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  • Donald J. Pagel

Low CO2 Cement: Making Concrete That Won’t Fry the Planet



Cement produced using traditional methods produces around 8% of the greenhouse gas carbon dioxide (CO2) emitted annually worldwide. Now, a new method for producing cement is under development that could reduce the amount of CO2 emitted during the cement manufacturing process. The new method is described in a recently published patent application (U.S. Patent Application No. US 2023/0174396 A1, published June 8, 2023) and in other publications. The new method uses electrochemistry to produce reagents and fuels that are used in manufacturing the cement and allows CO2 which is generated during manufacturing to be captured more easily while potentially using less energy overall than the old method.


The Old Method

The old method for producing Portland (hydraulic) cement involves mining rock (limestone) that contains calcium carbonate (CaCO3); crushing the limestone; heating the crushed limestone along with some aluminosilicate material, such as clay, to about 1,450 °C (2,642 °F) to produce clinker (nodules comprising a mixture of 3CaO·SiO2, 2CaO·SiO2, and some aluminates); and then grinding the clinker to a powder to yield cement. The cement is mixed with water and an aggregate, like sand or gravel, to produce concrete. During the heating phase, the calcium carbonate is converted to lime (referred to as “calcination”), and the lime fuses with silicates to form the clinker in a process known as “sintering”.


A significant source of unwanted CO2 comes during the heating/calcination phase when limestone (CaCO3) is converted to lime (CaO) with the release of CO2, probably before the clinker is formed. It’s difficult to capture the CO2 released during calcination because of the high temperature involved in the combined calcination and sintering process. Additionally, there is a significant CO2 footprint involved in supplying the heat required for calcination and sintering if hydrocarbon fuel is used to generate the heat used in these steps.


The New Method

The new method uses electrochemistry to produce calcium hydroxide (Ca(OH)2) from limestone which is then heated to form “alite” (3CaO·SiO2). Alite is the primary component in cement and is the main component of the clinker formed in the old method for producing cement. The new process makes it easier to capture the CO2 emitted during the conversion of limestone to cement because the calcination step is eliminated. Additionally, the new process potentially uses less energy overall if the gases produced during the electrochemistry phase are used to supply energy for the heating phase.


There are several approaches to implementing the new method. In one implementation, a single reactor is employed to electrolyze water containing limestone. The electrolysis of water causes acidic conditions (H+) to be formed at the anode, along with the emission of oxygen (O2) gas, and basic conditions (OH-) to be formed at the cathode along with the emission of hydrogen (H2) gas. The acidic conditions in the anode cell cause calcium carbonate from limestone to disassociate into calcium ions along with the emission of CO2 gas. The calcium ions migrate toward the cathode where they react with hydroxide ions to form calcium hydroxide which precipitates out of solution.


The CO2 and O2 emitted from the anode cell are easier to collect because the acid/limestone reaction is conducted at room temperature. Furthermore, the CO2 and O2 gases can be separated from each other and sequestered and/or used for other value-added purposes. The calcium hydroxide is collected and reacted with silicon dioxide (SiO2) at 1500°C to form the alite/cement. Energy savings can potentially be achieved by using the hydrogen gas generated at the cathode as a fuel for this high-temperature step. This implementation is described in the article by Leah D. Ellis et al., Toward electrochemical synthesis of cement – An electrolyzer-based process for decarbonating CaCO3 while producing useful gas streams, Proceedings of the National Academy of Sciences (PNAS), available online at https://doi.org/10.1073/pnas.1821673116 (first published September 16, 2019).


In another implementation of this technology, separate cells and reactors are used to carry out various operations. An electrolytic cell is used to produce an acid and base which are then sent to separate reactors. The limestone is dissolved in an acidic solution in one reactor and then converted to calcium hydroxide in a basic solution in another reactor. The calcium hydroxide is moved to a kiln where the sintering to alite occurs. This “multiple reactor” implementation of the electrolytic process is described in Chiang et al., Use of Reactor Outputs to Purify Materials, and Related Systems, U.S. Patent Application Publication No. US 2023/0174396 A1, published June 8, 2023.


In one example of this implementation, the acids and bases are formed in a chlor-alkali reactor which is an electrolysis reactor where a sodium chloride (NaCl) solution is converted at the anode into chlorine gas and sodium ions. The chlorine gas is reacted with hydrogen gas and water in an “acid burner” to form hydrochloric acid which is then sent to the reactor where the limestone is dissolved. Sodium ions from the anode cell migrate through a membrane to the cathode where they form sodium hydroxide. The sodium hydroxide is sent to the reactor where it reacts with the dissolved limestone to form calcium hydroxide. The calcium hydroxide is collected and reacted with silicon dioxide (SiO2) at 1500°C to form the alite/cement as previously described.


An advantage of this method of using separate reactors is that a pure stream of carbon dioxide is emitted from the acidic reactor where the limestone is dissolved and disassociated into calcium ions. This makes collection of the CO2 easier for sequestration or further use because the CO2 doesn’t have to be separated from oxygen as in the case where neutral water electrolysis is used. Additionally, the conversion of calcium hydroxide to alite uses less energy than traditional cement making and can be made very energy efficient if feedstocks from the acid/base production are used as fuels in the thermal step. Generating the acid and base separately from the cement making steps also makes the logistics of the process more flexible because the acid and base can be produced and stored for later use in the process.


An interesting aspect of this cement technology is that the basic calcination chemistry it employs is similar to the chemistry used in direct air capture (DAC) technology to remove CO2 from the atmosphere. How ironic that basic inorganic chemistry involving lye and chalk might be the key to mitigating the environmental damage caused by the industrial revolution.

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