Method of producing cement recycled material

Abstract

To provide a method of producing a cement recycled material, capable of increasing the content of calcium carbonate.SOLUTION: The method of producing cement recycled material comprises the steps of: mixing cement-containing waste and water to prepare a first slurry; mixing the first slurry for two hours or more; feeding exhaust including carbon dioxide into the first slurry while agitating the first slurry, to immobilize carbon dioxide included in the exhaust as calcium carbonate; and obtaining a second slurry including the calcium carbonate, formed in the immobilization step.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing recycled cement material. [Background technology]

[0002] Currently, cement-containing waste is solidified and disposed of as waste, but it contains a large amount of calcium, and there have been attempts to utilize this calcium as a source of counter cations for carbonation. For example, Patent Document 1 discloses a method for utilizing cement-containing waste, which is characterized by comprising a carbon fixation step in which waste containing cement is immersed in water to form a slurry-like waste material, and exhaust gas containing carbon dioxide discharged from a cement manufacturing facility is supplied into the waste material slurry to cause a reaction between the calcium components dissolved in the waste material slurry and the carbon dioxide in the exhaust gas, thereby fixing the carbon dioxide as calcium carbonate, and a recycling step in which solids are separated from the waste material slurry treated in the carbon fixation step and used as a cement raw material. Furthermore, Patent Document 2 discloses a method for producing a cement admixture, which includes a slurrying step in which fresh concrete sludge or fine waste concrete powder is heat-treated and then mixed with water to form a slurry, and a carbonation step in which carbon dioxide gas is passed through the slurry to carbonate it. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-117636 A [Patent Document 2] JP 2021-138574 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, neither Patent Document 1 nor Patent Document 2 discloses data on the amount of calcium carbonate produced by carbonation with carbon dioxide (i.e., the amount of fixed carbon dioxide), and sufficient consideration has not been given to increasing the amount of calcium carbonate. Furthermore, in the method described in Patent Document 2, the slurry is subjected to a heat treatment, for example at 100°C to 300°C, before passing carbon dioxide gas through it. This heat treatment results in the emission of carbon dioxide gas, and there is a problem in that the net amount of carbon dioxide fixation (the amount of carbon dioxide supplied minus the amount of carbon dioxide emitted by the fixation treatment) decreases.

[0005] An object of the present invention is to provide a method for producing a recycled cement material capable of increasing the content of calcium carbonate. [Means for solving the problem]

[0006] [1] A method for producing a cement recycled material, comprising: a step of mixing cement-containing waste with water to produce a first slurry; a step of stirring the first slurry for two hours or more; a step of supplying exhaust gas containing carbon dioxide into the first slurry while stirring the first slurry, thereby immobilizing the carbon dioxide contained in the exhaust gas as calcium carbonate; and a step of obtaining a second slurry produced in the immobilization step and containing the calcium carbonate.

[0007] [2] The method for producing cement recycled material according to [1], further comprising a step of separating the second slurry into a solid component containing the calcium carbonate and a liquid.

[0008] [3] Further comprising a step of drying the separated solid matter; The method for producing recycled cement material described in [2] above.

[0009] [4] In the stirring step, the first slurry is stirred for 5 hours or more. The method for producing cement recycled material according to any one of [1] to [3] above.

[0010] [5] In the immobilization step, the supply rate of the exhaust gas into the first slurry is 1 L / min or more per 1 kg of solid content (bone dry basis) in the cement-containing waste, and the supply time of the exhaust gas into the first slurry is 2 hours or more. The method for producing cement recycled material according to any one of [1] to [4] above.

[0011] [6] In the immobilization step, the exhaust gas is supplied to the first slurry by bubbling. The method for producing cement recycled material according to any one of [1] to [5] above.

[0012] [7] In the step of generating the first slurry, the ratio of the solid content (on an oven-dry basis) in the cement-containing waste to the water (the solid content (on an oven-dry basis) in the cement-containing waste / the water) is, in mass ratio, 0.01 or more and 0.43 or less; The method for producing cement recycled material according to any one of [1] to [6] above.

[0013] [8] The exhaust gas is a combustion exhaust gas. The method for producing cement recycled material according to any one of [1] to [7] above.

[0014] [9] The recycled cement material is used for any one of the following purposes: concrete products, road materials, embankment materials, blocks, mine drainage neutralizers, disinfectants for livestock farming, fillers for additives to plastics, fillers for additives to rubber, and fillers for additives to paints. The method for producing cement recycled material according to any one of [1] to [8] above. Effect of the Invention

[0015] According to one aspect of the present invention, it is possible to provide a method for producing recycled cement material that can increase the content of calcium carbonate. [Brief description of the drawings]

[0016] [Figure 1] Graph showing the relationship between stirring time and the proportion of CaCO3 in the product. [Diagram 2] Graph showing the relationship between stirring time and the proportion of CaCO3 in the product. [Diagram 3] Graph showing the relationship between CO2 supply time and the ratio of CaCO3 in the product. [Figure 4] Graph showing the relationship between stirring time and the proportion of CaCO3 in the product. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In this specification, a numerical range expressed using "to" means a range that includes the numerical value before "to" as the lower limit and the numerical value after "to" as the upper limit. In this specification, mass percent concentration (unit: mass % (mass%)) and weight percent concentration (unit: weight % (wt%)) are the same value.

[0018] [First embodiment] The manufacturing method of the cement recycled material according to the first embodiment (hereinafter also referred to as the manufacturing method of the first embodiment) includes a step of mixing cement-containing waste with water to generate a first slurry (hereinafter also referred to as a first slurry generation step), a step of stirring the first slurry for two hours or more (hereinafter also referred to as a stirring step), a step of supplying exhaust gas containing carbon dioxide into the first slurry while stirring the first slurry, and immobilizing the carbon dioxide contained in the exhaust gas as the calcium carbonate (hereinafter also referred to as an immobilization step), and a step of obtaining a second slurry generated in the immobilization step and containing the calcium carbonate.

[0019] The mechanism by which carbon dioxide in the exhaust gas is fixed as calcium carbonate in the fixation step of this embodiment will be described. Carbon dioxide in the exhaust gas dissolves and ionizes when it comes into contact with the water in the first slurry. The ionized carbon dioxide combines with calcium ions dissolved from the cement-containing waste to produce calcium carbonate (CaO + CO2 → CaCO3). The formation of calcium carbonate reduces the calcium ions in the water, lowering the pH of the slurry. Meanwhile, calcium hydroxide produced by the hydration reaction of cement with water dissolves in water within its solubility range. In other words, calcium dissolves from the cement-containing waste to make up for the calcium ions lost in the water due to the formation of calcium carbonate, up to the solubility of calcium hydroxide. Therefore, in theory, calcium carbonate will be continuously produced by continuing to supply carbon dioxide to the first slurry. The pH of the slurry is considered to be determined mainly by the "calcium ion concentration in water, which is determined by the dissolution rate of calcium hydroxide" and the "contact efficiency between carbon dioxide and calcium ions, which is determined by the carbon dioxide supply rate, bubble diameter, etc., as parameters."

[0020] The present inventors have found that in a carbon dioxide fixation technique, after mixing cement-containing waste with water to generate a first slurry, the first slurry is stirred for 2 hours or more, and then exhaust gas is supplied into the first slurry, thereby obtaining a second slurry having an increased calcium carbonate content, compared to when the first slurry is not stirred or when the first slurry is stirred for less than 2 hours (for example, 1 hour). It was confirmed that the solid content separated from the second slurry and the substance obtained by drying the solid content (solid content after drying) contain a large amount of calcium carbonate. At least one of the second slurry, the solid content separated from the second slurry, and the substance obtained by drying the solid content (solid content after drying) can be suitably used as a cement regenerator.

[0021] In the production method of this embodiment, the significance of including the stirring step and the significance of the stirring time being 2 hours or longer will be described. The calcium hydroxide produced in the stirring step serves to supply calcium ions in the immobilization step. In order to increase the amount of calcium carbonate produced, it is necessary to increase the amount of calcium hydroxide produced, and for this purpose, a stirring step in which the first slurry is stirred for a certain period of time or more is required. The present inventors have found that the amount of calcium carbonate produced increases when the stirring time of the first slurry is set to 2 hours or more. It is presumed that the reaction of producing calcium hydroxide proceeds sufficiently with a stirring time of 2 hours or more. On the other hand, when the stirring time is less than 2 hours, the amount of calcium hydroxide produced is small, which suggests that the reaction to produce calcium hydroxide does not proceed sufficiently.

[0022] Therefore, according to the manufacturing method of this embodiment, by setting the stirring time to 2 hours or more, a cement recycled material with an increased content of calcium carbonate can be obtained. The manufacturing method of this embodiment contributes to reducing carbon dioxide emissions and enables cement-containing waste, which was previously treated at high cost, to be recycled as recycled cement material. This recycled cement material can be used for concrete products, road materials, and civil engineering and building materials (e.g., banking materials, etc.).

[0023] (Cement-containing waste) The cement-containing waste used in the manufacturing method of this embodiment includes concrete sludge, residual concrete, returned concrete, coal ash, carbide slag, and the like discharged during the concrete product manufacturing process, and is not limited as long as it is a substance that causes calcium to dissolve into water. The calcium content of the solid matter (on an absolute dry basis) contained in the cement-containing waste is preferably 10 wt% or more calculated as CaO. An absolute dry basis refers to the mass of the solid matter without moisture. Cement-containing waste usually contains aggregate. However, even if the cement-containing waste contains aggregate, calcium carbonate is generated by supplying carbon dioxide to the first slurry. The more the amount of aggregate, the lower the calcium content in the cement-containing waste, and therefore the smaller the amount of calcium carbonate generated by fixation of carbon dioxide. Therefore, the cement-containing waste used in the manufacturing method of this embodiment is preferably one from which aggregate has been removed as much as possible. Examples of a method for removing aggregate include a specific gravity separation method.

[0024] Each step of the manufacturing method of this embodiment will be described.

[0025] (First slurry production process) The first slurry producing step is a step of mixing the cement-containing waste with water to produce the first slurry. The method of mixing the cement-containing waste with the water is not particularly limited. The water content of the first slurry to be obtained is determined taking into consideration both the amount of water (water content) contained in the cement-containing waste itself and the amount of water to be mixed with the cement-containing waste. For example, in the case of concrete sludge discharged during the concrete manufacturing process, the solid content (on an absolute dry basis) contained in the concrete sludge is usually 5% by mass or more. In the case of residual concrete and returned concrete, they are often squeezed using a filter press and discharged with a solid content (on an absolute dry basis) of around 50% by mass. In this way, the amount of moisture (moisture content) when discharged varies depending on the cement-containing waste, and even in the case of the same "concrete sludge," the moisture content fluctuates slightly from day to day. Therefore, in this specification, the water content of the first slurry is calculated as follows. For example, when 100 parts by mass of cement-containing waste having a moisture content (moisture content) of 40% by mass is mixed with 50 parts by mass of water, the moisture content (moisture content) of the first slurry is calculated to be 60% by mass ({(40+50) / (100+50)}×100=60).

[0026] The step of producing the first slurry is preferably a step of mixing the cement-containing waste with water so that the water content of the first slurry is 70% by mass or more and 99% by mass or less. The water content of the first slurry is more preferably 75% by mass or more and 98% by mass or less, and further preferably 80% by mass or more and 95% by mass or less.

[0027] In the first slurry, the ratio of the solids (on an oven-dry basis) in the cement-containing waste to the water (solids (on an oven-dry basis) in the cement-containing waste / the water) is, in mass ratio, preferably 0.01 or more and 0.43 or less, more preferably 0.02 or more and 0.33 or less, and even more preferably 0.05 or more and 0.25 or less. The water in the ratio (solids in the cement-containing waste (on an bone dry basis) / the water) refers to the combined amount of water contained in the cement-containing waste itself and the amount of water mixed with the cement-containing waste.

[0028] (stirring process) The stirring step is a step of stirring the first slurry for 2 hours or more. In the stirring step, the time for stirring the first slurry is preferably 2.5 hours or more, more preferably 3 hours or more, even more preferably 4 hours or more, even more preferably 5 hours or more, even more preferably 6 hours or more, even more preferably 8 hours or more, and even more preferably 12 hours or more. The upper limit of the time for stirring the first slurry is preferably 36 hours or less, and more preferably 24 hours or less, from the viewpoint of sufficiently suppressing solidification of the cement. The stirring means used in the stirring step (hereinafter also referred to as the first stirring means) is not particularly limited, and examples thereof include mechanical stirring using a stirring blade. In the stirring step, it is preferable to stir the first slurry so as to prevent the solid content in the cement-containing waste from settling. In the stirring tank used in the stirring step, it is necessary to obtain a flow of the first slurry such that the solid content in the first slurry does not settle and the solid content is unlikely to adhere to the wall surface of the stirring tank. The rotation speed of the first stirring means depends on the size and shape of the stirring vessel and the size and shape of the stirring blade, but is, for example, 10 rpm to 500 rpm, preferably 15 rpm to 250 rpm. When the rotation speed of the first stirring means is 15 rpm to 250 rpm, the flow of the first slurry is more easily obtained. In the stirring step, the first slurry is preferably stirred at room temperature from the viewpoint of suppressing the release of carbon dioxide due to the operation of the apparatus. Room temperature is a temperature at which no particular heating or cooling is performed, specifically, 5°C or higher and 35°C or lower. The temperature of the first slurry during stirring is 5°C or higher and 35°C or lower, preferably 10°C or higher and 35°C or lower. The temperature of the first slurry during stirring may be less than 5°C, as long as the first slurry does not freeze due to stirring. In the stirring step, the first slurry is preferably not heated.

[0029] (immobilization process) The immobilization step is a step of supplying exhaust gas containing carbon dioxide into the first slurry while stirring the first slurry, thereby immobilizing the carbon dioxide contained in the exhaust gas as calcium carbonate. The stirring of the first slurry in the fixation step is performed while supplying exhaust gas containing carbon dioxide after stirring for 2 hours or more in the stirring step. In the production method of the present embodiment, the immobilization step is carried out while stirring the first slurry, so that calcium hydroxide can be efficiently produced in the first slurry. The first slurry is preferably stirred at room temperature in the immobilization step. The temperature of the first slurry during stirring may be less than 5° C. as long as the first slurry does not freeze due to stirring. In the immobilization step, the first slurry is preferably stirred without being heated. In the stirring tank used in the immobilization step, it is necessary to stir the first slurry so as to efficiently bring the exhaust gas into contact with the calcium dissolved in the first slurry. The stirring means used in the immobilization step (hereinafter also referred to as the second stirring means) may be the same as the first stirring means. The second stirring means and the first stirring means may be the same or different. The rotation speed of the second stirring means depends on the size and shape of the stirring tank and the size and shape of the stirring blade, but is, for example, 10 rpm to 500 rpm, preferably 15 rpm to 450 rpm. When the rotation speed of the second stirring means is 15 rpm to 450 rpm, the exhaust gas can be more efficiently contacted with the calcium dissolved in the first slurry.

[0030] In the fixation process of this embodiment, if exhaust gas is continuously supplied to the first slurry, the pH will eventually reach a constant value, and it is presumed that this value is related to the balance between the "rate of calcium dissolution (which contributes to increasing the pH)" and the "rate of calcium carbonate production (which contributes to decreasing the pH)" which is determined by the contact efficiency between carbon dioxide and calcium ions. After the pH of the first slurry becomes constant, the pH gradually increases when the exhaust gas supply is stopped, which is likely due to either the redissolution of calcium carbonate or the continued dissolution of calcium from the cement-containing waste. The present inventors have found that by continuing to supply exhaust gas even after the pH of the first slurry becomes constant or falls into the neutral range (specifically, pH 9 or less), the amount of calcium carbonate produced gradually increases, and as a result, the content ratio of calcium carbonate in the second slurry can be increased.

[0031] In the immobilization step of this embodiment, the preferred ranges of the supply rate and supply time of the exhaust gas into the first slurry are as follows.

[0032] The supply rate of the waste gas into the first slurry is 1 L / min or more, preferably 1.5 L / min or more, per 1 kg of solid content (bone dry basis) of the cement-containing waste in the first slurry, from the viewpoint of reacting calcium with carbon dioxide in the first slurry in a short time and efficiently. The upper limit of the supply rate of the waste gas into the first slurry varies depending on the specifications of the apparatus, but is, for example, 10 L / min or less.

[0033] The supply time of the exhaust gas into the first slurry is preferably longer from the viewpoint of increasing the content ratio of calcium carbonate, and is preferably 50 minutes or more, more preferably 2 hours or more, further preferably 3 hours or more, further preferably 4 hours or more, and further preferably 5 hours or more. The upper limit of the supply time of the exhaust gas into the first slurry is, for example, 36 hours or less from the viewpoint of production efficiency. In the fixation process of this embodiment, it is preferable that the supply rate of the exhaust gas into the first slurry is 1 L / min or more per 1 kg of solids (bone dry basis) in the cement-containing waste, and the supply time of the exhaust gas into the first slurry is 2 hours or more.

[0034] In the fixation process, it is preferable to control the exhaust gas to be supplied into the first slurry until the pH of the first slurry becomes 6 or more and 9 or less, and it is more preferable to control the exhaust gas to be supplied into the first slurry until the pH of the first slurry becomes 6 or more and 8 or less. The pH of the first slurry can be controlled by adjusting, for example, the supply rate and supply time of the exhaust gas, the temperature of the first slurry, and the like.

[0035] In one embodiment of the immobilization step, it is preferable to supply the exhaust gas into the first slurry until the pH of the first slurry becomes constant. A constant pH means that, when the change (difference) in pH is ΔpH, ΔpH≦0.5 for at least 120 minutes. In one embodiment of the immobilization step, it is preferable to continue supplying the exhaust gas into the first slurry even after the pH of the first slurry becomes constant.

[0036] In the immobilization step, the exhaust gas is preferably supplied into the first slurry by bubbling.

[0037] In the immobilization step, the exhaust gas is preferably a combustion exhaust gas. Examples of combustion exhaust gas include combustion exhaust gas generated from the combustion of liquid fuels (heavy oil, kerosene, light oil, etc.), gases (liquefied petroleum gas, liquefied natural gas, solid fuel gasification gas, etc.), and solids (coal, biomass, garbage, etc.).

[0038] (Step of obtaining second slurry) A second slurry is produced in a fixing step. The step of obtaining a second slurry provides a second slurry containing calcium carbonate. Examples of materials contained in the second slurry other than calcium carbonate include fine aggregate, coarse aggregate, and crushed stone.

[0039] (separation process) The production method of the present embodiment preferably further includes a step of separating the second slurry into a solid content containing calcium carbonate and a liquid. The separation method is not particularly limited, but examples thereof include filtration, centrifugation, and decantation.

[0040] (drying process) The production method of the present embodiment preferably further includes a step of drying the separated solid content. The drying method is not particularly limited, and any known drying device can be used. The solids include calcium carbonate, water, aggregates, and the like. The content of calcium carbonate in the solid content (after drying) is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more.

[0041] (Uses of recycled cement materials) The recycled cement material obtained by the manufacturing method of the first embodiment can be used for any of the following applications: concrete products, road materials, embankment materials, blocks, tunnel drainage neutralizers, disinfectants for the livestock industry, fillers for additives to plastics, fillers for additives to rubber, and fillers for additives to paints. The recycled cement material of this embodiment can be used in construction and civil engineering materials (for example, concrete products, road materials, embankment materials, etc.) to reduce the amount of carbon dioxide released back into the atmosphere. The cement recycled material of this embodiment can be used as a substitute for natural limestone, thereby reducing the amount of carbon dioxide emitted during the mining of natural limestone, the production of cement using natural limestone, and the production of synthetic calcium carbonate.

[0042] [Modifications of the embodiment] The present invention is not limited to the above-described embodiment, and may include modifications and improvements within the scope of the present invention. EXAMPLES

[0043] EXAMPLES Hereinafter, examples of the present invention will be described, but the present invention is not limited to these examples.

[0044] In the following examples and comparative examples, recycled cement materials were produced using experimental equipment.

[0045] Example 1 A first slurry was generated by mixing 45 g of ordinary Portland cement with 450 g of water (first slurry generation step). Next, the first slurry was stirred with a stirrer at room temperature (20° C.) for 3 hours (stirring step). The stirrer rotation speed was set to 800 rpm. Next, the first slurry was transferred to a separable flask (500 mL), and while stirring the first slurry with a stirrer (rotation speed 800 rpm), carbon dioxide was supplied into the first slurry at a flow rate of 500 mL / min to be bubbled. A kerami filter (cylindrical gas injection tube) was used for bubbling carbon dioxide. During bubbling, the pH of the first slurry was monitored, and the flow rate of carbon dioxide at the inlet and outlet of the separable flask was monitored, and carbon dioxide was immobilized as calcium carbonate (immobilization process). During the bubbling of carbon dioxide, the pH of the first slurry decreased from 13 to 6.4, and remained almost constant until the bubbling was stopped.

[0046] After obtaining the slurry produced in the immobilization step (step of obtaining a second slurry), it was immediately filtered and separated into a solid content and a liquid (separation step), and the solid content was dried (drying step). In this manner, the recycled cement material (solid content after drying) of Example 1 was obtained.

[0047] [Examples 2 to 4] The recycled cement materials of Examples 2 to 4 were produced in the same manner as in Example 1, except that the stirring time of the first slurry in the stirring step was changed to the stirring times shown in Table 1, respectively.

[0048] Comparative Example 1 The recycled cement material of Comparative Example 1 was produced in the same manner as in Example 1, except that the first slurry was not stirred in the stirring step.

[0049] Comparative Example 2 The recycled cement material of Comparative Example 2 was produced in the same manner as in Example 1, except that the stirring time of the first slurry in the stirring step was changed to the stirring time shown in Table 1.

[0050] Example 5 The cement recycled material of Example 5 was produced in the same manner as Example 1, except that the stirring time of the first slurry in the stirring step, and the bubbling time and bubbling rate of carbon dioxide in the fixation step were changed to the stirring time, bubbling time and bubbling rate shown in Table 1. In the case of bubbling carbon dioxide in Example 5, the pH of the first slurry became constant 40 minutes after the start of bubbling, but bubbling was continued for another 80 minutes, so that bubbling was performed for a total of 120 minutes.

[0051] Comparative Example 3 The recycled cement material of Comparative Example 3 was produced in the same manner as in Example 5, except that the first slurry was not stirred in the stirring step.

[0052] Example 6 The recycled cement material of Example 6 was produced in the same manner as in Example 5, except that the stirring time of the first slurry in the stirring step was changed to the stirring time shown in Table 1. In the case of bubbling carbon dioxide in Example 6, the pH of the first slurry became constant 40 minutes after the start of bubbling, but bubbling was continued for another 320 minutes, so that bubbling was performed for a total of 360 minutes.

[0053] [Examples 7 to 8] The cement recycled materials of Examples 7 and 8 were produced in the same manner as in Example 1, except that the raw materials, the stirring time of the first slurry in the stirring step, and the bubbling time of carbon dioxide in the fixation step were changed to the raw materials, stirring times, and bubbling times shown in Table 1, respectively. In Examples 7 and 8, concrete sludge discharged from a cement product manufacturing plant was used as the raw material. The ratio (weight ratio) of solids to water in the concrete sludge was adjusted to be the same as that of ordinary Portland cement. Elemental analysis showed that the cement content of the solids in the concrete sludge was approximately 88 mass%, with the remainder estimated to be aggregate. Elemental analysis was performed by X-ray fluorescence spectrometry.

[0054] Comparative Example 4 The recycled cement material of Comparative Example 4 was produced in the same manner as in Example 7, except that the first slurry was not stirred in the stirring step.

[0055] 〔evaluation〕 (Calcium carbonate content) The calcium carbonate content (mass%) in the recycled cement material produced in each example was measured by thermogravimetric analysis. The measurement conditions were as follows. The evaluation results are shown in Table 1 and Figures 1 to 4. -Measurement conditions- Equipment: Thermogravimetric analyzer (Shimadzu Corporation, model: TGA-50) Heating temperature: Room temperature (25℃) to 900℃ Heating rate: 15℃ / min ·Measurement data: 10mg

[0056] [Table 1]

[0057] Examples 1 to 4 are compared with Comparative Examples 1 and 2 (FIG. 1). In Examples 1 to 4, in which the first slurry was stirred for 2 hours or more, the amount of calcium carbonate produced in the recycled cement material was increased compared to Comparative Example 1, in which the first slurry was not stirred, and Comparative Example 2, in which the first slurry was stirred for 1 hour.

[0058] In comparison with the test conditions of Example 3 and Comparative Example 1, Example 5 and Comparative Example 3 were used in which the bubbling speed was slowed down and bubbling was performed for a long time (FIG. 2). Similar to the results shown in FIG. 1, Example 5, in which the first slurry was stirred for 2 hours or more, showed a higher amount of calcium carbonate produced in the recycled cement material than Comparative Example 3, in which the first slurry was not stirred.

[0059] The test conditions are compared with those of Example 3, except that the bubbling speed was slowed down and the bubbling time was long (FIG. 3). It was found that, in the case of continuing the bubbling of carbon dioxide even after the pH of the first slurry became constant, the amount of calcium carbonate produced in the recycled cement material increased as the bubbling of carbon dioxide was continued for a longer period of time. In the case of Example 6, the ratio of calcium carbonate in the recycled cement material was 60 mass %. This means that 36 mass of carbon dioxide was fixed in the cement, assuming that the original cement mass was 100. The cement contains about 46 mass % of calcium, of which the amount of calcium that contributed to the production of calcium carbonate was calculated to be about 72 mass %, which is very large.

[0060] In contrast to the test conditions of Examples 2-3 and Comparative Example 1, Examples 7-8 and Comparative Example 4 use concrete sludge as the raw material and are bubbled for a long time (FIG. 4). Similar to the results shown in FIG. 1, in Examples 7 and 8, in which the first slurry was stirred for 2 hours or more, the amount of calcium carbonate produced in the recycled cement material was increased compared to Comparative Example 4, in which the first slurry was not stirred.

[0061] From the above results, it is evident that in order to increase the amount of calcium carbonate produced in the recycled cement material, it is effective to stir the first slurry for 2 hours or more before bubbling with carbon dioxide. [Industrial Applicability]

[0062] According to the method for producing recycled cement material of the present invention, the produced recycled cement material can be used for concrete products, road materials, civil engineering and building materials, etc. Therefore, the method for producing recycled cement material of the present invention has industrial applicability.

Claims

1. mixing cement-containing waste with water to form a first slurry; stirring the first slurry for at least 2 hours; supplying exhaust gas containing carbon dioxide into the first slurry while stirring the first slurry, thereby immobilizing the carbon dioxide contained in the exhaust gas as calcium carbonate; and obtaining a second slurry containing the calcium carbonate produced in the immobilizing step. Manufacturing method for recycled cement materials.

2. The method further comprises a step of separating the second slurry into a solid component containing the calcium carbonate and a liquid component. The method for producing recycled cement material according to claim 1.

3. The method further comprises a step of drying the separated solid content. The method for producing recycled cement material according to claim 2.

4. In the stirring step, the first slurry is stirred for 5 hours or more. The method for producing recycled cement material according to claim 1 or 2.

5. In the immobilization step, a supply rate of the exhaust gas into the first slurry is 1 L / min or more per 1 kg of solid content (bone dry basis) in the cement-containing waste, and a supply time of the exhaust gas into the first slurry is 2 hours or more. The method for producing recycled cement material according to claim 1 or 2.

6. In the immobilization step, the exhaust gas is supplied into the first slurry by bubbling. The method for producing recycled cement material according to claim 1 or 2.

7. In the step of producing the first slurry, a ratio of a solid content (bone dry basis) in the cement-containing waste to the water (solid content (bone dry basis) in the cement-containing waste / the water) is 0.01 or more and 0.43 or less in mass ratio. The method for producing recycled cement material according to claim 1 or 2.

8. The exhaust gas is a combustion exhaust gas. The method for producing recycled cement material according to claim 1 or 2.

9. The recycled cement material is used for any of concrete products, road materials, embankment materials, blocks, tunnel drainage neutralizers, disinfectants for livestock farming, fillers for additives to plastics, fillers for additives to rubber, and fillers for additives to paints. The method for producing recycled cement material according to claim 1 or 2.

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