Concrete composition, cured body, and method for producing cured body

Abstract

Provided is a concrete composition containing a binder, a curing accelerator for a hydraulic material, and water, wherein: the binder content is 300 kg / m3 or more; the water-binder ratio is 55% or less; the added amount of the curing accelerator for a hydraulic material is 1-50 kg / m3; and the curing accelerator for a hydraulic material contains 20.0-80.0 mass% of calcium formate, 15.0-70.0 mass% of an inorganic calcium compound, and 0.5-30.0 mass% of an inorganic sulfate excluding calcium sulfate.

Description

Concrete composition, hardened body, and method for producing the hardened body

[0001] The present invention relates to a concrete composition, a hardened concrete body, and a method for producing the same.

[0002] Hydraulic materials such as cement used in civil engineering and construction fields are usually mixed with water and allowed to harden by standing for a predetermined period of time. The hardening speed of hydraulic materials can be affected by the ratio of the material to water, the ambient temperature, and the curing method, but the time it takes for hydraulic materials to harden can be shortened by using admixtures that accelerate hardening, i.e., hardening accelerators.

[0003] Reducing the time it takes for hydraulic materials to harden leads to improved productivity at work sites. For example, hardened concrete used in precast construction methods for reinforced concrete buildings and the like can generally be obtained by pouring a cement composition into a formwork, leaving it to stand for a predetermined period of time, and then curing it by steam curing or the like. However, the use of a hardening accelerator can reduce the time it takes for the concrete to reach an initial strength that allows it to be removed from the form, allowing for efficient production of hardened concrete.

[0004] Furthermore, after pouring the cement composition, it is usually "compacted" using a vibrator or the like to distribute the cement composition evenly within the formwork. However, if the fluidity of the cement composition is reduced immediately after pouring, compaction requires time and effort, resulting in reduced productivity.

[0005] Regarding hardening accelerators, for example, Patent Document 1 discloses a hardening accelerator for hydraulic materials containing predetermined amounts of inorganic sulfate, calcium sulfoaluminate, and inorganic hydroxide. Patent Document 2 discloses a hardening accelerator for hydraulic materials containing a material having a Blaine specific surface area of ​​4000 cm ２ The document discloses a cement admixture containing calcium sulfoaluminate in an amount of 1 / g or more and one or more salts selected from the group consisting of formates, acetates and lactates.

[0006] JP 2014-19618 A JP 2010-235399 A

[0007] However, hydraulic materials using the above-mentioned hardening accelerators lose their fluidity immediately after mixing with water, and compaction after pouring requires effort and time, which raises concerns about reduced productivity.

[0008] In view of the above, an object of the present invention is to provide a concrete composition having good fluidity retention, setting properties and early strength development properties.

[0009] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that the above problems can be solved by a concrete composition containing a binder, a hydraulic material hardening accelerator, and water, in which the binder blend amount, water-binder ratio, and hydraulic material hardening accelerator addition amount are within predetermined ranges, and the hydraulic material hardening accelerator has a specific composition, thereby completing the present invention.

[0010] [1] A concrete composition containing a binder, a hardening accelerator for hydraulic materials, and water, wherein the binder blending amount is 300 kg / m ３ or more, the water-binder ratio is 55% or less, and the amount of the hardening accelerator for hydraulic materials added is 1 to 50 kg / m ３A concrete composition, wherein the hardening accelerator for hydraulic materials contains 20.0 to 80.0 mass% calcium formate, 15.0 to 70.0 mass% inorganic calcium compound, and 0.5 to 30.0 mass% inorganic sulfates excluding calcium sulfate. [2] The concrete composition according to [1] above, wherein the inorganic calcium compound is one or more selected from the group consisting of calcium sulfate, calcium hydroxide, calcium carbonate, and calcium oxide. [3] The concrete composition according to [1] or [2] above, wherein the inorganic sulfate is one or more selected from the group consisting of sulfate, thiosulfate, sulfite, bisulfite, pyrosulfate, and pyrobisulfite. [4] The concrete composition according to any one of [1] to [3] above, wherein the binder contains 1 to 15 mass% calcium sulfoaluminate. [5] A hardened body obtained by hardening the concrete composition according to any one of [1] to [4] above. [6] A method for producing a hardened concrete body, comprising curing the concrete composition according to any one of [1] to [4] above at a temperature of 40°C or less to harden it.

[0011] According to the present invention, it is possible to provide a concrete composition having good fluidity retention, setting properties and early strength development properties.

[0012] Hereinafter, one embodiment of the present invention (the present embodiment) will be described in detail, but the present invention is not limited to this embodiment. In addition, "%" and "parts" in this specification are based on mass unless otherwise specified.

[0013] [Concrete Composition] The concrete composition according to this embodiment is a concrete composition containing a binder, a hardening accelerator for hydraulic materials, and water, and the binder blending amount is 300 kg / m ３ or more, the water-binder ratio is 55% or less, and the amount of the hardening accelerator for hydraulic materials added is 1 to 50 kg / m ３The hardening accelerator for hydraulic materials contains 20.0 to 80.0 mass% of calcium formate, 15.0 to 70.0 mass% of an inorganic calcium compound, and 0.5 to 30.0 mass% of inorganic sulfates other than calcium sulfate. The concrete composition of the present invention has good fluidity retention, setting properties, and early strength development, and is suitable for producing precast concrete and the like. Furthermore, for example, precast concrete can be sufficiently hardened without using steam curing, and energy consumption and CO2 can be reduced. ２ This is advantageous in terms of emissions.

[0014] In the present invention, "binder" refers to a general term for substances that react with water to produce substances that contribute to the development of concrete strength, and examples thereof include cement, calcium aluminate, calcium sulfoaluminate, fly ash, blast furnace slag, fly ash, silica fume, limestone fine powder, metakaolin, allophane, etc., and the amount of binder in the binder blend amount and water-to-binder ratio refers to the total amount of these. Furthermore, the "hardening accelerator for hydraulic materials" of the present invention not only accelerates the hardening of the hydraulic material in the binder when it is mixed with water and hardens, but may also harden itself while accelerating the hardening.

[0015] The concrete composition of the present invention preferably contains at least cement as a binder. The type of cement is not particularly limited, and examples include various Portland cements such as normal, high-early-strength, ultra-high-early-strength, low-heat, and medium-heat cements; environmentally friendly cements (ecocements) produced from municipal waste incineration ash or sewage sludge incineration ash; commercially available fine cements; and white cements. Various cements can also be used in a finely powdered form. Furthermore, cements prepared by increasing or decreasing the amount of components (e.g., gypsum) typically used in cements can also be used. Furthermore, combinations of two or more of these can also be used. From the viewpoint of improving early strength development, it is preferable to select normal Portland cement or high-early-strength Portland cement.

[0016] The cement used in the present invention has a Blaine specific surface area value (hereinafter simply referred to as the "Blaine value") of 2,500 to 7,000 cm from the viewpoint of production cost and strength development. ２ / g, and 2,750 to 6,000 cm ２ / g, and more preferably 3,000 to 4,500 cm ２ In the present invention, the Blaine specific surface area value is measured in accordance with the specific surface area test specified in JIS R 5201:2015 "Physical testing methods for cement."

[0017] The cement content is preferably 20.0 parts by mass or more, more preferably 50.0 parts by mass or more, and even more preferably 60.0 parts by mass or more, per 100 parts by mass of binder in the concrete composition. When the cement content is within the above range, the fluidity retention and early strength development of the concrete composition can be improved.

[0018] The concrete composition of the present invention may contain calcium sulfoaluminate as a binder. Calcium sulfoaluminate has the chemical formula xCaO.yAl ２ O ３ zCaSO ４ ・mH ２ O (x, y, z are non-zero positive real numbers, m is 0 or a positive real number), and is a general term for hydraulic substances and hydrated salts. For example, hauyne (3CaO.3Al ２ O ３ CaSO ４ ), as well as ettringite (3CaO·Al ２ O ３ 3CaSO ４ ・32H ２ O), AFt phase, represented by monosulfate (3CaO.Al ２ O ３ CaSO ４ ・12H ２ Calcium sulfoaluminate may be amorphous. ２ O ３A small amount of Fe is included in part of ２ O ３ or SiO ２ etc., and CaSO ４ Part of Ca(OH) ２ or CaCO ３ In the present invention, the above chemical formula xCaO.yAl may be substituted. ２ O ３ zCaSO ４ ・mH ２ If z is 0, the fluidity of the hydraulic material cannot be maintained and there is a risk of a decrease in strength during hardening due to a phase transition, so z cannot be 0.

[0019] Calcium sulfoaluminate is produced using calcia raw materials such as lime, sulfate raw materials such as gypsum, and alumina raw materials such as bauxite (aluminum hydroxide), for example, as a calcium sulfoaluminate. ４ :Al ２ O ３ The raw materials can be blended in a predetermined ratio, such as a molar ratio of 3:3:1, fired at about 1,500°C in a kiln or the like, and pulverized to produce the product. Alternatively, silicon dioxide or the like may be added to the fired product, which may then be heat-treated and pulverized.

[0020] The Blaine specific surface area of ​​calcium sulfoaluminate is 1,000 to 6,000 cm ２ / g, and 2,000 to 4,000 cm ２ / g, and more preferably 2,200 to 3,800 cm ２ It is more preferable that the SiO2 content is 1 / g.

[0021] The content of calcium sulfoaluminate in the binder is preferably 1 to 15 mass %, more preferably 1 to 8 mass %, and even more preferably 2 to 5 mass %. When the content of calcium sulfoaluminate is within the above range, the fluidity retention and early strength development of the concrete composition can be improved.

[0022] The concrete composition of the present invention can contain a siliceous fine powder as a binder. The inclusion of the siliceous fine powder in the concrete composition facilitates improving fluidity retention and early strength development. Examples of the siliceous fine powder include latent hydraulic substances such as ground granulated blast furnace slag, fly ash, and pozzolanic substances such as silica fume, metakaolin, and allophane. Among these, ground granulated blast furnace slag, fly ash, and metakaolin are preferred.

[0023] The fineness of the siliceous powder is not particularly limited, but usually, the fine powder of blast furnace slag and fly ash has a Blaine value of 3,000 to 9,000 cm ２ / g, and silica fume has a BET specific surface area of ​​20,000 to 300,000 cm ２ It is preferable that the range is / g.

[0024] The content of the siliceous fine powder is preferably 5 to 60 parts by mass, more preferably 10 to 50 parts by mass, and even more preferably 20 to 40 parts by mass, per 100 parts by mass of the binder in the concrete composition. When the content of the siliceous fine powder is equal to or greater than the above-mentioned lower limit, it is easy to improve fluidity retention and early strength development. Furthermore, when the blending ratio of the siliceous fine powder is equal to or less than the above-mentioned upper limit, it is easy to further improve fluidity retention.

[0025] The amount of binder blended in the concrete composition of the present invention is 300 kg / m ３ The binder content is 300 kg / m ３ If the binder content is less than 320 kg / m, the initial strength development may be poor. ３ More than 350 kg / m ３ More preferably, 380 kg / m or more ３ More preferably, it is 550 kg / m or more. ３ Preferably, it is 580 kg / m or less. ３ More preferably, it is 600 kg / m or less. ３ It is more preferable that the binder content is within the above range, so that the initial strength development can be improved.

[0026] The water-binder ratio in the concrete composition of the present invention is 55% or less. The water-binder ratio (W / B) is the ratio of the amount of water to the amount of binder, and if the water-binder ratio exceeds 55%, there is a risk of poor setting properties. Furthermore, the water-binder ratio is preferably 55% or less, more preferably 48% or less, and even more preferably 45% or less. Furthermore, there is no particular lower limit, but it is preferably 20% or more. By having the water-binder ratio within the above range, setting properties can be made better.

[0027] (Hydraulic Material Hardening Accelerator) The hydraulic material hardening accelerator of the present invention contains 20.0 to 80.0 mass% calcium formate, 15.0 to 70.0 mass% inorganic calcium compound, and 0.5 to 30.0 mass% inorganic sulfates excluding calcium sulfate. If the calcium formate content is less than 20.0 mass%, the fluidity retention, setting performance, and early strength development of the concrete composition may not be improved. If the content of the inorganic calcium compound is more than 80.0 mass%, the fluidity retention, setting performance, and early strength development of the concrete composition may not be improved. If the content of the inorganic calcium compound is less than 15.0 mass%, the fluidity retention, setting performance, and early strength development of the concrete composition may not be improved. If it is more than 70.0 mass%, the fluidity retention, setting performance, and early strength development of the concrete composition may not be improved. If the content of inorganic sulfates excluding calcium sulfate is less than 0.5 mass%, the fluidity retention, setting properties, and early strength development of the concrete composition may not be improved, and if it exceeds 30.0 mass%, the fluidity retention, setting properties, and early strength development of the concrete composition may not be improved.

[0028] The hardening accelerator for hydraulic materials in the concrete composition of the present invention preferably contains calcium formate in an amount of 25.0 to 75.0 mass%, more preferably 30.0 to 70.0 mass%, and even more preferably 40.0 to 65.0 mass%. When the calcium formate content is within the above range, the fluidity retention, setting properties, and early strength development of the concrete composition can be improved.

[0029] The hardening accelerator for hydraulic materials in the concrete composition of the present invention preferably contains 18.0 to 65.0 mass %, more preferably 20.0 to 60.0 mass %, and even more preferably 22.0 to 40.0 mass % of an inorganic calcium compound. When the content of the inorganic calcium compound is within the above range, the fluidity retention, setting property, and early strength development of the concrete composition can be improved.

[0030] In this embodiment, the inorganic calcium compound is preferably one or more selected from the group consisting of calcium sulfate, calcium hydroxide, calcium carbonate, and calcium oxide. When two or more inorganic calcium compounds are used, the total content of each is taken as the content of the inorganic calcium compounds. In this embodiment, from the viewpoint of early strength development, calcium sulfate, calcium hydroxide, and / or calcium oxide are preferably used, and calcium sulfate is more preferred. When calcium sulfate is used, it is more preferred that it is anhydrous.

[0031] The hardening accelerator for hydraulic materials in the concrete composition of the present invention preferably contains 1.0 to 25.0 mass %, more preferably 2.0 to 15.0 mass %, and even more preferably 3.0 to 10.0 mass % of inorganic sulfates excluding calcium sulfate. When the content of inorganic sulfates excluding calcium sulfate is within the above range, the fluidity retention, setting properties, and early strength development of the concrete composition can be improved.

[0032] In this embodiment, the inorganic sulfate is preferably one or more selected from the group consisting of sulfate, thiosulfate, sulfite, bisulfite, pyrosulfate, and pyrobisulfite. When two or more inorganic sulfates are used, the total content of the inorganic sulfate is the sum of the respective contents. As inorganic substances that form salts, alkali metals and alkaline earth metals are preferred. In this embodiment, from the viewpoint of early strength development, sulfate and / or thiosulfate are preferably used, and sodium sulfate, aluminum sulfate, sodium thiosulfate, and potassium alum are more preferred. From the viewpoint of early strength development, sodium sulfate and aluminum sulfate are even more preferred. Among them, sodium sulfate is even more preferred from the viewpoint of improving fluidity retention. When sodium sulfate is used, it is more preferred that it is anhydrous.

[0033] The amount of the hardening accelerator for hydraulic materials added to the concrete composition of the present invention is 1 to 50 kg / m ３ The amount of hydraulic material hardening accelerator added is 1 kg / m ３ If it is less than 50 kg / m, the initial strength development may be poor. ３ If the amount of the hardening accelerator for hydraulic materials to be added is more than 5 to 48 kg / m, the fluidity retention may be poor. ３ is preferably 10 to 45 kg / m ３ More preferably, it is 13 to 43 kg / m ３ When the amount of the hardening accelerator for hydraulic materials added is within the above range, the fluidity retention, setting properties and early strength development of the concrete composition can be improved.

[0034] The concrete composition of the present invention essentially contains aggregate, and the aggregate used can be the same fine aggregate or coarse aggregate as that used in ordinary concrete. That is, river sand, river gravel, mountain sand, mountain gravel, crushed stone, crushed sand, limestone aggregate, lime sand, silica sand, colored sand, artificial aggregate, blast furnace slag aggregate, sea sand, sea gravel, artificial lightweight aggregate, heavy aggregate, etc., and combinations of these can also be used.

[0035] The mixing ratio of the aggregate is preferably 40 to 250 parts by mass, more preferably 50 to 230 parts by mass, and even more preferably 60 to 200 parts by mass, per 100 parts by mass of cement in the concrete composition. By having the mixing ratio of the aggregate within the above range, the fluidity retention and early strength development of the concrete composition can be improved.

[0036] The concrete composition may contain an alkali metal carbonate. By incorporating an alkali metal carbonate into the concrete composition, the fluidity retention and early strength development are likely to be improved. Examples of alkali metal carbonates include sodium carbonate, potassium carbonate, lithium carbonate, sodium bicarbonate, potassium bicarbonate, and lithium bicarbonate, and these may also be combined.

[0037] The alkali metal carbonate is preferably added in an amount of 1 to 6 parts by mass, more preferably 2 to 5 parts by mass, based on 100 parts by mass of cement in the concrete composition. When the alkali metal carbonate content is within the above range, it is easy to improve fluidity retention and early strength development.

[0038] A concrete composition may contain an antifoaming agent as long as it does not adversely affect performance. Antifoaming agents are used to reduce the amount of air entrained during mixing. The type of antifoaming agent is not particularly limited as long as it does not significantly adversely affect the strength characteristics of the hardened body, and both liquid and powder forms can be used. Examples include polyether-based antifoaming agents, polyhydric alcohol-based antifoaming agents such as polyhydric alcohol esters and alkyl ethers, alkyl phosphate-based antifoaming agents, and silicone-based antifoaming agents.

[0039] The blending ratio of the antifoaming agent is preferably 0.002 to 0.5 parts by mass, more preferably 0.005 to 0.45 parts by mass, and even more preferably 0.01 to 0.4 parts by mass, per 100 parts by mass of cement in the concrete composition. When the blending ratio of the antifoaming agent is equal to or greater than the lower limit, the antifoaming effect can be fully exerted, and when the blending ratio of the antifoaming agent is equal to or less than the upper limit, the fluidity retention can be easily improved.

[0040] In addition, the concrete composition may contain one or more of the following additives, within a range that does not adversely affect performance: gas foaming substances, water reducing agents, air entraining agents, rust inhibitors, water repellents, antibacterial agents, colorants, antifreeze agents, limestone fine powder, slowly cooled blast furnace slag fine powder, sewage sludge incineration ash and its molten slag, municipal waste incineration ash and its molten slag, and pulp sludge incineration ash; thickeners; shrinkage reducing agents; polymers; clay minerals such as bentonite and sepiolite; and anion exchangers such as hydrotalcite.

[0041] As for the fresh properties of the concrete composition before pouring, if the slump value and slump flow value are appropriate for the product to be manufactured, pouring and compaction can be easily carried out, workability can be improved, and the occurrence of material separation can be suppressed. The change in slump (slump change) after 1 hour of mixing the concrete composition is preferably less than 10%, more preferably less than 7.5%, and even more preferably less than 5%. If the slump change of the concrete composition is within the above range, pouring and compaction can be easily carried out and workability can be improved, even if the concrete is poured 1 hour after mixing the cement composition. Furthermore, the change in slump flow (slump flow change) after 1 hour of mixing the concrete composition is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less. In the present invention, slump can be measured in accordance with the method specified in JIS A 1101:2020 "Testing Method for Slump of Concrete," and slump flow can be measured in accordance with the method specified in JIS A 1150:2020 "Testing Method for Slump Flow of Concrete." The change in slump can be determined by subtracting the value calculated by (slump after 1 hour of mixing) / (slump immediately after mixing)×100(%) from 100%, and the change in slump flow can be determined by subtracting the value calculated by (slump flow after 1 hour of mixing) / (slump flow immediately after mixing)×100(%) from 100%.

[0042] The concrete composition may be prepared by mixing the individual materials at the time of construction, or by mixing some or all of them in advance. Any existing mixer, such as a tilting mixer, omni mixer, Henschel mixer, V-type mixer, Plosser mixer, or Nauta mixer, can be used.

[0043] [Hardened body] The hardened body according to this embodiment is obtained by hardening the concrete composition of the present invention. The hardened body is usually obtained by mixing the concrete composition, which causes a hydration reaction of the cement and hardens it. The concrete composition is obtained by mixing and then leaving it to harden, but it can also be obtained more efficiently by filling (casting) the concrete composition into a formwork and curing it, or by directly pouring it into the construction site, or by spraying or applying it.

[0044] The compressive strength of the hardened body depends on the type of cement used, but 16 hours after casting, it is 8.5 N / mm ２ It is preferable that the strength is 10.0 N / mm or more. ２ More preferably, it is 11.0 N / mm or more. ２ In the present invention, the compressive strength can be measured in accordance with the method specified in JIS A 1108:2018 "Test method for compressive strength of concrete."

[0045] [Method for producing a hardened body] The method for producing a hardened body according to this embodiment is a method for curing and hardening the concrete composition of the present invention at a temperature of 40° C. or less. The concrete composition is poured into a formwork and cured under the above conditions to obtain a hardened body.

[0046] The temperature of the concrete composition when poured is preferably 0 to 50° C., and more preferably 10 to 40° C. If the temperature of the concrete composition when poured is within the above range, the hardened body can be easily demolded.

[0047] In the method for producing a hardened concrete body of the present invention, it is preferable to further compact the concrete composition after pouring. While known methods can be used for compaction, it is preferable to use a vibrator from the viewpoint of workability. The concrete composition of the present invention maintains the fluidity of the concrete composition just before pouring, so compaction can be easily performed, the concrete composition can be uniformly distributed within the formwork, and air bubbles that may have been mixed in during pouring can be removed.

[0048] The curing conditions in the method for producing a cured body of the present invention are a temperature of 40°C or less, preferably 35°C or less. The humidity is preferably 80% RH or less, more preferably 70% RH or less. The curing time is preferably 24 hours or less, more preferably 16 hours or less, and even more preferably 12 hours or less. In addition, CO ２ From the viewpoint of reducing emissions, it is preferable not to use steam curing in the present invention.

[0049] The present invention will be further explained below based on experimental examples, but the present invention is not limited to these.

[0050] Experimental Example 1 A hardening accelerator for hydraulic materials containing 60% by mass of calcium formate, 35% by mass of calcium sulfate, and 5% by mass of sodium sulfate was prepared using the following materials, and the concrete composition shown in Table 1 was prepared. The slump of the resulting concrete composition was measured, and the slump change was calculated. The initial settling time was also measured. Both results are also shown in Table 1. The prepared cement composition was filled into a formwork measuring φ10 × 20 cm, and then cured for 24 hours under conditions of a temperature of 20°C and a humidity of 70% RH to obtain a hardened body. The compressive strength of the resulting hardened body was measured. The results are also shown in Table 1.

[0051] (Materials used) Cement: Ordinary Portland cement (commercially available), Blaine specific surface area 3,200 cm ２ / g, specific gravity 3.15g / cm ３Water: Tap water. Fine aggregate: River sand from the Himekawa River system in Niigata Prefecture. Coarse aggregate: River gravel from the Himekawa River system in Niigata Prefecture. Ground blast furnace slag: Commercially available. Fly ash: Commercially available. Metakaolin: Commercially available. Calcium sulfoaluminate: First-grade reagent calcium carbonate, calcium sulfate dihydrate, and aluminum hydroxide were used to prepare a CaO:CaSO ４ :Al ２ O ３ The mixture was mixed so that the molar ratio was 4:3:1, and the mixture was baked at 1,400°C for 2 hours and allowed to cool to room temperature, resulting in a Blaine specific surface area of ​​3,500 cm ２ / g. Calcium formate: Reagent. Calcium sulfate (inorganic calcium compound): Reagent. Sodium sulfate (inorganic sulfate): Anhydrous, Reagent.

[0052] (Measurement items) Slump change: According to the method specified in JIS A 1101:2020 "Concrete slump test method," the slump of the concrete was measured immediately after mixing and one hour after mixing. The slumps immediately after mixing were all about 12.0 cm. Using each measured slump, the slump change was calculated as (slump change) = (slump immediately after mixing) / (slump one hour after mixing), and the fluidity retention was evaluated.

[0053] Initial setting time: According to the method specified in JIS A 1147:2019 "Test method for concrete setting time", the penetration resistance value is 3.5 N / mm ２ The time when this occurs was set as the first train time.

[0054] Compressive strength: The compressive strength immediately after demolding was measured in accordance with the method specified in JIS A 1108:2018 "Test method for compressive strength of concrete."

[0055]

[0056] Experimental Example 2 Concrete compositions were prepared in the same manner as in Experimental Example 1, except that the hardening accelerator for hydraulic materials was changed to the composition shown in Table 2, and the slump change, initial set time, and compressive strength were measured. The results are also shown in Table 2.

[0057]

[0058] Experimental Example 3 Concrete compositions were prepared in the same manner as in Experimental Example 1, No. 1-2, except that calcium hydroxide, calcium carbonate, and calcium oxide were used in addition to calcium sulfate as the inorganic calcium compounds contained in the hardening accelerator for hydraulic materials in the proportions shown in Table 2, and the slump change, initial set time, and compressive strength were measured. The results are also shown in Table 3.

[0059] (Inorganic calcium compounds) Calcium hydroxide: Reagent. Calcium carbonate: Reagent. Calcium oxide: Reagent.

[0060]

[0061] Experimental Example 4 Concrete compositions were prepared in the same manner as in Experimental Example 1, No. 1-2, except that aluminum sulfate, potassium alum, and sodium thiosulfate were used in the proportions shown in Table 4 as inorganic sulfates contained in the hardening accelerator for hydraulic materials, and the slump change, initial set time, and compressive strength were measured. The results are also shown in Table 4.

[0062] (Inorganic sulfates) Aluminum sulfate: Reagent. Potassium alum: Reagent. Sodium thiosulfate: Reagent.

[0063]

[0064] The concrete composition of the present invention has good fluidity retention, setting properties and early strength development properties, and is therefore widely applicable to the fields of civil engineering and construction, such as hardened concrete used in precast construction methods.

Claims

1. A concrete composition containing a binder, a hardening accelerator for hydraulic materials, and water, wherein the binder content is 300 kg / m ３ or more, the water-binder ratio is 55% or less, and the amount of the hardening accelerator for hydraulic materials added is 1 to 50 kg / m ３ The concrete composition according to claim 1, wherein the hardening accelerator for hydraulic materials contains 20.0 to 80.0 mass% of calcium formate, 15.0 to 70.0 mass% of an inorganic calcium compound, and 0.5 to 30.0 mass% of an inorganic sulfate other than calcium sulfate.

2. The concrete composition according to claim 1, wherein the inorganic calcium compound is one or more selected from the group consisting of calcium sulfate, calcium hydroxide, calcium carbonate, and calcium oxide.

3. The concrete composition according to claim 1 or 2, wherein the inorganic sulfate is one or more selected from the group consisting of sulfates, thiosulfates, sulfites, bisulfites, pyrosulfates, and pyrobisulfites.

4. The concrete composition according to claim 1 or 2, wherein the binder contains 1 to 15% by mass of calcium sulfoaluminate.

5. A hardened body obtained by hardening the concrete composition according to claim 1 or 2.

6. A method for producing a hardened concrete product, comprising curing the concrete composition according to claim 1 or 2 at a temperature of 40°C or less to harden it.

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