Green Concrete
Green concrete,
eco-friendly concrete, or sustainable concrete refers to concrete that utilizes
industrial waste materials or recycled materials with a lower environmental footprint
as a partial replacement for concrete components such as cement and aggregate.
Green concrete could also refer to concrete that is more durable or lasts
longer than standard concrete so that it does not need to be replaced as
quickly, giving it lower life cycle emissions.
Concrete is one
of the most commonly used materials. Its production is responsible for about 8%
of global carbon emissions so there is room for improvement. Standard
concrete is composed of Portland cement, water, aggregate, and other
ingredients that are combined to set and cure over time. Traditional concrete
is composed of cement, sand, gravel, and water. “Portland cement contains a
mixture of crushed limestone and clay as well as minerals including calcium,
silicon, aluminum, and iron. These minerals must be mined, processed, and
transported before they’re incorporated, adding to the carbon dioxide load of
cement. Part of the carbon dioxide released from cement also comes as limestone
and clay are crushed and heated to form the final product, both from the energy
required to heat the materials and from the crushed materials themselves.”
Concrete is a composite product, typically with several components. Some can be
replaced with waste products, but studies need to be ongoing to determine the long-term
viability of some recipes. While a significant amount of CO2 emissions of cement
production come from the high temperatures that must be generated by the combustion
of dense hydrocarbons like oil and coal, most of the CO2 emissions (up to 66%)
come from the chemical process of calcination. These are known as process
emissions.
The recycled
materials added to make green concrete that replace cement or aggregate are
known as supplemental cementitious materials (SCMs). Typical SCMs are fly
ash, silica fume, post-consumer glass, and recycled concrete. These materials
would otherwise end up in landfills or sludge ponds in the case of fly ash.
They also give different properties to the concrete such as improved durability
or improved thermal insulation. Since cement is the component with the highest
emissions intensity, using materials that reduce the amount of cement needed has
been the focus. Other SCMs include rice husk ash, wood ash, sawdust, foundry
sand, sludge from the clarifiers of pulp and paper mills, de-inking solids from
paper recycling companies, and hemp.
Concrete with
fly ash SCM is known as ashcrete and has greater durability and is less prone
to shrinkage. Fly ash is a waste product from coal-fired power plants and has
been used in concrete since the 1930s. Fly ash typically replaces 15-25% of
cement but has been used to replace 40-60% of cement for its mechanical
properties and durability, and in large construction projects like dams for its
thermal properties. Granulated blast furnace slag waste from steel mills can replace
up to 70-80% of cement and requires less heat and water to produce. Rice husk
ash can replace 40% of cement and the resulting concrete is less prone to
cracking. Waste glass can replace up to 30% of cement and can also replace aggregate.
The resulting concrete has a desirable lower alkali-silica reactivity. Silica
fume from silicone and ferrosilicon alloy manufacturing is a micro silica
ultrafine powder that can replace 7-12% of cement. The resulting concrete has
increased compressive strength.
Another avenue
that has been explored for several years now is replacing Portland cement with magnesium
oxychloride cement (MOC). However, a major obstacle to its adoption has been
its lack of water resistance. MOC utilizes two wastes from magnesium mining: magnesium
oxide (MgO) powder and a concentrated solution of magnesium chloride (MgCl₂). MgO
can absorb CO2 from the atmosphere. Work has been underway since 2017 to
improve the water resistance of MOC. Addition of silica fume and fly ash could
be keys: “Both the fly ash and silica fume have a similar effect of filling
the pore structure in MOC, making the cement denser. The reactions with the MOC
matrix form a gel-like phase, which contributes to water repellence. The
extremely fine particles, large surface area and high reactive silica (SiO₂)
content of silica fume make it an effective binding substance known as a
pozzolan. This helps give the concrete high strength and durability.” However,
other additives are needed for resistance to warmer water and steel rebar corrosion
potential make MOC thus far unsuitable for structural concrete. Research is
ongoing.
In addition to
cement-replacing SCMs there are also aggregate-replacing SCMs. These include
post-consumer glass, recycled concrete aggregate, waste plastic, foundry sand,
and more recently the pyrolyzed biochar of used coffee grounds is being tested.
Glass can replace up to 100% of aggregate but the resulting concrete is weaker,
so other additives called plasticizers are added. Glass is also used as an aggregate
in asphalt in some places. Recycled aggregate can replace virgin aggregate,
requiring less transport and landfilling of the waste products. Recycled
aggregate is comparable to virgin aggregate but also contains hydrated cement
paste which reduces specific gravity, increases porosity, and increases absorption.
Fine recycled aggregate should be used sparingly. Waste plastic can replace up
to 20% of aggregate. The resulting concrete is lighter and weaker and is thus
used for non-structural applications such as sidewalks, highway median strips,
and highway pavement sub-bases. Foundry sand can replace virgin sand and similar
to recycled aggregate decreases transport and landfilling. Yet another recent aggregate
replacement is used up fiberglass wind turbines. This is being done by Iowa-based
company REGEN Fiber, a subsidiary of trucking company Travero. The company breaks
down the wind turbines mechanically, without heat or chemicals, into chopped glass
fibers and fiberglass reinforced plastic (FRP). The glass fibers can be used in
molded composites and asphalt and the FRP can be used in concrete and precast
products.
Calcined Clay Cement (LC3): SCM with Up to 40% Less
CO2 Emissions than Portland Cement
According to Wikipedia:
“Limestone Calcined Clay Cement (LC3) is a low-carbon cement developed
by the École Polytechnique Fédérale de Lausanne (EPFL), Indian Institute of
Technology (IIT) Delhi, IIT-Bombay, Technology and Action for Rural Development
(TARA), IIT-Madras, and the Central University of Las Villas (Cuba)” Manufacturing
emissions can be reduced by 30% compared to the ordinary Portland cement it is
intended to replace.
Basically,
calcined clay is a supplementary cementitious material (SCM). Clays are
activated and hydrated in the manufacturing process for LC3. These chemical
reactions are known as synergistic hydration.
LC3 is
composed of clinker, calcined clay, limestone, and gypsum. Clinker is the term
for small solid nodules or lumps made by sintering, or the fusing together of
limestone and aluminosilicates at high temperatures in cement kilns. Clays are
the source of the aluminosilicates that are a component of the clinker. The resulting
compounds include calcium silicates, tricalcium aluminate, and calcium
aluminoferrite. Portland cement clinker is ground to a fine powder and used as
the binder in many cement products. Calcined clay is clay that is thermally treated
under low oxygen conditions and high temperatures, not high enough to melt it,
but high enough to remove the carbon from the limestone and yield calcium oxide
(quick lime). The calcination reaction is as follows: CaCO3(s) → CaO(s) +
CO2(g). A small amount of gypsum is added to prevent the flash setting of the
most reactive component, tricalcium aluminate. The resulting cement, LC3, is
less porous, and some of the clinker is replaced by limestone and calcined clay.
LC3 has resistance
to chloride ingress and sulfate attack, which makes it useful in coastal areas.
It has a lower alkali content, which minimizes risks of alkali-silica reactions
and enhances longevity of concrete structures. LC3 can also solve other issues.
In places like Africa there is a shortage of limestone reserves to make
Portland cement clinker, so it has to be imported. Plans are underway for a plant
in Ghana that expects to substitute calcined clay for clinker up to 60-70%. A
new plant in France that includes calcined clay is making a product called
ECOPlanet green cement with 50% less carbon emissions than standard cement. In
some places SCMs like fly ash from coal plants is in short supply or will be in
the future so LC3 is one solution as an SCM.
India has been
one of the pioneers of LC3 and “the Bureau of Indian Standards (BIS) just recently
released an exclusive Indian Standard (IS) code (IS 18189: 2023) for LC3 in
India. The code provides comprehensive guidelines and specifications for the
production, testing, and usage of LC3 in concrete.” This new standard is
expected to accelerate LC3 adoption throughout the country, one of the largest
current makers of cement and concrete structures in the world.
Low Carbon Concrete and Cement
Most types of sustainable concrete have lower
carbon emissions. Most concrete emissions derive from the cement component, so
cement replacement is generally the best means of reducing emissions. Other ways
to reduce emissions include using less emissions-intense fossil fuels such as
natural gas and renewable electricity for a portion of the heat and increases
in efficiency. Calcined clay cement is one of the most promising forms of low
carbon cement. While a significant amount of cement’s emissions come from the
high temperatures required for its manufacture, it is estimated that as much as
two thirds of its emissions come from the CO2 released chemically during the
calcination reaction. Thus, the calcination component of cement manufacture is
responsible for about 5% of global CO2 emissions. To reiterate, the heat causes
limestone, or calcium carbonate (CaCO3) to chemically decompose into calcium
oxide (CaO) and carbon dioxide (CO2). Thus, solutions are aimed at replacing
the amount of clinker with calcined clay and limestone. Another approach is
treating powdered SCMs with chemicals called alkalis which makes the powders
more reactive and allows them to replace more cement. There is some debate
about how much emissions can be reduced by these alkali-activated cements. Estimates
vary but it could be comparable to calcined clay cement. The combination of
using highly reactive cement with superplasticizers, optimized particle size distribution,
granulated blast furnace slag, and lower water content allows for use of less
Portland cement.
An interesting
fact about concrete is that it absorbs CO2 from the air. However, it does this quite
slowly, and is not as good a carbon sink as other sinks like trees and plants.
The CO2 uptake by concrete does accumulate over decades so is significant over
time The reverse reaction of calcination, known as carbonation (or simply CaO +
CO2 > CaCO3), is how concrete uptakes CO2. A 2016 paper in Nature Geoscience
suggested that the world's concrete has been absorbing about 43 percent of the
original calcination emissions from an 83-year period from 1930 to 2013. As we
will see below, more CO2 can be absorbed in the mixing and curing process.
A 2023 article in Nature Communications concluded that a “combination of manufacturing and engineering decisions have the potential to reduce over 76% of the GHG emissions from cement and concrete production, equivalent to 3.6 Gt CO2-eq lower emissions in 2100.” The paper examined emissions reduction potential in several phases of concrete design and manufacture. The authors modeled different scenarios, including an increase in usage of SCMs from the 2015 average of 20.3% to 30% and 50%.
Fig. 2: Effects of binders, strength, and manufacturing improvements on greenhouse gas (GHG) emissions per m3 of concrete.
The authors also modeled emissions for different 28-day compressive strengths, with higher-strength concrete having significantly more emissions. Replacing some of the higher temperature thermal sources with natural gas and/or wind power was also modeled with both natural gas and electricity produced wind power. Both have similar emissions since gas provides higher direct heat than electricity. See graph above.
A 2023 report by the Rocky Mountain Institute - The 3Cs of Innovation in Low-Carbon Concrete: Clinker, Cement, and Concrete - considered all the ways to reduce emissions in concrete design and manufacture, including carbon capture at cement kilns. The following takeaways were presented:
1)
Innovations are actively happening in
each production phase of the concrete and cement value chain.
2)
There is no one-size-fits-all solution.
It is important to evaluate the technology applicability at regional and plant
levels.
3)
Using innovative low-carbon materials in
nonstructural components is a relatively low-risk way to start market
penetration.
4)
In this decisive decade, the cheapest and
fastest options should be deployed first while we prepare the industry for
carbon capture technology commercialization in the 2030s.
Since it is the
production of clinker in cement manufacture that is responsible for about 90%
of the carbon emissions of concrete, that is obviously the main focus of
emissions reduction efforts. Since clinker basically gives concrete its strength,
this is no easy task. One method is project designs that use less concrete while
maintaining structural integrity. Another method is to use less cement in
concrete while maintaining structural integrity. Additives can help here to
reduce what has become known as cement wastage. Replacing Portland cement
clinker with SCMs, including calcined clay cement is one of the best ways to
use less cement. Another Rocky Mountain Institute article – With Concrete, Less
is More - sums it up: “Given the global availability of limestone and
calcined kaolinite clay, LC3 cement — consisting of 50 percent clinker, 30
percent calcined clay, 15 percent limestone, and 5 percent gypsum — is seen as
one promising approach to the future of low-carbon concrete.”
Increasing
energy efficiency is another way to lower emissions in concrete and cement production.
This can happen in several phases and processes. Upgrading to equipment with higher
energy efficiency is an important method. The table below shows a few processes
where efficiency can be improved.
CCU Cement: Mineral Carbonation Via Mixing and
Curing Cement with Captured CO2
While CO2
curing of concrete offers emissions advantages, much is offset by the loss of
durability or compressive strength. In addition, there are other issues,
including corrosion of steel rebar which renders it unsuitable for structural
concrete, and it being non-applicable in sulfate-rich soils, cold regions, and acidic
environments. There has also been some debate as to whether this CCU cement
that utilizes mineral carbonation with captured CO2 actually produces a net
climate benefit. A paper published in Nature Communications in April 2021 by
researchers at the University of Michigan concluded that there are three main
challenges: 1) finding a CCU cement curing protocol that does not sacrifice
compressive strength; 2) decrease emissions from the curing process by
utilizing things like waste heat and natural drying; and 3) better understanding
of CO2 curing on durability. In this last regard they noted: “Future work
can prioritize standardizing the CO2 curing protocol (e.g., the steam curing
time, pre-hydration time, post-hydration time), and study the resulting
durability impact on different design mixes (e.g., use of different SCMs), with
the overall goal of identifying optimal curing conditions and design mixes to
maximize durability.”
References:
Unleashing
the Potential of Limestone Calcined Clay Cement (LC3). Swathi Shantha Raju,
Radhika Lalit. Rocky Mountain Institute. August 10, 2023. Unleashing the Potential of Limestone
Calcined Clay Cement (LC3) - RMI
With
Concrete, Less is More: Demand changes can drive the future of zero-carbon
concrete. Ben Skinner and Radhika Lalit. Rocky Mountain Institute. January
2023. With Concrete, Less
Is More - RMI
The
3Cs of Innovation in Low-carbon Concrete: Clinker, Cement, and Concrete. Zhinan
Chen and Radhika Lalit. Rocky Mountain Institute, 2023. The 3Cs of
Innovation in Low-Carbon Concrete - RMI
Wikipedia:
Limestone Calcined Clay Cement. Clinker. Calcined Clay. Limestone
Calcined Clay Cement - Wikipedia Cement clinker. Cement clinker - Wikipedia.
Calcination.
Calcination - Wikipedia
Calcined
Clay as Supplementary Cementitious Material. Roman Jaskulski, Daria
Jóźwiak-Niedźwiedzka, Yaroslav Yakymechko. Materials (Basel). 2020 Nov; 13(21):
4734. Published online 2020 Oct 23. Calcined Clay as
Supplementary Cementitious Material - PMC (nih.gov)
Scientists
Discover Amazing Practical Use For Leftover Coffee Grounds. Tessa Koumoundouros.
Science Alert. September 4, 2023. Scientists
Discover Amazing Practical Use For Leftover Coffee Grounds : ScienceAlert
What
is Green Concrete? Brittany Henneberry. What
is Green Concrete? (thomasnet.com)
Green
cement a step closer to being a game-changer for construction emissions. Chris
Calimlim, The Conversation. November 18, 2019. Green
cement a step closer to being a game-changer for construction emissions
(theconversation.com)
Green
Cement: Definition, Types, Advantages, and Applications. The Constructor. Green
Cement: Definition, Types, Advantages, and Applications - The Constructor
Low
Carbon Concrete and Its Advantages. The Constructor. Low
Carbon Concrete and Its Advantages - The Constructor
What
is Green Concrete: Its Applications and Advantages in Construction. The Constructor.
What is
Green Concrete? Its Applications and Advantages in Construction
(theconstructor.org)
Sustainable
Concrete Construction Methods and Practices. The Constructor. Sustainable
Concrete Construction - Methods and Practices (theconstructor.org)
Recycled
Aggregates. Recycled
Aggregates (cement.org)
Cement
Producers Are Developing a Plan to Reduce CO2 Emissions. Chelsea Harvey. Scientific
American. July 9, 2018. Cement
Producers Are Developing a Plan to Reduce CO2 Emissions - Scientific American
Eco-Friendly
Alternatives To Traditional Concrete. July 18, 2019. Eco-Friendly
Alternatives To Traditional Concrete | Specify Concrete
Substantial
global carbon uptake by cement carbonation. Fengming Xi, Steven J. Davis,
Philippe Ciais, Douglas Crawford-Brown, Dabo Guan, Claus Pade, Tiemao Shi, Mark
Syddall, Jie Lv, Lanzhu Ji, Longfei Bing, Jiaoyue Wang, Wei Wei, Keun-Hyeok
Yang, Björn Lagerblad, Isabel Galan, Carmen Andrade, Ying Zhang & Zhu Liu. Nature
Geoscience volume 9, pages880–883 (2016), November 21, 2016. Substantial global carbon
uptake by cement carbonation | Nature Geoscience
Company
Devises Ingenious Method of Repurposing Old Wind Turbines: ‘The perfect time’.
Andy Corbley. The Cool Down. MSN. August 31, 2023. Company
devises ingenious method of repurposing old wind turbines: ‘Entering the market
at the perfect time’ (msn.com)
Near-term
pathways for decarbonizing global concrete production. Josefine A. Olsson,
Sabbie A. Miller & Mark G. Alexander. Nature Communications volume 14,
Article number: 4574 (2023). Near-term pathways
for decarbonizing global concrete production | Nature Communications
Carbon
dioxide utilization in concrete curing or mixing might not produce a net
climate benefit. Dwarakanath Ravikumar, Duo Zhang, Gregory Keoleian, Shelie
Miller, Volker Sick & Victor Li. Nature Communications volume 12, Article
number: 855 (2021). Carbon
dioxide utilization in concrete curing or mixing might not produce a net
climate benefit | Nature Communications
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