Patent Publication Number: US-2021188711-A1

Title: Methods and systems for providing improved cement incorporating metal oxides and hydroxides

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application Nos. 62/953,251, filed Dec. 24, 2019, 63/083,311, filed Sep. 25, 2020, and 63/129,110, filed Dec. 22, 2020, which are incorporated by reference as if disclosed herein in their entireties. 
    
    
     BACKGROUND 
     Magnesium oxide (MgO)-based cement has over 150 years of history since Sorel cement was invented by Santislas Sorel in 1866. Typically, MgO-based cement is made by mixing calcined MgO powders with various types of concentrated solutions, such as magnesium chloride (MgCl 2 ), magnesium sulfate (MgSO 4 ), and phosphoric acid (H 3 PO 4 ), and forming the binary hydrated product (such as 5MgO.MgCl 2 .13H 2 O). When calcined reactive MgO is mixed with plain water, reactive MgO powder will react with water to form brucite (Mg(OH) 2 ), which has a porous structure and is typically quite low in strength. However, there exists a reawakened interest in MgO—H 2 O systems as they exhibit the ability to absorb CO 2 , forming carbonated products as strong as hydrated Portland cement. Recently, various aspects of the carbonation of reactive MgO have been investigated, including carbonation condition, microstructure, additives that enhance hydration and carbonation, durability, and life cycle assessment. MgO—H 2 O systems hold great promise as low-carbon alternative binders to replace conventional Portland cement in application, including porous blocks, concrete, and ground improvement. 
     The signs of global warming and climate change, such as melting glaciers, flooding, and heat waves, has sparked significant research efforts towards developing various technologies to stop or slow down the impacts of these climate disasters. Increasing levels of greenhouse gases like carbon dioxide (CO 2 ) in the atmosphere is the primary reason for global warming. Today, concrete is the second most consumed substance in the world after water, and its main ingredient, Portland cement, is the second highest industrial source of CO 2  on the planet. If nothing changes in the way cement is manufactured, cement is posed to contribute an even higher percentage of carbon emissions as concrete demand continues to increase, especially in developing countries. 
     Increased carbon dioxide (CO 2 ) emissions is the most significant environmental challenge of this century. Manufacturing of cement is responsible for about 5˜7 percent of global greenhouse gas emissions. Conventional feedstock for Portland cement manufacturing is limestone (CaCO 3 ), which is excavated and crushed. Then it is sintered with other materials (e.g., clays) at temperatures reaching ˜1450° C. in a cement kiln to produce clinker. This process directly releases CO 2  during calcination of CaCO 3  (such as via CaCO 3 →CaO+CO 2 ), which makes up ˜50-60% of the total emissions of Portland cement production. 
     Several cementitious materials such as alkali-activated materials, reactive belite, and carbonate binders have been developed to lower the carbon footprint of concrete. Carbon dioxide sequestration in concrete has recently been proposed as a potential solution due to the fact that CO 2  reacts with cations such as Ca 2+ , Mg 2+ , and Li 2+  that are present in cement to form highly stable carbonates, which can also improve mechanical properties over time. However, the potential CO 2  storage capacity of Portland cement concretes is significantly lower than the CO 2  emitted during their production. Therefore, the development and production of eco-efficient alternatives to carbon-intensive Portland cement is important. 
     Mg-based cements are promising given the high abundance of Mg in the earth&#39;s crust and seawater, which can be excavated or precipitated in forms such as magnesite, dolomite, and magnesium hydroxide. Although many forms of Mg-based cements exist, as discussed above, reactive magnesium oxide or magnesia (MgO) is best known for its ability to react with CO 2  in the presence of water to form carbonates. The key reactions are as follows: 
       MgO+H 2 O→Mg(OH) 2   (Brucite)
 
       Mg(OH) 2 +CO 2 +2H 2 O→MgCO 3 .3H 2 O  (Nesquehonite)
 
       5Mg(OH) 2 +4CO 2 →4MgCO 3 .Mg(OH) 2 .4H 2 O  (Hydromagnesite)
 
       5Mg(OH) 2 +4CO 2 +H 2 O→4MgCO 3 .Mg(OH) 2 .5H 2 O  (Dypingite)
 
     When sourced from magnesite (MgCO 3 ), it has been found that the carbon footprint of the resultant concrete can be higher than that of Portland cement due to the higher energy consumption of producing MgO from MgCO 3  than Portland cement clinker (4.096 GJ/ton vs. 2.52 GJ/ton, respectively). Also, similar to the calcination of limestone to produce CaO for Portland cement, calcination of MgCO 3  to produce MgO results in the direct release of CO 2 . From a sustainability perspective, the cement industry is calling for alternative binders that can reduce associated CO 2  emissions without compromising performance. 
     SUMMARY 
     Some embodiments of the present disclosure are directed to a method of making a cementitious composition including providing an aqueous source of metal ions, providing an aqueous demineralized feedstream to an electrochemical device, isolating an alkaline component and an acidic component from the aqueous demineralized feedstream, mixing the aqueous source of metal ions and the alkaline component to form an aqueous mixture, and precipitating metal hydroxides from the aqueous mixture to form a metal hydroxide product and a demineralized alkaline component, the metal hydroxide product including metal ions from the aqueous source of metal ions. In some embodiments, the method includes recycling at least a portion of the demineralized alkaline component to the electrochemical device as the aqueous demineralized feedstream. In some embodiments, the method includes combining the acidic component with at least a portion of the demineralized alkaline component as a demineralized aqueous source product. In some embodiments, the method includes incorporating the metal hydroxide product into a cementitious composition. In some embodiments, the aqueous source includes seawater, brine, brackish water, wastewater, or combinations thereof. In some embodiments, the metal ions include magnesium, calcium, or combinations thereof. 
     In some embodiments, incorporating the metal hydroxide product into a cementitious composition includes providing a composition of metal hydroxide product and contacting the metal hydroxide product composition with a source of carbon dioxide and a source of H 2 O. In some embodiments, contacting the metal hydroxide product with a source of carbon dioxide and a source of H 2 O includes providing H 2 O to the metal hydroxide product to form a slurry having a water-to-solids ratio of about 0.2 to about 0.3 by mass, compacting the slurry at a pressure to form a compacted slurry, and exposing the compacted slurry to an environment having a concentration of carbon dioxide of at least about 20%. In some embodiments, the pressure is about 3 MPa. In some embodiments, the compacted slurry is exposed to the carbon dioxide for about 2 to about 5 days. In some embodiments, the step of contacting the metal hydroxide product with a source of carbon dioxide and a source of H 2 O are repeated to additively form a multilayered concrete structural element. In some embodiments, contacting the metal hydroxide product with a source of carbon dioxide and a source of H 2 O includes pelletizing the metal hydroxide product composition with sprayed water and exposing the pelletized metal hydroxide composition to the source of carbon dioxide. 
     Some embodiments of the present disclosure are directed to a method of making a cementitious composition including providing an aqueous source of metal ions, providing an aqueous demineralized feedstream to an electrochemical device, isolating an alkaline component and an acidic component from the aqueous demineralized feedstream, mixing the aqueous source of metal ions and the alkaline component to form an aqueous mixture, precipitating metal hydroxides from the aqueous mixture to form a metal hydroxide product and a demineralized alkaline component, the metal hydroxide product including metal ions from the aqueous source of metal ions, heating the metal hydroxide product to form a metal oxide product, and incorporating the metal oxide product into a cementitious composition. In some embodiments, the method includes contacting at least a portion of the demineralized alkaline component with a source of carbon dioxide to form a carbonate product and providing the carbonate product to the slurry. 
     In some embodiments, incorporating the metal oxide product into a cementitious composition includes mixing the metal oxide product with a source of water to form a slurry and exposing the slurry to a source of carbon dioxide. In some embodiments, exposing the slurry to a source of carbon dioxide includes extruding a first layer of slurry in an environment having a concentration of carbon dioxide of about 20% and extruding at least one subsequent layer of slurry over the first layer of slurry. In some embodiments, incorporating the metal oxide product into a cementitious composition includes pelletizing the metal oxide product with sprayed water and exposing the pelletized metal oxide product to carbon dioxide. 
     Some embodiments of the present disclosure are directed to a system for making a cementitious composition including an aqueous source of metal ions; a membraneless electrolyzer, including a first input in fluid communication with an aqueous demineralized feedstream, a first outlet stream including an alkaline component, and a second outlet stream including an acidic component; a precipitation tank in fluid communication with the first outlet stream and the aqueous source of metal ions, the precipitation tank including a demineralized alkaline component outlet stream; a neutralization tank in fluid communication with the second outlet stream and the demineralized alkaline component outlet stream, the neutralization tank having a third outlet stream including a demineralized aqueous source product; and a recycle stream in fluid communication with the demineralized alkaline component outlet stream and the first input to provide least a portion of the demineralized alkaline component outlet stream to the membraneless electrolyzer as the aqueous demineralized feedstream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a system for making a cementitious composition according to some embodiments of the present disclosure; 
         FIG. 2A  is a chart of method of making a cementitious compositions according to some embodiments of the present disclosure; 
         FIG. 2B  is a chart of method of making a cementitious compositions according to some embodiments of the present disclosure; 
         FIG. 3A  is a chart of method of making a cementitious compositions according to some embodiments of the present disclosure; 
         FIG. 3B  is a chart of method of making a cementitious compositions according to some embodiments of the present disclosure; 
         FIG. 4A  is a graph of thermogravimetric analysis (TGA) results for standard Mg(OH) 2  and seawater-derived Mg(OH) 2  according to some embodiments of the present disclosure; 
         FIG. 4B  is a graph of X-ray diffraction (XRD) results for standard Mg(OH) 2  and seawater-derived Mg(OH) 2  according to some embodiments of the present disclosure; 
         FIG. 5  is a graph of particle size distribution of standard vs. seawater-derived precipitated Mg(OH) 2  according to some embodiments of the present disclosure; 
         FIG. 6A  is a graph of TGA results comparing standard versus seawater-derived samples cured in a pure CO 2  environment according to some embodiments of the present disclosure; and 
         FIG. 6B  is a graph of XRD results comparing standard versus seawater-derived samples cured in a pure CO 2  environment according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , some embodiments of the present disclosure are directed to a system  100  for making a cementitious composition. In some embodiments, system  100  produces one or more feedstocks for incorporation into and improvement of traditional cementitious compositions. In some embodiments, system  100  produces one or more feedstocks for use in novel cementitious compositions. The cementitious compositions produced by system  100  can then be used to create concrete structural elements, as will be discussed in greater detail below. In some embodiments, the feedstocks produced by system  100  include one or more metal hydroxides, one or more metal oxides, compositions including the one or more metal hydroxides, compositions including the one or more metal oxides, or combinations thereof. In some embodiments, the feedstocks produced by system  100  include one or more metal carbonates, compositions including the one or more metal carbonates, or combinations thereof. In some embodiments, the metal hydroxides and/or metal oxides include one or more metal ions. In some embodiments, at least a portion of the metal ions are from an aqueous source via a process that will be discussed in greater detail below. In some embodiments, the metal ions are any metal ion that, as part of a hydroxide or oxide compound, are suitable for use in cement and/or in concrete structural elements. In some embodiments, the metal ions include magnesium ions, calcium ions, or combinations thereof. In some embodiments, the aqueous source is any source having the above-identified metal ions dissolved therein. 
     Still referring to  FIG. 1 , system  100  includes an aqueous source of metal ions  102 . In some embodiments, the aqueous source includes seawater, brine, brackish water, wastewater, or combinations thereof. In some embodiments, the metal ions include magnesium ions, calcium ions, or combinations thereof. In some embodiments, system  100  includes one or more electrochemical devices  104 . In some embodiments, electrochemical device  104  includes at least a first input  104 A that is in communication with an aqueous demineralized feedstream. In some embodiments, the aqueous demineralized feedstream includes a reduced concentration of potential scalants, foulants, etc., or combinations thereof. In some embodiments, the aqueous demineralized feedstream includes a reduced concentration of metal ions. In some embodiments, electrochemical device  104  includes at least one outlet stream  104 B. In some embodiments, electrochemical device  104  includes a plurality of outlets streams  104 B. In some embodiments, at least one of outlet streams  104 B is includes an alkaline component, e.g.,  104 B′ from  FIG. 1 . In some embodiments, at least one of outlet streams  104 B is includes an acidic component, e.g.,  104 B″ from  FIG. 1 . 
     Electrochemical device  104  includes any suitable components to generate alkaline component  104 B′ from the aqueous demineralized feedstream and isolate the alkaline component from acidic component  104 B″. In some embodiments, electrochemical device  104  includes one or more electrolyzers, electrodialysis cells, or combinations thereof. In some embodiments, electrochemical device  104  is a membraneless electrolyzer. In some embodiments, aqueous source  102  is fed continuously, semi-continuously, as a batch process, or combinations thereof, to electrochemical device  104 . In some embodiments, to aid in precipitation of the metal ions from aqueous source  102 , additional hydroxide is added to the aqueous source, alkaline component  104 B′, or combinations thereof. In some embodiments, the additional hydroxide is NaOH or other suitable hydroxide. 
     Still referring to  FIG. 1 , in some embodiments, at least one outlet stream  104 B is in fluid communication with a precipitation tank  106 . In some embodiments, an amount of aqueous source  102  is fed to precipitation tank  106  to mix with alkaline component  104 B′ from outlet stream  104 B to form an aqueous mixture. In some embodiments, precipitation tank  106  includes an initial source of hydroxyls. In some embodiments, the initial source of hydroxyls aids in forming the aqueous mixture with aqueous source  102  where there is little to no alkaline component  104 B′ available. In some embodiments, the initial source of hydroxyls includes a concentration of KOH, NaOH, etc., or combinations thereof. Precipitation tank  106  can be of any suitable configuration to allow precipitation and isolation of a product from the aqueous mixture. In an exemplary embodiment, Mg 2+  ions can be converted into solid Mg(OH) 2  (brucite) through the following chemical reaction with OH − : 
       Mg 2+ +2OH − →Mg(OH) 2(s)  K sp   o =5.61×10 −12  M 3  
 
     where K sp   o  is the solubility product constant at 25° C. Consistent with Le Chatelier&#39;s principle, increasing pH shifts equilibrium of this reaction to the right, leading soluble Mg 2+  to rapidly convert into insoluble Mg(OH) 2  particles. Thus, by splitting the aqueous demineralized feedstream into reduced-pH acidic component  104 B′ and elevated-pH alkaline component  104 B′ via an electrolysis process and subsequent mixing of the alkaline component with aqueous source  102 , system  100  is able to induce precipitation of metal ions from that source, e.g., in the form of Mg(OH) 2  from the aqueous mixture with the Mg component coming from aqueous source  102 , which can then be collected as a product and repurposed. 
     As discussed above, in some embodiments, outlet stream  104 B includes alkaline component  104 B′. Alkaline component  104 B′ is provided to precipitation tank  106 , raising the pH and causing metal ions to convert to insoluble compounds and precipitate out of solution, forming a precipitated product and a demineralized stream, e.g., demineralized alkaline component outlet stream  106 B. In some embodiments, the precipitated product is separated from the demineralized stream via a separation module. The separation module can have any suitable configuration to isolate the precipitated product from the demineralized stream. In some embodiments, the separation module is integrated with precipitation tank  106 . In some embodiments, the separation module is a component of system  100  separate from precipitation tank  106 . In some embodiments, the separation module includes a rotary drum filter. In some embodiments, the precipitated product is removed from precipitation tank  106  continuously, semi-continuously, in a batch process, or combinations thereof. In some embodiments, precipitation tank  106  includes a product stream  106 A for removing the precipitated product from the tank. In some embodiments, the precipitated product includes one or more compounds including metal ions from aqueous source  102 . In some embodiments, the one or more compounds include metal hydroxides. In some embodiments, the metal hydroxides are incorporated into a cementitious composition, further processed for use in a cementitious composition, or combinations thereof, as will be discussed in greater detail below. 
     Referring again to  FIG. 1 , in some embodiments, as discussed above, precipitation tank  106  includes a demineralized alkaline component outlet stream  106 B. In some embodiments, at least a portion of demineralized alkaline component outlet stream  106 B (e.g.,  108 A in  FIG. 1 ) is recycled to electrochemical device  104  at input  104 A as the aqueous demineralized feedstream, where it again undergoes electrolysis, producing additional acidic component  104 B″ and alkaline component  104 B′. In some embodiments, a separate aqueous demineralized feedstream from any suitable external source (not pictured) can be provided to electrochemical device  104 , either in addition to or in place of stream  108 A. In some embodiments, at least a portion of demineralized alkaline component outlet stream  106 B (e.g.,  108 B in  FIG. 1 ) is fed to a neutralization tank  110 . In some embodiments, an outlet stream  104 B including an acidic component  104 B″ is also fed to neutralization tank  110 . In these embodiments, the addition of acidic component  104 B″ aids in neutralizing the pH of the portion of demineralized alkaline component outlet stream  108 B in neutralization tank  110  and produce a demineralized aqueous source product stream  110 A. In some embodiments, demineralized aqueous source product stream  110 A is suitable to be returned to the origin of aqueous source  102 , e.g., the ocean, sent to a water desalination plant, or combinations thereof. Demineralized seawater is of great interest for the water desalination industry because alkali earth metal hydroxides are well-known to result in scaling on the surfaces of membranes of water desalination units. In some embodiments, a portion of an acidic component  104 B″ is first removed as excess acid at  108 C. In some embodiments, the reduced pH of acidic component  104 B″ causes a concentration of carbon dioxide to come out of solution of acidic component  104 B″ as carbon dioxide effluent stream  108 D. 
     As discussed above, the precipitated product produced by system  100  is suitable for use in the production of cement. In an exemplary embodiment, metal hydroxides from precipitation tank  106  can be used as a substitute for limestone and other traditional feedstocks in the production of cement, which can help cut the carbon footprint by more than half while still utilizing existing plant infrastructure and mixing/building standards. In other exemplary embodiments, metal hydroxide and/or metal oxide products can be used as binders of inert aggregate for the formation of concrete and concrete structural elements having desired conformations. In some embodiments, at least a portion of carbon dioxide effluent stream  108 D is reused in processing of the precipitated product into cementitious compositions and/or concrete structural elements, as will be discussed in greater detail below. 
     Referring now to  FIG. 2A , some embodiments of the present disclosure are directed to a method  200  of making a cementitious compositions, concrete structural elements, etc., or combinations thereof. At  202 , an aqueous source of metal ions is provided. As discussed above, in some embodiments, the aqueous source includes seawater, brine, brackish water, wastewater, or combinations thereof. In some embodiments, the aqueous source is naturally occurring, i.e., is sourced from nature. In some embodiments, the aqueous source is the product of a synthetic process, e.g., waste effluent from an industrial process. At  204 , an aqueous demineralized feedstream is provided to an electrochemical device. At  206 , an alkaline component and an acidic component are isolated from the aqueous demineralized feedstream. In some embodiments, the alkaline component and the acidic component are generated via an electrolysis process and isolated via the electrochemical device. In some embodiments, the electrochemical device includes one or more electrolyzers, electrodialysis cells, or combinations thereof. In some embodiments, the electrolyzer is membraneless. Exemplary electrolyzers and electrolyzer systems suitable for use with the systems and methods of the present disclosure are described in U.S. Utility Pat. No. 10,844,494, which is incorporated by reference herein in its entirety. At  208 , the aqueous source of metal ions and the alkaline component are mixed to form an aqueous mixture. At  210 , metal hydroxides are precipitated from the aqueous mixture to form a metal hydroxide product and a demineralized alkaline component. As discussed above, the rising pH in the aqueous mixture shifts the equilibrium of metal ion conversion to metal hydroxides towards the metal hydroxide products, which then precipitate out of solution. In some embodiments, the reaction converting Ca 2+  to Ca(OH) 2  is shifted via the rising pH towards Ca(OH) 2 , causing metal ions from the aqueous source to precipitate out in the form of Ca(OH) 2 . In some embodiments, the reaction converting Mg 2+  to Mg(OH) 2  is shifted via the rising pH towards Mg(OH) 2 , causing metal ions from the aqueous source to precipitate out in the form of Mg(OH) 2 . Precipitation of the metal hydroxide product also produces a demineralized alkaline component with a reduced concentration of metal ions. This demineralized alkaline component is useful because it can be reused in method  200  itself, but also because it can be processed into a demineralized product, e.g., demineralized seawater. As discussed above, demineralized seawater can be used to limit scaling on the surfaces of membranes of water desalination units, but can also simply be returned to the sea. At  212 , at least a portion of the demineralized alkaline component is recycled to the electrochemical device as the aqueous demineralized feedstream. The demineralized alkaline component can then undergo additional electrolysis to generate higher pH alkaline components for subsequent precipitation of more metal hydroxide product at subsequent steps  210 . At  214 , at least a portion of the demineralized alkaline component is combined with the acidic component from the electrochemical device to neutralize the pH of the demineralized alkaline component, resulting in a demineralized aqueous source product, e.g., the demineralized seawater discussed above. 
     At  216 , the metal hydroxide product is incorporated into a cementitious composition. While the exemplary embodiments of the present disclosure describe incorporating metal hydroxide and metal oxide products that have been isolated from aqueous sources into cementitious products, the present disclosure is not limited in this regard, as other embodiments of systems and methods of the present disclosure incorporate metal hydroxide and metal oxide products from other sources, including, e.g., reagent grade products generally available to the public from a variety of providers. In some embodiments, both reagent grade metal hydroxide and metal oxide products and aqueous solution-sourced metal hydroxide and metal oxide products are incorporated into the cementitious composition. As discussed above, the metal hydroxide products, e.g., those produced via steps  202 - 210 , are highly useful in the production of cements and concretes. In some embodiments, Ca(OH) 2  is substituted for limestone in the production of traditional Portland cement. Without the need to utilize limestone in the traditional process, the CO 2  generated in the calcination of limestone to CaO is eliminated, reducing the overall carbon cost of producing Portland cement. 
     Referring now to  FIG. 2B , in some embodiments of method  200 , incorporating  216  the metal hydroxide product into a cementitious composition includes providing  216 A a composition of metal hydroxide product. In some embodiments, at  216 B, the metal hydroxide product composition is contacted with a source of water to form a slurry. In some embodiments, the source of water is liquid H 2 O, gaseous H 2 O, i.e., water vapor, or combinations thereof. In some embodiments, the slurry has a liquid-to-solids, e.g., water-to-solids, ratio of about 0.1 to about 0.5 by mass. In some embodiments, the slurry has a liquid-to-solids ratio of about 0.2 to about 0.3 by mass. In some embodiments, the slurry has a liquid-to-solids ratio of about 0.3 by mass. In some embodiments, at  216 C, the slurry is compacted. In some embodiments, the slurry is compacted at a pressure of about 0 MPa to about 6 MPa. In some embodiments, the slurry is compacted at a pressure of about 3 MPa. At  216 D, the metal hydroxide product composition is contacted with a source of carbon dioxide. The source of carbon dioxide can be any source having a suitable concentration of carbon dioxide therein. In some embodiments, at least a portion of the carbon dioxide is sourced from the acidic component. In some embodiments, the source of carbon dioxide is waste effluent from an industrial process. In some embodiments, the source of carbon dioxide has a CO 2  concentration of about 10% to about 30%. In some embodiments, the source of carbon dioxide has a CO 2  concentration of about 20%. When exposed to moisture and CO 2 , Mg(OH) 2  converts to MgCO 3  in various form, e.g., hydromagnesite and nesquehonite, which provides strength. In some embodiments, the slurry is maintained in contact with the source of carbon, i.e., is “cured,” for greater than about 5 days. In some embodiments, the slurry is cured for greater than about 2 days. In some embodiments, the slurry is cured to about 2 days to about 5 days. In some embodiments, the temperature during steps  216 B,  216 C, and/or  216 D is generally maintained at about 25° C. In some embodiments, the relative humidity in the environment around the slurry during steps  216 B,  216 C, and/or  216 D is generally maintained at about 75% to about 85%. 
     In some embodiments, other additives are incorporated into the slurry at steps  216 B,  216 C,  216 D, or combinations thereof In an exemplary embodiment, inert aggregate is included in the slurry which, upon curing, forms a concrete structural element. In some embodiments, steps  216 A- 216 D are repeated to form multilayered concrete structural elements. In these embodiments, a first layer of slurry is provided and cured as discussed above. Then, additional layers of slurry can be provided and cured on the first layer. Layer-by layer, a concrete structural element can be formed progressively, resulting in a densified structure with enhanced strength and transport properties and substantially uniform carbon curing throughout the resulting multilayered element. 
     Referring now to  FIG. 3A , some embodiments of the present disclosure are directed to a method  300  of making a cementitious composition utilizing metal oxides. Similar to method  200  discussed above, at  302 , an aqueous source of metal ions is provided. At  304 , an aqueous demineralized feedstream is provided to an electrochemical device. At  306 , an alkaline component and an acidic component are isolated from the aqueous demineralized feedstream, e.g., via an electrolyzer and an electrolysis process. At  308 , the aqueous source of metal ions and the alkaline component are mixed to form an aqueous mixture. At  310 , as the pH rises in the aqueous mixture, metal ions from the aqueous source precipitate out as metal hydroxides to form a metal hydroxide product and a demineralized alkaline component. At  312 , the metal hydroxide product is heated to form a metal oxide product. At  314 , the metal oxide product is incorporated into a cementitious composition. 
     Referring now to  FIG. 3B , in some embodiments of method  300 , incorporating  314  the metal oxide product into a cementitious composition includes providing  314 A a composition of metal oxide product. At  314 B, the metal oxide product composition is contacted with a source of water and a source of carbon dioxide. In some embodiments, the metal oxide product composition is mixed with water to form a slurry. In some embodiments, other additives are incorporated into the slurry. In some embodiments, the additives include inert aggregates. In some embodiments, the additives include one or more salts, e.g., NaCl. In some embodiments, the additives include one or more carbonates. In these embodiments, the carbonates can be provided from any suitable source. In some embodiments, the carbonates are waste materials from another industrial system or process. In some embodiments, the carbonates are naturally occurring minerals. In some embodiments, the carbonates are generated within method  300  itself. In these embodiments, at least a portion of the demineralized alkaline component is contacted with a source of carbon dioxide to form a carbonate product, which is then added to the slurry. In an exemplary embodiment, the demineralized alkaline component includes a concentration of alkali metal hydroxides, e.g., NaOH. After allowing the metal hydroxide product to settle out of solution, a portion of the demineralized alkaline solution can be sent to a carbonation tank where CO 2  reacts with NaOH to form sodium carbonate and/or sodium bicarbonate consistent with the reactions below: 
       NaOH+CO 2 →NaHCO 3  (pH&lt;8)
 
       2NaOH+CO 2 →Na2CO 3 +H 2 O (pH&gt;10)
 
     The sodium carbonate and/or sodium bicarbonate can then be provided to the slurry. These carbonates are beneficial in that they can accelerate the hydration of the metal oxides, forming additional reaction products that can provide strength gains in the structural elements made from the cementitious compositions of the present disclosure. In some embodiments, the source of water is liquid H 2 O, gaseous H 2 O, i.e., water vapor, or combinations thereof. In some embodiments, the slurry has a liquid-to-solids, e.g., water-to-solids, ratio of about 0.1 to about 0.5 by mass. In some embodiments, the slurry has a liquid-to-solids ratio of about 0.2 to about 0.3 by mass. In some embodiments, the slurry has a liquid-to-solids ratio of about 0.3 by mass. 
     The slurry is contacted with a source of carbon dioxide. The source of carbon dioxide can be any source having a suitable concentration of carbon dioxide therein. In some embodiments, at least a portion of the carbon dioxide is sourced from the acidic component. In some embodiments, the source of carbon dioxide is waste effluent from an industrial process. In some embodiments, the source of carbon dioxide has a CO 2  concentration of about 10% to about 30%. In some embodiments, the source of carbon dioxide has a CO 2  concentration of about 20%. In some embodiments, the slurry is cured for greater than about 2 days. In some embodiments, the slurry is cured to about 2 days to about 5 days. In some embodiments, the temperature during step  314 B is generally maintained at about 25° C. In some embodiments, the relative humidity in the environment around the slurry during step  314 B is generally maintained at about 75% to about 85%. 
     In some embodiments, other additives are incorporated into the slurry at steps  314 B. As discussed above, in an exemplary embodiment, inert aggregate is included in the slurry which, upon curing, thus forms a concrete structural element. In some embodiments, steps  314 A- 314 B are repeated to form multilayered concrete structural elements. In these embodiments, a first layer of slurry is provided and cured as discussed above. Then, additional layers of slurry can be provided and cured on the first layer. In some embodiments, the layers are extruded. Layer-by layer, a concrete structural element can be formed progressively, resulting in a densified structure with enhanced strength and transport properties and substantially uniform carbon curing throughout the resulting multilayered element. 
     When exposed to moisture, the metal oxide product hardens as oxides hydrate to form hydroxides. In the embodiments utilizing carbonate additives, this hydration is accelerated. When exposed to CO 2 , hydroxides convert to hydrated carbonates in various forms, e.g., hydromagnesite and/or nesquehonite in the case of magnesium, which can provide strength gain. Several studies have shown that MgO cement can capture 1.1 ton of CO 2  per 1 ton of MgO during accelerated CO 2  curing. Thus, embodiments of method  300  produce CO 2 -negative concrete considering zero CO 2  emission during the harvesting and calcination of Mg(OH) 2 , and carbon curing to form the final solid MgO binder. 
     In some embodiments, the metal oxide and/or metal hydroxide products are incorporated into a cementitious composition by pelletizing the products with sprayed water. In these embodiments, the metal oxide and/or metal hydroxide products are combined with water as it is sprayed on a surface, such as a pelletizer disc. Product powder particles stick to each other and start to agglomerate during the pelletizing process. In exemplary embodiments with a pelletizer disc , the rotation of the disc can be maintained until the aggregates reach the desired particle size. The particles are then exposed to a carbon dioxide consistent with the embodiments described above. The result is cementitious pellets including carbonates which are lightweight and advantageous for use as aggregate replacement in concrete structural elements. 
     EXAMPLE 
     Seawater-derived Mg(OH) 2  was obtained by precipitation using 4.86 g NaOH per L of seawater and rinsed 2 times with 48 L of deionized water. The seawater-derived Mg(OH) 2  was compared to reagent grade Mg(OH) 2  (Sigma Aldrich) via TGA and XRD in  FIGS. 4A-4B , respectively. 
     The precipitated Mg(OH) 2  formed a dried filter cake and was crushed into a fine powder that was white in color, as shown in the inset in  FIG. 4B . This powder was divided into two portions—one passing 75 μm and the other passing 53 μm sieves—and the combination proportion that gave the best match to the standard brucite control powder was used. Particle size analysis of a suspension of the cement powder showed that the particle size ranged from 0.4 μm to 75 μm with the bulk of material being less than 20 μm (see  FIG. 5 ). 
     From a TGA scan of the seawater-derived cement, the presence of Mg(OH) 2  is observed. However, besides the dominant peak associated with Mg(OH) 2 , smaller peaks associated with residual water and an unidentified peak were also observed. Consistent with TGA results, XRD patterns taken of the seawater-derived precipitate match brucite Mg(OH) 2  (PDF #44-1482). The only non-Brucite reflection is associated with a minor peak around 2θ=29.5°, which is consistent with CaCO 3  (Calcite, PDF #05-0586). The unidentified peak in the TGA was thus confirmed to be CaCO 3 . The composition of raw Mg-cement by weight was then determined to be: ˜84.1% Mg(OH) 2 , ˜3.5% water, and ˜9.3% CaCO 3 . Without wishing to be bound by theory, the expected inert nature of CaCO 3  in a carbon dioxide environment implies it will exist as an impurity and will effectively reduce the “active” proportion in the cement—the brucite. This may lead a lower strength as compared to purer Mg(OH) 2 . 
     The Mg(OH) 2  powder was mixed with water to yield a water-to-solids ratio of 0.3, then cast in cylindrical molds (1 in×1 in). Each cylindrical specimen was compacted at 3 MPa for two minutes, then demolded and cured in a carbon-rich environment (20% CO 2  concentration, 25° C.±1° C. operation temperature, and 80%±5% relative humidity). The cylinders were subjected to compression loads to observe their performance and were later processed for characterization. 
     Referring now to  FIGS. 6A and 6B , to understand product formation upon curing, the cylindrical specimens were crushed back into a powder and subject to TGA and XRD. TGA scans of cured samples showed noticeable new peaks over raw samples up to ˜250° C. (see  FIG. 6A ). These are expected to be due to carbonates. The characteristic peak from residual brucite is also visible. Some peaks from carbonates overlap with brucite peak. The peaks may belong to such carbonates as hydromagnesite, dypingite, or nesquehonite but given overlaps, TGA did not distinguish them individually. 
     The presence of carbonates was confirmed via XRD in both the seawater and standard Mg(OH) 2  (see  FIG. 6B ). Results from the cured standard brucite indicate presence of hydromagnesite/dypingite. On the other hand, XRD on cured seawater magnesium hydroxide samples showed that the predominant carbonate phase was nesquehonite, unlike the standard brucite. Without wishing to be bound by theory, the difference in carbonate phases observed between the two cured systems can be attributed to differences in the crystal structure, particle shape, particle size distribution, surface roughness, and trace impurities of the source brucite, and even under the same curing conditions and mix design, the attributes of the brucite can lead to different conditions locally within the material and result in the formation of different polymorphs, where temperature, pressure, CO 2  concentration, and relative humidity will influence carbonate phase formation. 
     The compressive strength of mixtures composed of standard Mg(OH) 2  subjected to carbonation curing for 3 hours, 2 days and 5 days were obtained. Additionally, compressive strength after 2 days carbonation curing was compared between mixtures composed of standard and seawater-derived Mg-cement. Each tested mix had a water-to-binder ratio of 0.3. 
     The mass, and density, of the cylinders were obtained prior to carbonation curing and were found to be comparable: 1.49 g/cm 3  for commercial brucite and 1.44 g/cm3 for seawater-derived precipitated Mg(OH) 2 . The mass change over 2 days was found to be the same at +13%. For the standard Mg-cement, the compressive strength increased with time, where the strength was 23 MPa at 3 hours, 33 MPa at 2 days, and reached 71 MPa at 5 days. The compressive strength of seawater specimens at 2 days was 23 MPa. Without wishing to be bound by theory, this may have occurred due to the properties of both the raw material and the carbonated material, namely, a difference in particle size distribution may affect packing behavior during compaction, which in turn may affect permeability for carbonation. Additionally, the PSD and particle morphology also affect the surface area available for products to form. CO 2  curing also leads to formation of different carbonates and these have distinctly different morphologies. Further, the strength of the binding matrix depends on product morphologies. For instance, rosette like structures are more favorable for strength, i.e., better integrity, than needle like structures. The strength observed in the exemplary embodiments described above is strong enough considering the early compressive strength of ordinary Portland cement concrete. With longer curing durations, the strength of the seawater magnesium samples should continue to enhance considering the compressive strength development results of standard magnesium hydroxide groups. 
     The methods and systems of the present disclosure are advantageous in that they utilize carbonated metal hydroxides and oxides to produce compositions with compressive strengths suitable for use as construction materials. The metal hydroxides and oxides are carbonated through a curing process to sequester environmental carbon dioxide, reducing or eliminating the carbon footprint associated with traditional methods of cement and concrete manufacturing. Additionally, the produced compositions reach compressive strengths comparable to those of traditional, reactive metal oxide fabrication techniques, but with reduced water demand and elimination of highly energy intensive calcination steps. By way of example, conventional limestone feedstock for Portland cement manufacturing is excavated, crushed, and then sintered with other materials at temperatures reaching ˜1450° C. in a cement kiln to produce clinker. This traditional process directly releases CO 2  during calcination of CaCO 3  (CaCO 3 →CaO+CO 2 ), which makes up ˜50-60% of total emissions of cement production. The systems and methods of the present disclosure can produce an electrochemically harvested Ca(OH) 2  powder that is fed into existing cement plants/kilns through partial to full replacement of the limestone. By using Ca(OH) 2  as a feedstock, indirect CO 2  emissions associated with the harvesting or direct CO 2  emissions during the calcination process to obtain CaO can be eliminated. 
     Finally, the systems and methods are advantageous in that the metal hydroxides and oxides used as construction materials can be sourced from mineralized aqueous solutions. Importantly, the production of these metal hydroxide/oxide products via electrolysis can be carried out without any CO 2  emissions if the electricity used to power electrolysis is generated by renewable solar and wind. Hydroxides generated by electrolysis can then be calcined to produce oxide products as necessary, releasing steam in the process. Obtaining the metal hydroxide/oxide product has the added benefit of evolving low pH demineralized streams as a byproduct, which can be recycled to help create yet more product, or can be neutralized for use in parallel water desalination systems and processes, or for return to the sea. 
     Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.