JP7615205B2 - Cement composition and method for producing same - Google Patents

Description

This disclosure relates to a cement composition and a method for producing the same.

As an effort to reduce carbon dioxide emissions, attention is being focused on a technology that chemically produces light calcium carbonate by dissolving carbon dioxide in a liquid phase and reacting it with calcium. Light calcium carbonate fixes carbon dioxide in the stable form of calcium carbonate, and therefore directly contributes to reducing carbon dioxide emissions.

When immobilizing a huge amount of carbon dioxide as calcium carbonate, it is possible to use waste and by-products that contain a lot of calcium and are discharged in large quantities, such as waste concrete and steelmaking slag, as a calcium source.
After calcium is extracted from the waste and by-products, a large amount of extraction residue is left behind. Generally, such extraction residue is often disposed of by landfill. However, if the extraction residue can be effectively utilized, it will contribute to extending the life of final disposal sites and reducing the environmental burden.

Possible uses for the extraction residue include substituting it as an admixture for part of the Portland cement, or using it as a blended cement. This will allow for effective use of the extraction residue and reduce the amount of clinker used, making it possible to reduce carbon dioxide emissions.

In recent years, from the perspective of preventing global warming and reducing carbon emissions, it has been desired to further increase the replacement ratio of admixtures in cement clinker in order to reduce the amount of cement clinker used. For example, Non-Patent Document 1 examines the mixing of admixtures with ordinary Portland cement in excess of 5 mass%, the amount stipulated by the JIS standard. In this case, it is necessary to consider the effects on various physical properties caused by increasing the amount of admixture.

Abstract of the 72nd Cement Technology Conference 2018, "3315 Quality evaluation of cement with increased amounts of minor mixed components"

As described above, when low-reactivity waste or by-products are mixed into cement clinker, the strength development may decrease. In particular, there is concern that the early strength of blended cement may decrease.
Therefore, an object of the present disclosure is to provide a cement composition that reduces the environmental impact by effectively utilizing waste and by-products, while having the same level of strength development as current Portland cement, and is particularly excellent in early strength.

One aspect of the present disclosure is a cement composition comprising a cement and a calcium-containing waste powder,
The calcium-containing waste powder is
The insoluble residue measured in accordance with JIS R 5202:2010 is 70.0% by mass or less,
Contains 5.0% by mass or more of calcium carbonate, and
The residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is 18.0% by mass or less,
The object of the present invention is to provide a cement composition containing the calcium-containing waste powder in an amount of 1.0 mass % or more and 7.5 mass % or less.

Another aspect of the present disclosure is a method for producing a cement composition comprising cement and a calcium-containing waste powder, the method comprising the steps of:
The calcium-containing waste powder is
The insoluble residue measured in accordance with JIS R 5202:2010 is 70.0% by mass or less,
Contains 5.0% by mass or more of calcium carbonate, and
The residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is 18.0% by mass or less,
The object of the present invention is to provide a method for producing a cement composition, which comprises mixing the cement composition with the calcium-containing waste powder so that the calcium-containing waste powder is contained in the cement composition in an amount of 0.5 mass % or more and 7.5 mass % or less.

According to the present disclosure, by effectively utilizing waste and by-products, a cement composition that has the same level of strength development as current Portland cement and is particularly excellent in early strength while reducing the environmental burden, and a method for producing the same are provided.

Unless otherwise specified, the materials exemplified in this disclosure may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the cement composition according to this disclosure, the content of each component in the cement composition means the total amount of the multiple substances present in the cement composition, unless otherwise specified.

One embodiment of the cement composition according to the present disclosure includes cement and calcium-containing waste powder.

As the cement, any cement known in the art can be used without any particular restrictions. For example, ordinary Portland cement, early-strength Portland cement, moderate-heat cement, low-heat cement, oil well cement, etc. can be used. From the viewpoint of ease of availability and further improving early strength, the cement preferably contains at least one of ordinary Portland cement and early-strength Portland cement, more preferably contains at least early-strength Portland cement, and only early-strength Portland cement may be used.

The cement clinker in the cement contains _C3S , _C2S , _C3A , and _C4AF . The contents of _C3S , _C2S , _C3A , and _C4AF can be calculated by the Bogue formula. The Bogue formula is a widely used formula for calculating the contents of major minerals in cement clinker from the content ratios of their chemical compositions. The Bogue formula shown below can be used to calculate the contents of tricalcium silicate ( _3CaO.SiO2 , _C3S ), dicalcium silicate ( _2CaO.SiO2 , _C2S ), tricalcium _aluminate ( _3CaO.Al2O3 , _C3A ), and tetracalcium _aluminate ferroferrate ( _{4CaO.Al2O3.Fe2O3} , _C4AF ) in cement clinker. The chemical formulas represent the content ratios (mass%) of each compound indicated by chemical analysis values according to JIS _R 5204: ₂₀₁₉ "Fluorescent X-ray analysis method for cement".

<Bogue Style>
C ₃ S [mass %] = (4.07 x CaO [mass %]) - (7.60 x SiO ₂ [mass %]) - (6.72 x Al ₂ O ₃ [mass %]) - (1.43 x Fe ₂ O ₃ [mass %]) - (2.85 x SO ₃ [mass %])
C ₂ S [mass %] = (2.87 x SiO ₂ [mass %]) - (0.754 x C ₃ S [mass %])
C ₃ A [mass %] = (2.65 x Al ₂ O ₃ [mass %]) - (1.69 x Fe ₂ O ₃ [mass %])
C ₄ AF [mass%] = 3.04×Fe ₂ O ₃ [mass%]

The amount of _C3S in the cement clinker in the cement is preferably 50.0 mass% or more, more preferably 55.0 mass% or more, even more preferably 60.0 mass% or more, and even more preferably 65.0 mass% or more. By making the value of the _C3S amount equal to or more than the above value, the early strength of the cement composition during hardening can be further improved.
The amount of _C3S in the cement clinker is preferably 80.0 mass% or less, more preferably 75.0 mass% or less, and even more preferably 70.0 mass% or less. By keeping the amount of _C3S at the above value or less, heat generation during hardening of the cement composition can be suppressed.

The amount of _C2S in the cement clinker is preferably 2.0 mass% or more, more preferably 5.0 mass% or more, and even more preferably 8.0 mass% or more. By making the value of the amount of _C2S equal to or more than the above value, the long-term strength of the hardened cement composition can be further improved.
The amount of _C2S in the cement clinker is preferably 40.0 mass% or less, more preferably 30.0 mass% or less, even more preferably 15.0 mass% or less, and even more preferably 10.0 mass% or less. By making the amount of _C2S less than the above value, the early strength of the cement composition during hardening can be further improved.

The amount of _C3A in the cement clinker is preferably 7.0 mass% or more, more preferably 8.0 mass% or more, even more preferably 9.0 mass% or more, even more preferably 9.5 mass% or more, and particularly preferably 10.0 mass% or more. By making the amount of _C3A equal to or more than the above value, it is possible to produce a cement composition that increases the amount of waste and by-products such as coal ash that are used as a raw material for cement clinker.
The amount of _C3A in the cement clinker is preferably 13.0 mass% or less, more preferably 12.0 mass% or less, even more preferably 11.0 mass% or less, even more preferably 10.5 mass% or less, and particularly preferably 10.0 mass% or less. By making the amount of _C3A less than the _above value, it is possible to suppress the regeneration of _ettringite (a compound represented by _{3CaO.Al2O3.3CaSO4.32H2O} ), and also to further reduce the increase in the _amount of heat generated when the cement composition hardens.

The amount of _C4AF in the cement clinker is preferably 5.0 mass% or more, more preferably 6.0 mass% or more, even more preferably 7.0 mass% or more, even more preferably 8.0 mass% or more, and particularly preferably 8.5 mass% or more. By making the amount of _C4AF equal to or more than the above value, it is possible to produce a cement composition that increases the amount of waste and by-products such as coal ash that are used as a raw material for the cement clinker.
The _C4AF content in the cement clinker is preferably 14.0 mass% or less, more preferably 12.0 mass% or less, even more preferably 11.0 mass% or less, even more preferably 10.0 mass% or less, and particularly preferably 9.0 mass% or less. By making the _C4AF content equal to or less than the above value, the increase in the amount of heat generated during hardening of the cement composition can be further reduced.

The cement clinker content is preferably 70% by mass or more and 95% by mass or less, more preferably 75% by mass or more and 93% by mass or less, and even more preferably 85% by mass or more and 90% by mass or less, assuming the total amount of cement to be 100% by mass.

The cement may contain gypsum. For example, gypsum dihydrate, gypsum hemihydrate, anhydrous gypsum, etc. may be used as the gypsum. One type of gypsum may be used alone, or a combination of two or more types may be used. The content of gypsum in the cement may be equivalent to the content of gypsum in general Portland cement. Specifically, the content of gypsum in the cement is preferably 0.5% by mass or more and _3.5 % by mass or less, more preferably 0.7% by mass or more and 3.0% by mass or less, and even more preferably 1.0% by mass or more and 2.5% by mass or less, based on the total amount of cement being 100% by mass, in terms of SO 3.

The cement may be commercially available Portland cement, or may be a mixture of cement clinker, gypsum, limestone and/or calcium carbonate, which have been separately ground and mixed. It may also be a mixture made by grinding two or more materials simultaneously.

The cement content in the cement composition according to the present disclosure is preferably 92.0 mass% or more, more preferably 93.5 mass% or more, and even more preferably 94.5 mass% or more, based on the cement composition. By making the cement content equal to or greater than the above value, the cement composition according to the present disclosure has a strength development at the same level as ordinary Portland cement, and is particularly excellent in early strength.
The cement content in the cement composition according to the present disclosure is preferably 99.5% by mass or less, more preferably 98.5% by mass or less, even more preferably 97.0% by mass or less, and even more preferably 95.5% by mass or less, based on the cement composition. By making the cement content equal to or less than the above value, waste materials such as waste concrete and waste mortar that have been discarded up to now can be effectively utilized, and the environmental load is reduced.

The cement composition according to the present disclosure contains calcium-containing waste powder in addition to cement. In this disclosure, "calcium-containing waste powder" refers to powder of various waste materials that have no specific use and are normally disposed of, and that contain calcium.

The calcium-containing waste powder is derived from waste materials such as waste concrete, waste mortar, waste cement paste, concrete sludge, paper sludge, blast furnace slag, converter slag, electric furnace slag, slowly cooled slag, chlorine bypass dust, coal ash, biomass ash, or incineration ash. The calcium-containing waste powder is not the waste material itself, but the residue after calcium is extracted from the waste material. In other words, the calcium-containing waste powder in the present disclosure is the calcium-containing waste powder after calcium extraction.
The waste material preferably contains calcium in an amount calculated as CaO of 5.0% by mass or more, more preferably 10.0% by mass or more, and even more preferably 20.0% by mass or more, from the viewpoint of enabling efficient recovery of calcium from the waste material and contributing to resource recycling.

In particular, the calcium-containing waste powder is preferably a residue remaining after calcium extraction from at least one waste material selected from waste concrete, waste mortar, and waste cement paste. By using the residues of these waste materials as components of the cement composition, it is possible to effectively utilize materials that have been discarded up until now, and reduce the environmental burden. In addition, the cement composition of the present disclosure containing calcium-containing waste powder has a strength development level equivalent to that of ordinary Portland cement, and is particularly excellent in early strength.

The calcium-containing waste powder preferably has a Blaine specific surface area value of 2500 cm ² /g or more and 20000 cm ² /g or less, more preferably 4000 m ² /g or more and 15000 cm ² /g or less, and even more preferably 6000 cm ² /g or more and 10000 cm ² /g or less. The Blaine specific surface area is measured in accordance with the description of JIS R 5201:2015 "Physical Measurement Method of Cement". A calcium-containing waste powder having a Blaine specific surface area value in such a range can be obtained by subjecting the residue after calcium extraction from waste materials such as waste concrete to processing such as crushing, sieving and classification. By having the calcium-containing waste powder have a Blaine specific surface area value in such a range, the cement composition according to the present disclosure has a strength development level equivalent to that of ordinary Portland cement while reducing the production cost, and is particularly excellent in early strength.

In relation to the Blaine specific surface area value of the calcium-containing waste powder, the Blaine specific surface area value of the cement composition is preferably 2500 cm ² /g or more and 6000 cm ² /g or less, more preferably 3000 cm ² /g or more and 5000 cm ² /g or less, and even more preferably 3500 cm ² /g or more and 4500 cm ² /g or less, from the same viewpoint as above. The Blaine specific surface area is measured in accordance with the description of JIS R 5201:2015 "Physical measurement method for cement".

The calcium-containing waste powder has an insoluble residue measured in accordance with JIS R 5202:2010 "Chemical Analysis Method for Cement" of preferably 70.0% or less, more preferably 60.0% or less, and even more preferably 55.0% or less. By setting the insoluble residue of the calcium-containing waste powder to the above value or less, the amount of highly crystalline and low-reactive components such as sand can be reduced, which has the advantage of increasing the strength development of the cement composition when added as an admixture.
In addition, the calcium-containing waste powder has no particular lower limit for the insoluble residue, but it is preferably 1.0% or more, more preferably 5.0% or more, even more preferably 10.0% or more, even more preferably 25.0% or more, and even more preferably 50.0% or more. By setting the insoluble residue of the calcium-containing waste powder to the above value or more, there is an advantage in that the cost of separating highly crystalline and lowly reactive components such as sand from the waste can be reduced.
The insoluble residue of the calcium-containing waste powder can be adjusted to the above value by, for example, crushing, grinding, sieving, and classifying the powder to separate soft components with relatively low strength, such as cement paste, from hard components with high crystallinity, such as sand.

The calcium-containing waste powder preferably contains 5.0% by mass or more of calcium carbonate, more preferably 10.0% by mass or more, more preferably 12.0% by mass or more, even more preferably 15.0% by mass or more, and even more preferably 20.0% by mass or more. By setting the calcium carbonate content in the calcium-containing waste powder to the above value or more, the cement composition according to the present disclosure has a strength expression level equivalent to that of ordinary Portland cement. In particular, it has excellent early strength. In addition, since a sufficient amount of calcium carbonate generated by the fixation of carbon dioxide is contained in the calcium-containing waste, it can contribute to reducing carbon dioxide emissions.
The higher the proportion of calcium carbonate contained in the calcium-containing waste powder, the more desirable it is from the viewpoint of improving the strength development of the cement composition according to the present disclosure. However, if the proportion of calcium carbonate is as high as about 40.0 mass %, the strength development is sufficiently improved.

The amount of calcium carbonate contained in the calcium-containing waste powder is measured, for example, by TG-DTA. In this measurement, the mass loss in the temperature range of 500°C to 800°C is assumed to be the decarbonation of _CO2 from calcium carbonate, and the amount of calcium carbonate can be calculated from the mass loss value.

In the calcium-containing waste powder, the residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is preferably 18.0% by mass or less, more preferably 15.0% by mass or less, even more preferably 12.0% by mass or less, and even more preferably 10.0% by mass or less. The residual amount of CaO means the degree of calcium extraction when obtaining calcium-containing waste powder from waste materials such as waste concrete and waste mortar. Setting the residual amount of CaO to the above value or less means that calcium is sufficiently extracted when obtaining calcium-containing waste powder from the waste materials, so it is advantageous from the viewpoint of effective use of calcium and from the viewpoint of effectively obtaining calcium carbonate by reacting calcium with carbon dioxide. From this viewpoint, there is no particular limit on the lower limit of the residual amount of CaO, but it may be 0% by mass or more, 3.0% by mass or more, 6.0% by mass or more, or 9.0% by mass or more.
The amount of remaining CaO is measured by the following method. First, the total CaO content is calculated from chemical composition analysis using fluorescent X-rays. Next, the amount of calcium carbonate is calculated from the following formula, assuming that the mass loss at 500°C to 800°C is decarbonation from calcium carbonate by TG-DTA measurement.
Amount of calcium carbonate (mass%)=(mass reduction from 500°C to 800°C)(mass%)×100/44
The remaining amount of CaO after subtracting the amount of CaO derived from calcium carbonate is calculated by the following formula.
Residual amount of CaO (mass%)=Total content of CaO (mass%)−(amount of calcium carbonate) (mass%)×56/100

The calcium-containing waste powder has an _R2O content defined by the following formula (1) of preferably 2.5 mass% or less, more preferably 2.0 mass% or less, and even more preferably 1.5 mass% or less. From the viewpoint of suppressing the alkali-silica reaction, it is desirable to reduce the total amount of alkali in cement and concrete, and from that viewpoint, it is desirable to reduce the _R2O content of the calcium-containing waste powder. The _R2O content of the calcium-containing waste powder is determined by chemical composition analysis using fluorescent X-rays.
Amount of R ₂ O [mass %] = Na ₂ O + K ₂ O × 0.658 [mass %] (1)
Na ₂ O and K ₂ O represent the contents [mass %] of Na ₂ O and K ₂ O, respectively.

The calcium-containing waste powder preferably has a basicity of 0.50 or more, more preferably 0.70 or more, and even more preferably 0.90 or more. In general, basicity is an index that indicates the reactivity of ground granulated blast furnace slag, and the higher the basicity, the higher the reactivity and the better the strength development. Similarly, the higher the basicity, the better the strength development when calcium-containing waste powder is added. From this viewpoint, there is no particular upper limit on the basicity, but it may be 3.0 or less, 2.0 or less, 1.5 or less, or 1.0 or less. The basicity of the calcium-containing waste powder is calculated from the component values of chemical component analysis using fluorescent X-rays using the following formula (2).
Basicity = (CaO + Al ₂ O ₃ + MgO) / SiO ₂ (2)
CaO, Al ₂ O ₃ , MgO and SiO ₂ represent the contents [mass %] of CaO, Al ₂ O ₃ , MgO and SiO _2, respectively.

The calcium-containing waste powder is the residue remaining after calcium is extracted from calcium-containing waste materials, and there is no particular restriction on the method of extracting calcium from the waste materials. As a method capable of efficiently and sufficiently extracting calcium from the waste materials, it is preferable to adopt a method in which the carbon dioxide partial pressure is made higher than atmospheric pressure. Specifically, it is preferable to adopt a method comprising the following steps (a) to (c).
(a) preparing a powder of calcium-containing waste material;
(b) A step of setting the partial pressure of carbon dioxide to more than 0.1 MPa and not more than 5.0 MPa, and dispersing or dissolving the powder in water under said partial pressure to prepare a liquid to be extracted.
(c) A step of separating a liquid component and a solid residue from the liquid to be extracted.

The powdering of the waste material in step (a) can be carried out using various conventionally known techniques. The average particle size of the powdered waste material may be set to, for example, 10 μm or more and 5 mm or less, so long as it has a large specific surface area that allows calcium to be efficiently extracted from the waste material. This step not only includes the actual crushing of the waste material, but also includes cases where finely powdered waste material containing calcium is already obtained, such as incineration ash discharged from a thermal power plant.

In step (b), by setting the partial pressure of carbon dioxide in the gas phase in contact with the liquid to be extracted within the above range, the calcium ion concentration extracted into the liquid phase can be increased, and the efficiency of recycling calcium from calcium-containing waste materials can be improved. From this perspective, it is even more preferable that the partial pressure of carbon dioxide is 0.1 MPa or more and 3 MPa or less.

The components other than carbon dioxide in the gas phase can be components generally contained in the atmosphere. That is, examples of components other than carbon dioxide in the gas phase include nitrogen and oxygen. The gas phase is conveniently a gas in which carbon dioxide has been supplied to the atmosphere to increase the partial pressure of carbon dioxide.

From the viewpoint of efficient extraction of calcium, it is preferable to carry out the (b) step by adding the fine waste material to water under the above-mentioned carbon dioxide partial pressure and stirring for a predetermined period of time, for example, from 1 minute to 90 minutes.
From the same viewpoint, the liquid to be extracted is preferably kept at 20°C or higher and 80°C or lower, more preferably 20°C or higher and 50°C or lower, and even more preferably 20°C or higher and 30°C or lower.

In step (c), it is preferable to perform solid-liquid separation while maintaining the partial pressure of carbon dioxide in the gas phase within the above-mentioned range and the temperature of the liquid to be extracted within the above-mentioned range. The solid-liquid separation can be performed using a conventionally known filtration technique. Seed crystals may also be added to the liquid phase.

The calcium-containing waste obtained in this manner before pulverization preferably has an average particle size of 10 μm or more and 1000 μm or less from the viewpoint of improving the strength development and pulverization efficiency of the cement composition according to the present disclosure. From the viewpoint of making this advantage more prominent, the average particle size of the calcium-containing waste is more preferably 300 μm or more and 600 μm or less, and even more preferably 400 μm or more and 500 μm or less.
The average particle size of the calcium-containing waste can be determined by particle size measurement by sieving, etc. As a specific method, for example, the sieving test method for aggregates in JIS A 1102:2014 can be used to measure the average particle size in the same manner as the sieving test for fine aggregates.

The calcium-containing waste obtained by the above-mentioned method may be used as it is, or may be crushed, sieved and classified to obtain the desired average particle size. In the latter case, the average particle size is preferably 2 μm or more and 50 μm or less from the viewpoint of strength development and fluidity retention. To make this advantage more prominent, the average particle size of the calcium-containing waste is more preferably 5 μm or more and 30 μm or less, and even more preferably 10 μm or more and 20 μm or less.

The content of calcium-containing waste powder in the cement composition according to the present disclosure is preferably 7.5% by mass or less, more preferably 6.5% by mass or less, and even more preferably 5.5% by mass or less, based on the cement composition. By making the content of calcium-containing waste powder equal to or less than the above value, the cement composition according to the present disclosure has a strength development at the same level as ordinary Portland cement, and is particularly excellent in early strength.
In addition, the content of calcium-containing waste powder in the cement composition according to the present disclosure is preferably 0.5% by mass or more, more preferably 1.5% by mass or more, even more preferably 3.0% by mass or more, and even more preferably 4.5% by mass or more, based on the cement composition. By making the content of calcium-containing waste powder equal to or more than the above value, waste materials such as waste concrete and waste mortar that have been discarded up to now can be effectively utilized, and the environmental load can be reduced.

The cement composition according to the present disclosure may contain limestone in addition to cement and calcium-containing waste powder. As the limestone, for example, powders containing calcium carbonate as the main component, such as limestone, limestone powder, and kansui stone powder, which are generally available on the market, may be used. The limestone preferably includes limestone that conforms to the minor mixing components described in JIS R 5210:2019 "Portland cement".

The limestone content in the cement composition according to the present disclosure is preferably 15.0 mass% or less, more preferably 10.0 mass% or less, and even more preferably 5.0 mass% or less, based on the cement composition. By setting the limestone content to the above value or less, the cement composition has excellent strength development.
The content of limestone in the cement composition according to the present disclosure is preferably 1.5% by mass or more, more preferably 3.0% by mass or more, and even more preferably 4.6% by mass or more, based on the cement composition. By making the content of limestone equal to or more than the above value, the strength development is improved compared to when limestone is not added, while contributing to the reduction of carbon dioxide emissions.

In addition to cement and calcium-containing waste powder, the cement composition according to the present disclosure may contain other components within the scope of the present disclosure. Examples of other components include calcium hydroxide, silica powder, other powders containing calcium (excluding gypsum and limestone), water reducers for concrete, accelerators, and retarders.

The cement composition according to the present disclosure is preferably obtained by a manufacturing method in which cement and calcium-containing waste powder are mixed so that the calcium-containing waste powder is contained in the cement composition at preferably 1.0% by mass or more and 7.5% by mass or less.

The cement composition according to the present disclosure may be mixed with ground granulated blast furnace slag, fly ash, silica fume, etc., and used as a mixed cement. It may also be used with cements other than JIS, such as pozzolan cement and LC ³ (Lime Calcinated Clay Cement).

The contents of this disclosure will be described in more detail below with reference to examples, comparative examples, and reference examples. However, this disclosure is not limited to the following examples.

[Raw materials for cement composition]
The following materials were used as raw materials for the cement composition.

(1) Preparation of Cement Clinker As the cement clinker, ordinary Portland cement clinker (cement clinker A) and high-early-strength Portland cement clinker (cement clinker B) were used, whose mineral compositions according to the Bogue formula are shown in Table 1 below. The chemical composition of the cement used in the calculation according to the Bogue formula was measured in accordance with the method described in JIS R 5204:2019 "Method for X-ray fluorescence analysis of cement".

(2) Preparation of Gypsum Gypsum satisfying the requirements described in JIS R 9151:2009 “Natural gypsum for cement” was used.

(3) Preparation of Limestone Powder The limestone (LS) used was a fine powder of limestone obtained by pulverizing limestone having a calcium carbonate content of 90.0% by mass or more, an aluminum oxide content of 1.0% by mass or less, and satisfying the requirements for minor mixed components described in JIS R 5210:2019 “Portland cement” (Blaine specific surface area value: 7,470 cm ² /g).

(4) Calcium-Containing Waste Powder Waste concrete was crushed and sieved to obtain a powder having a mesh size of 2 mm or less. The powder was then poured into pure water to obtain a dispersion.
The dispersion was airtightly packed in a pressure-resistant stirring tank type extraction apparatus, and while stirring, carbon dioxide was supplied into the apparatus and the partial pressure was increased to 1.0 MPa to extract calcium into the liquid phase. During this time, the pressure of the gas phase was monitored and the partial pressure of carbon dioxide was maintained.
Thereafter, the apparatus was subjected to solid-liquid separation into a liquid phase and an extraction residue, and the liquid phase to which the seed crystals had been added was depressurized to obtain calcium carbonate. The solid content obtained by the solid-liquid separation was recovered to obtain an extraction residue. The extraction residue was dried at 105° C. to remove moisture, and the intended calcium-containing waste was obtained.
Table 2 shows the particle size of calcium-containing waste before crushing. For reference, the particle sizes of standard sand from the Cement Association and representative silica sand (product name: Toyoura silica sand, manufacturer: Toyoura Silica Industry Co., Ltd.) are also shown. It can be seen that calcium-containing waste has a particle size distribution relatively close to that of standard sand. It can also be seen that calcium-containing waste contains about 5.0 mass% of fine powder of 0.063 mm or less.

Using a stainless steel disk mill, 50 g of calcium-containing waste was ground for 60 seconds to obtain calcium-containing waste powder. The Blaine specific surface area of the calcium-containing waste powder was 8090 cm ² /g. The Blaine specific surface area was measured in accordance with the description of JIS R 5201:2015 "Physical measurement method of cement". The average particle size of the calcium-containing waste powder was 17.0 μm. The average particle size was measured using a laser diffraction scattering type particle size distribution measuring device (Malvern, Mastersizer 3000E).

The physical properties of the calcium-containing waste powder are shown in Tables 3 and 4. The densities shown in these tables were measured using an automatic density meter (instrument: MAT-7000, Seishin Enterprise Co., Ltd.).

The chemical components of the calcium-containing waste powder were determined by chemical analysis using the fundamental parameter method of fluorescent X-rays (equipment used: Rigaku ZSX-100e). Ignition loss (ig. loss) and insoluble residue were measured according to JIS R 5202:2010 "Chemical analysis method for cement". The calcium carbonate content of the calcium-containing waste was calculated by assuming that the loss from 500°C to 800°C in TG-DTA measurement (apparatus name: Hitachi High-Technologies STA-7300, nitrogen gas flow environment, heating rate: 10°C/min) was the loss of carbon dioxide due to decarbonation of calcium carbonate.

[Production of cement composition]
Gypsum was added at 1.0% in terms of _SO3 to Portland cement clinker having the chemical composition shown in Table 1, and mixed and ground to obtain a base cement. The base cement, the calcium-containing waste powder, and limestone powder were mixed in the proportions shown in Table 5 to obtain a cement composition. The Blaine specific surface area value of the obtained cement composition is shown in Table 5.

[Evaluation of cement compositions]
The obtained cement composition was evaluated for its compressive strength at a predetermined age. The compressive strength was measured using a 10 x 10 x 60 mm square column specimen. The water-cement ratio was 0.50, the sand-cement ratio was 1.0, and silica sand (product name: Toyoura silica sand, manufacturer: Toyoura Silica Industry Co., Ltd.) with the particle size shown in Table 2 was used as sand. The mortar was formed by mixing 50 g of water, 100 g of cement, and 100 g of sand in a chemical mixer at 500 rpm for 3 minutes and pouring it into a 10 x 10 x 60 mm formwork. The mixture was sealed and cured for 24 hours, and then removed from the form and cured in water until the predetermined age.
The compressive strengths of Example 1 and Comparative Example 1 were evaluated as a compressive strength ratio when the compressive strength of the specimen of Reference Example 1 was set to 100. The compressive strengths of Example 2 and Comparative Example 2 were evaluated as a compressive strength ratio when the compressive strength of the specimen of Reference Example 2 was set to 100.

As is clear from the results shown in Table 5, the hardened bodies obtained in the respective Examples exhibited compressive strengths as high as that of the hardened bodies of the Reference Examples.
In particular, it can be seen that the hardened bodies obtained in each Example had high compressive strength at 7 days. Also, it can be seen that in the case of Example 2, which used high-early-strength Portland cement as the base cement, the compressive strength at 28 days was higher than in the case of Example 1, which used ordinary Portland cement.

Claims (8)

A cement composition comprising cement, calcium-containing waste powder , and limestone ,
The calcium-containing waste powder is
The insoluble residue measured in accordance with JIS R 5202:2010 is 60.0 % by mass or less,
Contains 5.0% by mass or more of calcium carbonate, and
The residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is 18.0% by mass or less,
The Blaine specific surface area is 4000 cm ² /g or more and 20000 cm ² /g or less,
The content of the limestone is 1.5% by mass or more and 15.0% by mass or less with respect to the cement composition,
The calcium-containing waste powder is contained in an amount of 0.5% by mass or more and 7.5% by mass or less. The cement composition according to claim 1, wherein the calcium-containing waste powder has an R ₂ O content of 2.5 mass% or less, the R 2 O content being represented by the following formula (1).
Amount of R ₂ O [mass %] = Na ₂ O + K ₂ O × 0.658 [mass %] (1) The cement composition according to claim 1, wherein the calcium-containing waste powder is derived from a waste material selected from waste concrete, waste mortar, and waste cement paste. The cement composition according to claim 1, wherein the C ₃ S content of cement clinker in the cement is 50.0 mass % or more and 80.0 mass % or less. 2. The cement composition according to claim 1, having a Blaine specific surface area of 2500 cm ² /g or more and 6000 cm ² /g or less. A method for producing a cement composition comprising cement, calcium-containing waste powder , and limestone , comprising the steps of:
The calcium-containing waste powder is
The insoluble residue measured in accordance with JIS R 5202:2010 is 70.0% by mass or less,
Contains 5.0% by mass or more of calcium carbonate, and
The residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is 18.0% by mass or less,
The Blaine specific surface area is 4000 cm ² /g or more and 20000 cm ² /g or less,
The content of the limestone is 1.5% by mass or more and 15.0% by mass or less with respect to the cement composition,
A method for producing a cement composition, comprising mixing the cement, the calcium-containing waste powder, and the limestone so that the calcium-containing waste powder is contained in the cement composition in an amount of 0.5 mass% or more and 7.5 mass% or less. The method according to claim 6 , wherein the calcium-containing waste powder is derived from a waste material selected from waste concrete, waste mortar and waste cement paste. A method for producing a cement composition comprising cement and a calcium-containing waste powder, comprising the steps of:
The calcium-containing waste powder is
Prepare a powder of waste material containing calcium;
The partial pressure of carbon dioxide is set to more than 0.1 MPa and not more than 5.0 MPa, and the powder is dispersed or dissolved in water under the partial pressure to prepare a liquid to be extracted;
The extract is obtained by separating a liquid component and a solid residue from the liquid to be extracted,
The insoluble residue measured in accordance with JIS R 5202:2010 is 70.0% by mass or less,
Contains 5.0% by mass or more of calcium carbonate, and
The residual amount of CaO, which is obtained by subtracting the amount of CaO derived from calcium carbonate, is 18.0% by mass or less,
A method for producing a cement composition, comprising mixing the cement and the calcium-containing waste powder so that the calcium-containing waste powder is contained in the cement composition in an amount of 0.5 mass % or more and 7.5 mass % or less.