Cement composition

Abstract

To provide a cement composition capable of exhibiting sufficient strength.SOLUTION: The cement composition according to the present invention is a cement composition containing calcium oxide, silicon dioxide, and calcium carbonate. The calcium carbonate comprises calcite, aragonite, and vaterite. The mass ratio [(V) / (C)] of calcite (C) to vaterite (V) is 0.2 to 1.1, and the degree of carbonation is 70 mass% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to cement compositions. [Background technology]

[0002] Inorganic hardened materials used in the construction industry are often cementitious hardened materials containing calcium oxide, silicon dioxide, and aluminum oxide. Examples of such cementitious hardened materials include concrete and mortar. Cement compositions are essential for the production of concrete and mortar, but the production of cement compositions emits a large amount of carbon dioxide due to, for example, the decomposition of calcium carbonate, a main component of limestone. Therefore, alternative materials for cement compositions have been investigated.

[0003] For example, the reuse of waste concrete as an alternative material for cement compositions has been investigated, and it has been reported that by crushing waste concrete and subjecting it to compression molding, it is possible to regenerate a hardened body with sufficient strength without sorting or separating aggregates (Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Yuya Sakai et al., Journal of the Japan Society of Civil Engineers, Vol. 72, No. 1, pp. 32-40 (2016) [Non-patent document 2] Yuya Sakai et al., Journal of the Japan Society of Civil Engineers, Vol. 76, No. 4, pp. 306-314 (2020) Summary of the Invention [Problem to be solved by the invention]

[0005] However, even if a hardened body is produced from crushed waste concrete using these methods, the strength is lower than that of ordinary concrete, and there is still room for improvement. An object of the present invention is to provide a cement composition capable of exhibiting sufficient strength and a method for producing a hardened product using the same. [Means for solving the problem]

[0006] The present inventors have conducted various studies in light of the above-mentioned problems and have found that, among cement compositions containing calcium oxide, silicon dioxide, and calcium carbonate, a cement composition containing calcite and vaterite, crystalline polymorphs of calcium carbonate, in a specific mass ratio and having a degree of carbonation not greater than a specific value can form a hardened body having sufficient strength without requiring a high molding pressure.

[0007] That is, the present invention provides the following [1] to [5]. [1] A cement composition containing calcium oxide, silicon dioxide, and calcium carbonate, calcium carbonate includes calcite, aragonite and vaterite; The mass ratio of calcite (C) to vaterite (V) [(V) / (C)] is 0.2 to 1.1, The carbonation degree is 70% by mass or less. Cement compositions. [2] The cement composition according to [1] above, wherein the mass ratio of calcite (C) to aragonite (A) [(A) / (C)] is 0.04 to 0.3. [3] The cement composition according to [1] or [2] above, wherein the content of calcite in calcium carbonate is 46 to 80 mass %. [4] The cement composition according to any one of [1] to [3] above, which is derived from a cementitious hardened body or a waste material thereof. [5] A first step of mixing the cement composition according to any one of [1] to [4] with water and pressurizing the mixture to produce a molded body; The second step is to cure the compact. A method for producing a hardened body, comprising: [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a cement composition that can develop sufficient strength. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Cement composition> (chemical composition) The cement composition of the present invention contains calcium oxide, silicon dioxide, and calcium carbonate. The cement composition of the present invention may contain inorganic compounds other than the three types mentioned above. Examples of inorganic compounds include, but are not limited to, aluminum oxide, calcium hydroxide, calcium silicate, aluminum hydroxide, calcium aluminate, aluminum silicate, silica gel, alumina gel, magnesium oxide, and iron oxide.

[0010] The content of calcium oxide in the cement composition of the present invention is preferably 25 to 65 mass %, more preferably 30 to 60 mass %, even more preferably 30 to 55 mass %, and even more preferably 35 to 50 mass %. The content of silicon dioxide in the cement composition of the present invention is preferably 5 to 30 mass %, more preferably 8 to 25 mass %, and even more preferably 12 to 20 mass %. The content of calcium carbonate in the cement composition of the present invention is preferably 30 to 60 mass %, more preferably 30 to 55 mass %, and even more preferably 35 to 50 mass %. The content of inorganic compounds other than the above three types can be appropriately set within a range that does not impair the object of the present invention.

[0011] (crystal polymorphism) The cement composition of the present invention contains three crystalline polymorphs of calcium carbonate: calcite, aragonite, and vaterite. Of these three types, vaterite is known to be thermodynamically unstable and readily transforms into calcite. For this reason, most of the calcium carbonate found in nature is calcite. However, the present inventors have discovered that when the mass ratio of calcite (C) to vaterite (V) [(V) / (C)] is within a specific range, a hardened body capable of exhibiting sufficient strength can be formed without requiring high molding pressure.

[0012] That is, the cement composition of the present invention has a mass ratio of calcite (C) to vaterite (V) [(V) / (C)] of 0.2 to 1.1, but from the viewpoint of further increasing the strength of the hardened body, it is preferably 0.25 to 0.9, more preferably 0.28 to 0.9, and even more preferably 0.3 to 0.8. In the cement composition of the present invention, the mass ratio of aragonite (A) to calcite (C) [(A) / (C)] is preferably 0.04 to 0.3, more preferably 0.06 to 0.25, and even more preferably 0.08 to 0.2, from the viewpoint of further increasing the strength of the hardened body.

[0013] The content of calcite in calcium carbonate is preferably 46 to 80 mass %, more preferably 48 to 75 mass %, and even more preferably 50 to 70 mass %, from the viewpoint of further increasing the strength of the hardened body. The content of vaterite in calcium carbonate is preferably from 15 to 45 mass %, more preferably from 18 to 43 mass %, and even more preferably from 20 to 40 mass %, from the viewpoint of further increasing the strength of the hardened body. The content of aragonite in calcium carbonate is the remainder excluding the total content of calcite and vaterite. For example, the content of aragonite in calcium carbonate is preferably 2 to 14 mass%, more preferably 3 to 12 mass%, and even more preferably 5 to 10 mass%, from the viewpoint of further increasing the strength of the hardened body.

[0014] The proportion of each crystalline polymorph in calcium carbonate can be determined by powder X-ray diffraction measurement. That is, the crystalline minerals (rock-forming minerals, cement hydrates, cement clinker minerals, calcium carbonate) contained in the sample can be identified from the diffraction pattern obtained by XRD measurement, and the amount of each crystalline polymorph of calcium carbonate can be quantified by Rietveld analysis. Specifically, the method described in the Examples below can be mentioned. The XRD measurement conditions are as follows:

[0015] XRD measurement conditions ·X-ray used: CuKα Tube conditions: Tube voltage 40kV - Tube current 40mA Scanning range: 5~65° Step width: 0.023° / step Measurement time: 0.13 seconds / step Rietveld analysis software: TOPAS Version 5.0 (Bruker AXS)

[0016] The proportion of each crystal polymorph in calcium carbonate can be calculated by the following formula based on the amount of mineral calculated by Rietveld analysis.

[0017] Percentage of calcite in calcium carbonate (%) = C / (C + V + A) x 100 Percentage of vaterite in calcium carbonate (%) = V / (C + V + A) x 100 Percentage of aragonite in calcium carbonate (%) = A / (C + V + A) x 100

[0018] [During the ceremony, C indicates the amount of calcite mineral (mass%), V indicates the amount of vaterite minerals (mass%), A indicates the amount of aragonite (mass%).

[0019] (carbonation degree) The cement composition of the present invention has a carbonation degree of 70% by mass or less, and from the viewpoint of further increasing the strength of the hardened body, it is preferably 65% ​​by mass or less, more preferably 60% by mass or less, even more preferably 55% by mass or less, even more preferably 50% by mass or less, even more preferably 45% by mass or less, and even more preferably 40% by mass or less. Note that, from the viewpoint of further increasing the strength of the hardened body, the lower limit of the carbonation degree is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more.

[0020] Here, in this specification, the term "carbonation degree" refers to the ratio of carbonated calcium oxide to calcium oxide in a cement composition. The "amount of calcium oxide" in a cement composition can be calculated by thermogravimetric analysis (TG measurement) under the following conditions, and the weight loss (amount of decarbonation) at 650 to 800°C can be calculated using the following formula (i). The "carbonation degree" can be calculated by calculating the amount of calcium carbonate from this weight loss (amount of decarbonation), and then using the following formula (ii). A commercially available thermogravimetric analyzer can be used, such as Thermo plus EV02 TG8121 (manufactured by Rigaku Corporation).

[0021] TG-DTA measurement conditions Sample amount: approx. 20 mg Heating rate: 10℃ / min N2 gas flow: 300 mL / min

[0022] Calcium carbonate amount (g) =Q / (PS)×100 / 44×100×100 / (100-R) (i)

[0023] [In formula (i), P indicates the amount of sample (g) used in the analysis, Q represents the weight loss (decarbonation amount) (g) between 650 and 800°C, R indicates the amount of insoluble residue (g) in the sample. S is the weight loss (g) from room temperature to 1000°C

[0024] Carbonation degree (mass%) =(X×56 / 100×100)÷[Y×100 / (100-R)] (ii)

[0025] [In formula (ii), X represents the amount of calcium carbonate in the sample (g), Y indicates the amount of calcium oxide (g) in the sample. R indicates the amount of insoluble residue (g) in the sample.

[0026] If the sample contains aggregate, the amount that did not dissolve in the solvent (insoluble residue) can be determined according to the method specified in "JIS R 5202:2010 (Methods for chemical analysis of cement)" "6. Method for determining insoluble residue by hydrochloric acid-sodium carbonate method."

[0027] As described in the section on the prior art, it has been reported that by crushing waste concrete and subjecting it to compression molding, it is possible to regenerate a hardened body with sufficient strength without sorting or separating the aggregates, but this is mainly due to the recovery of bonds due to hydrogen bonds on the CSH surface, and it was thought that waste concrete that has undergone a carbonation reaction would not bond. However, the present inventors have unexpectedly discovered that even with waste concrete that has undergone a carbonation reaction, a hardened body with sufficient strength can be formed by adjusting the mass ratio of calcite (C) to vaterite (V) [(V) / (C)] within the above-mentioned range.

[0028] <Method of manufacturing cement composition> (raw materials) In producing the cement composition of the present invention, for example, cementitious hardened body, incineration ash, slag (hereinafter also referred to as "cementitious hardened body, etc.") can be used as raw materials. Herein, the term "cementitious hardened body" means a product obtained by hardening a composition containing cement and water, and is a concept that encompasses not only completely hardened hardened bodies but also semi-hardened hardened bodies (in other words, those in the process of hardening).

[0029] Examples of cementitious hardened bodies include hardened bodies made of concrete or mortar, and hardened bodies made of cement paste. From the viewpoint of effective utilization of resources, waste cementitious hardened bodies can be preferably used. Examples of waste cementitious hardened bodies include demolished concrete and demolished mortar generated during civil engineering work or the demolition of structures, and sludge (fully hardened or semi-hardened sludge after dehydration) generated from ready-mixed concrete. When waste concrete is used as the waste cementitious hardened body, the aggregate content is preferably less than 85% by mass, and when waste mortar is used, the aggregate content is preferably less than 70% by mass. Furthermore, when using waste cementitious hardened bodies, it is preferable to separate the aggregate from the waste material, as this makes it easier to enjoy the effects of the present invention. The separated aggregate can be reused as recycled aggregate.

[0030] Examples of incineration ash include fly ash types I to IV specified in "JIS A 6201:2015 (Fly ash for concrete)," which are obtained by collecting coal ash generated by the combustion of pulverized coal at thermal power plants using an electrostatic precipitator or by classifying or pulverizing the collected coal ash. Other examples include clinker ash and biomass ash generated by the combustion of pulverized coal or biomass at thermal power plants. Furthermore, examples include municipal waste incineration ash and sewage sludge incineration ash.

[0031] Examples of slag include slag produced in melting and refining processes in various processes, including steel manufacturing processes, etc. Specific examples include blast furnace slag, steelmaking slag (e.g., converter decarburization slag, dephosphorization slag, desiliconization slag, desulfurization slag, electric furnace slag, and casting slag), smelting reduction slag (e.g., slag produced by melting and reducing iron ore, Cr ore, Ni ore, Mn ore, and the like), slag produced in other smelting furnaces and refining furnaces, refuse incineration ash molten slag, and waste gasification molten slag.

[0032] The size of the cementitious hardened body is not particularly limited as long as it can be mixed with water and pressurized, but the maximum particle size is usually less than 40 mm, preferably less than 35 mm, and more preferably less than 30 mm. Here, in this specification, the "maximum particle size" refers to the particle size represented by the smallest mesh size of a sieve through which the entire sample can pass. From the viewpoint of ease of handling, the form of the cementitious hardened body is preferably granular, powdery, or a mixture thereof, more preferably powdery. When powdery, the maximum particle size is preferably less than 5 mm, more preferably less than 3 mm, and even more preferably less than 1 mm.

[0033] In order to adjust the cementitious hardened material or the like to a desired particle size, one or more selected from crushing, grinding, and sieving can be carried out. Crushing can be performed using a crusher. Examples of such crushers include a jaw crusher, impact crusher, hammer crusher, roll crusher, and rotary crusher. In order to adjust the particle size of the cementitious hardened body or the like, the crusher may be equipped with a screen having a desired mesh size, or if no screen is equipped, the fixed teeth, rotating teeth, inner wall, and the like may be adjusted to the desired clearance. The grinding can be carried out using a grinder, such as a disc mill, a Wonder Blender, a rod mill, a ball mill, or a roller mill. The sieving may be performed using a sieve separator, and any of the vibrating, in-plane moving, rotary and stationary types may be used. Examples of the types of sieves include woven mesh, punched mesh, welded mesh, wedge wire screen and comb teeth made of metal or non-metal.

[0034] (Adjustment of mass ratio [(V) / (C)] and carbonation degree) The mass ratio [(V) / (C)] and the carbonation degree can be adjusted by subjecting the raw materials to a neutralization treatment. A carbonation accelerator can be used for the carbonation treatment. Note that the carbonation accelerator can be a commercially available device, such as a carbonation accelerator test device (manufactured by Marui Co., Ltd.). The treatment conditions may be appropriately selected to achieve the desired mass ratio [(V) / (C)] and carbonation degree, such as temperature, humidity, CO2 concentration, and carbonation period. For example, by setting the temperature at 45 to 60°C, humidity at 80%, CO2 concentration at 5%, and a carbonation period of 3 days, the mass ratio [(V) / (C)] can be 0.25 to 1.1 and the carbonation degree can be 70 mass% or less.

[0035] The carbon dioxide used in the neutralization treatment may be carbon dioxide filled in a commercially available cylinder, but is preferably derived from atmospheric carbon dioxide or carbon dioxide in various industrial exhaust gases. In particular, carbon dioxide derived from various industrial exhaust gases, including carbon dioxide emitted into the atmosphere and dispersed during cement production by calcination, can contribute to the fixation of atmospheric carbon dioxide. Note that carbon dioxide with a higher CO2 concentration is preferred because it accelerates the formation of carbonates.

[0036] The carbon dioxide injection method can be any conventionally known method that can dissolve carbon dioxide in a liquid to produce bicarbonate, including, but not limited to, methods using machines such as bubbling, a micropore method, a pressurized dissolution method, an ultrasonic method, a swirling liquid flow method, and a gas-liquid mixed shear method. The amount of carbon dioxide blown in is, for example, preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more, based on the saturation amount.

[0037] Alternatively, one or more types of cementitious hardened materials (for example, waste concrete, waste mortar) or waste materials thereof may be selected, and the mass ratio [(V) / (C)] and carbonation degree may be adjusted.

[0038] <Method of manufacturing the hardened body> The method for producing a cured product of the present invention includes a first step and a second step. (First step) This step involves mixing the cement composition of the present invention with water and then pressure-molding the mixture to produce a molded body. Examples of water include pure water, tap water as defined in JIS A 5308 Appendix C, and water other than tap water (for example, rainwater, river water, lake water, well water, groundwater, and industrial water). The amount of water used can be selected appropriately, but is preferably 5% by mass or more, more preferably 8% by mass or more, and is preferably 20% by mass or less, more preferably 15% by mass or less, based on the cement composition.

[0039] The cement composition and water may be mixed by hand or using a mixer. During mixing, in order to promote strength development, used alkanolamine obtained from a carbon dioxide capture device, an amine solution used as a cement grinding aid, or a solution of an alkaline earth compound such as calcium bicarbonate solution may be added. Aggregates, fibers, etc. may also be added. Examples of aggregates include crushed gravel, sand, and rocks collected from rivers and oceans, and recycled aggregates. Examples of fibers include commercially available concrete materials such as steel fibers for concrete, carbon fibers, glass fibers, basalt fibers (basalt fibers, which are made by melting and vitrifying basalt), and plastic fibers. The amounts of aggregate and fiber used may be appropriately selected within a range that does not impair the objectives of the present invention.

[0040] Pressure molding can be carried out, for example, by placing the mixture in a mold, etc. The shape of the mold can be appropriately selected depending on the application of the cured product. The molding pressure is preferably 1 MPa or more, more preferably 2 MPa or more, and even more preferably 3 MPa or more, and is preferably 100 MPa or less, more preferably 50 MPa or less, and even more preferably 25 MPa or less. By using such a molding pressure, sufficient strength can be imparted to the cured product.

[0041] (Second step) This step is a step of curing the compact. The curing method is not particularly limited as long as it can increase the strength of the compact, but examples include (1) drying, (2) carbonation treatment, (3) contact with a liquid containing a soluble alkaline earth metal salt, and (4) contact with a second mixed liquid containing a sulfate and bicarbonate prepared by blowing carbon dioxide into a first mixed liquid containing a Group 2 element-containing material and a sulfate. (1) to (4) may be used alone or in combination of two or more.

[0042] (1) Drying In this specification, the term "drying" refers to reducing the moisture content in a molded body. Therefore, the concept of drying includes not only heating and drying a molded body, but also air drying and drying under reduced pressure. The drying is preferably carried out until the moisture content of the dried molded body is preferably 24% by mass or less, more preferably 17% by mass or less, and even more preferably 12% by mass or less.

[0043] (2) Carbonation treatment The carbonation treatment is preferably carried out until the relative increase in the carbonation degree reaches preferably 3% or more, more preferably 5% or more, and even more preferably 9% or more. The carbonation treatment may be carried out in the same manner as the neutralization treatment described above.

[0044] (3) Contact with liquids containing soluble alkaline earth metal salts The liquid containing a soluble alkaline earth metal salt is not particularly limited as long as it contains a soluble alkaline earth metal, and may be a solution or a liquid in which the salt is suspended or dispersed, such as a slurry. Here, in this specification, "soluble" means soluble in water. The solvent contained in the liquid is usually water. Note that water is as described above.

[0045] The soluble alkaline earth metal salt is not particularly limited as long as it is soluble, and may be any of a soluble beryllium salt, a soluble magnesium salt, a soluble calcium salt, a soluble strontium salt, a soluble barium salt, and a soluble radium salt. However, in terms of making it easier to enjoy the effects of the present invention, it is preferable to include one or more soluble alkaline earth metal salts selected from soluble calcium salts and soluble magnesium salts.

[0046] Examples of soluble calcium salts include calcium bicarbonate, calcium sulfate, and calcium chloride, which may be in the form of a hydrate. One or more soluble calcium salts may be contained. Examples of soluble magnesium salts include magnesium bicarbonate, magnesium sulfate, magnesium nitrate, magnesium acetate, and magnesium chloride, which may be in the form of a hydrate. One or more soluble magnesium salts may be contained.

[0047] Among these, from the viewpoint of enhancing the strength of the hardened body, the soluble alkaline earth metal salt preferably contains one or more selected from calcium bicarbonate, calcium sulfate, calcium chloride, magnesium sulfate, magnesium nitrate, magnesium acetate, and magnesium chloride, and more preferably contains one or more selected from calcium bicarbonate, calcium chloride, magnesium bicarbonate, and magnesium chloride. The soluble alkaline earth metal salt may be in the form of a hydrate.

[0048] When a reagent is used as the soluble alkaline earth metal salt, the liquid may be prepared by, for example, simultaneously mixing the soluble alkaline earth metal salt and water, or by adding one to the other and mixing them, and the order of mixing is not particularly limited.

[0049] The soluble alkaline earth metal salt may also be derived from a composition containing, for example, a calcium compound or a magnesium compound. Examples of such compositions include limestone, waste concrete, waste mortar, and other hardened cementitious waste materials, sludge generated in ready-mix concrete (fully hardened or semi-hardened after dehydration), and seawater. A liquid containing calcium bicarbonate and / or magnesium bicarbonate can be prepared by injecting carbon dioxide into a liquid containing such a composition. The method for injecting carbon dioxide is as described in the neutralization treatment section, and the carbon dioxide is preferably derived from CO2 contained in various industrial exhaust gases.

[0050] From the viewpoint of enhancing the strength of the cured body, the content of soluble alkaline earth metal salt in the liquid is preferably 600 mass ppm or more, more preferably 800 mass ppm or more, even more preferably 1500 mass ppm or more, and even more preferably 2000 mass ppm or more, in terms of the alkaline earth metal ion content in the liquid. Since a higher alkaline earth metal ion content promotes the dissolution of calcium carbonate, there is no particular upper limit for the alkaline earth metal ion content in the liquid, and it may be the saturation amount. The calcium ion content can be measured using an ICP atomic emission spectrometer (Thermoscientific, iCAP Pro).

[0051] (4) Contact with a second mixture prepared by blowing carbon dioxide into the first mixture. The Group 2 element-containing material contained in the first mixed liquid is not particularly limited as long as it is a compound containing a Group 2 element of the periodic table, but it preferably contains one or more compounds selected from calcium compounds and magnesium compounds. The calcium compound and magnesium compound are preferably in the form of one or more selected from carbonates, oxides, hydroxides, silicates, and aluminum salts. Specific examples include carbonates such as calcium carbonate and magnesium carbonate, oxides such as calcium oxide and magnesium oxide, hydroxides such as calcium hydroxide and magnesium hydroxide, and silicates such as calcium silicate and magnesium silicate. Furthermore, examples of the Group 2 element-containing material that may be used include limestone, waste concrete, waste mortar, and other hardened cementitious materials, sludge generated in ready-mixed concrete (fully hardened sludge or semi-hardened sludge after dehydration treatment), and slag.

[0052] The sulfate contained in the first mixed solution may be a normal salt or an acid salt, and one or more kinds of sulfates may be contained. Examples of normal salts of sulfates include aluminum sulfate, ammonium sulfate, potassium sulfate, calcium sulfate, sodium sulfate, beryllium sulfate, magnesium sulfate, lithium sulfate, and rubidium sulfate. Examples of acid salts of sulfates include ammonium hydrogen sulfate, potassium hydrogen sulfate, and sodium hydrogen sulfate. Among these, from the viewpoint of increasing the strength of the cured product, normal salts are preferred, sulfates of Group 2 elements are more preferred, and one or two selected from calcium sulfate and magnesium sulfate are even more preferred.

[0053] The mass ratio of sulfate to water (sulfate / water) in the first mixed liquid is preferably 0.1 / 100 to 20 / 100, more preferably 1 / 100 to 15 / 100, and even more preferably 5 / 100 to 10 / 100, from the viewpoint of imparting sufficient strength to the cured body. The mass ratio of the Group 2 element-containing material to the sulfate and water in the first mixed liquid [Group 2 element-containing material / (water+sulfate)] is preferably 0.2 / 100 to 10 / 100, more preferably 0.5 / 100 to 5 / 100, and even more preferably 1 / 100 to 2 / 100, from the viewpoint of imparting sufficient strength to the cured body. The first mixed solution may be prepared by adding the Group 2 element-containing material, sulfate, and water simultaneously or in any order. Alternatively, at least one of the Group 2 element-containing material and sulfate may be in the form of an aqueous solution, and the two may be mixed in any order to prepare the mixed solution. The first mixed solution may be a suspension or dispersion.

[0054] Carbon dioxide is blown into the first mixture to prepare a second mixture containing sulfate and hydrogen carbonate. For example, if the Group 2 element-containing material and / or sulfate contains calcium as the Group 2 element, Ca(HCO3)2 will be produced in the second mixed liquid, and if it contains magnesium, Mg(HCO3)2 will be produced in the second mixed liquid. The second mixture contains HCO as the anion. 3- As long as a salt containing the above is formed, it may be in the form of a suspended or dispersed liquid, such as a slurry.

[0055] The method of blowing carbon dioxide into the first mixed liquid is as explained in the neutralization treatment, and the carbon dioxide is preferably derived from CO2 contained in various industrial exhaust gases.

[0056] The molded body is brought into contact with the liquid (3) or the second mixed liquid (4). Examples of the contacting method include, but are not limited to, the following methods. (i) A method of injecting the liquid (3) or the second mixed liquid (4) into a molded body filled in a mold. (ii) A method of spraying the liquid (3) or the second mixed liquid (4) onto the molded body removed from the mold. (iii) A method of immersing the molded body removed from the mold in the liquid (3) or the second mixed liquid (4). (iv) A method in which (ii) or (iii) is carried out, followed by drying, and (ii) or (iii) and drying are alternately repeated.

[0057] From the viewpoint of increasing the strength of the cured body, the contact temperature is preferably 5°C or higher, more preferably 10°C or higher, and even more preferably 15°C or higher, and from the viewpoint of economy, the contact temperature is preferably 90°C or lower, more preferably 80°C or lower, and even more preferably 70°C or lower. The contact time can be appropriately selected depending on the contact temperature. For example, at room temperature (20°C±15°C), from the viewpoint of increasing the strength of the cured body, the contact time is preferably 1 day or more, more preferably 3 days or more, and even more preferably 5 days or more. From the viewpoint of production efficiency, the contact time is preferably 50 days or less, more preferably 40 days or less, and even more preferably 30 days or less.

[0058] In this manner, a cured product can be produced, and the resulting cured product can have the following properties. The compressive strength of the hardened body is preferably 25N / mm 2 More preferably, it is 30 N / mm 2 More preferably, 35N / mm 2 That's all.

[0059] In this specification, the compressive strength of a hardened body is measured by compressing the hardened body (test specimen) using a conventional universal testing machine (tensile / compression testing machine), measuring the load, recording the maximum value, and dividing the load by the cross-sectional area to obtain the stress (compressive strength (N / mm 2 )) can be calculated as follows.

[0060] The hardened product of the present invention can be used as a substitute for concrete, for example, but is not limited to, construction materials, more specifically, building members such as pillars, beams, and slabs, solidified bodies such as blocks, foundations for buildings, and piles. [Example]

[0061] The following examples will explain the present invention in more detail, but the present invention is not limited to the examples below.

[0062] Examples 1 to 2 and Comparative Examples 1 to 4 (1-1) Production of cement composition First, a cement paste (W / C = 0.5) was mixed using ordinary Portland cement and water. The cement paste was then poured into a formwork and demolded after one day. The demolded hardened cementitious body was then cured in water for 28 days (28 d). The cured hardened cementitious body was then dried at 105°C for 7 days, then roughly crushed to 5 mm or less using a jaw crusher and pulverized using a ball mill. The crushed mortar was sieved using a 0.6 mm sieve, and the powder that fell through the sieve was collected and used in the experiment.

[0063] The powder that passed through the sieve was neutralized using a neutralization apparatus under the conditions shown in Table 1 below, or neutralized in an atmosphere of 20°C and 60% RH (CO2 concentration 0.04%) for the period shown in Table 1 to produce a cement composition. This cement composition was produced using waste cementitious hardened bodies simulating construction waste as the raw material.

[0064] [Table 1]

[0065] (1-2) Analysis of cement composition (i) Chemical composition The chemical composition was determined by measuring the loss on ignition in accordance with JIS R 5201. Furthermore, fluorescent X-ray analysis (calibration curve method) was performed using an X-ray fluorescence analyzer (ZSX primus II, manufactured by Rigaku Corporation) to determine the contents of SiO2, Al2O3, Fe2O3, CaO, MgO, and SO3. The results are shown in Table 2.

[0066] [Table 2]

[0067] (ii) Analysis of calcium carbonate content and carbonation degree The amount of calcium carbonate was calculated by thermogravimetric analysis (TG measurement) under the following conditions from the weight loss (amount of decarbonation) at 650 to 800°C using the following formula (i). The degree of carbonation was calculated by the following formula (ii) after calculating the amount of calcium carbonate using the following formula (i). These results are shown in Table 3.

[0068] If the sample contains aggregate, it can be determined as the amount that did not dissolve in the solvent (insoluble residue) according to the method specified in "JIS R 5202:2010 (Methods for chemical analysis of cement)" "6. Method for determining insoluble residue by hydrochloric acid-sodium carbonate method." Note that the waste cementitious hardened body used in this example does not contain aggregate, so the amount is zero.

[0069] TG-DTA measurement conditions Sample amount: approx. 20 mg Heating rate: 10℃ / min N2 gas flow: 300 mL / min

[0070] Calcium carbonate amount (g) =Q / (PS)×100 / 44×100×100 / (100-R) (i)

[0071] [In formula (i), P indicates the amount of sample (g) used in the analysis, Q represents the weight loss (decarbonation amount) (g) between 650 and 800°C, R indicates the amount of insoluble residue (g) in the sample. S is the weight loss (g) from room temperature to 1000°C

[0072] Carbonation degree (mass%) =(X×56 / 100×100)÷[Y×100 / (100-R)] (ii)

[0073] [In formula (ii), X represents the amount of calcium carbonate in the sample (g), Y indicates the amount of calcium oxide (g) in the sample. R indicates the amount of insoluble residue (g) in the sample.

[0074] (iii) Analysis of the proportion of each polymorph in calcium carbonate The proportion of each crystalline polymorph in calcium carbonate was determined by powder X-ray diffraction (XRD) measurement using a powder X-ray diffraction (XRD) apparatus (D8-Advance A-25, manufactured by Bruker AXS) under the following conditions. From the obtained diffraction pattern, the crystalline minerals contained in the sample (rock-forming minerals, cement hydrates, cement clinker minerals, calcium carbonate) were identified, and Rietveld analysis was performed to quantify the mineral amounts of each crystalline polymorph of calcium carbonate. The powder X-ray diffraction measurement was performed using a sample to which 10% corundum (α-Al2O3) was added as an internal standard.

[0075] XRD measurement conditions ·X-ray used: CuKα Tube conditions: Tube voltage 40kV - Tube current 40mA Scanning range: 5~65° Step width: 0.023° / step Measurement time: 0.13 seconds / step Rietveld analysis software: TOPAS Version 5.0 (Bruker AXS)

[0076] In this example, the cementitious hardened body waste material did not contain any aggregate, so the minerals were set as hydrated minerals (ettringite, monosulfate, monocarbonate, hemicarbonate, portlandite), amorphous cement clinker minerals (belite, ferrite phase), and calcium carbonate (calcite, vaterite, aragonite). However, if aggregate is contained, the rock-forming minerals can be set appropriately. The analysis results are shown in Table 3.

[0077] Mx=Mx'×Cm / α1×100 / (100-Cm) (iii)

[0078] [In formula (iii), Mx indicates the mineral content (mass%) of each crystal polymorph of calcium carbonate in the sample, Mx' represents the quantitative value (mass%) of each crystal polymorph of calcium carbonate obtained by Rietveld analysis, Cm indicates the corundum (α-Al2O3) mixing ratio (mass%), α1 indicates the quantitative value (mass%) of corundum (α-Al2O3) obtained by Rietveld analysis.

[0079] The proportion of each crystal polymorph in calcium carbonate was calculated using the following formula based on the mineral amount calculated by Rietveld analysis.

[0080] Percentage of calcite in calcium carbonate (%) = C / (C + V + A) x 100 Percentage of vaterite in calcium carbonate (%) = V / (C + V + A) x 100 Percentage of aragonite in calcium carbonate (%) = A / (C + V + A) x 100

[0081] [During the ceremony, C indicates the amount of calcite mineral (mass%), V indicates the amount of vaterite minerals (mass%), A indicates the amount of aragonite (mass%).

[0082] (2-1) Manufacturing of hardened body Using the cement compositions obtained in each of the Examples and Comparative Examples, hardened bodies were produced according to the following procedure. i) Production of molded bodies The cement composition was mixed with 10% water, and a pellet molding machine was used to produce a pellet with a diameter of 10 mm and a size of 20 mm. The molding pressure was 10 MPa.

[0083] ii)Curing The molded body was dried in a dryer at 105°C for more than 12 hours. Next, 15 g of the above cement composition and 30 g of magnesium sulfate were dissolved in 0.5 L of ion-exchanged water. Next, CO2 gas was blown in at a rate of 0.5 L / min while stirring at 300 rpm for 1 hour to prepare a calcium bicarbonate + magnesium sulfate aqueous solution. Then, a test specimen prepared by compression molding was immersed in this aqueous solution for 2 hours. Note that CO2 gas blowing and stirring of the solution continued during immersion. After immersion, the test specimen was dried in a dryer at 105°C for more than 12 hours. The operation from immersion to drying was considered one cycle, and a total of three cycles of dry-wet repeated curing were performed. After curing, the test specimens were subjected to a compressive strength test.

[0084] (2-2) Analysis of hardened body (i) Compressive strength measurement The load was measured using a displacement-controlled universal testing machine (manufactured by Instron Japan Co., Ltd.). The maximum load was then divided by the cross-sectional area of ​​the hardened body to determine the compressive strength. The results are shown in Table 3.

[0085] (ii) Analysis of calcium carbonate content and carbonation degree The analysis was carried out in the same manner as in (1-2) Analysis of cement composition (ii). The results are shown in Table 3.

[0086] [Table 3]

[0087] Table 3 shows that by forming a molded body using a cement composition containing calcite and vaterite, crystalline polymorphs of calcium carbonate, in a specific mass ratio [(V) / (C)] and with a carbonation level below a specific value, and then curing it, a hardened body with sufficient strength can be obtained without requiring high molding pressure.

Claims

1. A cement composition comprising calcium oxide, silicon dioxide and calcium carbonate, calcium carbonate includes calcite, aragonite and vaterite; The mass ratio of calcite (C) to vaterite (V) [(V) / (C)] is 0.2 to 1.1; The carbonation degree is 70% by mass or less. Cement compositions.

2. 2. The cement composition according to claim 1, wherein the mass ratio of calcite (C) to aragonite (A) [(A) / (C)] is 0.04 to 0.

3.

3. 3. The cement composition according to claim 1, wherein the content of calcite in the calcium carbonate is 46 to 80 mass %.

4. 3. The cement composition according to claim 1, which is derived from a cementitious hardened body or a waste material thereof.

5. A first step of mixing the cement composition according to claim 1 or 2 with water and press-molding the mixture to prepare a molded body; The second step of curing the compact A method for producing a hardened body, comprising:

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