Method for manufacturing modified recycled aggregate and modified recycled aggregate

Abstract

To provide a manufacturing method of a modified recycled aggregate useful for reducing a carbon dioxide gas, the method obtaining a dense modified recycled aggregate with improved water absorption at a low energy consumption from a concrete mass, and also to provide a modified recycled aggregate having a dense structure with improved water absorption.SOLUTION: A manufacturing method of a modified recycled aggregate includes: a first step of obtaining a recycled aggregate having a water absorption over 3.0% from a crushed matter with a particle diameter 40 mm or less obtained by crushing or milling a concrete mass; and a second step of applying a carbonation treatment to the recycled aggregate obtained at the first step to obtain a modified recycled aggregate containing a calcium carbonate and a silicon dioxide. A modified recycled aggregate is also provided.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a method for producing modified recycled aggregates and modified recycled fine aggregates.

Background Art

[0002] Concrete blocks obtained by demolishing concrete of buildings and the like are waste materials that are difficult to process, and currently most of the recycling is used as roadbed materials. Therefore, a recycling method for concrete blocks with higher added value has become an issue. Currently, attempts have been made to crush concrete blocks to obtain recycled aggregates. However, recycled aggregates containing a large amount of cement-derived components on the surface may affect the quality of hardened bodies using the recycled aggregates. Therefore, currently, the production of recycled aggregates with as few cement-derived components as possible is targeted. However, when recovering recycled aggregates, in order to reduce cement-derived components, it requires a lot of energy, the recycled concrete fine powder separated from the recycled aggregates contains aggregate components, etc., resulting in the problem that the utilization of recycled concrete fine powder becomes difficult.

[0003] As a method for modifying recycled aggregates separated from fine powder, a method for producing recycled aggregates has been proposed in which aggregates with a high calcium hydroxide content obtained by crushing and sieving concrete waste are brought into contact with carbon dioxide gas to improve the water absorption rate of the recycled aggregates caused by cement-derived components (see Patent Document 1). A method for treating concrete waste has been proposed in which concrete waste is crushed, brought into contact with carbon dioxide gas, then immersed in water containing carbon dioxide gas, followed by solid-liquid separation, and the filtrate is boiled to obtain carbonates (see Patent Document 2). Also, as a method for reducing cement-derived components in recycled aggregates, a method for producing recycled aggregates has been proposed in which coarse aggregate raw materials derived from concrete waste are heated to 100°C or higher, stirred in a dry stirring device to wear the surface, and further the surface is worn in a second stirring device to remove the mortar content (see Patent Document 3).

Prior Art Documents

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-238792 [Patent Document 2] Japanese Patent Application Publication No. 11-319765 [Patent Document 3] Japanese Patent Application Publication No. 8-109052 [Overview of the project] [Problems that the invention aims to solve]

[0005] The method described in Patent Document 1 assumes that the aggregate obtained from concrete waste contains calcium hydroxide, and is a technique for converting the calcium hydroxide on the aggregate surface to calcium carbonate. However, when concrete blocks are crushed to obtain aggregate, the calcium hydroxide on the aggregate surface may be carbonated in the air, and in such cases, there is a problem that sufficient carbonation cannot be achieved even when brought into contact with carbon dioxide. Conventionally, for the purpose of obtaining carbonates from concrete waste or removing mortar components from recycled aggregate, as described in Patent Documents 2 and 3, high-temperature heating is required, such as boiling the filtrate after solid-liquid separation or heating it to over 100°C to abrade it, which increases the energy required for the production of recycled aggregate. Furthermore, our investigations have revealed that, as described in Patent Document 2, the method of carbonizing only the calcium hydroxide on the aggregate surface does not sufficiently suppress the decrease in water absorption rate and improve strength, especially when the amount of calcium hydroxide adhering to the surface of the recycled aggregate is small.

[0006] The object of one aspect of this disclosure is to provide a method for producing a modified recycled aggregate that is useful for reducing carbon dioxide emissions, as it can be obtained from concrete blocks with low energy consumption and improved water absorption, resulting in a dense modified recycled aggregate. Another aspect of the present disclosure addresses the problem of providing a modified recycled aggregate having improved water absorption and a dense structure. [Means for solving the problem]

[0007] The means for solving the above problem include the following embodiments. <1> A method for producing modified recycled aggregate, comprising: a first step of obtaining recycled aggregate with a water absorption rate exceeding 3.0% from crushed material with a particle size of 40 mm or less obtained by crushing or grinding concrete blocks; and a second step of carbonizing the recycled aggregate obtained in the first step to obtain modified recycled aggregate containing calcium carbonate and silicon dioxide.

[0008] <2> The first step includes classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain recycled fine aggregate with a water absorption rate exceeding 3.5%, wherein the modified recycled aggregate is modified recycled fine aggregate. <1> A method for producing the modified recycled aggregate described above. <3> The first step includes classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain recycled coarse aggregate with a water absorption rate exceeding 3.0%, wherein the modified recycled aggregate is modified recycled coarse aggregate. <1> A method for producing the modified recycled aggregate described above.

[0009] <4> The carbonation treatment in the second step is either a dry treatment in which the recycled aggregate obtained in the first step is placed in a closed space and brought into contact with carbon dioxide gas at a concentration of 5% or more under a temperature atmosphere of 5°C to 200°C, or a wet treatment in which the recycled aggregate obtained in the first step is immersed in water and carbon dioxide gas is supplied to the water until the pH of the water reaches a range of 6 to 8. <1> ~ <3> A method for producing modified recycled aggregate as described in any one of the following.

[0010] <5> The process further comprises a third step of mechanically grinding the modified and recycled fine aggregate obtained in the second step. <2> A method for producing the modified and recycled fine aggregate described above.

[0011] <6> This modified recycled aggregate is a reaction product of recycled aggregate derived from concrete blocks with a water absorption rate exceeding 3.0% and carbon dioxide, and contains calcium carbonate and silicon dioxide. [Effects of the Invention]

[0012] According to one aspect of this disclosure, it is possible to obtain dense modified recycled aggregate with improved water absorption rate and low energy consumption from concrete blocks, and a method for producing modified recycled aggregate useful for reducing carbon dioxide emissions can be provided. According to another aspect of this disclosure, it is possible to provide a modified recycled aggregate having improved water absorption and a dense structure. [Modes for carrying out the invention]

[0013] The following describes in detail the method for producing the modified recycled aggregate and the modified recycled aggregate as described in this disclosure, with specific examples. The descriptions below may be based on typical embodiments of this disclosure, but the following descriptions are examples only, and this disclosure is not limited to the following descriptions. In this disclosure, numerical ranges indicated using "~" represent a range that includes the numbers before and after "~" as the lower and upper limits, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples. In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In this disclosure, the term "process" includes not only independent processes but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved. In this disclosure, coarse aggregate refers to aggregate that, when classified using a 5 mm sieve, fails to pass through at least 85% by mass, and fine aggregate refers to aggregate that passes through a 10 mm sieve completely and contains at least 85% by mass of particles 5 mm or smaller. For aggregate size, the provisions of JIS A5022 (2016) "Recycled Aggregate Concrete M" or JIS A5023 (2016) "Recycled Aggregate Concrete L" shall be applied. In this disclosure, unless otherwise specified, room temperature refers to the ambient temperature without any particular temperature control, and more specifically, it is used to mean a temperature range of 20°C to 30°C.

[0014] <Method for Producing Modified Recycled Aggregate> The method for producing modified recycled aggregate of the present disclosure (hereinafter sometimes referred to as "the production method of the present disclosure") includes a first step of obtaining recycled aggregate having a water absorption rate exceeding 3.0% from crushed materials having a particle size of 40 mm or less obtained by crushing or grinding concrete blocks, and a second step of subjecting the recycled aggregate obtained in the first step to carbonation treatment to obtain modified recycled aggregate containing calcium carbonate and silicon dioxide.

[0015] (First Step) The first step is a step of obtaining recycled aggregate having a water absorption rate exceeding 3.0% from crushed materials having a particle size of 40 mm or less obtained by crushing or grinding concrete blocks. The concrete blocks used in the present disclosure are块状解体混凝土 (block-shaped disassembled concrete) obtained by disassembling buildings, structures, etc. containing concrete. Concrete blocks have various compositions due to the influence of the composition of the original concrete, the environment where the buildings, structures, etc. are placed, the elapsed time, etc. According to the production method of the present disclosure, regardless of the composition of the concrete blocks, modified recycled aggregate suitable for reuse can be obtained with low energy.

[0016] In the first step, first, the concrete blocks are crushed or ground to obtain crushed materials having a particle size of 40 mm or less. Hereinafter, in the present disclosure, "crushing or grinding" may sometimes be collectively referred to as "crushing or the like". There is no particular limitation on the method of crushing or grinding concrete blocks to obtain crushed materials having a particle size of 40 mm or less, and known crushing devices or grinding devices can be used. Multistage crushing or the like may be performed, in which the concrete blocks are first roughly crushed and the obtained roughly crushed materials are further crushed or ground. For the rough crushing of disassembled concrete, for example, known crushers such as jaw crushers and impeller breakers can be used. When further crushing or the like is to be performed on the coarse crushed material, for example, a crushing device using a shot blasting method, an impact crusher, a grinding device such as a mechanical grinding method without heating, etc. can be used.

[0017] The crushed material with a particle size of 40 mm or less obtained by crushing or grinding the concrete mass serves as a raw material for recycled aggregates. For the crushed material which is a recycled aggregate raw material with a particle size of 40 mm or less, further crushing or the like can be performed, and then the crushed material can be classified by particle size to obtain recycled coarse aggregates, recycled fine aggregates, and recycled powder. There are no particular limitations on the methods for crushing or the like and for classifying the recycled coarse aggregates, recycled fine aggregates, and recycled powder, and known methods can be selected and applied according to the conditions for crushing or the like and classification or the like. The first step of obtaining the recycled aggregate includes a step of classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain a recycled fine aggregate having a water absorption rate exceeding 3.5%. The modified recycled aggregate may be a modified recycled fine aggregate. Also, the first step includes a step of classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain a recycled coarse aggregate having a water absorption rate exceeding 3.0%. The modified recycled aggregate may be a modified recycled coarse aggregate. More specifically, after coarse crushing, for the crushed material having a particle size of 40 mm or less, further crushing or the like is performed to obtain a recycled coarse aggregate having a size such that 85 mass% or more remains on a 5 mm sieve, and a recycled fine aggregate that completely passes through a 10 mm sieve and contains 85 mass% or more of particles of 5 mm or less. As a method for further crushing the crushed material with a particle size of 40 mm or less, there are methods such as crushing using a crusher such as a jaw crusher or an impeller breaker, a method using a mechanical grinding method without heating, a method using a heated grinding method, a blast shot method, etc. From the viewpoint of lower energy consumption, crushing by a crusher, a mechanical grinding method without heating, a blast shot method, etc. are preferable. There are no particular limitations on the maximum size of the recycled coarse aggregate, but from the viewpoint of usability, the maximum size of the recycled coarse aggregate is preferably 25 mm or less, and more preferably 20 mm or less.

[0018] After separating the recycled coarse aggregate, a mixture of recycled fine aggregate and powder derived from concrete blocks is obtained. Recycled fine aggregate can be obtained by separating recycled concrete powder (also called recycled powder) from the crushed material of 5 mm or less obtained after classifying recycled coarse aggregate. The separation of recycled concrete powder can be carried out by known methods, such as sieving and wind classification. Recycled aggregate obtained by mechanical crushing and classification has cement-derived components remaining on its surface. Consequently, the water absorption rate of the resulting recycled aggregate exceeds 3.0% due to the cement-derived components adhering to it. In the manufacturing method of this disclosure, the obtained recycled aggregate preferably has a water absorption rate of more than 3.0% and 3.5% or higher. There is no particular upper limit on the water absorption rate.

[0019] Furthermore, taking the case where the resulting recycled aggregate is recycled fine aggregate as an example, from the viewpoint that the carbonation treatment in the second step reduces the influence of the modified recycled fine aggregate on the quality of the concrete composition, etc., it is preferable that the water absorption rate of the recycled fine aggregate obtained through the first step be 15% or less, and more preferably 9% or less. In the first step, if the water absorption rate of the recycled aggregate is kept below 3.0%, more specifically, if the water absorption rate of the recycled coarse aggregate is kept below 3.0% and the water absorption rate of the recycled fine aggregate is kept below 3.5%, the recycled fine aggregate will be more suitable for use in cement compositions, concrete compositions, etc. However, in order to further remove cement-derived components from the classified recycled fine aggregate, additional processing such as heating and mechanical grinding is required, which is undesirable from the standpoint of energy consumption.

[0020] One indicator of particle size in recycled aggregate is the coarseness ratio. Hereafter, in this disclosure, particle size will be used as a guideline for the size of recycled coarse aggregate and recycled fine aggregate. The particle size of recycled coarse aggregate and recycled fine aggregate can be measured in accordance with JIS A1102 (2014). From the obtained particle size, the coarseness ratio of the recycled aggregate can be determined. The coarseness ratio of recycled aggregate is the value obtained by dividing the sum of the mass fractions (%) of aggregate particles that remain on sieves of 80 mm, 40 mm, 20 mm, 10 mm, 5 mm, 2.5 mm, 1.2 mm, 0.6 mm, 0.3 mm, and 0.15 mm by 100.

[0021] Among the aggregates obtained by mechanical crushing and classification, recycled fine aggregate has a larger surface area than recycled coarse aggregate. Over time, due to crushing and storage after crushing, it reacts with carbon dioxide in the air, causing a decrease in the calcium hydroxide content among the cement-derived components. The recycled aggregate obtained in the first step may have a calcium hydroxide content of 5% by mass or less, 2% by mass or less, or 1% by mass or less. For example, the carbonation treatment described in Patent Document 2 is a process that generates calcium carbonate on the surface by carbonating calcium hydroxide present near the surface of the aggregate. However, in the manufacturing method of the present disclosure, the recycled aggregate subjected to carbonation treatment does not depend on the calcium hydroxide content. As will be clear from the examples described later, even recycled aggregate without calcium hydroxide near the surface can be carbonized to reduce its water absorption rate and obtain a modified recycled aggregate with a dense structure. Accordingly, according to the manufacturing method of this disclosure, by performing a carbonation treatment under specific conditions in the second step described later, even if the recycled aggregate has a water absorption rate exceeding 3.0%, and even if the calcium hydroxide content due to cement-derived components of the recycled aggregate is 5% by mass or less, a modified recycled aggregate having physical properties suitable for cement compositions, concrete compositions, etc., can be obtained.

[0022] (2nd process) The second step involves carbonizing the recycled aggregate obtained in the first step to modify it to contain calcium carbonate and silicon dioxide. Recycled aggregate This is the process of obtaining [something].

[0023] The carbonation treatment in the second step may be carried out by a dry method or a wet method. The carbonation treatment is preferably a dry treatment in which the recycled aggregate obtained in the first step is placed in a closed space and brought into contact with carbon dioxide gas at a concentration of 5% or more under a temperature atmosphere of 5°C to 200°C, or a wet treatment in which the recycled aggregate obtained in the first step is immersed in water and carbon dioxide gas is supplied to the water until the pH of the water reaches 6 to 8.

[0024] (Carbonation treatment: dry treatment) In the second step, when the carbonation treatment is performed dry, the recycled aggregate can be placed in a closed space and the closed space can be filled with carbon dioxide gas to bring it into contact with the aggregate.

[0025] The temperature inside the enclosed space is preferably between 5°C and 200°C, and more preferably between 20°C and 100°C. For example, the carbonation treatment can also be performed in an enclosed space without specific temperature control, under a temperature atmosphere of 0°C to 40°C. The humidity in a closed space can be set to 30%RH to 90%RH, and preferably to 50%RH to 80%RH, from the viewpoint of improving carbonation efficiency.

[0026] The carbon dioxide concentration in the gas to be brought into contact is 5% or more, preferably 10% or more, and more preferably 50% or more. The carbon dioxide gas to be brought into contact may be 100% carbon dioxide. The carbon dioxide used in the carbonation process may be exhaust gas containing carbon dioxide. Examples of exhaust gas include exhaust gas emitted from heating furnaces and exhaust gas emitted from power engines, and there are no particular restrictions as long as it contains carbon dioxide at a concentration of 5% or more. By using exhaust gas containing carbon dioxide in the carbonation treatment, it is possible to simultaneously reduce the amount of carbon dioxide in the exhaust gas and modify the recycled aggregate by adsorbing the carbon dioxide onto it. When the exhaust gas temperature exceeds 100°C, efficient carbonation treatment can be performed without heating a closed space, which has the advantage of simultaneously achieving reduced energy consumption and a reduction in carbon dioxide in the exhaust gas. Furthermore, in order to efficiently carbonate the recycled aggregate, a closed space may be pressurized and carbon dioxide may be supplied and brought into contact with the recycled aggregate.

[0027] The preferred processing time in the dry process is appropriately selected in relation to the carbon dioxide concentration and supply rate. For example, when using carbon dioxide at a concentration of 10% to 50%, the processing time can be 1 hour or more, preferably 24 hours or more, more preferably 2 days or more, even more preferably 5 days or more, and still more preferably 10 days or more. There is no particular upper limit to the processing time, but from the viewpoint of manufacturing efficiency, it can be 20 days or less. The preferred processing time can be appropriately selected depending on the condition of the aggregate and the required physical properties of the modified recycled aggregate. For example, if the amount of cement components adhering to the recycled aggregate is small, and if only the cement powder adhering to the surface and vicinity of the recycled aggregate is to be carbonated, the desired modified recycled aggregate can be obtained in a processing time of 1 to 2 hours. Furthermore, when the processing time in the dry process is 5 days or longer, it is preferable to stir the recycled aggregate, which has been left to stand in a closed space, at a frequency of once every few hours to 3 days, from the viewpoint of making the carbonation process more efficient. Stirring ensures more uniform contact between the recycled aggregate and carbon dioxide, and reduces the amount of unreacted areas, such as parts of the recycled aggregate or parts of individual recycled aggregates that are not carbonized.

[0028] (Carbonation treatment: wet treatment) The wet carbonation treatment can be carried out by immersing the recycled aggregate obtained in the first step in water and supplying carbon dioxide to the water until the pH of the water reaches 6 to 8. Specific methods of wet processing include filling a container with water to immerse the recycled aggregate, and then bubbling the water with carbon dioxide. There are no particular restrictions on the water used for immersion. From the viewpoint of treatment efficiency, it is preferable that the water used contains few impurities that come into contact with carbon dioxide and form salts. For example, ion-exchanged water or tap water can be used.

[0029] In the second step, when performing wet processing, the carbon dioxide concentration supplied to the water can be 5%, preferably 10% or more, and more preferably 20% or more. There is no particular upper limit to the carbon dioxide concentration, and the carbon dioxide concentration may be 100%. Higher carbon dioxide concentrations tend to result in better carbonation efficiency of recycled aggregates. The supplied carbon dioxide may be exhaust gas containing carbon dioxide, similar to the dry treatment process. When exhaust gas is used as the source of carbon dioxide, the carbon dioxide concentration depends on the amount of carbon dioxide contained in the exhaust gas. Even when using exhaust gas, it is preferable that the carbon dioxide concentration in the exhaust gas be 5% or higher.

[0030] The amount of carbon dioxide supplied to the water in which the recycled aggregate is immersed, and the rate at which the carbon dioxide of the above concentration is supplied, can be appropriately selected according to the purpose. By optimizing the total amount and rate of carbon dioxide supply and blowing it in, contact between the recycled aggregate immersed in water and the carbon dioxide is efficiently achieved, and the carbonation of the recycled aggregate progresses. Furthermore, the solubility of carbon dioxide in water increases at lower water temperatures. Therefore, the water temperature does not need to be specifically controlled. Furthermore, when exhaust gas is used as the source of carbon dioxide, the water temperature may rise depending on the temperature of the exhaust gas, but the water temperature will not reach 100°C, so it is not expected to affect the progress of the carbonation treatment of the recycled aggregate.

[0031] There are no particular restrictions on the water temperature in the wet process, and it can be carried out at room temperature. Furthermore, when exhaust gas is used to supply carbon dioxide, the water temperature may rise to, for example, around 90°C due to the temperature and supply rate of the exhaust gas, but the carbonation process can still be performed under these temperature conditions.

[0032] There are no particular restrictions on the processing time for wet processing. For example, the wet processing time can be between 5 minutes and 10 hours, and is preferably between 30 minutes and 6 hours. As the carbonation of cement-derived components in recycled aggregate progresses due to contact with carbon dioxide, the pH of the water tends to decrease. Therefore, the end point of the carbonation treatment may be set when the pH of the water reaches 6 to 8. It is preferable to continue the wet treatment until the pH of the water reaches 6.5 to 7.0, and more preferably until the pH reaches 6.5 to 6.8. It is even more preferable to continue the wet treatment for another 30 to 90 minutes after the pH of the water reaches the target value, for example, when the pH of the water reaches 6.5 to 7.0. The pH of water in wet processing can be measured with a known pH meter. In this disclosure, the values ​​used are those measured with a glass electrode type hydrogen ion concentration indicator (TPX-999Si) from Toko Chemical Research Institute Co., Ltd., at a liquid temperature of 20°C.

[0033] According to the manufacturing method of this disclosure, a modified recycled aggregate containing calcium carbonate and silicon dioxide is obtained by performing the second step on the recycled aggregate obtained in the first step.

[0034] (Optional steps) The manufacturing method of the present disclosure may optionally include, in addition to the first and second steps described above, further optional steps. Optional steps include a step of surface-treating the modified recycled aggregate and a step of impregnating the modified recycled aggregate. Furthermore, if the modified recycled aggregate obtained through the first and second steps is modified recycled fine aggregate, the method may further include a step of mechanically grinding the modified recycled fine aggregate.

[0035] The manufacturing method of the present disclosure may further include a third step of mechanically grinding the modified recycled fine aggregate obtained in the second step.

[0036] (3rd step) The third step is to mechanically grind the modified and recycled fine aggregate obtained in the second step. The mechanical grinding may or may not involve heating, but from the viewpoint of reducing manufacturing energy, it is preferable to perform the mechanical grinding without heating. The recycled fine aggregate obtained through the first process has a weakened layer of aggregate, a cement hydrate layer, and fine powder generated during crushing attached to the surface of the recycled fine aggregate, which has been weakened by crushing and classification. In the second process, the carbonation treatment may cause these weakened layers to lift off. In such cases, it is believed that the properties of the modified recycled fine aggregate will be further improved by peeling and removing the weakened layers before use.

[0037] Mechanical grinding can be performed by sealing modified and recycled fine aggregate in a rolling mill and operating it for a predetermined time. A known rolling mill can be used for mechanical grinding. Examples of such mills include ball mills and rod mills, and in this disclosure, the MK-ARM7100 model (product name) from Matsu Koken Co., Ltd. is used. The operating time of the rolling mill can be 1 to 20 minutes, preferably 2 to 10 minutes. Within this range, effective processing can be performed with low energy consumption. There are no particular restrictions on the amount of recycled fine aggregate processed in a single step when it is subjected to the third process; it can be selected as appropriate from the perspective of the processing capacity of the equipment, productivity, etc. The third step is performed to remove the fragile layer, powder, and other impurities present on the surface of the modified and recycled fine aggregate. Therefore, it is not necessary to enclose any media or other medium in the rolling mill, and it is believed that the fragile layer is easily removed by the contact between the particles of the modified and recycled fine aggregate within the rolling mill.

[0038] (Physical properties of the obtained modified and recycled fine aggregate) First, the physical properties of the modified recycled fine aggregate obtained through the first and second steps, and an optional third step, will be described. The modified and recycled fine aggregate obtained through the first step, the second step, and an optional third step, as desired, contains calcium carbonate and silicon dioxide. The calcium carbonate and silicon dioxide contained in the modified recycled fine aggregate are thought to be produced by the carbonation of the cement components contained in the recycled fine aggregate, specifically calcium hydroxide and calcium silicate hydrate (hereinafter also referred to as CSH). Silicon dioxide may be contained in the modified recycled fine aggregate as silica gel, which is a hydrate. Generally, the calcium hydroxide present on the surface of recycled fine aggregate accounts for only about 20% by mass of the cement components, and simply carbonating calcium hydroxide to obtain calcium carbonate is insufficient for modifying recycled fine aggregate. On the other hand, CSH accounts for about 50% by mass of the cement components. It is said that CSH has various compositions, and as an example, the following composition is shown, and the reaction in which CaO contained in CSH is carbonated is explained. For example, as shown in the following equation, CaO contained in CSH is carbonated, producing calcium carbonate and silicon dioxide. 1.7CaO·SiO2·2.17H2O + 1.7CO2 (carbonation) → 1.7CaCO3 + SiO2 + 2.17H2O

[0039] By performing the first and second steps, not only calcium hydroxide present near the surface of the recycled fine aggregate, but also CSH present in the fine voids of the recycled fine aggregate is carbonated, and hard calcium carbonate fills not only the surface of the modified recycled fine aggregate but also the fine internal voids. As a result, the modified recycled fine aggregate obtained by the manufacturing method of this disclosure has a lower water absorption rate and improved density compared to untreated recycled fine aggregate, and has physical properties that make it suitable for use in cement compositions and concrete compositions, thus promoting the use of recycled fine aggregate. Furthermore, as the recycled fine aggregate is modified, a large amount of carbon dioxide contributes to the formation of calcium carbonate and is immobilized in the modified recycled fine aggregate. Therefore, the manufacturing method disclosed herein is useful for reducing carbon dioxide through immobilization.

[0040] The calcium carbonate content in the modified and recycled fine aggregate obtained by the manufacturing method of this disclosure can be confirmed by thermal analysis. (Quantitative determination of calcium carbonate in modified and recycled fine aggregate) In this disclosure, the calcium carbonate content is calculated from the weight loss at 600°C to 800°C using a thermal analyzer TG-DTA (Rigaku Thermo plus EVO2 TG-DTA8122).

[0041] The silicon dioxide content in the modified recycled fine aggregate obtained by the manufacturing method of this disclosure can be confirmed by emission spectroscopy. Since silica gel is a hydrate of silicon dioxide, the content is measured on a Si element basis. (Quantitative determination of silicon dioxide in modified and recycled fine aggregate) In this disclosure, the carbonation of CSH is confirmed by measuring the silica gel (SiO2 gel) content. First, the modified recycled fine aggregate is crushed to prepare a sample. The obtained sample is dissolved in an HCl(1+4) aqueous solution, which is obtained by mixing reagent-grade hydrochloric acid (concentration approximately 35%) with pure water in a volume ratio of 1:4. Any undissolved components are further dissolved with 0.2N KOH, and the Si content in the resulting solution is measured using an inductively coupled plasma (ICP) emission spectrometer to confirm the SiO2 gel content. In this disclosure, the values ​​used are those measured using the SPECTROBLUE® EOP ICP emission spectrometer manufactured by Hitachi High-Tech Science Corporation.

[0042] The modified and recycled fine aggregate obtained by the manufacturing method of this disclosure contains calcium carbonate and silicon dioxide. The presence of calcium carbonate and silicon dioxide in the modified and recycled fine aggregate can be confirmed by the quantitative method described above.

[0043] From the viewpoint of further improving the density and further reducing the water absorption rate of the modified recycled fine aggregate obtained through the second process, the calcium carbonate content in the modified recycled fine aggregate is preferably 4% to 20% by mass, more preferably 8% to 20% by mass, and even more preferably 10% to 20% by mass, based on the total mass of the modified recycled fine aggregate. From the viewpoint of improving strength through density and structural compactness of the modified recycled fine aggregate obtained through the second process, the silicon dioxide content in the modified recycled fine aggregate is preferably 1.5% to 5% by mass, and more preferably 2% to 5% by mass, based on the total mass of the modified recycled fine aggregate.

[0044] The modified recycled fine aggregate contains calcium carbonate and silicon dioxide in the above-mentioned amounts, resulting in a lower water absorption rate compared to recycled fine aggregate before carbonation treatment. The water absorption rate of the modified recycled fine aggregate is lower than that of recycled fine aggregate, thus achieving the above effect. Therefore, it does not necessarily have to be 3.5% or less, which is the water absorption rate of ordinary fine aggregate as defined in JIS.

[0045] (Manufacturing of modified recycled coarse aggregate) Next, we will describe the modified recycled coarse aggregate obtained by the manufacturing method of this disclosure. In the first step, concrete blocks are crushed to a size of 40 mm or less, and the crushed material is further crushed to separate the aggregate that remains on a 5 mm sieve at a rate of 85% or more by mass, thereby obtaining recycled coarse aggregate. The recycled coarse aggregate obtained through the first step preferably has a water absorption rate of more than 3%, and more preferably 4% or higher. There is no particular upper limit on the water absorption rate, but from the viewpoint of minimizing the impact on concrete quality due to the carbonation treatment in the second step, it is preferable that it be 7% or less, and more preferably 6% or less.

[0046] In the second step, the recycled coarse aggregate is subjected to carbonation treatment to obtain modified recycled coarse aggregate. The carbonation treatment conditions for the recycled coarse aggregate in the second step are as described above for the second step, and the preferred example is also the same.

[0047] (Physical properties of modified recycled coarse aggregate) From the viewpoint of improving density and reducing water absorption rate, the modified recycled coarse aggregate obtained through the second process preferably contains 2.5% to 15% by mass, and more preferably 5% to 15% by mass, relative to the total mass of the modified recycled coarse aggregate. From the viewpoint of improving strength through density and structural compactness of the modified recycled coarse aggregate obtained through the second process, the silicon dioxide content in the modified recycled coarse aggregate is preferably 0.5% to 4% by mass, and more preferably 1% to 4% by mass, based on the total mass of the modified recycled coarse aggregate.

[0048] <Modified recycled aggregate> The modified recycled aggregate disclosed herein has a water absorption rate exceeding 3.0% derived from concrete blocks. Recycled aggregate It is a reaction product of carbon dioxide and contains calcium carbonate and silicon dioxide.

[0049] (Modified recycled fine aggregate) When the modified recycled aggregate of this disclosure is modified recycled fine aggregate, from the viewpoint of improving strength due to the density and compactness of the modified recycled fine aggregate, it is preferable to contain silicon dioxide in an amount of 1.5% to 5% by mass, and more preferably 2% to 5% by mass, based on the total mass of the modified recycled fine aggregate. Furthermore, it is preferable to contain calcium carbonate in an amount of 4% to 20% by mass, more preferably 8% to 20% by mass, and even more preferably 10% to 20% by mass, based on the total mass of the modified recycled fine aggregate. Furthermore, from the viewpoint of further improving the density and structural compactness of the modified recycled fine aggregate and further reducing the water absorption rate, it is preferable that the modified recycled fine aggregate contains 1.5% to 5% by mass of silicon dioxide and 4% to 20% by mass of calcium carbonate relative to the total mass of the modified recycled fine aggregate, and it is more preferable that it contains 2% to 5% by mass of silicon dioxide and 8% to 20% by mass of calcium carbonate relative to the total mass of the modified recycled coarse aggregate. The modified recycled fine aggregate of this disclosure, containing silicon dioxide and calcium carbonate within the above range, exhibits better density and can be suitably used as fine aggregate in cement compositions, concrete compositions, and the like.

[0050] (Modified recycled coarse aggregate) When the modified recycled aggregate of this disclosure is modified recycled coarse aggregate, from the viewpoint of further improving the density and compactness of the modified recycled coarse aggregate and further reducing the water absorption rate, it is preferable to contain silicon dioxide in an amount of 0.5% to 4% by mass, and more preferably 1% to 4% by mass, relative to the total mass of the modified recycled coarse aggregate. Furthermore, it is preferable to contain calcium carbonate in an amount of 2.5% to 15% by mass, and more preferably 5% to 15% by mass, relative to the total mass of the modified recycled coarse aggregate. In this disclosure, the modified recycled coarse aggregate preferably contains 0.5% to 4% by mass of silicon dioxide and 2.5% to 15% by mass of calcium carbonate relative to the total mass of the modified recycled coarse aggregate, from the viewpoint of further improving density and structural compactness and further reducing water absorption, and more preferably contains 1% to 4% by mass of silicon dioxide and 5% to 15% by mass of calcium carbonate relative to the total mass of the modified recycled coarse aggregate. The modified recycled coarse aggregate of this disclosure, which contains calcium carbonate and silicon dioxide within the above range, has better density and lower water absorption compared to general recycled coarse aggregate, and can therefore be suitably used as coarse aggregate in cement compositions, concrete compositions, etc. [Examples]

[0051] The manufacturing method and modified recycled aggregate of this disclosure will be described in detail below with specific examples, but these examples are merely examples, and various modifications can be made in accordance with the spirit of this disclosure.

[0052] [Comparative Example 1, Comparative Example 2, Comparative Example 3] (1st step) (Comparative Example 1) The concrete blocks generated during the building's demolition were crushed to pieces under 40mm in size. The obtained crushed material, which was less than 40 mm in size, was further crushed and classified using a 5 mm mesh sieve. The powder was then separated using a dust collector to obtain recycled fine aggregate A (Comparative Example 1: Coarse particle ratio: 2.48, Water absorption rate: 6.67%). (Comparative Example 2) The obtained crushed material, which was less than 40 mm in size, was collected using an 8 mm mesh sieve to obtain recycled fine aggregate B (Comparative Example 2: Coarse particle size ratio: 3.51, Water absorption rate: 12.35%). (Comparative Example 3) In the recycled fine aggregate B described above, powder particles smaller than 0.15 mm were removed to obtain recycled fine aggregate C (Comparative Example 3: Coarse particle size: 3.71, Water absorption rate: 9.96%).

[0053] The first step was carried out under the above conditions, and three types of recycled fine aggregate with different water absorption rates were obtained. The particle size of the recycled fine aggregate was measured in accordance with JIS A1102 (2014), and the coarseness ratio of each recycled fine aggregate was determined.

[0054] The calcium hydroxide content of the obtained recycled aggregates A, B, and C was confirmed using a thermal analyzer TG-DTA (Rigaku Thermo plus EVO2 TG-DTA8122). No endothermic peaks associated with calcium hydroxide dehydration were observed around 400°C to 500°C, and calcium hydroxide was not detected.

[0055] [Example 1] (2nd process) Comparative Example 1 involved placing 5 kg of recycled fine aggregate A obtained in the first step into a sealed space (a chamber with a volume of 850 L) and treating it with 10% carbon dioxide for 7 days to obtain modified recycled fine aggregate. The particle size of the modified recycled fine aggregate after processing was measured in the same manner as the recycled fine aggregate obtained in the first step, and the coarseness ratio was 2.46.

[0056] [Example 2] (2nd process) Comparative Example 1, i.e., 5 kg of recycled fine aggregate A obtained in the first step, was placed in a sealed space (a chamber with a volume of 850 L) and treated with 10% carbon dioxide for 14 days to obtain modified recycled fine aggregate. The particle size of the modified recycled fine aggregate after processing was measured in the same manner as the recycled fine aggregate obtained in the first step, and the coarseness ratio was 2.46.

[0057] [Example 3] (2nd process) In Comparative Example 1, 5 kg of recycled fine aggregate A obtained in the first step was placed in a 20 L plastic bottle, and the bottle was filled with water at 20°C. The pH of the water was measured using a glass electrode type hydrogen ion concentration indicator (TPX-999Si) from Toko Chemical Research Institute Co., Ltd., and was found to be pH 11.1. Using a tube that reached the bottom of the bottle, 100% concentration carbon dioxide was injected into the bottle and bubbled. The bubbling process was stopped when the pH of the water reached 6.8, and after standing for 1 hour, modified recycled fine aggregate was obtained. The time it took for the pH of the water to reach 6.8 was approximately 2 minutes. When the particle size of the modified recycled fine aggregate after processing was measured in the same manner as the recycled fine aggregate obtained in the first step, the coarseness ratio was 2.45.

[0058] (Evaluation of recycled fine aggregate) 1.Particle size The particle size of the modified recycled fine aggregate obtained by the manufacturing methods of Examples 1 to 3 was measured in the same manner as the recycled fine aggregate obtained in the first step, and the coarse particle ratio was determined.

[0059] 2. Absolute dry density and water absorption rate The oven-dry density and water absorption rate of recycled fine aggregate A obtained by the manufacturing methods of Comparative Example 1 and Examples 1 to 3, and the modified recycled fine aggregate treated with carbonation, were measured in accordance with JIS A 1109 (2020).

[0060] 3. Calcium carbonate content in fine aggregate The calcium carbonate content was calculated from the weight loss at temperatures between 600°C and 800°C using a thermal analyzer (TG-DTA, Rigaku Thermo plus EVO2 TG-DTA8122).

[0061] 4. Silicon dioxide content in fine aggregate The recycled fine aggregate was crushed to prepare a sample. The obtained sample was dissolved in an HCl(1+4) aqueous solution, which was obtained by mixing reagent-grade hydrochloric acid (concentration approximately 35%) with pure water in a volume ratio of 1:4. Any undissolved components were further dissolved with 0.2N KOH, and the Si content in the resulting solution was measured using an ICP emission spectrometer to confirm the SiO2 gel content. For the ICP emission spectroscopy analysis, we used the SPECTROBLUE® EOP system from Hitachi High-Tech Science Corporation.

[0062] The results of each evaluation are shown in Table 1 below.

[0063] [Table 1]

[0064] As is clear from Table 1, the modified recycled fine aggregate obtained by the carbonation treatment showed improved oven-dry density and reduced water absorption compared to recycled fine aggregate A obtained in Comparative Example 1, which was not subjected to carbonation treatment. This is thought to be because the silicon dioxide content in the modified recycled fine aggregate increased to a favorable range, and furthermore, both the calcium carbonate content and the silicon dioxide content increased, bringing both the calcium carbonate content and the silicon dioxide content into the favorable range described above. From this, it can be seen that the physical properties of the recycled fine aggregate were improved by the carbonation treatment in the second step. The power consumption required for the carbonation treatment in Example 1 was limited to the power required for carbon dioxide production and the supply of carbon dioxide into the chamber. Considering the power consumption required to heat the recycled aggregate to 100°C, as described in Patent Document 1, it is clear that the manufacturing method of the present disclosure allows for the production of high-quality modified recycled aggregate with lower energy consumption.

[0065] [Example 4] (2nd process) In Comparative Example 2, 5 kg of recycled fine aggregate B obtained in the first step was placed in a sealed space (a chamber with a volume of 850 L) and treated with 10% carbon dioxide for 7 days to obtain modified recycled fine aggregate.

[0066] [Example 5] (2nd process) In Comparative Example 2, 5 kg of recycled fine aggregate B obtained in the first step was placed in a sealed space (a chamber with a volume of 850 L) and treated with 50% carbon dioxide for 7 days to obtain modified recycled fine aggregate.

[0067] The recycled fine aggregates obtained by the manufacturing methods of Comparative Example 2, Example 4, and Example 5 were evaluated in the same manner as in Example 1. The results of each evaluation are shown in Table 2 below.

[0068] [Table 2]

[0069] As is clear from Table 2, the modified recycled fine aggregate obtained by carbonation treatment showed improved oven-dry density and reduced water absorption compared to recycled fine aggregate B obtained in Comparative Example 2, which did not undergo carbonation treatment. This is thought to be because the silicon dioxide content in the modified recycled fine aggregate increased to a favorable range, and furthermore, both the calcium carbonate content and the silicon dioxide content increased, bringing both the calcium carbonate content and the silicon dioxide content into the favorable range described above. Furthermore, a comparison between Example 4 and Example 5 reveals that Example 5, which used a higher concentration of 50% carbon dioxide in the carbonation treatment, has a higher oven-dry density and lower water absorption rate than Example 4, which used a 10% concentration of carbon dioxide.

[0070] [Comparative Example 4] In Comparative Example 3, 4 kg of recycled fine aggregate C obtained in the first step was placed in a rolling mill and mechanically ground by rolling for 3 minutes. The rolling mill used was the MK-ARM7100 model (product name) from Matsu Koken Co., Ltd.

[0071] [Example 6] (2nd process) In Comparative Example 3, 5 kg of recycled fine aggregate C obtained in the first step was placed in a sealed space (a chamber with a volume of 850 L) and treated with 5% carbon dioxide for 7 days to obtain modified recycled fine aggregate.

[0072] [Example 7] (2nd process) In Comparative Example 3, 5 kg of recycled fine aggregate C obtained in the first step was placed in a sealed space (a chamber with a volume of 850 L) and treated with 5% carbon dioxide for 7 days to obtain modified recycled fine aggregate. Subsequently, the modified and recycled fine aggregate obtained was subjected to mechanical grinding for 3 minutes, in the same manner as in Comparative Example 4.

[0073] The recycled fine aggregates obtained by the manufacturing methods of Comparative Example 3, Comparative Example 4, Example 6, and Example 7 were evaluated in the same manner as in Example 1. The results of each evaluation are shown in Table 3 below.

[0074] [Table 3]

[0075] As is clear from Table 3, the modified recycled fine aggregate obtained by carbonation treatment in Example 6 showed improved oven-dry density and reduced water absorption compared to recycled fine aggregate C obtained in Comparative Example 3 without carbonation treatment. This is thought to be because the silicon dioxide content in the modified recycled fine aggregate increased to a favorable range, and furthermore, both the calcium carbonate content and the silicon dioxide content increased, bringing both the calcium carbonate content and the silicon dioxide content into the favorable range described above. Furthermore, in Example 7, where the modified recycled fine aggregate obtained in the same manner as in Example 6 was subjected to mechanical grinding treatment, it can be seen that, compared to Example 6, the absolute dry density was higher and the water absorption rate was lower due to the mechanical grinding treatment. This indicates that the physical properties of the modified recycled fine aggregate obtained can be improved by the optional third step, mechanical grinding treatment. On the other hand, even when the recycled fine aggregate obtained in Comparative Example 3 was subjected to mechanical grinding treatment, the oven-dry density did not improve, and the water absorption rate hardly improved. From this, it is considered that performing mechanical grinding treatment after the second step, which is carbonation treatment, is effective in further improving the physical properties of the modified recycled fine aggregate.

[0076] [Comparative Example 5] (1st step) The concrete blocks generated during the building's demolition were crushed to a size of 40 mm or less. The obtained crushed material, which was less than 40 mm in size, was further crushed and classified using a sieve with a 5 mm perforation to obtain recycled coarse aggregate of 5 mm or larger (Comparative Example 1: Coarseness ratio 6.69%, Water absorption rate: 4.69%).

[0077] [Example 8] (2nd process) Comparative Example 5 involved placing 10 kg of recycled coarse aggregate obtained in the first step into a sealed space (a chamber with a volume of 850 L) and treating it with 10% carbon dioxide for 7 days to obtain modified recycled coarse aggregate. The particle size of the modified recycled coarse aggregate after processing was measured in the same manner as the recycled coarse aggregate obtained in the first step, and the coarseness ratio was 6.68.

[0078] [Example 9] (2nd process) Comparative Example 5 involved placing 10 kg of recycled coarse aggregate obtained in the first step into a sealed space (a chamber with a volume of 850 L) and treating it with 10% carbon dioxide for 14 days to obtain modified recycled coarse aggregate. When the particle size of the modified recycled coarse aggregate after processing was measured in the same manner as the recycled coarse aggregate obtained in the first step, the coarseness ratio was 6.67.

[0079] (Evaluation of recycled coarse aggregate) The recycled coarse aggregates of Comparative Example 5, Example 8, and Example 9 were evaluated in the same manner as the recycled fine aggregate of Example 1. The results are shown in Table 4 below.

[0080] [Table 4]

[0081] As is clear from Table 4, the modified recycled coarse aggregates of Examples 8 and 9, obtained by carbonation treatment, showed improved oven-dry density and lower water absorption compared to the recycled coarse aggregate obtained in Comparative Example 5, which was not subjected to carbonation treatment. This is thought to be because the silicon dioxide content in the modified recycled coarse aggregate increased to a favorable range, and furthermore, both the calcium carbonate content and the silicon dioxide content increased, bringing both the calcium carbonate content and the silicon dioxide content into the favorable range described above. In addition, a comparison of Examples 8 and 9 shows that in the carbonation treatment, Example 9, which underwent carbon dioxide treatment for 14 days, had a higher oven-dry density, lower water absorption, and higher calcium carbonate and silicon dioxide content compared to Example 8, which underwent carbon dioxide treatment for 7 days.

Claims

1. The first step involves obtaining recycled aggregate from crushed material with a particle size of 40 mm or less obtained by crushing or grinding concrete blocks, wherein the water absorption rate is greater than 3.0% and less than or equal to 9.96%. A second step involves carbonizing the recycled aggregate obtained in the first step to obtain a modified recycled aggregate containing calcium carbonate and silicon dioxide. A method for producing modified recycled aggregate having [a certain characteristic].

2. The method for producing a modified recycled aggregate according to claim 1, wherein the first step includes classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain recycled fine aggregate having a water absorption rate of more than 3.5% and 9.96% or less, and the modified recycled aggregate is modified recycled fine aggregate.

3. The method for producing a modified recycled aggregate according to claim 1, wherein the first step includes classifying the recycled aggregate into recycled coarse aggregate and recycled fine aggregate to obtain recycled coarse aggregate having a water absorption rate of more than 3.0% and 9.96% or less, and the modified recycled aggregate is modified recycled coarse aggregate.

4. A method for producing a modified recycled aggregate according to any one of claims 1 to 3, wherein the carbonation treatment in the second step is a dry treatment in which the recycled aggregate obtained in the first step is placed in a closed space and brought into contact with carbon dioxide gas at a concentration of 5% or more under a temperature atmosphere of 5°C to 200°C, or a wet treatment in which the recycled aggregate obtained in the first step is immersed in water and carbon dioxide gas is supplied to the water until the pH of the water reaches a range of 6 to 8.

5. The method for producing modified recycled aggregate according to claim 2, further comprising a third step of mechanically grinding the modified recycled fine aggregate obtained in the second step.

6. A modified recycled aggregate containing carbon oxides derived from concrete blocks with a water absorption rate exceeding 3.0%, containing calcium carbonate filling the surface and internal voids, and also containing silicon dioxide.

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