JP7598966B2 - 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 part of efforts to promote low carbonization in the field of cement technology, cement compositions are being used in which part of ordinary Portland cement, which has a large carbon dioxide emission rate during its production, is replaced with an admixture. Limestone is widely used as an admixture to replace ordinary Portland cement because it is easy to obtain and inexpensive. However, the current JIS standard stipulates that the amount of admixture that can be added to ordinary Portland cement is 5% by mass or less.

From the perspective of further preventing global warming and reducing carbon dioxide emissions, it is desirable 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. However, it is necessary to consider the effects on various physical properties caused by increasing the amount of admixture mixed.

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

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

When a huge amount of carbon dioxide is fixed as light calcium carbonate, one possible use for the light calcium carbonate is to add it to cement, which is produced in large quantities. When light calcium carbonate is used as a cement admixture, in addition to the amount of carbon dioxide fixed as light calcium carbonate, it is possible to reduce carbon dioxide emissions due to the reduced amount of cement clinker used.

However, the light calcium carbonate produced by the above-mentioned conventional technology has a problem that when the light calcium carbonate is added to cement and used as a cement composition, the yield stress and viscosity of the cement composition before hardening increase. The increase in yield stress leads to problems such as an increase in the amount of admixture used in the cement composition, and the increase in viscosity leads to problems such as a decrease in pumpability.

The present disclosure therefore aims to provide a cement composition that uses light calcium carbonate as an admixture, thereby enabling a reduction in carbon dioxide emissions compared to the use of limestone (heavy calcium carbonate), and that can reduce the yield stress and viscosity to levels equal to or lower than those achieved when limestone is used, and a method for producing the same.

One aspect of the present disclosure is a cement composition comprising precipitated calcium carbonate, Portland cement clinker, and gypsum,
The light calcium carbonate is
The volume-based cumulative 50% particle size is 10 μm or more and 35 μm or less,
The 1 μm passing fraction is 3.0% or less on a volume basis, and
The 35 μm residue is 5.0% or more and 50.0% or less by volume,
When the cement composition is 100% by mass,
The content of the precipitated calcium carbonate is 1% by mass or more and 20% by mass or less, and
The present invention provides a cement composition in which the total content of the Portland cement clinker and the gypsum is 80 mass % or more and 99 mass % or less.

Another aspect of the present disclosure is a method for producing a cement composition including precipitated calcium carbonate, Portland cement clinker, and gypsum, the method comprising the steps of:
The light calcium carbonate is
The volume-based cumulative 50% particle size is 10 μm or more and 35 μm or less,
The amount passing through 1 μm is 3% or less on a volume basis, and
The 35 μm residue is 5.0% or more and 50.0% or less by volume,
When the cement composition is 100% by mass,
The precipitated calcium carbonate is contained in the cement composition in an amount of 1% by mass or more and 20% by mass or less, and
The object of the present invention is to provide a method for producing a cement composition, which comprises mixing the light calcium carbonate, the Portland cement clinker, and the gypsum so that the Portland cement clinker and the gypsum are contained in the cement composition in a total amount of 80 mass % or more and 99 mass % or less.

According to the present disclosure, by using a specific light calcium carbonate as an admixture, it is possible to contribute to reducing carbon dioxide emissions more than when limestone is used, and it is possible to provide a cement composition with reduced yield stress and viscosity, and a manufacturing method thereof. Furthermore, compared to current ordinary Portland cement, it is possible to provide a cement composition with reduced yield stress and viscosity even when the amount of admixture is increased.

FIG. 1 is a scanning electron microscope image of the precipitated calcium carbonate used in Reference Examples 1 and 2. FIG. 2 is a scanning electron microscope image of the ground calcium carbonate used in Comparative Examples 1 and 2. FIG. 3 is a scanning electron microscope image of the precipitated calcium carbonate used in Examples 1 and 2.

Unless otherwise specified, the raw 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 light calcium carbonate, Portland cement clinker, and gypsum. The cement composition uses light calcium carbonate as a partial substitute for the Portland cement clinker, and is therefore useful as a cement composition that can reduce carbon dioxide emissions compared to the use of limestone as a partial substitute.

As the portland cement clinker contained in the cement composition according to the present disclosure, any portland cement clinker known in the art can be used without any particular restrictions. For example, ordinary portland cement clinker, high-early-strength portland cement clinker, moderate-heat portland cement clinker, low-heat portland cement clinker, oil well portland cement clinker, etc. can be used. From the viewpoint of ease of availability and further improving early age strength, the portland cement clinker preferably contains at least one of ordinary portland cement clinker and high-early-strength portland cement clinker, more preferably contains ordinary portland cement clinker, and only ordinary portland cement clinker may be used.

Portland cement clinker 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.SiO ₂ , denoted by C ₃ S), dicalcium silicate (2CaO.SiO ₂ , denoted by C ₂ S), tricalcium aluminate (3CaO.Al ₂ O ₃ , denoted by C ₃ A), and tetracalcium aluminate ferroferrate (4CaO.Al ₂ O ₃ .Fe ₂ O ₃ , denoted by C ₄ AF) in Portland 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 Portland cement clinker is preferably 30.0 mass% or more, more preferably 40.0 mass% or more, even more preferably 50.0 mass% or more, even more preferably 54.0 mass% or more, and particularly preferably 56.0 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 Portland cement clinker is preferably 70.0 mass% or less, more preferably 66.0 mass% or less, even more preferably 63.0 mass% or less, even more preferably 59.0 mass% or less, and particularly preferably 57.0 mass% or less. By making the amount of _C3S equal to or less than the above value, heat generation during hardening of the cement composition can be suppressed.

The amount of _C2S in the Portland cement clinker is preferably 5.0 mass% or more, more preferably 10.0 mass% or more, even more preferably 12.0 mass% or more, even more preferably 15.0 mass% or more, and particularly preferably 18.0 mass% or more. By making the value of the _C2S amount 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 Portland cement clinker is preferably 50.0 mass% or less, more preferably 40.0 mass% or less, even more preferably 35.0 mass% or less, even more preferably 30.0 mass% or less, and particularly preferably 22.0 mass% or less. By making the value of the amount of _C2S less than the above value, the early strength of the cement composition when hardened can be further improved.

The amount of _C3A in the Portland cement clinker is preferably 7.0 mass% or more, more preferably 8.0 mass% or more, even more preferably 8.5 mass% or more, even more preferably 9.0 mass% or more, and particularly preferably 9.5 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 Portland 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 10.0 mass% or less, and particularly preferably 9.8 mass% or less. By making the value of the amount of _C3A less than the above value, the yield stress and viscosity of the cement composition can be further reduced.

The amount of _C4AF in Portland cement clinker is preferably 7.0% by mass or more, more preferably 8.0% by mass or more, even more preferably 9.0% by mass or more, even more preferably 9.5% by mass or more, and particularly preferably 10.0% by 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 in which the amount of waste and by-products such as coal ash used as a raw material for cement clinker is increased.
The amount of _C4AF in the Portland cement clinker is preferably 14.0 mass% or less, more preferably 13.0 mass% or less, even more preferably 12.0 mass% or less, even more preferably 11.0 mass% or less, and particularly preferably 10.5 mass% or less. By making the value of the amount of _C4AF less than the above value, the yield stress and viscosity of the cement composition can be further reduced.

The Portland 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, when the total amount of the cement composition is 100% by mass.

As the gypsum contained in the cement composition according to the present disclosure, any gypsum known in the art can be used without any particular limitation. For example, gypsum dihydrate, gypsum hemihydrate, anhydrous gypsum, etc. can be used. The gypsum may be used alone or in combination. The content of gypsum in the cement composition may be equivalent to the content of gypsum in general Portland cement. Specifically, the content of gypsum in the cement composition 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 the cement composition being 100% by mass, in terms of SO 3.

In the cement composition according to the present disclosure, the total amount of the Portland cement clinker content and the gypsum content is preferably within a predetermined range. Specifically, when the cement composition is taken as 100 mass%, the total amount of the Portland cement clinker content and the gypsum content is preferably 80 mass% or more, more preferably 85 mass% or more, even more preferably 88 mass% or more, and even more preferably 90 mass% or more. It is preferable that the total amount of the Portland cement clinker content and the gypsum content is 80 mass% or more from the viewpoint of strength development.
When the cement composition is taken as 100 mass%, the total amount of the Portland cement clinker content and the gypsum content is preferably 99 mass% or less, more preferably 97 mass% or less, even more preferably less than 95 mass%, and even more preferably 93 mass% or less. When the total amount of the Portland cement clinker content and the gypsum content is 99 mass% or less, a sufficient amount of light calcium carbonate can be used to reduce carbon dioxide emissions.

The cement composition according to the present disclosure preferably has a light calcium carbonate content, which will be described later, in a predetermined range. Specifically, when the cement composition is taken as 100% by mass, the light calcium carbonate content is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably more than 5% by mass, and even more preferably 7% by mass or more. By having a light calcium carbonate content of 1% by mass or more, it is possible to contribute to a reduction in carbon dioxide emissions compared to the case where limestone is used instead of the light calcium carbonate, and it is also possible to reduce the yield stress and viscosity of the cement composition.
When the cement composition is taken as 100% by mass, the content of the light calcium carbonate is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 12% by mass or less, and even more preferably 10% by mass or less. A light calcium carbonate content of 20% by mass or less is preferable from the viewpoint of strength development.

Calcium carbonate is generally classified into light calcium carbonate and heavy calcium carbonate, and the cement composition according to the present disclosure contains light calcium carbonate. Heavy calcium carbonate is obtained by physically crushing limestone and classifying the resulting crushed material. In contrast, light calcium carbonate is often obtained industrially by chemically synthesizing it by injecting carbon dioxide into an aqueous slurry of calcium hydroxide, and contains mainly calcium carbonate. In principle, the liquid calcium source is not limited to calcium hydroxide, and light calcium carbonate can also be chemically produced by injecting carbon dioxide into an aqueous solution in which calcium is dissolved.
Light calcium carbonate and heavy calcium carbonate have different shapes due to their manufacturing methods. Specifically, the shape of light calcium carbonate is relatively uniform, whereas the shape of heavy calcium carbonate is irregular.

By including light calcium carbonate in the cement composition according to the present disclosure, particularly light calcium carbonate synthesized using carbon dioxide as a raw material, it is possible to contribute to reducing carbon dioxide emissions compared to using limestone instead of the light calcium carbonate, and it is also possible to reduce the yield stress and viscosity of the cement composition.

The light calcium carbonate preferably has a volume-based cumulative 50% particle diameter D ₅₀ (hereinafter also simply referred to as "particle diameter D ₅₀ ") within a predetermined range. Specifically, the particle diameter D ₅₀ of the light calcium carbonate is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, and even more preferably 25 μm or more. When the particle diameter D ₅₀ of the light calcium carbonate is 10 μm or more, the yield stress and viscosity of the cement composition can be reduced.
The particle size _D50 of the light calcium carbonate is preferably 35 μm or less, more preferably 32 μm or less, even more preferably 30 μm or less, and even more preferably 28 μm or less. The particle size _D50 of the light calcium carbonate is preferably 35 μm or less from the viewpoints of the strength expression of the cement and the productivity of the light calcium carbonate.

The particle diameter D ₅₀ of precipitated calcium carbonate can be measured, for example, by the following method. Specifically, first, a sample to be measured is put into a laser diffraction scattering type particle size distribution measuring device (Malvern Instruments, Mastersizer 3000E) whose circulation path is filled with ethanol (refractive index 1.36, 25°C), and the sample is dispersed by an ultrasonic dispersing device built into the device to obtain a dispersion liquid and measure the particle size distribution. From the obtained chart of the volume-based particle size distribution, the volume-based cumulative 50% particle diameter D ₅₀ is obtained. The D ₅₀ thus obtained is defined as the particle diameter D ₅₀ in this specification. In the above-mentioned measuring device, the particle information is set as follows: refractive index: 1.57 (25°C), shape: aspheric, and transmission/non-transmission: transmission.

The light calcium carbonate preferably satisfies a predetermined particle size distribution. The particle size distribution of the light calcium carbonate can be determined, for example, by using a laser diffraction scattering type particle size distribution measuring device and based on the ratio of light calcium carbonate having a particle size less than a predetermined particle size D ₅₀ or a particle size equal to or greater than a predetermined particle size D _50. The particle size of the light calcium carbonate in the present disclosure means a value determined by the laser diffraction scattering type particle size distribution measuring device.

The light calcium carbonate has a volumetric content of 3.0% or less, more preferably 1.5% or less, and even more preferably 0%. By having the volumetric content of the light calcium carbonate passing through 1 μm of 3% or less, the yield stress and viscosity of the cement composition can be reduced.
The "1 μm passing fraction" refers to the volume ratio (%) of precipitated calcium carbonate having a particle diameter _D50 of less than 1 μm to the total volume of precipitated calcium carbonate.

The 1 μm passing portion can be calculated based on the following formula (1).
(Volume of light calcium carbonate having a particle diameter _D50 of less than 1 μm) / (Total volume of light calcium carbonate) × 100 (1)

The light calcium carbonate has a 35 μm residue of preferably 5.0% or more, more preferably 8.0% or more, even more preferably 9.0% or more, and even more preferably 10.0% or more, on a volume basis. When the light calcium carbonate has a 35 μm residue of 5.0% or more on a volume basis, the yield stress and viscosity of the cement composition can be reduced.
The light calcium carbonate has a 35 μm residue of preferably 50.0% or less, more preferably 40.0% or less, even more preferably 25.0% or less, and even more preferably 15.0% or less, on a volume basis. It is preferable that the light calcium carbonate has a 35 μm residue of 50.0% or less, on a volume basis, from the viewpoints of strength expression of cement and productivity of the light calcium carbonate.
The "35 μm residue" refers to the volume ratio (%) of precipitated calcium carbonate having a particle diameter _D50 of 35 μm or more to the total volume of precipitated calcium carbonate.

The 35 μm residue can be calculated based on the following formula (2).
(Volume of light calcium carbonate having a particle diameter _D50 of 35 μm or more) / (Total volume of light calcium carbonate) × 100 (2)

In addition to the 1 μm passing fraction and 35 μm remaining fraction being within the above-mentioned ranges, the light calcium carbonate also preferably has a predetermined ratio of the 15 μm passing fraction. Specifically, the light calcium carbonate has a 15 μm passing fraction of preferably 60% or less, more preferably 40% or less, and even more preferably 20% or less, on a volume basis. By having the 15 μm passing fraction of the light calcium carbonate of 60% or less on a volume basis, the yield stress and viscosity of the cement composition can be reduced.
The light calcium carbonate has a volumetric content of 5% or more, more preferably 7% or more, and even more preferably 10% or more. When the volumetric content of the light calcium carbonate is 5% or more, the strength of the cement composition is increased and the productivity of the light calcium carbonate is improved.
The "15 μm passing fraction" refers to the volume ratio (%) of precipitated calcium carbonate having a particle diameter _D50 of less than 15 μm to the total volume of precipitated calcium carbonate.

The 15 μm passing portion can be calculated based on the following formula (3).
(Volume of light calcium carbonate having a particle diameter _D50 of less than 15 μm) / (Total volume of light calcium carbonate) × 100 (3)

The particle size distribution of light calcium carbonate is a value calculated, for example, based on the Rosin-Rammler formula, and can also be determined by the n value, which indicates the uniformity of particle size. The Rosin-Rammler formula is an equation that represents the cumulative distribution of light calcium carbonate on a sieve, and is shown by the following equation (4).

In formula (4), b and n are constants. x is the representative particle diameter _D50 of precipitated calcium carbonate. The larger the n value calculated based on formula (4), the narrower the particle size distribution and the more uniform the particle sizes. Although the Rosin-Rammler equation is usually considered to be an equation based on mass, in this specification, for the purpose of comparing particles of the same quality, the Rosin-Rammler equation is also considered to be valid based on volume, and the n value is calculated using the integrated value based on volume for each particle diameter _D50 .

The n value of the Rosin-Rammler equation in the particle size distribution of the light calcium carbonate is preferably 2.2 or more, more preferably 2.8 or more, and further preferably 3.0 or more. The n value of 2.2 or more is preferable because the light calcium carbonate contains fewer fine particles, which can reduce the yield stress and viscosity of the cement composition.
The n value of the Rosin-Rammler equation in the particle size distribution of the precipitated calcium carbonate is preferably 4.0 or less, more preferably 3.8 or less, and further preferably 3.5 or less. The n value of 4.0 or less is preferable from the viewpoint of productivity of the precipitated calcium carbonate.

In order to set the particle size distribution of the light calcium carbonate within the above-mentioned range, the light calcium carbonate can be subjected to grinding, classification, sieving, etc., within the scope of the present disclosure. Alternatively, the light calcium carbonate may be adjusted by mixing multiple types of light calcium carbonate having different particle size distributions. Alternatively, the light calcium carbonate may be obtained by the manufacturing method described below.

As described above, the light calcium carbonate mainly contains calcium carbonate, but may contain various components in addition to calcium carbonate, as long as the effects of the present disclosure are not impaired. Examples of the various components include calcium hydroxide, quicklime, sand derived from raw materials, hardened cement paste, and the like. The various components may be contained alone or in combination. The various components may be contained as necessary based on the properties required for the cement composition, and the amount of each component contained in the cement composition is preferably 0.1% by mass or more and 40% by mass or less, more preferably 0.5% by mass or more and 20% by mass or less, even more preferably 1% by mass or more and 10% by mass or less, and particularly preferably 2% by mass or more and 5% by mass or less.
The content of calcium carbonate in the precipitated calcium carbonate is preferably 60.0 mass% or more, more preferably 80.0 mass% or more, even more preferably 90.0 mass% or more, particularly preferably 95.0 mass% or more, and even more preferably 97.0 mass% or more.

The content of calcium carbonate in precipitated calcium carbonate is measured, for example, by TG-DTA. In this measurement, the weight 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 based on the weight loss value. The amount of calcium carbonate is calculated by the following formula (5).
Amount of calcium carbonate (mass%) = (weight loss from 500 ° C to 800 ° C) (mass%) × 100/44 (5)

When the light calcium carbonate contains quicklime, the content of the quicklime in the light calcium carbonate is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, even more preferably 1.0 mass% or less, and even more preferably 0.1 mass% or less. The content of quicklime in the light calcium carbonate of 5.0 mass% or less is preferable from the viewpoint of controlling fluidity and setting in the cement composition. For this reason, the lower the content of quicklime in the light calcium carbonate, the more preferable it is, but if the content is as low as about 0.01 mass%, the above-mentioned effect is sufficiently exhibited.
The content of quicklime in the precipitated calcium carbonate can be quantified, for example, by Rietveld analysis of an X-ray diffraction (XRD) pattern.

When the light calcium carbonate contains calcium hydroxide, the content of the calcium hydroxide in the light calcium carbonate is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, and further preferably 1.7 mass% or less. The content of calcium hydroxide in the light calcium carbonate being 5.0 mass% or less is preferable from the viewpoint of increasing the amount of carbon dioxide fixed in the light calcium carbonate.
When the light calcium carbonate contains calcium hydroxide, the content of the calcium hydroxide in the light calcium carbonate is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and further preferably 1.0 mass% or more. It is preferable that the content of calcium hydroxide in the light calcium carbonate is 0.1 mass% or more from the viewpoint of productivity of the light calcium carbonate.

Light calcium carbonate is obtained by chemical synthesis by reacting slaked lime with carbon dioxide, and there is no particular restriction on the synthesis method. For example, light calcium carbonate may be obtained by reacting calcium hydroxide with carbon dioxide in a liquid phase, as in the conventional technology. As a method for efficiently and sufficiently synthesizing light calcium carbonate having the above particle size distribution, it is preferable to adopt a method including the following steps (a)-(e). The reason is that in the conventional technology, light calcium carbonate is precipitated by reacting calcium hydroxide, which is a calcium source with low solubility, with carbon dioxide, so that the particle diameter D ₅₀ of the light calcium carbonate becomes relatively small, and the yield stress and viscosity of the cement composition increase. In contrast, according to the following method, light calcium carbonate with a large particle diameter D ₅₀ , a small amount of 1 μm passing, and a large amount of 35 μm residue can be obtained, which is preferable because it can reduce the yield stress and viscosity of the cement composition. In addition, light calcium carbonate with a small amount of 15 μm passing can be obtained, so that the yield stress and viscosity of the cement composition can be further reduced.
(a) providing a calcium containing waste or by-product;
(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 waste or by-product in water under said partial pressure to prepare a liquid to be extracted.
(c) A step of recovering liquid components from the liquid to be extracted.
(d) mixing the liquid component with calcium carbonate seed crystals.
(e) A step of setting a partial pressure of carbon dioxide to 36 Pa or more and 1.0 MPa or less, and precipitating precipitated calcium carbonate from the liquid component under the partial pressure condition.

The waste or by-products prepared in step (a) may contain calcium, and there is no particular restriction on the type. The waste or by-products referred to here are waste or by-products generated secondarily, for example, during the manufacture of cement. Examples of such waste or by-products include waste or by-products containing natural raw materials such as limestone, quicklime, and calcium hydroxide. Alternatively, examples include waste or by-products that contain a large amount of calcium, such as waste concrete, waste mortar, waste cement paste, ready-mix concrete sludge, paper sludge, blast furnace slag, electric furnace slag, slowly cooled slag, converter slag, chlorine bypass dust, coal ash, biomass ash, and incineration ash. From the viewpoint of reducing the burden on the global environment, it is preferable to use the latter waste or by-products, and it is particularly preferable to use waste concrete, waste mortar, or waste cement paste.

In step (a), the waste or by-product may be powdered as necessary. Powdering can be performed using various conventionally known techniques. The particle size _D50 of the powdered waste or by-product may be set to a value that has a specific surface area large enough to efficiently extract calcium from the waste or by-product, for example, 10 μm or more and 5 mm or less.

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 or by-products can be improved. From this perspective, it is even more preferable that the partial pressure of carbon dioxide is more than 0.1 MPa and not more than 3 MPa.

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 waste or by-product 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 a solid-liquid separation operation 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 to recover the liquid component. The solid-liquid separation can be performed using a conventionally known filtration technique. The recovered liquid component contains calcium derived from waste or by-products.

In the step (d), from the viewpoint of efficiently and sufficiently precipitating light calcium carbonate from calcium in the liquid component, it is preferable to mix 0.5 g of calcium carbonate seed crystals with respect to 100 mL of the liquid component, more preferably 2 g or more, and further preferably 5 g or more.
From the same viewpoint, the purity of the calcium carbonate seed crystals is preferably 98% by mass or more, more preferably 99.5% by mass, and further preferably 99.9% by mass.
From the same viewpoint, the particle size _D50 of the calcium carbonate seed crystals is preferably 0.1 μm or more and 50 μm or less, more preferably 0.1 μm or more and 10 μm or less, and further preferably 0.1 μm or more and 5 μm or less.
From the same viewpoint, the specific surface area of the calcium carbonate seed crystals is preferably 6×10 ⁴ m ² /m ³ or more and 3×10 ¹² m ² /m ³ or less, more preferably 3×10 ⁵ m ² /m ³ or more and 3×10 ¹² m ² /m ³ or less, and even more preferably 6×10 ⁵ m ² /m ³ or more and 3×10 ¹² m ² /m ³ or less.

In the step (e), by setting the partial pressure of carbon dioxide in the gas phase in contact with the mixed solution of the liquid component and the calcium carbonate seed crystals within the above range, light calcium carbonate can be efficiently and sufficiently precipitated from calcium in the liquid component. From this viewpoint, the partial pressure of carbon dioxide is more preferably 36 Pa or more and 0.3 MPa or less, and further preferably 36 Pa or more and 0.2 MPa or less.
The components in the gas phase other than carbon dioxide may be components generally contained in the atmosphere, similar to step (b).

The step (e) is preferably performed by stirring the mixed liquid under the above-mentioned carbon dioxide partial pressure for a predetermined time, for example, from 1 minute to 90 minutes, from the viewpoint of efficient precipitation of precipitated calcium carbonate.
In addition, in the step (e), it is preferable to keep the mixed liquid at a temperature of preferably 20° C. or more and 80° C. or less, more preferably 20° C. or more and 50° C. or less, under the above-mentioned carbon dioxide partial pressure, from the viewpoint of quickly precipitating precipitated calcium carbonate.

In this way, light calcium carbonate having the above-mentioned particle size distribution can be obtained. Thereafter, if necessary, it is preferable to perform a solid-liquid separation operation on the mixed liquid to recover the light calcium carbonate. The solid-liquid separation can be performed using a conventionally known filtration technique. The recovered light calcium carbonate may be subjected to a drying treatment. After the drying treatment, if necessary, a classification treatment may be performed.

In addition to the Portland cement clinker, gypsum, and light calcium carbonate described above, the cement composition may contain other components within the scope of the present disclosure. Examples of other components include heavy calcium carbonate, calcium hydroxide (excluding calcium hydroxide contained in light calcium carbonate), ground granulated blast furnace slag, fly ash, silica powder, other calcium-containing powders (excluding gypsum, limestone, and allophane), water reducers for concrete, accelerators, and retarders. The other components may be contained alone or in combination. The other components may be contained as necessary based on the properties required for the cement composition, and the amount of each component contained in the cement composition may be preferably 0.1% by mass or more and 20% by mass or less.

As the heavy calcium carbonate, any heavy calcium carbonate known in the art can be used without any particular restrictions. For example, powders containing calcium carbonate as the main component, such as limestone, limestone powder, and limestone powder, which are generally available on the market, can be used. The limestone preferably includes limestone that conforms to the minor mixing components described in JIS R 5210:2019 "Portland cement".

Heavy calcium carbonate may be mixed with light calcium carbonate and used in the cement composition. When the total amount of the cement composition is taken as 100 mass%, the total amount of the light calcium carbonate content and the light calcium carbonate content is, for example, preferably 1 mass% or more, more preferably 2 mass% or more, even more preferably more than 5 mass%, and even more preferably 7.5 mass% or more. It is preferable that the total amount of the light calcium carbonate content and the light calcium carbonate content is 1 mass% or more from the viewpoints of reducing carbon dioxide emissions and developing strength of the cement composition.
When the total amount of the cement composition is taken as 100 mass%, the total amount of the light calcium carbonate content and the heavy calcium carbonate content is, for example, preferably 20 mass% or less, more preferably 15 mass% or less, even more preferably 12 mass% or less, and even more preferably 10 mass% or less. A total amount of the light calcium carbonate content and the heavy calcium carbonate content of 20 mass% or less is preferable from the viewpoint of strength development of the cement composition.

When the total amount of the light calcium carbonate content and the heavy calcium carbonate content is taken as 100 mass%, the light calcium carbonate content is preferably 10 mass% or more, more preferably 20 mass% or more, even more preferably 30 mass% or more, and still more preferably 40 mass% or more. A light calcium carbonate content of 10 mass% or more is preferable from the viewpoints of reducing carbon dioxide emissions and strength development of the cement composition.
When the total amount of the light calcium carbonate content and the heavy calcium carbonate content is taken as 100 mass%, the light calcium carbonate content is preferably 90 mass% or less, more preferably 80 mass% or less, even more preferably 70 mass% or less, and even more preferably 60 mass% or less. A light calcium carbonate content of 90 mass% or less is preferable from the viewpoint of strength development of the cement composition.

The cement composition preferably has an n value calculated based on the Rosin-Rammler formula within a predetermined range. Specifically, the n value of the Rosin-Rammler formula in the particle size distribution of the cement composition is preferably 1.14 or more, more preferably 1.15 or more, and even more preferably 1.17 or more. The n value of 1.14 or more is preferable because it makes the cement composition easier to manufacture.
The n value of the Rosin-Rammler equation in the particle size distribution of the cement composition is preferably 1.50 or less, more preferably 1.30 or less, and even more preferably 1.20 or less. The n value of 1.50 or less is preferable from the viewpoint of reducing the yield stress and viscosity of the cement composition.
The n value of the Rosin-Rammler equation in the particle size distribution of the cement composition is calculated based on the above formula (4). In the formula (4), b and n are constants, and x is the representative particle diameter _D50 of the cement composition.

In order to bring the particle size distribution of the cement composition into the above-mentioned range, it can be adjusted by crushing, classifying, sieving, etc., of the raw materials in the composition, within the scope of the present disclosure. Alternatively, it may be adjusted by mixing multiple types of raw materials with different particle size distributions.

The cement composition preferably has a Blaine specific surface area within a predetermined range. Specifically, the cement composition preferably has a Blaine specific surface area of 2000 cm ² /g or more, more preferably 2500 cm ² /g or more, and even more preferably 2800 cm ² /g or more. From the viewpoint of strength development of the cement composition, it is preferable that the cement composition has a Blaine specific surface area of 2000 cm ² /g or more.
The Blaine specific surface area of the cement composition is preferably 4000 ^cm2 /g or less, more preferably 3800 ^cm2 /g or less, even more preferably 3500 ^cm2 /g or less, still more preferably 3200 ^cm2 /g or less, and most preferably 2950 ^cm2 /g or less. A Blaine specific surface area of 4000 ^cm2 /g or less is preferred from the viewpoint of reducing the production cost of the cement composition.
The Blaine specific surface area can be measured, for example, in accordance with the description of JIS R5201:2015 "Physical measurement method of cement".

In order to set the Blaine specific surface area of the cement composition within the above-mentioned range, it can be adjusted by crushing, classifying, sieving, etc., within the scope of the present disclosure. Alternatively, it can be adjusted by mixing multiple cement compositions.

By having the above-mentioned configuration, the cement composition according to the present disclosure can exhibit an apparent viscosity at 0.1 s ^-1 at 20°C of preferably 100 Pa·s or less. The apparent viscosity at 0.1 s ^{-1 of the cement composition at 20°C is more preferably 80 Pa·s or less, even more preferably 75 Pa·s or less, and even more preferably 70 Pa·s or less. The apparent viscosity at 0.1 s-1 at} 20°C is an index of the yield stress of the cement before hardening, and the lower this value, the less the amount of admixture can be added to the cement composition to achieve a predetermined slump.
The method for measuring the apparent viscosity at 20° C. and 0.1 s ⁻¹ will be explained in the examples below.

By having the above-mentioned configuration of the cement composition according to the present disclosure, the cement composition can preferably exhibit an apparent viscosity at 100 s ^{- 1} at 20°C of 0.55 Pa·s or less. The apparent viscosity of the cement composition at 100 s-1 at 20°C is more preferably 0.53 Pa·s or less, even more preferably 0.50 Pa·s or less, and even more preferably 0.45 Pa·s or less. The apparent viscosity at 0.1 s ^-1 at 20°C is an index of the viscosity of the cement before hardening, and the lower this value, the smaller the resistance during high-speed flow can be, and the better the pumpability.
The method for measuring the apparent viscosity at 20° C. and 100 s ⁻¹ will be explained in the Examples below.

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 those specified by JIS, such as pozzolan cement and LC ³ (Lime Calcinated Clay Cement).

The cement composition according to the present disclosure can be produced by mixing the above-mentioned Portland cement clinker, gypsum, and light calcium carbonate. The light calcium carbonate may be mixed in powder form or in slurry form. The cement composition may also be produced by separately grinding the Portland cement clinker, gypsum, and light calcium carbonate and then mixing them. Alternatively, the cement composition may be produced by simultaneously grinding two or more raw materials among the Portland cement clinker, gypsum, and light calcium carbonate and then mixing them. Alternatively, the cement composition may be produced by mixing light calcium carbonate and Portland cement in the cement finishing process. If necessary, other components may be mixed in the cement composition in addition to the Portland cement clinker, gypsum, and light calcium carbonate. In this case, the cement composition may be produced by feeding the light calcium carbonate powder or slurry into a finishing mill together with the Portland cement clinker, gypsum, and other components such as limestone, blast furnace slag, and fly ash, and mixing and grinding them. Alternatively, two or more of the above-mentioned raw materials may be mixed and ground, and then the remaining raw materials may be mixed to produce the product. One or more of the raw materials may be mixed and ground with the material, and then mixed to produce the product.

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 Portland cement clinker As the Portland cement clinker, ordinary Portland cement clinker was used, whose mineral composition according to the Bogue formula is 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 fluorescent X-ray analysis of cement."

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

(3) Preparation of Calcium Carbonate As calcium carbonate, the following calcium carbonate A, calcium carbonate B, and calcium carbonate C were used.

(3-1) Calcium carbonate A
Heavy calcium carbonate produced by pulverizing limestone was used as calcium carbonate A. Calcium carbonate A was a limestone fine powder having a calcium carbonate content of 90% by mass or more and 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".

(3-2) Calcium carbonate B
As calcium carbonate B, commercially available light calcium carbonate was used.

(3-3) Calcium carbonate C
As calcium carbonate C, precipitated calcium carbonate produced by the method including the above-mentioned steps (a) to (e) was used.
Specifically, waste concrete was crushed and sieved to obtain powder with mesh sizes of 2 mm or less. This powder was then added to pure water to obtain a dispersion.
The dispersion was airtightly packed in a pressure-resistant stirring tank type extraction device, and carbon dioxide was supplied into the device while stirring, 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 liquid phase and the extraction residue were separated from the apparatus by solid-liquid separation to recover the liquid component. Calcium carbonate seed crystals were added to the liquid component, and the partial pressure of carbon dioxide in the gas phase in contact with the liquid component was reduced to obtain calcium carbonate. The calcium carbonate was dried at 105° C. to remove moisture, thereby producing calcium carbonate C.

The calcium carbonate used was observed under a scanning electron microscope (SEM) at a magnification of 500. SEM images are shown in Figures 1 to 3.
The physical properties of the calcium carbonate used are shown in Table 2 below.
The calcium carbonate content was calculated based on formula (5). The calcium hydroxide content was calculated by assuming that the weight loss at 400°C to 500°C in the TG-DTA measurement (in a nitrogen gas flow environment) was the weight loss due to the dehydration of calcium hydroxide.
The particle size _D50 of calcium carbonate was measured according to the method described above.
The 1 μm passing fraction of calcium carbonate was calculated based on formula (1). The 35 μm remaining fraction of calcium carbonate was calculated based on formula (2). The 15 μm passing fraction of calcium carbonate was calculated based on formula (3).
The n value of the Rosin-Rammler equation for the particle size distribution of calcium carbonate was calculated based on formula (4).
The content of quicklime in calcium carbonate was measured according to the above-mentioned method. In the obtained XRD peak profile, no quicklime peak was observed in any of calcium carbonates A, B, and C, confirming that no quicklime was contained.

[Production of cement composition]
Base cement was obtained by grinding the Portland cement clinker and gypsum shown in Table 1. The base cement and calcium carbonate were mixed in the proportions shown in Table 3 to obtain cement compositions.
The n value of the Rosin-Rammler equation and the Blaine specific surface area value of the particle size distribution of the obtained cement composition are shown in Table 5. The n value was calculated based on the formula (4). The Blaine specific surface area was measured in accordance with the description of JIS R 5201:2015 "Physical measurement method of cement".

[Evaluation of cement compositions]
The apparent viscosity of the obtained cement composition was evaluated. Specifically, 50 g of water and 100 g of silica sand (product name: Toyoura silica sand, manufacturer: Toyoura Silica Industry Co., Ltd., particle size: shown in Table 3) were added to 100 g of cement composition and mixed to prepare a cement paste having fluidity. Mixing was performed in a thermostatic chamber at 20°C, after measuring water into a stirring vessel, the cement composition and silica sand were added, and the mixture was mixed at 500 rpm for 3 minutes with a chemical mixer (manufactured by Shinto Scientific Co., Ltd., model number: BL1200).
The apparent viscosity was measured using a coaxial double cylinder rheometer (Anton Paar, product name: MCR101, fixture: CC27/T200/SS and CC27/P6) in a thermostatic room at 20 ° C., with the shear rate gradually changed from 10 ^-5 to 10 ³ s ^-1 . More specifically, the change in the shear rate was divided into 36 points on a logarithmic axis, and at each of the 36 points, the apparent viscosity was measured by applying shear at the shear rate at that point for 5 seconds each, and the viscosity was increased from 10 ^-5 to 10 ³ s ^-1 for a total of 180 seconds (rising curve). Thereafter, the shear rate was similarly lowered in stages (falling curve), and the apparent viscosity at that time was measured. The temperature inside the measurement container was kept at 20 ° C. during the measurement of fluidity. The apparent viscosity after applying shear at 0.1 s ^-1 for 5 seconds on the descending curve (apparent viscosity at 0.1 s ^-1 ) was used as an index of yield stress, and the apparent viscosity after applying shear at 100 s ^-1 for 5 seconds on the descending curve (apparent viscosity at 100 s ^-1 ) was used as an index of viscosity, and in both cases the lower the apparent viscosity, the better the evaluation.
The yield stress is the stress necessary for flow, and affects the concrete slump, etc. The lower the yield stress, the less the amount of admixture that can be added to the cement composition to achieve a specified slump. Also, the smaller the viscosity (plastic viscosity), the smaller the resistance during high-speed flow, improving pumpability.

As shown in Table 4, it was found that the cement composition using the light calcium carbonate (calcium carbonate B) produced by the conventional method had a higher viscosity at both the 5% and 10% addition amounts, compared to Reference Example 1 (cement containing 5% added limestone fine powder) which is equivalent to ordinary Portland cement.
On the other hand, when compared with the cement using calcium carbonate A at the same addition amount, the cement using calcium carbonate C had a lower apparent viscosity at 0.1 s ^-1 and the apparent viscosity at 100 s ^-1 was the same or lower, indicating that the cement composition had good yield stress and viscosity.
Furthermore, in the cements using calcium carbonate A or B in Comparative Examples 1 and 2 and Reference Examples 1 and 2, when the amount of calcium carbonate A or B added was increased from 5% to 10%, both the yield stress and the apparent viscosity corresponding to viscosity increased. In contrast, in the cements using calcium carbonate C in Examples 1 and 2, increasing the amount of calcium carbonate C added from 5% to 10% rather improved the yield stress and viscosity.
It has generally been thought that the viscosity of cement before hardening can be reduced by broadening the particle size distribution and lowering the n value of the Rosin-Rammler equation in the particle size distribution of the entire cement composition. However, in the present disclosure, a higher n value of the entire cement composition showed better viscosity. Therefore, it was suggested that the effect of adding light calcium carbonate on the viscosity of cement before hardening may be due not only to its effect as a filler, but also to the effect of the reactivity of the cement.
As can be seen from the results shown in Figures 1 to 3, the light calcium carbonate (calcium carbonate C) produced by the method comprising the above-mentioned steps (a) to (e) had a larger particle diameter _D50 than the light calcium carbonate (calcium carbonate A) obtained by the conventional technology. Furthermore, calcium carbonate A contained many spindle-shaped particles with a particle diameter _D50 of 1 µm or less, while heavy calcium carbonate (calcium carbonate B) had an angular shape with long sides. In contrast, calcium carbonate C had a relatively uniform shape that was close to a cube with short long sides. It is presumed that the fluidity also influenced these differences in shape.

Claims (11)

A cement composition comprising light calcium carbonate, Portland cement clinker, and gypsum,
The light calcium carbonate is
The volume-based cumulative 50% particle size is 10 μm or more and 35 μm or less,
The 1 μm passing fraction is 3.0% or less on a volume basis, and
The 35 μm residue is 5.0% or more and 50.0% or less by volume,
When the cement composition is 100% by mass,
The content of the precipitated calcium carbonate is 1% by mass or more and 20% by mass or less, and
The cement composition, wherein the total amount of the Portland cement clinker content and the gypsum content is 80 mass% or more and 99 mass% or less. The cement composition according to claim 1, wherein the light calcium carbonate has a volumetric content of 5.0% to 60.0% passing through a 15 μm filter. The cement composition according to claim 1, wherein the n value of the Rosin-Rammler formula in the particle size distribution of the light calcium carbonate is 2.2 or more and 4.0 or less. The cement composition according to claim 1, wherein the calcium hydroxide content in the light calcium carbonate is 5% by mass or less. It also contains heavy calcium carbonate,
2. The cement composition according to claim 1, wherein the total content of the light calcium carbonate and the heavy calcium carbonate is 1% by mass or more and 20% by mass or less, when the cement composition is taken as 100% by mass. The cement composition according to claim 1, wherein the n value of the Rosin-Rammler equation in the particle size distribution of the cement composition is 1.14 or more and 1.50 or less. The cement composition according to claim 1, wherein the cement composition has a Blaine specific surface area value of 2000 cm ² /g or more and 4000 cm ² /g or less. A method for producing a cement composition containing light calcium carbonate, Portland cement clinker, and gypsum, comprising the steps of:
The light calcium carbonate is
The volume-based cumulative 50% particle size is 10 μm or more and 35 μm or less,
The 1 μm passing fraction is 3.0% or less on a volume basis, and
The 35 μm residue is 5.0% or more and 50.0% or less by volume,
When the cement composition is 100% by mass,
The precipitated calcium carbonate is contained in the cement composition in an amount of 1% by mass or more and 20% by mass or less, and
A method for producing a cement composition, comprising mixing the light calcium carbonate, the Portland cement clinker, and the gypsum so that the Portland cement clinker and the gypsum are contained in the cement composition in a total amount of 80 mass% or more and 99 mass% or less. The manufacturing method according to claim 8, wherein the light calcium carbonate has a volumetric content of 5.0% to 60.0% passing through a 15 μm filter. Providing a waste or by-product containing calcium;
A partial pressure of carbon dioxide is set to more than 0.1 MPa and not more than 5.0 MPa, and the waste or by-product is dispersed or dissolved in water under the partial pressure to prepare a liquid to be extracted;
Recovering liquid components from the liquid to be extracted;
Mixing the liquid component with calcium carbonate seed crystals;
The method according to claim 8 , wherein a partial pressure of carbon dioxide is set to 36 Pa or more and 1.0 MPa or less, and the precipitated calcium carbonate is precipitated from the liquid component under the partial pressure state. The method for producing a cement composition according to claim 10, wherein the waste material and the by-product are waste concrete, waste mortar, or waste cement paste.