By Claude Goguen, P.E., LEED AP
CO2 could become an important ingredient for manufacturing durable and sustainable precast concrete.
Take a deep breath and exhale. Your breath produced about .0001 pounds of carbon dioxide, a colorless gas, that is essential to life on earth. CO2 is formed when one atom of carbon combines with two atoms of oxygen and is produced by a wide variety of natural and man-made sources. We use it in products ranging from fire extinguishers to soda. It’s even used to create the “pop” in Pop Rocks candy.
CO2 is a primary greenhouse gas that makes the earth habitable, but in increasing amounts it can also cause global warming by trapping the sun’s radiant energy in our atmosphere. However, some companies are making progress in harnessing CO2 as an ingredient in concrete.
CO2 and concrete
CO2 in the earth’s environment is constantly being released and absorbed in a continuous cycle. Concrete plays a role in that cycle, as CO2 is released during production of cement in two ways: the calcination process where limestone is heated creates a direct emission of CO2, while burning fossil fuels to heat the kilns contributes indirectly.
Concrete can also absorb CO2 and store it in a process known as carbonation. Over time, carbonation usually results in the slow reduction of the concrete’s pH. The rate of absorption of CO2 depends on the environmental conditions surrounding the structure and the concrete’s porosity.
Using CO2 to manufacture concrete
For many years, technology has existed to capture waste CO2 from large point sources such as fossil fuel power plants. This process is often referred to as carbon capture and sequestration. The captured CO2 is kept from entering the atmosphere by depositing it in large underground geological formations. Once CO2 is injected deep underground, it is trapped in minute pores or spaces in the rock structure. Impermeable cap rocks above the storage zones act as seals to ensure the safe long-term storage of CO2.
Researchers are working on finding other ways to store and use the waste CO2. CarbonCure Technologies of Dartmouth, Nova Scotia, claims injecting it into fresh concrete not only makes the concrete “greener,” but can also make it stronger. The company found putting purified and liquefied CO2 into concrete during mixing converts the CO2 to a solid mineral and captures the gas within the concrete. When CO2 is added to concrete while mixing, it reacts with water and cement and converts into calcium carbonate minerals. CarbonCure claims that the nano-sized calcium carbonate minerals act as a nucleation site for the hydration reaction, giving concrete its strength. They go on to state that ready-mixed concrete producers see an average strength improvement of approximately 10% when comparing concrete injected with CO2 to ordinary concrete. Also, compressive strength improvement enables ready-mixed concrete producers to optimize mix design, which typically includes reducing cement content.
Carbicrete, based in Montreal, Quebec, goes a step further and uses CO2 to create a cement-free concrete. Carbicrete uses steel slag, a byproduct from the steel industry, as the primary binder and injects CO2 into wet concrete. The company claims that products made in this fashion meet all the same specifications as cement-based concrete products, yet have lower material costs and increased durability. Due to the absence of portland cement and the sequestering of CO2, Carbicrete states it manufactures a carbon-negative concrete product which absorbs more CO2 than is produced.
Another version was created by a team of researchers at UCLA. Their product, called CO2NCRETE, uses a binder system based on calcium hydroxide (hydrated lime), which is mixed with aggregates and admixtures to form shape-stabilized building elements. The captured
CO2 is combined with the mixture via a carbonation reaction to form a solid building component. These elements are similar to Legos® and can be rapidly assembled to construct buildings, bridges and other infrastructure. The researchers even predicted this material could be used by 3-D printing machines.
All three technologies are semi-finalists in a $20-million global competition, called NRG COSIA Carbon XPRIZE, to develop breakthrough technologies that convert CO2 emissions from natural gas and power plant facilities into products with high net values.
What this means for precast
CarbonCure is the only technology that has gone to market and is primarily used in masonry and ready-mix industries. A representative from Carbicrete presented to NPCA’s Sustainability Committee at the The Precast Show 2017 in Cleveland and stated the sustainable benefits alone of any of these products would be a huge asset for the precast concrete industry.
Not enough is currently known to predict impacts on cost and performance. NPCA will continue to monitor progress to determine quality and durability. For the time being, we can continue to breathe easy knowing that good old-fashioned precast concrete is still the material of choice in terms of performance and sustainability.
If you have any questions about this or any other sustainability-related topics, contact Claude Goguen, director of sustainability and technical education, at [email protected] or (317) 582-2328.
Claude Goguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.