Method for producing powder for concrete and composition for concrete

Abstract

The present invention provides a method for producing powder for concrete, which can ensure workability when used in concrete, and a concrete composition. A powder material (2) derived from concrete sludge is subjected to a ball mill treatment so as to reduce the specific surface area, thereby obtaining a powder (1) for concrete.

Description

[Technical Field]

[0001] The present invention relates to a method for producing powder for concrete and a concrete composition. [Background technology]

[0002] Concrete sludge, a by-product of the concrete manufacturing process, has traditionally been disposed of as industrial waste, but in recent years, technologies for recycling concrete sludge have been attracting attention from the perspective of reducing environmental impact and recycling resources.

[0003] For example, a method is known in which synthetic calcium carbonate is produced by reacting concrete sludge with carbon dioxide gas (see, for example, Patent Document 1).

[0004] Also known is a method of using synthetic calcium carbonate as a raw material for concrete by reacting concrete sludge with carbon dioxide gas (see, for example, Patent Document 2).

[0005] Furthermore, a technique has been proposed in which concrete sludge is solidified and dried, and then crushed to obtain a powder material, which is then reused in concrete (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2023 / 190372 [Patent Document 2] Japanese Patent Application Publication No. 2025-20759 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-204194 Summary of the Invention [Problem to be solved by the invention]

[0007] However, because these powder materials have a porous structure and a large BET specific surface area, they absorb a large amount of water during mixing, resulting in an increase in the water demand of concrete. This leads to problems such as reduced workability and the need to use excessive amounts of chemical admixtures such as water-reducing agents and superplasticizers to ensure the required fluidity.

[0008] The present invention has been made in consideration of the above points, and an object of the present invention is to provide a method for producing powder for concrete and a concrete composition that can ensure workability when used in concrete. [Means for solving the problem]

[0009] The method for producing powder for concrete of the present invention comprises: It is a pulverized powder obtained by pulverizing solidified concrete sludge. Powder materials, BET By ball milling to reduce the specific surface area, powder for concrete is obtained. A saturated aqueous solution of calcium hydroxide and sand are added during the ball mill treatment. . [Effects of the Invention]

[0010] According to the present invention, workability can be ensured when used in concrete. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are schematic diagrams showing an example of a ball mill used in a method for producing powder for concrete according to one embodiment of the present invention, in which (a) is a front view thereof and (b) is a perspective view thereof. [Figure 2] 2 is an explanatory diagram showing the process of converting powder material into concrete powder according to the same method for producing concrete powder. FIG. [Figure 3] (a) is an SEM photograph showing an example of pulverized powder used in the manufacturing method of the powder for concrete, (b) is an SEM photograph showing an example of synthetic calcium carbonate used in the manufacturing method of the powder for concrete, (c) is an SEM photograph showing an example of fly ash as a comparative example, and (d) is an SEM photograph showing an example of sewage sludge incineration ash as another comparative example. [Figure 4]4 is a graph showing the relationship between age and compressive strength during underwater curing in a compressive strength test of the first embodiment of the method for producing powder for concrete, where (a) shows the embodiment and (b) shows the comparative example. [Figure 5] 4 is a graph showing the relationship between the age of material during steam curing and compressive strength in a compressive strength test of the above-mentioned manufacturing method of powder for concrete, where (a) shows an example and (b) shows a comparative example. [Figure 6] 10 is a graph showing the relationship between age and compressive strength during steam curing and underwater curing in a compressive strength test of the second embodiment of the method for producing powder for concrete. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] The present embodiment shown in Figures 1(a), 1(b), and 2 is a method for producing concrete powder 1, in which a powder material 2 is subjected to a pulverization process using a ball mill 3, thereby reducing the specific surface area (BET specific surface area) of the powder material 2.

[0014] The powder material 2 used in this embodiment is derived from concrete sludge and is at least one of two types: synthetic calcium carbonate and pulverized powder. Therefore, the concrete powder 1 is an environmentally friendly powder.

[0015] Concrete sludge includes concrete generated during concrete production (e.g., centrifugal molding) or construction, residual concrete (residual concrete) in ready-mix concrete plants, returned concrete (returned concrete), washing water from agitator trucks, etc. Preferably, concrete sludge from which coarse and fine aggregates have been removed is used.

[0016] Synthetic calcium carbonate is produced by injecting carbon dioxide (CO2) into sludge water with an adjusted pH from the above-mentioned concrete sludge, causing it to react with calcium hydroxide (Ca(OH)2) and other substances in the sludge. The synthetic calcium carbonate produced in this way contains light calcium carbonate, cement carbonated powder, and uncarbonated powder of cement. The ratio of these components is, for example, 2% light calcium carbonate, 94% or more cement carbonated powder, and 4% or less uncarbonated powder of cement. Furthermore, the weight ratio of carbon dioxide components resulting from the carbon dioxide adsorption reaction is 44 wt% CO2 for light calcium carbonate, 25 wt% CO2 for cement carbonated powder, and 1 wt% CO2 for uncarbonated powder of cement.

[0017] This synthetic calcium carbonate has a porous structure and a BET specific surface area of ​​30 to 60 m 2 / g and has high water absorption. This synthetic calcium carbonate is alkaline but stable with few unreacted components, and contains cement hydrate (calcium silicate hydrate (hereinafter simply referred to as CSH), Ca(OH)2, etc.) and unhydrated particles.

[0018] The pulverized powder is obtained by pulverizing the solid content of the concrete sludge. Preferably, the pulverized powder is obtained by pulverizing the solid content obtained by suspending the unhardened concrete sludge in water and then separating the solid content using a filter press. In this case, the concrete sludge is preferably a so-called slag fraction obtained by centrifugal molding. This slag fraction is less likely to contain fine aggregate powder from the concrete, so it has a higher cement content and excellent weighing properties.

[0019] This crushed powder contains unhardened cement (residual solids) that has not yet hardened like ordinary concrete, and its main components are hydration products derived from cement (CSH and ettringite).

[0020] Even if this pulverized powder contains fine powder contained in concrete, this does not cause any problems in the concrete powder 1 produced by the manufacturing method of this embodiment. Because the fine powder is inert, it does not have any negative effect on the activity of the solids, including cement. If you want to quantify the amount of fine powder, you can quantify the component called crystalline SiO2 (mineral name: quartz), which is contained as a crystalline substance, using powder X-ray diffraction or other methods. The crystalline SiO2 content contained in the slag is generally about 1 to 10%. Even if this level of crystalline SiO2 is contained, it does not have any effect on the concrete powder 1.

[0021] The pulverized powder has a porous structure and a BET specific surface area of ​​40 to 90 m 2 / g and has high water absorption. This pulverized powder contains cement hydrates (CSH, Ca(OH)2, etc.) and unhydrated particles. Furthermore, due to the structure of pulverized aggregates, this pulverized powder tends to have a wide particle size distribution, and if carbonation has progressed, it partially contains calcium carbonate.

[0022] The ball mill 3 has a container (pot) 5 that is rotated by a rotating stand equipped with a rotary drive source such as a motor. The container 5 has a cylindrical main body with both ends closed by end plates. Inside the container 5, balls (media) 6 are placed for pulverizing the object to be pulverized. Suitable materials for the balls 6 include ceramic, alumina, high-purity alumina, silicon nitride, steel, natural stone, zirconia, and MC nylon (with an iron core).

[0023] The pulverization process using the ball mill 3 in this embodiment, i.e., ball milling, is performed to solve the problem of poor moisture control and fluidity, i.e., workability, of concrete when the powder material 2, which has high water absorption and fine particles, is used as a concrete composition (admixture). The ball mill 3 breaks down the needle-shaped ettringite crystals and monocarbonate needle crystals in the powder material 2, thereby reducing the specific surface area and improving the stability of the material. Furthermore, in this ball milling process, the powder material 2 is atomized by the collision of balls 6 as shown in FIG. 2. At the same time, a mechanochemical reaction occurs, causing cement hydrate (CSH) 7 to be supported on the surface of the atomized powder material 2a. This improves the performance of the concrete powder 1 as a material.

[0024] Because powder material 2 contains a large amount of calcium, modifying and fixing its physical structure through ball milling can improve the reactivity of concrete powder 1. Furthermore, carbonation in the atmosphere of CSH, which is formed by the hydration of cement, produces a large amount of calcium carbonate, but this process leaves behind silicic acid, or siliceous components. Activation by ball milling regenerates this siliceous component into CSH, promoting the hydration of Alite and increasing strength. The residual siliceous components from carbonation, common to powder material 2, also act as a cover to inhibit carbonation. However, by activating them with ball mill 3 to produce CSH, the increased amount of CSH produced increases strength, and the disappearance of the cover effect exposes the cement, which is expected to regenerate CSH through the hydration of the cement. That is, in the concrete powder 1, the silica content, i.e., amorphous silica content, left behind in the powder material 2 due to carbonation is activated to generate CSH, and in addition, the silica content that had stopped carbonation reacts through the activation treatment, exposing new unhydrated cement content, causing a new hydration reaction, which then causes another hydration reaction and generates CSH.

[0025] As conditions for this ball mill treatment, for example, a rotation speed of 30 rpm to 80 rpm and a treatment time of about 0.3 hours (18 minutes) to 2 hours are preferable.

[0026] If the rotation speed is less than 30 rpm, the kinetic energy of the balls 6 in the ball mill 3 will be insufficient, resulting in insufficient impact and shear forces, and the effects of pulverization, mixing, and surface modification may be insufficient. Furthermore, if the rotation speed exceeds 80 rpm, the balls 6 will be in a "centrifugal motion state" in which they stick to the inner surface of the container 5 due to centrifugal force, making it difficult for impact pulverization to occur. This reduces the degree of micronization and surface activation of the powder material 2 and significantly reduces energy efficiency.

[0027] Furthermore, if the treatment time is less than 0.3 hours (18 minutes), mixing, crushing, and interface modification will be insufficient, resulting in insufficient improvement in the dispersibility and cohesiveness of the powder material 2. Furthermore, if the treatment time exceeds 2 hours, energy efficiency will decrease, while the powder material 2 will be over-crushed into excessively fine particles, which may increase the water demand due to an increase in specific surface area and cause a decrease in the fluidity of the concrete, as well as increasing treatment costs.

[0028] The ball mill treatment may be a dry type in which no liquid is added, or a wet type in which a liquid is added. In the wet type, for example, water, a saturated aqueous solution of calcium hydroxide, or an alkaline solution (e.g., sodium hydroxide, sodium carbonate, etc.) is added in an amount of 10 to 100 mass % relative to the mass of the powder material 2.

[0029] If the amount of water, saturated calcium hydroxide solution, or alkaline solution added is less than 10% by mass, the wet dispersion and interface modification effects of the liquid are insufficient, and the surface activation of the powder material 2 and the defibration of aggregates are not effectively achieved. In particular, when a calcium hydroxide solution is used, the supply of calcium hydroxide to the surface of the powder material 2 is limited, making it impossible to expect an improvement in the affinity of the concrete powder 1 with cement. Furthermore, if the amount of water or saturated calcium hydroxide solution added exceeds 100% by mass, the excess liquid causes slurry formation in the ball mill 3, significantly reducing the grinding efficiency. Furthermore, the resulting concrete powder 1 becomes excessively wet, making it difficult to dry and measure it in subsequent processes and to adjust the moisture content during concrete mixing.

[0030] By using a saturated aqueous solution of calcium hydroxide during the ball mill treatment, calcium is added to the powder material 2, which can further enhance the activation effect when used as concrete powder 1.

[0031] Furthermore, if necessary, sand, for example, fine aggregate (e.g., JIS standard sand) as a particle size adjuster or dispersion aid, may be added in an amount of 30 to 100 mass % relative to the mass of the powder material 2. Adding fine aggregate suppresses adhesion of the powder to the balls 6, allowing the ball mill treatment to proceed smoothly, making it easier for the concrete powder 1 to become a slurry, and also allowing CSH to be supported on the surface of the fine aggregate through a mechanochemical reaction caused by the ball mill treatment, and also allowing the balls 6 to be easily recovered after the ball mill treatment.

[0032] If the amount of fine aggregate added is less than 30% by mass, the particle size adjustment effect will be insufficient and ineffective, and the resulting concrete powder 1 will be biased toward fine powder, which may reduce the workability of the concrete. Furthermore, if the amount of fine aggregate added is more than 100% by mass, the aggregate ratio will be too high, reducing the frequency of contact between the ball 6 and the powder material 2, making it difficult to achieve sufficient grinding, mixing, and surface modification effects. Furthermore, the functional components of the powder material 2 will be diluted, making it difficult for the powder material 2 to demonstrate its performance as a concrete material 1.

[0033] The powder material 2 is composed of tiny particles, primarily calcium carbonate, and is highly porous and rough. The formation and support of CSH proceeds in a complex manner, resulting in strong mechanical adhesion and the formation of a new interfacial layer through chemical reactions. This makes the powder material highly reactive, serving as the nucleus for subsequent CSH growth. Furthermore, the powder material 2 may contain residual reactive components, such as unhydrated cement particles and aluminates, which may trigger secondary reactions that promote CSH formation.

[0034] In contrast, fine aggregates such as JIS standard sand have a non-porous structure, and the adhesion and support of CSH is mainly limited to physical adsorption or surface reaction, and they do not contain reactive components like powder material 2. Therefore, although the effect of additional CSH formation and growth is limited, CSH is supported in a monodispersed manner, and therefore has excellent reactivity, just like when supported on powder material 2, and acts as a nucleus for CSH to grow thereafter.

[0035] In this way, by ball milling at least one of powder material 2, which is synthetic calcium carbonate obtained by reacting carbon dioxide with concrete sludge, and powder material 2 obtained by pulverizing solidified concrete sludge, to reduce the specific surface area, concrete powder 1 is obtained, which makes it possible to adjust the particle size, make the particle size distribution uniform, and improve the aggregation state, thereby improving the usability of concrete powder 1.

[0036] Even if the ball mill treatment is dry and only pulverization is performed, the amount of water absorption can be reduced by improving the particle size distribution, making it easier to adjust the moisture content when mixing concrete.

[0037] On the other hand, when the ball mill treatment is carried out with the powder material 2 having been added with a saturated aqueous solution of calcium hydroxide or an alkaline solution, in addition to adjusting the particle size and making the particle size distribution uniform, calcium ions (Ca 2+The powder material 2 is chemically activated by the generation or exposure of functional groups such as hydroxyl groups (-OH) and reactive sites with chemical compositions and structures that have high chemical affinity with cement hydrates (particularly CSH) are formed on the surface of the powder material 2, making it easier to control moisture during mixing and improving dispersibility, thereby further improving the workability and fluidity of concrete.

[0038] In particular, by using a saturated aqueous solution of calcium hydroxide or the like, surface activation is promoted, and the powder for concrete 1 has affinity with cement-based materials and an effect of promoting the hydration reaction.

[0039] Furthermore, the slurry-like concrete powder 1 produced by wet ball mill processing can also be dried and used as dry powder. The physical properties, such as compressive strength, remain the same as in the slurry state. To produce the dry powder, the slurry-like concrete powder 1 is transferred to a stainless steel dish or the like and dried in a dryer at 100-110°C for a specified drying time. The drying time is generally about 24 hours. It dries more easily if the powder is spread thinly. There are no particular restrictions on the dryer, and a general-purpose dryer can be used. However, since blower-type dryers that blow air can scatter dust, it is preferable to use a constant-temperature dryer with little airflow.

[0040] The concrete powder 1 obtained in this manner has a controlled specific surface area and reduced cohesion, enabling the reuse of powder material 2 derived from concrete sludge as a concrete material. This improves dispersibility in concrete and suppresses excessive water absorption and increases in the viscosity of the mixture. Therefore, concrete or mortar containing this concrete powder 1 as a concrete composition (admixture) containing the powder 1 as an admixture or partial replacement for cementitious materials exhibits stable initial water absorption behavior, improving slump retention and workability during mixing. This facilitates fluidity and reduces the amount of water-reducing agent and superplasticizer used, thereby enhancing its utility as a low-carbon material and contributing to reducing environmental impact. Specifically, the concrete powder 1 obtained from concrete sludge uses powder material 2 in which carbon dioxide is stably immobilized, making it reusable as a building material. This provides environmental benefits such as waste reduction, greenhouse gas emission suppression, and resource recycling. Therefore, the powder for concrete 1 contributes to sustainable building material technology that recycles concrete sludge and reduces environmental load through fixation of carbon dioxide.

[0041] Here, the effectiveness of the concrete powder 1 of one embodiment will be explained using comparative examples. The comparative examples include one comparative example in which fly ash is used as a powder material and is subjected to a ball mill treatment, and another comparative example in which sewage sludge ash (SSA) is used as a powder material and is subjected to a ball mill treatment.

[0042] Fly ash is a fine ash contained in the exhaust gas from coal-fired power plant boilers. Fly ash has a spherical particle structure, with most particles being smooth and uniform, although some are hollow and lightweight. The particle size distribution is broad, ranging from a few microns to 100 microns. Some particles have smooth, dense, and glossy surfaces, while others contain glassy components and have an amorphous structure. An SEM image of an example of fly ash is shown in Figure 3(c). When fly ash is used in concrete compositions, its spherical and smooth shape improves flowability and workability, while its partially hollow particles enable lightweight construction and affect porosity. Furthermore, when ball milling fly ash to produce powder, it is necessary to optimize the ball material and mixing time to find conditions that minimize changes in particle shape and specific surface area. This depends on the type of fly ash and the amount of fly ash mixed, so trial and error is required. The smaller the activity index, the higher the pozzolanic reactivity tends to be.

[0043] Sewage sludge incineration ash is obtained by incinerating sewage sludge at temperatures between 800 and 1000°C. The particle structure of sewage sludge incineration ash is largely irregular and porous, not spherical, but angular (crushed) or flaky or agglomerated. Furthermore, the organic matter vaporizes and burns during incineration, resulting in a porous and rough structure. The particle size distribution is approximately 10–100 μm, with significant variation. It contains a large amount of crystalline components (aluminates, phosphates, etc.), and XRD (X-ray diffraction) analysis tends to produce distinct peaks. An SEM image of an example of sewage sludge incineration ash is shown in Figure 3(d). When sewage sludge incineration ash is used in concrete compositions, its irregular shape and rough surface result in poor filling and low fluidity. Its porous nature also leads to high water absorption and adsorption, which can affect self-hardening properties. Sewage sludge incineration ash also contains a high amount of phosphorus, and its phosphate has a wafer-like layered structure. The phosphate reacts with calcium hydroxide to form an insoluble calcium phosphate film initially, inhibiting the hydration reaction and slowing down the setting process. Furthermore, when sewage sludge incineration ash is ball-milled, the particle shape changes from agglomerates to amorphous, flat particles, destroying the layered phosphate structure, reducing water absorption, and binding calcium and phosphorus, thereby suppressing the setting delay caused by phosphorus. The pozzolanic reaction caused by the reaction with calcium hydroxide solution depends on the amount of amorphous silica in the incineration ash. While the amount of amorphous silica is generally small, the pozzolanic reaction produces reaction products, and the main effect of ball-milling is often to destroy the layered phosphate structure.

[0044] In contrast, the particle structure of the powder material 2 (pulverized powder, synthetic calcium carbonate) of this embodiment is composed of crushed, irregularly shaped particles, with almost no spherical particles observed. The surface is rough, particles are often aggregated and re-aggregated, and sharp-edged, fragmented particles, and plate-like and needle-like crystals may be observed. These particles originate from hydration products such as CSH residue and ettringite, and contain a mixture of amorphous and crystalline phases. Because the original cement sludge has a porous structure, the particles have a high internal surface area (BET specific surface area) due to the micropores and layered structure. The particle size distribution is approximately 1 to 20 μm, but aggregates can reach several tens of μm. Figure 3(a) shows an SEM photograph of an example of the pulverized powder used as the powder material 2, and Figure 3(b) shows an SEM photograph of an example of synthetic calcium carbonate. When powder material 2 is used as a concrete composition, its large BET specific surface area and many irregularities result in high water absorption and viscosity. Its irregular shape and sharp angles reduce flowability, and particles that have undergone hydration reactions may also lose their activity. Furthermore, concrete powder 1 produced by applying the manufacturing method of this embodiment to powder material 2 loses its porous structure due to the fracture of the crystalline structure, resulting in a reduced BET specific surface area. However, the particle surfaces become more compact and refined, resulting in easier-to-handle particles due to the destruction of agglomerates and the crushing of needle-like crystals. Furthermore, the exposure of unreacted components and changes in surface structure result in the activation of the particle surfaces. Furthermore, powder material 2 is originally rich in calcium, but adding more calcium with a calcium hydroxide solution can enhance the activation effect when used as a concrete material. [Example]

[0045] Tests were conducted on the pulverized powder produced by the manufacturing method of the above embodiment, or on mortar or concrete to which synthetic calcium carbonate had been added, and the results of the compressive strength tests with and without treatment (activation treatment) were compared to examine the difference in compressive strength enhancement properties due to ball mill treatment.

[0046] {First Example} The experimental material was ordinary Portland cement (C, density 3.16 g / cm 2 ), JIS standard sand (S, for Cement Association strength tests) as fine aggregate, powder material 2 or concrete powder 1 (R), tap water (W), high-performance AE water reducer (Sp, polycarboxylic acid type), and saturated calcium hydroxide aqueous solution (commercially available calcium hydroxide dissolved in it) were used.

[0047] The mortar mixes containing powder material 2 without ball mill treatment had one water-cement ratio (W / C) of 50% by mass and five powder material 2 addition rates (R / B) of 0, 10, 20, 30, and 40% by mass. Powder material 2 was added at the mass ratio of cement, and was externally substituted with JIS standard sand with the cement mass kept constant. These comparative examples (Comparative Examples 1 to 5) are shown in Table 1.

[0048] [Table 1]

[0049] The powder material 2 was a pulverized powder obtained by pulverizing the remaining solids in concrete sludge. The ball mill 3 had a wide-mouthed reagent bottle (73 mm diameter, 168 mm height, 500 ml capacity, made of low-density polyethylene) as the container 5, alumina balls 6, and a two-stage rotating stand with ten 15 mm diameter balls 6 and five 13 mm diameter balls 6. The conditions for the ball mill treatment were a rotating stand rotation speed of 30 rpm and a mixing time of 1 hour. The composition of the powder material 2 in the container 5 was 50% by mass of saturated calcium hydroxide aqueous solution, 63% by mass of tap water, and 45% by mass of JIS standard sand.

[0050] The mortar mix using the slurry (RS) containing the ball milled concrete powder 1 had one water-cement ratio (W / C) of 50 mass % and five addition rates (R / B) of concrete powder 1 of 0, 10, 20, 30, and 40 mass %. The mixing ratio in the container 5 during ball milling was adjusted to be the same depending on the addition rate of concrete powder 1. These examples (Examples 1 to 5) are shown in Table 2.

[0051] [Table 2]

[0052] The casting conditions for the test specimens were in accordance with JIS R 5201. A JIS mortar mixer was used for mixing, and the formwork was a square pillar of 40 x 40 x 160 mm. After molding, the test specimens were demolded the next day, and then underwater cured and steam cured.

[0053] For underwater curing, the test materials were placed in water in a constant temperature and humidity room at 20°C and 60% RH for 7, 14, and 28 days.

[0054] For steam curing, the temperature was raised to 65°C over 3 hours and 15 minutes at a rate of 20°C / h, and after maintaining 65°C for 3 hours, the specimens were gradually cooled to 20°C and then removed. The age periods after steam curing were 7, 14, and 28 days, and the specimens were cured in air in a constant temperature and humidity room at 20°C and 65% RH.

[0055] The compressive strength test method was measured in accordance with JIS R 5201 Appendix C. The measurement results are shown in Figs. 4 and 5.

[0056] Mixtures using large amounts of Powder Material 2 and Concrete Powder 1 resulted in a thick mixture that was difficult to mix, so an experiment was conducted to add a water reducer to maintain fluidity above a certain level in order to prevent overloading the mixer shaft and motor. A water reducer was used to maintain fluidity above a certain level, but it was found that the amount of water reducer required could be reduced with Concrete Powder 1 compared to Powder Material 2. These results showed that ball mill processing had the effect of improving the state of aggregation.

[0057] Furthermore, in the comparative example in which powder material 2 was added to mortar using ordinary Portland cement, a decrease in compressive strength was confirmed as the addition rate increased when cured underwater, but in the example in which concrete powder 1 was added, an increase in compressive strength was confirmed compared to the comparative example.

[0058] Between underwater curing (shown in Figures 4(a) and 4(b)) and steam curing (shown in Figures 5(a) and 5(b)), steam curing was more effective in preventing the decrease in compressive strength, and the increase in compressive strength was equivalent to that of JIS mortar.

[0059] [Second Example] The experimental material was ordinary Portland cement (C, density 3.16 g / cm 2 ), crushed sand, crushed stone, and powder material 2, synthetic calcium carbonate (Ca), tap water (W), and a high-performance water-reducing agent (Sp, naphthalene-based) were used.

[0060] Regarding the concrete mix with powder material 2 added without ball mill treatment, the water-cement ratio (W / C) was set at one level of 28%, and the amount of powder material 2 added was set at 0, 40, and 80 kg / m 3 The following three levels were set. Powder material 3 was substituted with crushed sand in a proportion with the cement mass kept constant. These comparative examples (Comparative Examples 6 to 8) are shown in Table 3.

[0061] [Table 3]

[0062] The powder material 2 used was a pulverized powder obtained by pulverizing synthetic calcium carbonate. The ball mill treatment involved only pulverization. The container 5 was a wide-mouthed reagent bottle (73 mm diameter, 168 mm height, 500 ml capacity, made of low-density polyethylene), the balls 6 were made of alumina, and a two-stage rotating stand was used, with ten 15 mm diameter balls 6 and five 13 mm diameter balls 6. The conditions for the ball mill treatment were a rotating stand rotation speed of 30 rpm and a mixing time of 1 hour. These examples (Examples 6 and 7) are shown in Table 4.

[0063] [Table 4]

[0064] The casting conditions for the test specimens were in accordance with JIS A 1132. A test mixer was used for mixing, and the formwork was a cylindrical shape with dimensions of φ10 x 20 cm. After the specimens were cast, they were demolded the next day and then underwent underwater curing and steam curing.

[0065] The test materials were submerged in water in a constant temperature and humidity room at 20°C and 60% RH for 7 and 28 days.

[0066] For steam curing, the temperature was raised to 65°C at a rate of 20°C / h, and after maintaining 65°C for 3 hours, the specimen was gradually cooled to 20°C and then removed.The material was left to age for 7 days after steam curing, and then cured in air in a constant temperature and humidity room at 20°C and 65% RH.

[0067] The compressive strength test method was measured in accordance with JIS A 1108. The measurement results are shown in Figure 6.

[0068] Powder material 2, which uses synthetic calcium carbonate, is highly water-absorbent, and the more it is used, the harder the mixture becomes, making it difficult to mix.In order to prevent overloading the mixer shaft and motor, an experiment was conducted to add a water-reducing agent to increase the slump above a certain level.Although a water-reducing agent was used to increase the slump above a certain level, it was found that the amount of water-reducing agent used could be reduced when the ball mill was treated compared to when it was not.This result showed that it has the effect of dispersing agglomerated admixtures.

[0069] In the comparative example where powder material 2 was added to concrete using ordinary Portland cement, an increase in compressive strength was confirmed as the addition rate increased. Furthermore, when activation treatment was performed using a ball mill, a tendency for the compressive strength to increase was confirmed compared to the untreated case.

[0070] Furthermore, between water curing and steam curing, steam curing was more effective in increasing the compressive strength in the activation treatment using powder material 2 using synthetic calcium carbonate.

[0071] The above first and second examples demonstrate the effectiveness of the ball mill treatment according to the above embodiment. [Explanation of symbols]

[0072] 1. Concrete powder 2 Powder material 3. Ball mill

Claims

1. A powder material, which is a pulverized powder obtained by pulverizing solidified concrete sludge, is subjected to a ball mill treatment so as to reduce the BET specific surface area, thereby obtaining a powder for concrete, and a saturated aqueous solution of calcium hydroxide and sand are added during the ball mill treatment. A method for producing powder for concrete, comprising:

2. A powder material, which is synthetic calcium carbonate obtained by reacting concrete sludge with carbon dioxide, is ball milled to reduce the BET specific surface area, thereby obtaining powder for concrete. A method for producing powder for concrete, comprising:

3. During ball milling, a saturated aqueous solution of calcium hydroxide and sand are added.

3. The method for producing concrete powder according to claim 2.

4. After ball milling, the material is dried to produce dry powder.

3. The method for producing concrete powder according to claim 2.

5. Ball mill processing is dry 3. The method for producing concrete powder according to claim 2.

6. The concrete powder produced by the method according to any one of claims 1 to 5 is used as an admixture or partial substitute for cement-based materials. A concrete composition characterized by:

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