Co 2 sequestered aggregate and a production method thereof

Abstract

According to an example aspect of the present invention, there is provided an aggregate and a method of producing the same, as well as uses thereof. The aggregate comprises a plastic soil, a carbonaceous material and a binder, and is produced by forming an aggregate mixture and subjecting such mixture to carbon dioxide curing.

Description

[0001] CO2SEQUESTERED AGGREGATE AND A PRODUCTION METHOD

[0002] THEREOF

[0003] Background of the Invention

[0004] Field of the Invention

[0005] The present invention concerns an aggregate and a method of producing the same. In particular, the present invention concerns a modified aggregate comprising a plastic soil, a carbonaceous material and a binder, and uses thereof. The method is based on carbon dioxide curing of an aggregate comprising said components.

[0006] Thus, according to one aspect the field of the invention is working and improving the weak soil conditions by adding into a plastic soil a carbonaceous material, in particular biochar (a carbon negative by-product from the pyrolysis process), moderate amounts of a binder (cement / lime / fly ash), and sequestered CO2gas to develop robust lightweight aggregates that can be used as fill materials and for developing low-carbon masonry cementitious blocks.

[0007] Description of Related Art

[0008] Weak soft sensitive clay constituting most of the Finnish geological landscape is often landfilled or stabilized using cement before conducting any construction activities. The Finnish government mandates the construction sector to reduce the CO2 emissions from the industry by practicing circular economy strategies of available waste resources. The addition of Portland cement (PC) as a binder material in cementitious composites contributes to the third largest global emitter of CO2 (0.8-1-ton CO2 per ton of concrete). The European Union and Finnish parliament target to achieve net-zero CO2 emissions by 2050. Apart from cement usage in the construction sector, another source of CO2 emission in Finland is in the transportation and landfilling of waste soft sensitive clay. This waste clay, along with mined earthworks, accounts for 86 million tons of annual unused inorganic waste as per the latest governmental report on national economy material flows. Finnish soft clays are geologically widespread and are notoriously known to be a weak stratum (disturbed compressive strength < 20 kPa) for infrastructure development and need to be either strengthened by cement stabilization or landfilled after excavation. In practice, the excavated soft clay in shallow depths is backfilled as waste by either mined or artificial lightweight aggregates. Both mined and artificial aggregates [lightweight expanded clay aggregate and expanded glass foam (EGF)] are cost-intensive, location-dependent, and CCE-intensive.

[0009] H2 has been envisaged by the EU as a futuristic fuel source that can help Europe reach its goal of net-zero CO2 emission. Among the different processes of EE production, turquoise EE (produced from the thermo-catalytic decomposition of CEE gas) has recently gained attention. The process of CEE decomposition is generally attributed to be low CO2 intensive or even, in some cases, net neutral. However, scaling up of this methane pyrolytic hydrogen would also have a three-fold production of pyrolytic carbon. For instance, Hycamite, a Finnish company producing hydrogen from this process, estimates that 6000 tons of carbon will be produced annually from their upcoming plant. Thus, the value addition of this side product can have economic value for its operations. The utility of other carbonaceous residues as a partial replacement for PC has been recently explored in the past decade. For instance, biochar (a carbonaceous residue from wood pyrolysis) usage in limited amounts has proved to be an efficient additive for pore filling, internal hydration curing, and enhancing the carbonation curing potential.

[0010] Further, Intergovernmental Panel on Climate Change (IPCC) has touted accelerated CO2 curing (ACC) in cementitious composites as one of the efficient and stable CO2 sequestering strategies. The mechanism involves artificially injecting CO2 in a porous matrix that has suitable alumino-oxides that can result in the formation of stable carbonates (Ca, Al, Mg-based). Thus, there is an urgent scope to develop low-emission lightweight aggregates utilizing available side streams in the Finnish context, like waste excavated soft clay, leveraging the CO2 sequestration potential of carbon.

[0011] Thus, there is a need to come up with lightweight aggregates that are low emission in nature, locally available, and sufficient for strength requirements in the construction sector. Summary of the Invention

[0012] The present invention aims at solving at least some of the problems of the prior art. In particular, the present invention provides a novel aggregate that acts as an improved alternative for the prior art solutions.

[0013] It is an object of the present invention to provide a new kind of aggregate, especially a modified aggregate, comprising the combination of a plastic soil and a carbonaceous material. Further, it is an object of the present invention to provide a method of producing such modified aggregate and uses of such modified aggregate.

[0014] Thus, according to the first aspect the present invention relates to a modified aggregate comprising a plastic soil, a carbonaceous material and a binder. In particular, the present invention relates to a CO2 sequestered lightweight aggregate. Especially, the present invention relates to soft clay-based aggregates (SCAs), i.e. to aggregates where the plastic soil is waste clay, in particular excavated waste clay, especially excavated soft waste clay. Thus, in one particular embodiment, the present invention explores the potential of valorizing excavated soft clay as a construction material through the development of novel SCAs by incorporating a side stream biochar and carbon for example from the methane pyrolysis process.

[0015] According to the second aspect, the present invention relates to a method of producing a modified aggregate, the method comprising the steps of providing a mixture of plastic soil, carbonaceous material and binder; subjecting the mixture into an aggregate pelletization to provide an aggregate; and subjecting the aggregate to a carbon dioxide curing to provide a modified aggregate.

[0016] According to the third aspect, the present invention relates use of the modified aggregate as described above. In particular, the present invention relates to use of the modified aggregate as a backfill material as such or as a component in insulation materials, cementitious composites and pedestrian asphalt, for example. Thus, the present invention is based on an idea of including a carbonaceous material, in particular carbonaceous residue, in an aggregate in order to increase CO2 absorption and, thus, mineralized calcite formation in the aggregate. Mineralized calcite formation, in turn, provides improved strength properties. Thus, the present invention explores the utility of developing cold-bonded lightweight aggregates using a plastic soil, a carbonaceous material and a binder.

[0017] In particular, the present invention is characterized by what is stated in the independent claims. Some specific embodiments are defined in the dependent claims.

[0018] Thus, several advantages are reached using the present invention. First of all, the practical implication includes the potential valorising of a plastic soil, in particular an annual amount of waste soft clay, generated during geotechnical infrastructure development at the site itself, which is otherwise landfilled in Finland. In addition, the present invention preferably takes advantage of carbonaceous side streams, such as carbon residues, in particular biochar. Further, the present invention sequesters CO2 thereby being environmentally friendly.

[0019] The present invention also provides improved energy efficiency. Unlike LECA (lightweight expanded clay aggregate) or foam glass aggregates that require firing at 1,000-1,200 °C, the aggregates of the present invention are produced at low temperature through (accelerated) carbonation curing, cutting energy use by up to 90%. Thus, the aggregates of the present invention can be cold-bonded, i.e. mixed without any heat required. Further, the aggregates have improved mechanical performance. The CO2 mineralization step increases aggregate strength and durability by forming stable calcium carbonate phases, making them suitable for both geotechnical and concrete applications. In addition, the present invention enables uniform CO2 curing since mineralization occurs at the aggregate scale (not large concrete blocks), wherein CO2 diffuses evenly without surface passivation, ensuring consistent carbon storage throughout the particle.

[0020] The present invention also eliminates transport and landfill since aggregates are produced directly on-site using excavated soils, avoiding expensive hauling and landfill, and supports sustainable land management by minimizing the need for new quarrying of virgin aggregates, preserving natural resources and reducing landscape degradation.

[0021] Finally, the present invention facilitates circular economy integration by converting three waste streams (excavated clay waste, biochar, and CO2) into a high-value construction material.

[0022] Next, embodiments will be discussed in more detail.

[0023] Brief description of the drawing

[0024] FIGURE 1(a) presents the schematic description of the pyrolysis process and aggregate formation according to one embodiment of the present invention.

[0025] FIGURE 1(b) presents the SEM micrographs of carbon according to one embodiment of the present invention.

[0026] FIGURE 1(c) presents FTIR spectra of carbon according to one embodiment of the present invention.

[0027] FIGURE 2 presents the following aggregate properties according to one embodiment of the present invention; (a) BET isotherms, (b) TGA spectra, and (c) Hydrated and carbonated phase.

[0028] FIGURE 3 presents (a) cross-sections of samples after phenolphthalein test,

[0029] (b) SE images of SC As, and (c) strength and of aggregates, according to one embodiment of the present invention.

[0030] FIGURE 4 presents the following final cast concrete properties according to one embodiment of the present invention (a) cross-section, (b) strength and pH, and (c) net CO2 emissions. Embodiments

[0031] The present invention relates to an aggregate, in particular called as a modified aggregate, comprising a plastic soil, a carbonaceous material and a binder.

[0032] Thus, the modified aggregate comprises a plastic soil. Plastic soil refers to soil, in particular fine-grained soil, that can be molded into different shapes, in particular molded without cracking due to the presence of at least some amount of water, which allows the particles to slip over one another instead of breaking. Therefore, any plastic soil can be used in the present invention. However, in one embodiment the plastic soil is clay, in particular waste clay, more particularly excavated waste clay, especially soft waste clay. Thus, according to one embodiment, the present invention concerns soft clay aggregates (SCAs), in particular modified SCAs. However, also other plastic soils can be used, such as tailings or peat.

[0033] Thus, according to one embodiment, the present invention concerns use of excavated soft clays, in particular excavated soft waste clays, to make modified aggregates.

[0034] In one embodiment, the amount of plastic soil is at least 50, 55, 60, 65, 70, 75, 80, 85 or 90 wt.%, calculated from the dry weight of the modified aggregate. In one embodiment, the amount of plastic soil in 60 to 90, preferably 70 to 80 wt.%, calculated from the dry weight of the modified aggregate.

[0035] The modified aggregate comprises a carbonaceous material. The carbonaceous material can be any carbon containing material, preferably having a carbon content of at least 50 wt.%, preferably at least 60 wt.%, calculated from the total weight of the carbonaceous material. In particular, carbonaceous residues are used as the carbonaceous material, wherein industrial byproducts can be utilized in the present invention. According to one embodiment, the carbonaceous material comprises or is porous carbon, such as biochar. Biochar is typically a carbon negative by-product from the pyrolysis process. According to another embodiment, the carbonaceous material is carbon black, in particular pyrolyzed carbon black. Porous carbon, such as biochar, is an ideal material for CO2 mineralization since it efficiently sequence CO2, as it generally have high surface are (95 m2 / g), which attributes to the presence of fibrous carbon nanotubes (CNTs). These CNTs possess suitable CO2 absorbing surface functional groups and high pore volume distribution in the nanoscale, which facilitates more CO2 mineralization in the aggregate material. Thus, porous carbon, in particular the one included in carbonaceous residues, comprises carbon particles with high surface area, catering as reaction sites for hydration and carbonation.

[0036] In one embodiment, the carbonaceous material has a surface are of at least 80 m2 / g, preferably at least 90 m2 / g, such as 95 m2 / g, more preferably at least 100 m2 / g, such as 100 to 200 m2 / g, the surface area can be determined by BET (Brunauer-Emmett-Teller) analysis.

[0037] In one embodiment, the amount of the carbonaceous material is in the range of 1 to 30 wt.%, preferably 2 to 25 wt.%, more preferably 5 to 20 wt.% or 3 to 15 wt.%, such as 3 to 10 wt.%, calculated from the dry weight of the modified aggregate. In one embodiment, the amount of the carbonaceous material is in the range of 10-20 wt.%, calculated from the dry weight of the modified aggregate.

[0038] The modified aggregate comprises a binder, any binder material can be used. According to one embodiment, the function of the binder is to lock the aggregate firmly in place, creating extremely rough, hard and durable surface capable of withstanding stress. The binder is preferably selected based on the application of the aggregate, it may be for example asphalt binder (i.e. petroleum by-product), inorganic binder, such as cement, some specific resin like epoxy or polyurethane, or natural or organic stabilizer. In one embodiment, the binder is an inorganic binder, such as cement, lime, gypsum, fly ash, slag or a combination thereof. In one embodiment, the binder is a polymer binder. In a preferred embodiment, the binder is cement, in particular Portland cement or geopolymer cement.

[0039] In one embodiment, the binder, preferably inorganic binder, such as cement, has a calcium oxide content of at least 60 wt.%, calculated from the total weight of the cement. In one embodiment, the amount of the binder is in the range of 5 to 40 wt.%, preferably in the range of 10 to 30 wt.%, such as in the range of 15 to 25 wt.%, calculated from the dry weight of the modified aggregate.

[0040] In one embodiment, the modified aggregate of the present invention is a soft clay aggregate (SCA) comprising clay, preferably waste clay, a carbonaceous material and a binder; in particular clay, biochar and cement; especially waste excavated clay, biochar and Portland cement. Thus, according to one embodiment, the present invention concerns developing a modified aggregate, in particular, cold-bonded lightweight aggregate, comprising soft clay, biochar and cement. In one embodiment, the present invention concerns a CO2 sequestered lightweight biochar amended soft clay aggregate. Preferably the aggregate comprises low dosages (such as 10 to 30 wt.%) of cement, wherein carbon, that is preferably produced from a methane pyrolysis process, is used as a partial replacement of binder, such as cement, in developing SCAs. The SCAs are preferably produced by accelerated CO2 curing (ACC), as described in more detail later on.

[0041] In one embodiment, the modified aggregate comprises the plastic soil, the carbonaceous material and the binder in the weight ratio between 4: 1 : 1 and 2: 1 : 1. In particular embodiment, the aggregate material comprises clay, biochar, especially carbon, and cement in said weight ratio.

[0042] In one embodiment, the modified aggregate comprises the plastic soil, the carbonaceous material and the binder in the weight ratio between 3: 1 : 1 and 2: 1 : 1. In particular embodiment, the aggregate material comprises clay, biochar and cement in said weight ratio.

[0043] In one embodiment, the modified aggregate comprises or is derived from a composition comprising:

[0044] 60 to 90 wt.% of plastic soil, such as clay,

[0045] 10 to 20 wt.% of carbonaceous material, such as carbon residue, and

[0046] 10 to 30 wt.% of binder, such as cement, calculated from the dry weight of the aggregate. In one embodiment, the modified aggregate has, preferably after carbon dioxide curing, a calcium carbonate content of at least 5 wt.%, preferably at least 6 wt.%, more preferably at least 7 wt.%, calculate from the dry weight of the modified aggregate.

[0047] In one embodiment, the modified aggregate has a particle size in the range of 3 to 35 mm, in particular in the range of 10 to 35 mm. The particle size depends on the end use of the aggregate. For example, in embankments and geological fills a suitable particle size may be in the range of 10 to 30 mm, whereas in insulation applications in the range of 10 to 20 mm. In one embodiment, the particle size can be determined by inclination and speed of the pelletizer.

[0048] Also, the strength of the modified aggregate can be adapted based on the desired application. In one embodiment, the aggregate has a point load compressive strength of 0.3 to 1.5 MPa. The point load compressive strength can be measured by point loas test according to ASTM D5731.

[0049] The present invention also relates to a method of producing a modified aggregate. In particular the present invention relates to a method of producing the modified aggregate described above. Thus, all the above-described embodiments also relate to the method of the present invention.

[0050] The method comprises mixing the components together, forming an aggregate and subjecting such aggregate to a curing. Thus, the first step is mixing the starting materials, i.e. components, i.e. a plastic soil, a carbonaceous material and a binder. In one particular embodiment, the mixture is formed by mixing together clay, biochar and cement. Mixing can be performed in a separate vessel / mixer, however, in a preferred embodiment, the components are mixed in the pelletizer, in particular in disc pelletizer.

[0051] According to one embodiment, the components are cold-bonded, i.e. mixed without heating. The second step of providing, i.e. forming, an aggregate is performed by subjecting the mixture of the starting materials into an aggregate pelletization. This is performed in pelletizer, preferably in disc pelletizer.

[0052] The curing step of the present invention is a carbon dioxide (CO2) curing, in particular accelerated CO2 curing, i.e. accelerated carbonation curing, (ACC). Carbon dioxide curing is a process where carbon dioxide, preferably recycled carbon dioxide, is injected into a material where it chemically reacts with calcium to form calcium carbonate, a solid mineral that strengthens the material matrix. This technology can reduce the amount of binder needed, cut curing time, and permanently store carbon in the finished product. ACC is a specific type of CO2 curing method that uses high concentration of carbon dioxide (CO2) in a pressurized chamber, preferably at a pressure of 1 atm, to strengthen the aggregate and sequester the gas. Thus, in CO2 curing, the aggregate is exposed to CO2 gas that reacts with calcium and water, and possibly other compounds, in the aggregate to form calcium carbonate, and possibly other beneficial hydration products, and thereby enhance material's strength and durability. Thereby, CO2 curing, in particular ACC, provides a sustainable alternative for tradition curing by reducing CO2 emissions while improving material properties.

[0053] Thus, in particular embodiment, the method comprises the steps of providing a mixture of plastic soil, carbonaceous material and binder, subjecting the mixture to an aggregate pelletization to provide an aggregate, and subjecting the aggregate into a carbon dioxide curing to provide a modified aggregate.

[0054] In one embodiment, the CO2 curing is performed at a carbon dioxide content of 50 to 100 %, preferably 100 %, in particular in a chamber having a carbon dioxide content of 50 to 100 vol.%, preferably 100 vol.%, calculated from the total gas volume of the chamber. The curing may occur for at least 2, 3, 4, 5 or 6 hours, such as for 3 to 5 hours, 6 to 10 hours, 4 hours or 8 hours.

[0055] In one embodiment, the method comprises producing SCAs, in particular modified SCAs.

[0056] In one exemplary embodiment, the SCAs can be formed by mixing clay, carbonaceous material, such as biochar or carbon, and cement, preferably at a ratio (weight ratio) between 4: 1 : 1 and 2: 1 :1 or between 3: 1 : 1 and 2: 1 :1, through cold-bonded aggregate pelletization. Thereafter, these SCAs are cured, preferably at CO2, CO2 content preferably being in the range of 50 to 100 %, for preferably 5 to 10 hours to precipitate calcium carbonate (CaCCh).

[0057] In one embodiment, the method comprises producing SCAs from the combination of clay, a carbonaceous material and a binder.

[0058] According to one embodiment, the inclusion of carbon into the aggregate and CO2 curing of the aggregate increases the calcium carbonate precipitation by at least 50, 100, 150 or 200 %, compared to an aggregate without carbon.

[0059] In one embodiment, the aggregate is subjected to hydration treatment prior to the curing step. Water content of the aggregate after the hydration is preferably in the range of 20 to 40 wt.%, such as 30 wt.%, calculated from the total weight of the aggregate. According to one embodiment, the hydration is performed by subjecting the aggregate to a high humidity (> 90 % RH), preferably at room temperature. The hydration treatment may occur for days, such as at least for 1, 2 or 3 days, such 1 to 2 or 1 to 3 or 1 to 4 days.

[0060] Thus, in one embodiment, the method comprises the steps of mixing the components together, forming an aggregate and subjecting such aggregate to hydration, followed by curing, in particular CO2 curing.

[0061] Further, the present invention relates to the use of the modified aggregate. Such modified aggregate finds several uses in the construction field. It can be used as a backfill material as such or it can be used as a reinforcement to add strength to the resulting material, such as in concrete or asphalt. They can also be used in drainage systems, as a base layer for foundations and pavement, and for decorative purposes like landscaping. Other uses include for example insulation, water filtration, erosion control and railway ballast. In general, the method of the present invention can be used to produce CO2 sequestered lightweight aggregate materials.

[0062] In particular, the present invention encourages use of SCAs as a non-structural backfill material and medium-strength cementitious composite development for geotechnical and municipality infrastructure.

[0063] Numerous other variations and modifications in the invention as illustrated in the specific examples will be apparent to those skilled in the art, and hence it is not intended that the invention be limited to the examples but only as required by the spirit and scope of the appended claims.

[0064] Example - Production of SCAs

[0065] Starting materials

[0066] Waste soft sensitive clay collected from the construction site in Malmi airfield (Northeastern Helsinki), taken at 0.5-2 m depth, was used as the plastic soil.

[0067] Pyrolyzed carbon black provided by Hycamite TCD Oy (Finland) was used as the carbonaceous material. It is obtained as a by-product of hydrogen production through the thermo-catalytic decomposition of methane based on the proprietary technology of Hycamite.

[0068] Commercial Portland cement, PC, (CEM-I-52.5R, later on CEM I) complying with (SFS- EN 197-1, 2011) was procured from Finnsementti (Parainen, Finland) and used as the binder.

[0069] Production of the aggregate material

[0070] The schematic illustration of how the SCAs were developed is shown in Figure la. A dry mixture of clay, PC, and carbon (pyrolyzed carbon black) at mass ratios between 4: 1 :1 and 2: 1 : 1, preferably 3: 1 :1 (clay 600 kg, cement 200 kg, carbon 200 kg) was mixed initially for 5 minutes in an-inhouse developed disc pelletizer rotating at a speed of 45 rpm and at an angle of 45°. Once the samples were initially mixed, water was sprayed slowly on the mixture in the pelletizer using a spray bottle for 10 mins to prepare SCAs with particle sizes in the range of 10 - 35 mm. The SCAs formed spherical shapes due to the initial plasticity of the PC as well as the Coriolis effect caused by rotation. After this, the SCAs were placed at room temperature (25 °C) and high relative humidity (> 90 % RH) for 3 days to undergo some curing strength. Thereafter, the SCAs were put under a pressure- controlled carbonation chamber with 100% CO2 concentration at 0.1 MPa for 8 hours

[0071] Trial tests were done to ensure full carbonation on SCAs with these ACC conditions, as the objective was to maximize the CO2 sequestration in SCAs. After ACC, the samples were left to cure through any possible hydration for the next 4 days. As a comparison, the SCAs were only hydrated for 7 days. This allowed for a comparison of the ACC effects on SCAs. The CO2-cured SCAs and those that were not CCh-cured are hereby designated as SCA-C and SCA-NC, respectively.

[0072] Further, the produced SCAs were used in concrete casts. The mix design of the cast concretes is provided in Table 1.

[0073] Table 1. Mix design of concretes [kg / m3]. Characterization measures

[0074] Physical properties of the clay, like density, particle size distribution, and specific gravity, were determined as per standards ASTM El 109 and ASTM D2974, respectively. The tested physical and chemical properties of the clay are provided in Table 2, and the detailed characterization measure is reported in relevant scientific documents. In table 2, the clay content of 71 % indicates particle size less than 2 microns.

[0075] The elemental oxides of the samples were conducted using X-ray fluorescence (Axios mAX 3kW, Malvern Panalytical, UK), whereas micrographs of the samples were assessed using Scanning Electron Microscopy (SEM) (ZEISS Sigma VP, ZEISS Microscopy, Germany). Fourier Transform Infrared (FTIR) spectroscopy (NICOLET i S50, ThermoFisher Scientific, USA) was employed to identify the surface functional groups between 400 cm'1to 4000 cm'1. Thermogravimetric analysis (TGA) (951).

[0076] Therm ogravimetric Analyzer, DuPont Instruments, USA) was carried out to assess the phases of cementitious composites formed based on the thermal decomposition curve. The specific surface area and pore size distribution of carbon black and clay were measured by Brunauer-Emmett-Teller (BET) analysis (TriStar II 3020, Micromeritics Instrument, USA) using N2 adsorption / desorption isotherms. A single-point load test to obtain the compressive strength of SCA was conducted as per ASTM D5731-16. The peak load value was recorded as the individual crushing load (N), and compressive strength (MPa) was calculated using the following Eq (1). where G is the single particle compressive strength of the aggregates (MPa), P is the peak load (N), and d is the distance between the upper and small surfaces of the aggregates (mm). pH value was measured as per SFS-EN ISO 10390 to assess the efficacy of ACC indirectly using a Fischer Accumet AE150 Benchtop pH meter. The compressive strength of the final concrete blocks was carried out as per SFS-EN 12390-3.

[0077] Characterization results

[0078] Table 2. Physical and chemical properties of the clay.

[0079] Figure lb presents the SEM micrographs of the carbonaceous residue used to make the SCAs. The micrographs indicate that the individual carbonaceous particles constituted both spherical and irregularly shaped particles with the maximum diameter / length ranging between 400 to 975 pm. This individual particle size indicates that it would be a fine filler material in the clay-cement matrix. Upon higher magnification at 12KX, the carbon exhibited the presence of carbon nano tube-like structures within the individual carbon particles. This indicates that the overall specific surface area of these carbon particles would likely be very high, catering as reaction sites for hydration and carbonation. This hypothesis is further discussed based on the pore size distribution of the carbonaceous material as well as the final hydrated products observed of the SCA-C.

[0080] The XRF results presented in Table 3 indicate that the carbon consisted mostly of oxides of Si, Al, and Ni with almost no calcium oxide (CaO) that may be helpful for hydration. The clay exhibited higher concentrations of oxides of Al, Si, and Fe that moderately contribute to the formation of hydrated products. Most of the hydration and carbonation would be caused by the PC, which consisted predominantly of CaO (= 70 %). Table 3. Elemental oxides of materials.

[0081] The FTIR spectra of the carbonaceous material in Figure 1c clearly indicates the presence of CO2 adsorbing groups having strong to medium peaks of amides (N-H), carboxyl (C=O), alkyne (C=C), and alkene (C=C). The carbon had moderate peaks of hydroxyl (- OH) that render the material hydrophilic.

[0082] Overall, the BET analysis revealed that the surface area was relatively high at 95 m2 / g, likely due to the presence of CNTs inside the carbonaceous residue. This is even higher than that of the clay, which was 26 m2 / g. The measured pore size distribution (Fig. 2a) revealed that the carbon had predominant pores between 2 to 50 nm that likely occurred due to the presence of CNTs. The analytical assessment of the carbonaceous residue indicates that the presence of CNTs, suitable surface functional groups, and a relatively high surface area would be ideal CO2 adsorbents in the clay matrix under ACC conditions. Figure 2b presents the TG (thermogravimetric) and DTG (derivative thermogravimetric) curves of the SC As. The analysis indicates that the decomposition of SCAs can be categorized into three main stages indicative of three different hydrated phases. Stage I (70 °C - 300 °C) is primarily associated with the thermal decomposition of C-S-H gel and ettringite products (Aft, AFm). Stage II (400 °C - 500 °C) involves the dihydroxylation of the hydroxides [Ca(OH)2 and Mg(0H)2]. Stage III (600 °C - 800 °C) is characterized by the thermal decomposition of carbonates (e.g., CaCCh, MgCCh). This last phase is associated with the amount of CO2 sequestered in the SCAs. It is evident from the DTG curve of the SCA-C that carbonate formation is higher than in the other two samples. This is likely due to the observed CNTs in the carbon (refer to Fig. lb), which would sequester more CO2 in the form of CaCOv Thereafter, a quantitative amount of CaCCF was calculated from the TGA curve according to Eq. (2).

[0083] The relative distribution of hydrated products in the SCAs studied is presented in Fig. 2c based on thermal decomposition observed between 50 - 300 °C for C-S-H, ettringite (Aft, AFm), and 400 - 500 °C for portlandite. Compared to the CaCCh formed in SCAs with no carbon (2.48 % and 4.39 % respectively), the CaCCh formed in the SCAs with carbon is almost twice (4.61 % and 7.38 %) [Note: C and NC in indicative of carbonated and noncarbonated samples]. This marks almost a 200 % increase in CaCCh trapped in the carbonated SCA-C samples with respect to the reference (non-carbonated samples). This increase in carbonate mineralization in SCA-C is attributed to the mesoporous volume and pore area of the CNTs, which expedited the ingress of CO2. The least ettringite, CSH, and AFm content in SCA-Cs are also indicative that the CO2 mineralization was the dominant mechanism for strength gain. The CO2 mineralization was further investigated based on the henolphthalein indicator test done on split SCAs samples (Fig. 3a). Regardless of the aggregate diameter, the carbonated DCA (SCA-C) showed no trace of the phenolphthalein, while the reference aggregate was stained with the indicator reagent. The pH of crushed SCA-C containing carbon drastically reduced to 8.8 as compared to the reference SCAs without carbon (12.11). The microstructure of SCA without carbon and no CO2 curing in Figure 3b clearly demonstrates that the majority of the classical hydration products surrounding the clay particles were majorly reticulated C-S-H (calcium silicate hydrate; blue sections) gels and hexagonal prismatic large crystals of C-H (calcium hydroxide, orange). However, SCA-C containing carbon predominantly showcases CaCCF formation in a mesh of the CNTs, as also observed in the literature. It can be inferred that in SCAs-C with the carbon, CaCCF effectively fills the pore space around the CNTs, working as a mesh and densifying the structure. This increase in density may likely affect the mechanical properties of the SCAs and consequent concrete, as discussed in the next section.

[0084] Figure 3c shows the maximum point load compressive strength (stress and load) of SCAs with respect to their effective diameter (10 - 34 mm) based on 41 test samples. The majority of SCAs failed in shear, branched into three parts, and exhibited irregular fractures. The maximum strength (1.4 MPa) is achieved for smaller sizes of SCAs at around 12 mm, and it gradually tapers down to 0.4 MPa with higher diameter particles between 25 - 34 mm. Similar effects of particle size on point load compressive strength are observed in the literature. The observed strength range of SCAs qualifies as Class-II as per rock classification standard cores and seems to be equivalent to expanded glass foam (refer Fig. 3c for a comparable range of sizes. The point load strength is lower than those observed by literature (i.e. 4 - 8 MPa for 9 - 13 mm effective diameter). However, this is expected as the binder content for current SCAs was at 20 % while those developed in literature had a binder content of 85 % with even lower carbon content in the form of biochar at 15 %. Additional details are provided in Table 4. It is interesting that the water absorption of ACC-induced SCA with the pyrolyzed carbon had lower moisture capabilities than that of natural hydration. This reflects why workability in terms of slump test was higher for carbonated SCA than its counterparts. Table 4. Physical properties of the SCAs (10-20 mm fraction size)

[0085] Figure 4a presents the cross-sectional images of final concrete samples prepared with SCAs with carbon (carbonated and non-carbonated). The cross-section of a sawed concrete is demonstrated with one side applied with a phenolphthalein indicator. The images show that the ACC-induced SCAs exhibit little to no staining from the phenolphthalein indicator, indicating a minimal effect on the surrounding concrete. In contrast, the non-carbonated SCAs show visible staining from the phenolphthalein indicator. However, no distinct interfacial features between the aggregates and the surrounding concrete are visible at the given resolution. Fig. 4b illustrates the compressive strength and pH measurements of the hardened concrete samples tested on the 7th day after casting. It is evident that replacing natural aggregates with both SCAs resulted in a reduction in compressive strength. Another takeaway observation is that the crushed concrete pH of all samples with SCA exhibited a pH greater than 12. The 7-day compressive strength of the concrete cast with carbonated SCAs (SCA-C) was 22.2 MPa, while that with non-carbonated SCAs (SCA- NC) was 24.8 MPa. According to SFS-EN 206, the minimum compressive strength for lightweight aggregate concrete at 28 days is 13 MPa (indicated as a red line in Figure 4b). The concrete with 20% SCAs already exhibits a higher strength of 22 MPa on the 7th day. This strength also meets the required 28-day compressive strength for aggregate concrete bricks used in earth retaining walls (minimum day compressive strength being 16.5 MPa, indicated as a purple in line in Figure 4b) as well as in drainage and sewage water inspection chambers or manholes (minimum day compressive strength being 22 MPa, indicated as a green line in Figure 4b) according to EN-1996. These comparisons indicate that concrete formed by replacing natural aggregates with SCAs can be used in various applications in compliance with European Standards for building design and other civil engineering works. Figure 4c presents the estimated CO2 emission equivalent considering only material properties. The without SCA-concrete is indicative of the cast concrete with natural aggregates, which showcased positive net emissions (0.09 kgCCh / kg) comparable to traditional ready mixed cast concrete with a similar mix design (0.12 kgCCh / kg, indicated as a red line in Figure 4c) obtained from CO2 database (CO2data.fi) for Finland. On the contrary, the utilization of SC As using the carbon under both carbonated and noncarbonated conditions resulted in an equivalent net CO2 emission at (-0.52 kgCO2 / kg). The details of the calculation of CO2 emissions are provided in tabular form in Table 5. The calculation of CO2 emissions was done like the work in the literature (Wyrzykowski, M. et al., Cold-bonded biochar-rich lightweight aggregates for net-zero concrete, Journal of Cleaner Production (2024), vol 434). Our results resonate with their findings of negative CO2 emissions (-0.07 kgCO2 / kg, indicated as a green line in Figure 4c) wherein wood waste biochar was used to make cold bonded aggregates for the same mix design adopted in the current study. The results imply that the partial replacement of cement by pyrolyzed carbonaceous material can be an effective CO2 sequestration strategy. Note that the current CO2 emission calculation only accounts for the material properties of the SCAs.

[0086] Table 5. Estimation of the CO2 emission of concretes with SCAs [kg CCh / m3]. Summary of the results

[0087] The analytical investigation described above revealed that the inclusion of carbon constituted fibrous nano tube-like structures with CO2 adsorbing functional groups and high specific surface area resulted in increased CaCCh precipitation. Mineralized calcite formation in SCAs (both carbonated and non-carbonated) increased by 85 - 197 % with respect to control SCAs without any carbon. The CaCCh formation in these carbonated SCAs with carbon amendment was verified using suitable analysis and means, such as thermogravimetric analysis, phenolphthalein reagent indicator, and surface micrographs. This calcite formation and densification resulted in the point load compressive strength (1.3 MPa) of medium-sized (> 15 mm) SCAs equivalent to those of commercially used expanded glass foam. A proof-of- concept investigation of the cast concrete blocks partially amended with SCAs revealed that the 7-day compressive strength was greater than 22 MPa, indicating its use as a masonry construction material as per Eurocode 6 provisions. The incorporation of carbon-amended SCAs in concrete can potentially act as a CO2 sink with negative emission factors as compared to traditional concrete. This invention promotes the use of SCAs as a non- structural backfill material and mediumstrength cementitious composite for geotechnical and municipality infrastructure satisfying UN sustainable development goals 11, 12, and 13.

[0088] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0089] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0090] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0091] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In this description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc.

[0092] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0093] The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention.

Claims

Claims1. A modified aggregate comprising a plastic soil, a carbonaceous material and a binder.

2. The aggregate according to claim 1, wherein the amount of the carbonaceous material is in the range of 1 to 30 wt.%, preferably 2 to 25 wt.% or 10 to 20 wt.%, calculated from the dry weight of the modified aggregate.

3. The aggregate according to claim 1 or 2, wherein the amount of the binder is in the range of 5 to 40 wt.%, preferably in the range of 10 to 30 wt.%, such as in the range of 15 to 25 wt.% calculated from the dry weight of the modified aggregate.

4. The method according to any of the preceding claims, wherein the plastic soil is clay, in particular waste clay, such as soft waste clay.

5. The aggregate according to any of the preceding claims, wherein the binder is an inorganic binder, such as cement.

6. The method according to any of the preceding claims, wherein the binder has a calcium oxide content of at least 60 wt.%, calculated from the total weight of the binder.

7. The aggregate according to any of the preceding claims, wherein the carbonaceous material has a carbon content of at least 50 wt.%, preferably at least 60 wt.%, calculated from the weight of the carbonaceous material.

8. The aggregate according to any of the preceding claims, wherein the carbonaceous material has a surface are of at least 80 m2 / g, preferably at least 90 m2 / g, such as 95 m2 / g, more preferably at least 100 m2 / g, determined by BET analysis.

9. The aggregate according to any of the preceding claims, wherein the carbonaceous material is biochar.

10. The aggregate according to any of the preceding claims, wherein the aggregate comprises:60 to 90 wt.% of plastic soil,10 to 20 wt.% of carbonaceous material, and10 to 30 wt.% of binder, calculated from the dry weight of the aggregate.

11. The aggregate according to any of the preceding claims, wherein the aggregate is cured aggregate.

12. The aggregate according to any of the preceding claims, wherein the aggregate has a calcium carbonate content of at least 5 wt.%, preferably at least 6 wt.%, more preferably at least 7 wt.%, calculate from the dry weight of the modified aggregate.

13. A method of producing a modified aggregate, the method comprising the steps of: providing a mixture of a plastic soil, a carbonaceous material and a binder, subjecting the mixture to an aggregate pelletization to provide an aggregate, and subjecting the aggregate to a carbon dioxide curing to provide a modified aggregate.

14. The method according to claim 13, wherein the mixture is obtained by mixing together clay, biochar and inorganic binder, such as cement.

15. The method according to claim 13 or 14, wherein the curing is accelerated CO2 curing (ACC).

16. The method according to any of claims 13 to 15, wherein the aggregate mixture is subjected to hydration treatment prior to the curing step, wherein the water content of the aggregate after the hydration is preferably 20 to 40 wt.%, such as 30 wt.%, calculated from the total weight of the aggregate.

17. Use of the aggregate according to any of claim 1 to 12 as a backfill material or in insulation materials, cementitious composites or pedestrian block asphalt.

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