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Cement additive containing superplasticizer and bentonite

Abstrict

A cement additive contains a superplasticizer, an inorganic strength increasing agent such as calcium sulfate or silica fume and bentonite.

Claims

What is claimed is:

1. A cement additive comprising a superplasticizer, an inorganic strength increasing agent, an organic acid and bentonite, said superplasticizer being selected from the group consisting of polyalkylaryl sulfonate superplasticizer and melamine/formalin resin sulfonate superplasticizer, said inorganic strength increasing agent being selected from the group consisting of calcium sulfate, silica fumes and a mixture thereof, said organic acid being selected from the group consisting of oxalic acid, maleic acid, fumaric acid, malonic acid, succinic acid, glutaric acid, adipic acid, lactic acid, heptonic acid, gluconic acid, glycolic acid, malic acid, tartaric acid, citric acid, salts thereof, derivatives thereof and a mixture thereof, said superplasticizer being contained in an amount of 0.25 to 5 parts by weight based on 100 parts by weight of cement, said calcium sulfate being contained in an amount not more than 15 parts by weight, calculated as anhydride, based on 100 parts by weight of cement, said silica fume being contained in an amount of not more than 20 parts by weight based on 100 parts by weight of cement, said organic acid being contained in an amount of 0.005 to 0.5 parts by weight based on 100 parts by weight of cement, and said bentonite being contained in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of cement.

2. The cement additive according to claim 1 wherein said polyalkylaryl sulfonate superplasticizer is selected from the group consisting of a condensation product of naphthalene sulfonic acid with formalin and a salt thereof, a condensation product of methylnaphthalene sulfonic acid with formalin and a salt thereof, a condensation product of anthracene sulfonic acid with formalin and a salt thereof, and a mixture thereof.

3. The cement additive according to claim 1 wherein said calcium sulfate has a specific surface area of not less than 2,000 cm.sup.2 /g in terms of Blaine value.

4. The cement additive according to claim 1 wherein said calcium sulfate is selected from the group consisting of Type II anhydrous calcium sulfate, Type III anhydrous calcium sulfate, calcium sulfate hemihydrate, calcium sulfate dihydrate and a mixture thereof.

5. The cement additive according to claim 1 wherein said cement additive further comprises an inorganic salt selected from the group consisting of carbonates, bicarbonates, silicates and aluminates of alkali metals, alums of sodium and potassium, aluminum sulfate, magnesium sulfate and a mixture thereof.

6. The cement additive according to claim 5 wherein said inorganic salt is contained in an amount of not more than 1.0 part by weight to 100 parts by weight of cement.

Description

BACKGROUND OF THE INVENTION

This invention relates to a cement additive containing a superplasticizer and bentonite and more particularly to a cement additive for eliminating tackiness of a cement composition and preventing the cement composition from cracking at an initial stage upon drying before the cement composition is hardened.

As compared with common plasticizers, such as ligninsulfonate or polyol type plasticizers, now in popular use in ready-mixed concrete plants, superplasticizers have desirable properties in that a cement composition or concrete having only low set retarding and air entraining properties, a high water reducing ratio and high strength may be easily produced even when a larger amount of the superplasticizer is added to the cement. However, the concrete admixed with the superplasticizer exhibits properties totally different in the unhardened state from those of the concrete admixed with the common plasticizers. Specifically, the former exhibits strong tackiness and non-bleeding. Thus, the following problems are presented in construction works employing such concrete.

More specifically, on account of strong tackiness, the concrete sags or slides down along a tilt if it exists on the construction site, so that the construction works are obstructed. In addition, on account of non-bleeding properties, the water in the concrete is lost on drying before the concrete is hardened, thus causing cracking upon drying due to plastic shrinkage. While such cracking occurs after lapse of more or less time interval since the time of casting, depending on the ambient temperature, humidity and the wind force prevailing at the time of construction, it may occur after lapse of a shorter time interval, such as 20 to 30 minutes, in an extreme case, with the width of crack being as large as 2 to 5 mm. In addition, because of strong tackiness and non-bleeding properties, coarse aggregates float on the concrete surface, while a skin is formed on drying on the concrete surface so that the troweling operation is obstructed, thus resulting in only poor surface finishing. Even when water is sprayed on the concrete surface to improve its surface finishing, the aggregates cannot be sunk because floating of the aggregates in the concrete is mainly caused by tackiness of the concrete. On the contrary, the superplasticizers in the concrete may float to cause an irregular color and adversely affect the surface finish such as uniform color and smoothness.

It is thus seen from the above that although the superplasticizers are superior to ordinary plasticizers in certain aspects when used for preparation of high strength concrete, they have not come into customary use in civil engineering and construction works in general, only because of the above problems. Although it is strongly desired to solve these problems, the art or technology which provides direct solution of these problems has not been evolved.

On the other hand, strength of the concrete admixed only with the superplasticizer ceases to increase when the concrete is dried during curing, with the resulting strength being fluctuated appreciably and occasionally reaching only a lower value depending on the degree of drying. Thus, the strength cannot be designed in large-sized civil engineering and construction works where it is not possible to perform sufficient wet curing.

It is known to add calcium sulfates, above all, Type II anhydrous calcium sulfate, in order that a sufficient design strength be obtained even upon drying, while it is also known to produce a high strength concrete by using citric acid or salts thereof, carbonates of alkali metals or siliceous materials in combination, as shown for example Japanese Laid-open Patent Publication No. 167460/1983. However, the above problems resulting from strong tackiness of the concrete employing the superplasticizer cannot be solved by these methods.

Although the art of improving concrete strength by using the superplasticizer and calcium sulfates simultaneously is now in popular use in factories producing concrete articles with the use of steam curing, the fact is that only a centrifugal casting mold is actually employed which is not affected by various problems resulting from concrete tackiness, while the art cannot be applied extensively to pre-cast members for which finishing is required, because the finishing properties are low while cracking on drying may be produced during pre-curing.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a cement additive effective in eliminating tackiness and improving the workability of a cement composition.

It is another object of the present invention to provide a cement additive effective in preventing a cement composition from cracking upon drying, especially from cracking at an initial stage upon drying before the cement composition is hardened.

It is a further object of the present invention to provide a cement additive for obtaining a hardened mass of the cement composition capable of exhibiting high strength in stability in spite that the composition contains superplasticizers.

It is yet another object of the present invention to provide a cement additive for obtaining a hardened mass of a cement composition having improved surface finish and smoothness and free from irregular color.

The above and other objects of the invention will become apparent from the following description.

According to the present invention, there is provided a cement additive comprising a superplasticizer, an inorganic strength increasing agent and a bentonite.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a diagrammatic elevational view partly in section showing a device for measuring tackiness of a concrete composition.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be explained in detail.

The present inventors conducted eager researches into eliminating tackiness, preventing cracking upon drying before hardening and improving surface finishing properties of a hardened mass of a cement composition containing a superplasticizer and an inorganic strength improving agent, without drastically lowering the high strength properties of the hardened mass. As a result, we have found that a cement composition having excellent properties can be obtained by employing bentonite in combination with the superplasticizer and the inorganic strength improving agent and, based on this finding, arrived at a cement additive of the present invention.

The superplasticizers that may be employed in accordance with the present invention include polyalkylaryl sulfonate type superplasticizer and melamine/formalin resin sulfonate type superplasticizer. The superplasticizers presently on the market are composed mainly of one or more of these components. However, any plasticizers not composed of the above components but exhibiting strong tackiness and non-bleeding properties may be employed as the superplasticizer of the present invention.

The polyalkylaryl sulfonate type superplasticizer includes a condensation product of naphthalene sulfonic acid with formalin or a salt thereof, a condensation product of methylnaphthalene sulfonic acid with formalin or a salt thereof, and a condensation product of anthracene sulfonic acid with formalin or a salt thereof.

The polyalkylaryl sulfonate type superplasticizers are marketed under the trade names of "MIGHTY 100", "MIGHTY 150" and "MIGHTY 200" by KAO Corporation; "SELFLOW-110P" by Dai-ichi Kogyo Seiyaku Co., Ltd.; "POLFINE 510N" by Takemoto Oil and Fat Co. Ltd.; "SANFLO-PS" and "SANFLO-PSR" by Sanyo-Kokusaku Pulp Co. Ltd.; and "FT-500" by Denki Kagaku Kogyo KK. The melamine/formalin resin sulfonate type superplasticizers are marketed under the trade names of "MELMENT F-10" and "MELMENT F-20" by Showa Denko KK. and "NL-4000" by Nisso Master Builders Co., Ltd.

These superplasticizers are preferably added in an amount of 0.25 to 5 parts by weight as solid contents to 100 parts by weight of the cement. With the amount of the superplasticizer less than 0.25 parts by weight, the properties of the superplasticizer, such as water reducing properties, cannot be exhibited. The contents of the superplasticizer in excess of 5 parts by weight are also not desirable from economical considerations and because the increase in water reducing properties cannot be expected. The superplasticizers are preferably added in an amount of 0.35 to 3 parts by weight and more preferably in an amount of 0.45 to 2 parts by weight.

The inorganic strength improving agents that may be employed in accordance with the present invention include calcium sulfates, silica fume and mixtures thereof.

The calcium sulfates include Type II anhydrous calcium sulfate, Type III anhydrous calcium sulfate, calcium sulfate hemihydrate and dihydrate. Usually, one of these calcium sulfates is used alone or two or more of these calcium sulfates are used in combination. There are no special restrictions as to the amounts or kinds of the industrially inevitable impurities insofar as calcium sulfates are reacted with aluminate in the cement and the resulting hydrates are converted into ettrigite.

The calcium sulfates may be contained in an amount of 15 parts by weight or less, preferably 1 to 13 parts by weight and more preferably 2 to 10 parts by weight calculated as anhydrous form to 100 parts by weight of cement. From the viewpoint of strength, the optimum amount of addition of the calcium sulfates to cement depends mainly on the curing temperature, such that, the higher the temperature, the more the amount of the calcium sulfates that need be added to cement to achieve the maximum strength. For steam curing at 60.degree. to 80.degree. C., the peak value of addition of the calcium sulfates is 10 to 13 parts by weight. The amount of the calcium sulfates in excess of 15 parts by weight is not desirable since the strength is not increased but rather starts to be lowered slightly. For steam curing at about 20.degree. C., the peak value of addition of the calcium sulfates is 4 to 6 parts by weight. For steam curing at lower temperatures, it is 3 to 5 parts by weight. The amount of addition of the calcium sulfates less than 1 part by weight has little effect even for lower steam curing temperatures. The specific surface area of the calcium sulfates may be not less than 2,000 cm.sup.2 /g (Blaine value, porosity of 0.5), preferably not less than 3,000 cm.sup.2 /g and more preferably 4,000 cm.sup.2 /g.

The silica fumes employed in the present invention are spherical fine amorphous SiO.sub.2 by-produced during production of Si metal or Si-Fe alloy in an electric furnace. The silica fumes have high pozzolanic activity and may be reacted easily with Ca(OH).sub.2 (Portlandite) in the hardened mass of cement to form calcium silicate hydrates to improve the strength. It is preferred that 20 parts by weight or less and desirably 2 to 15 parts by weight of the silica fumes be added to 100 parts by weight of cement. The particle size of the silica fumes is so small that, with the amount of the silica fumes in excess of 20 parts by weight, concrete tackiness is increased, and more bentonite is needed with the unit volume of water increased and with the concrete strength lowered. The amount of the silica fumes less than 2 parts by weight is not desirable since it has only a little effect on increasing the concrete strength.

Addition of the calcium sulfate simultaneously with silica fumes has a favorable effect on the strength even when the amount of addition of the silica fumes is reduced. Such reduction in the amount of addition of the silica fumes is convenient since a smaller amount of bentonite is sufficient to lower the tackiness. In such a case, the amount of 3 to 10 parts by weight of the silica fumes is preferred.

The bentonite is contained in concrete in combination with the superplasticizer to lower the tackiness, while elevating water retention properties, preventing cracking and vaporization of water contents and improving finishing properties. While it is preferred that the bentonite be added in an amount of 0.1 to 10 parts by weight to 100 parts by weight of cement, the optimum amount of addition of bentonite cannot be determined monistically since it may differ depending on the swelling degree of bentonite, concrete temperature or the amount of addition of the superplasticizer to cement. Even in consideration of possible variable factors, the amount of bentonite less than 0.1 part by weight is ineffective and hence not desirable, while the amount of bentonite in excess of 10 parts by weight is also not desirable since the percentage of the water reduction proper to the superplasticizer may be lowered significantly. The preferred amount of addition of bentonite is 0.3 to 6 parts by weight.

There are no restrictions as to the particle size of bentonite and the particle sizes of marketed products may be employed without any adjustment. If bentonite has approximately the same swelling degree, bentonite with 200-mesh or 400-mesh particle size may be used without any difference in effect.

According to the present invention, organic acids and/or salts and/or derivatives thereof may be added besides the above enumerated essential ingredients. The organic acids that may be used in the present invention include soluble organic acids including COOH-group or groups or simultaneously including COOH- and OH-groups, metal salts and/or derivatives thereof, and may contain unsaturated organic acids with chemical addition of alkyl- or aryl groups and condensation or copolymerization products thereof. Lower carboxylic acids available on the market include oxalic acid, maleic acid, fumaric acid, malonic acid, succinic acid, glutaric acid, adipic acid and metal salts thereof. Oxymonocarboxylic acids include lactic acid, heptonic acid, gluconic acid, glycolic acid and metal salts thereof. Polyvalent oxycarboxylic acids include malic acid, tartaric acid, citric acid and metal salts thereof. The common feature of these organic acids and salts thereof is that although they retard the setting of cement, if used in moderate amounts and simultaneously with bentonite, these organic acids and salts thereof are effective to reduce the lowering of the concrete strength even if the water to cement ratio is increased slightly. Above all, the polyvalent oxycarboxylic acids are effective to prevent faster slump drop which presents another inconvenience of the concrete admixed with the superplasticizer. The preferred polyvalent oxycarboxylic acids include malic acid, tartaric acid, citric acid and salts thereof, with the citric acid and salts thereof being most preferred. It is preferred that the amount of addition of the organic acids and salts thereof be 0.005 to 0.5 part by weight to 100 parts by weight of cement. More preferably, it is 0.01 to 0.35 part by weight and most preferably it is 0.05 to 0.25 part by weight. The amount of the organic acids and salts thereof less than 0.005 part by weight is not desirable since it has only a little effect in improving the strength, while the amount larger than 0.5 part by weight is not desirable since it retards cement setting and hence evolution of strength excessively.

According to the present invention, certain inorganic salts may be added to cement. These inorganic salts include carbonates, bicarbonates, silicates or aluminates of alkali metals such as lithium, sodium or potassium, alums of sodium or potassium, aluminum sulfate and magnesium sulfate. These inorganic salts assisting the 5 effect of bentonite are preferably employed in an amount not more than 1.0 part by weight, calculated as the anhydride, to 100 parts by weight of cement. The amount of the inorganic salts above the upper limit is not desirable since it is not effective and there are certain components that cause false or rapid setting. The most preferred range of addition is 0.1 to 0.5 part by weight.

The cements to which the cement additive of the present invention may be added include various Portland cements, mixed cements and hydraulic materials containing slag particles as essential components. The rapid hardening Portland cement is not effective in view of high strength since it contains alumina cement, 12CaO.7Al.sub.2 O.sub.3, 11CaO.7Al.sub.2 O.sub.3.CaF.sub.2 or amorphous calcium aluminate including CaO.Al.sub.2 O.sub.3, 12CaO 7Al.sub.2 O.sub.3, etc. as well as Type II anhydrous calcium sulfate. The rapid hardening is accompanied by rapid heat evolution so that the effect of the organic acids and salts thereof in decreasing the amount of water necessary for cement hydration and in improving the strength is lost.

It is noted that any desired method may be employed for mixing or adding the above enumerated various components or ingredients. For example, the various components may be previously mixed within the prescribed ranges of addition to cement before ultimate addition to cement. If the superplastcizer is in the liquid form, a moderate amount of water may be added thereto to form a slurry composition. Alternatively, the various components may be added separately during production of concrete products.

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