Abstract Cement production is currently the largest single industrial emitter of CO 2 , accounting for ∼8% (2.8 Gtons/y) of global CO 2 emissions. Deep decarbonization of cement manufacturing will require remediation of both the CO 2 emissions due to the decomposition of CaCO 3 to CaO and that due to combustion of fossil fuels (primarily coal) in calcining (∼900 °C) and sintering (∼1,450 °C). Here, we demonstrate an electrochemical process that uses neutral water electrolysis to produce a pH gradient in which CaCO 3 is decarbonated at low pH and Ca(OH) 2 is precipitated at high pH, concurrently producing a high-purity O 2 /CO 2 gas mixture (1:2 molar ratio at stoichiometric operation) at the anode and H 2 at the cathode. We show that the solid Ca(OH) 2 product readily decomposes and reacts with SiO 2 to form alite, the majority cementitious phase in Portland cement. Electrochemical calcination produces concentrated gas streams from which CO 2 may be readily separated and sequestered, H 2 and/or O 2 may be used to generate electric power via fuel cells or combustors, O 2 may be used as a component of oxyfuel in the cement kiln to improve efficiency and lower CO 2 emissions, or the output gases may be used for other value-added processes such as liquid fuel production. Analysis shows that if the hydrogen produced by the reactor were combusted to heat the high-temperature kiln, the electrochemical cement process could be powered solely by renewable electricity.

As discussed at the 2018 Sackler Colloquium “Status and Challenges in Science for Decarbonizing our Energy Landscape” and in other recent analyses (1⇓⇓–4), deep decarbonization of today’s energy system will require addressing not only energy generation (24% of global greenhouse gas emissions) and transportation (14% of global greenhouse gas emissions) but also difficult-to-decarbonize sectors such as large industry, which today is responsible for about 21% of global greenhouse gas emissions (5). Industry uses fossil fuels for heat and to drive chemical and thermochemical reactions but may become less reliant on fossil fuel if 1) electrical alternatives become available and 2) the cost and reliability of renewable electricity continues to improve (6, 7). The rise of very low-cost renewable electricity has already motivated the search for electrochemical methods to innovate industrial processes (1, 8, 9). Among these, electrochemical pathways to the production of cement have heretofore been limited; 1 previous example is high-temperature electrochemical decarbonation using molten salts, operating in the same temperature range as thermal calciners (10, 11). Here, we propose and demonstrate proof of concept for an ambient-temperature electrochemically based process that decarbonates CaCO 3 , precipitates solid Ca(OH) 2 from which desired calcium silicates are synthesized, and produces concentrated gas streams of H 2 and O 2 + CO 2 that are amenable to CO 2 capture and sequestration, and/or used in other value-added processes (Fig. 1).

Fig. 1. Scheme for a low-emission, electrochemically based cement plant. An electrochemical decarbonation reactor powered by renewable electricity converts CaCO 3 to Ca(OH) 2 for use in cement synthesis. The decarbonation cell (Fig. 2) uses the pH gradient produced by neutral-water electrolysis to dissolve CaCO 3 at the acidic anode and precipitate Ca(OH) 2 where pH ≥ 12.5. Simultaneously, H 2 is generated at the cathode and O 2 /CO 2 are generated at the anode. These gas streams can serve several alternative roles in a sustainable production system. CO 2 can be directly captured from the inherently concentrated stream (CCS). Electricity or heat can be generated from the H 2 and O 2 via fuel cells or combustors. The O 2 /CO 2 oxy-fuel can be recirculated to the kiln for cleaner combustion in the cement sintering cycle. CO 2 reuse and utilization (CO 2 U) concepts can be employed, such as use in EOR or production of liquid fuels.

Portland cement (12) is the most widely produced man-made material in the world, produced at a rate of 4 billion metric tons per year (13). Excluding agriculture, cement production is the largest industrial source of greenhouse gases (steelmaking follows closely), accountable today for 8% of global greenhouse gas emissions (14). About one-half of the emitted CO 2 is due to the use of CaCO 3 (generally, limestone) as a key component, with the balance being mainly due to combustion of fossil fuels in the cement kiln (15). Demand for cement is growing as the world’s population increases and becomes more urban, and as emerging economies develop infrastructure (16). By 2060, the number of buildings on Earth is expected to double; this is equivalent to building a New York City each 30 days for the next 40 years (17). Since each kilogram of cement produced emits nearly 1 kg of CO 2 (15), several gigatons of CO 2 per year will be released from new infrastructure, highlighting the urgency of decarbonizing cement production.

Current efforts to reduce cement’s carbon footprint include carbon capture from flue gases, use of alternative fuels, or development of supplementary cementitious materials (14, 18⇓⇓–21). Currently, the flue gas from cement plants is too impure for economical carbon capture through amine scrubbing; use of alternative fuels (such as used tires) does not alleviate the primary emissions from CaCO 3 ; and use of supplementary materials in the concrete has limited impact on the carbon emissions from Portland cement and may simultaneously compromise the physical properties (14, 19⇓–21). Another family of approaches uses the cement to capture and sequester more CO 2 , producing a carbonate-enriched cement or concrete product (22⇓⇓–25). In contrast with the above approaches, we were motivated to seek electrochemically based approaches that have the potential to produce the most widely accepted and used cements, thereby minimizing adoption risk, while taking advantage of emerging very low-cost renewable electricity to alleviate both the chemical and thermal sources of CO 2 . As we show, our process can work synergistically with other scientific and technological tools of a sustainable energy system discussed in the Sackler Colloquium, including wind and solar electricity, water splitting and fuel creation, and chemical and electrical energy storage.

Our reactor takes advantage of the inherent pH gradients in an electrolysis cell to carry out CaCO 3 decarbonation and Ca(OH) 2 precipitation and collection (Fig. 2). We show that the Ca(OH) 2 produced in this manner, which requires less energy to dehydrate to CaO than is required to calcine CaCO 3 , is readily reacted with SiO 2 to produce alite, the major active phase (50 to 70% by weight) in Portland cement (12). Near-stoichiometric operation, where every 2 protons electrolytically produced at the oxygen-generating anode decarbonates 1 CaCO 3 formula unit, is demonstrated at laboratory scale. We propose several pathways by which this electrochemical decarbonation reactor can be integrated into a low- or zero-carbon-emission cement plant (Fig. 1), including powering by renewable electricity and using the gases produced in any of several alternative functions such as 1) direct capture and sequestration of the inherently concentrated CO 2 stream, 2) generation of electricity or heat from the H 2 (and optionally the O 2 ) via fuel cells or combustors, 3) providing oxy-fuel for cleaner combustion in the cement sintering cycle, and 4) liquid fuel production. A first-order technoeconomic analysis of the energy consumption and fuel cost of such a process as a function of the cost of renewable electricity is presented.

Fig. 2. Schematic of electrolyzer-based decarbonation cell. Reactions 1 and 2 are oxygen evolution and hydrogen evolution half-cell reactions respectively, under near-neutral pH. Reaction 3 is formation of water from its component ions. Reactions 4 and 5 represent decomposition of calcium carbonate and release of CO 2 ; see the text for intermediate steps. In reaction 6, hydroxide ions in reaction 3 instead go toward formation of calcium hydroxide, and protons protonate carbonate ions (reaction 5). The overall reaction in which CaCO 3 is converted to Ca(OH) 2 with attendant release of H 2 , O 2 , and CO 2 is shown at the bottom.

Discussion In addition to producing a reactive Ca(OH) 2 suitable for cement synthesis, our electrolysis-based decarbonation reactor produces concentrated gas streams of H 2 at the cathode and O 2 and CO 2 (in a 1:2 molar ratio when operating at high coulombic efficiency) at the anode. These gases are important components in a wide range of sustainable technologies that are currently being pursued worldwide and open up several possible synergies between cement production and these technologies, which we now discuss. Carbon capture and sequestration (CCS) at the cement plant level has to date focused on postcombustion capture of CO 2 , combined with the use of oxy-fuel combustion. The O 2 /CO 2 stream from our decarbonation reactor can make these processes simpler and more efficient. Postcombustion capture refers to technologies that capture CO 2 from the kiln exhaust, such as calcium looping, amine scrubbing, and membrane filtration (36⇓–38). Oxy-fuel, or oxygen-enhanced combustion, refers to the burning of fossil fuels (here, primarily coal) with oxygen instead of air (37, 39). Oxy-combustion first results in improved fuel efficiency, since the nitrogen content of air does not have to be heated. Second, the absence of nitrogen permits higher flame temperatures without emitting nitrous oxides (NO x ), which have a global warming potential 298 times that of CO 2 on a per-mass basis (40) and also contribute to smog, acid rain, and ozone depletion. Third, the flue gas from oxy-fuel combustion has a higher concentration of CO 2 and fewer NO x impurities (37, 41), making carbon capture more efficient. In a cement plant using our decarbonation reactor, the O 2 /CO 2 gas mixture could therefore be used as oxy-fuel in the high-temperature kiln to lower energy consumption and NO x emissions. Among other benefits of oxygen enrichment, 1 commercial-scale experiment using 30 to 35% oxygen enrichment resulted in a 25 to 50% increase in cement kiln production (42). Also, oxy-fuel combustion has negligible, if not beneficial, effects on Portland cement clinker quality (39, 43⇓⇓⇓–47). The concentration of CO 2 in the flue gas from conventional cement kilns is ∼25% (48). For chemical absorption with amines, the most technologically mature postcombustion capture method for a combined stream (37, 38), increasing the concentration of CO 2 up to 60% has been shown to decrease heat requirements, solvent regeneration energy, and steam costs of capture (49⇓⇓⇓–53). The gas stream from our decarbonation cell is higher still (67%), which should make amine scrubbing more efficient. However, a greater benefit may be the ability to avoid expensive CCS processes like amine scrubbing altogether. Since here the CO 2 is delivered in a highly concentrated form mixed only with O 2 (and some H 2 O vapor), direct capture using the same simple compression processes (54, 55) now used for purified and concentrated CO 2 , could be used. The hydrogen gas produced at the cathode in our decarbonation cell has value as a feedstock in major industries such as ammonia and fertilizer production, oil and gas refining, and process metallurgy and is considered a key component of developing technologies that could decarbonize heavy-duty transportation, aviation, and heating (56, 57). The combined gas streams could also be used in CO 2 utilization processes that produce liquid fuels, such as those that also use hydrogen and produce alcohols. The hydrogen could also be looped back to support the cement process (Fig. 1). It could be directly combusted to provide heat or electric power back to the cement operation, or the H 2 and O 2 /CO 2 gas streams could supply a fuel cell that generates on-site electricity to power the electrochemical reactor or other plant operations such as grinding, mixing, and handling. By using a solid oxide fuel cell (SOFC) (58), which has the highest electrical efficiency of all fuel cell types (60 to 80%) (59), the deleterious effects of the CO 2 on proton exchange membrane fuel cells (60, 61) is averted, and typical SOFC operating temperatures of 500 to 1,000 °C could be readily maintained using heat from the cement kiln (which typically operates at 1,450 to 1,500 °C). Simultaneously, oxygen would be removed from the O 2 /CO 2 gas stream, further purifying the CO 2 and simplifying sequestration. Note that this combination of an electrochemical reactor and SOFC creates a regenerative fuel cell (62), which has the ability to store energy if storage of the reactants is provided, and could thereby smooth the intermittency of renewable electricity used to power the cement plant. The CO 2 stream produced from the decarbonation cell may also have value in applications that up-cycle captured CO 2 . CO 2 is already used to enhance oil recovery (EOR) (63, 64) and to make chemicals such as urea, salicylic acid, methanol, carbonates (65), synthetic fuel (via the Fischer–Tropsch process) (66), and synthetic natural gas (via the Sabatier reaction) (67). There is growing interest in finding ways to react CO 2 electrochemically or photochemically to create chemicals and fuels from captured CO 2 using renewable electricity (68). For example, a model plant that uses captured CO 2 to make synfuel has been demonstrated (69). We also considered the feasibility of operating an electrochemically based cement manufacturing process purely with renewable electricity. Perhaps the least capital-intensive way to use the output gases of the decarbonation reactor is through combustion to heat the cement kiln. We analyzed the energy flows in this configuration; details are given in SI Appendix. Assuming a decarbonation reactor operating with 85% coulombic efficiency, an electrolyzer operating at 60 to 75% efficiency, and combustion of the resulting H 2 and O 2 to heat the sintering kiln with 60 to 80% efficiency, the input electrical energy required to make 1 kg of cement is 5.2 to 7.1 MJ. This assumes no energy benefit from the substitution of Ca(OH) 2 for CaCO 3 in the high-temperature sintering process or other potential benefits such as reduction of capital and energy cost for grinding limestone (given that this function is replaced by chemical dissolution). At 80% efficient combustion of the H 2 and O 2 produced from the decarbonation cell, the thermal energy produced slightly exceeds that required for sintering. If combustion is only 60% efficient, 90% of the thermal energy required for sintering can be supplied from the electrolyzer gases (i.e., ∼0.5 MJ/kg of supplemental energy is required). This energy deficit, as well as electric power for supporting operations, could be made up with an excess of electrolyzer capacity above that stoichiometrically needed for decarbonation. This analysis suggests that a renewables-powered electrochemical cement process would not require large amounts of supplemental energy, if any. An important related question is, of course, the cost of the electrochemically based process. Given the numerous possible configurations discussed above, a complete techno-economic analysis is beyond the scope of this paper. The lifetime cost and economic return for a complete system or any of its subunits depends on capital cost, efficiency, and durability, as well as the value of the cement and gaseous byproducts. Many of the cost factors are currently unknown; for example, the lifetime cost of the decarbonation reactor will depends on its specific design and performance, none of which have yet been optimized. We therefore limit our techno-economic analysis to a comparison of the energy cost of the electrochemical process with its coal-fired counterpart. The 5.2 to 7.1 MJ/kg cement estimated for the electrochemical process does exceed the energy required for the conventional cement process in the average US kiln, which is 4.6 MJ/kg (70). At a coal price of $61 per ton (for bituminous coal) (71), the energy cost for the conventional process is ∼$28 per ton of cement, which is 25% of the average US cement selling price of $113 per metric ton (13). The corresponding cost for the electrochemical process naturally depends on the price of electricity and could in some instances be zero or even negative if obtained from renewable resources. However, for electricity costs of $0.02, $0.04, and $0.06 per kW⋅h, and assuming an energy requirement for the electrochemical process of 6 MJ/kg, which is in the middle of our estimated range, the energy cost is $35, $60, and $100 per ton of cement, respectively. This suggests that, in the absence of other considerations, the electrochemical process would be cost-competitive with conventional plants (∼$28 per ton of cement) if electricity is available at <$0.02 per kW⋅h. Note that the wholesale cost of wind electricity is now at or slightly below $0.02 per kW⋅h across much of the interior of the United States (72). We assume that wind electricity will be available at this price for the proposed cement plants, for example from a colocated wind farm. However, this cost comparison neglects the cost of carbon capture and sequestration, which for amine scrubbing of conventional cement flue gas has been estimated to be on the order of $91 per ton (50). In the electrochemical sequence modeled above, where electrolytic H 2 is combusted to heat the kiln, the cost of directly capturing CO 2 from the O 2 /CO 2 stream exhibiting the decarbonation reactor should be less than $40 per ton (50). This would swing net energy costs in favor of the electrochemical process, in an environment where policies require carbon remediation, and where low-cost renewable electricity is available. Finally, the water intensity of such an electrolyzer-based process should be considered. Each kilogram of cement made using the proposed decarbonation cell would require 0.4 kg of water; this means that the average US kiln, producing 1,800 tons of cement per day, would require ∼760 tons of water per day. However, half of this water would be recovered upon the dehydration of Ca(OH) 2 . If H 2 was used to fuel the kiln, the other half of the water could be condensed from the flue gas. In principle, all of the water used for electrolysis could be recycled.

Conclusions We propose and demonstrate an electrochemically based cement synthesis process in which CaCO 3 is decarbonated and Ca(OH) 2 is precipitated in the pH gradient produced by a neutral-water electrolyzer, while concentrated gas streams of H 2 and O 2 /CO 2 are simultaneously produced. The fine powder Ca(OH) 2 is used to synthesis phase-pure alite, the majority cementitious phase in ordinary Portland cement. The concentrated gas streams from this process may be used synergistically with other processes under development for sustainable industrial technologies. Among several alternatives, the CO 2 may be directly captured and sequestered; the H 2 and/or O 2 may be used to generate electric power via fuel cells or combustors; the O 2 may be used as a component of oxy-fuel to further lower CO 2 and NO x emissions from the cement kiln; or the output gases may be used to synthesize value-added products such as liquid fuels. Our laboratory-scale prototype decarbonation reactors are shown to be capable of operating with near-theoretical coulombic efficiency, wherein every 2 protons produced at the anode during electrolysis dissolves 1 CaCO 3 formula unit. Under such conditions, the electrolytic hydrogen produced, if combusted, can supply most or all of the thermal energy required in high-temperature sintering of the cement. These results suggest a pathway to cost-competitive emissionless cement manufacturing wherein all energy is supplied by renewable electricity.

Materials and Methods Decarbonation Cells. Custom-designed H-cells were fabricated by James Glass, Inc. The electrolyte was 1 M NaClO 4 or NaNO 3 (Sigma-Aldrich, ≥98%) dissolved in deionized water. These electrolytes were chosen because their calcium salts are soluble, and because they do not decompose at high voltage. Both electrodes were made from platinum: a rod at the cathode, and a wire at the anode (MW-1032; BASi). Platinum was chosen because it has a high catalytic activity for hydrogen and oxygen evolution in both acid and base. Alternative low-cost electrode materials might include Ni, Cu, or stainless steel for the cathode (pH 12.5) and Al, Sn, or Pb for the anode (pH 6). CaCO 3 powder (Sigma-Aldrich, ≥99%) was added to the anode compartment. Filter paper (28310-015, particle retention 5 µm; VWR) was used as the porous separator. Potentiostatic experiments were conducted using a Bio-Logic Science Instruments VMP3 potentiostat. All tests were done at room temperature. XRD Characterization. XRD patterns were collected using a PANalytical X’Pert PRO XRPD, using Cu radiation and a vertical circle theta:theta goniometer with a radius of 240 mm. The default configuration of this instrument is in Bragg–Brentano geometry with a high-speed high-resolution X’Celerator position-sensitive detector, using the Open Eulerian Cradle sample stage. XRD data were analyzed using Highscore, version 4.7. SEM Characterization. SEM imaging and compositional analysis of the samples was conducted using a Phenom XL instrument equipped with an energy-dispersive X-ray detector (nanoScience Instruments), operating at 10-kV accelerating voltage for imaging and 15 kV for energy-dispersive X-ray spectroscopy analysis. BET Characterization. A Quantachrome Instruments NOVA 4000E (Anton Paar QuantaTech) was used to perform multipoint BET analysis of powder specific surface areas. Alite Synthesis. Electrochemically precipitated Ca(OH) 2 or CaCO 3 (Sigma-Aldrich, ≥99%) was mixed with SiO 2 (99.5%, 2 µm; Alfa Aesar) in a 3:1 molar ratio. The powders were mixed into a slurry with ethanol then dried. The resulting well-mixed powders were pressed into pellets. The pellets were placed in platinum crucibles and heated at 2 °C per min to 1,500 °C in a muffle furnace (Thermolyne F46120-CM). The temperature was held at 1,500 °C for 2 h, then the pellets were furnace-cooled by turning off the power. The resulting powders were confirmed to be alite by XRD.

Acknowledgments This publication is based on work funded by the Skolkovo Institute of Science and Technology (Skoltech), “Center for Research, Education and Innovation for Electrochemical Energy Storage” program under contract 186-MRA. L.D.E. acknowledges support from the Banting Postdoctoral Fellowships program, administered by the Government of Canada. We thank Isaac Metcalf, Nathan Corbin, Kindle Williams, and Karthish Manthiram (Massachusetts Institute of Technology) for experimental assistance; Muhammad Adil and 24 M Technologies, Inc., for performing the BET measurements; and Form Energy, Inc., for providing access to the Phenom XL SEM. This work made use of the Shared Experimental Facilities supported in part by the Materials Research Science and Engineering Centers Program of the National Science Foundation under award DMR-1419807.

Footnotes Author contributions: L.D.E. and Y.-M.C. designed research; L.D.E., A.F.B., M.L.C., and R.J.-Y.P. performed research; L.D.E., A.F.B., and Y.-M.C. analyzed data; and L.D.E., A.F.B., and Y.-M.C. wrote the paper.

Conflict of interest statement: Y.-M.C., L.D.E., and A.F.B. are inventors on patent applications filed by Massachusetts Institute of Technology in relation to certain subject matter in the paper.

This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Status and Challenges in Decarbonizing our Energy Landscape,” held October 10–12, 2018, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. NAS colloquia began in 1991 and have been published in PNAS since 1995. From February 2001 through May 2019 colloquia were supported by a generous gift from The Dame Jillian and Dr. Arthur M. Sackler Foundation for the Arts, Sciences, & Humanities, in memory of Dame Sackler’s husband, Arthur M. Sackler. The complete program and video recordings of most presentations are available on the NAS website at http://www.nasonline.org/decarbonizing.

This article is a PNAS Direct Submission.

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