Direct CO2 capture by alkali-dissolved cellulose and sequestration in building materials and artificial reef structures

the emission of anthropogenic CO 2 and approach a net-zero society. [1] Carbon Capture and Storage (CCS) is a reasonable climate action in this context. [1] Further, the capture and use of carbon dioxide to create valuable products (Carbon Capture and Utilization, CCU) is an attractive pathway to reduce the costs associated with emissions removal and incentivize new industrial investments. [1,2] CO 2 capture from the atmosphere has been carried out by Direct Air Capture (DAC), [3] for instance, involving physical or chemical absorption of CO 2 by alkaline solutions based on sodium hydroxide, [4] or ammonia. [5] Despite their simplicity, these systems have limited absorption capacity, are water-intensive


Introduction
The need to mitigate > 700 Gt CO2 by 2050 and meet the twodegrees scenario (2DS) proposed by the International Energy Agency (IEA) has incentivized policies designed to reduce area.From the accessible technologies, the highest retention reported (1.47 gCO 2 g −1 ) has been possible by reticular chemistry, for example, using metal-organic frameworks (MOF). [10]nfortunately, high cost, energy consumption, and poor scalability, [10] limit MOF applicability at the industrial scale, though some advances have been achieved, such as in natural gas upgrading, [11] and wet flue gas CO 2 capture. [12]imple approaches, such as direct carbon dioxide mineralization, are cost-efficient and easily scalable. [13]Moreover, industrial alkaline and solid streams are cost-effective and maximize the carbon economy. [14]Particularly, alkaline industrial streams from the cement and paper industries are attractive to capture CO 2 via direct mineralization, translating to 2100 and 187 Mt of CO 2 yr −1 , respectively (a total reduction in global anthropogenic CO 2 emissions by ≈6.8%). [14]Such options are profitable for the cement industry (32 € per tonne considering only the CO 2 mineralization). [15]Moreover, given the multiple synergies between cementitious materials and cellulose fibers from the pulp and paper industry, it is reasonable to expect new opportunities to emerge in construction materials, allowing better CO 2 mitigation (and economic) outcomes. [16]We note that pathways involving added-value chemicals might reduce carbon dioxide emissions but have limited removal potential.By contrast, construction materials can both utilize and remove carbon dioxide. [2]Within this latter context, cellulose mineralization offers an unprecedented opportunity for CO 2 removal and storage, for instance, in building materials.Cellulose a biopolymer that can be sourced from plants and biomass residuals, [17,18] has been reported for uses in advanced materials and energy-efficient construction. [19,20]Importantly, using mineralized cellulose as a building matrix may add new functionalities and properties to materials such as cement, [15] ceramic materials, [21] or even coral reef stones, [18,22] which are subjects of the present work.
Motivated by the recently reported ability of alkali-dissolved cellulose to absorb CO 2 , [23,24] we propose Mineralized Cellulose Materials (MCM) by direct mineralization in solutions in the presence of stoichiometric amounts of Ca(OH) 2 , [24] as a Direct Air Capture strategy.Here, a significant challenge is the presence of moisture in the air, which hinders absorption and delays the mineralization kinetics as a consequence of cellulose gelation. [25]n the present study, we used CO 2 gas to produce MCM in alkali conditions under a controlled atmosphere. [24]Upon dissolution, the mineralization process took place at room temperature, in a single step, under continuous agitation, and until reaching CO 2 solution absorption capacity (saturation or constant weight), Figure 1a. [24]e found that the CO 2 absorption capacity tracked inversely with cellulose concentration, reaching a maximum of 6.5 gCO 2 g cellulose −1 (61 mgCO 2 g solution −1 ) at 1 wt.%, Figure 1b.This cellulose-lean MCM precursor (MCMl) resulted in a solid powder that was effective as a flux material for ceramic glazes, improving the glaze viscosity and reducing glaze cracking.The same formulation was used in a cement paste, demonstrating workability and dense microstructure, suggesting great prospects in building materials.By increasing the cellulose loading (7 wt.% cellulose), moldable/printable pastes were produced (MCM of high cellulose content, MCMh), which solidified as tough (stone-like) materials that reached a maximum compressive strength of 31 MPa at a 30% strain.This latter material was deployed as a substitute for coral stones in the Gulf of Mexico, allowing the implantation of three coral species that grew healthy for at least nine months.
A cradle-to-gate LCA analysis showed that MCM has a specific global warming potential (GWP) of −0.74 gCO 2 eq g −1 .
Results indicated a further GWP between 0.9 and 7.2 gCO 2 eq g −1 , including a cellulose-dissolving process (upstream stage), representing up to 62% reduction of the environmental impact originated at the dissolution stage.
In sum, MCM utilization as building material brings new opportunities for residual alkaline streams, expanding the prospects of cellulose to reduce emitted CO 2 and achieve ocean coral reef ecosystem restoration.

MCM Physicochemical Properties
Alkali-dissolved cellulose was prepared at different weight solid concentrations, leading to samples that were thereafter referred to as SN (where N is a digit corresponding to cellulose concentration, 0, 1, 3, 5, 7 wt.% in the prepared dope, see Experimental Section).The dissolved cellulose was observed by optical microscopy, which indicated an off-white, homogeneous fluid (Figure 1a; Figure S1, Supporting Information).The complete dissolution of cellulose in alkali conditions can be perceived as a clear solution, but variations ranging from transparent, translucent to off-white also occur, depending on physicochemical conditions such as temperature, [24,26] CO 2 concentration, [27] carbohydrates composition, [28] and gelation state. [29]After dissolution, stoichiometric amounts of Ca(OH) 2 , were added according to the respective CO 2 absorption capacity. [24]Simultaneously, CO 2 gas was bubbled, leading to cellulose coagulation and mineralization, [24,27] generating a two-phase system containing solid (non-soluble minerals) and aqueous (soluble minerals) phases (Figure 1a).The CO 2 absorption by cellulose dissolved in NaOH(aq) solutions has been described as a chemisorption process, as discussed by Gunnarsson et al. [23,30,31] , who also indicated that the hydroxyl group on the C6 carbon of cellulose reacts to carbonate and carbonate ions, leading to a small pH drop (from 13.90 to 13.46).The present study confirmed this small pH drop, maintained even after adding Ca(OH) 2 during the CO 2 absorption process.The small pH drop promotes cellulose coagulation, as shown in Figure S1 (Supporting Information).Hence the cellulose coagulation coincides with CO 2 absorption and Ca(OH) 2 addition.Continuous pH monitoring was not carried out, but the initial and final pH values were recorded.The final pH value, after CO 2 saturation, fell to pH = 12, for all samples, in agreement with the values reported by Kozlowski et al. [27] Lastly, a mineralized heterogeneous material, rich in cellulose and calcium carbonate (Figure 1a), was obtained by decantation.The water and soluble minerals were separated and recycled from the upper phase, rich in Na(OH), and soluble carbonates.The presence of ZnO in cellulose alkali has been suggested to exist as Zn(OH) 4 ; such hydroxide forms stronger

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© 2023 The Authors.Advanced Materials published by Wiley-VCH GmbH hydrogen bonds with cellulose compared with hydrated NaOH, enhancing the dissolution power. [32]Furthermore, Väisänen et al. [33] suggested that the addition of ZnO in this system aids the dissolution of cellulose and delays self-aggregation by coordinatively binding to cellulose chains' C2 and C3 OH groups, forming a ring-like structure similar to zinc glycerolate.The latter observations suggest a low probability of the reaction of zinc to carbonates compared to the more reactive, available ions such as sodium and calcium.However, the coprecipitation of zinc carbonates is possible under the mixture conditions. [34]In the present study, the FTIR and WAXS fingerprints of such carbonates might be obscured by the presence of other calcium or sodium carbonates or even not bonded ZnO.Hence, more detailed studies are needed for proper elucidation.
Figure 1b includes the absorption capacity of the systems with different cellulose content, and

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© 2023 The Authors.Advanced Materials published by Wiley-VCH GmbH the stoichiometric addition of Ca(OH) 2 and CO 2 absorption capacity; additionally, Table S1 (Supporting Information) presents the mechanical properties and morphologies of the MCM samples (S0-S7) after drying.
The amount of calcium hydroxide added was calculated according to the absorption capacity, [24] and following the stoichiometric conversion to calcium carbonate Equation 1: [35,36] The CO 2 adsorbed exceeded the stoichiometric conversion of calcium hydroxide to calcium carbonate (Figure 1b and Table 1) due to the chemical equilibrium involving CO 2 , and CaCO 3 in alkali rich environment. [27]At low cellulose concentrations, nonassociated Na + existed at a high concentration, [26] which reacted with carbonic acid, undergoing dissociation, [35] and producing different sodium carbonates and bicarbonates. [27,37]In contrast, at high cellulose concentrations (≥5 wt.%), there were no available Na + .Hence, CO 2 primarily reacted with available calcium ions once absorbed.
The chemical composition of MCM was studied by Fourier Transform Infrared Spectroscopy, FTIR (Figure S2, Supporting Information), which revealed a peak ≈1400 cm −1 for all samples (of decreasing intensity for S3-S7), attributed to bicarbonate ions. [39]The presence of cellulose was evidenced by the vibrational band at ≈1060, attributed to the CH-O-CH 2 group, and the broad peak associated with OH vibration, between 3000 and 3500 cm −1 . [40]The typical carbonate ion stretching vibration, [40] appearing for calcite at 1460 cm −1 , was absent for the sample without calcium hydroxide (SO*).This signal intensity was inversely correlated with the cellulose concentration.
According to Figure S2 (Supporting Information), the formation of carbonate-based compounds depended on the level of adsorbed CO 2 and the availability of reactive calcium or sodium ions.In general, sodium bicarbonate and calcite carbonate peaks were more intense at low cellulose concentrations.
The apparent difference in chemical compositions found in MCM samples suggests their possible uses.For instance, mixtures rich in carbonate powder were considered for fastcuring cement-based materials, [41] or in ceramic bodies and ceramic glazes. [42]Meanwhile, cellulose-rich MCMh was more suitable for molding and printing hard calcium/carbonate composites. [43]CM rheological behavior is a crucial aspect to consider for its deployment.As a consequence, the storage (G′) and loss (G″) moduli of the decanted MCM (15 wt.%) were measured at 1% strain, with frequencies between 0.1 and 100 rads −1 , Figure 1c.The dynamic viscosity was also assessed at 25 °C, Figure S3 (Supporting Information).
The storage modulus increased (from 10 to 10 5 Pa), tracking with cellulose loading (from 0 to 7 wt.%), Figure 1c.Similar values have been measured for TEMPO-oxidized nanocellulose. [44]Furthermore, samples S3 and S5 showed a shearthinning behavior (Figure S3, Supporting Information), confirming their suitability for molding/printing.In contrast, S1 did not display a stable solid-like structure (G″ > G′), and sample S7 presented a non-uniform viscosity typical of heterogeneous mixtures. [45]aking advantage of the flow behavior, the decanted MCM samples were dried in molds (Figure S4, Supporting Information).The compressive strength of the heterogeneous carbonated material was measured at a 30% strain right before reaching a strength plateau. [46]Sample S1 presented a fragile powder structure with an average Young's modulus of 3.4 MPa and average compressive strength of 0.13 MPa.The compressive strength reached a maximum of 31 MPa for sample S5.Samples S3 and S7 performed similarly, with average compressive strength of 14 and 16 MPa, respectively (Figure 1d).Conversely, a higher drying shrinkage was noted for the MCM samples of higher mechanical performance, with a volumetric shrinkage ratio of up to 85% for S3-S7 samples.The mechanical performance of the MCM samples was noted to lie between that of Flexible Polymer Foams (M.D.) [47] and polyethylenebased materials. [48]MCM showed a better performance compared to cork, [49] and carbon black reinforced styrene-butadiene rubber (SBR), [48] Figure S5 (Supporting Information).
The MCM materials revealed a wide window of porosities as measured by the ethanol adsorption method, [50] from 74% (sample S0) to 18% (sample S7).This result reflects that the cellulose-rich samples underwent a significant shrinkage during drying, yielding compact structures with few open pore areas.In contrast, all the samples exhibited similar BET pore size,   S1, Supporting Information), implying a significant porosity contribution assigned to the macroscopic pore size. [51]he chemical composition affected the MCM microstructure due to the complex mineralization process in the alkalidissolved cellulose environment.Figure 2 and Figure S6 (Supporting Information) show the mineral species identified by Wide Angle Scattering (WAXS).
In particular, Figure 2a presents the scattering peaks for Sample S5, which reveals the presence of cellulose II hydrate (green color), [52] crystallized calcite (blue color), [53] and vaterite (light blue color). [54]The diffraction spectra identified other mineral species, mainly composed of sodium-derived carbonates such as sodium carbonate, [37] sodium carbonate hydrate, [55] and gaylussite. [56]Figure 2a and Figure S6 (Supporting Information) revealed the unique composition of Sample S5, with a wide variety of calcium and sodium carbonates.The formation of highly crystalline calcium carbonate (calcite and vaterite polymorphs) was favored at higher cellulose concentrations, confirming FTIR observations (Figures S2, S6, and S7, Supporting Information).By contrast, sodium carbonates were dominant at low cellulose concentrations (<3 wt.%).The thermal degradation profiles confirmed the noted chemical compositions.Three distinctive thermal stages were observed: stage I, between 80 and 180 °C, corresponds to carbonate dehydration reactions and sodium bicarbonate thermal degradation into sodium carbonate, CO 2, and water. [55,57]Stage I evidenced peaks for samples S1 and S3.Stage II, between 210 and 310 °C, is associated with cellulose thermal degradation, [58] which is prominent in cellulose-rich samples, such as S7.This sample showed a small broad peak ≈400 °C, associated with vaterite degradation, especially notorious for biogenic vaterite. [59]The last stage, stage III, from 680 °C up to 790 °C, corresponded to calcium carbonate degradation into calcium oxide and CO 2 , [60] as shown for samples with a higher calcium carbonate content (Figure 2b; Figure S6, Supporting Information).
The X-ray diffraction peaks for MCM S0-S7 (Figure 2a; Figure S6a, Supporting Information) confirmed the appearance of calcium carbonate framboids, [35] correlated with cellulose content (Figure 2c,d,f; Figure S6b, Supporting Information).These framboids (turquoise color, Figure 2c) were embedded in the cellulose matrix (green color, Figure 2c), which were the places for calcite formation, starting from amorphous calcium carbonate. [38,53]The framboid structures (Figure 2d) were built up from smaller vaterite aggregates that appeared from the nucleation mechanism promoted by CO 2 diffusion, [35] Figure 2f.It is expected that the first spherical particles nucleated at the interface between the alkali solution and the CO 2 gas bubble.During the nucleation, CO 2 was consumed, and new particles were formed and transported, building a hierarchical and chiral structure, framboids. [61,62]In this study, the framboids detected in samples S5 indicated the coexistence of adjacent imbricated, calcitic plate layers oriented in the counterclockwise direction, resembling the natural coccolith skeletal structure found in Umbilicosphaera foliosa. [63]These chiral helicoids exhibited a plate inclination of ≈150 o , Figure 2d.
2D-Wide Angle X-ray Scattering (WAXS, Figure S7, Supporting Information) shows the isotropic lattice orientation of calcium carbonate crystallites, mainly from vaterite polymorph planes (3 1 0).The appearance of the isotropic azimuthal intensities for vaterite planes correlates with a low cellulose concentration (appearing only for samples S0-S5).In contrast, sample S7 possesses well-defined and homogenous azimuthal peak distributions with no indication of vaterite peaks (Figure S7, Supporting Information).Nevertheless, this aspect deserves further exploration, considering a detailed lattice contribution at different synthesis conditions, such as temperature and calcium carbonate polymorph concentrations.
The appearance of these isotropic structures in samples S0-S5 explains its outstanding mechanical performance.The latter correlated with a low calcite/cellulose ratio (Figure S6c, Supporting Information) and a well-balanced occurrence of calcite/ vaterite framboids.The samples with low cellulose concentrations showed a higher sodium carbonate and ZnO content (Figure 2e), displaying poorer mechanical strength.The superb mechanical properties of some natural materials involving cellulose or biopolymers usually result from highly ordered, multiscale, and hierarchical architectures, [64] mimicking nacrelike structures. [65]The latter also relates to the cohesive forces between supramolecular cellulose fibril bundles and virtually any type of particle. [66]These samples could be used in cementitious, [67] and ceramic [68] materials, as discussed next.

MCM's Addition to Ceramic Glaze and Cement
The dry MCM samples were added to a ceramic cracking glaze formulation.The samples (SN) replaced the total calcium carbonate amount (see methods).The ceramic glaze formulations were applied to bisque-fired (900 °C) precooked ceramic test tiles (10.5 × 5.5 cm).The test tile's visual appearances are shown in Figure 3a (left); Figure 3a (right) also shows the results for ceramic glaze viscosity (Experimental Section).
The glaze tests showed a dramatic reduction of the glaze cracks compared to the reference sample (reference, Ref).Cellulose loading improved the appearance of the glaze and reduced the surface cracks.Nevertheless, the samples with the highest cellulose content (S5 and S7) exhibited bulky agglomerations of glaze (S5, Figure 3a).Sample S3 presented the best appearance regarding crack reduction and glaze coverage (see also Figure S8, Supporting Information).The viscosity and ceramic porosity (Table S2, Supporting Information) confirmed that Sample S3 had better flowability and lower ceramic porosity, making this material a suitable flux additive for ceramic bodies. [69]Furthermore, adding 10 wt.% of the dry MCM samples to a liquid cement paste revealed similar changes in the material structure (Figure 3b-d).
The SEM observations revealed that the addition of Samples S0-S3 promoted the formation of a dense microstructure, with a slight increase in porosity and decreased amount of Ca(OH) 2 compared to the reference cement material (Figures S9-S11, Supporting Information).Moreover, all samples presented similar flowability compared to the reference cement paste (Figure S12, Supporting Information).
The reduction or refinement of Ca(OH) 2 improved the durability of the cementitious composites, given its highly-soluble hydrate nature. [70]In contrast, samples S5 and S7 stimulated the formation of pores with large diameters > ≈30 um, Figure 3b.The inspected pores were loosely filled with agglomerations of cement hydrates, which could be promoted by the high cellulose content.The cement hydrates primarily form on these available porous structures. [71]Cellulose hydrophilicity promotes an ideal environment where water molecules react with calcium ions from the mineralized paste and available calcium ions from cement grains producing calcium hydroxide. [72]dditionally, in the presence of air bubbles, calcium hydroxide can be transformed into carbonate following the reaction presented in Equation 1.The EDX analyses in Figure 3c support the formation of hydration phases.The point EDX analysis of these hydrates indicated a concentration of Ca + > 85%, suggesting the formation of Ca(OH) 2 and CaCO 3 (Figures S10 and  S11, Supporting Information).Based on the TGA/DTG analysis (Figure 3d), it is concluded that the amount of carbonate increased with cellulose concentration, confirming the high CaCO 3 content compared to the reference cement paste.The calcium existing in the loose formations in the cement matrix provided an easily accessible source and alkaline environment that could be beneficial for applications related to the production of coral stones, see next. [73]v.Mater.2023, 35, 2209327

MCMs for Coral Reef Stones
The MCMh material from sample S5 was chosen as a source of coral reef stones, given its flow properties and suitable mechanical performance.Sample S5 was molded and printed (Figure 4a,b).The samples, named Coral Stones (CS), were used without further modifications to implant coral larvae.A preliminary test was conducted with two juvenile Balanophyllia elegans pacific cup corals.The test indicated CS as suitable for coral attachment and growth in seawater with no signs of toxicity, as observed for approximately eight months in the Pacific coastal location in California (Figure S13, Supporting Information).
Three additional coral species fragments Solenastrea bournoni (SBOU), Orbicella faveolata (OFAV), and Porites astreoides (PAST) were tested by gluing coral fragments to CS on top of ceramic plugs.Additionally, one control fragment per specie was glued directly to the ceramic plug.Complete survival was observed for fragments of all species during seven months on CS.Furthermore, the OFAV species Mean Monthly

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© 2023 The Authors.Advanced Materials published by Wiley-VCH GmbH Growth (MMG) was 1.5 mm 2 , superior to the control sample of 0,7 mm 2 (Figure 4c).The MMG for PAST and SBOU were similar to the reference sample (Figure 4d,e).The observations of survival and growth of coral fragments on these CS materials suggest possible uses as substrates for coral growth to be tested in the future at a larger scale, monitoring the kinetics of ions diffusion and degradation.Finally, CS was fixed with biopolymer filaments, [18] on an electrolytic coral restoration structure on the Cozumel coast (Mexico, August 8th to September 29th, 2021).Observations of low macroalgae covering CS in both wet lab and oceanic conditions suggested that the MCM might have contributed to the inhibition of macroalgae settlement and growth, contributing to coral colony fitness and health, particularly in nutrient-rich waters. [74]

Life Cycle Assessment (LCA)
An environmental analysis was carried out following ISO 14.040-44:2006 standard in a cradle-to-gate approach.The system boundary covered the extraction of natural feedstock until the final fixation of CO 2 through the mineralization process at the laboratory scale (Figure 5a).The process was divided into two modules, dissolution or upstream and mineralization or core.The dissolution module included the dissolution in alkali conditions, as described earlier. [24]The process separation in two modules was made based on the assumption that some industrial processes already produce residual alkaline cellulose streams, [14] with the potential to serve as starting feedstock for mineralization.Hence, the mineralization process

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© 2023 The Authors.Advanced Materials published by Wiley-VCH GmbH Adv.Mater.2023, 35, 2209327   was considered to produce added-value products and mitigate related industrial environmental impacts. [14]he environmental LCA simulations were made for MCM samples S0 and S7, associated with two boundary conditions of low and high environmental impact, respectively (Figure 5b; Table S3, Supporting Information).LCA from S7 presented an environmental impact 2-4 times higher than that of Sample S0.This difference is related to the higher CO 2 chemisorption capacity at low cellulose concentration and the more limited use of water and electricity demanded for mixing.Generally, the most significant environmental burdens are produced at the cellulose dissolution stage, derived from the alkali and electricity required for cooling (−5 °C). [24]The electricity and alkali requirements during the cellulose dissolution were responsible for more than 90% of the impact on the mineral resource scarcity, freshwater eutrophication, and freshwater ecotoxicity categories (Figure 5b).Hence, the dissolution of cellulose in alkali conditions remains the main challenge to ensure environmental sustainability. [26]In contrast, the mineralization stage decreased the global warming potential by up to 31% (Figure 5b, left), producing new calcium carbonate and replacing its natural exploitation.Independently of the cellulose content, the mineralization stage significantly reduced mineral and fossil resource scarcity, as well as freshwater eutrophication, neutralizing ≈10% of the impact of the non-optimized dissolution stage for every impact category in sample S7 (Figure 5b, right).
The global warming potential (GWP) for MCM production ranged from 0.9 g CO 2 eq g −1 to 7.2 g CO 2 eq g −1 for S0 and S7 samples, respectively.GWP was primarily linked to the refrigeration and mixing needs at the dissolution stage, [24,26] yet a 31% reduction was achieved at the mineralization stage.Overall, the proposed Direct Air Capture (DAC) strategy tackles the environmental impact of the forestry and construction industries, offering a broad opportunity for material development.

Conclusion
Mineralized Cellulose Materials (MCM) were presented as platforms to develop structures with various mechanical properties and applications.LCA results showed promising possibilities to tackle global warming effects originated in the cement industries through MCM production.Furthermore, low cellulose materials exhibited suitable properties as an additive for ceramic and cement formulations.Meanwhile, a high cellulose loading allowed the production of moldable and printable artificial coral stones that hosted three species of corals for seven months.Ongoing research is being directed to produce hybrid materials with ceramic/cement/MCM, i.e., to synergize the properties of the different components, allowing, for instance, proper control of ions (essential in the coral growing process) and extending the materials' lifespan in the ocean environment.

Experimental Section
Analytical Instruments: Microscopy imaging was carried out using a Leica DM 750 unit (Microsystems, Germany, camera ICC50HD).Fourier-Transform Infrared Spectroscopy (FT-IR) characterization was conducted with a Thermo Fisher Scientific Nicolet Avatar, 380 FTIR spectrometer.Surface morphology was inspected using SEM (ZEISS SIGMA VP, Germany); before imaging, samples were vacuum-dried for 18 h overnight and subsequently sputtered with a ≈7 nm Au/Pt layer (Emitech K100X); the final images were colored according to their EDX spectra, using Adobe Photoshop®.The mechanical properties were evaluated using a Universal Tensile Tester Instron 4204 with a 5 kN load cell; samples were prepared according to the ASTM C1424 and tested until 30% strain in a conditioned room at 50% relative humidity, 23 °C.The X-ray crystallography data was obtained using a small-angle and wideangle X-ray scattering (SAXS/WASX) device (Xenocs, Xeuss 3.0, U.K.) bench beamline equipment.The generator worked at 45 kV and 200 mA with Cu Kα radiation; the background correction due to the sample holder and the air was made by subtracting the sample diffractogram data with the corresponding blank data (without sample).The shear rheology of MCM liquid paste at 15 wt.% solid content was monitored in the steady and oscillatory modes using an Anton Paar Physica MCR 302 (Anton Paar GmbH, Austria).The tests were carried out with a parallel plate geometry (25 mm diameter and 1 mm gap) and included viscosity measurement and frequency swept at 0.1% amplitude.The thermal stability of the samples was studied using a thermogravimetric analyzer (sensitivity of 0.001 mg) (TA7.Instruments, Q500, USA), with ≈10 mg of each sample placed in the thermogravimetric system under Nitrogen flow (60 mL min −1 ).The programmed temperature procedure was maintained at 30 °C for 15 min, then increased to 900 °C (ramp rate 10 °C min −1 ), and finally kept at 900 °C for 30 min.Ethanol porosity was determined using a previously reported method, [50] and BET porosity was measured by Nitrogen sorption measurements performed at 77 K on a Micromeritics ASAP 2020 and TriStar II 3020 instruments.The surface area was calculated according to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) theories; the pore size distribution was applied to the adsorption and desorption branches of the BET isotherm.
Cement Composite: The MCM suspensions at 15 wt.% solid content were mixed with the liquid cement paste using CEMI and a total waterto-binder ratio of 0.5.Three prisms 40 × 40 × 160 mm were cast with each cementitious binder, unmolded after one day of hardening, and stored in water for the next 27 days.After 28 days, the cement paste cylinder with a diameter of 25 mm was drilled out from each prism.The middle section of the cylinders with a thickness of 3 mm was cut out and stored in isopropanol for five days to stop the hydration.The sample preparation included epoxy impregnation, and polishing was done following reported procedures. [75]BSE images were obtained with 1000x magnification (36 images per sample) to analyze porosity.In total, three replicas were analyzed per each cement paste mixture.
CS Printing: A bioprinter (CELLINK, Sweden) with a pneumatic printhead was used to print 15 wt.% solid content MCM suspensions.The samples were designed as rectangular grids with a hexagonal infill pattern and 25% infill density.The system utilized clear pneumatic 3 mL syringes and a sterile tapered nozzle, 16G from Drifton.The solid support for 3D printing consisted of plastic Petri dishes (100 mm diameter).The printing parameters, including nozzle size, print speed, and extrusion pressure, were adjusted to achieve suitable conditions for 3D printing of alkali-dissolved cellulose of high visual quality.
Coral Fragmentation and Growing Test: All coral tests were performed in locally authorized laboratories by U.S. and Mexico Governments.University of California-Santa Cruz, Long Marine Laboratory, and Brinton Environmental Center, Boy Scouts of America High Adventure Camp 23800 Overseas Highway, Summerland Key, FL 33042, Federal permit FKNMS-2021-101-A1.Also, Cozumel coast (Mexico, August 8th to September 29th, 2021, Flanigan, C. Zoe a living sea sculpture.https://zoecoral.com/),SEMARNAT permit 23QR2011TD083 Mexico Government.UN actions 43755 and 37902.MCM samples S5 were tested in rectangular molded stones or Coral Stone (CS).The CS was used for seven months as a substrate for coral growth in a wet lab.One coral of each of the three coral species Solenastrea bournoni (SBOUR), Orbicella faveolata (OFAV), and Porites astreoides (PAST), were fragmented into four micro fragments each (<0.5 cm 2 ) using a diamond-band saw.Three fragments per species were glued to the CS on top of ceramic plugs, and one control fragment per species was glued directly to the ceramic plug.Survival and monthly growth rate of coral colonies area per month were calculated between seven months, comparing growth between photos taken on 16th December 2021 and 3rd July 2022.The corals will continue to be assessed monthly as this is an ongoing experiment at the time of writing.They are still alive, being monitored and documented.
LCA Simulation Model: The process was simulated in SimaPro v.9.0.0.49using Ecoinvent v.3.5/2018 as a support database, evaluating ten mid-point impact categories with ReCiPe(H) v1.03 method and considering CO 2 natural emissions.The impact categories evaluated included global warming potential (GWP), stratospheric ozone depletion (SOD), fine particulate matter formation (FPM), ozone formation-terrestrial ecosystem (O3F-T), terrestrial acidification (TA), freshwater eutrophication (FWEU), freshwater ecotoxicity (FWET), human carcinogenic toxicity (HCT), mineral resource scarcity (MRS), and fossil resources scarcity (FRS), selected as most representative.Other results related to other impacts can be found in Table S3 (Supporting Information).The main simulation assumptions were that all the database processes were considered 'cut-off' and addressed to European conditions.The electricity low voltage values were considered from available information from the Finnish system.Additionally, all the environmental burdens were allocated to MCM production, ignoring other stages such as transportation, infrastructure, equipment maintenance, and product end life.

Figure 1 .
Figure 1.Mineralized materials of low (l) and high (h) cellulose content (MCMl, MCMh) and their structural features.a) Mineralization process illustration.b) CO 2 absorption capacity (N = 3, p < 0.001) with MCM as shown by SEM images (left, scale bars are 1 µm) where cellulose and calcium carbonate are shown in green and cyan color, respectively; errors bars are standard deviations (box chart is presented in Figure S14).The proposed applications are included (right, scale bars are 1 cm).c) Wet MCM rheology (frequency test at 1% strain).d) Dry MCM compression test (up to 25% strain) from molded stones (15 mm x 15 mm x 7 mm).e) BET nitrogen adsorption isotherms for solid MCM samples.
between 20 and 23 nm, and a specific surface area of ≈10 m 2 g −1 (Figure1e; Table

Figure 2 .
Figure 2. Crystal structure and composition of MCM samples.a) X-rays diffraction peaks for sample S5. b) Thermogravimetric profile for MCM samples (25 °C to 900 °C).c) SEM image of S5 showing the three main components, cellulose (green), calcite (light blue), calcium carbonate framboids (turquoise).d) S5 calcium carbonate framboids chiral structure.e) Sample S0 minerals in the presence of sodium carbonate platelets (yellow) and unreacted ZnO micro flower (red).f) Calcium carbonate framboid formation from CO 2 diffusion.Scale bars in c, d, e, and f are 1 µm.

Figure 3 .
Figure 3. MCM applications in ceramic glazes and Portland cement paste.a) Appearance of ceramic glaze tested tiles (left), with the original appearance also shown in Figure S8.Included is also the glaze viscosity for samples S3 and S1 test (right).b) Scanning electron microscopy (SEM) of samples used as cement paste (sample S5), including porosity % (right), errors bars are standard deviations (N = 3, p < 0.01).c) SEM image of sample S5 showing the porous microstructure.d) Thermogravimetric analysis of cement composites (25 °C to 1000 °C).Scale bars in a, are 1 cm; scale bars in b, and c are 1 mm and 50 µm, respectively.