Background
Cement production accounts for ∼8 % (2.8 gigatons/year) of global CO2 emissions, the largest single industrial emitter. About half of this CO2 results from using CaCO3 (decomposes to CaO) as a key component while the rest is mainly due to the combustion of fossil fuels in the cement kiln during calcining (∼900 °C) and sintering (∼1450 °C).
Current efforts to reduce cement’s carbon footprint include carbon capturing from flue gases, using alternative fuels, or developing supplementary cementitious materials. However, these methods suffer economic challenges and may compromise the physical properties of cement with limited mitigation of carbon emissions.
Alternatively, electrochemical methods have the potential to produce the most widely accepted and used types of cement, thereby reducing adoption risk while utilizing emerging economical renewable electricity to alleviate both the chemical and thermal sources of CO2.
Thus, Sublime Systems employed an electrochemical process to break down calcium silicate rocks at ambient temperature. Abundant raw materials were used to create reactive calcium and silicates that were dried and blended into cement. This system exhibited the same final strength and hardened phases as Portland cement.
Methods
The researchers employed custom-designed H-cells for the decarbonization process, using NaClO4 or NaNO3 dissolved in deionized water as the electrolyte, which do not decompose at high voltages. The electrodes consisted of platinum rods as the cathode and platinum wires as the anode, chosen for their high catalytic activity in both acidic and basic conditions. CaCO3 powder was introduced into the anode compartment, separated by a 5 μm filter paper as the porous separator.
After conducting potentiostatic experiments at room temperature, the resulting materials were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. The electrochemically precipitated Ca(OH)2, along with commercial CaCO3 as a control sample, was mixed with SiO2 in a 3:1 molar ratio, then pressed into pellets and dried. These pellets were sintered at 1500 °C in a muffle furnace for two hours. The sintered powders were analyzed by XRD and identified as alite (3CaO·SiO2), a major component of Portland cement.
Results and Discussion
The designed decarbonization cell simultaneously functioned as an electrolyzer and a chemical reactor, converting CaCO3 to Ca(OH)2. The reactor utilized inherent pH gradients in an electrolysis cell for CaCO3 decarbonization and Ca(OH)2 precipitation and collection. Overall, this was a near-stoichiometric operation, with every two protons produced at the oxygen-generating anode decarbonating one CaCO3 unit.
Different H-cell reactors (with and without a porous separator between the cathode and anode chambers) were constructed to test the proposed method of decarbonating CaCO3. In the cell without a separator, Ca(OH)2 precipitated across the cell length, including directly on the platinum wire cathode, eventually passivating this cathode. This passivation led to a sharp drop in cell current after a few hours of operation.
Alternatively, a porous paper separator at the intersection of each chamber limited convection in the cell. Significant amounts of white precipitate were collected directly on the separator in front of the cathode. XRD analysis confirmed this precipitate to be predominantly Ca(OH)2 with a small amount of CaCO3.
The electrochemically produced Ca(OH)2 exhibited particle sizes and morphologies comparable to those of the control alite sample. Consequently, Ca(OH)2 from the decarbonization reactor is an effective precursor for Portland cement, capable of synthesizing the primary hydrating calcium silicate phase. Additionally, its fine morphology and decomposition temperature of more than 300 °C lower than that of CaCO3 likely enhance its reactivity.
Apart from reactive Ca(OH)2 suitable for cement synthesis, the proposed electrochemical decarbonization reactor produced concentrated gas streams of H2 at the cathode and O2 and CO2 (in a 1:2 molar ratio) at the anode. These gases could be used in various sustainable technologies.
Conclusion
Overall, Sublime Systems’ electrochemical process offers a novel approach to decarbonizing cement production by eliminating the need for high temperatures and reducing dependence on limestone. This method not only lowers CO2 emissions but also produces concentrated gas streams with various sustainable applications. For example, CO2 can be captured and sequestered, while H2 and O2 can be utilized in fuel cells or combustors for electricity generation. Additionally, O2 can be employed in oxy-fuel cement kilns to reduce CO2 and NOx emissions, or the gases can be used in the synthesis of liquid fuels.
Recently, three tons of cement produced using this method were utilized in Boston’s largest net-zero commercial building in Seaport. Sublime Systems is also working on a commercial-scale manufacturing plant in Massachusetts, which will have an annual production capacity of 30,000 tons. The researchers suggest that future developments should focus on designing more advanced reactors to better control convection and chemical gradients and to improve the efficiency and continuity of Ca(OH)2 collection.
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