JP7569816B2 - Concrete composition and centrifugal concrete products - Google Patents

Description

The present invention relates to a concrete composition and a centrifugal concrete product that contain a binder containing Portland cement and ground granulated blast furnace slag.

Cement is generally produced by burning raw materials such as limestone and clay at high temperatures to produce a composition (intermediate) called cement clinker, which is then crushed and gypsum is added. During the cement clinker production stage, limestone, which is primarily composed of calcium carbonate (CaCO ₃ ), is burned to become cement clinker, which is primarily composed of calcium oxide (CaO), and carbon dioxide (CO ₂ ) is emitted when this is turned into the resulting product, so concrete that uses cement is said to be environmentally hazardous.

For this reason, in order to reduce the amount of carbon dioxide generated from cement production, concrete compositions with a low environmental impact have been proposed that use blast furnace slag or fly ash instead of cement, thereby reducing the amount of cement used (see, for example, Patent Documents 1 to 4).

JP 2019-151517 A JP 2010-285293 A JP 2014-148434 A JP 2009-269786 A

However, in concrete compositions in which a large amount of cement has been replaced with blast furnace slag, or in concrete compositions that do not use cement, the amount of clinker is reduced, which delays the initial reaction and reduces the initial strength, making it difficult to apply this to high-strength secondary concrete products that require early aging.

One possible solution to this problem is to use early-strength agents such as silica fume, anhydrous gypsum, gypsum dihydrate, and C-S-H-based agents to accelerate the initial reaction, but this increases the cost of the product and makes it more susceptible to carbonation. In addition, when used in centrifugally compacted concrete molding, there is a risk that compaction will become more difficult.

The present invention has been made in consideration of these points, and aims to provide a concrete composition and centrifugal concrete molded product that can be produced inexpensively and reduce carbon dioxide emissions.

The concrete composition according to claim 1 comprises water, a binder, a filler material, and an aggregate, the binder containing Portland cement and ground granulated blast furnace slag, a water-binder ratio being 20% to 35%, the binder being present at 370 kg/m3 ^to 750 kg/m3 ^, the filler material being light calcium carbonate in which carbon dioxide has been fixed in waste material , the filler being present ^at 10 kg/m3 ^to 100 kg/m3 , and a mass replacement rate of the ground granulated blast furnace slag to the Portland cement in the binder being 40% to 60%.

The present invention relates to a centrifugal concrete molded product, which comprises a concrete portion obtained by hardening the concrete composition according to the present invention, and a reinforcing material embedded in the concrete portion.

The centrifugal force concrete formed product according to the third aspect of the present invention is the centrifugal force concrete formed product according to the second aspect of the present invention, wherein the concrete portion has a compressive strength of 55 N/mm2 ^or more at an age of 7 to 14 days.

The present invention makes it possible to produce a concrete composition that is inexpensive and reduces carbon dioxide emissions.

1 is a graph showing compressive strength versus replacement rate of ground granulated blast furnace slag for each test specimen in a first example of an embodiment of the present invention. 1A is a perspective view showing a test specimen in the second embodiment, and FIG. 1B is a side view showing the test specimen. 13 is a graph showing an example of bending performance of a test specimen in the third example.

The following describes one embodiment of the present invention.

The concrete composition of one embodiment of the present invention contains water, a binder, a filler material, and an aggregate.

The binder contains Portland cement and ground granulated blast furnace slag.

Portland cement is as specified in JIS R 5210, and preferably ordinary Portland cement or high-early-strength Portland cement is used.

The ground granulated blast furnace slag is ground granulated blast furnace slag for concrete as specified in JIS A 6206, and preferably ground granulated blast furnace slag 3000, ground granulated blast furnace slag 4000, or ground granulated blast furnace slag 6000 is used.

As the filler material, light calcium carbonate powder is used. Light calcium carbonate powder is usually produced from limestone. It is produced by crushing and calcining limestone, reacting it with water to make milk of lime, and passing carbon dioxide discharged by calcination through the milk of lime to generate crystals of light calcium carbonate. In contrast, the light calcium carbonate used in the present embodiment is derived from waste. For example, the light calcium carbonate of the present embodiment is produced by immobilizing carbon dioxide contained in boiler exhaust gas from a factory or the like in a liquid obtained by solid-liquid separation of concrete sludge or washing water derived from a concrete product factory or a ready-mixed concrete plant. Alternatively, light calcium carbonate is produced by immobilizing carbon dioxide in waste seawater. For example, light calcium carbonate produced by adding an alkali agent as a pH adjuster to waste seawater after producing magnesium hydroxide from seawater, and reacting carbon dioxide contained in exhaust gas from a factory or power plant with a calcium component is used. Preferably, light calcium carbonate having a specific surface area of 2000 cm ² /g to 4000 ^{cm 2} /g is used.

Preferably, in the concrete composition of this embodiment, an admixture is used depending on the state of the concrete. As the admixture, a high-strength admixture, an expansive agent, or a shrinkage reducing agent is used. The admixture is a chemical admixture for concrete specified in JIS A 6204, for example, a polycarboxylic acid-based high-performance water reducer or a naphthalene sulfonate-based high-performance water reducer is used. The admixture is contained, for example, at 0.5% to 3.5% of the total binder content. The admixture is contained, for example, at 3% to 10% by mass of the total binder content.

The concrete composition of this embodiment has a water-binder ratio of 20% to 35%. If the water-binder ratio is below 20%, the plastic viscosity increases and handling decreases, and if the water-binder ratio is above 35%, the plastic viscosity decreases and formability by centrifugal molding decreases. In short, the upper and lower limits of the water-binder ratio are set based on the plastic viscosity (handling and formability).

The mass replacement rate of the granulated blast furnace slag powder to the Portland cement in the binder is 40% to 60%. In other words, the proportion of the granulated blast furnace slag powder in the total content of the Portland cement and the granulated blast furnace slag powder is 40% to 60% of the total mass.

The replacement rate of ground granulated blast furnace slag is set so that the strength development is equivalent to that of Portland cement alone and the strength reduction is limited to about 10 N/ ^mm2 , and the amount used is within a range that allows easy adjustment of the water-cement ratio when the compressive strength is reduced compared to that of Portland cement alone. In other words, the higher the replacement rate of ground granulated blast furnace slag, the greater the strength reduction, so the upper limit of the replacement rate of ground granulated blast furnace slag is set based on the compressive strength at a specified material age. Also, the lower limit of the replacement rate of ground granulated blast furnace slag is set based on the amount of carbon dioxide emissions, as described below.

Precipitated calcium carbonate prevents the decrease in early strength caused by the use of ground granulated blast furnace slag. In other words, there are cases where it has been reported that fine limestone powder promotes the hydration of alite (tricalcium silicate) in concrete made with fine limestone powder, and since the main component of fine limestone powder is considered to be calcium carbonate, a similar effect can be obtained by using the above-mentioned precipitated calcium carbonate.

The prescribed mix ratio for precipitous calcium carbonate is considered as aggregate replacement. The more precipitous calcium carbonate is used, the more carbon dioxide emissions from the concrete composition can be reduced, so the more it is used, the better.

The carbon dioxide emissions of the materials used in the concrete composition of this embodiment are shown in Table 1. The values are taken from JIS Q 13315-4, Appendix C. The data for the precipitated calcium carbonate was calculated based on the chemical formula and the amount of electricity used during the reaction.

Table 2 also shows an example of the composition of each material based on the reduction rate of carbon dioxide emissions.

Based on the above data, the lower limit of the replacement rate of ground granulated blast furnace slag and the lower limit of the blending ratio of light calcium carbonate are set so that the reduction rate of carbon dioxide emission is within a predetermined range, for example, 30% to 60 ^% , based on the case where the binder is Portland cement only (for example, No. 1 in Table 2). For example, as shown in No. 2 in Table 2, when the blending ratio of light calcium carbonate is 10 kg/m3 , the replacement rate of ground granulated blast furnace slag is 40% and the reduction rate of carbon dioxide emission is about 35%. Therefore, if the replacement rate of ground granulated blast furnace slag falls below 40%, it is predicted that the reduction rate of carbon dioxide emission will fall below 30% regardless of the blending ratio of light calcium carbonate, so the lower limit of the replacement rate of ground granulated blast furnace slag is set to 40%. Therefore, from the above range of the water-binder ratio, the total content of the binder of the concrete composition of this embodiment, i.e., Portland cement and ground granulated blast furnace slag , is 370 kg ^{/ m3} to 750 kg/ m3 .

On the other hand, when used as an aggregate replacement, light calcium carbonate has a large specific surface area compared to aggregate and a high water absorption rate, so the more light calcium carbonate is used, the more stiff the finished product will be. Therefore, the amount of light calcium carbonate used should be within a range that can be adjusted with a water reducer. In short, the lower limit of the light calcium carbonate blend is set based on the amount of carbon dioxide emissions, and the upper limit is set according to the state of the finished product.

For example, in this embodiment, the light calcium carbonate is contained in an amount of 10 kg/m ³ to 100 kg/ m ³ .

The aggregate includes coarse aggregate and fine aggregate. The coarse aggregate is preferably crushed stone. The fine aggregate is preferably crushed sand.

Then, a centrifugal concrete molded product is manufactured using the concrete composition of this embodiment. Examples of centrifugal concrete molded products include concrete piles, concrete columns, walls, and other centrifugally molded products. The centrifugal concrete molded product has a concrete part made by hardening the concrete composition, and a reinforcing material embedded in the concrete part. The concrete part is a high-strength concrete part having a compressive strength of 55 N/mm2 or ^more at a short-term material age (material age of 7 to 14 days). The reinforcing material is a reinforcing bar or the like.

Next, we will explain the manufacturing method for the above centrifugal concrete molded product.

The manufacturing method of this embodiment includes a manufacturing process (step S1) for manufacturing a concrete composition, a centrifugal molding process (step S2) using the concrete composition manufactured in this manufacturing process, and a curing process (step S3) after this centrifugal molding process.

In the manufacturing process, commercially available Portland cement, ground granulated blast furnace slag, light calcium carbonate, and aggregate are mixed, and then mixing water and admixtures are added to produce the concrete composition.

At this time, because the specific gravity of light calcium carbonate is lighter than that of cement, if sludge is generated by centrifugal compaction in the next centrifugal molding process, some of it will be discharged. This is not a problem when molding without sludge and the finished product has a hard mix (slump 0cm to 5cm), but when molding with sludge, it is necessary to assume that 10% to 25% of the amount of light calcium carbonate mixed in will be discharged, so molding with a slump of 0 to 5cm is desirable.

Light calcium carbonate is used in a powdered or slurried state, and has a moisture content of 1% to 4% in the powdered state. For example, when 50 kg/m ³ of light calcium carbonate is used, the light calcium carbonate contains moisture of 0.5 kg/m ³ to 2.0 kg/m ^3. Therefore, the amount of water to be mixed must be adjusted taking into account the moisture content contained in the light calcium carbonate powder. When light calcium carbonate is used in a slurried state, the moisture content must be understood. For example, when the moisture content is 50%, in order to use 50 kg/m ³ of light calcium carbonate, 100 kg/m ³ of the slurry containing light calcium carbonate must be used. In this case, the slurry contains moisture of 50 kg/m ³ , so the amount of water to be mixed must be adjusted taking into account the moisture content contained in the slurry containing light calcium carbonate.

In the centrifugal molding process, the concrete composition produced in the manufacturing process is poured into a specified formwork in which reinforcing materials are placed, and centrifugal molding is performed using a known centrifuge. In centrifugal molding, the centrifugal acceleration caused by the rotation of the formwork is gradually increased, for example, from initial speed (1G to 2G), low speed (3G to 5G), medium speed (10G to 15G), and high speed (20G to 35G). The time for each speed is appropriately determined depending on the diameter and thickness of the centrifugal concrete product.

In the curing process, the intermediate product centrifugally molded in the centrifugal molding process is steam cured at normal pressure in a steam curing tank. The steam curing is performed by first leaving the product at room temperature for 3 to 5 hours, then increasing the temperature by 20°C/hour, and then curing the product at a maximum temperature of 60°C to 70°C for 3 to 5 hours, after which the product is allowed to cool naturally.

The centrifugal concrete molded product produced in this manner has a concrete portion that exhibits high compressive strength of 55 N/mm2 ^{or more} at a short period of time (7 to 14 days).

In this way, the concrete composition of this embodiment contains 370 kg/m3 ^to 750 kg/m3 ^of binder, has a mass replacement rate of 40% to 60% of the Portland cement with ground granulated blast furnace slag in the binder, and contains 10 kg/m3 ^to 100 kg/m3 ^of light calcium carbonate as a filler, thereby suppressing delay in the initial reaction during hardening without using an early-strengthening agent, making it possible to produce the concrete composition at low cost and reducing carbon dioxide emissions by 30% to 60% compared to concrete products produced using only Portland cement as a binder.

In addition, centrifugal concrete molded products made using this concrete composition achieve early aging and are suitable as high-strength secondary concrete products.

In particular, the concrete portion of the centrifugal concrete molded product of this embodiment can achieve a high compressive strength of 55 N/mm2 ^or more at a short period of steam curing (7 to 14 days) as described above, and is not vulnerable to carbonation.

First Example
In this example, the relationship between the amount of ground granulated blast furnace slag used and the compressive strength was evaluated by comparing the compressive strength at a specified age, for example, 7 days, of centrifugal concrete products obtained by centrifugally molding a concrete composition using only Portland cement and centrifugal concrete products obtained by centrifugally molding a concrete composition in which part of the Portland cement was replaced with ground granulated blast furnace slag.

The concrete composition was made using the same common materials and mixed using a diagonal mixer or a twin-shaft mixer.

The compressive strength test was carried out using specimens with a diameter of 20 cm, a length of 30 cm, and a wall thickness of 4 cm. The specimens were manufactured using each concrete composition and the same manufacturing method.

Examples of test results are shown in Figure 1 and Table 3.

As can be seen from FIG. 1 and Table 3, the higher the substitution rate of ground granulated blast furnace slag in the concrete composition, the lower the compressive strength. For example, a centrifugal concrete product made using a concrete composition with a substitution rate of 75% ground granulated blast furnace slag showed a strength decrease of about 25 N/ ^mm2 compared to a centrifugal concrete product made using a concrete composition containing only cement.

Therefore, in order to bring out the hydraulic properties of granulated blast furnace slag, an alkaline stimulant such as cement or gypsum is required, but since Portland cement alone is insufficient as a stimulant, it has been shown that by keeping the replacement rate of granulated blast furnace slag to 60% or less, it is possible to limit the decrease in compressive strength of centrifugal concrete products to 10 N/ ^mm2 without using a stimulant.

Second Example
In this example, fresh properties and compressive strength were evaluated depending on the amount of light calcium carbonate used. Specifically, 45 to 50% of Portland cement was replaced with ground granulated blast furnace slag, and the amount of light calcium carbonate substituted for aggregate was increased, and the fresh properties and compressive strength were evaluated.

Light calcium carbonate is treated as a filler material, and when used, its blending is considered as an aggregate replacement. The test specimens were manufactured using the same manufacturing method as in Example 1. The test results are shown in Table 4. The carbon dioxide reduction rate is based on Test Example 2-1.

The more light calcium carbonate is used, the more carbon dioxide emissions from the concrete composition can be reduced, so it is preferable to use a larger amount, but the results showed that the more the amount used, the more stiff the finished product became. Although adjustments were made with a water reducing agent, mixing became difficult in Test Example 2-5, so it was shown that the amount of light calcium carbonate used should be 100 kg/m3 ^{or less} . In addition, there was a tendency for an increase in the amount of light calcium carbonate used to slightly improve early strength.

<Third Example>
In this example, a centrifugal concrete product using ground granulated blast furnace slag and light calcium carbonate, which have similar compressive strength, was compared with a centrifugal concrete product not using these materials to determine whether there was any effect on the amount of drying shrinkage, creep coefficient, and carbonation.

The mix proportions for these concrete compositions are shown in Table 5.

As with the above examples, the same materials were used except for the ground granulated blast furnace slag and light calcium carbonate, and the test specimens were manufactured using the same manufacturing method.

First, the drying shrinkage test method was based on "Part 3: Dial gauge method" of JIS A 1129-3 "Methods for measuring changes in length of mortar and concrete," and a 10 x 10 x 40 cm test specimen was manufactured. Gauge plugs were embedded into both ends of the test specimen, and the length change was measured at each age (1 week, 2 weeks, 4 weeks, 2 months, 3 months, 6 months) using a dial gauge measuring device to calculate the amount of shrinkage. During the measurement, the test specimens were stored in a constant temperature and humidity room at a temperature of 20±1°C and a humidity of 60±5%.

For the creep test method, a hollow cylindrical centrifugal specimen with a diameter of 20 cm x 30 cm was manufactured according to JIS A 1136, wire strain gauges were attached to the surface of the specimen at two opposing longitudinal positions on the outer circumference, the surface was sealed with sealing tape, and the specimen was loaded with 10 N/ ^mm2, which is the maximum prestress force applied to centrifugal concrete molded products, and the strain was measured at each age. At the same time, unloaded specimens were also prepared, and the strain was measured at each age (1 week, 2 weeks, 4 weeks, 2 months, 3 months, and 6 months). The creep strain is the value obtained by subtracting the strain of the unloaded specimen from the strain of the loaded specimen. The specimens during the measurement were stored in a constant temperature and humidity room at a temperature of 20 ± 1 ° C and a humidity of 60 ± 5%.

Furthermore, for the carbonation test method, a hollow cylindrical centrifugal specimen measuring 20 cm in diameter x 30 cm was manufactured in accordance with JIS A 1136, and this specimen was cut in the axial direction to form an arch-shaped specimen 1 as shown in Figures 2(a) and 2(b), and the cut surface 2 was coated 3 with epoxy resin. Three specimens 1 were manufactured for each test example, and an accelerated carbonation test was carried out on each of them. This accelerated carbonation test was carried out in accordance with JIS A 1153, and the arch-shaped specimen 1 was split or cut when it reached the specified accelerated period (1 week, 4 weeks, 8 weeks, 13 weeks, 26 weeks), and measured in accordance with JIS A 1152.

The results of the drying shrinkage test, creep test, and carbonation test are shown in Tables 6 to 8, respectively.

Compared to Test Example No. 3-1, Test Example No. 3-2 had a nearly equivalent rate of length change in terms of drying shrinkage up to an age of six months. Also, although the creep coefficient was somewhat large, it showed a tendency to converge after an age of four weeks, so there is no particular problem with it as prestressing concrete. Furthermore, with regard to carbonation, while no change was observed on the outer periphery, carbonation progressed on the inner surface. However, since the durability guideline in the High Durability Guidelines (draft) specifies that the carbonation depth at an age of 26 weeks is 25 mm or less, there is no problem with it as concrete.

This shows that centrifugal concrete molded products made from the concrete composition of this embodiment have sufficient performance against drying shrinkage, creep, and carbonation.

<Fourth Example>
Under the conditions of Test Example No. 3-2 of the third embodiment, a PHC pile (Type B) with a diameter of 60 cm was manufactured as a test specimen, and its bending performance was evaluated. An example of the test result is shown in Figure 3.

No concrete sludge was generated during centrifugal molding, and the molded product could be manufactured without cracks on the inside surface. As a result of the bending performance test according to JIS A 5373, Appendix E "(Regulations) Pile", the initial crack moment was 1.29 times the design value M _cf , and the fracture moment was 1.19 times the design value _Mu , indicating that the test specimen has the performance required for a PHC pile (Type B).

The above first to fourth examples show that the various conditions of the concrete composition of this embodiment are suitable for providing centrifugal concrete molded products with the required performance.

Claims (3)

Water, a binder, a filler, and an aggregate;
The binder contains Portland cement and ground granulated blast furnace slag,
The water-binder ratio is 20% to 35%;
The binder is 370 kg/m ³ to 750 kg/m ³ ,
The filler material is light calcium carbonate in which carbon dioxide is fixed in waste , and has a density of 10 kg/m ³ to 100 kg/m ³ ;
A concrete composition, characterized in that a replacement rate by mass of the ground granulated blast furnace slag to the Portland cement in the binder is 40% to 60% . A concrete part obtained by hardening the concrete composition according to claim 1 ;
A reinforcement material embedded in the concrete portion,
A centrifugal concrete molded product comprising: The centrifugal concrete molded product according to claim 2, characterized in that the concrete portion has a compressive strength of 55 N/ ^mm2 or more at an age of 7 to 14 days.

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