JP4234610B2 - Cement admixture and concrete composition - Google Patents

Description

  The present invention relates to a cement admixture. More particularly, the present invention relates to an admixture of a hydraulic composition containing cement such as cement paste, mortar and concrete.

In recent years, various cement admixtures have been developed due to the need to increase the strength of concrete and maintain the fluidity for a long period of time, along with the increase in technology for building super high-rise buildings. As a cement admixture for maintaining fluidity for a long time, a polymer composed of a (meth) acrylate monomer having a specific sulfonic acid group has been proposed (see Patent Document 1).
Japanese Patent Laid-Open No. 62-119147

  However, although the above cement admixture has an effect of reducing the viscosity, there is a problem that it is insufficient in terms of the slump loss prevention performance.

The present inventors have conducted intensive research and found that a cement admixture comprising a copolymer having a specific sulfate ester monomer as a structural unit can achieve a viscosity reduction effect and a slump loss prevention effect even with a small addition amount. The present invention has been reached. That is, the present invention is a cement comprising a water-soluble vinyl copolymer (A) having a monomer (a) represented by the following general formula (1) and another monomer (b) as constituent monomers. An admixture; and a concrete composition containing the cement admixture.
R ¹ —CH═C (R ² ) —COO— (A ¹ O) _m —SO ₃ M _{1 / r} (1)
In the formula, R ¹ and R ² are a hydrogen atom or a methyl group, A ¹ is an alkylene group having 2 to 4 carbon atoms, M is a hydrogen atom or an r-valent cation, r is 1 or 2, and m is an integer of 1 to 300. Represents.

  The concrete prepared using the cement admixture according to the present invention exhibits a high fluidity even when the admixture is added in a smaller amount than the conventional cement admixture, and has a large viscosity reducing effect. Moreover, there is little change in the slump over a long period of time.

In the present invention, the monomer (a) is represented by the general formula (1), R ¹ and R ² in the general formula (1) are a hydrogen atom or a methyl group, and from the viewpoint of the polymerizability of (a), R ¹ is preferably a hydrogen atom.

A ¹ is an alkylene group having 2 to 4 carbon atoms, specifically ethylene group, 1,2- and 1,3-propylene group, 1,2-, 1,3- and 1,4-butylene. Groups. Among these, an ethylene group and a 1,2-propylene group are preferable, and a 1,2-propylene group is particularly preferable. m is an integer of ^{1 to} 300 and represents the degree of polymerization of (A ¹ O). m is preferably 1 to 100, more preferably 1 to 50, particularly preferably 1 to 15, and particularly 2 to 12 from the viewpoint of the polymerizability of (a). Further, m A ^{1 s} may be the same or different, and in the case where they are different,-(A ¹ O) _m- may be added in any of random addition, block addition, and alternate addition.

  The r-valent cation represented by M in the general formula (1) includes an alkali metal (for example, sodium, potassium, lithium) cation, an alkaline earth metal (for example, calcium, magnesium, barium) cation, an organic amine cation, and a fourth cation. Secondary ammonium cations.

  Examples of the organic amine constituting the organic amine cation include aliphatic mono- or diamine, alicyclic mono- or diamine, heterocyclic mono- or diamine, alkanolamine, and alkylene oxide (hereinafter abbreviated as AO) adducts. It is done.

Aliphatic monoamines include alkyl groups such as methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, methylhexylamine, methyloctylamine, dimethylhexylamine, dimethyloctylamine, dimethyllaurylamine and dimethylcetylamine. Examples include mono-, di- and tri-alkylamines having 1 to 18 carbon atoms.
Examples of the aliphatic diamine include ethylene diamine and propylene diamine.
Examples of alicyclic monoamines include cycloalkylamines having 4 to 12 carbon atoms in the cycloalkyl group such as cyclobutylamine, cyclohexylamine, cyclopentylamine, cyclooctylamine, N-methylcyclohexylamine and N-ethylcyclohexylamine, and An alkyl (C1-C6) substitution product is mentioned.
Examples of the alicyclic diamine include cyclohexylenediamine.
Examples of the heterocyclic monoamine include heterocyclic amines having 4 to 10 carbon atoms such as morpholine, examples of the heterocyclic diamine include piperazine, and examples of the alkanolamine include monoethanolamine, diethanolamine and triethanol. Examples include mono-, di-, and tri-hydroxyalkylamines in which the hydroxyalkyl group such as amine has 2 to 8 carbon atoms.
Examples of AO (2 to 4 carbon atoms) of these AO adducts include ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO), and butylene oxide. EO is preferred. The number of added moles is usually 1 to 5 moles, preferably 1 or 2 moles per active hydrogen (hydroxyl group and —NH— group). Specific examples of these AO adducts include dihydroxyethylhexylamine and hydroxyethylmethylhexylamine.

Specific examples of the quaternary ammonium cation include trimethylethylammonium, triethylmethylammonium, trimethylhexylammonium, trimethyloctylammonium, tributyloctylammonium, trimethyldecylammonium, trimethyltetradecylammonium, trimethylcetylammonium, and monomethyltrioctylammonium. A tetraalkylammonium cation having 1 to 18 carbon atoms of the alkyl group; a cycloalkyl group having 4 to 12 carbon atoms and an alkyl group such as N, N-dimethylcyclohexylammonium and N, N-diethylcyclohexylammonium C1-C6 cycloalkyldialkylammonium cation; hydride such as trihydroxyethylhexylammonium Carbon number of 2 to 8 and an alkyl group carbon number of Kishiarukiru group and 1 to 6 trihydroxy alkyl alkylammonium cation.
Preferred among M are alkali metal cations, organic amine cations and quaternary ammonium cations, more preferably sodium cations, potassium cations, aliphatic and alicyclic amines or cations of AO adducts thereof, and Quaternary ammonium cations, particularly preferred are sodium cations.

  Specific examples of (a) include sulfuric acid ester of (poly) ethylene glycol mono (meth) acrylic acid ester, sulfuric acid ester of (poly) propylene glycol mono (meth) acrylic acid ester, (poly) butylene glycol mono (meth) Examples thereof include sulfuric acid ester of acrylic acid ester, sulfuric acid ester of (poly) ethylene propylene glycol mono (meth) acrylic acid ester, sulfuric acid ester of (poly) ethylene glycol monocrotonic acid ester and salts thereof. Preference is given to sulfate salts of (poly) ethylene glycol mono (meth) acrylate and sulfate salts of (poly) propylene glycol mono (meth) acrylate.

  The production method of (a) is, for example, a method in which AO is added to (meth) acrylic acid or crotonic acid or the like in the presence of a catalyst (an alkaline catalyst or an acidic catalyst), a sulfate is formed by a sulfating agent, and a salt is formed if necessary. And a method of esterifying polyalkylene glycol monosulfate (salt) with (meth) acrylic acid or crotonic acid, and the like, and a method of sulfate formation after addition of AO is preferred.

  Examples of the sulfating agent include fuming sulfuric acid, sulfuric acid, chlorosulfuric acid or sulfamic acid, and chlorosulfuric acid is preferred. Moreover, as a method of making a salt if necessary, a method of neutralizing with an alkali metal or alkaline earth metal hydroxide, ammonia water, an organic amine, etc., a method of reacting with a quaternary ammonium carbonate, a salt exchange, etc. Is mentioned. A method of neutralizing with sodium hydroxide is preferred.

The monomer (b) is a vinyl monomer that can be copolymerized with (a), and is particularly limited as long as the copolymer (A) obtained by copolymerization is water-soluble. Not.
Examples of (b) include the following anionic hydrophilic monomers (b1), cationic hydrophilic monomers (b2), nonionic hydrophilic monomers (b3) and hydrophobic monomers (b4). And the salt of (b1) is also included.

(B1) Anionic hydrophilic monomer (b11) Carboxylic acid group-containing monomer (b111) Monocarboxylic acid monomer Acrylic acid, methacrylic acid, crotonic acid and the like.
(B112) Dicarboxylic acid monomers maleic acid, itaconic acid, citraconic acid, fumaric acid and the like.
(B113) monoalkyl (carbon number 1 to 12) ester of dicarboxylic acid monomethyl maleate, monoethyl itaconate, monoethyl citraconic acid, monobutyl fumarate and the like.
(B12) Sulfonic acid group-containing monomer (b121) Aromatic sulfonic acid monomer styrene sulfonic acid, α-methylstyrene sulfonic acid, (meth) acrylamide benzene sulfonic acid and the like.
(B122) Aliphatic sulfonic acid monomers and salts thereof Alkenyl sulfonic acids such as vinyl sulfonic acid and (meth) allyl sulfonic acid; sulfomethyl (meth) acrylate, sulfoethyl (meth) acrylate, sulfopropyl (meth) acrylate, sulfobutyl Sulfoalkyl or hydroxyalkyl (C1-6) (meth) acrylate monomers such as (meth) acrylate and 2-hydroxy-3- (meth) allyloxypropanesulfonic acid; and 2- (meth) acrylamide- (Meth) acrylamidoalkyl (C2-6) sulfonic acid monomers such as 2-methylpropanesulfonic acid (hereinafter abbreviated as AMPS).
(B13) Phosphoric acid group-containing monomer Monophosphate ester of hydroxyethyl methacrylate (hereinafter abbreviated as HEP), monophosphate ester of hydroxypropyl methacrylate (hereinafter abbreviated as HPP), and monophosphate ester of hydroxybutyl methacrylate (hereinafter abbreviated as HBP) ) And the like.

Examples of the salt of (b1) include salts having a cation represented by M described above as a counter ion, and preferred ones are also the same.

(B2) Cationic hydrophilic monomer (b21) Quaternary ammonium base-containing monomer Trimethyl (meth) acryloyloxyethylammonium chloride, triethyl (meth) acryloyloxyethylammonium chloride, dimethylbenzyl (meth) acrylic (Meth) acrylate or (meth) acrylamide-based quaternary ammonium base-containing monomers such as leuoxyethylammonium chloride and dimethyl (meth) acrylamidepropyltrimethylammonium chloride.
(B22) Amino group-containing monomer Dialkyl (C1-C4) aminoalkyl (C2-C4) (meth) acrylate such as dimethylaminoethyl (meth) acrylate and diethylaminoethyl (meth) acrylate and these ( Amino group-containing (meth) acrylamide corresponding to (meth) acrylate.

(B3) Nonionic hydrophilic monomer (b31) (poly) alkylene (2 to 3 carbon atoms) glycol mono (meth) acrylate:
Droxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, poly (degree of polymerization 2 to 100) ethylene glycol mono (meth) acrylate, poly (degree of polymerization 2 to 100) ethylene propylene glycol mono (meth) acrylate and the like.
(B32) Monoalkoxy (C1-4) polyalkylene (C2-3) glycol mono (meth) acrylate:
Methoxy polyethylene glycol mono (meth) acrylate and methoxy polyethylene propylene glycol mono (meth) acrylate.
(B33) Amide group-containing nonionic monomer For example, the amide group-containing nonionic monomer:
(Meth) acrylamide and N-methylol (meth) acrylamide.

(B4) Hydrophobic monomer (b41) Alkyl having 1 to 18 carbon atoms or alkyl or alkenyl (meth) acrylate having alkenyl methyl, ethyl, n- and iso-propyl, n-, iso-, sec- and tert -Alkyl such as butyl, octyl, 2-ethylhexyl, nonyl, decyl, dodecyl, 2-methylundecyl, tridecyl, 2-methyldodecyl, tetradecyl, 2-methyltridecyl, pentadecyl and 2-methyltetradecyl (meth) acrylate (Meth) acrylate; alkenyl (meth) acrylate such as allyl, octenyl, decenyl, dodecenyl (meth) acrylate.
(B42) Aromatic hydrocarbon monomers Styrene monomers such as styrene, α-methylstyrene and vinyltoluene.
(B43) Aliphatic hydrocarbon monomers, olefins having 2 to 24 carbon atoms such as ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene and octadecene; chain dienes such as butadiene and isoprene.
(B44) Cycloaliphatic unsaturated hydrocarbons having 5 to 18 carbon atoms such as cyclopentadiene, pinene, limonene, indene, bicyclopentadiene and lidene norbornene.
(B45) Alkenyl ester of carboxylic acid C2-C4 alkenyl ester of C2-C18 carboxylic acid, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl octoate, vinyl laurate, and allyl acetate.

  Of (b), (b1) and (b3) are preferred, (b1) is more preferred, (b111) of (b11) and (b121) and (b122 of (b12) are particularly preferred. ), And (b13) HEP.

The copolymerization ratio (molar ratio ) of (a) and (b) is preferably (a) / (b) = 1-60 / 99-40 from the viewpoint of the viscosity reduction effect and slump loss prevention performance of (A). , more preferably 2-30 / 98-70, especially preferably 5 to 20/95 to 80.
When (b111) is used for at least a part of (b), (b111) in (b) is 60 mol% of (b) from the viewpoint of the slump loss prevention performance of (A). It is preferable to set it as the above, and 70 mol% or more is particularly preferable.
In addition, when the hydrophobic monomer (b4) is used for at least a part of (b), (b4) in (b) is (b) from the viewpoint of making (A) water-soluble. ) Is preferably 30 mol% or less, more preferably 10 mol% or less, and particularly preferably 1 mol% or less, but particularly preferably (b4) is not used in combination.

  (A) in the present invention is water-soluble and has a solubility in water at 25 ° C. of 5 or more. The solubility of (A) is preferably 10 or more, more preferably 30 or more, particularly preferably freely mixed with water, from the viewpoint of the slump loss prevention performance of (A).

  The weight average molecular weight (hereinafter abbreviated as Mw) of (A) is preferably in the range of 5,000 to 300,000, and more preferably 10,000 to 300,000. If Mw is 300,000 or less, the viscosity reducing effect tends to be improved, and if it is 10,000 or more, the slump loss preventing effect tends to be improved. Mw in the present invention is measured by a method based on polyethylene glycol by gel permeation chromatography.

(A) in the present invention can be produced by a known production method. For example, a method in which a monomer is polymerized in a solvent in the presence of a polymerization initiator (for example, JP-A-62-1119147) Described polymerization method).
Polymerization solvents include water, alcohols (such as methyl alcohol, ethyl alcohol, and isopropyl alcohol), aromatic hydrocarbons (such as benzene, toluene, xylene, ethylbenzene, trimethylbenzene, and ethyltoluene), and aliphatic hydrocarbons (such as cyclohexane, n-hexane, heptane, octane and decalin), esters (such as ethyl acetate), ketones (such as acetone, methyl ethyl ketone and methyl isobutyl ketone) and the like. In particular, water, methyl alcohol, ethyl alcohol, isopropyl alcohol and a mixed solvent thereof are preferable.

  Polymerization initiators include azo-based [2,2′-azobisisobutyronitrile, 2,2′-azobis (2-methylbutyronitrile), 2,2′-azobis (2,4-dimethylvaleronitrile]. ), 2,2′-azobisisobutyrate, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis (2-methylpropionamide) dihydrate, and the like [T-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneoheptanoate, t-butyl peroxyneodecanoate, t-butyl peroxy 2-ethylhexanoate, 1 , 1,3,3-tetramethylbutylperoxy 2-ethylhexanoate, dibutylperoxytrimethyladipate, benzoylpa Inorganic polymerization initiators such as ammonium salts or alkali metal salts of persulfate and hydrogen peroxide; and the like oxides, such as cumyl peroxide and lauryl peroxide, organic polymerization initiator such as. Of these, inorganic polymerization initiators, 2,2'-azobis (2-amidinopropane) dihydrochloride, and 2,2'-azobis (2-methylpropionamide) dihydrate are preferable.

  It is also possible to use a polymerization accelerator and / or a chain transfer agent in combination. Examples of the polymerization accelerator include sodium bisulfite, and examples of the chain transfer agent include mercapto compounds (such as lauryl mercaptan, 2-mercaptoethanol, mercaptoacetic acid, 1-mercaptoglycerin, and mercaptosuccinic acid).

  The polymerization temperature is usually 30 to 150 ° C, preferably 50 to 120 ° C. In addition to the above solution polymerization, it can also be obtained by bulk polymerization. Furthermore, the mode of copolymerization may be either random addition polymerization or alternating copolymerization, and may be either graft copolymerization or block copolymerization.

  The cement admixture of the present invention contains (A) and optionally water. The form of the cement admixture of the present invention is in the form of a lump, melt, or aqueous solution, and is preferably an aqueous solution from the viewpoint of ease of addition. The weight concentration of (A) in the aqueous solution is preferably 1 to 90%, more preferably 5 to 70%, and particularly preferably 10 to 50%.

  The addition amount of the cement admixture of the present invention based on the weight of the cement is usually 0.02 to 1% by weight, preferably 0.05 to solid amount, based on the cement, in terms of the addition amount of (A). The amount added is 0.5% by weight.

  The cement admixture of the present invention may be used in combination with a conventionally known high-performance water reducing agent. As the high-performance water reducing agent, high-performance water reducing agents such as naphthalene-based, melamine-based, aminosulfonic acid-based and polycarboxylic acid-based materials, for example, “Development Trend of High-performance AE Water-Reducing Agent”, Fine Chemical February 1, 2002, Vol. 31 , No. 2, the composition described in p15, or the composition described in “Chemical Admixture for Concrete” edited by the Japan Society of Materials Science (Asakura Shoten: published in 1972) can be used in combination. When the admixture of the present invention is used in combination with these known high-performance water reducing agents, the ratio of the high-performance water reducing agent based on the total weight (in terms of active ingredient) is 30% by weight or less.

The cement admixture of the present invention can be used in combination with other conventionally known additives for concrete. For example, AE agent, AE water reducing agent, high performance AE water reducing agent, water reducing accelerator, water reducing delay agent, curing described in p1-17 of “Chemical admixture for concrete” edited by Japan Society of Materials Science (Asakura Shoten: published in 1972) Accelerators, cure retarders, quick setting agents, waterproofing agents, foaming agents, foaming agents, antifreeze agents, rust preventives, water retention agents, blast furnace slag, pozzolans, gypsum and CSA. The composition of these additives is resin acid, wood resin, synthetic detergent, lignin sulfonic acid, petroleum acid, fatty acid, protein, sulfonated hydrocarbon, higher polyhydric alcohol sulfonic acid, alkylbenzene sulfonic acid, polyoxyethylene alkylphenol ether, Naphthalene sulfonic acid formalin condensate, melamine sulfonic acid formalin condensate, lignin sulfonic acid, polyacrylamide partial hydrolyzate, polycarboxylic acid, aromatic aminosulfonic acid and vinyl copolymers other than (A) in the present invention (for example, sulfone) Vinyl chloride copolymer, maleate ester copolymer and styrene copolymer) and salts thereof.
When the cement admixture of the present invention is used in combination with a known high-performance water reducing agent and / or other concrete admixture, the proportion of the cement admixture of the present invention based on their total weight (in terms of active ingredient) is 30 to 100% by weight, preferably 50 to 100% by weight.

The concrete composition of the present invention is obtained by curing a slurry comprising the cement admixture of the present invention, cement, aggregate, water and, if necessary, the above-mentioned high-performance water reducing agent and / or other additives. From the viewpoint of the effect of preventing slump loss, the weight ratio of cement to cement is preferably 10 to 50/90 to 50, and more preferably 20 to 40/80 to 60.
The content of (A) in the concrete composition of the present invention is usually 0.005 to 1% by weight, preferably 0.01 to 0.5% by weight. The concrete composition of the present invention constitutes unreinforced concrete, reinforced concrete, prestressed concrete and the like used for various uses such as for construction, civil engineering, road, irrigation drainage, and river sea.

<Example>
EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following examples, parts represent parts by weight.

The composition of the monomer (a) used is shown below.

(A-1): Poly (polymerization degree = 9) sulfate ester sodium salt of propylene glycol monoacrylate (a-2): Poly (polymerization degree = 9) propylene glycol monomethacrylate sulfate sodium salt

Examples 1-5, Comparative Examples 1 and 2;
A reaction vessel equipped with a thermometer, a stirrer, two dropping funnels, a nitrogen gas inlet tube and a reflux condenser was charged with 38 parts of water, purged with nitrogen while stirring, and heated to 95 ° C. under a nitrogen atmosphere. In a beaker, the monomers listed in Table 1 were dissolved and mixed in 30 parts of water to prepare a monomer solution. The total amount of the monomer solution and 8 parts of 5% ammonium persulfate aqueous solution were dropped from a separate dropping funnel over 2 hours. After completion of dropping, 4 parts of 5% aqueous ammonium persulfate solution was further dropped over 1 hour, and then kept at 95 ° C. for 1 hour for aging to obtain a 20% by weight aqueous solution of the vinyl copolymer in the present invention. The cement admixtures (A-1) to (A-5) of the present invention were obtained, and a 20% by weight aqueous solution of a comparative vinyl copolymer was obtained by the same method. -1) and (Q-2).

Performance evaluation example;
The cement admixture obtained above was actually mixed with concrete and its performance (viscosity reduction effect and slump loss prevention effect) was evaluated. First, a preliminary test was performed to determine the amount of cement admixture added so that the initial slump value was 20 ± 1 (cm). For concrete, cement, aggregate and water are mixed in the proportions shown in Table 2, and cement admixtures are 0.10%, 0.15%, 0.20%, 0.30%, 0.40%,. 50% and 0.60% (vs. cement weight) were added, and kneaded for 3 minutes at a rotation speed of 25 rpm with a tilting mixer. The slump value of the concrete immediately after kneading was measured to determine the amount of cement admixture added. After that, as in the preliminary test, a cement admixture with an addition amount (described in Table 3) such that the initial slump value is 20 ± 1 (cm) is added to the concrete having the mixing ratio described in Table 2, and the preliminary test is performed. Kneaded under the same conditions. The slump value of the concrete immediately after kneading was measured to obtain the slump value immediately after the kneading, and the slump value was measured for the concrete after further kneading at 3 rpm for 60 minutes and 90 minutes with a tilting mixer. The slump value was measured according to JIS A1101 method.

  As is apparent from Table 3, the concrete prepared using the cement admixture according to the present invention shows high fluidity even when the amount of the admixture added is small compared to the comparative product, and the viscosity reducing effect is large. It was. Moreover, it can be seen that the concrete prepared using the cement admixture according to the present invention has little change in the slump value over a long period of time and has a large effect of preventing the slump loss.

  The concrete prepared using the cement admixture of the present invention exhibits high fluidity and a large viscosity reduction effect even if the additive amount of the admixture is smaller than that of the conventional cement admixture. Moreover, there is little change in the slump over a long period of time. Therefore, the concrete using the cement admixture of the present invention is excellent in workability and extremely effective for the production of concrete. The concrete composition of the present invention is used for construction, civil engineering, road use, irrigation drainage, river sea It can be suitably used for unreinforced concrete, reinforced concrete, prestressed concrete, and the like used for various purposes such as.

Claims (8)

A cement admixture comprising a water-soluble vinyl copolymer (A) having a monomer (a) represented by the following general formula (1) and another monomer (b) as constituent monomers.
R ¹ —CH═C (R ² ) —COO— (A ¹ O) _m —SO ₃ M _{1 / r} (1)
Wherein R ¹ and R ² are a hydrogen atom or a methyl group, A ¹ is an alkylene group having 2 to 4 carbon atoms, M is a hydrogen atom or an r-valent cation, r is 1 or 2, and m is 1 to 300. Represents an integer.) 2. (b) is at least one monomer selected from the group consisting of carboxylic acid group-containing monomers, sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, and salts thereof. The cement admixture described. The blending of cement according to claim 1 or 2, wherein the copolymerization ratio [(a) / (b)] of (a) and (b) in the copolymer (A) is 1 to 60/99 to 40 in terms of molar ratio. Agent. The cement admixture according to any one of claims 1 to 3, wherein the copolymer (A) has a weight average molecular weight of 5,000 to 300,000. The cement admixture according to any one of claims 1 to 4, wherein the solubility of the copolymer (A) in water is 10 or more at 25 ° C. Furthermore, 1 or more types of additives for cement chosen from the group which consists of an AE agent, a water reducing agent, a high performance water reducing agent, an AE water reducing agent, and a high performance AE water reducing agent are contained, Any one of Claims 1-5 characterized by the above-mentioned. A cement admixture according to claim 1. The cement admixture according to any one of claims 1 to 6, which is used for concrete having a water / cement weight ratio of 20 to 40/80 to 60 . A concrete composition containing the cement admixture according to claim 1.