Concrete is everywhere. As the world’s most widely used construction material, it forms bridges, buildings, roads and other infrastructure. But concrete, and its primary ingredient, cement, are responsible for roughly 8% of the world’s CO2 emissions. In fact, if the cement industry were a country, it would rank as the fourth largest greenhouse gas emitter, behind China, the United States and India. 

Now, new scholarly research from Mehdi Khanzadeh, assistant professor of civil and environmental engineering in the College of Engineering, could help expand the use of carbonatable concrete, a more eco-friendly concrete alternative. 

“Carbonatable concrete is really only used in CMU blocks, which is a very limited portion of our industry,” Khanzadeh said. “If we can address limitations through the method we are proposing, then we can open a much larger portion of our industry to implement carbonatable systems.” 

Current methods for producing carbonatable concrete make it difficult to achieve high-strength and durable material, which is why carbonatable concrete is primarily used to produce CMU blocks (also known as cinder blocks) and other small-scale building materials that are non-load bearing. 

In his latest research, Khanzadeh addresses these limitations and presents breakthrough findings that could one day lead to the material being more widely used in construction. 

The scholarly research “Unlocking the Depth-Dependent Limitation of External CO2 Curing in Carbonatable Cementitious Materials Using Enzymatic Solution-Impregnated Hydrogels” was published in ACS Sustainable Chemistry and Engineering. 

Traditional concrete is made through a reaction between cement and water, called hydration. In carbonatable concrete, however, cement interacts with CO2 instead of water in a process called carbonation. This process absorbs CO2 into the material, making it more sustainable than traditional concrete. Carbonatable concrete also uses a different kind of cement, which requires less energy to produce and generates lower CO2 emissions. 

The deeper CO2 is absorbed into the concrete (i.e., uniform carbonation over depth), the stronger and more durable it becomes, but current methods for producing carbonatable concrete have achieved limited carbonation depth. 

Khanzadeh has developed a new method for making carbonatable concrete, called internal-external CO2 curing, which increases carbonation depth. Initial tests show that concrete made with this process has an 80–100% increase in its mechanical and durability performance compared to current carbonatable concrete. 

“We’re hoping that by using this process we can move from only using carbonatable concrete for blocks and pavers to instead using the material for large-scale beams and columns,” Khanzadeh said. 

Khanzadeh has been working on this research since 2021, and in 2024 he received an NSF CAREER Award to continue the work. He began by producing small-scale, simplified carbonatable concrete systems, starting with solutions and progressing to pastes and mortars. At each step, Khanzadeh performed advanced characterization methods and other tests to examine the material’s performance. 

He is currently working on scaling up production of the carbonatable concrete using his method, taking into account material-supply challenges, cost effectiveness and sustainability. 

“I try to keep in mind, even if this is successful, is it going to be applicable?” Khanzadeh said. “Is this material going to be scalable? This is especially important for something like concrete. We use it so much, so it needs to be accessible everywhere.” 

The research is still in the proof-of-concept stage, and Khanzadeh says more testing will be required to determine the material’s long-term durability. He also plans on conducting additional tests to determine the extent to which the material is carbon negative or carbon neutral. 

But Khanzadeh is hopeful that his research will help the construction industry move toward a more sustainable future.