Process for the preparation of carbonated recycled concrete fines

Abstract

A process for the preparation of carbonated recycled concrete fines, particularly recycled concrete fines from construction and demolition waste, and carbonated recycled concrete fines prepared by the process, is described. The process relates to accelerated mineral carbonation of recycled concrete fines having a specific moisture content. Also described is further processing the carbonated recycled concrete fines into secondary aggregate as well as uses and methods involving the carbonated recycled concrete fines and / or aggregate.

Description

[0001] Process for the preparation of carbonated recycled concrete fines

[0002] Field of the Invention

[0003] The present invention relates to a process for the preparation of carbonated recycled concrete fines; and carbonated recycled concrete fines prepared by the process. The invention also relates to further processing the carbonated recycled concrete fines into secondary aggregate and to secondary aggregate prepared by the process. The invention also relates to uses and methods involving carbonated recycled concrete fines and / or aggregate.

[0004] Background

[0005] Construction and demolition waste (CDW) generation around the world has reached more than 3 billion tonnes annually. Fortunately, some of the materials in construction and demolition waste, such as wood, glass and metal can be directly recycled. However, concrete, which can reach up to 80% of the total CDW, has different characteristics which make concrete waste exceptional and difficult to handle. It is desirable to upcycle concrete waste because the reuse of recycled concrete aggregate not only reduces the huge consumption of natural resources during concrete production by substituting natural aggregates, but it also minimises greenhouse gas emissions. Additionally, it serves as a solution with outstanding potential for reducing landfill volume.

[0006] Recycled concrete waste consists of fine and coarse aggregates (-70-80 vol.% of waste) and hydrated cement paste (up to 30 vol.% of waste).

[0007] The recycled concrete fines (RCF) retrieved from a demolition concrete waste site are typically already well-hydrated and extensively carbonated under atmospheric carbonation conditions, therefore it can be difficult to carbonate them further. A few scientific investigations have been conducted on the CO2mineralisation and carbonation of RCF, with wet carbonation being the most well-regarded method. However, most studies have concentrated on freshly crushed hydrated cement paste that has been cast and cured in tightly controlled laboratory conditions and hence preserved from carbonation. These materials can be considered as an "idealised” model for the real RCF as they have sufficient carbonatable compounds (e.g. portlandite) to achieve an ideal mineral carbonation performance. Such materials differ significantly from recycled concrete materials collected from demolition waste sites, which undergo extensive, long-term carbonation under atmospheric conditions. In addition, wet carbonation has many technical disadvantages such as increased water usage to form the slurry or suspension, adding to overall operational costs and environmental considerations associated with the process, and dealing with slurry handling and separation of RCF as a relatively dense and abrasive material. Also, pumping, agitation and solid-liquid separation can be energy-intensive and require specialised equipment.

[0008] There therefore remains a need for an improved method of mineral carbonation of RCF and processing the carbonated RCF into secondary aggregate.

[0009] Summary

[0010] In a first aspect, there is provided a process for the preparation of carbonated recycled concrete fines, the process comprising the steps of (a) mixing damp recycled concrete fines having a moisture content of between about 5% w / w and about 12% w / w in a mixer; and (b) accelerated mineral carbonation of the damp recycled concrete fines in the presence of carbon dioxide to form carbonated recycled concrete fines.

[0011] In one embodiment, the damp recycled concrete fines have a moisture content of between about 8% w / w and about 10% w / w.

[0012] In another embodiment, the recycled concrete fines have a maximum particle size of about 4mm.

[0013] In another embodiment, step (a) involves mixing dry or partially dried recycled concrete fines with water to provide the damp recycled concrete fines.

[0014] In another embodiment, the process further comprises a step of initially milling the dry or partially dried recycled concrete fines before mixing the milled recycled concrete fines with water.

[0015] In another embodiment, the recycled concrete fines are milled to a maximum particle size of up to about 500 pM.

[0016] In some embodiments, the recycled concrete fines have a particle size distribution of d(10) of up to about 2 pm and / or d(50) of up to about 55pm and / or d(90) of up to about 360pm. Preferably, the recycled concrete fines have a particle size distribution of d(10) of up to 2 pm, d(50) of up to 55pm and d(90) of up to 360pm. In one embodiment, the concentration of carbon dioxide in the carbonation step is between about 15% to about 99.9%

[0017] In another embodiment, the carbonation occurs at ambient temperature.

[0018] In another embodiment, the carbonation occurs at ambient pressure.

[0019] In another embodiment, the recycled concrete fines are obtained from concrete demolition waste.

[0020] In another embodiment, the damp recycled concrete fines are carbonated for up to 45 minutes, preferably between 30 minutes and 45 minutes.

[0021] In a second aspect, the process further comprises the steps of (c) blending the carbonated recycled concrete fines with a binder to produce a blended mixture; and (d) pelletizing the blended mixture; to produce secondary aggregates.

[0022] In one embodiment, the binder is a low-carbon hydraulic binder, optionally selected from the group consisting of Portland cement, blended cement comprising Portland fly ash cement, Portland slag cement and Portland limestone cement; blast-furnace cement (incorporating ground granulated blast-furnace slag), pozzolanic cement (incorporating natural or synthetic pozzolanic material); composite cement, limestone calcined clay cement (LC3), calcium sulfoaluminate cement, magnesium-based cement, ternesite-rich cement, belitic calcium sulfoaluminate cement and / or carbonation-hardened cement.

[0023] In another embodiment, the carbonated recycled concrete fines are blended with up to 15 wt.% binder.

[0024] In another embodiment, the carbonated recycled concrete and binder are blended together in up to about 12% w / w water, preferably between about 5% w / w and about 12% w / w water.

[0025] In a third aspect, there is provided carbonated recycled concrete fines prepared by the process of the first aspect.

[0026] In a fourth aspect, there is provided use of the carbonated recycled concrete fines according to the third aspect as supplementary cementitious materials and / or filler. In a fifth aspect, there is provided secondary aggregate prepared according to the second aspect, or prepared using the carbonated recycled concrete fines according to the third aspect.

[0027] In a sixth aspect, there is provided use of the secondary aggregate according to the fifth aspect in the manufacture of concrete.

[0028] Brief Description of the Figures

[0029] Figure 1 is an image showing the samples of RCF (a) as received (as received RCF), (b) as dried (as received dried RCF) and (c) as pulverized by vibratory disk mill (RCF Milled).

[0030] Figure 2 is a graph showing the particle size distribution of the as received RCF sample.

[0031] Figure 3 is a schematic of the experimental design used in the examples.

[0032] Figure 4 is a graph showing the particle size distribution of binders and ground materials before carbonation.

[0033] Figure 5 is images of manufactured aggregates from the post-carbonated materials for Mix 2 & 3 in the examples.

[0034] Figure 6 is images of manufactured aggregates from the post-carbonated materials for Mix 4-7 in the examples.

[0035] Figure 7 is images of manufactured aggregates from the post-carbonated materials for Mix 8-11 in the examples.

[0036] Figure 8 is images of manufactured aggregates from the post-carbonated materials for Mix 12 and 13 in the examples

[0037] Figure 9 is a graph showing the results of individual aggregate strength development measured at 1 day, and 3, 7, 14, and 28 days for “case 1” mixes from carbonated as- received RCF (AR RCF).

[0038] Figure 10 is a graph showing the results of individual aggregate strength development measured at 1 day, and 3, 7, 14, and 28 days for “case 2” mixes from carbonated dried RCF (Dried RCF).

[0039] Figure 11 is a graph showing the results of individual aggregate strength development measured at 1 day, and 3, 7, 14, and 28 days for “case 3” mixes from carbonated RCFmilled (RCFmilled).

[0040] Detailed Description

[0041] The present invention is concerned with methods to maximise the accelerated carbonation of recycled concrete fines (RCF) derived from concrete demolition. The invention is also concerned with processes for manufacturing supplementary cementitious material (SCM) and / or artificial secondary aggregate using the carbonated RCF. As outlined above, carbonation of RCF can be challenging because the RCF retrieved from a demolition concrete waste site are typically already well-hydrated and extensively carbonated under atmospheric carbonation conditions.

[0042] The inventors have found a process for the preparation of carbonated RCF, the process comprising the steps of (a) mixing damp RCF having a moisture content of between about 5% w / w and about 12% w / w; and (b) accelerated mineral carbonation of the damp RCF in the presence of carbon dioxide to form carbonated RCF fines.

[0043] The claimed process provides a quick and efficient, technically viable, environmentally friendly and low-cost method for carbon dioxide sequestration in RCF. The claimed method provides an additional option for the management of RCF and contributes to a more sustainable construction industry.

[0044] Recycled concrete fines

[0045] RCF are an advantageous starting material. Firstly, unlike many thermal residues and byproducts, RCF do not typically exhibit the challenges associated with high chloride and sulphate contents, as well as elevated concentrations of heavy metals. Secondly, RCF are an advantageous starting material for this process from an operational point of view because the carbonation reaction does not generate significant exothermic heat. This circumvents the need for specific considerations and measurements, such as the control of the exothermic heat generated by the lime hydration reaction and the subsequent carbonation of portlandite and calcium silicate phases during the carbonation process of industrial by-products like cement bypass dust or paper sludge ash.

[0046] In the present invention, any RCF can be used as the starting material. Preferably, the RCF are obtained from concrete demolition waste.

[0047] The mixing of RCF with the low amount of water required in the process forms damp RCF. This is not therefore a wet (aqueous) mineral carbonation process where minerals react with CO2dissolved in water (typically in a suspension or slurry while the CO2gas is bubbled in the water) to form stable carbonate minerals. Instead, the process utilises thin-film carbonation, where a thin film of CO2 gas directly interacts with the mineral surface, leading to carbonate formation. As a result, the claimed process has significant advantages over wet carbonation methods known in the art. For example, there is no need to form a slurry or suspension and no need to remove or filter excess amounts of water (drying) at the end of the carbonation stage. As a result, operation costs are reduced and the process is more environmentally friendly. Furthermore, the proposed mineral carbonation process exhibits faster kinetics compared to wet or slurry carbonation methods. It is hypothesized that this is mainly attributed to the replacement of a thick water layer with an extremely thin water film, which significantly enhances the gas-liquid-solid interaction between carbon dioxide and the microstructure of RCF, leading to improved mineral carbonation efficiency. The carbonated RCF can also be directly incorporated into the granulation / aggregation process to produce secondary carbonated aggregates, eliminating the need for additional processing steps.

[0048] The damp RCF have a moisture content (also referred to as a water content) of between about 5% w / w and about 12% w / w. As used herein, "% w / w” refers to the weight of free water contained in a mixture (or material) relative to the total weight of the mixture (dry weight of materials). “% w / w” can also be referred to as ''wt.%” or “weight percent”.

[0049] Preferably, the damp RCF have a moisture content of between about 6% w / w and about 11 % w / w, for example between about 8% w / w and about 10% w / w. In some embodiments, the damp RCF have a moisture content of about 8% w / w, or about 9% w / w, or about 10% w / w, or about 11 % w / w, or about 12% w / w.

[0050] The damp RCF may have a moisture content of between about 6% w / w and 12% w / w, for example, between about 7% w / w and 12% w / w, or about 8% w / w and 12% w / w, or about 9% w / w and 12% w / w, or about 10% w / w and 12% w / w, or about 11 % w / w and 12% w / w.

[0051] The damp RCF may already have the required moisture content before being added to the mixer. Alternatively, the RCF may initially be dry or partially dry and step (a) involves mixing dry or partially dry RCF with water to provide the damp RCF. The RCF may initially be wet and then be partially dried (or completely dried) to a lower moisture content than desired. In this case, step (a) can involve mixing the partially dried, or completely dried, RCF with water to provide the damp RCF.

[0052] Where the RCF are initially wet and then partially or completely dried, the drying step is preferably gentle drying. Gentle drying may be at a temperature of about 40°C to about 60°C, preferably about 40°C to about 50°C. In some embodiments, the drying is at about 45°C.The drying can be air drying. In some embodiments, the RCF are dried to a moisture content of about 0% w / w to about 3% w / w. The dried RCF are then mixed with water to a moisture content of between about 5% w / w and about 12% w / w.

[0053] The moisture content of the damp RCF is crucial in the claimed process for optimum mineral carbonation. In particular, the amount of water and the thickness of the water film around particles of the RCF exposed to CO2 are crucial factors that impact the efficiency and kinetics of the carbonation process, is the amount of water and thickness of the water film is also essential for the orderly arrangement and uniform distribution of calcium carbonate phases formed within the microstructure of RCF particles which is linked to minimising the occurrence of decalcification of calcium silicate hydrates without the formation of calcium carbonates.

[0054] The specific moisture content disclosed herein confers several advantages:

[0055] Firstly, a specific moisture content according to the present disclosure provides an optimum reaction surface area and acts as a medium to bring the CO2and minerals into contact for the carbonation reaction. Too much moisture would lead to a thicker water film which can affect the liquid-solid interfaces, limiting the diffusion of CO2 and slowing the transport of ions which in turn lowers the reaction kinetics and degree of carbonation conversion. Too little moisture would lead to a water film that is too thin and insufficient water would be present to dissolve ions and to facilitate the speciation of carbon dioxide in a liquid-solid interface. This also reduces diffusion rates and carbonation efficiency.

[0056] Secondly, a specific moisture content according to the present disclosure provides optimum dissolution of CO2and minerals. The thin water film provided by the disclosed specific moisture content ensures that there is ample dissolved CO2and carbonic acid. The thin water film also allows effective dissolution and leaching of mineral phases (i.e. calcium ions mainly from calcium silicate hydrates) and subsequently enables diffusion of the leached mineral phases to react with carbonate ions. If the water film is too thick, it slows the dissolution and diffusion of phases, lowering the transport of ions and consequently reducing the carbonation reaction kinetics. If the water film is too thin, there may not be sufficient water to dissolve CO2 and ions from the mineral phases, reducing diffusion rates and carbonation efficiency.

[0057] Thirdly, a specific moisture content according to the present disclosure provides optimum particle wetting. The thin water film provided by the disclosed specific moisture content enables proper wetting of mineral particles and reactive phases which leads to an even exposure to CO2and maximises total surface area contact while keeping dissolution paths short enough to facilitate fast ion transport. This in turn enables a thorough and uniform carbonation to occur. A thick water film increases the diffusion distance ions must travel between mineral phase surfaces, slowing down the mass transport rates of dissolution and hence carbonation efficiency. If the water film is too thin, it may not fully wet or hydrate the mineral phase surfaces, reducing the accessible surface area for carbonation.

[0058] Additives

[0059] In some embodiments, additives may be added to the damp RCF. Such additives can enhance the mineral carbonation and / or modify the properties of any secondary aggregates produced from the carbonated recycled concrete fines.

[0060] Additives may increase the efficiency of the carbonation step by, for example, influencing the reaction kinetics, reactant dispersion, or overall process conditions. Examples of suitable additives include but are not limited to, catalytic additives (for example weak acids like acetic, oxalic, or citric acid, room temperature ionic liquid anions like acetate or formate which can dissolve calcium and catalyse rapid calcium carbonate precipitation, amino acids like lysine or synthetic catalysts like imidazole which can form activated complexes with CO2and accelerate carbonation), surfactants and dispersants (for example polycarboxylate ether-based superplasticizer (PCE), sodium salt of poly(acrylic) acid (PAANa), Sodium Dodecyl Sulfate (SDS), Triton X-405 (70% Active Octylphenol Ethoxylate in water), and / or Tween 80 ( >58.0% Oleic acid, balance primarily linoleic, palmitic, and stearic acids)), pH modifiers, cationic (CTAB) or nonionic surfactants which can alter surface energetics between aqueous and gas phases to improve wetting, additives for nucleation and crystal growth etc.

[0061] Additionally or alternately, additives may promote the dissolution of Ca components in RFC.

[0062] Examples of suitable additives include but are not limited to, strong acids (e.g., HCI, HNO3, and H2SO4), organic acids (e.g., acetic acid, formic acid, succinic acid, oxalic acid, etc.), and / or salts (e.g., NaCI, NH4CI, trisodium citrate, disodium EDTA, sodium oxalate, sodium).

[0063] Mixing

[0064] The damp RCF are mixed in a mixer. Any suitable mixer can be used but a rotary pan-type mixer with a motorised propeller and angled blades or a planetary mixer to maximise the agitation and shearing action applied to RCF particles is preferable to other types of mixer, such as drum-type mixers.

[0065] The mechanical agitation used in this invention during the mineral carbonation process offers some distinguished advantages: First, it increases mass transfer by creating turbulence and shear forces within the reaction mixtures, leading to an enhanced reaction surface area of damp RCF particles to CO2 throughout the carbonation process. Second, it enhances gas-liquid-solid interaction and contact by promoting continuous movements of the reaction mixture, ensuring thorough contact between carbon dioxide gas, water, and the reactant mineral phases. This accelerates the carbonation process and leads to faster uptake of CO2 by the RCF. Third, the aggressive mixing action leads to improved particle dispersion and to break up agglomerates as well as faster homogenisation of RCF particles and water specifically when the addition of water is needed for carbonation. All these benefits contribute to faster, more efficient, and controlled mineral carbonation processes, leading to enhanced carbonation efficiency. The whole process of carbonation occurs while the damp RCF are mixed in the mixer. The agitator speed can be up to 1200 rpm. The mixing time (equivalent to carbonation time) can be up to 30-45 minutes.

[0066] Carbonation

[0067] The damp RCF are subject to accelerated mineral carbonation in the presence of carbon dioxide to form carbonated RCF. This is also known as thin-film mineral carbonation or gas- liquid-solid interface accelerated carbonation. Following the start of mixing and the introduction of CO2into the mixer, the CO2efficiently dissolves in the thin water film (around 500 nm thick) coating the surface of RCF particles (or when additional water is added in the mixer), forming carbonic acid which reacts with calcium ions leaching from calcium-bearing compounds (mainly calcium silicate hydrates) in the RCF to form carbonate minerals. The carbonates (mainly calcite phase) precipitate and deposit on the particle surfaces, pores, voids and microcracks in the RCF’s microstructure which leads to modification and densification of the microstructure and interfacial transition zone (ITZ) between the old cement paste and aggregates and reduction in overall porosity.

[0068] As outlined above, RCF are an advantageous starting material for mineral carbonation from an operational point of view because the carbonation reaction does not generate significant exothermic heat compared to many industrial by-products like cement bypass dust or paper sludge ash. This eliminates the need for specific considerations and measurements during the process, as discussed above. Any concentration of carbon dioxide suitable for accelerated mineral carbonation can be used. In some embodiments, the concentration of carbon dioxide in the carbonation step is between about 15% to about 99.9%. In some embodiments, the concentration of carbon dioxide is within the range of 10-20%. This is a range that corresponds closely to the exhaust gas emissions of cement manufacturing plants. Therefore, exhaust gas can be used as the CO2 source, further reducing waste emissions.

[0069] The carbonation may occur at any temperature. In preferred embodiments, the carbonation occurs at ambient temperature. Ambient temperature is also referred to as room temperature. Generally, ambient temperature is understood to mean a temperature between about 15° and about 25°C.

[0070] The carbonation may occur at any pressure. In preferred embodiments, the carbonation occurs at ambient pressure. Ambient temperature is also referred to as atmospheric pressure. Generally, ambient pressure is understood to mean a pressure of about 1 bar.

[0071] The carbonation may occur for as long as necessary to carbonate the RFC. In some embodiments, the damp RFC are carbonated for up to 45 minutes, preferably between 30 minutes and 45 minutes.

[0072] Particle size

[0073] RCF typically have a maximum particle size of 5 mm. Therefore in some embodiments, it can be said that the RFC have a maximum particle size of about 5 mm. Preferably, the RFC have a maximum particle size of about 4.75 mm, for example, about 4.5 mm, or about 4 mm. It can alternatively be said that the RFC have a maximum particle size of up to about 5 mm, preferably up to about 4.75 mm, for example up to about 4.5 mm, or up to about 4 mm.

[0074] The small maximum particle size leads to a faster and more efficient carbonation reaction. This is due to the larger surface area, providing more reactive sites for the reaction of speciated carbon dioxide with the adhesive cement paste matrix in the RFC particles. This increased surface area enhances the gas-liquid-solid contact between CO2, water and the RCF particles, accelerating the carbonation reaction.

[0075] In some embodiments, the process may further comprise a step of initially milling the recycled concrete fines before mixing with water. Preferably, the RFC are dry or partially dried before milling. This can be done using any suitable energy-efficient grinding methods / processes. In some embodiments, the milling can be carried out using a ball mill to crush and pulverise the RCF with moderate energy consumption. Attrition mills can also be used which are less energy-intensive than ball mills. Generally, the milling step proposed in this invention does not necessitate a high energy-intensive milling process because super fine particle sizes are not required.

[0076] In some embodiments, the RFC are milled to a maximum particle size of up to about 500 pm. A maximum particle size of up to about 500 pm provides excellent carbonation efficiency due to the enhanced particle fineness and hence the surface area. Preferably the RFC are milled to a particle size distribution of d(10) of up to about 2 pm, and / or d(50) up to about 55 pm and / or d(90) of up to about 360 pm. Preferably, the recycled concrete fines have a particle size distribution of d(10) up to about 2 pM, d(50) up to about 55pM and d(90) up to about 360pm.

[0077] Supplementary cementitious material (SCM) and / or filler

[0078] The claimed invention also relates to carbonated RCF prepared by the process described above. The carbonated RCF produced by the claimed processes can be used as supplementary cementitious material and / or filler which can have a pozzolanic reactivity stemming from the reactive alumina-silica gel formed after the carbonation of hydrated cement compartment. This SCM can be used directly as a partial replacement of Portland cement for producing low-carbon concrete. It also has the potential to be used as a partial replacement for Portland clinker to develop new low-carbon composite cement or blended cement.

[0079] As outlined above, in contrast to many thermal residues and byproducts, RCF, as the proposed feedstock for carbonated material production, generally do not pose the challenges associated with high chloride and sulfate contents, or elevated concentrations of heavy metals.

[0080] Secondary aggregates

[0081] The carbonated RCF can be further processed into artificial secondary carbonated aggregate. This has the advantage of preserving natural resources used for aggregate production in the construction industry and reduces the demand for virgin aggregates after the use of RCF for the sequestration of CO2.

[0082] Accordingly, in some embodiments, the process further comprises (c) blending the carbonated RFC with a binder to produce a blended mixture; and (d) controlled pelletising (throughout an aggregation or granulation process) of the blended mixture; to produce secondary aggregate. The addition of binder gives the carbonated material structural integrity and particle binding. The pelletising process facilitates the collision of particles and makes them adhere together to form artificially aggregated granulates.

[0083] Any binder that can be used to form synthetic aggregates can be used in the present invention. In some embodiments, the binder is a low-carbon hydraulic binder. In these embodiments, the binder may be selected from the group consisting of Portland cement, blended cement comprising Portland fly ash cement, Portland slag cement and Portland limestone cement; blast-furnace cement (incorporating ground granulated blast-furnace slag), pozzolanic cement (incorporating natural or synthetic pozzolanic material); composite cement, limestone calcined clay cement (LC3), calcium sulfoaluminate cement, magnesium- based cement, ternesite-rich cement, belitic calcium sulfoaluminate cement and / or carbonation-hardened cement. In some embodiments, the binder is the carbonated milled RFC proposed in part of the processes in this invention.

[0084] The amount of binder required in the process to produce secondary aggregate can be varied depending on the required quality and physical properties of the aggregates including, but not limited to, strength development, crushing resistance value, density, and water absorption. Generally, a higher binder content leads to enhanced early strength gain, higher crushing value, higher density and lower water absorption of the produced aggregates. However, the use of low-carbon binders requires a balance between the quality and physical properties of the produced secondary aggregates and their carbon footprint. A higher proportion of Portland clinker in the composite binder generally results in improved properties but also increases the carbon footprint.

[0085] The properties of aggregates can therefore be adjusted by changing the amount and type of binder. In some embodiments, the carbonated RFC are blended with up to 15 wt.% binder.

[0086] The carbonated RCF and binder are preferably blended together in a mixer while a specific percentage of water may be added to this mixture. This water addition may be required to facilitate the granulation of the mixture during the controlled palletisation stage. It also is critical to the progress of hydration of the hydraulic binder. In some embodiments, the carbonated RFC and binder are blended together in up to about 12% w / w water, for example up to about 10% w / w water, or about 8% w / w water. In some embodiments, the carbonated RFC and binder are blended together in between about 5% w / w water and about 12% w / w water. Following the addition of water, the mixing is continued for up to 2 minutes to ensure ample homogeneity of the mixture with water. During this stage, the granulation may be simultaneously triggered.

[0087] Addition of fine inert filler (like limestone) might be helpful to modify the particle size distribution of carbonated RCF and improve the particle packing when manufacturing secondary aggregates.

[0088] The blended mixture with optional added water is subsequently pelletised. This may be done by transferring the blended mixture into a rolling drum granulator, where it is lightly rolled and tumbled. As the carbonated particles tumble, they collide and adhere together. The continued repetitive collisions, aided by the rolling and cascading motion gradually build up granules as more and more carbonated recycled fines stick together with the help of a binder (as proposed above). This controlled tumbling agglomeration process finally transfers the mixture of carbonated RCF with a binder to the secondary aggregates with robust structure and integrity.

[0089] Following approximately 5 to 10 minutes of residence time in the rotating drum, the final product emerges as granulated secondary aggregate pellets with established mechanical integrity. The physical properties of these produced secondary aggregates continue to enhance over time, driven by the ongoing hydration of binder as well as the improvement in the ITZ interface between the old cement paste, carbonated matrix and new hydrated binder.

[0090] Accordingly, there is also defined herein, secondary aggregate prepared by the process described above. There is also defined herein secondary aggregate prepared using the carbonated RFC prepared by the process described above.

[0091] The secondary aggregate can be screened to desired aggregate size fractions and reused as a low-carbon construction material in any application known in the art. In some embodiments, the secondary aggregate can be used in the manufacture of concrete, for example, concrete blocks and / or non-structural low-carbon concrete.

[0092] Examples

[0093] The following examples are specific embodiments of the present invention but are not intended to limit the present invention.

[0094] Introduction This example outlines the carbonation of a RCF sample sourced from a demolished concrete waste site in the UK using the accelerated mineral carbonation method claimed herein. Also, secondary carbonated aggregates were manufactured using the carbonated RCF. The results of various key charactersiations carried out on the carbonated materials as well as the produced aggregates are provided.

[0095] Materials

[0096] Images of the sample are shown in Figure 1 . Moisture content, density, and particle size distribution of the as-received RCF sample were tested and are shown in Table 1. Figure 2 shows the particle size distribution of the as-received RCF sample.

[0097] Table 1: Physical properties of as received RCF sample

[0098] Property RCF sample

[0099] Moisture Content (%) 9.1 e Bulk Density (kg / m3) 1020 d Bulk Density (kg / m3) 1288

[0100] RCF mineralogy was examined using the X-ray diffraction (XRD) method and the results of quantitative phase analysis using the Rietveld refinement method are shown in Table 2.

[0101] Table 2: XRD Rietveld quantification phase analysis of three independent samples taken from RCF material

[0102] RCF - Sample 1 RCF - Sample 2 RCF - Sample 3

[0103] Sample 1 Sample 2 Sample 3

[0104] Calcite 46.8 40.6 42.8

[0105] Quartz 16.8 15.1 18.1

[0106] Portlandite 1.2 1.4 1.3

[0107] Dolomite 4.5 3.5 4.1

[0108] Illite 3.5 - 3.9

[0109] Albite 2.7 1.8 3.0

[0110] Microcline 2.3 2.1 1.1

[0111] Kaolinite - 0.9

[0112] Fluorite 1.6 1.8 1.6

[0113] Chlorite 0.8 - 0.8

[0114] Hydrotalcite 0.4 - 0.5

[0115] Ettringite - 1 .0

[0116] Gypsum - 0.9 Amorphous content 19.4 30.9 22.8

[0117] Table 2 presents the data indicating that RCF contains an insignificant amount of portlandite, which is a desirable phase for carbonation. Consequently, the carbonation efficiency and yield of this RCF sample primarily depend on the amorphous phases, predominantly associated with the calcium silicate hydrates (C-S-H) phases. The relatively high presence of calcite phase suggests the occurrence of carbonate minerals, mainly originating from dolomitic rocks, and natural carbonation of the materials (as it has been subjected to natural carbonation). Additionally, proportions of phases related to feldspar-rich igneous rock (primarily from the feldspar-rich aggregates present in the recycled concrete), quartz, and minor phases of clay minerals were detected.

[0118] A portion of the as-received RCF was dried in the oven at 50°C (to avoid changes in C-S-H delicate phases) and a portion of the dried RCF was ground using a vibratory disk mill. The former was labelled as “driedRCF” and the latter as “RCFmilled”. The images of these materials are shown in Figure 1 ((b) and (c)) and a summary of properties is given in Table 3.

[0119] To manufacture secondary aggregates, two types of Portland cement including ordinary Portland cement, CEM I and a blended cement CEM lll / A (Hanson cement UK) as a low- carbon binder were used as binders (see Table 3 for their properties). A commercial sample of ground limestone fine (labelled as “LF”) was also used to make a mix.

[0120] Table 3: Physical properties of dried RCF, RCFmilled sample as well as binders

[0121] ‘Measured by a gas pycnometer (nitrogen)

[0122] *CEM lll / A contains 40% Ground Granulated Blast-furnace Slag (GGBS)

[0123] Methodology and experimental design

[0124] Three scenarios were designed and explored to process RCF samples in this example.

[0125] These included the use of as-received RCF (labelled as “AR RCF”), dried RCF and

[0126] RCFmilled throughout the processes. Each case involved accelerated carbonation followed by the manufacture of small batches (-450 g) of aggregate. The aggregate was produced in the laboratory and cured in a closed chamber for up to 28 days, enabling the development of the optimum process conditions to manufacture the highest quality aggregate possible. Figure 3 shows a schematic of the experimental design used in this example.

[0127] Individual aggregate strength was tested using a modified ASTM D4179 procedure. It involved crushing ten single aggregates with a force gauge and normalising the maximum failure force to the aggregate dimensions.

[0128] Mixing and carbonation were performed using a planetary pan-type mixer in the lab. A mix loading of 440g was used to treat as received RCF (AR RCF) for which the 9.1 % moisture content was taken into account. For the dried RCF and RCFmilled, 400g of materials were used in each run. The carbonations using room temperature gas were carried out at 100 Vol.% CO? and a total gas flow of 10 litre / min.

[0129] A star-shaped revolving blade was used in the mixer at a speed of 1200 rpm or the whole period of mixing and carbonation. Then, a speed of 800 rpm was used after the addition of water required for the aggregation stage and just before the start of pelletising.

[0130] Table 4 shows the composition of mixes indicating the proportions of components used during processing. Initial mixes (Mix 02 and Mix 03) were trials to examine the aggregation of as-received RCF in the presence of binders. Then, the binders' percentages were adjusted in the remaining mixes to 10 and 15 % for CEM I and CEM III, respectively. Following trials, it was found that producing secondary aggregate from RCF requires a relatively high proportion of binder. It is hypothesized that this is due to the coarse particle size distribution of RCF.

[0131] To keep the binder proportion as low as possible and facilitate the aggregation, it can be helpful to add powdered filler to modify the particle size distribution. Two ground fillers were added at 10 wt.% (of dried RCF weight in each mix) along with binders to the postcarbonated materials within the groups of the as-received RCF and dried RCF mixes. These included ground commercial limestone (containing nearly 78 wt.% calcite) and RCF milled.

[0132] This study included two mixes (Mix07 and Mix08) designed to isolate the sole effect of carbonation on secondary aggregate properties. Made using as received RCF (Mix07) and dried RCF (Mix08 - containing 5% added water and therefore about 8% total moisture content), these control mixes serve as direct comparisons to Mix05 and Mix09 (also containing 5% added and therefore about 8% total moisture content), respectively. Notably, they share the same composition as their counterparts except for the absence of carbonation treatment. This enables a clear assessment of how carbonation alters the properties of secondary aggregates made from RCF.

[0133] To determine the CO2 uptake during the carbonation process for each mix, the direct quantification of CO2 contents was carried out using a CO2 / H2O analyser (ELTRA CW-800, Germany). The samples dried at 50°C for 24 hours before testing. About 800 mg of ground sample was used for the analysis.

[0134] True Density (skeletal density) of samples was measured using a gas pycnometer (Ultrapyc 1200e nitrogen Pycnometer; Quantachrome Instruments, UK).

[0135] The particle size distribution of the ground materials and binders used in this example were determined using a Mastersizer 3000 optical instrument coupled to an Aero S dry powder dispersion accessory (Malvern Instrument Ltd, UK).

[0136]

[0137] Results and discussion

[0138] Particle distribution of fine materials

[0139] The particle size distribution (PSD) for the binders (CEM I and CEM III) and ground materials used as PSD modifiers (LF and RCFmilled) are shown in Figure 4. Binders have a monomodal distribution with 100% and 50% of the volume of particles smaller than 100 pm and 10 pm, respectively. However, the ground limestone and RCFmilled have a broader particle size distribution than the binders, despite having a similar distribution for particles below 5 pm. The size distribution for LF and RCFmilled particles are nearly identical. This was deliberately achieved by attempting a couple of milling trails to make sure the results of Mix 04 would be comparable to the other mixes incorporated with RCFmilled.

[0140] Approximately 60% of the volume of particles is finer than 100 pm for RCFmilled which was also used for the direct carbonation in the scenario of case #3 (Mixes 12 and 13 in Table 4). The particle size below 100 pm is preferred for increased surface area, maximising the surface for carbonation reactions.

[0141] Properties of post-carbonated materials

[0142] The CO2 content of samples taken from the post-carbonated materials for each mix was measured and the average values corresponding to each scenario of accelerated carbonation are given in Table 5. (a) the average value from 5 different samplings and 18 measured replicates.

[0143] (b) the average of 6 replicates taken from 2 different mixes

[0144] (c) the average of 9 replicates taken from 3 different mixes

[0145] (d) all values in this group of data are the average of 3 replicates

[0146] The results indicate that, in the context of the accelerated carbonation process studied, the order of CO2 captured is as follows: RCFmilled > dried RCF > AR RCF. The carbonation of RCFmilled resulted in a higher CO2 capture, with an increase of up to 1 .68 wt.% and an equivalent of 7.0% CO2 uptake compared to the non-carbonated material. In contrast, the carbonation of as-received RCF and dried RCF captured 0.20 wt.% and 0.60 wt.% of CO2, corresponding to 0.8% and 2.5% CO2 uptake, respectively. The better performance of RCFmilled in capturing CO2 is thought to be due to exposing a larger surface area of the cement paste particles (mainly C-S-H gel as a reactant material) to carbonation reaction when an optimum amount of water was added to promote thin film carbonation. The relatively higher quantity of moisture content present in AR RCF (9.1%) than the water added to dried RCF mixes (see Table 4 - total moisture content of 7-8%) can be the reason for lower CO2 captured for AR RCF post-carbonated material. This is because of the adverse effect of the excess water film around the particles in thin film carbonation.

[0147] Properties of the aggregates manufactured from post-carbonated materials Physical properties

[0148] Images of manufactured aggregates from the post-carbonated materials for each mix are shown in Figures 5-8 and their loose bulk density as well as the average size are given in Table 6. Efforts were made to manufacture aggregates within a similar size range to ensure accurate and reliable comparisons of their compressive strength. The addition of ground filler (LF and RCFmilled) promotes the overall aggregation performance, enabling to reduction of the content of the binder from 16 % and 20 % down to 10 % and 15% for CEM I and CEM III incorporated mixes, respectively.

[0149] Despite the reduction in binder content, the addition of fillers as particle size distribution modifiers slightly increased the bulk density of aggregates up to 3 % (for example see the results of Mix05 compared to Mix03). It is hypothesized that this could be due to better particle packing and a reduction in large pores and voids in the microstructure of aggregates. Also, the aggregates manufactured using carbonated AR RCF obtained nearly 6 % higher bulk density compared to the non-carbonated mix (Mix07).

[0150] Table 6: Loose bulk density and the average size of aggregates manufactured from the post-carbonated materials

[0151]

[0152] Compressive strength

[0153] Figures 9 to 11 depict the results of individual aggregate strength development measured at 1 day, and 3, 7, 14, and 28 days for different aggregate mixes designed with three carbonation scenarios.

[0154] Overall, all mixes manufactured with binders (except Mix04 with LF) achieved 1 .0 Mpa for compressive strength by 3 days onwards.

[0155] Among the AR RCF mixes (case 1 ), Mix02 exhibited the highest early compressive strength due to its larger binder proportion. When the binder was reduced and RCFmilled was added as a filler, Mix05 outperformed both the non-carbonated control mix (Mix07) and the mix incorporating LF (Mix04) at 3 and 7 days. This improvement can be attributed to the enhanced cement hydration achieved by the addition of RCFmilled, which provided additional nucleation sites for the formation of newly precipitated C-S-H gel, unlike limestone fines. A similar trend was observed in the comparison of Mix03 and Mix06, where the reduction of CEM III binder and the addition of 10% RCFmilled in Mix06 resulted in increased compressive strength at 3, 7, 14, and 28 days. Notably, the mix containing 15% CEM III exhibited the highest mid-term and long-term compressive strength among the mixes with adjusted binder content. This can be attributed to the pozzolanic reaction between the GGBS in OEM III (as blended cement) and portlandite, leading to the formation of additional C-S-H gel and improved strength performance at later ages.

[0156] An optimum amount of water was initially added in mixes to facilitate the accelerated carbonation of dried RCF (case 2 in Table 4). Within the mixes manufactured from carbonated dried RCF (with an additional optimum amount of water for carbonation) shown in Figure 10, Mix09 and Mix10 showed a better compressive strength at later ages than the non-carbonated control mix (Mix08) with the same initial water for carbonation. It is hypothesized that this could be the result of an improved Interfacial transition zone (ITZ) interface between the RCF (as an old interface) and to new cement paste, leading to better interlocking and the lowest overall porosity. This effect was seen to be even more stressed in the case of Mix10, when a lower percentage of added water (4%) was used for carbonation of driedRCF, (making the total moisture content of the damp RCF about 7%) but slightly more water was needed to manufacture the same size of aggregate within these three mixes. The explanation for this observation can be attributed to the presence of a slightly higher amount of free water added at the stage of post-carbonation and the start of aggregation in the mixer and pelletised. The marginally higher water can potentially promote the hydration of CEM I as this system can already be classified as a low-water to binder cement-based system. Thus, any excess water can significantly accelerate the hydration of Portland cement, leading to a considerably improved strength. Similar to what was seen in the results of the as-received case (Figure 9), Mix11 containing CEM III exhibited an enhanced later-age strength, increasing the 28-day compressive strength by 65% compared to the same age in Mix08 with CEM I.

[0157] The mixes that were produced using carbonated RCFmilled demonstrated considerably higher compressive strength starting from 3 days onwards, in comparison to all the mixes examined in both case 1 and case 2 groups (refer to the results of Mix12 and Mix13 in Figure 11 ). This enhancement can be attributed to the reduction in the overall porosity of the system when fine particles, containing already carbonated C-S-H gel, are bound (and glued) together using a cement-based binder such as Portland cement. However, the presence of relatively coarse particles (up to 4 mm particle size of RCF) in the first two scenarios investigated in this example, can result in the formation (and existence) of voids and entrapped air during the aggregation process in the pelletisation stage, leading to reduced mechanical performance.

[0158] Conclusions

[0159] The following concluding remarks can be drawn from the results of this example: 1) The accelerated carbonation process applied to ground RCF resulted in the capture of up to 1.68 % of CO2, whereas dried RCF and as-received RCF captured 0.6 % and 0.2 %, respectively. This suggests that 1 tonne of ground-dried RCF can potentially capture and store up to 17 kg of CO2 using the accelerated carbonation method proposed in this invention at ambient pressure and temperature. The large disparity in the capacity of capturing CO2 between ground RCF and non-ground RCF can be attributed to the increased surface area of particles when the sample is milled, leading larger surface of C-S-H gel exposed to the carbonation reactions.

[0160] 2) It was shown to be feasible to manufacture carbonated product aggregates from RCF with sufficient strength, spherical irregular shape and a size range of 10-12 mm to be potentially utilised in concrete block manufacturing and / or a partial replacement of natural aggregates in structural and non-structural concrete mixes. Aggregates with individual compressive strengths of 2.0 MPa, 2.6 Mpa and 2.7 MPa at 28 days were manufactured from as-received, dried and ground RCF, respectively.

[0161] 3) The addition of fine filler was found to be helpful to modify the particle size distribution of non-ground RCF and improve the particle packing when manufacturing aggregates.

[0162] 4) Out of the three scenarios explored in this example, the most favourable case, which achieved maximum CO2 capture, superior aggregation performance, and compressive strength, while minimizing the required binder content, was found to be the production of aggregates from carbonated ground RCF with an initial water addition of 7.5% for efficient carbonation (moisture content of damp RCF about 10.5%) - Mix 12. It can be suggested that even with slightly lower proportions of binders, such as less than 10% for CEM I and CEM III, the desired properties and sufficient strength level could still be maintained.

[0163] 5) It became evident that the aggregates created from dried RCF can have a better strength development than those produced from as-received RCF. That can be linked to better control of the optimum water required to thin-film carbonation as well as the relatively higher amount of free water available to enhance the hydration of the binder.

[0164] 6) The utilisation of blended low-carbon cement (CEM III) was discovered to be very advantageous to improve the strength development of manufactured aggregates. Aggrgetes incorporating 15% CEM III outperformed those with 10% CEM I starting from 7 days onwards in almost all three scenarios examined in this example. For the avoidance of any doubt, the terms “a”, “an” and “the” are intended, unless specifically indicated otherwise or the context requires otherwise, to include plural alternatives, e.g., at least one.

[0165] "Optional" or "optionally" means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0166] Various other modifications to the present invention will be readily apparent to those skilled in the art.

Claims

Claims1 . A process for the preparation of carbonated recycled concrete fines, the process comprising the steps of:(a) mixing damp recycled concrete fines having a moisture content of between about 5% w / w and about 12% w / w in a mixer;(b) accelerated mineral carbonation of the damp recycled concrete fines in the presence of carbon dioxide to form carbonated recycled concrete fines.

2. The process according to claim 1 , wherein the damp recycled concrete fines have a moisture content of between about 8% w / w and about 10% w / w.

3. The process according to claim 1 or 2, wherein the recycled concrete fines have a maximum particle size of about 4mm.

4. The process according to any one of claims 1 to 3, wherein step (a) involves mixing dry or partially dried recycled concrete fines with water to provide the damp recycled concrete fines.

5. The process according to 4, further comprising a step of initially milling the dry or partially dried recycled concrete fines before mixing the milled recycled concrete fines with water.

6. The process according to claim 5, wherein the recycled concrete fines are milled to a maximum particle size of up to about 500 pM.

7. The process according to claim 5 or claim 6, wherein the recycled concrete fines have a particle size distribution of d(10) up to about 2pM, and / or d(50) up to about 55pM, and / or d(90) up to about 360pm.

8. The process according to any one of claims 1 to 7, wherein the concentration of carbon dioxide in the carbonation step is between about 15% to about 99.9%9. The process according to any one of claims 1 to 8, wherein the carbonation occurs at ambient temperature.

10. The process according to any one of claims 1 to 9, wherein the carbonation occurs at ambient pressure.11 . The process according to any one of claims 1 to 10, wherein the recycled concrete fines are obtained from concrete demolition waste.

12. The process according to any one of claims 1 to 11 , wherein the damp recycled concrete fines are carbonated for up to 45 minutes, preferably between 30 minutes and 45 minutes.

13. The process according to any one of claims 1 to 12, further comprising:(c) blending the carbonated recycled concrete fines with a binder to produce a blended mixture; and(d) pelletizing the blended mixture; to produce secondary aggregate.

14. The process according to claim 13, wherein the binder is a low-carbon hydraulic binder, optionally selected from the group consisting of Portland cement, blended cement comprising Portland fly ash cement, Portland slag cement and Portland limestone cement; blast-furnace cement (incorporating ground granulated blast-furnace slag), pozzolanic cement (incorporating natural or synthetic pozzolanic material); composite cement, limestone calcined clay cement (LC3), calcium sulfoaluminate cement, magnesium-based cement, ternesite-rich cement, belitic calcium sulfoaluminate cement and / or carbonation- hardened cement.

15. The process according to claim 13 or 14, wherein the carbonated recycled concrete fines are blended with up to 15 wt.% binder.

16. The process according to any one of claims 13 to 15, wherein the carbonated recycled concrete and binder are blended together in up to about 12% w / w water, preferably between about 5% w / w and about 12% w / w water.

17. Carbonated recycled concrete fines prepared by the process of any one of claims 1 to 12.

18. Use of the carbonated recycled concrete fines according to claim 17 as supplementary cementitious materials and / or filler.

19. Secondary aggregate prepared by the process of any one of claims 13 to 16, or prepared using the carbonated recycled concrete fines according to claim 17.

20. Use of the secondary aggregate according to claim 19 in the manufacture of concrete.

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