Sequestering CO2 in concrete

The production of cement, the binding element in concrete, accounts for 7% of total global carbon dioxide (CO₂) emissions. Concrete is one of the most-used resources on Earth, totaling an estimated 26 billion tons produced annually worldwide. Due to global demand growth, production isn’t expected to slow down for at least two more decades.  

Concrete is made up of aggregate materials, primarily rocks and sand, along with cement and water. As about 80% of concrete’s carbon footprint comes from the cement component, researchers have been working to find more suitable materials to reduce such carbon emissions. Apart from developing blended cements and algae-based cements to reduce CO₂ emissions, researchers and private companies are also focusing on tapping into new ways to use captured CO₂ as an ingredient in the cement itself, locking it away and preventing it from entering the atmosphere. 

What are carbon-conditioning and CO₂ curing?

Currently, in order to accomplish this, CO₂ is either added to aggregates by injection during mixing in a process called carbon-conditioning or after concrete has been cast in a process called carbonation curing or CO₂ curing. These processes transform gaseous CO₂ to carbonate minerals that can be stored indefinitely without causing global warming. Precast concrete is capable of storing more CO₂ than cast-in-place concrete. Whereas cast-in-place can store up to 3.8% of its total mass in CO₂, precast can store as much as 9.6% of its total mass in CO₂.  

Aside from reduced CO₂ emissions, this process also improves the strength of concrete approximately 10 to 20 percent depending on the mix. Therefore, concrete made using CO₂ curing actually requires less cement, further reducing amounts of related CO₂ emissions. Moreover, as less materials are required, CO₂ curing leads to lower production costs and larger revenue margins for concrete manufacturers. 

In the following, we will explain both the carbon-conditioning and carbon-curing processes and how Alicat’s MC-Series of mass flow controllers provide all-in-one metering solutions for these applications.  

Carbon-conditioning using MC-Series mass flow controllers

In carbon-conditioning, CO₂ is injected directly into aggregates during the mixing stage.

For this process, different CO₂ sources with different advantages and disadvantages can be used, including pure CO₂ gas, flue gas, or blended gas mixes. Whereas pure CO₂ tanks are more costly and must either be purchased externally or created internally using CO₂ separation procedures, flue and blended gases are more affordable options that reclaim direct emissions from industrial processes. Conversely, pure CO₂ gases offer faster reaction times and are easier to store. 

Regardless of which CO₂ source is used, aggregate materials are placed into a temperature and humidity controlled curing chamber and then pressurized by the gas source using a pressure controller, such as an Alicat MC-Series mass flow controller running in pressure control mode. Depending on which gas source is used, this generally varies somewhere in the range of 150 kPaG-500 kPaG (although sometimes ambient conditions are used). Greater pressures will allow for quicker reactions. MC-Series mass flow controllers regulate the process pressure while taking mass flow and temperature data.  

Open-inlet systems

In open-inlet systems, or constantly flowing systems, chamber pressure and temperature are roughly unchanged during the reaction due to fresh CO₂ consistently replacing sequestered CO₂. The reaction therefore concludes when the flow rate of gas drops near to zero, signaling that no more CO₂ can be stored in the concrete. Additionally, temperature drops also determine that the process has ended. Once these predetermined flow rate or temperature endpoints are reached, the mass flow controller running in pressure control mode signals via a PLC or computer to close off the gas supply.

Alicat’s MC-Series of mass flow controllers running in pressure control mode can provide continuous, fast-response, pressure control through a PID-control loop to automate the process while simultaneously recording critical temperature and mass flow data which also monitor and control the process via a PLC or computer. 

Closed-inlet systems

In closed-inlet systems, CO₂ is added until reaching a chamber pressure setpoint, after which, the gas supply is turned off. Since the system is now closed after pressurization and airtight, the temperature, the CO₂ concentration, and the gas pressure in the chamber will be reduced as the concrete absorbs CO₂. CO₂ sensors attached to the chamber signal when all CO₂ has been used and all remaining gas is vented via an electric valve. New CO₂ gas is then added and the process is repeated cyclically until CO₂ is still detectable by the CO₂ sensors and no significant temperature or pressure change is observed anymore.

In addition to controlling the cyclical gas flow, just as in open-inlet systems, Alicat’s MC-Series of mass flow controllers running in pressure control mode with CO₂ sensors in closed-inlet systems signal to the control loop when the process is over based on either temperature or pressure data. When the pressure and temperature no longer decrease over time and CO₂ levels do not drop towards zero, the control loop ends the process. Alternatively, when the temperature and pressure decrease over time but CO₂ levels drop towards zero, the control loop vents remaining chamber gas via an electric valve and adds more CO₂.

Carbon-conditioning using PC3-Series pressure controllers

Alternatively, an Alicat PC3-Series pressure controller can automate the process based on just pressure setpoints. In the open-inlet process, the PC3-Series pressure controller sets the chamber pressure and ends the process when it begins to rise above the setpoint. In the closed-inlet configuration, the PC3-Series pressurizes the chamber and then batch adds more CO₂ as the chamber pressure decreases and CO₂ levels decrease over time. When the chamber pressure and CO₂ levels remain constant, the PC3 signals to the control loop to vent all remaining gases via an electric valve and to end the process.   

To track the amount of CO₂ or mixed gases being used in either open-inlet or closed-inlet systems, Alicat’s MC-Series includes totalizer and batching options, allowing for easy, automated calculations of how much CO₂ is being sequestered, creating a traceable record which may be useful for reasons such as tax incentives.  

CO₂ curing using MC-Series of PC3-Series controllers

Carbonation curing, or CO₂ curing, describes CO₂ sequestering in entire concrete blocks after the mixing stage. The process is similar to carbon-conditioning aside from this difference. Just as in the carbon-conditioning system configurations, a humidity, temperature, and pressure controlled chamber is utilized in combination with CO₂ gas sources, pressure transducers or controllers, and CO₂ sensors. 

Just as in carbon-conditioning, Alicat’s MC-Series of mass flow controllers can help to reap the full benefits of CO₂ curing.

• Accuracy up to ±0.5% of reading and ±0.1% of full scale
• Repeatability up to ±0.1% of reading + 0.02% of full scale
• Response times fast as 1 ms

Alicat’s PC3-Series of pressure controllers can help to automate CO₂ curing.

• Accuracy up to ±0.125% of full scale
• Repeatability up to ±0.08% of full scale
• Control range of 0.01–100% of full scale
• Sensor response times <1 ms

Moreover, Alicat’s devices are compatible with a wide selection of communication options, allowing for various automation options when using either a computer or PLC.