plastic and concrete debris. They also save on virgin resources.
Overall, these efforts are helping to lower the environmental impact than standard concrete without sacrificing quality and structural integrity.
With the floor area of buildings transforming more and more landscapes over the world, I sincerely hope the the eco concrete industry develops rapidly.
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I’m the Creator and Editor of Tiny Eco Home Life. I write and publish information about more sustainable, environmentally friendly living in and around the home. Alongside this website, I love spending time in the natural world, living a simple life and spending time with my young family (Murphy the dog!) I round up my thoughts and recent blogs on the Eco Life Newsletter.