Method for producing cement hardened body with co2 fixed

Abstract

To provide a method for fixing more CO2 to a cement hardened body than before.SOLUTION: A method for producing a cement hardened body with CO2 fixed according to the present invention includes a water supplying step of supplying water to a cement hardened body to obtain a water-containing cement hardened body, and a curing step of curing the water-containing cement hardened body in an air-curing atmosphere under a high-concentration CO2 atmosphere. According to the present invention, the fixation efficiency of CO2 is improved compared to the case where CO2 is fixed in the cement hardened body from CO2-dissolved water, and since it is an in-air curing, calcium in the cement hardened body basically does not elute out of the cement hardened body during curing.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing a cement hardened body with CO2 fixed therein, and more particularly, to a method for producing a cement hardened body with CO2 fixed therein, which is obtained by fixing ambient CO2 in the cement hardened body.

Background Art

[0002] In recent years, in various industries, efforts have been made to realize a decarbonized and low-load recycling society. Among them, in the cement and concrete industries, since the greenhouse gas emissions from the entire cement and concrete industries account for about 7% or more of the anthropogenic emissions, reduction thereof has been demanded.

[0003] As an effort to reduce greenhouse gas emissions in the cement and concrete industries, the utilization of low-carbon concrete that reduces the amount of cement used by using by-products such as blast furnace slag fine powder and fly ash as cement substitutes has been promoted. Furthermore, in recent years, the development of technologies for forcibly immobilizing CO2 in concrete and cement hardened bodies, such as CO2-SUICOM (registered trademark), has been actively carried out, and the development of carbon pool concrete (hereinafter referred to as CP concrete), which is an effort to achieve both resource circulation and carbon neutrality by fixing CO2 in recycled materials derived from concrete such as recycled aggregates, has been promoted. CP concrete is concrete obtained by fixing CO2 generated in the cement firing process and the like in the hydration products of cement and using it as recycled aggregate.

[0004] Since water is required for fixing CO2 in the hydrate of cement, conventionally, it has been proposed to fix CO2 in the hydration products of cement with CO2-dissolved water.

[0005] Patent Document 1 discloses a carbonation curing method for a concrete water storage structure in which carbonated water is poured into the water storage area of a concrete water storage structure having a water storage area capable of storing water. According to the method of Patent Document 1, the inner wall surface of the water storage area is carbonated, and the long-term durability of the concrete water storage structure can be improved.

[0006] Patent Document 2 discloses a method of introducing carbonated bubble water into a substance having a cement hardened body by immersing the substance having a cement hardened body in carbonated bubble water or applying it with carbonated bubble water. According to the method of Patent Document 2, CO2 in the carbonated bubble water reacts with calcium hydroxide present in the cement hardened body, and calcium carbonate is formed in the cement hardened body, thereby improving properties such as the density of the cement hardened body.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, if the cement hardened body is carbonated with CO2 dissolved water as in Patent Documents 1 and 2, the amount of CO2 that can dissolve in water is about 1.71 g per 1 L of water under the environment of 20°C and 1 atm, and calcium carbonate is formed only on the surface of the cement hardened body in contact with water. Therefore, the fixation of CO2 is limited.

[0009] In addition, since the pH decreases when CO2 is dissolved in water, calcium in the cement hardened body is likely to elute into the CO2 dissolved water, and there is also a problem that carbonation inside the cement hardened body is difficult to proceed. Therefore, a method for efficiently carbonating a cement hardened body has been demanded.

[0010] An object of the invention of the present application made in view of the above problems is to provide a method capable of fixing more CO2 with a cement hardened body than in the past.

Means for Solving the Problems

[0011] The method for producing a CO2-fixed cement hardened body according to the invention described in claim 1 for achieving the above object comprises a water supply step of supplying water to the cement hardened body to obtain a water-containing cement hardened body, and a curing step of performing air curing on the water-containing cement hardened body in a high-concentration CO2 atmosphere.

[0012] According to this invention, the CO2 fixation efficiency is improved as compared with the case of fixing CO2 from CO2-dissolved water to the cement hardened body, and since it is air curing, basically calcium in the cement hardened body does not elute outside the cement hardened body during curing.

[0013] Further, it is preferable that the high-concentration CO2 atmosphere is an atmosphere containing CO2 at a concentration of 10% by volume or more. This makes it possible to further improve the CO2 fixation efficiency to the cement hardened body.

[0014] Furthermore, it is preferable that the water supplied in the water supply step is fine bubble water containing fine bubbles having a bubble diameter of less than 100 μm. By using fine bubble water, the surface tension of the water is reduced, and the necessary water can penetrate into the inside of the cement hardened body. Therefore, by using fine bubble water as the water supplied in the water supply step, it becomes possible to fix CO2 to a deeper part of the cement hardened body.

[0015] Moreover, it is preferable that the water supplied in the water supply step has a total carbonate concentration of 1 mg / L or more. By the water supplied in the water supply step having a total carbonate concentration of 1 mg / L or more, in addition to the CO2 in the atmosphere supplied in the curing step, the CO2 in the water supplied in the water supply step is also fixed to the cement hardened body, and the CO2 fixation efficiency to the cement hardened body can be further improved.

[0016] Also, after the curing process, it is preferable to repeat the water supply process and the curing process at least once or more. By repeating the water supply process and the curing process in this way, a cycle of supplying water necessary for CO2 fixation, CO2 fixation in the presence of water (curing process), resupplying the evaporated water, and CO2 fixation in the presence of water is repeated, and CO2 can be efficiently fixed in the cement hardened body.

Advantages of the Invention

[0017] According to the present invention, compared with the case of fixing CO2 from CO2-dissolved water to a cement hardened body, the CO2 fixation efficiency is improved, and since it is air curing, basically calcium in the cement hardened body does not elute outside the cement hardened body during curing.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0019] <Method for Manufacturing a Cement Hardened Body Having Fixed CO2> Figure 1 is a flowchart showing a method for manufacturing a cement hardened body having fixed CO2 according to the present invention. As shown in the figure, the method for manufacturing a cement hardened body having fixed CO2 according to the present invention includes a water supply step (S110) and a curing step (S120).

[0020] [Water Supply Step (S110)] In this step, water is supplied to the cement hardened body to obtain a water-containing cement hardened body.

[0021] The cement hardened body is a hardened body of a cement composition containing cement. The cement composition may optionally contain admixtures such as fine aggregates (such as sand), coarse aggregates (such as gravel), AE agents, water reducers (AE water reducers, water reducers, high-performance AE water reducers, etc.), fluidizing agents, setting and hardening regulators, flash set retarders, rust inhibitors, waterproof agents, etc.

[0022] Examples of the cement hardened body include those obtained by newly preparing, placing, curing, and hardening a cement composition; concrete gall generated from ready-mixed concrete that remained unused or left over during placement (so-called residual concrete) or ready-mixed concrete returned without unloading from an agitator truck (so-called returned concrete); disintegrated concrete blocks; sludge (i.e., concrete sludge) discharged from the washing water for washing an agitator truck after transporting fresh concrete.

[0023] Among them, from the perspective of recycling materials that have been discarded as industrial waste as CP concrete, it is preferable that the cement hardened body is selected from the group consisting of the above concrete gall, disintegrated concrete blocks, and concrete sludge and mixtures thereof.

[0024] Regarding the shape of the cement hardened body, from the perspective of increasing the contact area between water and the cement hardened body, it is preferable that the surface area is large. Therefore, it is preferably a cement hardened body with a coarse grain ratio (by the test method specified in JISA5023:2018 Appendix A for recycled aggregate concrete L) of 8 or less, and more preferably 7 or less. On the other hand, from the perspective of recycling the CO2-fixed cement hardened body produced by the present invention as an aggregate, the above coarse grain ratio of the cement hardened body as the starting material is preferably 1 or more, and more preferably 2 or more.

[0025] The water supplied may be ordinary tap water. The fixation of CO2 to the cement hardened body, that is, the carbonation of the cement hardened body, is based on the following reaction formulas (I) and (II) CO2 + H2O + Ca(OH)2 → CO32- + 2H + + Ca 2+ + 2OH - (I) CO3 2- + 2H + + Ca 2+ + 2OH - →CaCO3↓ + 2H2O (II) As shown in , since carbon dioxide and calcium hydroxide in the cement hardened body dissolve in water and ionize, water supply is required.

[0026] Moreover, it is preferable that the water supplied in this step is fine bubble water containing fine bubbles with a bubble diameter of less than 100 μm.

[0027] Since the water supplied in this step is fine bubble water, the surface tension of the water is reduced, and the necessary water can penetrate into the cement hardened body. Therefore, it is possible to fix CO2 deeper into the cement hardened body.

[0028] The definition of the above fine bubbles shall be based on ISO 20480-1:2017.

[0029] Furthermore, it is preferable that the water supplied in this step has a total carbonate concentration of 50 mg / L or more, more preferably 100 mg / L or more, even more preferably 300 mg / L or more, and particularly preferably 500 mg / L or more.

[0030] The total carbonate concentration is a concentration index based on JIS K 1012:2019, Part 1, Test Methods for Industrial Water and Factory Wastewater, and is the concentration (mg / L) representing the total carbonate in water (the total amount of dissolved CO2 and carbonate substances) as CO2. The following formula (III)

Number

[0031] Since the water supplied in this step has a total carbonic acid concentration of 50 mg / L or more, in addition to the CO2 in the atmosphere supplied in the curing step, the CO2 in the water supplied in the water supply step can also be fixed in the cement hardened body, so that the CO2 fixation efficiency in the cement hardened body can be further improved.

[0032] In addition, when the water supplied in this step has a total carbonic acid concentration of 50 mg / L or more and is fine bubble water, it is preferable that the fine bubbles are generated by a method other than the ultrasonic method. This is because when fine bubbles are generated by the ultrasonic method, the CO2 dissolved in the water will escape due to the ultrasonic waves.

[0033] The water supply can be performed, for example, by spraying water onto the cement hardened body, immersing the cement hardened body in water, or pouring water over the cement hardened body.

[0034] The water supply amount is, for example, 0.5 mass% or more and 10 mass% or less with respect to the total mass of the cement hardened body, preferably 1 mass% or more and 6 mass% or less with respect to the total mass of the cement hardened body, and particularly preferably 2 mass% or more and 4 mass% or less. If the water supply amount is less than 0.5 mass% with respect to the total mass of the cement hardened body, the ionic dissociation of carbon dioxide and calcium hydroxide in the cement hardened body will not proceed, so the fixation of CO2 will not proceed. If the water supply amount exceeds 10 mass% with respect to the total mass of the cement hardened body, the water supply amount will be excessive, resulting in the formation of calcium carbonate only on the surface of the cement hardened body, the dissolution of CO2 into the water adhering to the surface of the cement hardened body and the accompanying pH decrease, and further the elution of calcium in the cement hardened body into the CO2 dissolved water, reducing the CO2 fixation efficiency in the cement hardened body.

[0035] Therefore, it is preferable that water is supplied in a small amount, and the water supply is preferably performed by spraying water onto the cement hardened body.

[0036] Furthermore, when the supplied cement hardened body is already in a wet state, it can be considered that the water supply step (S110) has been carried out and the curing step (S120) can be entered. Even in such a case, when the water supply step (S110) and the curing step (S120) are further repeated as described later after the curing step (S120), since the cement hardened body becomes dry after the curing step (S120), it goes without saying that the repeated water supply step (S110) and curing step (S120) need to be actually executed.

[0037] The supply of water to the cement hardened body is preferably carried out while stirring the cement hardened body so that water is uniformly supplied to the cement hardened body (the above is the water supply step (S110)).

[0038] [Curing step (S120)] In this step, the water-containing cement hardened body is subjected to air curing in a high-concentration CO2 atmosphere.

[0039] In the present invention, the high-concentration CO2 atmosphere is an atmosphere containing CO2 at a concentration of 8% by volume or more, preferably an atmosphere containing CO2 at a concentration of 10% by volume or more, more preferably an atmosphere containing CO2 at a concentration of 12% by volume or more, and particularly preferably an atmosphere containing CO2 at a concentration of 15% by volume or more. As such a high-concentration CO2 atmosphere, for example, exhaust gas containing high-concentration CO2 generated by firing limestone during cement production, or exhaust gas discharged from other lime production factories, cleaning factories, thermal power plants, etc. can be utilized.

[0040] The period of air curing is determined by the fact that CO2 fixation does not progress without water, and thus it is until the water on the surface of the water-containing cement hardened body evaporates. Therefore, although the amount of water supplied in the water supply step (S110) also affects this period, when water is supplied to the cement hardened body in an amount of 1% by mass or more and 6% by mass or less based on the total mass of the cement hardened body, under the conditions of a temperature of 20 ° C and a humidity of 40%, it is 8 hours or more and 48 hours or less, preferably 12 hours or more and 36 hours or less (above is the curing step (S120)).

[0041] Further, after the curing step (S120), it is preferable to repeat the water supply step (S110) and the curing step (S120) at least once, preferably two or more times. By repeating the water supply step (S110) and the curing step (S120) in this way, it becomes possible to efficiently fix CO2 in the atmosphere to the cement hardened body while avoiding the problem of excessive water supply amount.

[0042] As described above, according to the method for producing a cement hardened body having CO2 fixed therein of the present invention, the CO2 fixation efficiency is improved as compared with the case of fixing CO2 from CO2-dissolved water to the cement hardened body, and since it is air curing, basically calcium in the cement hardened body does not elute outside the cement hardened body during curing.

[0043] Therefore, for example, by using industrial wastes such as waste concrete and recycled concrete as the cement hardened body, supplying the CO2 generated in the cement firing process as a high-concentration CO2 atmosphere, and using the obtained cement hardened body having CO2 fixed therein as recycled aggregate for concrete or mortar production, it becomes possible to greatly improve the efficiency of carbon recycling in the cement-concrete industry.

[0044] Furthermore, the cement hardened body having CO2 fixed therein obtained by the method for producing a cement hardened body having CO2 fixed therein of the present invention can be used as asphalt mixture, roadbed material, etc. as recycled crushed stone in addition to the above recycled aggregate.

Example

[0045] Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples only.

[0046] <Test 1. CO2 fixation in recycled aggregates by combination of water supply and air curing> i. Spraying and curing with tap water For specimens made of recycled coarse aggregates produced at a waste processing plant, as shown in Figure 2, tap water was sprayed in an amount of 3% by mass of the total mass of the specimens, and the specimens sprayed with this tap water were cured for 1 day in a high-concentration CO2 atmosphere (CO2 concentration = 17% by volume). On the second day, the same amount of tap water as on the first day was sprayed, and the specimens were further cured for 1 day in the high-concentration CO2 atmosphere. On the third day, the same amount of tap water as on the first day was again sprayed, and the specimens were further cured for 1 day in the high-concentration CO2 atmosphere. After the curing for a total of 3 days, the CO2 content of the specimens was measured by thermal analysis (TG-DTA). This section is designated as Example 1.

[0047] Next, a section in which the test was conducted under exactly the same conditions as in Example 1 except that tap water was sprayed in an amount of 6% by mass of the total mass of the specimens made of recycled coarse aggregates was designated as Example 2.

[0048] Furthermore, sections in which tap water was not sprayed on specimens made of recycled coarse aggregates and the specimens were cured for 1 day, 2 days, and 3 days, respectively, in a high-concentration CO2 atmosphere with a CO2 concentration of 17% by volume were designated as Comparative Example 1, Comparative Example 2, and Comparative Example 3, respectively.

[0049] The physical property values of the recycled coarse aggregates used in this test, obtained by the test methods specified in JISA5023:2018 Appendix A for Recycled Aggregate Concrete L, are as follows.

[0050] Apparent dry density 2.28 (g / cm 3 ) Absolute dry density 2.09 (g / cm 3 ) Water absorption rate 9.0 (%) Fine particle content 0.5 (%) Coarse particle ratio 6.79 Adhesion paste amount 32.2(%)

[0051] ii. Measurement of CO2 content in the specimen by thermal analysis (TG-DTA) ii-1. Preparation of the measurement sample Using about 2 kg of the recycled coarse aggregate sample reduced by the quartering method or sample divider so that the material composition ratio of the measurement sample remains unchanged, the sample was prepared according to the following procedure.

[0052] 1 Pulverize to about 1 mm or less 2 Immerse in acetone for 6 hours or more to remove free water 3 Leave in a vacuum desiccator for 6 hours or more 4 Fine pulverize with a ball mill to about 100 μm 5 Vacuum freeze-dry and store in a sealed container until analysis

[0053] ii-2. Measurement Weigh about 50 - 70 mg of the measurement sample prepared in ii-1. above into a platinum cell, and using a differential thermal-thermogravimetric simultaneous analyzer TG-DTA (Hitachi High-Tech Science TG / DTA7200), while flowing nitrogen gas at a flow rate of 70 mL / min, the measurement was carried out at a heating rate of 20 °C / min. From the relationship curve between the temperature and the mass loss rate obtained by the measurement, assuming that the range of mass loss of carbon dioxide due to the decomposition of calcium carbonate is from 600 °C to 850 °C, the CO2 content rate (%) was determined from the mass loss amount in that temperature range.

[0054] ii-3. Calculation method of CO2 fixation amount The CO2 fixation amount, as the CO2 fixed per ton of recycled coarse aggregate, was calculated by formula (IV)

Equation

[0055] Note that by using the measured values before and after immobilizing CO2 in the carbonation test, the influence of the amount of CO2 fixed from natural sources is excluded. The test results are shown in Figure 3.

[0056] As shown in the figure, from the comparison of Comparative Examples 1 to 3, it can be seen that by performing air curing in a high-concentration CO2 atmosphere, the amount of CO2 fixed in the recycled coarse aggregate increased. Also, from the comparison between Example 1 and Comparative Example 3, it was found that the combination of water supply and subsequent air curing in a high-concentration CO2 atmosphere significantly increased the amount of CO2 fixed in the recycled coarse aggregate.

[0057] Furthermore, from the comparison between Example 1 and Example 2, since the CO2 fixation amount decreased when the water supply was excessive, even when curing was performed in a high-concentration CO2 atmosphere, if the water supply becomes excessive, on the surface of the cement hardened body, the same phenomena as the carbonation of the cement hardened body by conventional CO2-dissolved water occur, that is, the formation of calcium carbonate only on the surface of the cement hardened body, the dissolution of CO2 into the water adhering to the surface of the cement hardened body and the accompanying pH decrease, and furthermore, the elution of calcium in the cement hardened body into the CO2-dissolved water, and it is presumed that the CO2 fixation efficiency of the cement hardened body decreased.

[0058] <Test 2. CO2 Fixation in Recycled Aggregate by Air Curing> According to Comparative Examples 1 to 3 of Test 1, since the amount of CO2 fixed in the recycled aggregate increased as the air curing period extended without water supply, a test was conducted to explore the possibility of improving the CO2 fixation efficiency by supplying water as water vapor into the air curing atmosphere in addition to spraying water prior to air curing.

[0059] i. Air Curing Conditions For specimens made of the same recycled coarse aggregate as used in Test 1 above, they were cured for 3 days under the conditions of a high-concentration CO2 atmosphere (CO2 concentration = 17% by volume), humidity 40%, and temperature 20°C, and the CO2 content of the specimens was measured by thermal analysis (TG-DTA). This category is designated as Comparative Example 4.

[0060] Next, sections where tests were conducted under exactly the same conditions as Comparative Example 4 except that the humidity was changed to 60%, 80%, and 95% respectively were designated as Comparative Example 5, Comparative Example 6, and Comparative Example 7, respectively.

[0061] ii. Measurement of CO2 content in the test specimen by thermal analysis (TG-DTA) For the sections of Comparative Examples 4 to 7, samples were prepared in the same manner as in Test 1, and the CO2 content in the test specimen was measured by thermal analysis (TG-DTA). The results are shown in Fig. 4.

[0062] As shown in the figure, when water was not supplied prior to air curing, the CO2 fixation efficiency in the cement hardened body was maximized when air curing was carried out under the condition of 80% humidity.

[0063] However, even in Comparative Example 6 where the CO2 fixation efficiency was maximized, the CO2 fixation amount was small compared to Examples 1 to 2 where water was directly supplied to the recycled aggregate. Therefore, from the viewpoint of improving the CO2 fixation efficiency in the cement hardened body, it can be seen that spraying water on the cement hardened body prior to air curing in a high-concentration CO2 atmosphere is more effective than supplying water as water vapor into the air curing atmosphere.

Claims

1. A water supply step of supplying water to a cement hardened body to obtain a water-containing cement hardened body, For the cement hardened body containing water, high-concentration CO ２ A curing process of performing air curing in an atmosphere, and A method for manufacturing a cement hardened body characterized by having CO ２ fixed thereto.

2. High-concentration CO ２ The atmosphere is an atmosphere containing CO ２ at a concentration of 10% by volume or more. The method for producing a cement hardened body fixing CO according to claim 1 is characterized in that ２ is as described above.

3. The method for producing a cement hardened body according to claim 1, characterized in that the water supplied in the water supply step is fine bubble water containing fine bubbles with a bubble diameter of less than 100 μm. ２ for fixing CO.

4. The method for producing a cement hardened body fixing CO according to claim 1, wherein the water supplied in the water supply step has a total carbonic acid concentration of 100 mg / L or more. ２ ​

5. After the curing process, the method for manufacturing a cement hardened body according to any one of claims 1 to 4, characterized in that the water supply process and the curing process are repeated at least once more. ２ with CO fixed.

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