Manufacturing method for carbonated precast concrete products with enhanced durability

Abstract

A method for producing a carbonated precast concrete product is provided, the method comprising the steps of obtaining a mixture including at least one binder material, aggregate, and water; forming the mixture into a shaped intermediate; demolding the shaped intermediate to obtain a demolded intermediate, the demolded intermediate having a first water-to-binder ratio; conditioning the demolded intermediate to provide a conditioned article having a second water-to-binder ratio less than the first water-to-binder ratio of the demolded intermediate; moistening at least one surface of the conditioned article with an aqueous medium, thereby increasing the weight of the conditioned article and providing a moistened product, the first portion of the moistened product having a third water-to-binder ratio greater than a fourth water-to-binder ratio of a remaining portion of the moistened product; and curing the moistened product with carbon dioxide to obtain the carbonated precast concrete product.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. patent application Ser. No. 63 / 034,037, filed Jun. 3, 2020, which is incorporated herein by reference in its entirety. [Technical Field]

[0002] The present disclosure relates generally to precast concrete products such as, but not limited to, concrete pipes, traffic barriers, walls, boxes, culverts, tiles, pavers, hollow core slabs, patio slabs, steps, curbs, retaining walls, concrete masonry units, etc. More particularly, the present disclosure relates to carbonated precast concrete products and methods for making same. [Background technology]

[0003] Precast concrete is becoming widely adopted in the construction industry. Precast concrete is a construction product that is cast into reusable molds or forms in a factory under controlled conditions. The precast concrete hardens in a controlled curing environment within the factory. After curing, it is transported to the building site.

[0004] Precast concrete has a wide range of structural and architectural applications, common examples of which include pavers, foundations, slabs, beams, floors, columns, walls, retaining walls, manholes, sewer pipes, blocks, modular boxes, and bridge decks.

[0005] Traditionally, Portland cement has been used as a binder in the production of precast concrete products. However, producing Portland cement is known to have undesirable environmental impacts. Producing one ton of Portland cement produces approximately one ton of CO2. Obtaining Portland cement requires quarrying, which releases air pollutants and requires the use of large kilns, which require significant amounts of energy.

[0006] Cementitious precast products rely on cement hydration to achieve sufficient strength. Heat and steam curing techniques have been widely adopted as standard curing methods for conventional cementitious precast concrete products to promote early strength development.

[0007] Curing precast concrete in a CO2-rich environment can reduce the environmental impact of the precast concrete. This is known as carbonated precast concrete. While such carbonated precast concrete has certain environmental advantages, there remains a need to improve other physical properties of carbonated precast concrete, such as its ability to better withstand freeze-thaw cycles and abrasion. Summary of the Invention [Means for solving the problem]

[0008] In one aspect, a method for producing a carbonated precast concrete product is provided, the method including: obtaining a mixture including at least one binder material, aggregate, and water; molding the mixture into a shaped intermediate; demolding the shaped intermediate to obtain a demolded intermediate, the demolded intermediate having a first water-to-binder ratio; conditioning the demolded intermediate to provide a conditioned article having a second water-to-binder ratio less than the first water-to-binder ratio of the demolded intermediate; moistening at least one surface of the conditioned article with an aqueous medium, thereby increasing a weight of the conditioned article and providing a moistened product, the first portion of the moistened product having a third water-to-binder ratio greater than a fourth water-to-binder ratio of a remaining portion of the moistened product; and curing the moistened product with carbon dioxide to obtain the carbonated precast concrete product.

[0009] In another aspect, a method for producing a carbonated precast concrete product is provided, the method including the steps of obtaining a base mixture including at least one binder material, an aggregate, and water, and an outer layer mixture including at least one second binder material, a second aggregate, and water; molding the base mixture and the outer layer mixture into a multi-layer molded intermediate having an outer layer and a base material; and demolding the multi-layer molded intermediate to obtain a demolded multi-layer intermediate, wherein the demolded multi-layer intermediate has a first water-to-binder ratio for the base material and a second water-to-binder ratio for the outer layer. conditioning the demolded multilayer intermediate to provide a conditioned multilayer article having a first water-to-binder ratio and a second water-to-binder ratio, the conditioned multilayer article having a reduced amount of water compared to the demolded multilayer intermediate; moistening a surface of at least one outer layer of the conditioned multilayer article with an aqueous medium to increase the weight of the outer layer to provide a moistened multilayer product, wherein the outer layer of the moistened multilayer product has a third water-to-binder ratio greater than the second water-to-binder ratio; and curing the moistened multilayer product with carbon dioxide to obtain a carbonated precast concrete product.

[0010] In yet another aspect, a method for producing a carbonated precast concrete product is provided, the method including: obtaining a demolded intermediate from a molding mixture including at least one binding material, aggregate, and water, the demolded intermediate having a first water-to-binder ratio; conditioning the demolded intermediate to provide a conditioned article having a second water-to-binder ratio less than the first water-to-binder ratio of the demolded intermediate; moistening at least one surface of the conditioned article with an aqueous medium to increase the weight of the conditioned article to provide a moistened product, the first portion of the moistened product having a third water-to-binder ratio greater than a fourth water-to-binder ratio of a remaining portion of the moistened product; and curing the moistened product with carbon dioxide to obtain the carbonated precast concrete product.

[0011] The methods described herein and above may further include one or more of the following additional components and / or steps, in whole or in part, and in any combination:

[0012] In certain embodiments, moistening the at least one surface comprises applying the aqueous medium using an application method selected from partially or fully immersing the at least one surface in a water-containing liquid, spraying the aqueous medium onto the at least one surface, or applying the aqueous medium to the at least one surface with a roller device.

[0013] In certain embodiments, the step of moisturizing at least one surface of the conditioning article comprises moistening the conditioning article until the weight of the conditioning article is at least 10 g / m 2 moisturizing at least one surface until the

[0014] In certain embodiments, applying the aqueous medium comprises applying water, an aqueous solution and / or an aqueous slurry onto the at least one surface.

[0015] In certain embodiments, the step of applying the aqueous medium comprises applying the aqueous medium at 15 to 25°C.

[0016] In certain embodiments, conditioning the demolded intermediate comprises conditioning the demolded intermediate until 20% to 70% by weight of the initial moisture content of the demolded intermediate has been removed.

[0017] In certain embodiments, obtaining the mixture includes obtaining a dry portion and a liquid portion, the dry portion having at least one aggregate material and at least one binder material, and the liquid portion having water, and mixing the dry portion and the liquid portion to obtain the mixture.

[0018] In certain embodiments, the method includes mixing an additive with a dry portion and a liquid portion.

[0019] In certain embodiments, the method includes mixing the microfibers with a dryer.

[0020] In certain embodiments, obtaining the mixture includes obtaining a mixture having at least one bonding material, wherein the at least one bonding material is non-cementitious.

[0021] In certain embodiments, obtaining the mixture includes obtaining a mixture having at least one binder comprising a steel slag and cement, wherein the weight ratio of the steel slag to the cement is between 1:20 and 20:1.

[0022] In certain embodiments, obtaining the mixture includes obtaining a mixture having aggregate, wherein the weight ratio of the aggregate to the total weight of the mixture is between 0.3 and 0.8.

[0023] In certain embodiments, obtaining the mixture includes obtaining a mixture in which the weight ratio of the additive to the total weight of the mixture is between 0.005 and 0.010.

[0024] In certain embodiments, obtaining the mixture comprises obtaining a mixture that includes an admixture.

[0025] In certain embodiments, the admixture is a water repellent admixture.

[0026] In certain embodiments, the admixture is a plasticizer, a superplasticizer, or a polycarboxylic acid water reducer.

[0027] In certain embodiments, shaping the mixture comprises forming the mixture into a formed intermediate.

[0028] In certain embodiments, shaping the mixture includes consolidating the formed intermediate to provide a shaped intermediate.

[0029] Many further configurations and combinations of the present improvements will be apparent to those skilled in the art after reading this disclosure. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a flowchart showing the steps for manufacturing a carbonate precast concrete product. [Figure 2] 1 is a schematic diagram of a precast concrete product having two layers of different thicknesses. [Figure 3] 3 is a flowchart showing the manufacturing process of a two-layer carbonate precast concrete product as shown in FIG. 2. [Figure 4] 1 is a schematic diagram of a precast concrete product having three layers through its thickness. [Figure 5] 1 is a graph showing the average moisture content of precast concrete products at different stages of production using the process of the present disclosure. [Figure 6] This is a graph showing the change in moisture content of a precast concrete sample prepared with the mix design of sample A, measured at thickness after conditioning (solid line) and after surface moisturization (dashed line). DETAILED DESCRIPTION OF THE INVENTION

[0031] The present process for preparing carbonated precast concrete products may require less cement, or even no cement at all, and may allow the concrete to store CO2 within the product. The carbonated precast concrete products of the present disclosure may advantageously have improved resistance to physical attacks, such as resistance to freeze-thaw cycles and abrasion. Carbonation curing also offers the potential for incorporating industrial waste materials as cement substitutes. Such waste materials include steel slag. The presently disclosed process not only reduces the CO2 footprint of concrete, but also makes precast concrete products more affordable by using currently underutilized, low-cost industrial waste materials, such as steel slag and bottom ash. Steel slag is a by-product of steelmaking and is produced during the separation of molten steel from impurities in steel furnaces. Steel slag occurs as a molten liquid melt, a solution containing silicates and oxides that solidifies upon cooling. Bottom ash is a coarse, non-combustible by-product of coal combustion that is recovered from the bottom of the furnace.

[0032] Examples of precast concrete products that may be produced using the methods described herein include, but are not limited to, concrete pipes, traffic barriers, walls including retaining walls, boxes including modular boxes, culverts, tiles, pavers, foundations, slabs including hollow core slabs, patio slabs, steps, curbs, concrete masonry units, beams, floors, columns, manholes, sewer pipes, sleepers, and other precast concrete products.

[0033] The production of carbonated precast concrete products differs from the production of conventional cementitious precast concrete primarily in the application of carbon dioxide during the carbonation curing process and the wide selection of suitable binder sources (e.g., cement). Carbonated precast concrete obtains its strength primarily through the reaction between the introduced carbon dioxide gas and the calcium and / or magnesium oxides and / or hydroxides in the binder, with sufficient water in the mix. Precasts containing such minerals often harden rapidly when exposed to high or low concentrations of CO2.

[0034] [Freeze-thaw and abrasion resistance] The lack of durability of certain existing concrete structures can sometimes cause serious and detrimental damage, rendering the product unsuitable for its intended use. The present disclosure relates to a process that can produce concrete products with improved durability, more specifically, improved freeze-thaw and abrasion resistance. In some cases, the disclosed process can improve other mechanical / physical properties.

[0035] Freeze-thaw damage is induced by internal tensile stresses that build up during repeated freeze-thaw cycles. Water expansion and the hydraulic pressure caused by the expansive forces of unfrozen water particles contribute to the damage caused by freeze-thaw cycles. The presence of deicing salts in the freeze-thaw process can exacerbate the damage caused to precast concrete.

[0036] Surface wear is the gradual loss of mass from the surface of concrete due to repeated cycles of friction. Abrasion is one of the primary surface wear mechanisms and refers to the friction of other solid objects moving along the surface of the concrete. The cause of wear depends on the application of the precast concrete.

[0037] The durability of precast concrete products is measured by a series of standard tests. The tests and requirements vary depending on the type of product and its use. As an example, a summary of standard tests for precast concrete pavers and retaining walls is provided below. This disclosure references several standards (e.g., ASTM standards). It will be understood that these standards correspond to the versions available as of the filing date of this patent application.

[0038] There are several methods for evaluating the abrasion resistance of precast concrete. In addition to the sandblasting test method (ASTM C418) specified in ASTM C936 to evaluate the abrasion resistance of concrete pavers, the rotary cutter method (ASTM C944) can also be applied to measure the abrasion resistance of precast concrete products. In the rotary cutter method, the testing equipment consists of a drill press and a rotary cutter. A concrete sample is placed under the rotary cutter, and a constant vertical load of 98 N is applied to the spindle that turns the cutter. The sample is subjected to abrasive wear at a speed of 200 rpm for 2 minutes, and the mass loss rate is calculated (ASTM, 2019a).

[0039] The freeze-thaw durability of precast concrete pavers is tested according to CSA A231.2 or ASTM C1645 as required by ASTM C936. During the test, the precast concrete specimens are completely immersed in a 3% sodium chloride solution. They are subjected to 24-hour cycles of 16 hours of freezing and 8 hours of thawing. After 28 cycles of freeze-thaw testing, the mass of the precast concrete is 225 g / m of total surface area. 2 or after 49 cycles of freeze-thaw testing, the mass of the precast concrete shall not decrease by more than 500 g / m2 of the total surface area. 2should not be reduced by more than (ASTM, 2019b; ASTM, 2018a; CSA, 2005).

[0040] The freeze-thaw durability of precast concrete retaining walls is tested in accordance with ASTM C1262. During the test, the retaining wall specimens are placed face down in water to a depth of 13 ± 2 mm (ASTM, 2018b). The weight of a set of five specimens must not decrease by more than 1% of their original weight after 100 freeze-thaw cycles, or four of the five specimens must not decrease by more than 1.5% of their original weight after 150 freeze-thaw cycles (ASTM, 2017). One freeze-thaw cycle is defined as a complete freeze cycle followed by a complete thaw cycle. A portion of the retaining wall specimen may also be immersed in a 3% sodium chloride solution. In this case, the requirement of less than 1% mass loss is generally reduced to 40 freeze-thaw cycles.

[0041] Carbonate concrete has poor freeze-thaw resistance, which may make it unsuitable for applications such as paving and retaining walls. Additionally, carbonate precast concrete may perform poorly under abrasive conditions, making it unsuitable for paving applications.

[0042] Conventional methods often result in insufficient surface moisture in precast concrete prior to carbonation curing. While this lack of surface moisture does not significantly affect the compressive strength of carbonated precast concrete, it is believed to impair certain other properties of the carbonated concrete product, such as flexural strength, surface hardness, and surface durability (including abrasion resistance and / or freeze-thaw resistance). As a result, carbonated precast concrete products often do not meet the specific durability requirements for their intended applications.

[0043] It has been found that the processes disclosed herein can improve at least certain durability properties of carbonated precast concrete products as measured by freeze-thaw and abrasion resistance, and it has been observed that carbonated precast concrete products made in this manner can maintain their strength over time and freeze-thaw cycles.

[0044] This disclosure relates to a manufacturing process for carbonated precast concrete products. The main components for making carbonated precast concrete include binder, aggregate, and water. Depending on the type of raw materials and the specifications of the carbonated precast concrete, other additives may be included.

[0045] [Binding material] The cementitious and / or non-cementitious binders in carbonated precast concrete can include any of the carbonatable materials, such as ordinary Portland cement, other types of cement, non-hydraulic cement, hydraulic cement, ground granulated blast furnace slag (GGBFS), steel slag, fly ash, bottom ash, stainless steel slag, and other materials rich in CaO and / or MgO and / or calcium silicate content, as well as combinations thereof. Any suitable combination of two or more binders can be used. In some cases, a single binder can be used. In some embodiments, the binder is a cementitious binder and can include ordinary Portland cement, other types of cement, non-hydraulic cement, hydraulic cement, and combinations thereof. In some embodiments, the binder is a non-cementitious binder and can include steel slag, fly ash, bottom ash, stainless steel slag, and other materials rich in CaO and / or MgO, calcium silicate content, as well as combinations thereof. In one particular example, the binder is a non-cementitious binder and contains less than 5% by weight, less than 4% by weight, less than 3% by weight, less than 2% by weight, or less than 1% by weight cement.

[0046] For example, carbon dioxide precast concrete products can be produced herein using steel slag as the sole binder component, or optionally as the primary binder component, along with a proportion of cement, using carbon dioxide as a hardening agent. In other words, the steel slag replaces all or most of the cement. Carbon dioxide is also applied to strengthen, harden, and activate the slag.

[0047] In one embodiment, the binder comprises steel slag. In some cases, the binder comprises steel slag and other suitable components.

[0048] In one embodiment, the binder consists essentially of steel slag. As used herein, the term "consisting essentially of" can mean, in one example, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, or at least 98% by weight. In some cases, the binder may include a majority (e.g., greater than 50% by weight) of steel slag.

[0049] In one embodiment, the binder comprises steel slag and cement, and the weight ratio of steel slag to cement is about 1:20, about 1:15, about 1:10, 1:5, about 1:20 to about 20:1, about 1:10 to about 10:1, or about 1:5 to about 5:1. Alternatively, in some embodiments, the weight ratio of steel slag to cement is about 20:1, or about 15:1, or about 10:1, or about 5:1.

[0050] In further embodiments, particularly in multi-layer products as defined herein, the binder in the top / outer layer consists of or comprises essentially of steel slag, or has a weight ratio of steel slag to cement of about 1:20 to about 20:1, or a weight ratio of steel slag to cement of about 20:1, or about 15:1, or about 10:1, or about 5:1.

[0051] In one embodiment, the weight ratio of binder (e.g., any or all of steel slag, cement, and other carbonatable materials) to the total weight of the composition including aggregate, binder, water, and additives ranges from about 0.20 to about 0.60, preferably from about 0.25 to about 0.50, or more preferably from about 0.30 to about 0.50.

[0052] [cement] In the present disclosure, the following non-limiting list of cements can be used to make carbonate precast concrete: Portland cement (Type I-V), Portland limestone cement, rapid hardening cement, quick setting cement, low heat cement, blast furnace slag cement, Portland slag cement, high alumina cement, white cement, colored cement, pozzolanic cement (Portland-pozzolanic cement), air entraining cement, channel cement, non-hydraulic cement, ternary blend cement.

[0053] As used herein, an effective cement is one that contains calcium silicate phases, specifically C3S, that can gain strength when reacted with water. The presence of the calcium silicate phases ensures short-term and long-term strength development.

[0054] [Steel slag and stainless steel slag] As used herein, "steel slag" refers to a slag by-product produced during the manufacture of iron and steel. Steel slag may include slag produced from a basic oxygen furnace (BOF), also known as Linz-Donawitz (LD) process slag, or LD slag. Steel slag may also include slag produced from an electric arc furnace (EAF). As used herein, steel slag may further include ladle slag. Steel slag may be a combination of the above slags. It is understood that "steel slag" as used herein excludes iron slag and blast furnace slag, which are commonly generated during the manufacture of iron and may be used in the manufacture of cement, such as pozzolana slag.

[0055] As used herein, "ladle slag" refers to a type of steel slag. Ladle slag is produced as a by-product of ladle refining operations. In various steel processes, the molten steel produced in the EAF or BOF process undergoes additional refining steps based on the desired steel quality.

[0056] As used herein, "EBH slag" refers to an EAF-BOF hybrid, which is a type of steel slag formed from a mixture of EAF-produced slag and BOF-produced slag.

[0057] Stainless steel slag may include slag generated during stainless steel manufacturing, primarily from the argon oxygen decarburization (AOD) and / or ladle metallurgy (LM) processes.

[0058] [Chemical composition of steel slag] In one embodiment, the steel slag used herein has a cumulative calcium silicate content (e.g., CS+C2S+C3S phase concentration) of at least about 15% by weight.

[0059] In one embodiment, the steel slag used herein has a cumulative calcium silicate content (e.g., CS+C2S+C3S phase concentration) of at least about 20% by mass.

[0060] In one embodiment, the steel slag used herein has a cumulative calcium silicate content (e.g., CS+C2S+C3S phase concentration) of at least about 30% by mass.

[0061] In one embodiment, the steel slag used herein has a cumulative calcium silicate content (e.g., CS+C2S+C3S phase concentration) of at least about 40% by mass.

[0062] In one embodiment, the iron and steel slag used herein is S i The O2 content is at least about 6% by weight, or more preferably at least about 15% by weight.

[0063] [Chemical composition of stainless steel slag] In one embodiment, the stainless steel slag used herein is S i The O2 content is at least about 15% by weight, or more preferably at least about 20% by weight.

[0064] In one embodiment, the stainless steel slag used herein has a calcium oxide content of at least about 30% by weight, or more preferably at least about 35% by weight.

[0065] [Physical properties of steel slag] Iron and steel slag may contain a mixture of coarse and fine slag pieces. Coarse slag pieces have a Blaine fineness of about 50 m. 2 / kg, and the fine slag pieces have a Blaine fineness of about 50m 2 / kg. Coarse slag pieces, fine slag pieces, or both may be landfilled as a result of typical steel processing. Steel slag originating from waste (such as landfill and / or post-industrial waste) can be received and optionally refined.

[0066] Refining the steel slag may include filtering the received steel slag to separate it into coarse and fine slag fractions.

[0067] Alternatively or additionally, refining the received steel slag may include grinding the steel slag into a fine powder. In some exemplary embodiments, the filtered fine fraction is ground, while the coarse fraction is not ground. For example, in the case of EAF steel slag, the slag is ground to a fine powder of at least 50 ml. 2 / kg, preferably about 180m 2 For example, in the case of EBH steel slag (a mixture of EAF, BOF and ladle slag), the slag can be ground to a fineness of at least 100m 2 / kg, preferably about 240m 2 / kg Blaine fineness. In other exemplary embodiments, the steel slag may be ground to an even finer size. In another example, at least 50 percent of the ground slag is less than 100 microns, with D(50)<100 microns.

[0068] [aggregate] Aggregates can be natural or artificial, and can be normal-weight or lightweight. They can be divided into coarse aggregate and fine aggregate based on size. The type, distribution, and size of coarse and fine aggregates vary depending on their availability, cost, and gradation, as well as the required workability of the concrete mix and the desired surface texture and properties of the carbonated precast concrete. In some embodiments, the fine aggregate has less than 5% particles with a diameter greater than 3 / 8". In some embodiments, the diameter of the coarse aggregate is greater than 1 / 4". In further embodiments, the diameter of the coarse aggregate is greater than 1 / 2". In some embodiments, the term "normal-weight" as used herein refers to naturally occurring or crushed gravel or sand, such as limestone or granite, having a specific gravity of about 2.7. In some embodiments, the term "lightweight aggregate" as used herein refers to natural or artificial particles with a specific gravity ranging from about 0.3 to about 1.9. As used herein, the term "about" refers to a plus or minus 10% variation of the value.

[0069] In one embodiment, the weight ratio of aggregate to the total weight of the composition, including aggregate, binder, water, and additives, ranges from about 0.3 to about 0.8, preferably from about 0.4 to about 0.7, or more preferably from about 0.40 to about 0.65.

[0070] [Additives] Additives as used herein include, but are not limited to, air-entraining admixtures, water-reducing admixtures, shrinkage-reducing admixtures, corrosion-inhibiting admixtures, accelerating or retarding admixtures, viscosity modifiers, pigments, water-repellent admixtures and other natural or chemical additives. Further ingredients / additives include fibers (such as Euclid's PSI Multi-Mix 80) that may be added when formulating the carbonate precast concrete depending on the end use. The additive may also be a mineral admixture.

[0071] Water-reducing admixtures are added to concrete mixes to increase compressive strength, reduce water content, reduce void content, and reduce permeability. Water-reducing admixtures are classified as plasticizers or superplasticizers (i.e., polycarboxylic acid-based water-reducing admixtures). Any water-reducing admixture that can reduce the required water content by approximately 50% or increase compressive strength by approximately 60% can be used as a water-reducing admixture in the present invention. The admixtures used in the present invention must meet the requirements of ASTM C494 (Standard Specification for Chemical Admixtures for Concrete, ASTM International, West Conshohocken, PA, 2019).

[0072] Water-repellent admixtures are designed to provide integral water repellency to concrete by affecting the capillary action of water in and out of the concrete. Water-repellent admixtures can function as static pore plugs, creating more difficult pathways for water to move, or as reactive chemicals that not only plug the pores but also form "in situ" hydrophobic materials that chemically repel water from the concrete surface.

[0073] In certain embodiments of the present disclosure, an additive according to the description herein is present in a conditioned demolded product (i.e., a conditioned article) or part thereof intended to undergo a surface moistening step as described herein, the additive being an air-entraining admixture.

[0074] In one embodiment, the weight ratio of the air entraining admixture as additive to the total weight of the composition including aggregate + binder + water + additives ranges from about 0.0001 to about 0.001, or from about 0.0002 to about 0.0008, or from about 0.0004 to about 0.0006.

[0075] In certain embodiments, the water repellent admixture is present only in the conditioned demolded product (i.e., conditioned article) or part thereof, such as a multi-layer product, which is not intended / required to undergo a surface moistening step as described herein.

[0076] In certain embodiments, the water repellent admixture is present in the composition comprising the binder, aggregate, and water in a weight ratio (based on the total weight of the composition) of at least about 0.005, or at least about 0.006, preferably at least about 0.007, or at least about 0.008, or at least about 0.009, or at least about 0.010.

[0077] In certain embodiments, the water repellent admixture is present in a composition comprising binder, aggregate, and water in a weight ratio (based on the total weight of the composition) of at least about 0.005 and at most about 0.009-0.010.

[0078] [Water to binder ratio] The water-to-binder ratio depends on the manufacturing process (wet cast or dry cast), the binder and aggregate content, and the use and amount of water-reducing admixture in the mix design. Generally, the water-to-binder ratio varies from about 0.10 to about 0.50 (by weight).

[0079] In one embodiment, the water to binder ratio ranges from about 0.10 to about 0.25.

[0080] In one embodiment, the water to binder ratio may be at least about 0.10, or at least about 0.11, or at least about 0.12, or at least about 0.13, or at least about 0.14, or at least about 0.15, or at least about 0.20, or at least about 0.25.

[0081] In one embodiment, the weight ratio of water to the total weight of the composition including aggregate, binder, water, and additives ranges from about 0.01 to about 0.10, preferably from about 0.05 to about 0.10, or more preferably from about 0.06 to about 0.08.

[0082] Referring to FIG. 1 , a process 100 for producing carbonated precast concrete products according to the present disclosure is provided. As described in more detail below, the process 100 includes step 102 of mixing raw materials, including a binder, aggregate, water, and some optional additives. The process 100 also includes forming the mixture of at least one binder, aggregate, and water into a shaped intermediate. In the illustrated embodiment, forming the mixture into a shaped intermediate includes step 104 of forming the mixture to obtain a shaped intermediate and step 106 of consolidating the shaped intermediate to obtain a consolidated intermediate, which may correspond to the shaped intermediate. An optional step 108 of initial curing the shaped intermediate may also be performed. Step 110 of demolding the shaped intermediate to obtain a demolded intermediate is performed. The demolded intermediate has a first water-to-binder ratio. Step 112 conditions the demolded intermediate to provide a conditioned article having a second water-to-binder ratio. The second water-to-binder ratio of the conditioned article is less than the first water-to-binder ratio of the demolded intermediate. Step 114 moisturizes the surface of the conditioned article to obtain a moisturized product. Moisturizing step 114 increases the weight of the conditioned article. The third water-to-binder ratio of a first portion of the moisturized product is greater than a fourth water-to-binder ratio of a remaining portion of the moisturized product. In some embodiments, the first portion having the third water-to-binder ratio corresponds to an outer layer or surface of the moisturized product, and the remaining portion of the moisturized product having the fourth water-to-binder ratio may correspond to a core of the moisturized product that is at least partially surrounded or covered by the outer layer or surface. An optional step 116 of air-drying the moisturized product may also be included, followed by step 118 of curing the moisturized product with carbon dioxide. Optional steps are indicated by dashed lines in FIG. 1. Each of steps 102-118 is described in more detail herein below.

[0083] [Mixing of components] The mixing step 102 is performed using at least one binder material, aggregate, water, and optional additives, such as admixtures. In one embodiment, prior to the mixing step 102, the method includes the steps of: a) providing a dry portion and a liquid portion, the dry portion comprising at least one aggregate material and at least one binder material, and the liquid portion comprising water and optional additives; and combining the dry portion and the liquid portion.

[0084] In some embodiments, additives and other ingredients can optionally be introduced into the mix if needed as a function of the intended use of the concrete product.

[0085] [Formation and compaction] The molding step 104 involves adding a sufficient amount of the mixed material, containing a binder, suitable aggregate, moisture, and optional additives by any known means, to a mold to form the component into the desired shape. Although the terms "molding" or "molding" are used herein, this contemplates any hollow form or matrix to provide a shape, such as a frame / mold, and then optionally flatten. In other words, the mixture may be shaped in such a way as to define an internal cavity.

[0086] Consolidating 106 involves consolidating the formed intermediate body using, for example, vibration, compaction, compression, or a combination of forces, to a desired thickness, shape, and density, which may be selected depending on the desired application as known in the art.

[0087] Both the forming step 104 and the consolidating step 106 may be referred to herein as forming the product, or in other words, the forming step 104 and the consolidating step 106 may be considered two substeps of forming the mixture.

[0088] In one embodiment, prior to the forming step 104, the method includes a further step of mixing at least one binder material, aggregate, water, and optional additives to provide a base mixture, which may occur before, simultaneously with, or after the mixing step 102. In one embodiment, the method includes a step of forming the base mixture to provide a formed intermediate, which may occur before, simultaneously with, or after step 104. In further embodiments, one or more of the base mixture mixing step 102 and the base material forming step 104 may be repeated. In further embodiments, each of the mixtures of at least one binder material, aggregate, water, and optional additives may contain the same or different proportions of these components. However, at least one of these mixtures, particularly the one that undergoes the surface moistening step 114, includes a binder other than cement, such as a slag binder, or preferably, steel slag. Within the above embodiments, each of the molded product (i.e., molded intermediate), demolded product (i.e., demolded intermediate), adjusted demolded product (i.e., adjusted article), moisturized pre-hardened molded product (i.e., moisturized product), and carbonated precast concrete product is multi-layered (e.g., two or more layered products).

[0089] [Initial curing (optional)] In some cases, particularly when precast concrete is made by a wet casting process, an optional step 108 is performed in which the intermediate shaped body is first cured to provide a satisfactory early strength before it is removed from the mold in step 110. The precast concrete relies on the hydration / setting or other physical / chemical / activation of the binder to achieve the desired strength during this initial curing stage, which may last, for example, from 2 hours to several days.

[0090] [Adjustment (reducing the moisture content of demolded precast concrete)] Carbonation curing can be performed immediately after demolding the precast concrete, but it is more common to condition the compacted concrete for a period of time before introducing carbon dioxide gas. This conditioning step 112 begins after demolding the molded intermediate body 114 and before curing the demolded intermediate body with CO2 118. The conditioning step 112 involves controlled removal of excess water. Its primary purpose is to promote a rapid and uniform carbonation reaction within the concrete by removing excess water. Excess water can hinder the diffusion of CO2 to the reactants, limiting the reaction, while insufficient water content can cause a lack of water and stop the reaction. In this way, an optimal water content can be achieved before carbonation for optimal carbonation.

[0091] The conditioning step 112 is performed on the demolded intermediate product (optionally after initial curing) after the demolding step 110. The conditioning step 112 can be performed at room temperature, 15-25°C, and 30-60% humidity, with or without the assistance of forced air circulation. The duration of the conditioning step can vary from 10 minutes to 24 hours or more. This conditioning step 112 can help reduce the moisture content of the precast concrete by evaporating water. As moisture is released, numerous pores remain within the consolidated precast concrete. This is believed to be important for achieving the desired amount of CO2 uptake and uniform carbonation throughout the precast concrete product. A relatively high CO2 uptake and uniform carbonation distribution are believed to be critical for the physical-mechanical properties of carbonated precast concrete products. In one embodiment, 20-70% by weight, preferably 30-60% by weight, and particularly 40-50% by weight of the mixture's initial water content has been removed from the precast concrete by the end of the conditioning step 112. Other known methods of reducing moisture, such as heat, may alternatively be used during the conditioning step.

[0092] [Exposure of product surface to aqueous media] The process 100 of the present disclosure includes a step 114 of moisturizing the surface of the conditioned article. The moisturizing step 114 involves exposing at least one surface of the conditioned article to an aqueous medium to increase its weight. After conditioning the molded product 112, the moisture content of the precast concrete, i.e., the ratio of water to binder, is reduced. Sufficient moisture content in the matrix allows for uniform penetration of carbon dioxide and sufficient CO2 uptake, potentially resulting in satisfactory performance as carbonated precast concrete. However, as described in the art, a drawback associated with conditioning is that the exterior (exposed) surface of the molded product loses moisture much faster than the interior of the precast concrete. Thus, when the interior moisture content is at an ideal level, the moisture content on the exterior surface is likely to be lower. Insufficient moisture on the exterior surface will result in incomplete carbonation curing on the exterior surface. This is because a sufficient amount of water is required to promote the reaction between carbon dioxide and calcium silicate phases or calcium and magnesium oxides or hydroxides.

[0093] In operating the process defined herein, it is desirable to increase the surface moisture content of the exterior surface to an appropriate level prior to carbonation curing to ensure that a sufficient degree of carbonation can be achieved throughout the entire volume of the precast concrete product.

[0094] In step 114, moisturizing the surface of the conditioned article by exposing the surface of the conditioned article to an aqueous medium, an aqueous medium (i.e., a water-containing material or phase) can be applied to at least one surface of the conditioned article to increase the surface moisture content. The aqueous medium can be water alone, an aqueous solution, or an aqueous slurry. There are no specific requirements regarding the temperature at which the water-containing material is applied in the surface moisturizing step, but temperatures close to room temperature (i.e., about 15 to about 25°C) are preferred. As used herein, the aqueous medium for increasing the weight of the surface of the conditioned article includes any water-containing means (suspension / solution or other phase / material) suitable for concrete production, containing sufficient amounts of CaO / Ca(OH)2 / MgO, capable of providing moisture to the surface, and suitable for concrete production. Alternatively, a 1 to 25% aqueous chemical solution, such as sodium silicate (water glass), can be used to moisturize the surface. Slurries of hydrated cement, steel slag, GGBFS, stainless steel slag, lime, non-hydrated cement, fly ash, or any material containing sufficient amounts of CaO / Ca(OH)2 / MgO with a solids content of 1-80% can also be used for surface moistening. When using slurries, the solids content can be 1-25% by mass, with 5-10% being preferred. The slurry can be prepared by adding a CaO- and / or MgO-rich material to a proportional amount of water, such as tap water, in a container equipped with a stirrer at room temperature (15-25°C). The stirrer must be running during the slurry preparation and surface moistening process to maintain a constant solids content. The prepared aqueous medium should have a pH value of 6.5-13.5.

[0095] The aqueous medium can be applied using a variety of application methods, including, but not limited to, partially or completely immersing the surface in the water-containing liquid, spraying the aqueous medium onto the surface, or applying the aqueous medium to the surface with a roller or similar device. If immersion is selected, the immersion time of the precast concrete in the water-containing liquid can be from 1 second to 5 hours, preferably 3 to 5 seconds.

[0096] Whatever application method is chosen, the moisturizing product should be applied in an amount of at least 10 g / m during moisturizing step 114 after the surface moisturizing treatment is complete. 2 , preferably at least 50 g / m 2 , preferably 50 to 350 g / m 2 or 75 to 325 g / m 2 The weight (based on the total moisturizing surface area of ​​precast concrete) can be increased.

[0097] [CO2 curing and optional air drying] After the surface-moisturizing step 114, the moisturizing product can be cured (carbonated). A curing step 118 may be performed immediately after the moisturizing step 114. This is performed during the carbon dioxide curing step 118. In some embodiments, to account for the necessary waiting time in the carbonated precast concrete manufacturing process, such as transporting the moisturized precast concrete and loading it into the carbonation curing chamber, an optional air-drying step 116 of the moisturizing product can be performed after the moisturizing step 114 and before the curing step 118. This period can last up to one hour, preferably 5 to 20 minutes. During this period, the moisturized precast concrete should not be exposed to excessively hot, dry, or windy environments. If an air-drying time of more than one hour is required, the moisturizing operation should be delayed. Otherwise, the moisturized precast concrete should be covered with plastic sheeting or a similar material to prevent further moisture loss before carbonation curing. In some embodiments, the air temperature is in the range of 15 to 40°C, preferably about 22°C. The relative humidity may be in the range of 30% to 90%, preferably about 50%. The air velocity may be 0.1 m / s to 100 m / s, preferably about 2 m / s.

[0098] In some embodiments, the surface-moisturized precast concrete is placed in a pressure chamber where carbonation curing occurs. Carbon dioxide gas having a purity of 5-99.9% is introduced into the sealed pressure chamber. The concrete is cured with the gas at atmospheric or superatmospheric pressure. If superatmospheric, the pressure of the carbon dioxide gas in the pressure chamber is adjusted to 0.07-0.689 MPa (0.1-100 psi) during the carbonation curing process, which lasts for at least 5 minutes, preferably 2-24 hours.

[0099] [Layered carbonated product structure] As discussed herein, the process 100 of the present disclosure can favorably affect certain mechanical properties and durability of carbonated precast concrete products, specifically their resistance to wear and freeze and thaw. Therefore, such a process 100 can potentially provide products with a combination of beneficial properties. For example, layered carbonated product structures can be prepared. The present disclosure contemplates the fabrication / use of multiple layers in a given product, if desired. The carbonated product may contain at least a first / outer / top layer, which may be considered the outer layer depending on the product or configuration, and a second layer, which may be referred to as the second / inner / bottom / base layer depending on the product or configuration. The compositions of the two layers may differ with respect to binder (although the use of at least multiple binders is considered preferred herein), aggregate, mix proportions, additives, and other optional components. Furthermore, the thicknesses of the layers may differ, and thus their relative size ratios may also differ. The properties of the two layers may be different, with the first / outer / top layer having at least one improved property compared to the second / inner / bottom / substrate layer, the property being at least one of abrasion resistance, freeze resistance, and thaw resistance.

[0100] As shown in Figure 2, an exemplary embodiment of a carbonated precast concrete product 1 may be comprised of two layers 10, 20 of different thicknesses. On top is a denser, stronger / durable precast concrete layer 10. Beneath the top layer 10 is a substrate layer 20. In one embodiment, the thickness of the top layer 10 may be less than half the total thickness of the concrete product 1.

[0101] In the top layer embodiment, 1) the thickness can be 5 to 20 mm, preferably 6 to 13 mm. 2) The same type of binder as that of the base layer 20 can be used, or a different type of binder can be used. 3) If the same type of binder as that of the base layer 20 is used, the top layer 10 can have the same binder content as that of the base layer 20 but a higher first water-to-binder ratio. Alternatively, the top layer 10 can have a higher binder content than that of the base layer 20 but a similar or higher first water-to-binder ratio than that of the base layer 20. Such a distribution technique can be beneficial for improving the abrasion resistance of carbonated precast concrete. 4) After the conditioning process is complete, it is recommended that the above-mentioned surface moistening step be applied to this layer or the entire precast concrete. 5) If the surface moistening step 214 is applied, water-repellent admixtures should not be added to the precast concrete mixture to facilitate the moisture absorption process. 6) The carbonated precast concrete is designed as a surface that can withstand the main wear and tear processes during service. Embodiments of the base layer 20 include: 1) adding a water-repellent admixture in an amount of at least 0.1% of the binder mass, preferably 1.5-2.0%; 2) using a lower binder content than the top layer 10 to save on material costs; 3) having a lower water-to-binder ratio by mass than the top layer 10 to compensate for the lower carbon dioxide gas permeability of the denser top layer 10; and 4) being thicker than the top layer 10. The higher water-repellent admixture loading may improve both the freeze-thaw resistance and water absorption of the base layer 20. It may also provide a dimensionally stable substrate, which may be important for the durability of a relatively thin top layer.

[0102] In one embodiment, there is provided a method of making a carbonated precast concrete product, comprising:

[0103] 1) mixing at least one binder material, aggregate, water, and optional additives, preferably wherein the binder consists of cement, or consists essentially of cement, or the ratio of cement to steel slag or other carbonatable material is about 1:20 to about 20:1, or the weight ratio of cement to steel slag or other carbonatable material is about 20:1, or about 15:1, or about 10:1, or about 5:1.

[0104] 1A) Mixing at least one binder material, aggregate, water, and optional additives to provide a base mixture, preferably the binder consisting of, or consisting essentially of, steel slag and / or any other carbonatable material, or comprising a steel slag to cement ratio of about 1:20 to about 20:1.

[0105] Step 1A) is carried out before, simultaneously with, or after step 1).

[0106] 2) forming the mixed at least one binder material, aggregate, water, and optional additives of step 1) to provide a formed product.

[0107] 2A) forming the substrate mixture to provide a formed substrate mixture, wherein step 2A) can occur before, simultaneously with, or after step 2).

[0108] Optionally repeating one or more of steps 1) and 2) and / or 1A) and 2A), optionally wherein the mixed at least one binder material, aggregate, water, and optional additives each can contain the same or different components in the same or different proportions.

[0109] 3) Consolidating the formed product of steps 2) and 2A) into a molded product having a first water to binder ratio.

[0110] 4) demolding the molded product of step 3) having the first water to binder ratio to provide a demolded product.

[0111] 5) conditioning the demolded product of step 4) to provide a conditioned demolded product having a reduced amount of water compared to the molded product having the first water to binder ratio.

[0112] 6) A step of curing the conditioned demolded product of step 5) with carbon dioxide.

[0113] Referring now to FIG. 3 , a process 300 for manufacturing a carbonated precast concrete product having a base layer and a top layer is provided. As described in more detail below, process 300 includes step 302a of mixing binder, aggregate, water, admixtures, and other additives for the base layer (base mix) and step 302b of mixing binder, aggregate, water, admixtures, and other additives for the top layer (top or outer layer mix). Step 304 forms a concrete product (multi-layer concrete product) having the base layer from the base mix and the top layer from the top layer mix to obtain a multi-layer molded intermediate. Step 306 consolidates the multi-layer molded intermediate to obtain a multi-layer consolidated intermediate. Optional step 308 performs initial curing of the multi-layer consolidated intermediate. Step 310 demolds the multi-layer consolidated intermediate to obtain a demolded multi-layer intermediate. The demolded multilayer intermediate has a first water-to-binder ratio for the substrate and a second water-to-binder ratio for the top layer. Step 312 conditions the demolded multilayer intermediate to obtain a conditioned multilayer article. At least the top layer of the conditioned multilayer article has a reduced water-to-binder ratio during the conditioning step 312. Optionally, after conditioning the demolded multilayer intermediate in step 312, the water-to-binder ratios of both the substrate layer and the top layer are reduced from their respective first and second water-to-binder ratios. Step 314 moisturizes at least one surface (e.g., the top layer) of the conditioned multilayer article to obtain a moisturized multilayer product. After moisturizing the top layer in step 314, the top layer has a third water-to-binder ratio greater than the second water-to-binder ratio. Optional step 316 air-dries the moisturized multilayer product. And step 318 cures the moisturized multilayer product with carbon dioxide. Optional steps are indicated by dashed lines in FIG. 3 . Each of steps 302a / 302b through 318 is described in more detail herein below.

[0114] FIG. 3 shows another process for producing a two-layer carbonated precast concrete product 300. Process 300 may also be used to produce a multi-layer product. Process 300 includes step 302a, mixing the components for base layer 20 (FIG. 2), and step 302b, mixing the components for top layer 10 (FIG. 2). As previously mentioned, base layer mixing step 302a and top layer mixing step 302b may use different binders, different binder contents, and different water-to-binder ratios. In addition, base layer 20 may have a relatively high water-repellent admixture loading. Air-entraining admixtures are recommended for top layer 10, although the amount varies depending on the supplier. However, because water-repellent admixtures are commonly used in base layer 20, air-entraining admixtures may not be necessary for base layer 20. Optionally, microfibers may be added at a loading of 0.2–0.5% (based on the total amount of raw materials). The addition of microfibers reduces the likelihood of cracking but may also increase the abrasion resistance of carbonated precast concrete. Furthermore, if the surface moisturizing step 314 is performed, the top layer 10 does not need to contain a water-repellent admixture. After being batched and mixed separately, the top layer 10 and base layer 20 mixture are transported to a forming / molding station. The forming step 304 and the consolidation step 306 then follow, with the two layers being added to the mold in the required amounts and order. It should be noted that either the top layer mixture or the base layer mixture can be added to the mold first. To achieve a strong and durable bond between the two layers 10 and 20, the second layer mixture should be added immediately on top of the level but uncompressed first layer material. Other techniques can be implemented to improve the bond between the two layers, including placing a mesh on top of the first layer or roughening the surface of the first layer before casting the second layer. An initial curing step 308 is optionally performed as described herein with reference to step 108 described above with reference to Figure 1. Additionally, a demolding step 310 and conditioning step 312 are performed as described herein with reference to steps 110 and 112 of Figure 1.After conditioning step 312, the precast concrete can be subjected to carbonation curing step 318 without surface moistening step 314 if the top layer 10 contains a cementitious binder, e.g., hydraulic cement. Otherwise, the top layer 10 or the entire precast concrete undergoes surface moistening step 314 in the manner described hereinabove. The addition of a large amount of water-repellent admixture can render the base layer 20 very hydrophobic, making surface moistening step 314 unnecessary for the base layer 20. After surface moistening step 314, if desired, the precast concrete can optionally undergo a short air drying step 316 before being subjected to carbonation curing step 318. The duration of this air drying and the measures taken when the air drying time is longer than one hour are the same as those described above for step 116 of FIG. 1, for example. The two-layer precast concrete is loaded into a pressure chamber and then carbonation cured in step 318. Carbonation curing step 318 includes the parameters described herein for two-part precast concrete, for example, as described in step 118 of FIG.

[0115] In addition to adding a surface moisturizing step, adding a sufficient amount of water-repellent admixture to the precast concrete mix can also be an effective way to improve the freeze-thaw resistance of carbonate precast concrete. If water or deicing salt solutions cannot penetrate the carbonate precast concrete, damage caused by freeze-thaw cycles can be prevented. Adding a sufficient amount of water-repellent admixture can significantly reduce the water absorption rate of carbonate precast concrete at saturated conditions, potentially improving its freeze-thaw resistance. Typically, the recommended amount of water-repellent admixture used in precast concrete is less than 0.4% (based on the weight of the binder). Such low amounts of water-repellent admixture may have very limited effect on the freeze-thaw resistance of the concrete. The process of the present disclosure may also increase the amount of water-repellent admixture used to 1.0% or more.

[0116] This carbonate precast concrete with a high loading of water-repellent admixture may perform well in freeze-thaw tests without the additional surface moisturizing step mentioned above.

[0117] Referring now to FIG. 4, the two-layer design described above can be further expanded to a three-layer design. The top and bottom layers of this design are denser, stronger, and more durable precast concrete layers 10. Between these two layers is a core layer 30. The precast concrete for the top and bottom layers 10 may be prepared in the same manner as the first / outer / top layer 10 of the two-layer precast concrete 1 described above with reference to FIG. 2. The core layer 30 may have an entirely different raw material source and mix design than that used to prepare the second / inner / bottom / base layer 20 of the two-layer design 1 described above with reference to FIG. 2. Such a sandwich design may provide many benefits to carbonated precast concrete, including improved durability, improved flexural performance, a balanced structure, potentially shorter conditioning and carbonation curing times, and the addition of other desired features. The two-layer carbonated precast concrete manufacturing process 300 shown in FIG. 3 can also be employed for three-layer carbonated precast concrete by slightly modifying the casting order. Thus, three or more multi-layer carbonated precast concrete 2 can be formed.

[0118] In the following examples, the carbonate precast concrete products are formulated with some or all of the ingredients listed below. Steel slag: A mixture of EAF slag and BOF slag (EBH slag) with an average particle size (D50) of 25 μm and an estimated specific gravity of 3.3. The steel slag has a cumulative calcium silicate content of at least about 20%. The steel slag has a calcium oxide content of at least 20%. The steel slag has a silicon dioxide content of at least about 6%. · Ordinary Portland cement: Type I. ·Water: Tap water. Aggregate: Crushed stone with a specific gravity of 2.7. 100% passing through a 4.76 mm (No. 4) sieve. Moisture content: 0.25%, water absorption: 0.75%. Air entraining admixture (AEA): BASF (registered trademark), product name Micro Air. Water-repellent admixture (WRA): BASF (registered trademark), product name MasterPel 240. Slaked lime: A white powder of calcium magnesium tetrahydroxide, supplied by DAP Canada. Carbonation curing uses compressed CO2 in cylinders with a purity of >99.9%.

[0119] The properties of the produced carbonate precast concrete were evaluated and compared to the specifications of either ASTM C1372 for segmental retaining wall units or ASTM C936 for concrete interlocking paving units. These examples are intended only to demonstrate that desirable properties can be obtained with carbonate precast concrete made according to the methods of the present disclosure. The raw materials and processes, as well as the products and uses, are not limited to those shown in the examples.

[0120] [Example 1] Precast concrete sample A was prepared using the mix design shown in Tables 1a and 1b below.

[0121] [Table 1a]

[0122] [Table 1b]

[0123] The raw materials were mixed in a mixer for 5 minutes, molded, and consolidated to the desired density under compression vibration. After demolding, the precast concrete sample A was placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 / min at room temperature for 2 hours to reduce the initial moisture content by 50%. After that, Sample A was completely immersed in water for 1 second to moisten the surface, reducing the weight to 189 g / m 2The surface-moisturized precast concrete was then air-dried at ambient temperature for approximately 5 minutes before being loaded into a pressure chamber for carbonation curing. The weight gain before carbonation curing began was 172 g / m 2 Carbon dioxide gas adjusted to a pressure of 0.1 MPa (15 psi) was introduced into the sealed pressure chamber. The carbonation curing time was 24 hours. After carbonation curing, the following properties of the carbonated precast concrete were evaluated.

[0124] Density, water absorption, and compressive strength - according to ASTM C140 (ASTM, 2018c).

[0125] Freeze-thaw resistant - Complies with ASTM C1262.

[0126] Abrasion resistant - Conforms to ASTM C944.

[0127] For freeze-thaw resistance tests, carbonate precast concrete specimens were partially immersed in a 3% NaCl solution and subjected to 40 freeze-thaw cycles, with loose particles collected and the brine replaced every 10 cycles. While ASTM C1372 requires that segmented concrete retaining wall units have a cumulative mass loss of 1% or less after 100 freeze-thaw cycles, this specification is generally considered to apply to concrete retaining walls partially immersed in water rather than deicing salt solutions. Because concrete undergoes complex physical and chemical changes in deicing salt solutions, faster and more severe deterioration often occurs when deicing salts such as NaCl are present during the freeze-thaw process. Due to this difference, many state transportation departments specify a cumulative mass loss of 1% or less after 40 freeze-thaw cycles in the presence of 3% NaCl as the pass criterion for concrete retaining wall freeze-thaw testing. Generally, if carbonate precast concrete can meet this standard, it should easily meet the requirement of a mass loss of 1% or less after 100 freeze-thaw cycles in water.

[0128] For comparison, a conventional carbonate precast concrete sample, sample B, was prepared using the same mix as sample A, but without the surface moisturizing step. The moisture content after conditioning was reduced by 56%. The physical and mechanical properties and durability after carbonation curing were also evaluated.

[0129] [Table 1c]

[0130] [Table 2]

[0131] The test results for both Samples A and B are summarized in Table 1c. The obtained test results are compared with the specifications of ASTM C1372 (Table 2). Due to the surface strengthening provided by the surface moistening process, Sample A has much better freeze-thaw resistance and abrasion resistance than Sample B. Without the surface moistening step, carbonate precast concrete would have poor freeze-thaw resistance and would not be acceptable for segmental retaining wall applications, even though other properties would meet the requirements of ASTM C1372. Due to the surface strengthening effect, Sample A now meets all of the requirements of ASTM C1372 for concrete segmental retaining walls.

[0132] The moisture content of precast concrete was monitored at different stages of the specimen preparation process, and the results are shown in Figure 5. Approximately 50% of the initial moisture content of the precast concrete had evaporated by the end of the conditioning process. The surface moistening process returned some moisture to the precast concrete, resulting in a moisture reduction of approximately 40% of the initial moisture content. It can be seen that it is not a change in average moisture content but a change in moisture content distribution after the surface moistening process that significantly improves the interior and surface quality or freeze-thaw resistance of carbonated precast concrete. The moisture content was measured along the thickness of conditioned specimen B, and the results are shown in Figure 6. From the top to bottom of specimen B, the moisture content distribution resembles a slightly distorted bell curve. The moisture content at the center was 3%. This moisture content continues to decrease from the center to the outer surface as a result of the fan conditioning process. At the outermost part of specimen B, the moisture content dropped to 1%. This low moisture content leads to incomplete carbonation curing of the precast concrete because the carbonation reaction requires sufficient water as a reactant. This incomplete carbonation curing on the outer surface weakens the surface of the carbonate precast concrete, which is why Sample B had very poor freeze-thaw resistance and abrasion resistance.

[0133] The moisture content distribution along the thickness of the precast concrete was completely corrected by the surface moistening process. As shown in Figure 6, after immersion in water, the difference in moisture content between the center and the outer surface of the precast concrete narrowed to less than 0.5%. This indicates a more uniform moisture distribution along the thickness of the specimen. More importantly, the moisture content of the outer surface of the precast concrete increased from the initial 1% to approximately 4% after the surface moistening process. In fact, a moisture content of approximately 4% is considered optimal for precast concrete to achieve maximum carbonation curing. With sufficient moisture at and near the outer surface of the precast concrete, the carbonation reaction reaches its highest potential. This improved the internal and surface quality of the carbonated precast concrete. This explains why Sample A was significantly more resistant to both freeze-thaw and abrasion deformation than Sample B.

[0134] [Example 2] Precast concrete samples were prepared using the same mix and manufacturing method as in Example 1, but the surface moistening method was changed. After conditioning the conditioned precast concrete samples with a fan for 2 hours, instead of immersing them in water, they were immersed for 1 second in a 5% cement (i.e., Portland cement with the same mixed composition as above) slurry (Sample C) or a 5% hydrated lime slurry (Sample D). The moisture loss due to conditioning was 56% for Sample C and 55% for Sample D, respectively. The weight gain after the surface moistening step was 248 g / m2 for each. 2 and 234g / m 2 The surface-moisturized samples were then air-dried for approximately 6 minutes before being carbonated and cured for 24 hours at a pressure of 0.1 MPa (15 psi). The freeze-thaw and abrasion resistance of the carbonate precast concrete samples were evaluated in the same manner as for sample A, and the results are recorded in Table 3. When compared with the test results for conventional carbonate precast concrete (sample B in Table 1), it is clear that the proposed surface-moisturizing step improved the freeze-thaw and abrasion resistance.

[0135] [Table 3]

[0136] [Example 3] Precast concrete samples were prepared using the formulation and manufacturing method of Example 1, but with an increased surface moistening time. Instead of immersing the conditioned precast concrete samples in water for 1 second, they were immersed in water for 3 seconds (Sample E) or 5 seconds (Sample F). After conditioning in front of a fan for 2 hours, Samples E and F lost 55% and 54%, respectively, of their initial moisture content. After the surface moistening step, each weighed 231 g / m. 2 and 222 g / m 2 The surface moistening step was followed by approximately 6 minutes of air drying, followed by 24 hours of curing with carbon dioxide gas adjusted to 0.1 MPa (15 psi) pressure. The abrasion resistance of the carbonate precast concrete samples was evaluated in the same manner as for Sample A, and the results are recorded in Table 4. When compared with the test results for conventional carbonate precast concrete (Sample B in Table 1), it is clear that the addition of the surface moistening step improved abrasion resistance.

[0137] [Table 4]

[0138] [Example 4] Precast concrete samples were prepared using the same mix and manufacturing method as in Example 1, but the surface moistening method was changed. Instead of immersing in water, the conditioned precast concrete samples were immersed for 1 second in either a 25% cement slurry (Sample G), a 25% hydrated lime slurry (Sample H), or a 25% steel slag slurry (Sample I). After conditioning for 2 hours in front of a fan, Samples G, H, and I lost 47%, 50%, and 51% of their initial moisture content, respectively. After the surface moistening step, the weights were 213, 268, and 258 g / cm², respectively. 2 The surface moistening step was followed by approximately 6 minutes of air drying, followed by curing for 24 hours with carbon dioxide gas at a pressure of 0.1 MPa (15 psi). The abrasion resistance of the carbonate precast concrete samples was evaluated in the same manner as for Sample A, and the results are recorded in Table 5. When compared with the abrasion test results for conventional carbonate precast concrete (Sample B in Table 1), it is clear that the abrasion resistance improved with the addition of the surface moistening step. In addition, the use of steel slag slurry is more effective at improving the abrasion resistance of carbonate precast concrete than the use of cement or hydrated lime slurry.

[0139] [Table 5]

[0140] [Example 5] Precast concrete specimens were prepared using the mix design shown below.

[0141] [Table 6a]

[0142] [Table 6b]

[0143] The raw materials were mixed together, formed, and then consolidated to the desired density. After demolding, the precast concrete samples were placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 The precast concrete specimen J was then conditioned at room temperature for 2 hours at a temperature of 100°C / min. This conditioning process resulted in a 53% reduction in moisture content in both specimens J and K, and a 49% reduction in moisture content in both specimens L and M. The precast concrete specimen J was then completely immersed in water for 5 seconds to moisten the surface, reducing the weight to 245 g / m. 2 Precast concrete samples K, L, and M were completely immersed in 25% cement slurry, 25% steel slag slurry, and 25% hydrated lime slurry for 5 seconds to moisten the surface, and the weights were 283, 295, and 305 g / m, respectively. 2 The surface-moisturized precast concrete samples were then air-dried at ambient temperature for approximately 10 minutes before being loaded into a pressure chamber for carbonation curing. Carbon dioxide gas adjusted to a pressure of 0.1 MPa (15 psi) was introduced into the sealed pressure chamber. The carbonation curing time was 24 hours. After carbonation curing, the density, water absorption, compressive strength, and freeze-thaw resistance of the carbonate precast concrete samples were evaluated according to the method described in Example 1. The results are recorded in Table 6c. Compared with the ASTM C1372 specifications shown in Table 2, Samples J, K, L, and M are all acceptable for retaining wall applications. The surface-moisturizing step provided these samples with significantly higher freeze-thaw resistance than conventional carbonate precast concrete (Sample B in Table 1). Additionally, immersion in cement slurry or water enhances the freeze-thaw resistance of carbonate precast concrete compared to immersion in steel slag or hydrated lime slurry. Furthermore, when comparing the freeze-thaw test results of sample A in Table 1 with those of sample J, it is clear that the freeze-thaw resistance of carbonate precast concrete is higher when it is immersed in water for 5 seconds than when it is immersed in water for only 1 second.

[0144] [Table 6c]

[0145] [Example 6] Precast concrete sample N was prepared using the mix design shown below. The main features of this mix design were the use of a dual binder system (steel slag and ordinary Portland cement) and an increase in the amount of water-repellent admixture (WRA) from the conventional 0.2-0.4% to 2.0%.

[0146] [Table 7a]

[0147] [Table 7b]

[0148] The raw materials were mixed together, formed, and then consolidated to the desired density. After demolding, the precast concrete samples N were placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 The precast concrete samples were then immediately loaded into a pressure chamber for carbonation curing without surface moistening. Carbon dioxide gas adjusted to a pressure of 0.1 MPa (15 psi) was introduced into the sealed pressure chamber. The carbonation curing time was 24 hours. After carbonation curing, the density, water absorption, compressive strength, freeze-thaw resistance, and abrasion resistance of the carbonate precast concrete samples were evaluated according to the method described in Example 1. The results are recorded in Table 7c. Compared with the ASTM C1372 specifications shown in Table 2, Sample N is fully acceptable for retaining wall applications. Sample N has significantly better freeze-thaw resistance than conventional carbonate precast concrete (Sample B in Table 1). It also has better freeze-thaw resistance than carbonate precast concretes with surface moistening treatment (Sample A in Table 1, Samples C and D in Table 3, and Samples J, K, L, and M in Table 6c). Comparing the abrasion test results in Table 7c with the abrasion resistance of Sample B in Table 1, unfortunately, the use of a higher amount of water-repellent admixture does not result in any improvement in abrasion resistance.

[0149] [Table 7c]

[0150] [Example 7] Sample O was prepared using a layered structure with a dual binder composition containing 90% steel slag and 10% ordinary Portland cement. This carbonate precast concrete was designed for concrete paving or similar applications. The thickness of the top layer of Sample O was 6 mm. The thickness of the base layer was 24 to 45 mm and varied depending on the required physical property tests. The mix design for Sample O is shown below.

[0151] [Table 8a]

[0152] [Table 8b]

[0153] The concrete mixes for the top and base layers were prepared separately. After all the raw materials were mixed together, the base layer mix was formed into a mold, flattened, and then the top layer mix was added. The formed material was then consolidated to the desired density under compressive vibration. After demolding, the precast concrete specimens were placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 The precast concrete specimen O was then conditioned at room temperature for 2 hours at a rate of 1 / min, resulting in a 36% reduction in moisture content. The top layer of the precast concrete specimen O was then immersed in water for 3 seconds to moisten the surface, and the weight of the specimen was reduced to 84 g / m. 2 The surface-moisturized precast concrete was then air-dried at ambient temperature for approximately 10 minutes before being loaded into a pressure chamber for carbonation curing. Carbon dioxide gas adjusted to a pressure of 0.1 MPa (15 psi) was introduced into the sealed pressure chamber. The carbonation curing time was 24 hours. After carbonation curing, the following properties of the carbonated precast concrete were evaluated:

[0154] Density, moisture content, water absorption, compressive strength - conforms to ASTM C140.

[0155] Freeze-thaw resistance - Complies with ASTM C1645 using a 3% NaCl solution.

[0156] Abrasion resistant - Conforms to ASTM C944.

[0157] For comparison, a single-layer carbonated precast concrete sample, Sample P, was prepared with a mix design very similar to that of the base layer of Sample O. The only difference was that the mix for Sample P initially had a water-to-binder ratio of 0.11, while the mix for the base layer of Sample O had a water-to-binder ratio of 0.13. Sample P was prepared using the same proportions and manufacturing process as Sample N. Before carbonation curing, the sample was placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 / min at room temperature for 2 hours, resulting in a 26% moisture loss. The properties of Sample P were evaluated using the same test methods used for Sample O. The test results for both Samples O and P are summarized in Table 8c. The test results are then compared to the ASTM C936 specifications for concrete pavers, as shown in Table 9.

[0158] ASTM C936 specifies a sandblasting method (ASTM C418) for measuring the abrasion resistance of concrete pavement materials. However, due to the complexity and limited availability of ASTM C418, an alternative method, ASTM C944, was selected to evaluate the abrasion resistance of samples O and P.

[0159] To determine the abrasion resistance qualities of Samples O and P, commercially available concrete pavers, including four concrete interlocking paving units and five different types of paving slabs, were procured from Home Depot. The abrasion resistance of the purchased concrete pavers was tested according to ASTM C944. The commercial concrete pavers tested with the lowest mass loss due to friction were selected as the benchmark for evaluating the abrasion resistance of Samples O and P. The results are shown in Table 9.

[0160] Sample P, which lacks a layered structure, exhibits very poor abrasion resistance. The mass loss after the abrasion test is 25 times that of Sample O. Sample P also exhibits slightly lower strength than Sample O, but superior water absorption and freeze-thaw resistance. Compared to the best abrasion resistance of commercially available concrete paving materials, as shown in ASTM C936 specifications and Table 9, Sample P is unacceptable for concrete paving applications, primarily due to its inferior abrasion resistance. Conversely, Sample O, due to the surface strengthening effect of its layered structure and surface moisturization, exhibits abrasion resistance nearly equivalent to that of the best commercially available concrete paving materials (Table 9). The water absorption, strength, and freeze-thaw resistance of Sample O comply with the requirements of ASTM C936. It is clear that Sample O is well suited for concrete paving applications.

[0161] [Table 8c]

[0162] [Table 9]

[0163] Table 8c shows that Sample O lost more mass after 28 freeze-thaw cycles than Sample P. This result may be due to the use of a larger amount of water-repellent admixture in Sample P. Although the top surface of Sample O is strengthened by the additional surface moisturizing step, this surface strengthening effect may not be as effective in freeze-thaw resistance as a high amount of water-repellent admixture. Nevertheless, the mass loss due to freeze-thaw for Sample O is within the maximum allowable range in the ASTM standard for concrete paving materials.

[0164] [Example 8] Sample Q was prepared using a layered structure, with steel slag as the only binder. This carbonate precast concrete was designed for concrete paving or similar applications. The thickness of the top layer of Sample Q was 6 mm. The thickness of the base layer of Sample Q ranged from 24 to 45 mm and varied depending on the required physical property testing. The precast concrete samples were prepared with the mix design shown below.

[0165] [Table 10a]

[0166] [Table 10b]

[0167] The concrete mixes for the top and base layers were prepared separately. After all the raw materials were mixed together, the base layer mix was formed into a mold, flattened, and then the top layer mix was added. The formed material was then consolidated to the desired density under compressive vibration. After demolding, the precast concrete sample Q was placed in front of a commercial electric fan with an air flow rate of 43 m / s. 3 / min at room temperature for 2 hours, reducing the initial moisture content by 41%. The top layer of the precast concrete was then immersed in water for 3 seconds to moisten the surface, and the weight of the concrete decreased to 103 g / m2 due to water absorption. 2 The surface-moisturized precast concrete was then air-dried at ambient temperature for approximately 10 minutes before being loaded into a pressure chamber for carbonation curing. Carbon dioxide gas adjusted to a pressure of 0.1 MPa (15 psi) was introduced into the sealed pressure chamber. The carbonation curing time was 24 hours. After carbonation curing, the physical-mechanical properties and durability of the carbonated precast concrete were evaluated as described in Example 7.

[0168] For comparison, a single-layer carbonate precast concrete sample R was prepared using the same composition as the base layer of sample Q, but without the surface moisturizing step. 3 After conditioning at 1000 kJ / min for 2 hours at room temperature, sample R had a 47% reduction in initial moisture content. Sample R was also characterized. Test results for both samples Q and R are summarized in Table 10c.

[0169] Sample R, which lacks a layered structure and a surface moistening step, exhibits very poor abrasion resistance. Its mass loss after the abrasion test is nine times that of Sample Q. Sample R also exhibits higher water absorption than Sample Q, but its strength is comparable to that of Sample Q and its freeze-thaw resistance is slightly better. Compared to the best abrasion resistance of commercial concrete pavers as shown in ASTM C936 specifications and Table 9, Sample R is unacceptable for concrete paving applications, primarily due to its poor abrasion resistance. Conversely, Sample Q, due to the surface strengthening effect of the layered structure and surface moistening, exhibits better abrasion resistance than the best commercial concrete pavers (Table 9). The water absorption, strength, and freeze-thaw resistance of Sample Q meet the requirements of ASTM C936. It is clear that Sample Q is well suited for concrete paving applications.

[0170] [Table 10c]

[0171] [References] All of the following references are incorporated herein by reference in their entirety. ACI Committee. (2008). 201.2R-08: Guide to Durable Concrete Durability. Michigan, US: American Concrete Institute. ASTM. (2017). Standard Specification for Dry-Cast Segmental Retaining Wall Units (Vol. ASTM C1372-17). West Conshohocken, PA: ASTM International. ASTM. (2018a). Standard Specification for Solid Concrete Interlocking Paving Units (Vol. ASTM C936 / C936M-18). West Conshohocken, PA: ASTM International. ASTM. (2018b). Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry-Cast Segmental Retaining Wall Units and Related Concrete Units (Vol. ASTM C1262 / C1262M-18). West Conshohocken, PA: ASTM International. ASTM. (2018c). Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units (Vol. ASTM C140 / C140M-18). West Conshohocken, PA: ASTM International. ASTM. (2019a). Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method (Vol. ASTM C944 / C944M-19). West Conshohocken, PA: ASTM International. ASTM. (2019b). Standard Test Method for Freeze-thaw and De-icing Salt Durability of Solid Concrete Interlocking Paving Units (Vol. ASTM C1645-19). West Conshohocken, PA: ASTM International. CSA. (2005). CSA A231.2 Precast concrete paving slabs / Precast concrete pavers. Toronto, Ontario: CSA. Environmental Protection Agency. (1995). AP 42 - Compilation of Air Pollutant Emission Factors, 5th Edition, Volume 1. Research Triangle Park, NC: U. S. Environmental Protection Agency. Patel, H., Bland, C., & Poole, A. (1995). The microstructure of concrete cured at elevated temperatures. Cement and Concrete Research, 25(3), 485-490. Zhang, D., Ghouleh, Z., & Shao, Y. (2017). Review on carbonation curing of cement-based materials. Journal of CO2 Utilization, 21, 119-131. doi:10.1016 / j.jcou.2017.07.003

Claims

1. 1. A method for producing a carbonate precast concrete product, the method comprising: Obtaining a base layer mixture including at least one binder, aggregate, water, and a water-repellent admixture in an amount of at least 0.1% by weight of the binder, and a top layer mixture including at least one second binder, the content of which is equal to or greater than the content of the at least one binder in the base layer mixture, a second aggregate, and water; molding the base layer mixture and the top layer mixture into a multi-layered intermediate body having a top layer and a base layer that is thicker than the top layer; demolding the multilayer molded intermediate to obtain a demolded multilayer intermediate, the demolded multilayer intermediate having a first water-to-binder ratio for the base layer and a second water-to-binder ratio for the top layer that is equal to or greater than the first water-to-binder ratio; conditioning the demolded multilayer intermediate to provide a conditioned multilayer article, wherein a top layer has a third water-to-binder ratio that is less than the second water-to-binder ratio; moistening at least one surface of the top layer of the conditioned multilayer article with an aqueous medium to increase the weight of the conditioned multilayer article and the top layer to provide a moisturized multilayer product, wherein a fourth water-to-binder ratio of a first portion of the moisturized multilayer product including the top layer is greater than the third water-to-binder ratio; and curing the moisturizing multi-layer product with carbon dioxide to obtain the carbonated precast concrete product.

2. The step of moisturizing the at least one surface of the outer layer comprises: Immersing a portion or all of the at least one surface in a water-containing liquid; spraying the aqueous medium on the at least one surface; or 10. The method of claim 1, comprising applying the aqueous medium using an application method selected from: applying the aqueous medium to the at least one surface with a roller device.

3. The step of moisturizing the at least one surface of the conditioned multi-layer article comprises moistening the conditioned multi-layer article until the weight of the conditioned multi-layer article is at least 10 g / m ２ The method of claim 1 or 2, further comprising moistening the at least one surface until the surface is moistened.

4. moisturizing the at least one surface of the top layer includes applying the aqueous medium using a roller device to the at least one surface; The method of claim 1 , wherein applying the aqueous medium comprises applying water, an aqueous solution, and / or an aqueous slurry onto the at least one surface.

5. moisturizing the at least one surface of the top layer includes applying the aqueous medium using a roller device to the at least one surface; The method of claim 1 , wherein the step of applying the aqueous medium comprises applying the aqueous medium at 15 to 25° C.

6. 6. The method of claim 1, wherein conditioning the demolded multilayer intermediate comprises conditioning the demolded multilayer intermediate until 20% to 70% by weight of the initial moisture content of the demolded multilayer intermediate is removed.

7. obtaining the top layer mixture obtaining a dry portion and a liquid portion, the dry portion comprising at least one aggregate material and at least one binder material, and the liquid portion comprising water; and b. mixing the dry portion and the liquid portion to obtain the top layer mixture.

8. 8. The method of claim 7, including the step of mixing an additive with the dry portion and the liquid portion.

9. The method of claim 8 including the step of mixing microfibers with the dryer portion.

10. 10. The method of any one of claims 1 to 9, wherein obtaining the top layer mixture comprises obtaining the top layer mixture with the at least one bonding material, wherein the at least one bonding material is non-cementitious.

11. 11. The method according to claim 1, wherein obtaining the top layer mixture comprises obtaining the top layer mixture having the at least one binder comprising steel slag and cement, wherein a weight ratio of steel slag to cement is 1:20 to 20:

1.

12. 12. The method according to claim 1, wherein obtaining the top layer mixture comprises obtaining the top layer mixture having the aggregate, wherein a weight ratio of the aggregate to a total weight of the top layer mixture is between 0.3 and 0.

8.

13. 13. The method of any one of claims 1 to 12, wherein obtaining the top layer mixture comprises obtaining the top layer mixture in which the weight ratio of additive to the total weight of the top layer mixture is between 0.005 and 0.

010.

14. The method of any one of claims 1 to 13, wherein shaping the top layer mixture comprises forming the top layer mixture into a formed intermediate.

15. The method of claim 14 , wherein shaping the top layer mixture comprises consolidating the shaped intermediate to provide a multi-layer shaped intermediate.

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