Tech Briefs Magazine - January 2024 - 32

Sustainable Technology
Baking Soda Could Turn Concrete into an Effective
Carbon Sink
Engineers have discovered new carbonation pathways for creating greener concrete.
Massachusetts Institute of Technology, Cambridge, MA
D
espite the many advantages of
concrete as a modern construction
material, its production currently
accounts for approximately 8 percent
of global carbon dioxide emissions.
Recent discoveries by MIT engineers
have revealed that introducing
new materials into existing concrete
manufacturing processes could significantly
reduce this carbon footprint
without altering concrete's bulk
mechanical properties.
Approximately half of the emissions
associated with concrete production
come from the burning of
fossil fuels, which are used to heat
up a mix of limestone and clay that
ultimately becomes the familiar gray
powder known as ordinary Portland
cement (OPC). While the energy
required for this heating process
could eventually be substituted with
electricity generated from renewable
solar or wind sources, the other half
of the emissions is inherent in the
material itself; as the mineral mix is
heated to temperatures above 2,552
°F (1,400 °C), it undergoes a chemical
transformation from calcium carbonate
and clay to a mixture of clinker (consisting
primarily of calcium silicates) and
carbon dioxide - with the latter escaping
into the air.
Introducing additives to concrete manufacturing processes could reduce the sizeable carbon footprint
of the material without altering its bulk mechanical properties, an MIT study shows. (Image:
The researchers)
able pathways that accelerate concrete
deterioration.
" The problem with these postcuring
When OPC is mixed with water, sand,
and gravel material during the production
of concrete, it becomes highly
alkaline, creating a seemingly ideal environment
for the sequestration and
long-term storage of carbon dioxide in
the form of carbonate materials (carbonation).
Despite this potential of concrete
to naturally absorb carbon dioxide from
the atmosphere, when these reactions
normally occur, mainly within cured
concrete, they can both weaken the material
and lower the internal alkalinity,
which accelerates the corrosion of the
reinforcing rebar.
These processes ultimately destroy the
load-bearing capacity of the building
and negatively impact its long-term mechanical
performance. Thus, these slow
late-stage carbonation reactions, which
can occur over timescales of decades,
have long been recognized as undesir32
carbonation
reactions is that you disrupt
the structure and chemistry of the cementing
matrix that is very effective in
preventing steel corrosion, which leads to
degradation, " said Professor Admir Masic.
However, the team's new carbon dioxide
sequestration pathways rely on
the very early formation of carbonates
during concrete mixing and pouring,
before the material sets, which might
largely eliminate the detrimental effects
of carbon dioxide uptake after the
material cures.
The key to this new process is the addition
of sodium bicarbonate - more
commonly known as baking soda. In lab
tests, the team demonstrated that up to
15 percent of the total amount of carbon
dioxide associated with cement production
could be mineralized during these
early stages - enough to potentially
make a significant difference in its global
carbon footprint.
In addition, the resulting concrete sets
much more quickly via the formation
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of a previously undescribed composite
phase, without impacting its mechanical
performance.
The composite, a mix of calcium carbonate
and calcium silicon hydrate, " is
an entirely new material, " said Masic.
" Furthermore, through its formation, we
can double the mechanical performance
of the early-stage concrete.
" While it is currently unclear how
the formation of these new phases will
impact the long-term performance of
concrete, these new discoveries suggest
an optimistic future for the development
of carbon neutral construction
materials. "
" Our new discovery could further
be combined with other recent innovations
in the development of lower
carbon footprint concrete admixtures
to provide much greener, and even carbon-negative
construction materials for
the built environment, turning concrete
from being a problem to a part of a solution, "
added Masic.
For more information, contact Abby
Abazorius at abbya@mit.edu; 617-2532709.
Tech
Briefs, January 2024
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Tech Briefs Magazine - January 2024

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