Cement concrete admixture, cement composition, and hardened body

Abstract

To provide a cement concrete admixture capable of suppressing the permeability of chloride ions even by steam curing.SOLUTION: A cement concrete admixture for steam curing of the present invention contains calcium aluminate having a CaO / Al2O3 molar ratio of 0.15 to 0.7.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention mainly relates to a cement concrete admixture used in the civil engineering and construction industries, a cement composition using the admixture, a hardened body using the cement composition, and a method for producing the hardened body.

Background Art

[0002] In recent years, in the fields of civil engineering and construction, there has been an increasing demand for improving the durability of concrete structures. As one of the deterioration factors of concrete structures, there is salt damage in which steel bars corrode due to the presence of chloride ions. As a means of suppressing this, there is a method of imparting chloride ion penetration resistance to concrete structures.

[0003] As a method of suppressing the penetration of chloride ions into the interior of a hardened concrete body and imparting chloride ion penetration resistance, a method of reducing the water / cement ratio is known (see Non-Patent Document 1). However, the method of reducing the water / cement ratio not only impairs workability but may not be a radical countermeasure in some cases.

[0004] Also, for the purpose of imparting early strength to cement concrete and preventing corrosion of steel bars, a cement admixture mainly composed of CaO·2Al2O3 and gypsum and containing fine powder with a Blaine specific surface area value of 8000 cm 2 / g has been proposed (see Patent Document 1). Furthermore, a method has been proposed in which a cement admixture containing calcium aluminate with a CaO / Al2O3 molar ratio of 0.3 to 0.7 and a Blaine specific surface area value of 2000 to 7000 cm 2 / g is used, which has excellent chloride ion penetration resistance and suppresses temperature cracking of mass concrete (see Patent Document 2).

[0005] In addition, a cement admixture containing a calcium ferroaluminate compound with a CaO / Al2O3 molar ratio of 0.15 to 0.7 and an Fe2O3 content of 0.5 to 15% by mass has been proposed (see Patent Document 3). Furthermore, a method of adding nitrite or the like for the purpose of preventing rust on steel bars has also been proposed (see Patent Documents 4 and 5).

[0006] By the way, in recent years, the precast of concrete has advanced, and the construction method of manufacturing concrete products in factories, transporting them to the construction site, and assembling them has increased. In such precast concrete products, steam curing is carried out for the purpose of improving productivity. Steam curing is a method in which the concrete placed in the formwork is installed in a curing chamber, steam generated by a boiler or the like is introduced into this curing chamber, the concrete is heated under humid conditions, and the hydration reaction of the cement is promoted to accelerate the development of strength.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0009] In precast concrete products as described above, when steam curing is initially performed using conventional cement to accelerate the hardening of mortar and concrete, it has been confirmed that the shielding effect against the penetration of external chloride ions, which affects the long-term durability of reinforced concrete, decreases. This is presumably because the structure of the concrete becomes coarser and its salt resistance decreases due to steam curing. Therefore, it has been difficult to obtain concrete with an excellent shielding effect in precast concrete products subjected to steam curing with a short curing time. Under such circumstances, an object of the present invention is to provide a cement concrete admixture capable of suppressing the permeability of chloride ions even by steam curing in precast concrete products and the like that are steam cured.

Means for Solving the Problems

[0010] The inventors of the present invention conducted intensive research to solve the above problems and found that a cement concrete admixture suitable for steam curing can be obtained by using calcium aluminate having a specific CaO / Al2O3 molar ratio, thereby completing the present invention. That is, the present invention is as follows. [1] A cement concrete admixture for steam curing containing calcium aluminate having a CaO / Al2O3 molar ratio of 0.15 to 0.7. [2] The cement concrete admixture for steam curing according to [1] above, wherein the content of the calcium aluminate is 2 to 20% by mass. [3] The cement concrete admixture for steam curing according to [1] or [2] above, wherein the steam curing is performed at a maximum temperature of 40 to 70°C for 2 to 5 hours. [4] A cement composition comprising the cement concrete admixture for steam curing according to any one of [1] to [3] above and cement. [5] A cured body obtained by curing the cement composition according to [4] above by steam curing. [6] A method for producing a hardened body by steam curing using the cement composition described in [4] above, wherein the steam curing is carried out at a maximum temperature of 40 to 70°C for 2 to 5 hours. A method for producing a cement hardened body characterized by this. [7] The method for producing a cement hardened body according to [6] above, wherein the steam curing is preceded by pre-curing at 10 to 30°C for 1 to 3 hours, the maximum temperature is set to 50 to 60°C, steam curing is carried out for 2 to 4 hours, and natural cooling is carried out until 20°C.

Effects of the Invention

[0011] By using the cement concrete admixture for steam curing of the present invention, even in concrete products accompanied by steam curing, the penetration of chloride ions can be suppressed, and the durability of concrete structures can be improved.

Modes for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described in detail. Parts and % used in the present invention are based on mass unless otherwise specified.

[0013] [Cement Concrete Admixture] The cement concrete admixture of the present invention contains calcium aluminate having a CaO / Al2O3 molar ratio of 0.15 to 0.7.

[0014] <Calcium Aluminate> Calcium aluminate is a general term for compounds mainly composed of CaO and Al2O3, which are obtained by mixing raw materials containing calcia and raw materials containing alumina, and performing heat treatment such as firing in a kiln or melting in an electric furnace. The calcium aluminate according to the present invention is characterized in that the CaO / Al2O3 molar ratio is in the range of 0.15 to 0.7. When the CaO / Al2O3 molar ratio is less than 0.15, the progress of the hydration reaction is slow, and excellent chloride resistance cannot be obtained in a short curing period. On the other hand, when the CaO / Al2O3 molar ratio exceeds 0.7, the heat generation due to the hydration reaction becomes fast, and sufficient workable time cannot be ensured. From the above viewpoints, the CaO / Al2O3 molar ratio is preferably in the range of 0.3 to 0.7, and more preferably in the range of 0.4 to 0.6.

[0015] As the content of calcium aluminate, it is preferably in the range of 2 to 20% by mass as the content in the binder. When the content of calcium aluminate is 2% by mass or more, the addition effect of calcium aluminate can be obtained. That is, even for concrete products accompanied by steam curing, the penetration of chloride ions can be suppressed. On the other hand, when the content of calcium aluminate is 20% by mass or less, the content of cement becomes relatively large, and sufficient bonding strength can be obtained. From the above viewpoints, the content of calcium aluminate is more preferably in the range of 4 to 15% by mass, further preferably in the range of 5 to 12% by mass, even more preferably in the range of 6 to 10% by mass, and particularly preferably in the range of 7 to 9% by mass. In the present invention, the "binder" means a cement admixture for steam curing containing cement and calcium aluminate. In the present invention, cement concrete is a general term for cement paste, cement mortar, and concrete.

[0016] The calcium aluminate according to the present invention preferably has a Blaine specific surface area value in the range of 2000 to 6000 m 2 / g. When the Blaine specific surface area value of calcium aluminate is 2000 cm 2 / g or more, the chloride resistance can be sufficiently exhibited. On the other hand, when the Blaine specific surface area value of calcium aluminate is 6000 cm 2When it is 6000 cm 2 / g or less, it is easy to control the heat generation by the hydration reaction. Even when the Blaine specific surface area value exceeds 6000 cm 2 / g, no further improvement in salt damage resistance can be expected. From the above viewpoints, the Blaine specific surface area value of calcium aluminate is preferably 2500 to 5000 cm 2 / g, and more preferably 3000 to 4000 cm 2 / g. 2 Even if it exceeds 6000 cm 2 / g, no further improvement in salt damage resistance can be expected. From the above viewpoints, the Blaine specific surface area value of calcium aluminate is preferably 2500 to 5000 cm 2 / g, 2 and more preferably 3000 to 4000 cm 2 / g. 2

[0017] The calcium aluminate according to the cement admixture of the present invention may contain impurities in addition to CaO and Al2O3. Examples of the impurities include SiO2, Fe2O3, TiO2, K2O, Na2O, B2O, etc. The presence of these impurities may have an effect of promoting the formation reaction when firing calcium aluminate, and since there are also favorable aspects, they may be present in a total range of 10% by mass or less without any problem.

[0018] In addition to the calcium aluminate of the present invention, one or more of mixed materials such as limestone fine powder, blast furnace slag fine powder, metakaolin, sewage sludge incineration ash and its molten slag, municipal waste incineration ash and its molten slag, pulp sludge incineration ash, thickeners, shrinkage reducing agents, polymers, setting regulators, clay minerals such as bentonite, etc. can be used in combination within a range that does not substantially inhibit the object of the present invention.

[0019] [Cement Composition] The cement composition of the present invention contains the above cement admixture and cement. The cement admixture of the present invention can be used for any concrete. Examples of the cement used for concrete include various Portland cements such as ordinary, early strength, ultra-early strength, low heat, medium heat, and sulfate-resistant Portland cements, various blended cements obtained by mixing blast furnace slag, fly ash, silica, or limestone fine powder with these Portland cements, and waste utilization type cements, so-called eco-cements, etc. Among them, it has good compatibility with ordinary Portland cement or early strength Portland cement.

[0020] The content of the cement admixture is not particularly limited as long as it is within the range where the effects of the present invention can be achieved. Usually, it is preferably in the range of 1 to 50 parts by mass, more preferably in the range of 3 to 30 parts by mass, and even more preferably in the range of 5 to 10 parts by mass with respect to 100 parts by mass of the total amount of cement and cement admixture (binder amount). When it is at or above the above lower limit value, a sufficient rust prevention effect and chloride ion shielding effect can be obtained. On the other hand, when it is at or below the above upper limit value, a sufficient workable time can be ensured.

[0021] Water is usually mixed into the cement composition. The water content is preferably in the range of 25 / 75 to 70 / 30 (25 to 70% by mass) in terms of the water / binder ratio (mass ratio), and more preferably in the range of 30 / 70 to 65 / 35 (30 to 65% by mass). When the blending amount of water is at or above the above lower limit value, the pumpability and workability are good, and there are no problems such as shrinkage. On the other hand, when it is at or below the above upper limit value, sufficient strength can be obtained.

[0022] The cement admixture and cement composition of the present invention may be mixed immediately before steam curing, or may be partially or entirely premixed. As the mixing device, any existing device can be used. For example, a mortar mixer, a tilting drum mixer, an omnimixer, a V-type mixer, a Henschel mixer, a forced two-shaft mixer, and a Nauta mixer can be used. The kneading conditions are not particularly limited either. For example, it may be stirred at a low speed or a high speed at a temperature of 5 to 50°C for about 1 to 10 minutes.

[0023] In addition to cement, cement admixture, fine aggregates such as sand, and coarse aggregates such as gravel, the cement composition of the present invention can be used in combination with one or more of the group consisting of an expansive agent, a rapid hardening agent, a water reducing agent, an AE water reducing agent, a high-performance water reducing agent, a high-performance AE water reducing agent, an antifoaming agent, a thickening agent, a conventional rust inhibitor, an antifreeze agent, a shrinkage reducing agent, a polymer emulsion, a setting regulator, clay minerals such as bentonite, anion exchangers such as hydrotalcite, slag such as blast furnace granulated slag fine powder, and admixtures such as limestone fine powder, as long as the object of the present invention is not substantially inhibited.

[0024] [Hardened concrete] The mortar or concrete prepared by kneading the cement composition of the present invention, aggregates and water is molded and steam-cured to obtain a hardened concrete body.

[0025] (Steam curing) The cement composition of the present invention depends on the type of cement used. For example, pre-curing is carried out at 5 to 30°C for 1 to 3 hours, then the temperature is raised, and steam curing is carried out at a maximum temperature of 40 to 70°C for 2 to 5 hours, and then sealed curing is carried out at about 20°C. By this method, a hardened body with excellent strength characteristics can be obtained. Regarding the maximum temperature, by setting it to 40°C or higher, concrete having an excellent shielding effect can be obtained with a short curing time. On the other hand, by setting the maximum temperature to 70°C or lower, the penetration of chloride ions can be suppressed. From the above viewpoints, the maximum temperature of steam curing is more preferably in the range of 50 to 65°C, and even more preferably in the range of 50 to 60°C. Also, the curing time in steam curing is preferably in the range of 2 to 5 hours. Within this range, it is a sufficient curing time and the strength of the hardened concrete body becomes sufficient. From the above viewpoints, the curing temperature is more preferably in the range of 50 to 60°C, and the curing time is more preferably in the range of 2 to 4 hours. [Examples]

[0026] The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.

[0027] [Evaluation method] [Chloride ion penetration depth] From the age of 14 days, in accordance with JSCE-G 572-2010 "Test Method for Apparent Diffusion Coefficient of Chloride Ions in Concrete by Immersion", the test specimens were immersed in a 10% NaCl aqueous solution. At the time of 13 weeks of immersion, the test specimens were split, and the penetration depth of chloride ions was measured by the silver nitrate solution spraying method. Specifically, the parts where the cross-section of the specimen did not turn brown were measured at 8 points using vernier calipers, and the average value was taken as the chloride ion penetration depth.

[0028] (Materials Used) Cement: Commercially available ordinary Portland cement Water: 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

[0029] Example 1 The test specimens were prepared by the following method and evaluated by the above method. The evaluation results are shown in Table 1. (Preparation of Calcium Aluminate) Reagent-grade calcium carbonate, aluminum oxide, and magnesium oxide were blended at a predetermined ratio, melted in an electric furnace at 1500 °C, and then slowly cooled to synthesize calcium aluminate (denoted as "CA" in Table 1). It was pulverized to a Blaine specific surface area value of 3000 cm 2 / g and used as a cement admixture. The CaO / Al2O3 (molar ratio) of each example and comparative example is as shown in Table 1.

[0030] (Mixing) Next, mixing was carried out using a 55 L forced double-shaft mixer. As the procedure, 692 kg / m 3 of fine aggregate, 400 kg / m 3 of cement, and 1052 kg / m 3 of coarse aggregate were charged and dry-mixed for 15 seconds. Then, 160 kg / m 3 of water and kg / m 3 of calcium aluminate were charged and mixed for 120 seconds to produce cement concrete. Note that the calcium aluminate content is calculated as 8 parts by mass (8% by mass) in 100 parts by mass of the cement composition composed of cement and cement admixture. Also, the water / cement ratio and the water / binder ratio are calculated as 40.0 and 36.8, respectively.

[0031] (Steam curing) The cement concrete obtained as described above was molded using a mold in the shape of a 10 cm × 10 cm × 40 cm prism to obtain a specimen. The following steam curing methods (1) to (3) were applied to this specimen. (1) Steam curing temperature 55°C: Pre-curing was performed at 20°C for 2 hours, the temperature was raised to a maximum temperature of 55°C at a rate of 20°C / hour, held at 55°C for 3 hours, and then naturally cooled to room temperature (20°C). (2) Steam curing temperature 60°C: Pre-curing was performed at 20°C for 2 hours, the temperature was raised to a maximum temperature of 60°C at a rate of 20°C / hour, held at 60°C for 3 hours, and then naturally cooled to room temperature (20°C). (3) Steam curing temperature 65°C: Pre-curing was performed at 20°C for 2 hours, the temperature was raised to a maximum temperature of 65°C at a rate of 20°C / hour, held at 65°C for 3 hours, and then naturally cooled to room temperature (20°C). Thereafter, the mold was removed, and the specimen was sealed and cured until the age of 14 days.

[0032] Examples 2 to 21 and Comparative Examples 1 to 4 In Example 1, specimens were prepared and evaluated in the same manner as in Example 1, except that the CaO / Al2O3 (molar ratio), CA content, and steam curing temperature were changed as shown in Table 1. The evaluation results are shown in Table 1.

[0033] Comparative Examples 5 to 7 In Example 1, specimens were prepared and evaluated in the same manner as in Example 1, except that calcium aluminate was not blended and the steam curing temperature was changed as shown in Table 1. The evaluation results are shown in Table 1.

[0034]

Table 1

[0035] From Table 1, it can be seen that the cement admixture of the present invention has a small value of chloride ion penetration depth and can suppress the permeability of chloride ions even by steam curing. Also, the chloride ion penetration depth is smaller in the range of CaO / Al2O3 molar ratio of 0.3 to 0.7, and particularly shows good results in the range of 0.4 to 0.6. On the other hand, it can be seen that in the comparative examples where the content of calcium aluminate is outside the scope of the present invention or does not contain calcium aluminate, the penetration depth of chloride ions increases.

Industrial Applicability

[0036] The cement concrete admixture for steam curing of the present invention can suppress the penetration of chloride ions even by steam curing, so that precast concrete products having an excellent chloride ion shielding effect can be obtained in a short curing time. As a result, in civil engineering and construction work, precast concrete products can be used, and it becomes possible to shorten the construction period.

Claims

1. CaO / Al ２ O ３ A method for producing a cement hardened body by steam curing, using a cement composition containing a steam curing cement concrete admixture containing calcium aluminate with a molar ratio of 0.4 to 0.6 and cement, wherein the content of the calcium aluminate is 2 to 20% by mass as the content in the binder, and the steam curing is carried out by performing pre-curing at 5 to 30°C for 1 to 3 hours, then performing steam curing at a maximum temperature of 50 to 60°C for 2 to 5 hours, and naturally cooling to 20°C.

2. The method for producing a cement hardened body according to Claim 1, wherein the steam curing is carried out for 2 to 4 hours.

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