Cement additives, cement compositions, and methods of preparing such

Abstract

A cement additive comprises, consists essentially of or consists of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

Description

[0001] Cement additives, cement compositions, and methods of preparing such

[0002] Field of the Invention

[0003] The present invention relates to methods for preparing cement additives, to cement additives and cement compositions, and to methods for altering the solubility of metals in silicate rocks.

[0004] The development of increasingly efficient technologies for the capture of carbon dioxide (CO2) emissions is key to achieving global carbon targets.

[0005] Ex-situ direct mineral carbonation technologies aim to trap CO2 in the form of carbonates. Most research has produced direct carbonation by reaction of CO2 at elevated temperatures of 400-600 °C with the mineral olivine or with rocks rich in magnesium and calcium (carbonate-forming metals) such as dolerite, basalt and peridotite. However, the process requires substantial energy input; samples must first be powdered to create a large surface area, and then reacted with CO2 at temperatures of 500 °C, producing trapping efficiencies of 0.255-0.26 mg CO2 g-1for olivine minerals (forsterite) and 1.26-13.43 mg CC^ g-1for magnesium- and calcium-rich rocks such as olivine basalts, dunites and doleritic mine waste.

[0006] Mechanochemistry is a branch of chemistry whereby some, or all, of the energy required for a chemical reaction is provided by the deformation of a solid substance, in this case grinding of rocks and minerals, with the aim of reducing the external energy input to the process by reducing the heat or pressure increase required. In mechanochemistry, fracturing of the chemical bonds in solids releases charged particles and photons and is thought to produce an instantaneous localized temperature increase: a localized temperature of 648 °C is estimated (see Torre et al. Powder Technol. 364, 915-923 (2020)) when grinding olivine at ambient temperature in water with an atmosphere of CO2, resulting in the production of light hydrocarbons.

[0007] Recent research (M. Stillings, Z. K. Shipton, R. J. Lunn (2023) Mechanochemical processing of silicate rocks to trap CO2. Nature Sustainability, 6(7): 1-9. DOI: 10.1038 / s41893-023-01083-y) describes the mechanochemical CO2 trapping capability of commonly available polymineralic rocks, using various types of rock that are both high (basalt) and low (granite) in magnesium and calcium. It was demonstrated that, through mechanochemical processing of silicate rocks, CO2 is chemically trapped into the crystal structure, predominantly at the boundaries between different minerals. Further, it was shown that CO2 becomes permanently trapped in an insoluble form within the crystal structure. Under ambient temperature conditions, polymineralic rocks can capture >13.4 mg CO2 g"1as thermally stable, insoluble CO2. This study only relates to the capture of CO2 within silicate rocks via mechanochemical processing.

[0008] A problem with the use of metal-containing natural resources is the unintended release of toxic metals from those natural resources, for example upon exposure to water. For examples, silicate rocks typically contain various concentrations of transition metal species that may be released when mixed with water, which may cause heath and / or environmental concerns in using such materials.

[0009] Cement is a ubiquitous construction material, usually used to bind various construction components together. Typical commercially available cement combines Ordinary Portland Cement (OPC) and an additive material, commonly pulverised fly ash (PFA) or blast-furnace slag (BFS). The additive material is utilized for two main reasons: (i) it reduces the amount of ordinary Portland cement required and (ii) it can improve the workability, strength and / or durability of hardened concrete products formed using the cement mixture. A problem with the use of such additives is that they are inextricably linked with industries associated with a high carbon footprint, i.e. coal combustion in the case of PFA, and iron or steel production in the case of BFS.

[0010] US 2022 / 0396526 A1 (Sinha) discloses compositions comprising a mechanochemically carboxylated mineral filler and a binder, wherein the binder is cement and / or asphalt and wherein the filler is obtainable by mechanochemically carboxylating a silicate mineral. However, the method exclusively uses specific minerals either alone or in combination, and preferably uses fly ash.

[0011] It is an object of the invention to address and / or mitigate one or more problems associated with the prior art.

[0012] It is an object of the invention to provide alternative and more environmentally- friendly cement additives.

[0013] It is an object of the invention to alter the solubility of one or more metals in silicate rocks, e.g. to increase the solubility of certain valuable metals, and / or to decrease the solubility of certain toxic metals.

[0014] Summary

[0015] According to a first aspect, there is provided a cement additive, the cement additive comprising, consisting essentially of or consisting of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0016] Typically, the molar concentration of CO2 in the gas composition may be between about 0.05% and 100%.

[0017] The molar concentration of CO2 in the gas composition may be greater than 0.04%, e.g. at least 0.1%, e.g. at least 1 %, e.g. at least 10%, e.g. at least 50%, e.g. at least 90%, e.g. at least 99%.

[0018] In some embodiments, the gas atmosphere may be a pure or substantially pure CO2 atmosphere, e.g. may have a molar concentration of C020f at least 99%, e.g. at least 99.9%, e.g. at least 99.99%.

[0019] In other embodiments, the gas atmosphere may comprise, may be or may consist of an effluent gas stream or a mixture of effluent gas streams. The effluent gas stream(s) may be derived from or may be produced by industrial processes such as cement production, e.g. during calcination, asphalt production, blue hydrogen production, natural gas power generation, and biomass-fired power stations.

[0020] Advantageously, using a gas effluent stream as or in the gas atmosphere for milling the silicate rock, may allow the capture of carbon dioxide within the milled rock, thereby reducing the carbon footprint of the associated industrial activity generating the gas effluent stream.

[0021] The effluent stream may comprise, in addition to carbon dioxide, one or more additional gases including nitrogen (N2), carbon monoxide (CO), oxygen (O2), nitrogen Oxides (NOx), sulphur oxides (SOX) and methane (CH4).

[0022] Advantageously, the inventors have found that milled silicate rock compositions which have been generated through mechanochemical treatment with CO2 in a concentration above atmospheric concentration, trap CO2 in an insoluble form within the crystal structure. Further, and advantageously, it was discovered that, when mixed with cement, the chemical properties of the mechanochemically treated powder increase the mechanical properties of the resulting concrete composition, for example strength and / or durability, compared to concrete compositions including conventional cement additives. Thus, these compositions represent a lower carbon and globally abundant alternative to common cement additives such as PFA or BFS, whilst improving or at least maintaining the mechanical properties associated with such additives.

[0023] Typically, the milled silicate rock may have an average particle size, e.g. diameter, of less than about 1 mm, e.g. less than about 0.5mm, e.g. less than about 0.1 mm, e.g. less than about 50pm, e.g. less than about 20pm. The milled silicate rock may have an average particle size, e.g. diameter, of about 0.1-100pm, e.g. about 1-80pm, -e.g. about 5-50pm, e.g. about 10-20pm.

[0024] The term “silicate rock” will be herein understood to refer to a whole naturally- occurring silicate-mineral bearing rock type, rather than to a specific mineral constituent thereof. Therefore, the term “silicate rock” may herein be defined as a naturally-occurring silicate rock, a natural silicate rock, a polymineralic silicate rock, and / or a silicate rock containing a plurality of minerals. These take the form of rocks comprised of interlocking crystals or grains of a single mineral phase or a polymineralic silicate rock which is a silicate rock containing more than one type of mineral. Typically, a silicate rock includes two or more interlocking crystals or grains of a single mineral phase, which includes one or more types of mineral at least one of which contains silicon. Without wishing to be bound by theory, this is significant because the inventors surprisingly found that whole silicate rock types which typically comprise a mixture of two or more individual mineral types in varying amounts, behave differently from the specific mineral types taken individually or mixed together, in relation to one or more of the following properties:

[0025] When milling under a CC>2-rich atmosphere, the proportion of CO2 trapped in insoluble form is greater when milling silicate rocks than when milling specific mineral types such as olivine or mica, whether alone or in combination. In other words, it was found that CO2 trapped in milled specific mineral species found in silicate rocks is more susceptible to leaching out than CO2 trapped in milled (whole) silicate rocks;

[0026] The strength of a cement composition using CO2-milled silicate rock as a cement additive is greater than the strength of a cement composition using, as a cement additive, a mixture of CO2-milled mineral types found in silicate rock.

[0027] The milled silicate rock may comprise, may consist essentially of or may consist of one or more types of silicate rock, including for example felsic or granitic rock, andesitic or intermediate rock, mafic or basaltic rock, and / or ultramafic rock.

[0028] Typically, the milled silicate rock may comprise, may consist essentially of or may consist of a mixture of silicate rock-forming minerals, including for example orthoclase, quartz, plagioclase, pyroxene, and / or olivine. It will be understood that such rock-forming silicate minerals are typically found in varying amounts in different types of naturally occurring silicate rock. According to a second aspect, there is provided a cement composition comprising: a hydraulic cement component; and a cement additive, wherein the cement additive comprises, consists essentially of or consists of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0029] Typically, the hydraulic cement component may comprise, may consist essentially of or may consist of Portland cement. Portland cement may also be named Ordinary Portland cement (OPC).

[0030] The cement additive may be a cement additive as described in the first aspect, the features of which are not repeated here, merely for brevity.

[0031] Typically, the amount of cement additive may be about 1-90 wt%, e.g. about 5- 60 wt%, e.g. about 10-40 wt%, of the cement composition.

[0032] According to a third aspect, there is provided a method for manufacturing a cement additive, the method comprising: milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0033] The method may comprise milling the silicate rock under a gas atmosphere in which the concentration of CO2 is greater than 0.04%, e.g. at least 0.1%, e.g. at least 1 %, e.g. at least 10%, e.g. at least 50%, e.g. at least 90%, e.g. at least 99%.

[0034] The method may comprise milling the silicate rock in a milling apparatus, e.g. a planetary ball mill.

[0035] The method may comprise feeding the gas atmosphere to the milling apparatus.

[0036] The method may comprise providing at least part of the gas atmosphere from an effluent gas stream. Advantageously, using a gas effluent stream as or in the gas atmosphere for milling the silicate rock, may allow the capture of carbon dioxide within the milled rock, thereby reducing the carbon footprint of the associated industrial activity generating the gas effluent stream.

[0037] Advantageously, the method may comprise trapping carbon dioxide within the silicate rock during milling.

[0038] The method may comprise milling the silicate rock to an average particle size, e.g. diameter, of less than about 1mm, e.g. less than about 0.5mm, e.g. less than about 0.1mm, e.g. less than about 50pm, e.g. less than about 20pm, and / or to an average particle size, e.g. diameter, of about 0.1-100pm, e.g. about 1-80pm, e.g. about 5-50pm, e.g. about 10-20pm.

[0039] Typically, the method may comprise milling the silicate rock at ambient temperature. Advantageously, this may avoid the need for costly temperature control.

[0040] Typically, the method may comprise milling the silicate rock at ambient and / or atmospheric pressure. Advantageously, this may avoid the need for costly pressure control.

[0041] It will be appreciated that, in some embodiments, the method may comprise adjusting pressure and / or temperature, for example in order to optimise CO2 capture within the silicate rock.

[0042] According to a fourth aspect, there is provided a method for manufacturing a cement composition, the method comprising mixing a hydraulic cement component and a cement additive, wherein the cement additive comprises, consists essentially of or consists of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0043] The method may comprise manufacturing the cement additive according to a method of the third aspect.

[0044] According to a fifth aspect, there is provided a method of altering the water solubility of one or more metals in a silicate rock, the method comprising: milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0045] The method may comprise milling the silicate rock under a gas atmosphere in which the concentration of CO2 is greater than 0.04%, e.g. at least 0.1%, e.g. at least 1 %, e.g. at least 10%, e.g. at least 50%, e.g. at least 90%, e.g. at least 99%.

[0046] The method may comprise milling the silicate rock in a milling apparatus, e.g. a planetary ball mill.

[0047] The method may comprise feeding the gas atmosphere to the milling apparatus.

[0048] The method may comprise providing at least part of the gas atmosphere from an effluent gas stream. Advantageously, using a gas effluent stream as or in the gas atmosphere for milling the silicate rock, may allow the capture of carbon dioxide within the milled rock, thereby reducing the carbon footprint of the associated industrial activity generating the gas effluent stream. Advantageously, the method may comprise trapping carbon dioxide within the silicate rock during milling.

[0049] The method may comprise milling the silicate rock to an average particle size, e.g. diameter, of less than about 1mm, e.g. less than about 0.5mm, e.g. less than about 0.1 mm, e.g. less than about 50pm, e.g. less than about 20pm, and / or to an average particle size, e.g. diameter, of about 0.1-100pm, e.g. about 1-80pm, e.g. about 5-50pm, e.g. about 10-20pm.

[0050] Typically, the method may comprise milling the silicate rock at ambient temperature. Advantageously, this may avoid the need for costly temperature control.

[0051] Typically, the method may comprise milling the silicate rock at ambient and / or atmospheric pressure. Advantageously, this may avoid the need for costly pressure control.

[0052] It will be appreciated that, in some embodiments, the method may comprise adjusting pressure and / or temperature, for example in order to optimise CO2 capture within the silicate rock.

[0053] It was discovered that milling silicate rock under a carbon dioxide-enhanced atmosphere caused a modification of the water solubility of certain metals in the rock. Without wishing to be bound by theory, it is believed that the fracturing of silica bonds allows CO2 to form covalent bonds with silica in the mineral lattice. The formation of these bonds may act to either strengthen or weaken the ionic bonds of certain metals in the rock, making them either more, or less, soluble, in water.

[0054] The method may comprise reducing the solubility of one or more transition metals, e.g. one or more metals from any of groups 3 to 12, and / or or one or more posttransition metals, e.g. from any of groups 13 to 16. By such provision, the method may allow the reduction of release of such metals when the milled silicate rock is exposed to water. This may help reduce the risk of pollution and / or contamination by one or more toxic metals contained in the silicate rock prior to milling. In an embodiment, the method may comprise reducing the solubility of one or more transition metals selected from aluminium, iron and manganese. It will be appreciated that the specific transition metals and / or post-transition metals whole solubility is reduced by the present method will depend on the chemical composition of the particular type and geographical origin of the silicate rock being used / milled, as each type of silicate rock may have a different composition with a different combination of, and different amounts of, transition metals and / or post-transition metals. Thus, in an embodiment of the fifth aspect, there is provided a method of reducing the water solubility of one or more transition metals, e.g. one or more metals from any of groups 3 to 12, and / or or one or more post-transition metals, e.g. from any of groups 13 to 16, in a silicate rock, the method comprising: milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0055] The method may comprise increasing the water solubility of one or more metals, e.g. one or more metals from group 1 or group 2. By such provision, the method may allow the release or extraction of such metals when the milled silicate rock is exposed to water. Advantageously, this may allow the recovery of one or more valuable metals from the silicate rock. The method may comprise increasing the solubility of one or more metals selected from calcium, potassium, magnesium, strontium, and lithium. Advantageously, the method may comprise increasing the solubility of lithium.

[0056] Thus, in an embodiment of the fifth aspect, there is provided a method of increasing the water solubility of one or more group 1 or group 2 metals, in a silicate rock, the method comprising: milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

[0057] The method may comprise placing the milled silicate rock in water, e.g. in ultra- pure water, for example for at least 1 hour, e.g. at least 6 hours, e.g. at least 12 hours, e.g. at least 24 hours.

[0058] The method may comprise recovering from the milled silicate rock one or more metals selected from calcium, potassium, magnesium, strontium, and lithium. Advantageously, the method may comprise recovering lithium.

[0059] Advantageously, the present method(s) may allow the recovery of certain valuable metals such as lithium, the trapping of certain toxic metals such as aluminium or manganese, and the permanent capture of greenhouse gases from effluent gas streams in a product which can be used as a cement additive substitute.

[0060] The features described in relation to any aspect of the invention may equally apply to any other aspect and, merely for brevity, are not repeated. For example, features described in relation to compositions can apply in relation to methods, and vice versa. Brief Description of Figures

[0061] The present invention will now be further described in detail and with reference to the figures, which show:

[0062] Figure 1 : Mineral phase identification chart showing the variability in the minerals present in silicate igneous rocks highlighting two key end members granitic (left) and basaltic (right);

[0063] Figure 2: Mean and two standard deviations of compressive strength of mortar mixes using: 100% CEM1 cement; 50% CEM1 and 50% CC>2-milled basalt; 50% CEM1 and 50% air-milled basalt; 75% CEM1 and 25% CC>2-milled granite; 75% CEM1 and 25% air-milled granite;

[0064] Figure 3: Mean metal concentrations in the supernatant after 24 hours of leaching from CO2-milled rocks (red bars) and air-milled rocks (grey bars), for basalt (a) and granite (b). Leached metal concentrations for major metals Al, Ca, Fe, K and Mg are displayed in mg / g and minor metals Sr, Li, and Mn are displayed in pg / g

[0065] Figure 4a: For granite micromechanically reacted with CC^and air, raw XPS data (grey dots) and C1s component, showing deconvolved peaks (green lines), fitted curved (black line) and binding energies of C-C (284.9 eV), C-0 (286.9 eV) and C=O (289.6 eV carbon peaks (green lines);

[0066] Figure 4b: Fitted XPS data for air-milled (grey) and CO2-milled (coloured) granite at different temperatures, showing the evolution of the C-C, C-0 and C=O peaks with increasing temperature;

[0067] Figure 4c: IRRAS spectra of air-milled (grey) and CO2-milled (coloured) granite at different temperatures, with the upper and lower frequency of the IR unidentate (blue), bidentate (green) and bridging (pink) stretching modes of the free CO3 ion and the IR stretching frequency of each free CO3 ion peak (dashed black lines);

[0068] Figures 5(a)-5(b): Proportions of different mineral types present in granite determined by XRD and proportions of mineral feedstocks used in the granite mineral mixture;

[0069] Figures 6(a)-6(b): Proportions of different mineral types present in basalt determined by XRD and proportions of mineral feedstocks used in the basalt mineral mixture;

[0070] Figure 7: Mean 28-day compressive strength (n=6) of cured mortar cylinders (height 50mm, diameter 25mm) made from a mortar mix of 1 cement:3 sharp sand:1 water. CEMI (black), CEMII High 42.5N (dark grey) and Low 32.5N (light grey) Portland cement with 6-20% and 21-35% pulverised fly ash replacement respectively, cement replaced mechanochemically treated basalt and granite rock powders containing trapped CO2 (dark orange and dark blue), air milled controls (light orange and light blue), and the mineral mixtures, 50% cement replacement with basalt (orange) and 25% cement replacement with granite (blue). Cements containing 50% replacement of cement with basalt containing milled in CO2 retains >70% of the strength of CEMI and 25% replacement of cement with granite containing mechanochemically trapped CO2 retains >80% of the strength of CEMI in small mortar cylinder tests;

[0071] Figure 8: Concentration of carbon captured as milligrams of CO2 per tonne in basalt (Orange) and granite (blue) milled under a 2bar atmosphere in a planetary ball mill at 350rpm for 2.5 hours, before leaching (solid) and after leaching (horizontal lines);

[0072] Figure 9: XRD mineral composition as percentage by mass. Cement samples are taken after 28-days of curing. Results are for CEM1 , basalt rock, cement with a 50% CO2-milled basalt replacement, granite whole-rock and cement with a 25% CO2-milled granite replacement. Also shown in the dashed boxes are the mineral composition for a 50:50 mix of CEM1 and basalt, and the mineral composition for a 75:25 mix of CEM1 and granite.

[0073] Detailed Description

[0074] In the present disclosure, reference is made to a number of terms, which have the meanings provided below, unless a context indicates to the contrary. The nomenclature used herein for defining compounds, in particular the compounds according to the invention, is in general based on the rules of the IIIPAC organisation for chemical compounds, specifically the “IIIPAC Compendium of Chemical Terminology (Gold Book)”. For the avoidance of doubt, if a rule of the IIIPAC organisation is in conflict with a definition provided herein, the definition herein is to prevail. Furthermore, if a compound structure is in conflict with the name provided for the structure, the structure is to prevail.

[0075] The term “comprising” or variants thereof is to be understood herein to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0076] The term “consisting” or variants thereof is to be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps. The term “about” herein, when qualifying a number or value, is used to refer to values that lie within ± 5% of the value specified. For example, if a temperature is specified to be about 5 to about 13 °C, temperatures of 4.75 to 13.65 °C are included.

[0077] Reference to physical states of matter (such as liquid or solid) refer to the matter’s state at 25 °C and atmospheric pressure unless the context dictates otherwise.

[0078] Experimental

[0079] Rock materials

[0080] As explained above, previous research (M. Stillings, Z. K. Shipton, R. J. Lunn (2023) Mechanochemical processing of silicate rocks to trap CO2. Nature Sustainability, 6(7):1-9. DOI: 10.1038 / s41893-023-01083-y) describes the mechanochemical CO2 trapping capability of commonly available polymineralic rocks, using various types of rock that are both high (basalt) and low (granite) in magnesium and calcium. Figure 1 shows a mineral phase identification chart from Mineralogical Society of America 2022,

[0081] Mineralogy4Kids - The Rock Cycle , accessed 09 / 02 / 2024,

[0082] : / / min4kids.orq / iqneous-rock-identification-chart / Figure 1 illustrates the variability of minerals in silicate rocks, which are comprised of the five major (and most globally abundant) rock-forming minerals (i.e., Orthoclase, Quartz, Plagioclase, Pyroxene, and Olivine). Silicate rocks typically range from felsic (granitic rocks) to mafic / ultramafic (basaltic).

[0083] In the present experiments, two types of silicate rocks from both ends of the compositional spectrum were used, namely granitic and basaltic. As such, if positive results were obtained for these two types, then a person of skill in the art would reasonably expect positive results to apply for all other polymineralic silicate rocks types.

[0084] Milling Process

[0085] Prior to milling, an amount of silicate rock (granite and basalt) was crushed using a hydraulic press and sieved to less than 1mm. 60g samples of rock were then milled in a Retsch PM 100 planetary ball mill at ambient temperature and pressure (with a ball-to- rock ratio of 10:1 (w:w) at 350 rpm for 2.5 hours) within an atmosphere of pure CO2. This produces ‘mechanochemically-altered’ rock powders containing trapped CO2, that have an average grain size of 15 .m. The methodology for preparing the milled rock followed the methods described in M. Stillings, Z. K. Shipton, R. J. Lunn (2023) Mechanochemical processing of silicate rocks to trap CO2. Nature Sustainability, 6(7): 1-9. DOI: 10.1038 / s41893-023-01083-y, which is incorporated herein by reference in its entirety.

[0086] CO2 capture behaviour

[0087] Spectroscopic analysis was performed on CO2 and air-milled powders to identify how the carbon dioxide was trapped during milling, identifying how the carbon is bonded and what form the bonded carbon takes on the rock surfaces. Results are shown in Figures 4(a)-4(c).

[0088] Results

[0089] Use of Mechanochemically altered powders as partial cement replacement

[0090] Industry standard CEM1 , which is 100% Ordinary Portland Cement (OPC) was used in all experiments since this contains no cement replacement products (e.g. no pulverised fuel ash (PFA) or blast furnace slag (BFS)).

[0091] Experiments were conducted by replacing a percentage of the cement with either CO2-milled rock powder or air-milled rock powder (air-milled powders were prepared using the exact same methodology described above but milling in air instead of CO2).

[0092] All samples were prepared using a standard building mortar mix composed of one part ‘cement-mix’ to three parts sharp sand, to one-part water.

[0093] The cement-mixes used were (1) 100% CEM1 cement (i.e. 100% OPC), (2) 50% CEM1 and 50% CO2-milled basalt, (3) 50% CEM1 and 50% air-milled basalt, (4) 75% CEM1 and 25% CO2-milled granite, (5) 75% CEM1 and 25% air-milled granite. This resulted in a total of 30 samples for unconfined compressive strength (UCS) testing (the 5 different mortar types, each with 6 replicates). After mixing, each mortar was poured into six (replicate) cylinders with a diameter of 25mm and height of 50mm for testing. All samples were cured for 28 days underwater. Once cured, unconfined compressive strength tests were conducted using a constant loading rate of 250 N.s-1in ambient temperature and pressure conditions.

[0094] Figure 2 shows mean and two standard deviations of compressive strength measured for the mortar mixes described above. It can be observed that for both the basalt and granite rock types, the CO2-milled rock powders provide significantly higher strength than the air-milled powders with the same percentage of replacement. In the case of the basalt, the 50% CEM1 replacement with CC>2-milled rock powder produces a mortar with 73% of the strength of the 100% CEM 1 mix. For the CC>2-milled granite, the 25% CEM1 replacement results in a mortar with 88% of the 100% CEM1 mix. Most modern cement is now sold with some form of cement replacement, including for example - as explained above - PFA or BFS.

[0095] Currently, cement manufacturing companies produce several products including: high strength CEM2 / A-V or CEM2 / A-S, which contains 6-20% OPC replacement (using PFA and BFS respectively); and lower strength CEM2 / B-V or CEM2 / B-S which contains a 21 %-35% OPC replacement (using PFA and BFS respectively). According to the BRMCA British Ready-Mixed Concrete Association, manufacturers data typically guarantees 81% of the strength of CEM1 for high-strength CEM2 and 62% for low- strength CEM2. See “Cement type / early age properties. Concrete Today. Accessed 09 / 02 / 2024 URL

[0096] <https: / / brmca.org.uk / documents / Cement Type Early Age Properties 23 Jun 11.pdf >”.

[0097] Comparing these values with the results of Figure 2, it can be seen that the 25% replacement CO2-milled granite outperforms the high-strength CEM2 (which contains between 6% and 20% PFA / BFS) and the 50% CC>2-milled basalt outperforms the low- strength CEM2 (which has between 21% and 35% PFA / BFS). Therefore, it can be extrapolated that using CC>2-modified milled silicate rock as an alternative cement additive would at least match, if not improve, the strength of the resulting cement and / or mortar, compared to conventional cement additives. This is in addition to the environmental benefits associated with the use of such materials compared to conventional additives.

[0098] Metal Mobility and water solubility

[0099] The inventors have discovered that milling silicate rocks such as granite and basalt under a CO2 atmosphere changes the solubility of certain metals within the rock.

[0100] Milling was performed as described above under “Milling Process”.

[0101] Leaching experiments mixed 4.5g of rock / mineral with 45ml of ultra-pure water for 24 hours. After this 24-hour period, the supernatant was sampled for metal analysis. Metal ion concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS). Each leaching experiment was conducted in duplicate, and for each extracted sample of the supernatant, ICP-MS analysis was carried out in triplicate.

[0102] The results are shown in Figure 3, and in Table 1 below, for all metals that are above the measurable limit. Figure 3 plots the mean of the duplicates, with the raw data in Table 1. For both the basalt (Figure 3a) and the granite (Figure 3b) powders, the solubility of all metals after milling in CO2 is significantly different to that after milling in air. In particular, it can be observed that in both the basalt and the granite, milling in CO2 significantly increases the solubility of all group 1 and group 2 metals present (in the measured samples: Ca, K, Mg, Sr, and Li) and decreases the solubility of the transition or post-transition metals (in the measured samples: Al, Fe and Mn). Therefore, milling silicate rocks under a CO2 atmosphere represents an effective method for (i) reducing the solubility of toxic transition or post-transition metals thereby reducing the risk of contamination, and / or (ii) extracting certain valuable Gp1 or Gp2 metals, such as lithium, from silicate rocks.

[0103] Table 1 : Results of duplicate experiments assessing the leaching of metals leaching from basalt and granite milled in air and CO2

[0104] Rock Milling Condition Duplicate Al mg / g Ca mg / g Fe mg / g K mg / g Mg mg / g Sr pg / g Li pg / g Mn pg / g

[0105] A 0.556 0.124 0.103 0.292 0.109 0.90 0.60 1.40

[0106] Basalt Air

[0107] B 0.575 0.163 0.140 0.307 0.145 1.30 0.60 2.00

[0108] A 0.091 1.004 0.001 0.344 1.941 3.40 1.00 0.10

[0109] Basalt CO2

[0110] B 0.087 0.966 0.001 0.325 1.845 3.20 0.90 0.10

[0111] A 0.654 0.042 0.041 1.468 0.032 0.20 2.90 0.80

[0112] Granite Air

[0113] B 1.042 0.071 0.138 1.456 0.065 0.60 2.90 2.60

[0114] A 0.225 0.251 0.017 4.395 0.467 0.70 10.40 1.10

[0115] Granite CO2

[0116] B 0.190 0.228 0.013 4.087 0.430 0.60 9.60 1.00

[0117] CO2capture behaviour

[0118] X-ray Photoelectron spectroscopy (XPS) was performed on granite, as the results are shown in Figures 4(a)-4(c). These show that C-0 and C=O groups are formed on the rock surfaces when granite and basalt are milled in CO2. Infrared spectroscopy (IR) was performed to identify the chemical form of the CO2 bond. IR spectroscopy of CO2 milled granite (Figure 4(c)) shows peaks at IR stretching frequencies which correspond to the high and low stretching frequencies for unidentate, bidentate and bridging silicon carbonate groups (Santoro et al., 2011) IR peaks corresponding to silicon carbonate groups are not observed in the air-milled rock samples.

[0119] During spectroscopic analysis (XPS and IR) the CC>2-milled samples were heated stepwise to 700C. This heating process causes the trapped CO2 to be desorbed from the rock surfaces. Hence, when CO2 is desorbed from the rock the XPS peaks for C-0 and C=O in the XPS reduce relative to C-C (elemental carbon) and the peaks at IR stretching frequencies for differently bonded silicon carbonate groups also decrease in intensity. Stepwise heating of these samples confirms that the bonded CO2 is bonded in one of three forms of silicon carbonate on the powdered rock surface. Step-wise heating of the samples shows CO2 is stable to 300°C; from the IRRAS, unidentate-silicon carbonates are desorbed at 300-400°C, and bidentate- and bridged- at 500-700°C (Figure 4(b)).

[0120] Concrete strength of Rock vs mineral feedstocks

[0121] The compressive strength of concrete made with Portland cement replaced with either 25% granite or 50% basalt milled rock / mineral powders, by weight, were tested and compared against conventional CEMI (Portland cement) and CEMII high and low strength (Portland cement with supplementary Cementous material, i.e. PFA, GGBS).

[0122] Basalt and granite mixtures were also tested. These rock, mixtures were obtained by mixing the feedstock minerals present in the granite and basalt within the mill (as opposed to using the whole naturally-occurring rocks). The ratio of feedstock minerals used in each ‘rock mixture’, in comparison to the minerals found in the original rocks (using X-Ray Diffraction, XRD) is shown in Figures 5(a), 5(b), 6(a) and 6(b).

[0123] Both the rocks and the mineral mixtures (from the different raw feedstocks) were milled in CO2. The whole rocks were also milled in air (as a control). 28-day compressive strength of cured mortar cylinders was determined using different cement mixtures. Cement cylinders of height 50mm and diameter 25mm made from a mortar mix of 1 cement:3 sharp sand:1 water. In each case, the cement differed from CEMI (black), CEMII 42.5N (dark grey) and 32.5N (light grey), Portland cement with 6-20% and 21- 35% pulverised fly ash replacement respectively, cement replaced mechanochemically treated rock powders containing trapped CO2 (dark orange and dark blue) and air milled controls (light orange and light blue), 50% cement replacement with basalt (orange), 50% replacement with the basalt mineral mixture, 25% cement replacement with granite (blue) and 25% replacement with the granite mineral mixture.

[0124] The compressive strength of each of the mortar cylinders is shown in Figure 7. The basalt and granite milled in CO2 produced a stronger cement mixture than the same rocks milled in air. For the rock powders milled in CO2, 50% basalt and 25% granite cement replacement are both stronger than the high strength commercially available CEMII. In the case of the basalt and granite mineral mixtures, milled in CO2, both mixtures are weaker than the low strength CEMII cores. These findings are particularly interesting for the granite mixture where the same proportions of minerals is present as that in the whole-rock, implying that whole rock feedstocks milled in CO2 produce a more commercially beneficial cement mixture and behave differently to mineral feedstock mixtures.

[0125] Stability of captured CO2

[0126] The stability the CO2 trapped in the milled powders and its solubility in water, was investigated. Leaching tests were conducted by mixing the powders with water in a ratio of 1 :10, powderwater.

[0127] Figure 8 shows CO2 trapped in the powder before and after leaching for the basalt and granite rocks milled CO2 (before and after leaching in water) and for the basalt and granite mixtures (before and after leaching water). In the whole-rock powders, 96% - 98% of the trapped CO2is insoluble (and remains in the powder), whereas in the mineral mixtures 71 % of the CO2 is soluble in the granite mixture and 40% in the basalt mixture. These results are consistent with those found for the individual minerals where most of the trapped CO2is soluble; in olivine 92.6% and in biotite mica 60% of the trapped carbon is soluble (Stillings et al., 2023).

[0128] Investigation the mineralogy of tested compositions

[0129] To understand why additional strength gain is generated in the cores when cement is replaced by CO2 milled rock (granite and basalt), the mineralogy of the cores after 28-days curing was determined using XRD, as shown in Figure 9. For the pure CEMI (left hand bar on Figure 4) 33% of the minerals present are amorphous cement phases, such as calcium silicate hydrates which are known to be key to the compressional strength. The remaining 67% of CEM1 comprises five well-known crystalline cement minerals (portlandite, larnite, calcite, ettringite and alite). The second bar shows the mineral composition of the original basalt rock (l.e. the data from the pie chart in Figures 5-6). The third bar shows the XRD data after 28-days curing from the core with a 50% CC>2-milled basalt cement replacement. For comparison (in the adjacent dashed box) is the mineral composition for a 50:50 mixture of CEM1 and basalt rock. Visual observation shows that significantly more (at least double) amorphous minerals are present in the CC>2-milled basalt cement replacement core than would be present in a 50:50 mixture of cement and basalt; thus, additional amorphous minerals are forming during curing due to the presence of the CC>2-milled basalt powder in the cement.

[0130] Similar data are presented for the granite in the last three bars of Figure 9. Again, the percentage of amorphous minerals in the CC>2-milled granite 25% cement replacement core is higher than would be expected from a 75:25 mixture of cement and granite minerals.

[0131] It will be understood that the present embodiments are provided by way of example only, and that various modifications can be made to the present embodiments without departing from the scope of the invention. For example, whilst the experiments described herein used pure CO2 during milling, an alternative source of pure or diluted CO2 may be used, for example an effluent gas stream from an industrial facility.

Claims

CLAIMS:

1. A cement additive, the cement additive comprising, consisting essentially of or consisting of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

2. A cement additive according to claim 1 , wherein the molar concentration of CO2 in the gas composition is between about 0.05% and 100%.

3. A cement additive according to claim 1 or claim 2, wherein the milled silicate rock has an average particle size of about 0.1 -100pm, optionally about 5-50pm.

4. A cement additive according to any preceding claim, wherein the milled silicate rock comprises, consists essentially of or consists of one or more types of naturally- occurring silicate rock selected from felsic or granitic rock, andesitic or intermediate rock, mafic or basaltic rock, and ultramafic rock.

5. A cement composition comprising: a hydraulic cement component; and a cement additive according to any preceding claim.

6. A cement composition according to claim 5, wherein the hydraulic cement component comprises, consists essentially of or consists of Portland cement.

7. A cement composition according to claim 5 or claim 6, wherein the amount of the cement additive is about 1-90 wt%, optionally about 5-60 wt%, optionally about 10-40 wt%, of the cement composition.

8. A method for manufacturing a cement additive, the method comprising: milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

9. A method according to claim 8, comprising milling the silicate rock under a gas atmosphere in which the concentration of CO2 is between about 0.05% and 100%.

10. A method according to claim 8 or claim 9, the method comprising milling the silicate rock in a milling apparatus, and feeding the gas atmosphere to the milling apparatus.

11. A method according to any of claims 8 to 10, the method comprising providing at least part of the gas atmosphere from an effluent gas stream.

12. A method according to any of claims 8 to 11 , the method comprising trapping carbon dioxide within the silicate rock during milling.

13. A method according to any of claims 8 to 12, the method comprising milling the silicate rock to an average particle size of about 0.1 -100pm, optionally about 5-50pm.

14. A method according to any of claims 8 to 13, the method comprising milling the silicate rock at ambient temperature and / or ambient pressure15. A method for manufacturing a cement composition, the method comprising mixing a hydraulic cement component and a cement additive, wherein the cement additive comprises, consists essentially of or consists of a milled silicate rock, wherein the milled silicate rock has been milled under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

16. A method of altering the water solubility of one or more metals in a silicate rock, the method comprising milling a silicate rock under a gas atmosphere in which the concentration of CO2 is higher than atmospheric concentration.

17. A method according to claim 16, the method comprising reducing the solubility of one or more transition metals and / or or one or more post-transition metals, in the silicate rock.

18. A method according to claim 16, the method comprising increasing the water solubility of one or more group 1 or group 2 metals, in the silicate rock.

19. A method according to claim 18, the method comprising increasing the water solubility of lithium, in the silicate rock.

20. A method according to claim 18 or 19, the method comprising placing the milled silicate rock in water.

21. A method according to claim 20, the method comprising placing the milled silicate rock in water for at least 1 hour, optionally at least 6 hours, optionally at least 12 hours, optionally at least 24 hours.

22. A method according to claim 21 , the method comprising recovering lithium from the milled silicate rock.

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