PROCESSES AND SYSTEMS FOR CARBON DIOXIDE SEQUESTRATION AND RELATED CONCRETE COMPOSITIONS

Abstract

A ready-mixed composition and a pre-mix composition for the production of a concrete material containing sequestered carbon dioxide, a CO.sub.2-containing water used in such compositions, dry-batch and wet-batch processes for sequestering carbon dioxide in concrete material, general method and process for sequestering carbon dioxide in hardening concrete, system and ready-mixed truck to perform such processes and methods for the production of a ready-to-cure carbonated concrete. Compositions comprise a concrete mixture and a CO.sub.2-containing water. The CO.sub.2-containing water comprising water and at least one of blended CO.sub.2 gas bubbles, dissolved H.sub.2CO.sub.3, carbonate ions (CO.sub.3.sup.2), bicarbonate ions (HCO.sup.3−), nanosized alkaline earth metal carbonate and nanosized alkali metal carbonate particles. The concrete mixture comprises a cementitious material, aggregates and at least one CO.sub.2-sequestering chemical for accelerating a CO.sub.2 sequestration speed and maximizing the captured amount of the carbon dioxide.

Claims

1. A process for the preparation of a wet carbonated concrete mixture, the process comprising the steps of: mixing a concrete mixture with a CO.sub.2-containing water to produce the wet carbonated concrete mixture; wherein the CO.sub.2-containing water comprises CO.sub.2 gas bubbles; and wherein the concrete mixture comprises a cementitious material, aggregates and at least one CO.sub.2-sequestering chemical that releases species which are ready to be carbonated via the CO.sub.2-containing water, thereby increasing the captured amount of CO.sub.2 in the wet carbonated concrete mixture.

2. The process of claim 1, wherein mixing the concrete mixture with the CO.sub.2-containing water is performed to add at least 1 kg of CO.sub.2 per m.sup.3 of wet concrete mixture.

3. The process of claim 1, wherein the cementitious material comprises cement, supplementary cementitious materials (SCMs) or a mixture thereof.

4. The process of claim 3, wherein the SCMs comprise fly ash, ground granulated blast furnace slag (GGBFS), silica fume, siliceous material, aluminosilicate material, calcined clay, metakaolin, ground glass or any combination thereof.

5. The process of claim 1, wherein the aggregates comprise sand, gravel, crushed stone or any combinations thereof.

6. The process of claim 1, wherein the mixing of the concrete mixture with the CO.sub.2-containing water is performed with a CO.sub.2-containing water to total cementitious material weight ratio between 0.15 and 2.0.

7. The process of claim 1, wherein the CO.sub.2-containing water has a CO.sub.2 to water ratio between 5 g/L and 200 g/L.

8. The process of claim 1, further comprising dissolving and/or blending carbon dioxide into water to produce the CO.sub.2-containing water.

9. The process of claim 8, wherein the water is carried in a feed line at a water flow rate, and dissolving and/or blending carbon dioxide into the water comprises injecting the carbon dioxide into the feed line carrying the water and adjusting the water flow rate and a CO.sub.2 injection speed to deliver at least 1 kg of CO.sub.2 per m.sup.3 of wet concrete mixture.

10. The process of claim 9, wherein injecting carbon dioxide into the water comprises injecting the carbon dioxide in at least one of a gas phase and a liquid phase.

11. The process of claim 9, wherein the water flow rate ranges between 300 kg/minute and 1800 kg/minute, and the CO.sub.2 injection flow rate ranges between 1 kg/minute and 40 kg/minute of CO.sub.2 per min.

12. The process of claim 1, comprising providing the CO.sub.2-containing water and the concrete mixture in batched amounts to form a pre-mix composition before mixing thereof to produce the wet concrete mixture.

13. The process of claim 1, wherein the conditioning of the wet concrete mixture comprises batching the wet concrete mixture in a batched amount to form a batched ready-mix composition.

14. The process of claim 1, wherein the CO.sub.2-containing water further comprises at least one of nanosized alkaline earth metal carbonate particles or nanosized alkali metal carbonate particles.

15. The process of claim 1, wherein a weight ratio of the at least one CO.sub.2-sequestering chemical to the cementitious material is between 0.5% and 10%.

16. The process of claim 1, wherein the at least one CO.sub.2-sequestering chemical is under the form of ground or fine particles that comprise at least one alkali metal compound, at least one alkaline earth metal compound or a combination thereof, and that generate carbonate particles which densify a concrete microstructure upon carbonation of the at least one CO.sub.2-sequestering chemical.

17. A process for the preparation of a carbonated concrete material, the process comprising the steps of: mixing a concrete mixture with a CO.sub.2-containing water to produce a wet carbonated concrete mixture; and allowing the wet carbonated concrete mixture to harden and produce the carbonated concrete material upon hydration and carbonation thereof; wherein the CO.sub.2-containing water comprises CO.sub.2 gas bubbles; and wherein the concrete mixture comprises a cementitious material, aggregates and at least one CO.sub.2-sequestering chemical that releases species which are ready to be carbonated via the CO.sub.2-containing water, thereby increasing the captured amount of CO.sub.2 in the carbonated concrete material.

18. The process of claim 17, wherein allowing the wet carbonated concrete mixture to harden comprises conditioning the wet carbonated concrete mixture to produce a ready-to-cure concrete.

19. The process of claim 18, wherein the conditioning of the wet carbonated concrete mixture comprises at least one of batching, transiting, depositing, forming, consolidating, and finishing the wet concrete mixture.

20. The process of claim 17, wherein the at least one CO.sub.2-sequestering chemical is under the form of ground or fine particles that comprise at least one alkali metal compound, at least one alkaline earth metal compound or a combination thereof, and that generate carbonate particles which densify a concrete microstructure upon carbonation of the at least one CO.sub.2-sequestering chemical.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 is a schematic process flowchart of general steps for producing carbonated concrete material from a concrete mixture and CO.sub.2-containing water.

[0055] FIG. 2 is a schematic process flowchart of general steps for producing a wet concrete mixture in a central plant from a concrete mixture and CO.sub.2-containing water, which will be cured on a work site.

[0056] FIG. 3 is a schematic process flowchart of general steps for producing a ready-to-cure concrete on a work site from a pre-mix composition including a concrete mixture and CO.sub.2-containing water.

[0057] FIG. 4 is a schematic process flowchart of general steps for producing a ready-mixed composition from a concrete mixture and stored CO.sub.2-containing water.

[0058] FIG. 5 is a schematic process flowchart of general steps for producing a ready-mixed composition from independently fed concrete ingredients and CO.sub.2-containing water prepared by injecting CO.sub.2 in a flow of water.

[0059] FIG. 6 is schematic representation of a one-tank system for injecting CO.sub.2 into water before mixing with concrete mixture.

[0060] FIG. 7 is schematic representation of a two-tank system for injecting CO.sub.2 into water before mixing with concrete mixture.

[0061] FIG. 8 is a graph of the average one-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the one-tank system of FIG. 6.

[0062] FIG. 9 is a graph of the average seven-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the one-tank system of FIG. 6.

[0063] FIG. 10 is a graph of the average twenty-eight-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the one-tank system of FIG. 6.

[0064] FIG. 11 is a graph of the average one-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the two-tank system of FIG. 7.

[0065] FIG. 12 is a graph of the average seven-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the two-tank system of FIG. 7.

[0066] FIG. 13 is a graph of the average twenty-eight-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) for preparation thereof using the two-tank system of FIG. 7.

[0067] FIG. 14 is a graph of the average one-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) with varying lime content for preparation thereof using the two-tank system of FIG. 7.

[0068] FIG. 15 is a graph of the average seven-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) with varying lime content for preparation thereof using the two-tank system of FIG. 7.

[0069] FIG. 16 is a graph of the average twenty-eight-day compressive strength (in MPa) of concrete cylinders versus the content of CO.sub.2 in the CO.sub.2-containing water (in g/L) with varying lime content for preparation thereof using the two-tank system of FIG. 7.

[0070] FIG. 17 is a graph of the average one-day compressive strength (in MPa) of concrete cylinders versus the content of iron slag (in %) for preparation thereof using the two-tank system of FIG. 7.

[0071] FIG. 18 is a graph of the average seven-day compressive strength (in MPa) of concrete cylinders versus the content of iron slag (in %) for preparation thereof using the two-tank system of FIG. 7.

[0072] FIG. 19 is a graph of the average twenty-eight-day compressive strength (in MPa) of concrete cylinders versus the content of iron slag (in %) for preparation thereof using the two-tank system of FIG. 7.

[0073] FIG. 20 is a graph of the average one-day compressive strength (in MPa) of concrete cylinders (prepared with CO.sub.2 flakes) versus the content of CO.sub.2 using CO.sub.2 flakes for preparation thereof.

[0074] FIG. 21 is a graph of the average seven-day compressive strength (in MPa) of concrete cylinders (prepared with CO.sub.2 flakes) versus the content of CO.sub.2 using CO.sub.2 flakes for preparation thereof.

[0075] FIG. 22 is a graph of the average twenty-eight-day compressive strength (in MPa) of concrete cylinders (prepared with CO.sub.2 flakes) versus the content of CO.sub.2 using CO.sub.2 flakes for preparation thereof.

[0076] While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims.

DETAILED DESCRIPTION

[0077] It should be noted that the same numerical references refer to similar elements. Furthermore, for the sake of simplicity and clarity, namely so as to not unduly burden the figures with several references numbers, not all figures contain references to all the components and features, and references to some components and features may be found in only one figure, and components and features of the present disclosure which are illustrated in other figures can be easily inferred therefrom. The embodiments, geometrical configurations, materials mentioned and/or dimensions shown in the figures are optional, and are given for exemplification purposes only. Therefore, the descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only.

[0078] It is worth mentioning that throughout the following description when the article “a” is used to introduce an element it does not have the meaning of “only one” it rather means of “one or more”. It is to be understood that where the specification states that a component, feature, structure, step or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, step or characteristic is not required to be included.

[0079] In the following description, the term “about” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e. the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.

[0080] In the present description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features or implementations of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

[0081] Aspects of the invention described herein relate to concrete ready-mixed and pre-mix compositions, methods, processes, and systems, each of these aspects having at least one implementation. All implementations relate to sequestering CO.sub.2 in concrete, resulting in producing carbonated concrete.

Composition Implementations

[0082] Embodiments of the concrete composition include a wet composition that can be referred to as ready-mixed composition, and a dry composition that can be referred to as a pre-mix composition, both compositions including CO.sub.2-containing water for the production of a concrete material containing sequestered CO.sub.2.

[0083] More particularly, there is provided a ready-mixed composition including a concrete mixture and CO.sub.2-containing water. The ready-mixed composition may be also referred to as freshly mixed concrete, wet concrete mixture, wet carbonated concrete mixture or a wet concrete batch. In the ready-mixed composition, the CO.sub.2-containing water is fully mixed with the concrete mixture and the ready-mixed composition has a consistency allowing workability of the ready-mix composition.

[0084] Workability of the ready-mix composition directly refers to the ability to work with said composition. Workability of the ready-mix composition enables transport of the composition to a work site, mixing, pumping, pouring, molding, shaping, compacting, finishing thereof and/or any other operation needed to provide concrete at the desired location and into the desired shape. As known by one skilled in the art, workability can be determined by performing a slump and/or slump flow test on the ready-mixed composition, or by measuring the amount of energy required to overcome friction while compacting.

[0085] In some implementations, the ingredients of the ready-mixed composition are mixed in advance in a stationary mixer, from a concrete plant for instance, and can be loaded in a transit or a ready-mixed concrete truck for delivery to a work site. In some other implementations, the ready-mixed composition may also result from the mixing of concrete ingredients in a ready-mixed truck or another mixing device directly on a work site.

[0086] In another embodiment, a pre-mix composition is provided and includes a concrete mixture and CO.sub.2-containing water. The pre-mix composition includes the same ingredients as the ready-mixed composition but the CO.sub.2-containing water is not in mixed state with the remaining ingredients of the composition. The ready-mixed composition can be therefore prepared by mixing the CO.sub.2-containing water and the concrete mixture from the pre-mix composition.

[0087] In some implementations, the ingredients of the pre-mix composition are batched in a concrete plant, transited without being combined together. The pre-mix composition is mixable by a ready-mixed truck or another mixing device directly on the work site, to form a ready-mixed composition that will harden into concrete. The pre-mix composition may therefore be referred to as a dry concrete batch.

[0088] It should be noted that CO.sub.2 sequestration refers herein to the incorporation of an amount of CO.sub.2 in an early concrete production stage via water. The water acts as a medium which receives an initial amount of CO.sub.2, before being mixed with other concrete ingredients (at least a cementitious material and aggregates). Water including an initial amount of carbon dioxide is referred to herein as CO.sub.2-containing water. CO.sub.2-containing water as encompassed herein includes a CO.sub.2 to water ratio between about 2 g/L and 200 g/L, optionally between about 2 g/L to 100 g/L, further optionally between 5 g/L to 80 g/L. The CO.sub.2-containing water is tailored for reaction with MA.sup.a+ ions from the concrete mixture for the preparation of carbonated concrete, the MA.sup.a+ ions being at least one of an alkali metal ion and an alkaline metal earth ion.

[0089] It should be further noted that depending on the source of water used for the preparation of concrete, CO.sub.2 may be present in water under different forms which include CO.sub.2 gas bubbles, dissolved CO.sub.2 (H.sub.2CO.sub.3), carbonate ions (CO.sub.3.sup.2−), bicarbonate ions (HCO.sub.3.sup.−) and nanosized carbonate particles. Depending on the pH of the water and the nature of components contained in the water used for the preparation of the concrete, CO.sub.2 may already react with these components and start forming secondary components. The water may be sourced from a municipal water network and/or deriving from an upstream process as recycled process water. For example, the CO.sub.2-containing water can be prepared from process water derived from concrete production, which may include calcium-based minerals which react with carbonate and bicarbonate ions from the added CO.sub.2, thereby forming the CO.sub.2-containing water including nanosized calcium carbonate particles.

[0090] It should further be noted that the CO.sub.2 may be sourced as pure carbon dioxide, carbon dioxide from a flue gas deriving from the burning of fossil fuels or from other industrial processes including cement manufacture, or combinations thereof.

[0091] It should be noted that the processes and methods described herein can include the preparation of the CO.sub.2-containing water. Systems to prepare the CO.sub.2-containing water can include two feed lines for CO.sub.2 and water respectively (as seen on FIG. 5), each line being provided with a flow meter (not illustrated) so as to adjust the water flow rate in accordance to the CO.sub.2 injection speed depending on the desired CO.sub.2 dosage and desired CO.sub.2-containing water batch quantity for a concrete mixture batch.

[0092] Ingredients of the concrete mixture from the presently described compositions may vary depending on the properties to be conferred to the hardened concrete and the desired hardening time. The concrete mixture at least includes a cementitious material and aggregates. The cementitious material can include cement only or supplementary cementitious materials (SCMs). The cement is a hydraulic cement, optionally Portland cement. The SCMs may include fly ash, ground granulated blast furnace slag (GGBS), silica fume, siliceous material, aluminosilicate material, calcined clay, metakaolin, ground glass, natural Pozzolanic materials, or any combinations thereof. The aggregates may include, without being limited to, sand, gravel, crushed stone or any combinations thereof.

[0093] One skilled in the art will readily understand how to batch, i.e. measure a needed amount of, each ingredient of the concrete mixture and the CO.sub.2-containing water depending on the properties desired for the resulting concrete. For example, the cementitious material per cubic meter of concrete may range between about 10 kg/m.sup.3 and about 2000 kg/m.sup.3. The CO.sub.2-containing water to cementitious material weight ratio may range between about 0.15 and about 10.

[0094] The cementitious material may be hydraulic cement which is known to mainly include tricalcium silicate (C.sub.3S), dicalcium silicate (C.sub.2S), tricalcium aluminate (C.sub.3A), tetracalcium aluminoferrite (C.sub.4AF), and gypsum (CSH.sub.2). Hydration of hydraulic cement into secondary components provides strength to the resulting concrete according to the following chemical reactions during hydration. The cementitious material may be blended hydraulic cement, including Portland blast furnace slag cement (S), Portland fly ash cement (F); Portland silica fume cement (SF), ternary blended cement, and quaternary blended cement is also involved in the following reactions.

[0095] The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat according to chemical reaction (1).

Tricalcium aluminate÷gypsum+water.fwdarw.ettringite+heat C.sub.3A+3CSH.sub.2+26H.fwdarw.C.sub.6AS.sub.3H.sub.32,ΔH=207 cal/g  (1)

[0096] Once all the gypsum is used up as per reaction (1), the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals according to chemical reaction (2):

Tricalcium aluminate÷ettringite+water.fwdarw.monosulfate aluminate hydrate 2C.sub.3A+3C.sub.6AS.sub.3H.sub.32+22H.fwdarw.3C.sub.4ASH.sub.18,  (2)

[0097] The tricalcium silicate is hydrated to produce calcium silicate hydrates, lime and heat according to chemical reaction (3):

Tricalcium silicate÷water.fwdarw.calcium silicate hydrate+lime+heat 2C.sub.3S+6H.fwdarw.C.sub.3S.sub.2H.sub.3÷3CH,ΔH=120 cal/g  (3)

[0098] Calcium silicate hydrate, referred to as CSH, has a short-networked fiber structure which contributes greatly to the initial strength of the cement paste.

[0099] The dicalcium silicate also hydrates to form calcium silicate hydrates and heat according to chemical reaction (4):

Dicalcium silicates+water.fwdarw.calcium silicate hydrate+lime C.sub.2S+4H.fwdarw.C.sub.3S.sub.2H.sub.3+CH,ΔH=62 cal/g  (4)

[0100] Like in reaction (3), CSH contributes to the strength of the cement paste. Reaction (4) however generates less heat and proceeds at a slower rate than reaction (3), meaning that the contribution of dicalcium silicate to the strength of the cement paste will be slow initially. Dicalcium silicate is still responsible for the long-term strength of the resulting concrete.

[0101] The ferrite undergoes two progressive reactions with the gypsum. The ferrite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides according to reaction (5); and the ferrite further reacts with the ettringite formed according to reaction (1) to produce garnets according to reaction (6):

Ferrite+gypsum+water.fwdarw.ettringite+ferric aluminum hydroxide+lime C.sub.4AF+3CSH.sub.2+3H.fwdarw.C.sub.6(A,F)S.sub.3H.sub.32+(A,F)H.sub.3+CH  (5)

Ferrite+ettringite+lime+water.fwdarw.garnets C.sub.4AF+C.sub.6(A,F)S.sub.3H.sub.32+2CH+23H.fwdarw.3C.sub.4(A,F)SH.sub.18+(A,F)H.sub.3  (6)

[0102] As an initial amount of CO.sub.2 is provided to the water before contacting the concrete mixture, hydration of carbon dioxide occurs according to equation (7) and (8), such that H.sub.2CO.sub.3 is in equilibrium with carbonate and bicarbonate ions in dissolved state.

CO.sub.2+H.sub.2O=HCO.sub.3.sup.−(bicarbonate)+H.sup.+

HCO.sub.1.sup.−=CO.sub.3.sup.2−(carbonate ion)+H.sup.+  (7) and (8)

[0103] It should be understood that other reactions may occur within the water medium depending on the source of water used to prepare the CO.sub.2-containing water, i.e. depending on the nature and amount of components that may be included in the water. As above-mentioned, CO.sub.2 may be present in the CO.sub.2-containing water under different forms which include CO.sub.2 gas bubbles, dissolved CO.sub.2 (H.sub.2CO.sub.3), carbonate ions (CO.sub.3.sup.2−), bicarbonate ions (HCO.sub.3.sup.−) and nanosized carbonate particles. For example, nanosized calcium carbonate particles may be present in the CO.sub.2-containing water, if the source of water used for the preparation of the CO.sub.2-containing water is a process water which is recycled from concrete production and contains calcium-based compounds. Other carbonate particles may be present, mainly as alkaline earth metal carbonate and alkali metal carbonate particles, depending on the nature of the compounds present in the process water deriving from concrete production. It should be noted that the process water may also include suspended particles of cement and cement hydration products. It should be noted that all species deriving from the presence of CO.sub.2 in the CO.sub.2-containing water, before contacting the concrete mixture (cement, aggregates, etc.) can be referred to herein as carbon-based compounds, including CO.sub.2 gas bubbles, dissolved H.sub.2CO.sub.3, carbonate ions (CO.sub.3.sup.2−), bicarbonate ions (HCO.sup.3−), nanosized alkaline earth metal carbonate and nanosized alkali metal carbonate particles.

[0104] Carbonation reactions further occur between carbon-based compounds, and at least one of cementitious component and cement hydration product. Indeed, as the cementitious material comes into contact with the CO.sub.2-containing water, hydration reactions and carbonation reactions occur. Carbonation of the hydration products formed during chemical reactions (1) to (6) lead to the formation of alkaline earth metal carbonate particles and/or alkali metal carbonate particles. Calcium hydroxide (referred to as lime CH in equations (1) to (6)) releases calcium ions which are consumed by carbon-based compounds from the CO-containing water as calcium carbonate has a lower solubility compared to cement hydration products (see chemical equilibrium equations (9) to (11) for example). Calcium ions may further be released from cement hydration products, including CSH, to form calcium carbonate or hydration products with a lower calcium oxide/silicate oxide ratio. Formation of compounds through carbonation reactions may therefore impact the concrete properties such as strength development.

Ca(OH).sub.2.fwdarw.Ca.sup.2++2OH.sup.−  (9)

Ca.sup.2++CO.sub.3.sup.2−.fwdarw.CaCO.sub.3  (10)

H.sup.++OH.sup.−.fwdarw.H.sub.2O  (11)

[0105] In some implementations, the concrete mixture can include additional ingredients including CO.sub.2-sequestering chemicals.

[0106] CO.sub.2-sequestering chemicals can be added as part of the concrete mixture to be mixed with the CO.sub.2-containing water for accelerating a sequestration speed and maximizing the captured amount of CO.sub.2 from the CO.sub.2-containing water in the formed wet concrete mixture. Optionally, the CO.sub.2-sequestering chemicals comprise at least one alkali metal compound, at least one alkaline earth metal compound or a combination thereof. Optionally, the CO.sub.2-sequestering chemicals comprise grounded steel slag, cement kiln bypass dust (CKD) or a combination thereof. Optionally, the CO.sub.2-sequestering chemical to cementitious material weight ratio is between about 0.5% and about 10%.

[0107] Advantageously, the CO.sub.2-sequestering chemicals release an alkali ion and/or an alkaline earth metal ion (MA.sup.a+) which can also react with carbon-based compounds from the CO.sub.2-containing water to form alkali metal or alkaline earth metal carbonate compound which mitigates the competition of alkali metal or alkaline earth metal ions from cement hydration products. For example, grounded steel slag includes CaO, MgO, and other alkali oxides in small amounts, which when in contact with the CO.sub.2-containing water, release their alkali and alkaline earth metal ions (for example Ca.sup.+, Na.sup.+, K.sup.+, Mg.sup.2+, etc.), thereby reacting with carbon-based compounds from the CO.sub.2-containing water to form nano-sized carbonated compounds.

[0108] Once the concrete mixture is in contact with the CO.sub.2-containing water, reaction products resulting from at least the following chemical reaction (12) may be formed within the resulting wet batch concrete:

MA.sup.a+=CO.sub.3.sup.2−=MA.sub.xCO.sub.3  (12)

[0109] MA.sup.a+ is an alkali metal ion or an alkaline earth metal ion resulting from the reaction of water with at least one of the cementitious material and the CO.sub.2-sequestering chemicals (a being equal to 1 or 2), and the reaction products include at least one of alkaline earth metal carbonate particles and alkali metal carbonate particles (x being equal to 1 or 2). The carbonate particles form and take place mostly in the pores of the forming concrete which densifies the concrete microstructure and can further possibly enhance certain properties of the concrete including strength development.

[0110] Nanosized alkaline earth metal carbonate particles and alkali metal carbonate particles are therefore produced from carbonation of at least one of cement components, cement hydration products, and CO.sub.2-sequestering chemicals. Presence of carbon-based compounds in the CO.sub.2-containing water before contact with the concrete mixture, which optionally include CO.sub.2-sequestering chemicals, enables to enhance distribution of the nanosized alkaline earth metal carbonate particles and alkali metal carbonate particles within the forming concrete microstructure, thereby enhancing specific concrete properties such as strength, permeability, durability as the pores are filled with distributed nanosized material. The techniques described herein may enable to obtain an even distribution of nanosized carbonate compounds within the concrete network microstructure.

[0111] In some implementations, the concrete mixture can include admixtures.

[0112] The admixtures can be added as part of the concrete mixture to be mixed with the CO.sub.2-containing water to modify physico-chemical properties of the resulting wet concrete mixture including workability, hardening time and durability. The admixtures comprise at least one of a water reducer, an air entrainment agent, a plasticizer, a superplasticizer, an accelerator, a viscosity modifier, a waterproofing agent, and a retarder.

[0113] It should be noted that the concrete mixture may include one or more admixture(s) including an accelerating admixture which accelerates the setting and early strength development of concrete; a retarding admixture retards the setting of concrete; a water-reducing admixture which reduces the quantity of mixing water required to produce concrete of a given consistency; a water-reducing admixture—high range which reduces the quantity of mixing water required to produce concrete of a given consistency by 12% or greater; a water-reducing and accelerating admixture which reduces the quantity of mixing water required to produce concrete of a given consistency and accelerates the setting and early strength development of concrete; a water-reducing and retarding admixture which reduces the quantity of mixing water required to produce concrete of a given consistency and retards the setting of concrete; a water-reducing—high range—retarding admixture which reduces the quantity of mixing water required to produce concrete of a given consistency by 12% or greater and retards the setting of concrete; an air-entraining admixture which is added to the batch immediately before or during its mixing, for the purpose of entraining air; a specific performance admixture which provides a desired performance characteristic(s) other than reducing water content, or changing the time of setting of concrete, or both, without any adverse effects on fresh, hardened and durability properties of concrete as specified herein, excluding admixtures that are used primarily in the manufacture of dry-cast concrete products.

[0114] One skilled in the art will readily know how to select an amount of admixtures to be added as part of the concrete mixture for further mixing thereof with the CO.sub.2-containing water, according to the desired effects to be conferred to the resulting freshly mixed concrete as above-detailed.

[0115] In some implementations, the concrete mixture may further include at least one of mineral fillers, fibers, and colour pigments.

Method and Process Implementations

[0116] Various implementations of a method for sequestering CO.sub.2 in concrete and a process for the preparation of a carbonated concrete material derive from the preparation of the above-mentioned compositions.

[0117] In a first aspect, there is provided a general method of sequestering CO.sub.2 in hardening concrete. Hardening concrete refers a concrete in which chemical equilibrium is not reached yet and still undergoes hydration reactions with H.sub.2O and carbonation reactions with carbon-based compounds deriving from CO.sub.2.

[0118] The method can include providing a CO.sub.2-containing water comprising at least one of carbonic acid, carbonate ions, bicarbonate ions, nanosized alkaline earth metal carbonate particles and nanosized alkali metal carbonate particles. The CO.sub.2-containing water can be prepared by dissolving CO.sub.2 into water. Once an initial amount of CO.sub.2 is provided into the water, the CO.sub.2 being converted into at least one carbonic acid, carbonate ions, bicarbonate ions, nanosized alkaline earth metal carbonate particles and nanosized alkali metal carbonate particles by hydration reaction and potential additional chemical reactions. The compounds that are present in the CO.sub.2-containing water and deriving from the presence of CO.sub.2 in water can be referred to as carbon-based compounds. The amount of CO.sub.2 to be provided in the water depends on the amount desired to be present in the water as carbon-base compounds and the amount to be sequestered into the hardening concrete under the form of carbonate particles and/or CO.sub.2. For example, the CO.sub.2 may be present at a concentration between 2 g and 100 g per liter of water. One skilled in the art can readily understand that preparation of the CO.sub.2-containing water may be handled upstream of the main concrete production. For example, providing recycled process water such as process water from concrete production including carbonate particles is to be considered as providing CO.sub.2-containing water according to the present methods.

[0119] The method further includes preparing the concrete mixture as defined herein by mixing the cementitious material and the aggregates. Preparing the concrete mixture may further include mixing additional concrete ingredients such as admixtures, CO.sub.2-sequestering chemicals, mineral fillers, fibers, and colour pigments.

[0120] The method further includes mixing the CO.sub.2-containing water with the concrete mixture to induce hydration reaction of the cementitious material with water and carbonation reaction of the cementitious material (and additional components such as CO.sub.2-sequestering chemicals if any) with carbon-based compounds. This step allows for the production of the ready-mix composition as defined herein, thereby sequestering the initial amount of carbon dioxide into the hardening concrete. One skilled in the art will readily understand that the term “sequester”, and its synonyms that can be used herein such as “capture”, means that the CO.sub.2, that can be present in the water under the form of carbon-based compounds, is converted into secondary carbonate components (carbonate particles, bicarbonate ions, carbonate ions) that are part of the concrete network or as gas bubbles entrapped in the formed concrete.

[0121] In some implementations, preparing the concrete mixture may further include mixing at least one CO.sub.2-sequestering chemical with the remaining ingredients of the concrete mixture. The at least one CO.sub.2-sequestering chemical can be selected to mitigate the carbonation reaction of the cementitious material by releasing another carbonatable species, which will be carbonated instead of the cementitious material in presence of the CO.sub.2-containing water according to equation (12). The resulting carbonate compounds are more stable and possesses lower water solubility than cement hydration products. It should be noted that depending on the ingredients included in the concrete mixture, additional reactions may occur when the CO.sub.2-containing water is mixed with the concrete mixture. For example, other carbonation reactions may involve the CO.sub.2-sequestering chemicals instead of the cementitious material, thereby rendering an increased quantity of cementitious material available for hydration reactions instead of carbonation reactions. The method may include selecting operational conditions favoring hydration and/or carbonation reactions of specific compounds among the concrete mixture ingredients.

[0122] In another aspect, referring to FIGS. 1 to 3, there are provided general process 100 steps to prepare a carbonated concrete material. Implementations of such process steps include a wet batch process 200 wherein a ready-mixed composition as defined herein is prepared in advance and transported for delivery on work site; and a dry batch process 300 wherein the concrete mixture is mixed with the CO.sub.2-containing water directly on the work site.

[0123] Referring to FIG. 1, the process includes providing the ingredients of the pre-mix composition 2, including the concrete mixture 4 and the CO.sub.2-containing water 6. The process then includes mixing 101 the concrete mixture 4 with the CO.sub.2-containing water 6 to produce the wet concrete mixture 8. The concrete mixture 4 includes aggregates 14, cementitious material 16, and optionally at least one of admixtures 18 and CO.sub.2-sequestering chemical 20. This mixing step 101 is performed in mixing conditions allowing workability of the wet concrete mixture 8 (ready-mixed composition 8 in FIG. 1). The mixing conditions comprise at least one of a mixing temperature, a mixing time, a water to cementitious material weight ratio, an admixture to cementitious material weight ratio, and any other characteristic that is controllable to ensure that the wet concrete mixture is workable.

[0124] The workability of the resulting mixture is important as this mixture will undergo at least one of batching, transporting, mixing, pumping, pouring, discharging, depositing, molding, forming/shaping, compacting, finishing thereof and any other operation needed to provide concrete at the desired location and into the desired shape. All these potential operations that can be performed on the wet concrete mixture before the concrete is ready to be cured are referred to as conditioning operations. Still referring to FIG. 1, the process therefore further includes conditioning 110 the wet concrete mixture 8 to produce a ready-to-cure concrete 10. The ready-to-cure concrete 10 is a conditioned concrete which is finished and formed according to the needs of the work site (foundation, wall, building material, object, surface treatment, etc.). The ready-to-cure concrete 10 is however not fully hardened into carbonated concrete.

[0125] Still referring to FIG. 1, the process further includes allowing the ready-to-cure concrete 10 to harden in curing conditions and produce the carbonated concrete material 12 upon hydration and/or carbonation of the cementitious material. It should be understood that depending on the ingredients of the concrete mixture 4, carbonatable compounds may also derive from additional ingredients including CO.sub.2-sequestering chemicals. For example, the process may further include mixing at least one CO.sub.2-sequestering chemical 20 to produce the wet concrete mixture 8.

[0126] The amount of CO.sub.2 which is captured in the wet concrete mixture is increased by such addition of at least one CO.sub.2-sequestering chemical. Indeed, the CO.sub.2-sequestering chemical releases carbonatable compounds which are ready to be carbonated by the carbon-based compound from the CO.sub.2-containing water, and thereby produce nanosized carbonate particles. Hydration and hardening of the cementitious material is advantageously favored as carbon-based compounds from the CO.sub.2-containing water react with the carbonatable compounds from the CO.sub.2-sequestering chemical instead of from the cementitious material.

[0127] Curing conditions may be selected and controlled to favor hydration and hardening of the ready-to-cure concrete, while mitigating carbonation of the cementitious material. Curing conditions include curing time, curing temperature and CO.sub.2-sequestering chemical to cementitious material weight ratio. Optionally, the curing time may range between about 24 hours and about 7 days. Further optionally, the curing temperature may range between about −40° C. and about 55° C. Further optionally, the CO.sub.2-sequestering chemical to cementitious material weight ratio is between about 0.5% and about 10%.

[0128] In some implementations, the process may include the preparation of the CO.sub.2-containing water. Such preparation can be performed by injecting gaseous CO.sub.2, by blending liquid CO.sub.2, by dissolving solid CO.sub.2 or carbon-based compounds derived from CO.sub.2. Alternatively, the process may include providing a ready-to-use CO.sub.2-containing water, such as a process water that includes carbon-based compounds derived from CO.sub.2. An implementation wherein gaseous CO.sub.2 is injected into water before mixing with the concrete mixture is illustrated in FIGS. 6 and 7, and detailed in the experimentation section.

[0129] In some implementations, such as in a dry batch process 300 illustrated in FIG. 3, the process may include providing the CO.sub.2-containing water 6 and the concrete mixture 4 in batched amounts to form a pre-mix composition 2 as defined herein, before mixing 101 thereof to produce the wet concrete mixture 8 and then the ready-to-cure concrete 10. The batched amounts of concrete ingredients are selected according to an amount or volume of carbonated concrete material 12 to be prepared.

[0130] In other implementations, such as in a wet batch process 200 illustrated in FIG. 2, the conditioning 110 of the wet concrete mixture 8 may include batching of the wet concrete mixture 8 in a batched amount to form a ready-mix composition as defined herein. The batched amount of ready-mix composition is selected according to an amount or volume of carbonated concrete material to be prepared. The conditioning 110 further includes transporting and discharging the batched amount of ready-mix composition 8 to produce the ready-to-cure concrete 10 on the work site.

[0131] In one embodiment, there is provided a dry batch process for sequestering CO.sub.2 in concrete material. A dry batch process makes use of the pre-mix composition as defined herein such that the concrete mixture is contacted with the CO.sub.2-containing water directly on the work site. This process avoids handling a ready-mixed composition that has started to chemically react before reaching the work site where concrete is needed.

[0132] More particularly, the dry batch process includes feeding the pre-mix composition, including the concrete mixture and the CO.sub.2-containing water in an unmixed state, to a mixing device which is located on the work site. It should be understood that an unmixed state includes the feeding of the concrete mixture independently from the feeding of the CO.sub.2-containing water to the mixing device. As mentioned above, the concrete mixture includes at least a cementitious material and aggregates, and may further includes admixtures, CO.sub.2-sequestering chemicals, mineral fillers, fibers and/or coloring pigments.

[0133] In the implementation illustrated in FIG. 3, the process includes transporting 80, via a transit truck and delivering to the work site, batched amounts of concrete mixture 4 and CO.sub.2-containing water 6 in an unmixed state, as a pre-mix composition 2. The transit truck may be a ready-mixed truck or any vehicles able to transport batched amounts of ingredients in an unmixed state. Batching of the ingredients in advance can be performed via a ready-mixed batching system (not illustrated in FIG. 3). The dry batch process 300 further includes mixing 101 the batched concrete mixture 4 with the CO.sub.2-containing water 6 in a mixing device on site to produce the wet concrete mixture as defined herein. One skilled in the art will readily understand that the mixing device refers to a ready-mixed truck or a stationary mixer that is provided on the work site. One skilled in the art will know that a typical ready-mixed truck has an on-route mixing functionality to mix the concrete and maintain concrete in workable conditions.

[0134] It should be noted that depending on the ingredients of the concrete mixture, independent feeding lines may have to be used to provide ingredients of the concrete mixture into the ready-mixed drum of the truck or the stationary mixer. For example, depending on the compatibility of the admixtures with the water and/or cementitious material, admixtures may have to be fed independently from water and other concrete mixture ingredients.

[0135] In another embodiment, there is provided a wet batch process for sequestering CO.sub.2 in concrete material. A wet batch process makes use of a ready-mixed composition as defined herein which is batched and tailored to the needed properties of the concrete. The wet batch process avoids handling mixing concrete ingredients on site to prepare the ready-mixed composition.

[0136] In the implementation illustrated in FIG. 2, the wet batch process 200 includes mixing 101 the concrete mixture 4 with the CO.sub.2-containing water 6 in a mixing device of a central plant to produce the ready-mixed composition 8 as defined herein.

[0137] As mentioned above, the concrete mixture includes at least a cementitious material and aggregates, and may further includes admixtures, CO.sub.2-sequestering chemicals, mineral fillers, fibers and/or coloring pigments. One skilled in the art will readily understand that such mixing may be performed in a stationary mixer resulting in a ready-mixed composition that is batched to be transported onto worksite in a ready-mixed truck; or performed directly in a ready-mixed truck that receives batched amounts of the concrete mixture and CO.sub.2-containing water.

[0138] Still referring to FIG. 2, the wet batch process 300 further includes transporting 110 the ready-mixed composition in a transit truck unto the work site, the ready-mixed composition being transported under the mixing conditions maintaining workability thereof. One skilled in the art will readily understand that the transit truck may be the ready-mixed truck which is mentioned above.

[0139] Both dry batch process and wet batch process further include conditioning the wet concrete mixture from the mixing device to produce a ready-to-cure concrete material, and curing the ready-to-cure concrete material to produce the carbonated concrete material containing carbon dioxide from the CO.sub.2-containing water and sequestered under the form of carbonate compounds.

[0140] In some implementations, the conditioning may include discharging the wet concrete mixture onto a deposit zone of the work site. The discharging may be performed via at least one of a pump, a conveyor belt, a chute, an elephant trunk, and a discharge pipe.

[0141] In some implementations, the conditioning may further include pumping, depositing, molding, forming/shaping, compacting and/or finishing depending on the desired concrete end product.

System Implementations

[0142] Depending on embodiments of the process described herein, various embodiments of a system for the production of a wet carbonated concrete mixture may be encompassed as follows.

[0143] In one embodiment, there is provided a system for the production of the wet carbonated concrete mixture as defined herein. The system includes: [0144] a water feed tank providing a flow of water via an outlet thereof; [0145] a mixer receiving the concrete mixture; [0146] a feed line ensuring fluid communication between the outlet of the water feed tank and an inlet of the mixer; and [0147] a CO.sub.2-injector mounted on the feed line upstream of the mixer for injecting carbon dioxide into the flow of water to produce a flow of CO.sub.2-containing water;
wherein the flow of CO.sub.2-containing water is fed to the inlet of the mixer via the feed line for mixing thereof with the concrete mixture and production of a wet carbonated concrete mixture.

[0148] In another embodiment, there is provided a ready-mixed truck for the production of the ready-mixed composition as defined herein. The ready-mixed truck includes: [0149] a mixing drum receiving the concrete mixture; [0150] a water feed line in fluid communication with the mixing drum of the ready-mixed truck; and [0151] a CO.sub.2-injector mounted on the feed line upstream of the ready-mixed truck for injecting carbon dioxide into water flowing via the water feed line and produce a flow of CO.sub.2-containing water;
wherein the flow of CO.sub.2-containing water is fed to the ready-mixed truck via the water feed line for mixing thereof with the concrete mixture and production of the ready-mixed composition.

[0152] FIG. 5 illustrates general implementation of a system 500 including an assembly for producing the CO.sub.2-containing water. The preparation of the CO.sub.2-containing water 6 involves the use of a CO.sub.2 injector 24 to inject CO.sub.2 from a CO.sub.2 tank 23 via a first feed line 25 which may be provided in fluid communication with a water feeding line 22, the water feed line 22 joining a water feed tank 26 to mixing device 28, as seen on FIG. 5. The system 500 can further include a controller or batching computer 27 simultaneously controlling the amounts of the ingredients (14, 16, 18, 20) of the concrete mixture, water 22 and CO.sub.2 25. It should be noted that the batching computer may control other parameters such as the CO.sub.2 injection speed via the injector 24 or the water flow rate via a flow meter (not illustrated) operatively connected to the water feed tank 26.

[0153] The CO.sub.2 injection pressure can be made compatible with the water injecting pressure to ensure CO.sub.2 is efficiently blended with water and pumped into the mixing drum. For example, the CO.sub.2 injection pressure may be maintained higher than the water pressure.

[0154] Alternatively, as seen on FIG. 4, another system 400 including a CO.sub.2-containing water feed tank 30 may be used to feed the mixing device 28 via water feed line 22, the CO.sub.2-containing water being prepared in advance. The CO.sub.2-containing water tank 30 may be configured such that the CO.sub.2-containing water 6 is stored under a minimized temperature in order to maximize the solubility of CO.sub.2 in water and blending efficiency.

[0155] Various devices, machinery and operational conditions (temperature, pressure, amounts, etc.) may be used to perform preparation of the CO.sub.2-containing water depending on the source of water, the physical form of the CO.sub.2 and the ambient conditions of the site where the CO.sub.2-containing water is prepared. For example, a CO.sub.2 injector coupled with a device such as a loop reactor may be used to provide the desired CO.sub.2 concentration in a flow of circulating water.

[0156] As seen in the experimental set up illustrated in FIGS. 6 and 7, the system 510 can include preparing the CO.sub.2-containing water 6 before mixing with the concrete mixture (only a mixing drum 28 is illustrated in FIGS. 6 and 7). Water can be stored and fed from a water feed tank 30 and CO.sub.2 may be stored and fed from a CO.sub.2 tank 36. One or two CO.sub.2 tank(s) 36 can be provided to adjust the time needed to reach a desired concentration of CO.sub.2 in water to produce the CO.sub.2-containing water 6. A flowmeter 38 can be operatively connected to the CO.sub.2 tank 36 to control the CO.sub.2 injection flow rate of CO.sub.2 in water. Optionally, the CO.sub.2 injection flow rate can be between 40 L of CO.sub.2 per min to 1000 L/min, optionally between 100 L/min to 600 L/min in gas phase, or 1 L/min to 40 L/min, optionally between 5 L/min to 20 L/min in liquid phase.

[0157] The method can further include adjusting a water flow rate to the CO.sub.2 injection speed to ensure that the required amount of CO.sub.2 is injected before all the batched water is mixed with the concrete mixture. As the CO.sub.2 can be injected at the same time as the water starts flowing, mixing time and exposure time of the concrete mixture to the flowing CO.sub.2-containing water can vary and influence the chemical reactions leading to the formation of the carbonated concrete. The water flow rate can range between 10 L/minute to 1000 L/minute, preferably from 100 L/minute to 700 L/minute to cover various ranges of desired CO.sub.2 concentration in the CO.sub.2-containing water, in accordance with a given CO.sub.2 injection speed and physical state of the injected CO.sub.2 (liquid, gas, etc.).

[0158] As the process may include independently feeding the concrete ingredients to the mixing device, at least two feed lines are in fluid communication with said mixing device to provide the concrete mixture and the CO.sub.2-containing water respectively.

[0159] For example, referring to FIG. 4, the system 400 includes a concrete mixture feed tank 32 including batched amount of each and everyone of the ingredients for the concrete mixture 4 (aggregates 14, cementitious material 16, optional admixtures 18 and/or optional CO.sub.2-sequestering chemicals 20) which are fed to the mixing device 28 via a dedicated feed line 34 (e.g. conveyor). In another example, referring to FIG. 5, the system 500 may include independent feed lines (A to D) to feed each ingredient of the concrete mixture to the mixing device 28 independently. In other implementations, an admixture feed line in fluid communication with the mixing drum of a ready-mixed truck, which is independent from the water feed line, may be provided.

[0160] It should be understood that any one of the above-mentioned implementations or embodiments of each process, method, composition and system may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. For example, the various method steps and/or structural elements of the system described herein-above, herein-below and/or in the appended Figures, may be combined with any of the general process appearing herein-above, herein-below and/or in the appended Figures.

Experimentation Results

[0161] Several experiments have been performed according to a trial plan, according to which different wet concrete mixtures samples (referred to as mixes) have been prepared and cured, to evaluate the proposed methodologies and systems to sequester CO.sub.2 into concrete by measuring the strength development of produced concrete cylinders. An experimental system was configured where one or two CO.sub.2 tanks can be attached to a water container while the water flows out through an exit valve and is timed to control a flow rate thereof, as illustrated in FIGS. 6 and 7.

Standards

[0162] The following standards have been respected during experimentation and will be referred to further below. [0163] CSA-A23.2-2C: Making Concrete Mixes in the Laboratory [0164] CSA-A23.2-3C: Making and Curing Concrete Compression and Flexural Test Specimens [0165] CSA-A23.2-4C: Air content of Plastic Concrete by the Pressure Method [0166] CSA-A23.2-5C: Slump of Concrete [0167] CSA-A23.2-9C: Compressive Strength of Cylindrical Concrete Specimens [0168] CSA-A23.2-17C: Temperature of Freshly Mixed Hydraulic Cement Concrete [0169] CSA-A23.2-23C: Electrical Indication of Concrete's ability to Resist Chloride Ion Penetration [0170] C403M-16: Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance

Materials

[0171] The following materials were used for the purpose of experimentation:

Cement:

[0172] Portland Cement General Use (Type GU): Type GU (Type 1) is an all-purpose general use Portland cement that is commonly used in Canada and meets the CSA-A3001-13 specifications.

CO.SUB.2.-Sequestering Chemicals:

[0173] CRH Slag: CRH Slag cement is a ground granulated blast furnace slag (GGBFS) which is a by-product of the steel manufacturing industry. It is known to densify the concrete matrix and enhance strength as it forms more calcium silicate hydrate which is the paste needed to hold concrete together. The CRH Slag meets the CSA-A3001-13 requirements. [0174] Steel Slag: Steel slag is also a by-product of steel manufacturing but unlike CRH Slag, this slag is produced in a Basic Oxygen Furnace (BOF) or an Electric Arc Furnace (EAF). The hot iron is treated with lime and reacts with silicates and oxides of magnesium, manganese, and aluminum to form steel furnace slag. Once cooled and processed, the steel slag can be sized into desired products. Table 1 gives some of the wide range of chemical composition of steel slag:

TABLE-US-00001 TABLE 1 Chemical Composition of Steel Slag Comp. CaO Al.sub.2O.sub.3 FeO MgO SiO.sub.2 SO.sub.3 MnO TiO.sub.2 P.sub.2O.sub.5 Fe.sub.2O.sub.3 Range (%) 30-60 1-12 10-35 1-15 10-20 0.1-0.6 0.3-8 0.4-2 0-1.5 10-38 [0175] Lime: Hydrated lime that is normally used for curing concrete in tanks is also used as a raw material in some of the concrete mixes.

Aggregates:

[0176] Limestone: 19 mm limestone is used as coarse aggregate in this study and meets the OPSS 1001 material specification for aggregates. [0177] Sand: The concrete sand used throughout this study meets the OPSS 1001 requirements for aggregates.

Admixture:

[0178] Water Reducer: Eucon WR is used as a water reducer throughout this study which is a solution of lignosulfonate. It creates a plastic and cohesive mix that gives durability, reduced shrinkage, and less permeability in concrete. The water reducer meets the C494/C494M standard specification for chemical admixtures for concrete.
Carbon dioxide: Food grade certified CO.sub.2 is used.

General Procedure

[0179] Based on the mix design and trial plan, specific amounts of cement, slag, sand, and aggregates were batched. Lime was also batched according to the trial plan for certain mixes. Some mixes also included steel slag instead of regular CRH slag.

[0180] The proportioning of the raw materials per cubic meter of concrete consists of the following:

TABLE-US-00002 % of Total ml/100 kg Cement Kg/m.sup.3 of Cement by Weight CRH Water GU Slag Sand Limestone Water CO.sub.2 Reducer Lime 234 41 922 1070 165 Varies 250 Varies

[0181] The moisture content of the sand was measured, and the weight of the batched sand was adjusted by adding or removing sand from the concrete mix drum based on the measured moisture content. Water reducer was also batched based on the trial plan and the batched materials are placed in a concrete mixer drum. The water amount was determined by the moisture content and carbonated water was prepared via the one-tank or two-tank systems that will be described in the following sections. Once all the materials were placed in the concrete mixer drum, the mixing process is started according to CSA-A23-2C standard, while the mixing and resting time were adjusted based on trial plan.

Traditional Flake Injection (Comparative)

[0182] The first stage of the study consisted of directly adding CO.sub.2 flakes into the concrete ingredients and water. After the cement, sand, aggregate, water, and admixture is added into the mixer drum and started, CO.sub.2 flakes are then added so that they are directly in contact with the concrete. The mixer is run until the standard mixing time is completed. Cylinders are casted, cured, and test as per CSA-A23.2-9C.

One-Tank System

[0183] Referring to FIG. 6, this system 510 involves a custom made 10-L water tank 30 with an exit valve 42 attached to a T-shaped ball valve connector. The water tank has an opening on top which is used to fill the required amount of water (not illustrated in FIG. 6). A CO.sub.2 hose is inserted in one of the T-valve connectors (becoming CO.sub.2 valve 40) and the top of the water tank is closed. The CO.sub.2 flow rate is set to a constant rate of 50 ft.sup.3/hr. The CO.sub.2 valve 40 is open and the water exit valve 42 is opened once the CO.sub.2 tank 36 is stable at the set flow rate. The time is calculated when the water starts dropping into the mixture. The CO.sub.2 should be exposed to the water for an exposition time required to achieve the desired CO.sub.2 concentration level in the CO.sub.2-containing water. The CO.sub.2-containing water trickles down via the pressure exerted by the incoming CO.sub.2 gas through the hose, and falls into the mixer. Once the exposition time is over, the CO.sub.2 valve 40 is closed along with the exit water valve 42. The amount of water remaining in the water tank 30 is then poured down in the concrete mixture directly from the top opening of the water tank 30 into the mixing drum 28. The mixing is performed for a total of 10 minutes initially, followed by a 2-minute rest, and 3 minutes of final mixing.

[0184] Cylinders of carbonated concrete are casted and cured from the ready-to-cure mixture produced via such mixing, and strength and plastic performance of the carbonated concrete is tested as per CSA-A23.2-9C.

[0185] The flow rate of CO.sub.2-containing water fed to the mixing drum can therefore vary according to the injection rate of CO.sub.2 within water. For example, if the desired CO.sub.2 concentration is reached slowly, the flow rate of CO.sub.2-containing water fed to the mixing drum will also be slow, thereby forcing the mixing time to be elongated such that the needed quantity of CO.sub.2-containing water be mixed with the concrete ingredients from the mixing drum.

Two-Tank System

[0186] This two-tank system 512 uses the same custom made 10-L water tank as for the one-tank system 510. The only difference is that two CO.sub.2 tanks 36 are connected in the T-shaped ball valve connector. The same procedure is followed as per the one-tank system 510 and the CO.sub.2 injection flow rate from the second CO.sub.2 tank 36 is also set at 50 ft.sup.3/hr. The system 512 enables that the exposition time required to achieve the desired CO.sub.2 concentration in the CO.sub.2-containing water 6 decreases by half as much as it was with the one-tank system 510.

Experimental Results

One-Tank System

[0187] Production of different wet concrete mixtures were conducted using the tank system 510 illustrated in FIG. 6 to regulate the CO.sub.2 gas so that leakage does not occur and CO.sub.2-containing water 6 goes directly into the mixing drum 28. Five mixes were produced via mixing of batched concrete mixture with CO.sub.2-containing water having 0 (reference), 20, 40, 60, and 80 g of CO.sub.2 per liter of water respectively (2.4, 4.8, 7.2, and 9.6 kg/m.sup.3 of concrete).

[0188] Table 2 and FIGS. 8 to 10 summarize the results of the CSA-A23.2-9C test.

[0189] Referring to FIG. 8, 1-day compressive strength for the reference mix was higher than for all other mixes. Upon increasing the CO.sub.2 level in the CO.sub.2-containing water during preparation of the mix, the compressive strength of the resulting carbonated concrete cylinder slightly decreases and fluctuate around 8 MPa. Referring to FIGS. 9 and 10, by day 7 and 28, the strengths for mixes with 40 (4.8 kg/m.sup.3) and 60 g/L CO.sub.2 (7.2 kg/m.sup.3) were the highest and turned out to be the optimum dose in the experimental conditions. FIGS. 9 and 10 further show that even at the highest dose of 80 g/L (9.6 kg/m.sup.3), the compressive strength was comparable to the reference mix. This system showed that higher doses of CO.sub.2 could be sequestered as the strengths were comparable to the reference mix.

[0190] However, the one-tank system only allowed for slow carbonation of water to produce the CO.sub.2-containing water since the flow rate of CO.sub.2 could not exceed 50 ft.sup.3/hr per tank. Such flow rate for CO.sub.2 injection resulted in an increased mixing time as the CO.sub.2-containing water slowly dripped in the mixing drum to allow the desired carbonation level to be reached. Injection flow rate of CO.sub.2 can therefore be a challenge, especially in plant operation conditions as the concrete mixture are batched quickly in comparison to the time required for the water to reach the desired carbonation level.

TABLE-US-00003 TABLE 2 Average Strengths for Concrete Mixes with One-Tank System One Tank System CO2 in Water CO2 (kg/m.sup.3) in Avg. 1-Day Strength Avg. 7-Day Strength Avg. 28-Day (g/L) Concrete (MPa) (MPa) Strength 0 0 9.68 26.22 34.48 20 2.4 7.87 24.90 32.90 40 4.8 8.31 26.85 37.06 60 7.2 7.94 26.58 36.52 80 9.6 8.53 25.80 33.65

Two-Tank System

[0191] The same five mixes were prepared by using the two-tank system as illustrated in FIG. 7. Another CO.sub.2 tank 36 was connected to a second inlet valve 44 and set to 50 ft.sup.3/hr of injected CO.sub.2. The total CO.sub.2 injection rate was therefore doubled with respect to the one-tank system by reaching 100 ft.sup.3/hr. Table 3 and FIGS. 11 to 13 summarize the results of the CSA-A23.2-9C test with respect to concrete cylinders prepared from said mixes.

[0192] Referring to FIGS. 11 and 12, a decreasing trend in compressive strength is seen as the CO.sub.2 concentration is increased for 1-day and 7-day strength. For the 28-day strength shown in FIG. 13, the strength was similar for 40, 60, and 80 g/L CO.sub.2(4.8, 7.2, and 9.6 kg/m.sup.3) while the highest strength was observed for 20 g/L (2.4 kg/m.sup.3) CO.sub.2. Therefore, at an increased injection rate, the optimal dose has decreased to 20 g/L (in comparison with FIG. 10), which gives about 5% increase in strength relative to reference mix after 28-days. Upon injecting more CO.sub.2, the 28-day strength decreased as seen on FIG. 13. As seen on FIG. 11, the highest 1-day strength was achieved with the reference mix while the lowest strength of 5.88 MPa was achieved at 80 g/L (9.6 kg/m.sup.3) CO.sub.2.

TABLE-US-00004 TABLE 3 Average Strengths for Concrete Mixes with Two-Tank System Two Tank System CO.sub.2 (kg/m.sup.3) in Avg. 1-Day Avg. 7-Day Strength Avg. 28-Day Strength CO.sub.2 (g/L) Concrete Strength (MPa) (MPa) 0 0 9.68 26.22 34.48 20 2.4 7.65 25.84 36.21 40 4.8 6.79 24.56 32.37 60 7.2 6.04 23.55 31.20 80 9.6 5.88 22.26 32.54

[0193] Two tanks can advantageously be used to inject higher concentrations of CO.sub.2 in water in lesser time (than for the one tank system) to achieve more carbonation and to ensure the process replicates in plant since plant operations require quick batching of materials. However, the results indicate that increasing the injection speed can decrease the overall strength with increasing CO.sub.2 concentration. In the experimental set-up, when the concentration of CO.sub.2 is reached quicker via the use of two tanks, the water flow rate can be increased, which leads to mixing the concrete ingredients with a same amount of CO.sub.2-containing water in a lesser time (in comparison to the one-tank system). The 7-day and 28-day strengths at 20 g/L CO.sub.2 (2.4 kg/m.sup.3) was the mix that obtained higher strength than the reference mix which indicates that lower concentrations of CO.sub.2 might be suitable in this process. Therefore, at lower speeds, higher doses yield higher strengths while faster injection results in lower optimal dosage. Hence, it is necessary to maintain a balance between injection speeds, CO.sub.2 dosage and water flow rate.

Lime Addition and Two-Tank System

[0194] Referring to Table 4 and FIGS. 14 to 16, 1%, 2%, 3% of lime (by weight of total cement) was added to another set of newly prepared mixes to evaluate whether chemical reaction between carbonated water and lime would impact the compressive strength of the resulting concrete. Referring to Table 4 and FIG. 14, the 1-day strength for the mix with 1% lime with 20 g/L CO.sub.2 (2.4 kg/m.sup.3) concentration was higher than for the mixes with 2% and 3% lime, but significantly decreased with increasing CO.sub.2 content. Referring to FIGS. 14 and 15, concrete mix with 2% lime at 1-day showed an increase in strength at 80 g/L (9.6 kg/m.sup.3) CO.sub.2 compared to the strength at 20 g/L, but at 7-days the strength at 80 g/L was much lower than that at 20 g/L. Referring to FIG. 16, at 28-day strength, it was interesting to note that at 3% lime the strengths at higher CO.sub.2 dosage were much lower than those at 1% and 2% lime content.

[0195] The findings suggest that the speed of injection can have a negative impact on the strength development of the resulting concrete cylinders, which can be counterbalanced with the use of CO.sub.2-sequestering chemicals. Compared to both the reference mix and the mixes without lime at 60 g/L and 80 g/L, the results show that the strength is slightly higher with 2% lime at high doses of CO.sub.2. The results indicate that there is a slightly positive impact at 2% lime, suggesting that the objective of sequestering more CO.sub.2 can be achieved at low lime content while increasing the lime to 3% results in decreasing the strength. Overall, the reaction of lime in the mix was sensitive when combined with CO.sub.2 water.

TABLE-US-00005 TABLE 4 Average Strengths for Concrete Mixes with Varying Lime Content using Two-Tank System Two-Tank System CO.sub.2 (kg/m.sup.3) in Avg. 1-Day Strength Avg. 7-Day Avg. 28-Day Lime (%) CO.sub.2 (g/L) Concrete (MPa) Strength (MPa) Strength (MPa) 1 20 2.4 7.20 25.67 35.09 60 7.2 5.31 22.04 31.90 80 9.6 4.69 23.97 33.82 2 20 2.4 5.79 25.00 34.32 60 7.2 5.52 26.22 35.10 80 9.6 6.44 22.81 31.52 3 20 2.4 5.43 24.18 34.46 60 7.2 7.85 23.16 30.26 80 9.6 7.15 20.77 26.81

Steel Slag Addition and Two-Tank System

[0196] To test if low reactive lime would impact the compressive strength of concrete mixes with carbonated water, steel slag was used as a replacement of CRH slag in some of the mixes. Steel slag primarily consists of limestone, silica, alumina, magnesium oxide, and some sulphur. Concrete mixes were produced using the two-tank system with a fixed CO.sub.2 content of 60 g/L (7.2 kg/m.sup.3). Table 5 highlight the mixes that were produced.

TABLE-US-00006 TABLE 5 At 60 g/L CO.sub.2 (7.2 kg/m.sup.3) Mix No. GU (%) CRH Slag (%) Steel Slag (%) 1 100 0 0 2 85 15 0 3 95 0 5 4 90 0 10

[0197] Referring to Table 6 and FIGS. 17 to 19, the compressive strength results indicate that the mix containing 5% steel slag achieved the highest strength. Concrete mix consisting of purely GU cement had the lowest strength while the strength of the mix containing 10% steel slag was comparable to the one containing 5% at 7-days and 28-days. At 1-day, the strength of the 5% mix was considerably higher. Compared to the reference mix with 15% CRH slag, the 28-day strength is also slightly higher with steel slag. These results conclude that the use of steel slag can improve early strength in certain conditions and that an optimum dose can be found (for example 5% in the experimental conditions).

TABLE-US-00007 TABLE 6 Average Strengths for Concrete Mixes with Different Steel Slag Content At 60 g/L CO.sub.2 (7.2 kg/m.sup.3) Steel Slag Avg. 1-Day Strength Avg. 7-Day Strength (%) CRH Slag (MPa) (MPa) Avg. 28-Day Strength (MPa) 0 0 7.82 22.18 28.70 0 15 6.04 23.55 31.20 5 0 12.15 27.42 32.85 10 0 10.87 27.07 32.36

Traditional Flakes Injection

[0198] Table 7 and FIGS. 20, 21, and 22 summarize the results of the mixes conducted by mixing CO.sub.2 flakes to a wet concrete mixture.

TABLE-US-00008 TABLE 7 Average Strengths for Concrete Mixes with CO.sub.2 Flakes CO.sub.2 Flakes in Concrete CO.sub.2 (kg/m.sup.3) Avg. 1-Day Avg. 7-Day Strength Avg. 28-Day in Concrete Strength (MPa) (MPa) Strength (MPa) 0 9.68 26.22 34.48 0.5 5.84 22.05 33.12 1.5 5.71 22.57 33.54 2.5 3.83 19.66 29.73 4.0 3.58 20.08 29.97

[0199] The results show that the addition of CO.sub.2 flakes directly on top of the concrete mix significantly decreased the concrete compressive strength. For 1-day results, the reference mix had the highest strength of 7.62 MPa while it went down to 3.58 MPa at a CO.sub.2 level of 4 kg/m.sup.3. The decreasing trend continuing at 28-days as well where the reference mix had strength of 37.93 MPa while the mix with 4 kg/m.sup.3 CO.sub.2 had a strength of 29.97 MPa. The mixes with 0.5 and 1.5 kg/m.sup.3 CO.sub.2 had relatively similar strengths.

[0200] These results indicate at lower doses of CO.sub.2 and up to 1.5 kg/m.sup.3, the strength development at 28-days was similar to that of the reference mixes. Early strength data at 1-day and 7-days shows that lower doses also yield slow strength development. CO.sub.2 content higher than 1.5 kg/m.sup.3 results in lower strengths at both early and later stages. These results prove that adding CO.sub.2 flakes directly into the concrete surface can only be achieved at low dosages to avoid compromising the strength development while high doses will result in significant strength development reduction.

SUMMARY

[0201] The first phase of the study proved that the traditional way of adding CO.sub.2 flakes on top of concrete reduced the strength development at all ages, with the impact on strength was significant at CO.sub.2 doses higher than 1.5 kg/m.sup.3. The initial investigation using one tank of CO.sub.2 showed that 40 g/L and 60 g/L (4.8 and 7.2 kg/m.sup.3) were the optimum doses of CO.sub.2 where the strengths were higher than reference mix at 28-days. This system showed that even higher doses of CO.sub.2 could be sequestered as the strengths were comparable to the reference mix. However, the slow process of carbonation could not be replicated in plant due to challenging operations. This led to the use of another tank to speed up the process and increase carbonation. The results showed a decreasing trend of strength with increasing CO.sub.2 content, except at 20 g/L (2.4 kg/m.sup.3) where the 28-day strength was higher than the reference mix. This implies that this methodology could be utilized for lower doses of CO.sub.2. In all cases, 1-day strength decreased with the addition of CO.sub.2. Furthermore, the addition of lime showed its sensitivity in concrete with CO.sub.2. These mixes proved that lower lime percentage of 1% and 2% were optimal for high CO.sub.2 content of 60 g/L and 80 g/L (9.6 kg/m.sup.3) for later strength while it had less impact on early strength. Steel slag, on the other hand, had a significant impact on early strength while the 28-day strength also improved with an optimum dose of 5%. Therefore, steel slag addressed the early strength issues (1-day strength). The lab trials, therefore, deduce that CO.sub.2 sequestering can not only make a positive environmental impact but can also enhance strength when combined with steel slag or other sequestering chemicals such as lime that is compatible with carbonation.