Cement admixture and cement composition

Abstract

To provide a cement admixture capable of appropriately controlling the timing of hydration heat generation and a cement composition using the cement admixture.SOLUTION: A cement admixture of the invention has a CaO / Al2O3 molar ratio of 0.3 to 0.7 and contains calcium aluminate containing MgO.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to cement admixtures and cement compositions primarily used in the civil engineering and construction industries. [Background technology]

[0002] In recent years, there has been an increasing demand in the fields of civil engineering and construction for improving the durability of concrete structures. One of the causes of deterioration of concrete structures is salt damage, which occurs when reinforcing steel bars corrode due to the presence of chloride ions. One way to prevent this is to make concrete structures resistant to chloride ion penetration.

[0003] A method of reducing the water / cement ratio is known as a method for inhibiting the penetration of chloride ions into hardened concrete and imparting resistance to chloride ion penetration (see Non-Patent Document 1). However, the method of reducing the water / cement ratio not only impairs workability, but also does not always provide a fundamental solution.

[0004] In addition, for the purpose of imparting early strength to cement and concrete and preventing corrosion of reinforcing bars, a concrete mixture containing mainly CaO·2Al2O3 and gypsum with a Blaine specific surface area of ​​8000 cm 2 A method has been proposed in which a cement admixture containing fine powder of 0.1g / g is used (see Patent Document 1). Furthermore, the CaO / Al2O3 molar ratio is 0.3 to 0.7, and the Blaine specific surface area is 2000 to 7000 cm 2 A method has been proposed in which a cement admixture containing 0.1g / g of calcium aluminate is used to provide excellent resistance to chloride ion penetration and suppress thermal cracking of mass concrete (see Patent Document 2).

[0005] Also, a cement admixture containing a calcium ferroaluminate compound with a CaO / A12O3 molar ratio of 0.15 to 0.7 and an Fe2O3 content of 0.5 to 15 mass % has been proposed (see Patent Document 3). Furthermore, methods of adding nitrites and the like have been proposed for the purpose of preventing rust on reinforcing bars (see Patent Documents 4 and 5).

[0006] However, in order to provide excellent rust prevention to the reinforcing bars inside hardened cement / concrete using conventional technology and to provide a shielding effect against the penetration of chloride ions from the outside, it is important to promote the hydration reaction of the cement composition, and to this end, it is necessary to ensure a sufficient curing period. On the other hand, in order to rapidly impart salt damage resistance, it is preferable that the first peak of heat generation due to hydration of the cement admixture occurs earlier than the first peak of heat generation due to hydration of the cement.

[0007] On the other hand, when a cement admixture having salt damage resistance is mixed with cement and used, it is preferable to control the first peak of heat generation due to hydration of the cement admixture so that it approaches the first peak of heat generation due to hydration of cement in order to ensure fresh properties and setting time. When calcium aluminate with a large CaO / Al2O3 molar ratio is used, the first peak of heat generation due to hydration rises early, so it is necessary to appropriately delay this first peak. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] "Durability of Concrete Series, Salt Damage (I)", Gihodo Publishing, Koichi Kishitani, Noriaki Nishizawa et al., pp.34-37, May 1986 [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 47-035020 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-104828 [Patent Document 3] Patent No. 5688073 [Patent Document 4] Japanese Patent Application Publication No. 53-003423 [Patent Document 5] Japanese Patent Application Publication No. 01-103970 Summary of the Invention [Problem to be solved by the invention]

[0010] In order to obtain cement and concrete that has excellent rust prevention properties for the reinforcing bars inside the hardened cement and concrete in a short curing period while maintaining fresh properties and setting time, and that has excellent blocking properties against the penetration of chloride ions from the outside, it is necessary to properly control the hydration heat generation period of the cement admixture. However, using conventional cement admixtures, it has been difficult to delay the first peak of heat generation due to the hydration reaction of the cement admixture so that it approaches the first peak of heat generation due to the hydration reaction of the cement, while controlling the hydration reaction of the cement admixture so that it does not delay too much, and to obtain cement concrete that has excellent rust prevention effects on the rebar inside the hardened cement concrete and excellent shielding effects against the penetration of chloride ions from outside, in a short curing period. Furthermore, it has been even more difficult to obtain cement and concrete with excellent resistance to salt damage using small amounts of conventional cement admixtures. Therefore, an object of the present invention is to provide a cement admixture capable of appropriately controlling the time of heat generation during hydration, and a cement composition using the same. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the timing of heat generation during hydration can be appropriately controlled by adding MgO to calcium aluminate, thereby solving the above-mentioned problems, and have thus completed the present invention. [1] A cement admixture containing calcium aluminate having a CaO / Al2O3 molar ratio of 0.3 to 0.7 and containing MgO. [2] The cement admixture according to the above [1], wherein the content of the MgO is 0.1 to 5.0 parts by mass per 100 parts by mass of the cement admixture. [3] The cement admixture according to [1] or [2] above, wherein the first peak of the heat of hydration in the hydration heat rate spectrum from the start of the hydration reaction to the end of the hydration reaction appears between 500 minutes and 900 minutes from the start of the hydration reaction. [4] The Blaine specific surface area of ​​the calcium aluminate is 2000 to 6000 cm 2 The cement admixture according to any one of the above [1] to [3], wherein the value is / g. [5] A cement composition comprising the cement admixture according to any one of [1] to [4] above and cement. [Effects of the Invention]

[0012] By using the cement admixture of the present invention, it is possible to provide a cement composition, and furthermore a cement / concrete, which has an excellent rust prevention effect on the reinforcing steel bars inside the hardened cement / concrete body, with a short curing period and a small amount of addition, while minimizing the effect on the fresh properties, and which has an excellent shielding effect against the penetration of chloride ions from the outside. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below. In the present invention, parts and % are based on mass unless otherwise specified.

[0014] [Cement admixture] The cement admixture of the present invention contains calcium aluminate.

[0015] <Calcium aluminate> Calcium aluminate is a general term for compounds whose main components are CaO and Al2O3, which are obtained by mixing raw materials containing calcia and raw materials containing alumina, and then subjecting them to heat treatment such as firing in a kiln or melting in an electric furnace. The calcium aluminate according to the present invention is characterized by a CaO / Al2O3 molar ratio in the range of 0.3 to 0.7. If the CaO / Al2O3 molar ratio is less than 0.3, the hydration reaction proceeds slowly, and excellent salt damage resistance cannot be ensured with a short curing period. On the other hand, if the CaO / Al2O3 molar ratio exceeds 0.7, heat generation due to the hydration reaction occurs quickly, making it impossible to appropriately control the hydration heat generation time of the cement admixture. From the above viewpoints, the CaO / Al2O3 molar ratio is preferably in the range of 0.4 to 0.6.

[0016] The calcium aluminate of the present invention is characterized by containing MgO. It is believed that MgO partially dissolves in CaO, and that upon cooling, the MgO solid solution precipitates at the grain boundaries of the CaO solid solution, enveloping the CaO and protecting it from hydration. This is believed to control the hydration heat generation period of the cement admixture of the present invention. The content of MgO is preferably in the range of 0.1 to 5.0 parts by mass relative to 100 parts by mass of the cement admixture. When the content of MgO is 0.1 part by mass or more, the above-mentioned effect of adding MgO is fully exhibited, and when the content is 5.0 parts by mass or less, the content of calcium aluminate becomes relatively sufficient, resulting in sufficient salt damage resistance.

[0017] In the cement admixture according to this embodiment, in the hydration heat release rate spectrum from the start of the hydration reaction to the end of the hydration reaction, the peak of the hydration heat release rate preferably appears 500 minutes or more but within 900 minutes from the start of the hydration reaction. If the hydration heat release rate peak appears at 500 minutes or more, it will be close to the peak of the hydration heat release rate of the cement, and fresh properties and setting time can be ensured. On the other hand, if the hydration heat release rate peak appears within 900 minutes, salt damage resistance can be quickly imparted. From the above points, the hydration heat release rate peak is more preferably 550 minutes or more but within 800 minutes, and even more preferably 600 minutes or more but within 750 minutes. Here, the hydration heat generation rate spectrum from the start to the end of the hydration reaction refers to a spectrum obtained by adding water to a cement admixture in a thermostatic container at 20°C and recording the change in heat generation rate, i.e., the amount of heat generated by hydration per unit time (J / hour g), over time, and can be measured using a microcalorimeter, specifically the MMC-511C6 multi-microcalorimeter. The amount of water added to the cement admixture can be, for example, equivalent to half the mass of the cement admixture. The peak of the heat release rate of hydration refers to the position where the heat release rate of hydration shows the maximum value in the heat release rate of hydration spectrum.

[0018] As described above, the cement admixture of this embodiment uses MgO to suppress the hydration of calcium aluminate and adjusts the peak of the heat release rate of hydration to within the above range, thereby appropriately delaying the rapid hydration reaction that occurs immediately after the start of the hydration reaction and in the early stages, and suppressing the peak of the reaction before the cement hardens. Therefore, it is presumed that the cement admixture does not require a long curing period before it can impart excellent salt damage resistance, and even with a small addition amount, it can impart excellent rust prevention effects to rebars inside hardened cement / concrete, and can produce cement / concrete with excellent shielding effects against the penetration of chloride ions from the outside. The peak position of the hydration heat generation rate can be controlled by adjusting the types and amounts of raw materials used when preparing the cement admixture, as well as the manufacturing conditions such as the firing conditions and grinding conditions of the cement admixture. In the present invention, cement concrete is a general term for cement paste, cement mortar, and concrete.

[0019] The calcium aluminate according to the present invention has a Blaine specific surface area of ​​2000 to 6000 m 2 / g. The Blaine specific surface area of ​​calcium aluminate is preferably in the range of 2000 cm 2 / g or more, it can exhibit sufficient resistance to salt damage. On the other hand, when the Blaine specific surface area of ​​calcium aluminate is 6000 cm2 / g or less, it is easy to control the heat generated by the hydration reaction. 2 / g, further improvement in salt damage resistance cannot be expected. From the above viewpoint, the Blaine specific surface area value of calcium aluminate is 2500 to 5000 cm 2 / g is preferred, and 3000 to 4000 cm 2 / g is more preferred.

[0020] The calcium aluminate used in the cement admixture of the present invention may contain impurities in addition to CaO, Al2O3, and MgO. Examples of impurities include SiO2, Fe2O3, TiO2, KO, Na2O, and BO. The presence of these impurities may have the effect of promoting the formation reaction when calcium aluminate is fired, and may therefore be desirable, so that their total content may be up to 10% by mass.

[0021] In addition to the calcium aluminate of the present invention, one or more of the following admixtures may be used in combination, insofar as they do not substantially impair the object of the present invention: fine limestone powder, finely divided blast furnace slag 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 modifiers, clay minerals such as bentonite, etc.

[0022] [Cement composition] The cement composition of the present invention comprises the above cement admixture and cement. The cement admixture of the present invention can be used in any type of concrete. Examples of cements that can be used in concrete include various types of Portland cement, such as normal, high-early-strength, ultra-high-early-strength, low-heat, medium-heat, and sulfate-resistant cements, various blended cements in which Portland cement is mixed with blast furnace slag, fly ash, silica, or limestone powder, and waste-recycled cement, so-called ecocement. In particular, it is compatible with normal Portland cement or high-early-strength Portland cement.

[0023] The amount of cement admixture used is not particularly limited as long as it is within a range that achieves the effects of the present invention, but is usually preferably in the range of 1 to 50 parts by mass, more preferably 5 to 30 parts by mass, per 100 parts by mass of the total amount of cement and cement admixture. If it is equal to or greater than the lower limit, sufficient rust prevention effect and chloride ion shielding effect can be obtained. On the other hand, if it is equal to or less than the upper limit, sufficient usable time can be ensured.

[0024] Water is usually mixed into cement compositions, and the water content, in terms of the water / binder ratio (mass ratio), is preferably in the range of 25 / 75 to 70 / 30 (25 to 70 mass%), and more preferably in the range of 30 / 70 to 65 / 35 (30 to 65 mass%). When the amount of water is equal to or greater than the lower limit, pumpability and workability are improved, and problems such as shrinkage are avoided. On the other hand, when the amount of water is equal to or less than the upper limit, sufficient strength is obtained. Here, the binder means cement and calcium aluminate.

[0025] The cement admixture and cement composition of the present invention may be prepared by mixing the respective materials at the time of application, or may be mixed in part or in whole in advance. Any existing mixer can be used, such as a mortar mixer, a tilting mixer, an omni mixer, a V-type mixer, a Henschel mixer, a forced twin-screw mixer, a Nauta mixer, etc. The kneading conditions are not particularly limited, and may be, for example, a temperature of 5 to 50°C, low-speed or high-speed stirring for about 1 to 10 minutes. The method for curing the cement composition of the present invention is not particularly limited, and may be steam curing, autoclave curing, or in-situ curing in a poured state.

[0026] In addition to cement, cement admixtures, and fine aggregates such as sand and coarse aggregates such as gravel, the cement composition of the present invention can contain one or more additives selected from the group consisting of expansive additives, rapid hardening additives, water reducing agents, air-entraining water reducing agents, high-performance water reducing agents, antifoaming agents, thickeners, conventional rust inhibitors, antifreeze agents, shrinkage reducing agents, polymer emulsions, setting modifiers, clay minerals such as bentonite, anion exchangers such as hydrotalcite, slags such as slowly cooled blast furnace slag ground powder, and admixtures such as limestone ground powder, within a range that does not substantially impair the objects of the present invention. [Example]

[0027] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0028] [Evaluation method] <Peak position of heat of hydration> 2.5 g of water was added to 5.0 g of the cement admixture obtained in each Example and Comparative Example. The time when the cement admixture came into contact with the water was considered the start of the hydration reaction, and the hydration heat generation rate spectrum from the start to the end of the hydration reaction was measured using a multi-microcalorimeter MMC-511C6. Measurements were carried out at a constant temperature of 20°C. From the obtained hydration heat generation rate spectrum from the start to the end of the hydration reaction, the peak position of the hydration heat generation rate (the time after the start of the hydration reaction when the position where the maximum value of the hydration heat generation rate (heat generation per unit time) was recorded) was determined. The results are shown in Table 1.

[0029] <Flow retention rate> The flow was measured immediately after mixing and after 1 hour in accordance with JIS R 5201. The flow retention rate was calculated according to the following formula. Flow retention rate (%) = Flow after 1 hour (mm) / Flow immediately after mixing (mm) x 100

[0030] <Compression strength> Mortar specimens in the shape of square pillars measuring 4cm x 4cm x 16cm were prepared, and the compressive strength after 28 days was measured in accordance with JIS R 5201.

[0031] <Chloride ion penetration depth> Cylindrical mortar specimens measuring φ10cm x 20cm were prepared, and after immersion in simulated seawater (a salt solution with a chloride ion concentration of 3.5% by mass) for 13 weeks, chloride ion penetration resistance was evaluated by measuring the chloride ion penetration depth, with respect to two cases: one with a pre-curing period of 0 days and one in which the specimen was cured by immersion in water at 20°C for up to 28 days.The chloride ion penetration depth was measured using the fluorescein-silver nitrate method, with calipers at eight points on the cross section of the mortar specimen where the color had not turned brown, and the average value was taken as the chloride ion penetration depth.

[0032] <Rust prevention effect> 10 kg / m in mortar 3 The rust prevention effect was confirmed in an accelerated test in which chloride ions were added so that the concrete was 100% rust-resistant, round steel rebars were inserted, and the material was heated and cured at 50°C. After one year, if no rust had developed on the rebar, it was rated as good, if rust had developed within 1 / 10 of the area, it was rated as acceptable, and if rust had developed beyond 1 / 10 of the area, it was rated as unacceptable.

[0033] (Materials used) Cement: Commercially available ordinary Portland cement Water: Tap water Fine aggregate: Standard sand for cement strength testing used in JIS R 5201

[0034] Examples 1 to 10 and Comparative Examples 1 to 4 First-grade reagent calcium carbonate, aluminum oxide, and magnesium oxide are mixed in a specified ratio, melted in an electric furnace at 1500°C, and then slowly cooled to synthesize calcium aluminate, with a Blaine specific surface area of ​​3000cm. 2 The CaO / Al2O3 (molar ratio) and MgO content of each example and comparative example are shown in Table 1. The peak positions of the heat generated by hydration of the cement admixture were measured using a multi-microcalorimeter using the method described above. The results are shown in Table 1. Next, 10 parts by mass of calcium aluminate, which is a cement admixture, was mixed with 100 parts by mass of a cement composition consisting of cement and a cement admixture to prepare a cement composition, and a mortar was prepared by mixing 300 parts by mass of fine aggregate with 10 parts by mass of the cement composition and having a water-binder ratio of 50%. The prepared mortar was evaluated for flow retention, compressive strength, chloride ion penetration depth, and rust prevention effect by the methods described above. The results are shown in Table 1.

[0035] Comparative Example 5 Using commercially available Portland cement as is, the peak position of the heat of hydration was measured using a multi-microcalorimeter according to the method described above. The results are shown in Table 1. Next, mortar was prepared using 10 parts by mass of cement, 300 parts by mass of fine aggregate, and a water-binder ratio of 50%. The prepared mortar was evaluated for flow retention, compressive strength, chloride ion penetration depth, and rust prevention effect using the methods described above. The results are shown in Table 1.

[0036] [Table 1]

[0037] From Table 1, it can be seen that the cement admixture of the present invention delays the onset of the first peak of heat generation due to hydration by adding an appropriate amount of magnesium oxide. Furthermore, it can be seen that the cement admixture of the present invention has excellent anti-rust effects and chloride ion blocking effects while suppressing a decrease in flow, and is particularly effective under conditions where the pre-curing period is short.

[0038] On the other hand, in Comparative Example 1, the CaO / Al2O3 molar ratio was less than 0.3, so the peak position of the heat release rate of hydration exceeded 900 minutes, the hydration reaction proceeded slowly, and excellent salt damage resistance was not secured with a short curing period. Therefore, the chloride ions penetrated deeply, and the rust prevention effect was not obtained. In Comparative Examples 2 and 3, which did not contain MgO, the peak position of the heat release rate of hydration was less than 500 minutes, and although a rust-preventing effect was observed, the flow rate was significantly reduced and the penetration depth of chloride ions was also greater than in Examples 1 and 2. Furthermore, in Comparative Example 4, the CaO / Al2O3 molar ratio exceeded 0.7, so the peak position of the heat release rate of hydration was less than 500 minutes, the flow rate was significantly reduced, and the penetration depth of chloride ions was also greater than in Examples 1 and 2. [Industrial Applicability]

[0039] The cement admixture of the present invention significantly improves the salt damage resistance and rust prevention effect of concrete even under conditions where it is difficult to ensure a sufficient curing period, and therefore can be used in a wide range of civil engineering and construction applications, contributing to the extension of the service life of concrete structures.

Claims

1. CaO / Al ２ O ３ A cement admixture containing calcium aluminate having a molar ratio of 0.3 to 0.7 and containing MgO, wherein the Blaine specific surface area of ​​the calcium aluminate is 2000 to 6000 cm ２ / g, and in a hydration heat generation rate spectrum from the start of the hydration reaction to the end of the hydration reaction, a first peak of the amount of heat generated by hydration appears between 500 minutes and 900 minutes from the start of the hydration reaction, and the content of the MgO is 0.1 to 5.0 parts by mass per 100 parts by mass of the cement admixture.

2. A cement composition comprising the cement admixture according to claim 1 and cement.

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