Methods of producing cement and concrete

Abstract

A method of producing cement is provided The method comprises contacting brine with an alkali material to extract Mg2+ in the form of brucite from the brine, wherein the contacting is carried out at a pH of at least 10 and in the presence of a modifier for modifying a surface of the brucite. A method of producing concrete, and a cement and a concrete produced by the methods are also provided.

Description

METHODS OF PRODUCING CEMENT AND CONCRETECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore patent application no. 10202401723R, filed 14 June 2024, the contents of which being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments relate to methods of producing cement and concrete, and a cement and a concrete produced by the methods.BACKGROUND

[0003] Calcination, which is an indispensable process in conventional cement manufacture, constitutes the main source of carbon dioxide (CO2) emission in the construction industry, raising concerns over global greenhouse effect.

[0004] Traditional methods of producing ordinary Portland cement (OPC) or magnesium- based cement usually involve energy-intensive calcination. Although alternative cements have been developed at rapid pace in the research community over the decades, removal of calcination process from cement manufacturing remains a challenge. For example, reactive magnesium oxide (MgO) cement (RMC), which is one of the most promising alternative cements, still relies on high-temperature calcination to decompose magnesium carbonates to gain reactivity in the dry route. Despite a decrease in calcination temperature from about 1350 °C for OPC to less than 1000 °C for RMC, calcination still accounts for a major portion of energy consumption, and involving large amounts of CO2 emission.

[0005] In light of the above, there remains a need for an improved method for cement production that addresses or at least alleviates one or more of the above-mentioned problems.SUMMARY

[0006] In a first aspect, a method of producing cement is provided. The method comprises contacting brine with an alkali material to extract Mg2+in the form of brucite from the brine, wherein the contacting is carried out at a pH of at least 10 and in the presence of a modifier for modifying a surface of the brucite.

[0007] In a second aspect, a cement produced by a method according to the first aspect is provided.

[0008] In a third aspect, a method of producing concrete is provided. The method comprises providing the cement produced by a method according to the first aspect, and (a) mixing the cement with water to form a brucite mixture, and curing the brucite mixture in carbon dioxide to form hydrated magnesium carbonate, or (b) mixing the cement with silica fume and water to form a brucite-silica mixture, and curing the brucite-silica mixture in air to form magnesium silicate hydrate.

[0009] In a fourth aspect, a concrete produced by a method according to the third aspect is provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

[0011] FIG. 1 is a graph showing derivative thermogravimetric (DTG) curves of brucite synthesized from reject brine (B: Brine), for varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0012] FIG. 2 is a graph showing X-ray diffraction (XRD) patterns of brucite synthesized from reject brine, for varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0013] FIG. 3A is a graph showing specific surface area (SSA) (m2 / g) of brucite synthesized from reject brine at varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0014] FIG. 3B is a graph showing pore volume (cc / g) of brucite synthesized from reject brine at varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0015] FIG. 4A is a graph depicting reactivity results of brucite synthesized from reject brine, whereby reactivity (s'1) of the low carbon cement at different modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L is shown.

[0016] FIG. 4B is a graph depicting reactivity results of brucite synthesized from reject brine, whereby neutralization time (s) of the low carbon cement at different modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L is shown.

[0017] FIG. 5 is a graph showing relationship between specific surface area (SSA) and reactivity of brucite at varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0018] FIG. 6A shows field emission scanning electron microscope (FESEM) image of low carbon cement synthesized at 0.0 mol / L urea, i.e. no modifier. Scale bar denotes 100 nm.

[0019] FIG. 6B shows FESEM image of low carbon cement synthesized with 0.1 mol / L urea as modifier. Scale bar denotes 100 nm.

[0020] FIG. 6C shows FESEM image of low carbon cement synthesized with 0.2 mol / L urea as modifier. Scale bar denotes 100 nm.

[0021] FIG. 6D shows FESEM image of low carbon cement synthesized with 0.3 mol / L urea as modifier. Scale bar denotes 100 nm.

[0022] FIG. 6E shows FESEM image of low carbon cement synthesized with 0.4 mol / L urea as modifier. Scale bar denotes 100 nm.

[0023] FIG. 6F shows FESEM image of low carbon cement synthesized with 0.5 mol / L urea as modifier. Scale bar denotes 100 nm.

[0024] FIG. 7 is a graph showing compressive strength (MPa) of low carbon concrete (hydrated magnesium carbonate, HMC) made with the low carbon cement at varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.

[0025] FIG. 8 is a graph showing XRD of low carbon concrete (HMC) made with the low carbon cement at varying modifier (urea) dosages of 0.0 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, and 0.5 mol / L.DESCRIPTION

[0026] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0027] As disclosed herein, a method of producing cement is provided. The term “cement” may refer to a cementitious binder composition that sets or hardens, and adheres to other materials. In so doing, the cement binds the other materials together. The cement may otherwise be termed as a low carbon cement or LCC, as it may be produced with less energy,hence having a smaller carbon footprint, as compared to conventional cement. The low carbon cement disclosed herein may be a brucite-based low carbon cement, in that brucite may form at least a major component of the cement. As calcination is not required, energy for calcination can be saved. There is also lower carbon dioxide emission as a result. Apart from the lower carbon dioxide emission, the low carbon cement is able to absorb carbon dioxide when subjected to a hardening process to produce a low carbon concrete, thereby removing carbon dioxide in the process.

[0028] To enhance production efficiency and quality, both nanostructure and microstructure of the low carbon cement may be tailored using a modifier in the harvesting process. Exemplary calcination-free low carbon cement disclosed herein has achieved similar strength to that of OPC concrete.

[0029] The method may comprise contacting brine with an alkali material to extract Mg2+in the form of brucite from the brine, wherein the contacting may be carried out at a pH of at least 10 and in the presence of a modifier for modifying a surface of the brucite.

[0030] The term “brine” as used herein refers to an aqueous solution comprising one or more salts dissolved therein. Examples of salt may include salts that produce dissociated ions, such as halides, sulphates, carbonates and nitrates of monovalent, divalent or trivalent metal cations such as lithium, potassium, sodium, magnesium, calcium, zinc and / or aluminium.

[0031] The brine may, for example, be magnesium-rich resources such as seawater, subsurface brine, lake brine, or reject brine obtained from desalination of seawater.

[0032] By precipitating the abundant magnesium resources from natural sources such as the ocean, or from reject brine generated from desalination plants into Mg(OH)2 through alkali addition, direct CO2 emission does not take place. Therefore, methods disclosed herein may be regarded as a greener process for producing cement. Recovery of magnesium-based low carbon cement from marine feedstock is able to address global challenge on carbon neutrality.

[0033] In various embodiments, the brine comprises or consists of reject brine obtained from desalination of seawater. Reject brine obtained from desalination of seawater may comprise a high concentration of Mg2+, such as a concentration of more than 2000 ppm. Other components in reject brine may include Cl’, SOT’, Na+, Ca2+, an example of which is shown in TABLE 1. Given that majority of the reject brine is currently being disposed of by discharging directly back into the ocean, which may adversely affect the marine ecosystem, methods disclosed herein may allow reject brine to be utilized in a more efficient and sustainable manner.

[0034] The alkali material may be one or more of sodium hydroxide, ammonia, lime, and dolomite lime. In various embodiments, the alkali material comprises or consists of sodium hydroxide. Sodium hydroxide may advantageously be used, as it allows a higher extraction efficiency and purity. This is due to a much higher solubility of sodium hydroxide in water as compared to calcium hydroxide (Ca(OH)2). By using sodium hydroxide in lieu of calcium hydroxide, Ca-based compounds which may be insoluble in water are also not introduced. When considering health and safety aspects, use of sodium hydroxide is advantageous as it may not require the level of care as in the case for ammonia.

[0035] The alkali material may be in the form of a liquid or a solid. Accordingly, the alkali material may be an alkali solution or in the form of pellets as alkali pellets.

[0036] In various embodiments, contacting brine with the alkali material comprises adding the alkali material to the brine to form a mixture, and adding the modifier to the mixture. For example, the alkali material may first be added to the brine to form a mixture in the absence of the modifier, followed by addition of the modifier to the mixture.

[0037] For example, alkali pellets may be added to the brine and be allowed to dissolve. This may be carried out before addition of the modifier. In so doing, pH of brine may be increased from a value such as about 7 or about 8, to a value of 10 or more, such as at least 10.4, at least10.5, at least 11, at least 11.5, at least 12, at least 12.2, at least 12.5, or at least 13. At a pH of 10 or more, brucite may precipitate from the brine.

[0038] The alkali material may be mixed with brine for a time period in the range of a few minutes, such as about 1 minute to about 10 minutes, about 2 minutes to about 5 minutes, or about 3 minutes. A homogenous mixture may be formed, whereby at least substantially all of the alkali material may be mixed with or dissolved in the brine. The modifier may be added to the mixture for modifying or changing a surface structure of the brucite.

[0039] In various embodiments, the alkali material is sodium hydroxide. In some embodiments, the alkali material is sodium hydroxide pellets. Molar ratio of the sodium hydroxide to Mg2+may be in the range of 2 to 4, such as 2, 3, or 4.

[0040] The term “modifier”, otherwise termed herein as a “micro modifier” or “nano modifier”, refers to a substance which is able to alter or change a surface morphology of the brucite. The modifier may function as a cross-linking agent, whereby two or more brucite particles precipitated from the brine may be linked or attached to one another via the modifier. Brucite of a different surface morphology may result due to action of the modifier on the brucite particles as they are being precipitated from the brine. Accordingly, a surface structure of the brucite may be modified by the modifier. As mentioned above, both nanostructure and microstructure of the low carbon cement may be tailored using a modifier. The modifier may also result in a higher particle packing density and higher particle surface exposure of the brucite.

[0041] At various dosages of modifier, in particular at an optimum dosage of modifier, particles of the brucite are able to align with each other with high surface exposure, thus presenting high reactivity, pore volume and specific surface area.

[0042] In various embodiments, the modifier is one or more of urea and polyethylene glycol(PEG).

[0043] The modifier may, for example, be a cross-linking agent formed from organic polymers such as polyethylene glycol, which is oligomer or polymer of ethylene oxide. The hydroxide (OH) group of PEG can draw adjacent brucite crystals together in one crystal orientation to enable the self-assembly of nano lamellas of brucite to form various plate-like morphologies. Addition of PEG 200 (1 wt%, 2 wt%, and 5 wt% by mass of solvent) in brucite solution was evaluated and it was concluded that the use of 2 wt% PEG 200 was the optimal because the lateral arrangement caused by PEG prevented agglomeration of brucite particles and ensure the best modifier adsorption.

[0044] In various embodiments, the modifier comprises or consists of urea. Urea, being an organic compound with chemical formula CO(NH2)2, has two -NH2 groups joined by a carbonyl (C=O) functional group. Due to its lower cost as compared to organic polymers such as polyethylene glycol (PEG), use of urea as a modifier in tailoring the nanostructure and / or microstructure of brucite particles during brucite precipitation is advantageous.

[0045] The modifier may, for example, be present at 0.1 mol / L to 0.5 mol / L based on volume of the brine, such as 0.1 mol / L to 0.4 mol / L, 0.2 mol / L to 0.4 mol / L, or 0.1 mol / L to 0.3 mol / L based on volume of the brine. It was surprisingly found by the inventors that modifier present at less than 0.3 mol / L, such as 0.1 mol / L to 0.2 mol / L, based on volume of the brine results in brucite with higher reactivity. The higher reactivity may result from a higher porosity of the brucite, with the brucite particles of regular shape and being separated from each other to increase number of air voids between the brucite particles.

[0046] In various embodiments, contacting brine with the alkali material to extract Mg2+in the form of brucite from the brine is carried out in a vessel, wherein the contacting comprises settling the brucite into a bottom portion of the vessel as precipitated brucite while removing a supernate from the vessel. The vessel may be a receptacle for containing or holding a liquid or slurry, and may be in the form of a container, a tube, a duct, or a cavity. The supernate may bepresent above the precipitated brucite, and removing the supernate may comprise removing the supernate from a top portion of the vessel above the precipitated brucite.

[0047] Removing the supernate may be carried out by siphoning. By using siphoning, disturbance and dilution of the precipitated brucite at the bottom portion of the vessel may be minimized or avoided.

[0048] Following removal of the supernate, the precipitated brucite may be centrifuged to form colloidal brucite. Conditions for carrying out the centrifuging is not particularly limited, and may, for example, be carried out at a speed in the range of 4000 rpm to 6000 rpm, such as 5000 rpm. The centrifuging may be carried out for any suitable time period, such as a time period in the range of about 5 minutes to about 15 minutes, or about 10 minutes.

[0049] As a result of removal of the supernate, the colloidal brucite may have a solid content in the range of 5 wt% to 15 wt%, such as 7 wt% to 15 wt%, 10 wt% to 15 wt%, 12 wt% to 15 wt%, 5 wt% to 12 wt%, 5 wt% to 10 wt%, 5 wt% to 8 wt%, 8 wt% to 12 wt%, or 10 wt%.

[0050] Methods disclosed herein may further comprise drying the colloidal brucite. The drying may be carried out in an oven, such as a vacuum oven, at a temperature in the range of 40 °C to 60 °C, such as 40 °C to 55 °C, 40 °C to 50 °C, 40 °C to 45 °C, 45 °C to 60 °C, 50 °C to 60 °C, 55 °C to 60 °C, or 42 °C to 52 °C.

[0051] The drying may be carried out for a time period in the range of 60 hours to 80 hours. A suitable time period may, for example, be in the range from 65 hours to 80 hours, such as 70 hours to 80 hours, 75 hours to 80 hours, 60 hours to 75 hours, 60 hours to 70 hours, 65 hours to 75 hours, 68 hours to 72 hours, or about 72 hours.

[0052] In various embodiments, the drying is carried out at a temperature in the range of 40 °C to 60 °C for a time period in the range of 60 hours to 80 hours.

[0053] In various embodiments, the method disclosed herein further comprises grinding the dried brucite into dried brucite powder having a particle size of 150 pm or less, such as 120pm or less, 100 m or less, 80 pm or less, or particles having a size in the range of 50 pm to 150 pm, 100 pm to 150 pm, or about 150 pm. Advantageously, dried brucite powder having a finer particle size, such as one in the range of 150 pm or less, may allow reaction for forming concrete to be better carried out.

[0054] Various embodiments refer in a second aspect to a cement prepared by a method according to the first aspect.

[0055] The cement may be used for producing low carbon concrete, such as hydrated magnesium carbonate (HMC) and magnesium silicate hydrate (MSH). Both the cement and the concrete may be used in construction, for constructing infrastructure, such as buildings, roads, drains, and bridges.

[0056] Various embodiments refer to a third aspect to a method of producing concrete. The method comprises providing the cement produced by a method according to the first aspect, and (a) mixing the cement with water to form a brucite mixture, and curing the brucite mixture in carbon dioxide to form hydrated magnesium carbonate, or (b) mixing the cement with silica fume and water to form a brucite-silica mixture, and curing the brucite-silica mixture in air to form magnesium silicate hydrate.

[0057] The cement may be in the form of dried brucite powder. In various embodiments, to form hydrated magnesium carbonate, the method further comprises mixing the dried brucite powder with water to form a brucite mixture, and curing the brucite mixture in carbon dioxide.

[0058] The dried brucite powder may be mixed with water at a water to binder (w / b) ratio of 0.3 to 0.6, such as 0.3, 0.4, 0.5 or 0.6. A low w / b ratio such as that in the range of 0.3 to 0.6 may advantageously result in higher strength of the resulting concrete material. By reacting the brucite with carbon dioxide, strength may be developed. Therefore, brucite synthesizedfrom a method disclosed herein may be directly carbonated to gain strength. Advantageously, high levels of carbon dioxide gas may be sequestered.

[0059] Curing the brucite mixture in carbon dioxide may be carried out with concentration of carbon dioxide in the range of 10% to 20%., such as 12 wt % to 20 wt%, 15 wt % to 20 wt%, 10 wt % to 18 wt%, 10 wt % to 15 wt%, 12 wt % to 18 wt%, or 10 wt%. The curing may be carried out at room temperature, such as a temperature in the range of 20°C to 35°C, such as 25°C to 35°C, 30°C to 35°C, 20°C to 30°C, 20°C to 25°C, 25°C to 30°C, or 30°C.

[0060] In various embodiments, to form magnesium silicate hydrate, the method may further comprise mixing the dried brucite powder with silica fume and water to form a brucite-silica mixture, and curing the brucite-silica mixture in air.

[0061] As in the case for preparing hydrated magnesium carbonate, the dried brucite powder may be mixed with water at a water to binder ratio of 0.3 to 0.6, such as such as 0.3, 0.4, 0.5 or 0.6. As mentioned above, a low w / b ratio such as that in the range of 0.3 to 0.6 may advantageously result in higher strength of the resulting concrete material.

[0062] The curing may be carried out at room temperature, such as a temperature in the range of 20°C to 35°C, such as 25°C to 35°C, 30°C to 35°C, 20°C to 30°C, 20°C to 25°C, 25°C to 30°C, or 30°C.

[0063] Various embodiments refer in a further aspect to a concrete produced by a method according to the third aspect.

[0064] The concrete may be a low carbon concrete, such as hydrated magnesium carbonate (HMC) and magnesium silicate hydrate (MSH). As mentioned above, the concrete may be used in construction, for constructing infrastructure, such as buildings, roads, drains, and bridges.

[0065] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.EXAMPLES

[0066] Various embodiments relate to calcination free magnesium-based low carbon cement (LCC) harvested from the ocean or reject brine.

[0067] As disclosed herein, a possibility to developing calcination free cement lies in magnesium-based binder systems such as reactive magnesium oxide cement (RMC), evidenced by chemical evolution before and after calcination.

[0068] RMC may be produced using dry or wet routes. In the dry route, magnesium carbonates may be decomposed into MgO as shown in equation (1):

[0069] MgCOi -> MgO + CO2T (1)

[0070] In the wet route, MgO may be formed by thermal decomposition of Mg(OH)2 as shown in equation (2):

[0071] Mg(0H)2MgO I +H,0 t (2)

[0072] Using either dry or wet route, the obtained MgO may be used as alternative cement to form hydrated magnesium carbonates (HMC), with the following reaction mechanism shown in equations (3) to (6):

[0073] MgO 4- H20 -> Mg(OH)2(brucite) (3)

[0074] Mg(OH)2+ C024- 2H2O -» MgCO3■ 3 H20 (nesquehonite) (4)

[0075] 5Mg(OH)24- 4CO2-> 4MgCO3■ Mg(OH)2■ 4H2O (hydromagnesite) (5)

[0076] 5Mg(OH)z4- 4CO24- H2O -► 4MgCO3• Mg(OH)2■ 5H,0 (dypingite) (6)

[0077] Formation of nesquehonite, hydromagnesite and dypingite in hydrated magnesium carbonate (HMC) are based on the intermediate reaction production of Mg(OH)2. Direct use of Mg(0H)2may also trigger reaction mechanism of hydrated magnesium carbonate, and eventually provide strength.

[0078] Magnesium hydroxide (Mg(0H)2) is a double-layered hydroxide with a brucite structure. The structure is composed of magnesium (Mg) layers coordinated octahedral by O- H groups, with the hydrogen pointing in the direction of the next adjacent layer. The (001) of hydroxide layers formed by O-H groups is present as the large plane of Mg(0H)2, and the two adjacent hydroxide layers are stabilized by weak hydrogen bond, thus microscopically forming the hexagonal thin sheet crystal of Mg(0H)2.

[0079] Brucite, being the mineral form of Mg(0H)2, exhibits low solubility in water. Under aqueous carbonation, however, the hydroxide groups in Mg(OH)2 are prone to interact with hydrogen ions supplied by the dissolved CO2 as shown in equations (7) and (8), accompanied by reaction between Mg2+and CCh2-as shown in equation (9):

[0080] CO2(g) CO2(aq) (7)

[0081] C02(aq) + H20 H2CO3(aq) HCO3“ + H+->■ CO32-+ 2H+(8)

[0082] Mg2+ + C0 / “ -> MgCO3(s) (9)

[0083] It was found that at calcination temperature of less than 650 °C, many original Mg(0H)2frameworks or pseudomorphs remained. It was also found that a lower calcination temperature (500 °C to 700 °C) and a shorter duration (2 hr to 12 hr) benefited the reactivity of MgO. Importance of low calcination temperature of 325 °C to 470 °C in changing the morphologies of Mg(0H)2 laminae to MgO cubelets and foils was also found. Dehydration of Mg(0H)2into MgO happens in a short time. Tess calcination of Mg(0H)2may lead to a broader (111) plane surface, otherwise the transformation from (111) plane to stable (100),(010), and (001) planes reduced the surface area and coordinative unsaturated sites. It was found that less calcination of Mg(OH)2 generally favors its surface reactivity.

[0084] It is demonstrated herein that direct use of Mg(0H)2, i.e. without involving a calcination process, as cement is possible.

[0085] Regarding the feedstock for producing a low carbon cement disclosed herein for the making of concrete, the ocean may provide a new raw material source of low carbon cement, after taking into account the total reserves, easiness of acquisition, and environmental concerns.

[0086] Magnesium (Mg) is the third most abundant element in seawater and the eighth most abundant element in the Earth's crust which represents approximately 2.1 % of its composition. At present, the main raw material for the production of Mg binders is terrestrial magnesite minerals (mainly MgC() <) of which China geographically accounts for 27 %, North Korea accounts for 24%, and Russia accounts for 22 % in its total reserves. However, most magnesium resources exist in the ocean, rather than on land. The limited magnesium resources stored on land can hardly satisfy huge demand in the construction industry when MgO is used as an alternative cement or expansion agent.

[0087] In contrast, National Oceanic and Atmospheric Administration's (NOAA) National Geophysical Data Center estimates a total of 1 ,338,000,000 cubic kilometres of seawater in the ocean, which implies a total Mg reserves of 1.686xl015ton in the ocean (assume a typical Mg2+concentration of 1.26 g / L in seawater). This may mean that the total Mg reserves in the ocean can supply MgO for 0.685 million years at the present consumption rate of cement.

[0088] Currently, calcination of magnesite (MgCOo is the principal route to produce MgO which generates approximately 1 ton of carbon dioxide (CO2) when producing one ton of MgO, deviating the global action from carbon neutrality. In contrast, by precipitating the abundant magnesium resources in the ocean into Mg(OH)? through alkali addition and thencalcined into MgO under the wet route, direct CO2 emission does not take place. This can therefore be regarded as a greener process for obtaining Mg cement in comparison to the dry route.

[0089] Apart from the option of seawater as the feedstock, reject brine generated from desalination plants can be recycled as well for Mg cement harvesting. Reject brine may be used to produce MgO. In fact, magnesia cement extracted from seawater or seawater-derived brine may be more reactive and purer than that obtained by dry method due to the synthetic method used in wet route.

[0090] Globally, the daily production of desalinated water is approximately 95.4 million m3and the daily production of brine is approximately 141.5 million m3, which are still increasing. These reject brine implies an annual production of approximately 103 million tons of Mg (equivalent to 172 million tons of MgO or 250 million tons of Mg(OH)2) by assuming a typical Mg2+concentration of 2000 ppm in reject brine. However, approximately 4.1 billion tons of cement are consumed annually, indicating a huge difference between the supply and the demand worldwide. However, locally in Singapore, five desalination plants provide a total of around 0.85 million m3of water per day, implying an annual production capacity of around 1.53 million tons of MgO or 2.22 million tons of Mg(OH)2. According to the Singapore Building and Construction Authority (BCA), the annual cement consumption in Singapore is estimated to be around 4.4 million tons in 2022.

[0091] To increase annual supply of Mg cement to approach the annual demand of OPC, feedstock of Mg cement is evaluated and explored. Apart from the above, seawater, or other magnesium-rich resources (e.g., seawater, subsurface brine and lake brine) can be considered. From the perspective of demand, Mg cement does not necessarily fully replace the OPC. Instead, the Mg cement can be used as: (i) partial replacement of OPC; (ii) an expansion agent; and (iii) a component of functional concrete (e.g., rapid-hardening concrete).

[0092] To precipitate Mg2+from seawater or seawater-derived brine, the pH value may be increased from around 8, to a value of 10 or higher by the use of alkalis, such as sodium hydroxide (NaOH), ammonia (NH3OH), and lime (CaO) / do1omite lime (CaO-MgO).

[0093] As mentioned above, sodium hydroxide may advantageously be used, given that it allows a higher extraction efficiency and purity as sodium hydroxide does not introduce additional Ca-based compound, and solubility of sodium hydroxide is much higher as compared to calcium hydroxide (Ca(OH)2) in water. This takes into consideration that dolomite (lime) is more economic in comparison to sodium hydroxide. The handling of ammonia needs a higher level of care in comparison to sodium hydroxide, when considering health and safety aspects.

[0094] To enhance material efficiency of Mg(OH)2-based LCC, modifications on the nanostructure of Mg(OH)2 may be carried out to gain a high particle packing density and high particle surface exposure. This may be carried out through use of a modifier or nano modifier in brucite precipitation.

[0095] The modifier may, for example, be a cross-linker, which may be formed from organic polymers such as polyethylene glycol (PEG), which is oligomer or polymer of ethylene oxide. The hydroxide (OH) group of PEG can draw adjacent brucite crystals together in one crystal orientation to enable the self-assembly of nano lamellas of brucite to form various plate-like morphologies. Addition of PEG 200 (1 wt%, 2 wt%, and 5 wt% by mass of solvent) in brucite solution was evaluated and it was concluded that the use of 2 wt% PEG 200 was the optimal because the lateral arrangement caused by PEG prevented agglomeration of brucite particles and ensure the best modifier adsorption.

[0096] In various embodiments, the modifier comprises or consists of urea. Urea, being an organic compound with chemical formula CO(NH2)2, has two -NH2 groups joined by a carbonyl (C=O) functional group. Due to its lower cost as compared to organic polymers suchas polyethylene glycol (PEG), use of urea as a modifier in tailoring the microstructure of brucite particles during brucite precipitation is advantageous.

[0097] The method may comprise providing reject brine, adding alkali to the reject brine to precipitate Mg(0H)2 (brucite), and adding a modifier to the resulting mixture to modify the microstructure of the Mg(0H)2.

[0098] The alkali may be sodium hydroxide (NaOH), ammonia, or lime (CaO) / dolomite lime (CaO MgO), preferably NaOH. The modifier may be urea.

[0099] Residual water from a method disclosed herein may be removed by a siphon. The precipitated brucite may then be loaded into centrifuge tubes for centrifuging. After centrifuging, colloidal brucite may be obtained. The colloidal brucite may be put into a vacuum oven. The dried brucite may then be grounded into powder and used as cement, and / or be used to produce hydrated magnesium carbonate (HMC) and magnesium silicate hydrate (MSH).

[0100] In particular, at optimum dosages of modifier, particles of the low carbon cement may align with each other with high surface exposure, thus presenting high reactivity, pore volume and specific surface area. Thermogravimetric (TG), X-ray diffraction (XRD) and scanning electron microscopic (SEM) analysis were performed on LCC. Chemical analysis of carbon-negative concrete made with low carbon cement revealed hydromagnesite and nesquehonite as the main hydration products. Strength of the carbon negative concrete showed a peak point at the optimum urea dosage, and the reason was revealed by microstructural analysis.

[0101] A novel method to produce calcination-free low carbon cement is disclosed herein. The developed calcination-free cement disclosed herein involves no calcination which implies the calcined energy can be saved, and it can absorb carbon dioxide (CO2) in the hardening process. Availability of the calcination-free low carbon cement is rich consideringthe ocean / marine-derived feedstocks which can possibly sustain 0.6 million years. Most importantly, the developed calcination -free low carbon cement can achieve a similar strength to that of OPC concrete, which still have a large room to increase.

[0102] Materials and sample preparation

[0103] Reject brine as brine was collected locally as the feedstock for low carbon cement. pH value of the reject brine was measured as about 8, and chemical composition of the reject brine is shown in TABLE 1. Sodium hydroxide (NaOH) (pellet, Sigma- Aldrich) was used as alkali source and urea (powder, Sigma-Aldrich) was used as modifier. To test the reactivity, citric acid (0.25 mol / L) and phenolphthalein (pH indicator) solution were prepared.

[0104] TABLE 1 Chemical composition of reject brineComposition CP SO42' Mg2+Na+Ca2+K+Si Sr Li pHConcentration (ppm) 30904 4332 2125 17770 569 754 0.15 12.6 0.2 8.17

[0105] Chemical synthesis of LCC

[0106] To investigate effect of modifier on the brucite precipitation, different dosages of urea (0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 mol / L) in terms of the volume of brine were adopted. In the brucite precipitation, the NaOH / Mg2+molar ratio was kept at 2.0.

[0107] To trigger brucite precipitation, sodium hydroxide was mixed with brine as a solute for 3 min until a homogeneous solution was obtained. Immediately after that, the modifier was added to the solution and mixed for another 2 min. When the brucite was precipitated on the bottom of the reaction container, the upper residual water was removed by a siphon. The use of the siphoning effect is to avoid the disturbance and dilution of the precipitated brucite at the bottom. The precipitated brucite was then loaded into centrifuge tubes for centrifuging at 5000 rpm for 10 min. After centrifuging, colloidal brucite (with a solid content of about 10 %) was obtained. Then, the colloidal brucite was put into vacuumoven (50 °C) for drying for 72 h. The dried brucite was then ground into powder finer than 150 pm and used as low carbon cement (LCC).

[0108] Exemplary procedures of producing and using low carbon cement are described below.

[0109] Specifically, in the wet route, marine-derived feedstocks can be used for the precipitation of brucite. In the precipitation, both the nano and the microstructure of the low carbon cement particles can be modified by using a novel and low-cost modifier. The low carbon cement synthesized with the aid of modifier can then be used as calcination free cement to produce concrete of various types. The concrete so produced involves raw materials with less or no calcination and less carbon dioxide emission in comparison to ordinary Portland cement, and can absorb carbon dioxide in the hardening process, thus can be called as low carbon concrete.

[0110] To further reduce the production cost, waste alkaline can be used for precipitation, and the low carbon cement may also be calcined to obtain MgO-based expansion agent for functional design of concrete. Modifier was used in the synthesis of low carbon cement, which was then used to produce low carbon concrete.

[0111] Preparation of low carbon concrete using low carbon cement

[0112] Low carbon cement disclosed herein was used to produce hydrated magnesium carbonate and magnesium silicate hydrate samples. To investigate the influence of modifier in the production of low carbon cement on the mechanical performance of resulting low carbon concrete, three cubic samples were prepared at each urea dosage.

[0113] To prepare the hydrated magnesium carbonate samples, the low carbon cement powder was mixed with water at a water to binder (w / h) ratio of 0.5 consistently herein. Immediately after casting, the samples were cured in a carbon dioxide chamber (CO2concentration of 10 %, relative humidity of 85 %, and temperature of 30 °C) for one day and then put back after demolding until the age of 7 days.

[0114] To prepare the magnesium silicate hydrate samples, the low carbon cement powder and silica fume were mixed with water at a water to binder (w / b) ratio of 0.5 consistently herein. The magnesium silicate hydrate samples were demolded one day after casting and cured in air (at temperature of 30 °C and relative humidity of 85 %,). The strength tests were conducted at the age of 28 days for all the samples. For each of the hydrated magnesium carbonate and magnesium silicate hydrate mixes, one intact sample was taken for microstructural tests.

[0115] Characterization methods

[0116] To evaluate the chemical composition of the low carbon cement, thermogravimetry analysis (TGA) was performed by Pyris Diamond TGA 4000 at a heating rate of 10 °C / min under continuous nitrogen flow. To investigate the crystal characteristics of the low carbon cement, X-ray powder diffraction (XRD) was performed by Bruker D8 Advance under a Cu-Ka source ( / . = 1.5405 A) at 40 kV and 40 mA. The scan rate was 0.02° s-!, and the 20 ranged from 5 to 100°. To investigate the physical properties of the low carbon cement, the specific surface area (SSA) was obtained by Brunauer-Emmett-Teller (BET) analysis from nitrogen adsorption -desorption isotherms using a Quadrasorb Evo automated surface area and pore size analyzer.

[0117] To evaluate the reactivity of the low carbon cement, acid neutralization was used. First, the citric acid (0.25 mol / L) solution and phenolphthalein (pH indicator) was magnetically stirred at a constant rate of 600 rpm. The low carbon cement was added into the tube in one go with video recording turned on to determine the time elapsed for color changing from white to pink. The time elapsed for this neutralization is an indicator of the reactivity. To evaluate the microstructure of the low carbon cement, a JSM-7600F fieldemission scanning electron microscope (FESEM) was used to obtain microstructural images. To obtain the mechanical properties of low carbon concrete made with low carbon cement, the cubic samples were used for compression tests and the three repeated results were averaged to the compressive strength result for each mix.

[0118] Discussion

[0119] Formation of Mg(0H)2 was observed via the reaction between the Mg2+in the brine and OH- provided by NaOH. The kinetics of the reaction between reject brine and NaOH reflected by the change of pH. A rapid reaction was observed, which was completed in half an hour as the pH reached an equilibrium state of 11.5.

[0120] FIG. 1 depicts the TG-DTG results of the low carbon cement synthesized at different dosages of modifier. The results showed that the mass loss at the temperature range of 50 °C to 300 °C was mainly due to the loss of water. The mass loss was generally increased with the increased dosage of modifier, which was mainly attributed to the increased water retention ability in the presence of modifier. The mass loss due to transformation of Mg(0H)2 to MgO at the temperature range of 500 °C to 900 °C was increased from 4.6 %, 5.3 % to 6.8 % at increasing modifier dosages of 0.0 %, 0.1 % and 0.2 %, showing the increased water retention ability in the presence of modifier.

[0121] FIG. 2 shows the X-ray diffractograms of the low carbon cement. The main peak positions of the synthesized brucite were located at the 20 of 18.5°, 38.5°, and 51°, matching well with the reference peaks of brucite documented in JCPDS card no. 89-7746. The brucite peaks were sharp and clear at the modifier dosage of 0.0 mol / L to 0.3 mol / L, indicating that the optimum modifier dosage range of 0.0 mol / L to 0.3 mol / L. At the modifier dosage of 0.0 mol / L to 0.3 mol / L, the chemical composition of low carbon cement varied at a small range. Above 0.3 mol / L, the brucite peaks became unobvious and the relativeproportion of B(101) and B(102) became less, indicating that the modifier dosage of above 0.3 mol / L may adversely affect production efficiency and quality of the low carbon cement.

[0122] FIG. 3A shows the relation between the specific surface area (SSA) of the low carbon cement and the modifier dosage, while FIG. 3B shows the relation between the pore volume of the low carbon cement and the modifier dosage. The figures revealed that the specific surface area of the low carbon cement first increased and then decreased with the modifier dosage, which showed an optimum point. The increased specific surface area and pore volume of the low carbon cement can be ascribed to the morphological modification effect and water retention effect of the modifier. The optimum specific surface area of the low carbon cement obtained was around 60 m2 / g, which may not be as high as that of magnesium cement reported in literature.

[0123] For instance, synthesis of MgO by adding NaOH into MgCh solution resulted in a SSA of 22.1 m2 / g for the MgO calcined at 500 °C for 2.0 h. As another example, addition of NaOH and NH4OH to reject brine (from a Singapore desalination plant) resulted in SSA of 51.4 m2 / g and 78.8 m2 / g for the synthesized MgO (under the same calcination condition of 500 °C for 2 h), respectively.

[0124] Notwithstanding the above, the low SSA of the low carbon cement synthesized did not necessarily lead to a low reactivity because of the morphology of the low carbon cement, as can be seen from the following.

[0125] FIG. 4A and FIG. 4B show the neutralization time and reactivity of the low carbon cement at different modifier dosages. Likewise, the figures revealed that the neutralization time first decreased and then increased with the modifier dosage, while that of the reactivity first increased and then decreased with the modifier dosage. This phenomenon, being similar to that of SSA, implied that there may be a critical modifier dosage that yielded the highest reactivity. Correlating the reactivity results to the XRD results of the MgO, it canbe found that the low carbon cement synthesized at a modifier dosage exceeding 0.3 mol / L exhibited less brucite (001), brucite (101) and brucite (102). However, the relative quantities of the crystals revealed by XRD does not fully explain the reactivity of the MgO and explanation may be sought from the perspective of SSA.

[0126] FIG. 5 shows the reactivity results are correlated to SSA results of the low carbon cement. The correlation in FIG. 5 revealed that the reactivity of the low carbon cement has a positive relation with the SSA.

[0127] FIG. 6A to FIG. 6F show the typical morphology of the microstructure of the low carbon cement produced at different dosages of modifier. At nanoscale, the microstructure of the low carbon cement was composed of a variety of grains with different shapes and sizes, depending on the modifier dosage. In general, with the increase of modifier dosage, the particle size of the low carbon cement became larger but the particles became agglomerated together. Tn addition, there existed a critical modifier dosage (0.2 mol / L herein) that roughly distinguished small thick isolated brucite pellets and large thin agglomerated brucite pellets.

[0128] More specifically: at the modifier dosage of 0.0 mol / L, the particles were adhered to each other; when the modifier dosage was increased from 0.0 to 0.1 mol / L, the brucite particles became separated from each other; when the modifier dosage was increased from 0.1 to 0.2 mol / L, the particles became larger and flaky; when the modifier dosage was increased from 0.2 to 0.3 mol / L, the particles became larger, flaky, and agglomerated with each other; when the modifier dosage was increased from 0.3 to 0.5 mol / L, the particles were adhered to each other again with few air voids. Above all, the modifier dosage of 0.1 mol / L to 0.2 mol / L was able to yield brucite particles of regular in shape and separated from each other, which implies a high porosity that favors carbon dioxide absorption when used as binder.

[0129] FIG. 7 shows the 28-day compressive strength of the hydrated magnesium carbonate cubes (at the same water to binder ratio of 0.50) made with low carbon cement, and the low carbon cement was synthesized with different dosage of modifier. The results revealed that the compressive strength first increased until the modifier dosage of 0.2 mol / L and then decreased with the modifier dosage. The trend of compressive strength was similar to the trend of specific surface area, pore volume and reactivity of the low carbon cement particles. Although previous studies showed that strength of hydrated magnesium carbonate was not solely determined by reactivity of the magnesium-based cement particles, it was demonstrated herein that a high reactivity of the magnesium-based cement particles from the same source leads to a higher strength of hydrated magnesium carbonate. The increased strength of the hydrated magnesium carbonate can be ascribed to the increased carbon dioxide absorption ability of the low carbon cement at an appropriate modifier dosage.

[0130] FIG. 8 shows the XRD results of the hydrated magnesium carbonate made with low carbon cement. The figure revealed that the main reaction products were dypingite and nesquehonite, and there were some unreacted low carbon cement particles. The peaks of each crystal in hydrated magnesium carbonate varied a lot with the low carbon cement synthesized at different modifier dosage.

[0131] More specifically: there was no significant changes in the peaks when the modifier dosage was increased from 0.0 mol / L to 0.1 mol / L: when the modifier dosage was increased from 0.1 mol / L to 0.2 mol / L, the brucite peaks at 18.5°, 38.5°, and 51° became lower and less sharp and the peaks of dypingnite and nesquehonite became clearer, implying a decrease in the relative amount of unreacted low carbon cement particles and a larger consumption of in the formation of various types of hydrated magnesium carbonates (note that when the modifier dosage was increased from 0.1 mol / L to 0.2 mol / L, the low carbon cement particles became larger and flaky from the FESEM images); when the modifierdosage was increased from 0.2 mol / L to 0.5 mol / L, the peaks changed minorly in general (note that when the modifier dosage was increased from 0.2 mol / L to 0.5 mol / L, the low carbon cement particles became larger, flaky, and agglomerated with each other until adhered to each other with few air voids from the FESEM images).

[0132] To produce 1 ton of MgO from brine via the addition of alkali base, the total calcination energy demand is 2.08 GJ (equivalent to 577.8 kWh), respectively, at calcination temperatures of 500 °C. Nowadays, about 95 % of Singapore's electricity is generated by natural gas and the rate is about 0.15 USD / kWh, resulting in an energy cost of 79 to 90 USD for the calcination of Mg(OH)j) into 1 ton of MgO. In other words, the development of calcination free low carbon cement disclosed herein can save the high production cost caused by calcination.

[0133] Use of modifier in the synthesis of low carbon cement, which was then used to produce low carbon concrete, is disclosed herein. The low carbon cement is harvested from the ocean / marine-derived feedstocks, in which Mg is abundant and the abundance is in the similar order to current cement consumption.

[0134] The above results indicate that synthesis of calcination free low carbon cement via the use of wastewater and low-cost modifier is viable, and potentially has high commercial values. The low carbon cement is particularly useful in addressing the global climate changes and solve the local heavy reliance of importing ordinary Portland cement. Feedstocks for producing the calcination free low carbon cement is abundant locally in Singapore, for example, using waste brine discharged from desalination plants and waste alkaline water from concrete producers or recycled concrete. The potential of the low carbon cement is promising, because if all the local brine produced in Singapore was fully utilized, about half of the local cement consumption can be satisfied. In addition, the low carboncement has high recyclability. The low carbon cement has great commercial application potential in local construction, such as for the construction of HDB houses.

[0135] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0136] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0137] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including”, “containing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0138] By “about” in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

[0139] The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso ornegative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0140] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Claims

CLAIMS1. A method of producing cement, the method comprising contacting brine with an alkali material to extract Mg2+in the form of brucite from the brine, wherein the contacting is carried out at a pH of at least 10 and in the presence of a modifier for modifying a surface of the brucite.

2. The method according to claim 1 , wherein the contacting comprises adding the alkali material to the brine to form a mixture, and adding the modifier to the mixture.

3. The method according to claim 1 or 2, wherein the alkali material is one or more of sodium hydroxide, ammonia, lime, and dolomite lime.

4. The method according to any one of claims 1 to 3, wherein the alkali material is sodium hydroxide.

5. The method according to any one of claims 1 to 4, wherein the alkali material is in the form of pellets.

6. The method according to any one of claims 1 to 5, wherein the alkali material is sodium hydroxide pellets.

7. The method according to claim 6, wherein molar ratio of sodium hydroxide to Mg2+is in the range of 2 to 4.

8. The method according to any one of claims 1 to 7, wherein the modifier is one or more of urea and polyethylene glycol.

9. The method according to any one of claims 1 to 8, wherein the modifier is present at 0.1 mol / L to 0.3 mol / L based on volume of the brine.

10. The method according to any one of claims 1 to 9, wherein the brine is reject brine obtained from desalination of seawater.

11. The method according to any one of claims 1 to 10, wherein the contacting is carried out in a vessel, and wherein the contacting comprises settling the brucite into a bottom portion of the vessel as precipitated brucite while removing a supernate from the vessel.

12. The method according to claim 11, wherein removing the supernate is carried out by siphoning.

13. The method according to claim 11 or 12, wherein the contacting comprises centrifuging the precipitated brucite to form colloidal brucite.

14. The method according to claim 13, wherein the colloidal brucite has a solid content in the range of 5 wt% to 15 wt%.

15. The method according to claim 13 or 14, further comprising drying the colloidal brucite.

16. The method according to claim 15, wherein the drying is carried out at a temperature in the range of 40 °C to 60 °C for a time period in the range of 60 hours to 80 hours.

17. The method according to claim 15 or 16, further comprising grinding the dried brucite into dried brucite powder having a particle size of 150 pm or less.

18. A cement produced by a method according to any one of claims 1 to 17.

19. A method of producing concrete, the method comprising providing the cement produced by a method according to any one of claims 1 to 17, and(a) mixing the cement with water to form a brucite mixture, and curing the brucite mixture in carbon dioxide to form hydrated magnesium carbonate, or(b) mixing the cement with silica fume and water to form a brucite-silica mixture, and curing the brucite-silica mixture in air to form magnesium silicate hydrate.

20. A concrete produced by the method according to claim 19.

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