JP7779766B2 - Concrete composition and method for increasing compressive strength of concrete composition - Google Patents

Description

The present disclosure relates to a concrete composition and a method for increasing the compressive strength of a concrete composition.

In recent years, increasing demands for measures to combat global warming have led to calls for a reduction in _CO2 emissions during cement production. To reduce _CO2 emissions, cement production methods that replace part of the cement clinker, which generates a large amount of _CO2 during preparation, with admixtures have been widely studied. Among admixtures, steel slag, such as granulated blast furnace slag (BFS), is expected to improve the long-term strength of concrete and its salt-shielding effect. Therefore, research is underway into cement that uses steel slag as an admixture and increases its mixing ratio.

However, blast furnace cement containing a higher proportion of granulated blast furnace slag tends to have lower initial strength when hardening compared to cement containing only ordinary Portland cement (OPC). Therefore, various methods are being investigated to achieve the same level of strength as cement containing only OPC, even in the case of blast furnace cement.

For example, a method of further adding calcium hydroxide fine powder (e.g., Non-Patent Document 1) and a method of producing blast-furnace cement using Portland cement clinker with a high C ₃ S content (e.g., Non-Patent Document 2) have been reported. In addition, the use of inorganic or organic accelerators other than calcium hydroxide fine powder has also been investigated as a measure to improve the early strength of cement compositions.

Furthermore, in blast furnace cement containing a high proportion of granulated blast furnace slag, etc., gypsum is incorporated to improve early strength (see, for example, Non-Patent Document 3). Aiming to achieve strength equivalent to that achieved when OPC is used alone, the amount of gypsum is increased, and further studies are being conducted to improve early strength by incorporating accelerators that accelerate the hardening reaction of cement clinker (see, for example, Patent Document 1). Patent Document 2 also discloses that in low-carbon concrete without accelerators, blast furnace slag powder with added gypsum has superior quality stability.

It has been confirmed that the hydration reaction of ground granulated blast furnace slag in blast furnace cement mixed with granulated blast furnace slag is activated by the use of nitrite. Patent Document 3 discloses a cementitious hydraulic composition in which part of the cement is replaced with ground granulated blast furnace slag, which has latent hydraulic properties as a cement substitute, and in which nitrite is added to activate the hydration reaction of the ground granulated blast furnace slag.

JP 2014-125371 A JP 2016-141614 A Japanese Patent Application Laid-Open No. 2018-076203

Hiroaki Yoshiga et al., "Effect of Fine Calcium Hydroxide Powder on Cement Containing Blast Furnace Slag," Cement & Concrete Journal, Vol. 67, 2013, pp. 151-156 Atsushi Yatagai et al., "Effect of specific surface area of ground granulated blast furnace slag on the properties of blast furnace cement using high C3S clinker," Cement & Concrete Journal, Vol. 67, 2013, pp. 296-303 Etsuro Sakai et al., "Effect of calcium nitrite and alkanolamines on the hydration of high-blast furnace slag cement," Cement & Concrete Journal, Vol. 73, 2019, pp. 52-57

In concrete compositions containing water, admixtures, and aggregates mixed with a hydraulic composition, a commonly known method for improving compressive strength is to reduce the amount of water added (reducing the water/cement ratio). However, reducing the amount of water added reduces the interparticle distance of the alkali activator in the concrete composition, making it difficult to ensure fluidity and potentially worsening the workability of the concrete. For this reason, from the perspective of balancing compressive strength and fluidity, there is a limit to how much water can be reduced with the sole purpose of increasing compressive strength. It would be useful to have a method for improving compressive strength other than adjusting the amount of water added.

Furthermore, in blast furnace cement containing a high proportion of granulated blast furnace slag, etc., even when gypsum and an accelerator are used in combination as described above, the expected improvement in compressive strength may not be achieved by adding an accelerator. For example, when blast furnace cement prepared with an accelerator is hardened, an improvement in initial strength is observed compared to when no accelerator is added, but as the material ages, the strength may not be as strong as expected.

The present disclosure aims to provide a concrete composition containing a relatively high proportion of granulated blast furnace slag, which has sufficient fluidity for concrete construction and is capable of exhibiting excellent compressive strength both at an early stage and over the long term. The present disclosure also aims to provide a method for increasing the compressive strength of a concrete composition containing a relatively high proportion of granulated blast furnace slag, both at an early stage and over the long term.

The inventors investigated the above-mentioned issues and found that while increasing the amount of gypsum and adding an accelerator are both believed to be beneficial for improving early strength, when both are used in combination, gypsum can inhibit the action of the accelerator, and this inhibiting effect can affect accelerators in general, and can have a strong effect on alkaline earth metal nitrites, which are intended to accelerate the hardening of granulated blast furnace slag.

A more detailed explanation will be given. Gypsum in the hydraulic composition is a component for controlling the reactivity of the alkaline activator and adjusting the hydration reaction of the hydraulic composition. However, it also reacts with components (Ca, Al, etc.) contained in granulated blast furnace slag to form ettringite (3CaO·Al ₂ O ₃ ·3CaSO ₄ ·32H ₂ O), which has low solubility. The ettringite covers the particle surfaces of the granulated blast furnace slag, suppressing its dissolution and slowing down the reaction rate. In particular, in the case of a hydraulic composition containing an accelerator, the accelerator not only promotes the dissolution and reaction of the granulated blast furnace slag in the early stages of the reaction, but also promotes the formation of the above-mentioned ettringite due to the reaction with gypsum present in large amounts in the same system. In other words, compared to a case where an accelerator is not included, the reaction for the formation of ettringite as described above is promoted, and the amount of ettringite produced becomes excessive, which may actually suppress the reaction of the granulated blast furnace slag. Generally, it is believed that increasing the amount of gypsum is beneficial for increasing strength because ettringite formation increases bound water and reduces voids in the early stages of the hardening reaction. However, it was discovered that the amount of gypsum used in combination with an accelerator can actually have a negative effect. This led to the new discovery that by reducing the gypsum content in a system in which an accelerator is used in combination, contrary to conventional common knowledge, the resulting hydraulic composition can exhibit excellent compressive strength both in the early stages and over the long term. The present disclosure is based on these new findings.

One aspect of the present disclosure provides a concrete composition including a hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate, wherein the hydraulic composition includes cement including an alkali activator and granulated blast furnace slag, and an accelerator, the content of the granulated blast furnace slag being 40.0 to 95.0 mass% based on the total amount of the cement, the content of gypsum in the cement being 0.05 to 1.70 mass% in terms of _SO3 , the content of the accelerator being 0.2 to 10.0 parts by mass relative to 100 parts by mass of the cement, the content of the hydraulic composition being 250 to 600 kg/ ^m3 , the content of the water relative to the content of the hydraulic composition being 0.25 to 0.60, and the content of the water-reducing agent being 0.5 to 3.0 mass% based on 100 mass% of the hydraulic composition.

By keeping the gypsum content in the hydraulic composition low and using it in combination with an accelerator, the above concrete composition can reduce the gypsum's inhibition of the accelerator's hardening-accelerating effect. This effect allows the concrete composition to exhibit excellent compressive strength both in the early stages and over the long term when hardened, even when using a hydraulic composition containing a relatively large amount of granulated blast furnace slag.

The water-reducing agent may include a high-performance air-entraining water-reducing agent.

The content of the granulated blast furnace slag and the content of the gypsum may satisfy the relationship of the following formula (1): By adjusting the content of the granulated blast furnace slag and the content of the gypsum so as to satisfy the relationship of the following formula (1), it is possible to achieve a higher level of both early-stage and long-term compressive strength when the concrete composition is hardened.
[Gypsum content]≦1.5−2.0([granulated blast furnace slag content]−60)/100...Equation (1)

The accelerator may contain at least one selected from the group consisting of alkali metal salts and alkaline earth metal salts. By including the accelerator, the reactivity of granulated blast furnace slag can be further improved.

The accelerator may contain a salt having a monovalent anion. By including a salt of a monovalent anion in the accelerator, the formation of hydrates on the surface of the granulated blast furnace slag can be controlled, further improving the reactivity of the granulated blast furnace slag.

The accelerator may contain a calcium salt, which increases the calcium ion (Ca ²⁺ ) concentration in the aqueous solution formed by contacting the hydraulic composition with water, and promotes the production of calcium silicate hydrate (C-S-H), which is a major component of the hardened body, resulting in superior initial compressive strength when the concrete composition is hardened.

The accelerator may contain at least one selected from the group consisting of nitrites, nitrates, and chlorides. By including the accelerator, the concrete composition will have better initial compressive strength when hardened.

The alkali activator may contain at least one selected from the group consisting of Portland cement clinker, hydrated lime, and quicklime. By including the above-mentioned components in the alkali activator, the hydration reaction of the granulated blast furnace slag can be accelerated, further accelerating the hardening reaction in the concrete composition.

The gypsum may contain at least one selected from the group consisting of gypsum dihydrate and gypsum hemihydrate. The inclusion of at least one of gypsum dihydrate and gypsum hemihydrate in the gypsum can further promote the initial reaction between the alkaline activator (e.g., Portland cement clinker) and granulated blast furnace slag. While gypsum can be any of gypsum anhydrite, gypsum dihydrate, and gypsum hemihydrate, the reason why the above-mentioned effect is significant when the concrete composition according to the present disclosure contains at least one of gypsum dihydrate and gypsum hemihydrate is believed to be due to the following reason. That is, gypsum dihydrate and gypsum hemihydrate have a faster dissolution rate than gypsum anhydrite and can contribute to the reaction between the alkaline activator and granulated blast furnace slag at an earlier stage of the hardening reaction. Therefore, the effect can be significantly achieved when at least one of gypsum dihydrate and gypsum hemihydrate is contained. This difference is particularly evident in systems with a reduced gypsum content, such as the hydraulic composition according to the present disclosure.

The basicity of the granulated blast furnace slag may be less than 1.75. Since the hydraulic composition can fully utilize the accelerator effect by reducing the gypsum content, it is possible to use granulated blast furnace slag, so-called low-grade slag, which is generally avoided in order to improve early strength. The hydraulic composition can use granulated blast furnace slag with a basicity of less than 1.75, which can be classified as low-grade slag. Even in this case, the concrete composition can exhibit excellent compressive strength both in the early stages and over the long term when hardened.

The basicity of the granulated blast furnace slag may be 1.75 to 1.95. By ensuring that the basicity of the granulated blast furnace slag is within this range, the latent hydraulic properties of the granulated blast furnace slag can be more fully utilized, allowing for the production of concrete with excellent strength development. It is generally known that low basicity reduces the reactivity and strength development performance of granulated blast furnace slag. However, in the hydraulic composition disclosed herein, even when granulated blast furnace slag with low basicity is used, adjusting the basicity to fall within the above range allows for a concrete composition that can demonstrate superior strength development performance.

The aluminum oxide content of the granulated blast furnace slag may be 14.5% by mass or less. Using granulated blast furnace slag with a relatively low aluminum oxide content further reduces the amount of ettringite that can inhibit the reaction of the granulated blast furnace slag, and allows the reaction of the granulated blast furnace slag to be promoted over a longer period of time. Furthermore, using granulated blast furnace slag with a relatively low aluminum oxide content also makes it possible to reduce the amount of accelerator used, further reducing the cost of concrete production.

The content of the granulated blast furnace slag may be 60 to 95 mass% based on the total amount of the cement. By reducing the gypsum content, the hydraulic composition can fully utilize the effects of the accelerator. Therefore, even compositions with a high content of granulated blast furnace slag, which are generally not widely used from a practical standpoint, can exhibit compressive strength superior to that of conventional blast furnace cement.

One aspect of the present disclosure provides a method for increasing the compressive strength of a concrete composition, the method comprising: measuring a gypsum content in a composition containing cement that contains an alkaline activator and granulated blast furnace slag, the granulated blast furnace slag content being 40.0 to 95.0% by mass; measuring a gypsum content in the cement and adjusting the gypsum content in the cement to 0.05 to 1.70% by mass, calculated as _SO3 ; measuring a content of an accelerator in the composition and adjusting the content of the accelerator to 0.2 to 10.0 parts by mass per 100 parts by mass of the cement; and blending water, a water-reducing agent, fine aggregate, and coarse aggregate into the composition so that the blending amount of the composition is 250 to 600 kg/ ^m3 , the blending amount of the water relative to the blending amount of the composition is 0.25 to 0.60, and the content of the water-reducing agent is 0.5 to 3.0% by mass, based on 100% by mass of the composition.

The method for increasing the compressive strength of the above concrete composition involves first adjusting the amount of gypsum in the cement and the amount of accelerator in the composition for a composition containing a specified cement, and then blending water, a water-reducing agent, fine aggregate, and coarse aggregate in specified amounts, thereby increasing the compressive strength of the resulting concrete composition when it is hardened.

In the above-mentioned compressive strength enhancement method, the above-mentioned composition may contain an accelerator. Even if the composition already contains an accelerator, i.e., if the composition is itself a hydraulic composition, the above-mentioned compressive strength enhancement method can be applied.

The present disclosure provides a concrete composition that contains a relatively high proportion of granulated blast furnace slag, has sufficient fluidity for concrete construction, and is capable of exhibiting excellent compressive strength both in the early and long term. The present disclosure also provides a method for increasing the compressive strength of a concrete composition that contains a relatively high proportion of granulated blast furnace slag, both in the early and long term.

Embodiments of the present disclosure are described below. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following content. In the following description, when it is stated as "X to Y" (X and Y are arbitrary numbers), it means "X or greater and Y or less" unless otherwise specified.

Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

[Concrete composition]
One embodiment of the concrete composition includes a hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate. In the concrete composition, the content of the hydraulic composition is 250 to 600 kg/ ^m3 . In the concrete composition, the content of the water relative to the content of the hydraulic composition is 0.25 to 0.60. In the concrete composition, the content of the water-reducing agent is 0.5 to 3.0 mass% based on 100 mass% of the hydraulic composition. The concrete composition according to the present disclosure may be fresh concrete. In this specification, fresh concrete means concrete that has fluidity from immediately after mixing until setting and hardening. Each component of the concrete composition is described below.

The hydraulic composition contains cement containing an alkali activator and granulated blast furnace slag, and an accelerator. In the hydraulic composition, the content of the granulated blast furnace slag is 40.0 to 95.0 mass% based on the total amount of the cement, the content of gypsum in the cement is 0.05 to 1.70 mass% in terms of _SO3 , and the content of the accelerator is 0.2 to 10.0 parts by mass per 100 parts by mass of the cement.

In this specification, cement refers not only to cases where the alkaline activator contains cement clinker, but also to powder containing granulated blast furnace slag as the main component and containing an alkaline activator (and in some cases, powder that further contains gypsum). The above cement may be composed of an alkaline activator, granulated blast furnace slag, and gypsum.

The alkali activator is a component that stimulates the hardening reaction of granulated blast furnace slag and promotes the hardening reaction of the hydraulic composition. The alkali activator may contain, for example, at least one selected from the group consisting of Portland cement clinker, slaked lime, and quicklime, or may be any one of Portland cement clinker, slaked lime, and quicklime, or may be Portland cement clinker.

The Portland cement clinker can be the Portland cement clinker used to prepare various Portland cements specified in JIS R 5210:2003 "Portland Cement." Examples of the various Portland cements include ordinary Portland cement, high-early-strength Portland cement, moderate-heat Portland cement, and low-heat Portland cement. The Portland cement clinker can be the Portland cement clinker used to prepare ordinary Portland cement and high-early-strength Portland cement.

The mineral composition of Portland cement clinker can be calculated using the Bogue formula. The Bogue formula is a widely used formula for calculating the content of major minerals in Portland cement clinker from the content ratios of their chemical compositions. Using the Bogue formula shown below, the contents of tricalcium silicate (represented by 3CaO·SiO ₂ , C ₃ S), dicalcium silicate (represented by 2CaO·SiO ₂ , C ₂ S), and tricalcium aluminate (represented by 3CaO·Al ₂ O ₃ , C ₃ A) in Portland cement clinker can be calculated. Note that "%" in the formula below means "mass %." The chemical formula represents the content ratio (mass %) of each compound indicated by chemical analysis values according to JIS R 5204:2019 "Method for X-ray fluorescence analysis of cement."

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

The _C3A content in the Portland cement clinker may be preferably 0.5 to 11.0 mass%, more preferably 0.5 to 10.5 mass%, even more preferably 0.5 to 10.0 mass%, and particularly preferably 0.5 to 9.5 mass%. When the _C3A content in the Portland cement clinker is within the above range, it is possible to further reduce the amount of gypsum that inhibits the hydration reaction in the hydraulic composition, and also to more fully exhibit the hydration reaction of granulated blast furnace slag.

The fineness of the Portland cement clinker may be adjusted from the viewpoint of further improving the performance of the hydration reaction in the hydraulic composition. The lower limit of the Blaine specific surface area of the Portland cement clinker may be, for example, 2800 cm ² /g or more, or 3000 cm ² /g or more. By setting the lower limit of the Blaine specific surface area of the Portland cement clinker within the above range, the hydration reaction with granulated blast furnace slag can be further promoted. The upper limit of the Blaine specific surface area of the Portland cement clinker may be, for example, 10000 cm ² /g or less, 5000 cm ² /g or less, 4000 cm ² /g or less, or 3500 cm ² /g or less. Setting the upper limit of the Blaine specific surface area of the Portland cement clinker within the above range can reduce the production cost of the hydraulic composition and further reduce CO ₂ emissions in the production of the Portland cement clinker. The Blaine specific surface area of the Portland cement clinker may be adjusted within the above ranges, for example, 2800 to 10000 cm ² /g, 3000 to 5000 cm ² /g, 3000 to 4000 cm ² /g, or 3000 to 3500 cm ² /g.

For example, commercially available granulated blast furnace slag may be used, or slag equivalent to granulated blast furnace slag may be prepared and used. The hydraulic composition described above can fully utilize its accelerator capabilities by adjusting the amount of gypsum in the cement. Therefore, regardless of the quality of the granulated blast furnace slag, when a concrete composition is prepared and hardened, it can exhibit superior compressive strength compared to conventional hydraulic compositions that contain a relatively high amount of gypsum.

The upper limit of the aluminum oxide content (also referred to as _{the Al2O3} content) in the granulated blast furnace slag may be, for example, 14.5% by mass or less, 14.0% by mass or less, 13.5% by mass or less, 13.0% by mass or less, or _12.5 % by mass or less. When the _Al2O3 content in the granulated blast furnace slag is within the above range, it is possible to further suppress a decrease in the long-term strength development when the resulting concrete composition is hardened. The lower limit of _{the Al2O3} content in the granulated blast furnace slag may be, for example, 8% by mass or more, 10% by mass or more, or 12% by mass or more. When the lower limit of _{the Al2O3} content in the granulated blast furnace slag is within the above range, it is possible to more fully exhibit the latent hydraulic properties of the granulated blast furnace slag. The term "latent hydraulic properties" refers to the property of initiating a hydration reaction by adding an alkaline activator. The amount of Al ₂ O ₃ in the granulated blast furnace slag may be adjusted within the above range, for example, 8 to 14.5 mass %, or 10 to 12.5 mass %.

The lower limit of the silicon dioxide content (also referred to as the _SiO2 content) in the granulated blast furnace slag may be, for example, 30.0 mass% or more, 34.0 mass% or more, 34.5 mass% or more, or 35.0 mass% or more. By ensuring that the lower limit of the _SiO2 content of the granulated blast furnace slag is within the above range, it is possible to suppress a decrease in initial and long-term strength development. The upper limit of the _SiO2 content of the granulated blast furnace slag may be, for example, 40.0 mass% or less, 38.0 mass% or less, 36.5 mass% or less, or 35.5 mass% or less. By ensuring that the upper limit of the _SiO2 content of the granulated blast furnace slag is within the above range, it is possible to suppress a decrease in initial strength development. The _SiO2 content of the granulated blast furnace slag may be adjusted within the above range, for example, 34.5 to 40.0 mass%.

The lower limit of the calcium oxide content (also referred to as the CaO content) in granulated blast furnace slag may be, for example, 35.0 mass% or more, 38.5 mass% or more, or 40.0 mass% or more. Having the lower limit of the CaO content in granulated blast furnace slag within the above range can further improve initial strength development. The upper limit of the CaO content in granulated blast furnace slag may be, for example, 45.0 mass% or less, 43.5 mass% or less, 43.0 mass% or less, 42.5 mass% or less, 42.0 mass% or less, or 41.5 mass% or less. Having the upper limit of the CaO content in granulated blast furnace slag within the above range can suppress a decrease in long-term strength development. The CaO content in granulated blast furnace slag may be adjusted within the above range, for example, 38.5 to 45.0 mass%.

The lower limit of the magnesium oxide content (also referred to as MgO content) in granulated blast furnace slag may be, for example, 4.0% by mass or more, 5.0% by mass or more, 5.5% by mass or more, 6.0% by mass or more, 7.0% by mass or more, or 7.2% by mass or more. By ensuring that the lower limit of the MgO content in granulated blast furnace slag is within the above range, it is possible to suppress a decrease in the initial and long-term strength development of the concrete composition when it is hardened. The upper limit of the MgO content in granulated blast furnace slag may be, for example, 10.0% by mass or less, 9.0% by mass or less, less than 7.5% by mass, or less than 7.4% by mass. By ensuring that the upper limit of the MgO content in granulated blast furnace slag is within the above range, it is possible to suppress a decrease in the initial strength development of the concrete composition when it is hardened. The MgO content in granulated blast furnace slag may be adjusted within the above range, for example, 4.0 to 10.0% by mass.

Granulated blast furnace slag may contain, as other components, for example, sulfur trioxide (SO ₃ ), sodium oxide (NaO ₂ ), potassium oxide (K ₂ O), and titanium oxide (TiO ₂ ).

The chemical composition of granulated blast furnace slag in this specification refers to values measured in accordance with JIS R 5202:2015 "Methods for chemical analysis of cement."

The reactivity of granulated blast furnace slag is evaluated by an index called basicity, which is expressed as the value of (CaO + MgO + Al ₂ O ₃ )/SiO ₂ (the ratio of the total content of calcium oxide, magnesium oxide, and aluminum oxide to the content of silicon dioxide in granulated blast furnace slag). Granulated blast furnace slag with either high or low basicity can be used.

As a high-basicity granulated blast furnace slag, for example, one with a basicity of 1.75 or greater can be used. The upper limit of the basicity of granulated blast furnace slag may be, for example, 1.95 or less, less than 1.95, less than 1.90, less than 1.85, or less than 1.80. The lower limit of the basicity of granulated blast furnace slag may be, for example, greater than 1.75 or greater than 1.78. Having the lower limit of basicity within the above range can more easily improve the early strength of the concrete composition when it is hardened. The basicity of granulated blast furnace slag can be adjusted within the above range, and may be, for example, 1.75 to 1.95, or 1.75 or greater but less than 1.80.

In the above-described hydraulic composition, granulated blast furnace slag with low basicity can also be used. Low-basicity granulated blast furnace slag is generally not used as low-grade slag because it is difficult to achieve sufficient compressive strength. However, in the above-described hydraulic composition, such low-grade slag can be used because it can suppress the inhibitory effect on the accelerator. The upper basicity limit of such low-grade granulated blast furnace slag may be, for example, less than 1.75, less than 1.70, or less than 1.65. The lower basicity limit of low-grade granulated blast furnace slag is not particularly limited, but may be, for example, 1.55 or more, 1.60 or more, or 1.65 or more. The basicity of low-grade granulated blast furnace slag may be adjusted within the above-described range, for example, 1.55 or more but less than 1.75.

The basicity in this specification is a value measured in accordance with the description in JIS A 6206:2013 "Ground granulated blast furnace slag for concrete," and specifically means the value of (CaO + MgO + Al ₂ O ₃ )/SiO ₂ (the ratio of the total content of calcium oxide, magnesium oxide, and aluminum oxide to the content of silicon dioxide).

The Blaine specific surface area of the granulated blast furnace slag may be, for example, 2500 to 10,000 cm ² /g, 2500 to 8,000 cm ² /g, 2500 to 6,000 cm ^{2 /} g, 2500 to 5,000 cm ² /g, 3,000 to 5,000 cm ^{2 /g, 4,000 to 5,000 cm 2} /g, or 4,000 to 4,500 cm ² /g.

In this specification, "Blaine specific surface area" refers to the value measured in accordance with the method specified in JIS R 5201:2015, "Physical testing methods for cement."

The content of the granulated blast furnace slag is 40.0 to 95.0 mass% based on the total amount of the cement, which can contribute to reducing _CO2 emissions during the production of the concrete composition. The hydraulic composition reduces the amount of gypsum in the cement and can exhibit a hardening acceleration effect when used in combination with an accelerator, which can prevent a significant decrease in compressive strength when the concrete composition is hardened, and can increase the proportion of cement replaced by granulated blast furnace slag.

The lower limit of the granulated blast furnace slag content may be, for example, 45.0% by mass or more, more than 45.0% by mass, 50.0% by mass or more, 55.0% by mass or more, 60.0% by mass or more, 65.0% by mass or more, or 70.0% by mass or more, based on the total amount of the cement. Having the lower limit of the granulated blast furnace slag content within the above range allows for further reduction in the amount of alkaline activator used. The upper limit of the granulated blast furnace slag content may be, for example, less than 95.0% by mass, 90.0% by mass or less, 85.0% by mass or less, or 80.0% by mass or less, based on the total amount of the cement. Having the upper limit of the granulated blast furnace slag content within the above range allows for further suppression of deterioration in early strength and further improves the effect of using an accelerator in combination. The content of granulated blast furnace slag may be adjusted within the above range, and may be 45.0 to 95.0 mass%, 60.0 to 95.0 mass%, or 60.0 to 90.0 mass% based on the total amount of the cement.

In this specification, the content of granulated blast furnace slag refers to a value determined by the method described below. Specifically, a measurement sample is prepared by first heating the hydraulic composition at 900°C for 1 hour to crystallize the granulated blast furnace slag (glass). The measurement sample is then subjected to X-ray diffraction measurement, and each crystalline phase in the measurement sample is quantified using Rietveld analysis to quantify gehlenite, merwinite, and other crystalline phases formed by crystallization of the granulated blast furnace slag. The total amount of these is taken as the content of granulated blast furnace slag. If you manufacture your own hydraulic composition, the amount (measured value) of granulated blast furnace slag added during the manufacturing process corresponds to the above content.

In the hydraulic composition according to the present disclosure, the gypsum content in the cement is kept relatively low. The gypsum content in the cement is 0.05 to 1.70 mass% in terms of _SO3 . By keeping the gypsum content within this range, the effect of the gypsum on improving the compressive strength of the concrete composition as it hardens can be exhibited, while the action of the accelerator on inhibiting the effect of improving the compressive strength of the concrete composition as it hardens can be reduced.

The upper limit of the gypsum content in the cement, calculated as _SO3 , may be, for example, 1.6 mass% or less, 1.5 mass% or less, 1.4 mass% or less, 1.3 mass% or less, 1.2 mass% or less, 1.0 mass% or less, or 0.8 mass% or less. When the upper limit of the gypsum content is within the above range, the hydraulic composition containing the accelerator can exhibit better strength development. The lower limit of the gypsum content in the cement, calculated as _SO3 , may be, for example, 0.10 mass% or more, 0.20 mass% or more, 0.30 mass% or more, 0.40 mass% or more, or 0.50 mass% or more. When the lower limit of the gypsum ( _SO3 ) content is within the above range, the hydration reaction of the cement is more favorable, the fluidity of the concrete composition mixed with water is further improved, and the early strength development upon hardening can be further improved. The gypsum content in the cement may be adjusted within the above range, for example, 0.10 to 1.5 mass %, or 0.50 to 1.3 mass %.

The gypsum content in this specification refers to the total amount of gypsum components that may be contained in the alkaline activator and granulated blast furnace slag, as well as the gypsum components that are blended with the cement. The gypsum content in this specification refers to a value determined by the method shown below. Specifically, the gypsum content is measured in accordance with the _SO3 analysis method specified in JIS R 5202:2015 "Methods for Chemical Analysis of Cement."

For example, gypsum dihydrate, gypsum hemihydrate, and anhydrous gypsum can be used. The gypsum may contain at least one selected from the group consisting of gypsum dihydrate and gypsum hemihydrate, or may be one selected from the group consisting of gypsum dihydrate and gypsum hemihydrate.

The Blaine specific surface area of the cement may be, for example, 2800 to 10,000 cm ² /g. Furthermore, when the alkali activator constituting the cement includes Portland cement clinker, the Portland cement clinker, granulated blast furnace slag, and gypsum may be simultaneously ground to produce cement. When simultaneously ground, the lower limit of the Blaine specific surface area of the cement may be, for example, 2800 cm ² /g or more, or 3,000 cm ² /g or more. By setting the lower limit of the Blaine specific surface area of the cement within the above range, the hydration reaction with the granulated blast furnace slag can be further promoted. The upper limit of the Blaine specific surface area of the cement may be, for example, 10,000 cm ² /g or less, 5,000 cm ² /g or less, 4,000 cm ² /g or less, or 3,500 cm ² /g or less.

Accelerators are compounds that accelerate the reaction of granulated blast furnace slag and improve its early strength.

The accelerator may contain at least one selected from the group consisting of alkali metal salts and alkaline earth metal salts. By including an alkali metal salt or alkaline earth metal salt in the accelerator, the reactivity of granulated blast furnace slag can be further improved. Examples of alkali metals include sodium and potassium, and examples of alkaline earth metals include magnesium and calcium. From the perspective of promoting hydrate formation and improving compressive strength, it is preferable for the alkaline earth metal to contain calcium, and calcium is more preferable.

The accelerator may contain a salt having a monovalent anion, and may also contain a calcium salt. The accelerator may contain at least one selected from the group consisting of nitrites, nitrates, and chlorides, and preferably contains nitrites. By including nitrites in the accelerator, the early strength of the concrete composition when it is hardened can be further improved. By including nitrites in the accelerator, the amount of heat generated by hydration when the concrete composition is hardened can also be reduced.

Specific examples of accelerators include calcium nitrite, calcium nitrate, calcium chloride, calcium hydroxide, sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, sodium chloride, and potassium chloride. Among the above compounds, the accelerator preferably contains an alkali metal nitrite, more preferably calcium nitrite, and even more preferably calcium nitrite.

The upper limit of the accelerator content may be, for example, 10.0 parts by mass or less, 8.0 parts by mass or less, 6.0 parts by mass or less, 5.0 parts by mass or less, 4.0 parts by mass or less, 3.5 parts by mass or less, or 3.3 parts by mass or less, per 100 parts by mass of the cement. By keeping the upper limit of the accelerator content within the above range, the occurrence of abnormal setting, which may occur when the reaction of granulated blast furnace slag or the like is excessively accelerated, can be more reliably suppressed. The lower limit of the accelerator content may be, for example, 0.2 parts by mass or more, 0.3 parts by mass or more, 0.5 parts by mass or more, 1.0 parts by mass or more, 2.0 parts by mass or more, or 2.5 parts by mass or more, per 100 parts by mass of the cement. By keeping the lower limit of the accelerator content within the above range, the reaction of granulated blast furnace slag can be more effectively accelerated. The accelerator content may be adjusted within the above range, and may be, for example, 0.2 to 10.0 parts by mass, 0.3 to 5.0 parts by mass, or 0.5 to 3.5 parts by mass per 100 parts by mass of the cement.

In the hydraulic composition, the content of the granulated blast furnace slag and the content of the gypsum may satisfy the relationship of the following general formula (X). In the following general formula (X), the content of gypsum is the content of gypsum in cement (unit: mass%), expressed as an _SO3 equivalent value. In the following general formula (X), the content of the granulated blast furnace slag is the content (unit: mass%) based on the total amount of cement. In the following general formula (X), A is a constant and may be, for example, 1.5 or less. When the content of the granulated blast furnace slag and the content of the gypsum satisfy the relationship of general formula (X) in which the constant A is a smaller value, the effects of the present disclosure can be more pronounced. For example, the constant A may be 1.5, 1.0, 0.8, or 0.6. For reference, the formula when the constant A is 1.5 is shown below (see formula (1)).
[Gypsum content]≦A−2.0([granulated blast furnace slag content]−60)/100...General formula (X)
[Gypsum content]≦1.5−2.0([granulated blast furnace slag content]−60)/100...Equation (1)

Examples of water include tap water, distilled water, and deionized water. The water content may be 20 to 100 parts by weight, or 25 to 60 parts by weight, per 100 parts by weight of the hydraulic composition.

Because the concrete composition according to the present disclosure contains the hydraulic composition described above, it is not necessarily necessary to reduce the water content in order to achieve excellent compressive strength. The upper limit of the water-to-cement ratio (100 x water mass/cement mass) may be, for example, 60% by mass or less, 55% by mass or less, 50% by mass or less, or 45% by mass or less. By keeping the upper limit of the water-to-cement ratio within the above range, the compressive strength of the concrete composition when hardened can be further improved. The lower limit of the water-to-cement ratio may be, for example, 25% by mass or more, 30% by mass or more, 35% by mass or more, or 40% by mass or more. By keeping the lower limit of the water-to-cement ratio within the above range, the fluidity of fresh concrete can be further improved, thereby further improving the workability of concrete construction.

The fine aggregate specified in JIS A 5005:2020 "Crushed Stone and Crushed Sand for Concrete" can be used. Examples of fine aggregate include river sand, land sand, sea sand, crushed sand, silica sand, copper slag fine aggregate, and electric furnace oxidizing slag fine aggregate. When fine aggregate is used, the amount of fine aggregate used may be, for example, 50 to 500 parts by mass, 100 to 300 parts by mass, or 200 to 250 parts by mass per 100 parts by mass of the hydraulic composition.

The coarse aggregate specified in JIS A 5005:2020 "Crushed stone and crushed sand for concrete" can be used. Examples of coarse aggregate include gravel and crushed stone. When using coarse aggregate, the amount of coarse aggregate used may be, for example, 50 to 500 parts by mass, 100 to 300 parts by mass, or 200 to 250 parts by mass per 100 parts by mass of the hydraulic composition.

Fine aggregate and coarse aggregate can also be used in combination. In this case, the total amount of fine aggregate and coarse aggregate used may be 100 to 300 parts by mass, or 200 to 250 parts by mass, per 100 parts by mass of the hydraulic composition described above.

The lower limit of the fine aggregate ratio may be, for example, 40% by volume or more, 43% by volume or more, 45% by volume or more, or 47% by volume or more. By setting the lower limit of the fine aggregate ratio within this range, the material separation resistance of fresh concrete can be improved. The upper limit of the fine aggregate ratio may be, for example, 56% by volume or less, 54% by volume or less, 52% by volume or less, or 50% by volume or less. By setting the upper limit of the fine aggregate ratio within this range, the fluidity of fresh concrete can be further improved, and workability in concrete construction can be further improved.

The lower limit of the coarse aggregate bulk volume may be, for example, 0.500 ^{m 3} /m ³ or more, 0.520 ^{m 3} /m ³ or more, 0.540 m ³ /m ³ or more, or 0.560 ^{m 3} /m ³ or more. When the lower limit of the coarse aggregate bulk volume is within the above range, the fluidity of fresh concrete can be further improved, and the workability in concrete construction can be further improved. The upper limit of the coarse aggregate bulk volume may be, for example, 0.620 ^{m 3} /m ³ or less, 0.600 ^{m 3} /m ³ or less, 0.580 ^{m 3} /m ³ or less, or 0.565 ^{m 3} /m ³ or less. When the upper limit of the coarse aggregate bulk volume is within the above range, the material separation resistance of fresh concrete can be improved.

Examples of water-reducing agents include water-reducing admixtures, air-entraining (AE) water-reducing agents, high-performance water-reducing agents, and high-range air-entraining (AE) water-reducing agents. AE water-reducing agents and high-range air-entraining (AE) water-reducing agents can be of standard, delayed, or accelerated types, with delayed types being preferred. AE water-reducing agents and high-range air-entraining (AE) water-reducing agents can also be of Type I, Type II, or Type III types, with Type I being preferred. The water-reducing agent may include a high-performance air-entraining (AE) water-reducing agent, or may be a high-range air-entraining (AE) water-reducing agent.

The content of the water-reducing agent is 0.5 to 3.0% by mass, based on 100% by mass of the hydraulic composition. The lower limit of the water-reducing agent content may be, for example, 0.6% by mass or more, 0.7% by mass or more, or 0.8% by mass or more, based on 100% by mass of the hydraulic composition. Having the lower limit of the water-reducing agent content within this range can further improve the fluidity of fresh concrete. The upper limit of the water-reducing agent content may be, for example, 1.5% by mass or less, 1.2% by mass or less, 1.0% by mass or less, or 0.9% by mass or less, based on 100% by mass of the hydraulic composition. Having the upper limit of the water-reducing agent content within this range can impart material separation resistance without impairing the fluidity of fresh concrete.

Other admixtures besides water-reducing agents include, for example, air-entraining agents, superplasticizers, antifoaming agents, shrinkage-reducing agents, set accelerators, set retarders, thickeners, silica powder, other inorganic powders containing calcium, fly ash, and inorganic minerals containing Si and Al. The amount of other admixtures used may be, for example, 0.01 to 2% by mass based on 100% by mass of the above-mentioned hydraulic composition.

The concrete composition described above can be hardened to give a hardened concrete body with excellent compressive strength. There is no difference in composition between the concrete composition and the hardened concrete body. In other words, the present disclosure provides a hardened concrete body comprising a hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate, wherein the hydraulic composition comprises cement containing an alkali activator and granulated blast furnace slag, and an accelerator, the content of the granulated blast furnace slag being 40.0 to 95.0 mass% based on the total amount of the cement, the content of gypsum in the cement being 0.05 to 1.70 mass% in terms of _SO3 , the content of the accelerator being 0.2 to 10.0 parts by mass relative to 100 parts by mass of the cement, the content of the hydraulic composition being 250 to 600 kg/ ^m3 , the content of the water relative to the content of the hydraulic composition being 0.25 to 0.60, and the content of the water-reducing agent being 0.5 to 3.0 mass% based on 100 mass% of the hydraulic composition.

[Method of manufacturing concrete composition]
The above-mentioned concrete composition can be produced, for example, by the following method. One embodiment of a method for producing a concrete composition includes a kneading step (kneading step) in which a water-reducing agent and water are blended with a mixture containing the above-mentioned hydraulic composition, fine aggregate, and coarse aggregate. In the kneading step, the water, water-reducing agent, fine aggregate, and coarse aggregate are blended so that the blending amount of the hydraulic composition is 250 to 600 kg/ ^m3 , the blending amount of the water relative to the blending amount of the hydraulic composition is 0.25 to 0.60, and the content of the water-reducing agent is 0.5 to 3.0 mass% based on 100 mass% of the hydraulic composition.

In the method for producing a concrete composition, the hydraulic composition described above may be prepared in advance, or a hydraulic composition may be prepared and used by the following method. That is, the method for producing a concrete composition may further include a first step of preparing cement by mixing raw materials including an alkali activator and granulated blast furnace slag so that the blending amount of the granulated blast furnace slag is 40.0 to 95.0 mass%, and a second step of mixing 0.2 to 10.0 mass parts of an accelerator per 100 mass parts of the cement. The first step includes adjusting the gypsum content in the cement to 0.05 to 1.70 mass% in terms of _SO3 .

The raw materials in the first step are a mixture of alkaline stimulant and granulated blast furnace slag, and may also contain gypsum.

In the first step, each component constituting the raw material may be crushed. When crushing is performed in the first step, the order of mixing and crushing is not particularly limited. That is, the various components may be mixed and then crushed, or they may be crushed and then mixed, or the various components may be mixed and crushed simultaneously. The various components may be mixed in the first step using a mixer such as a pan mixer, tilting mixer, or ribbon mixer, or may be mixed and crushed using a crusher such as a ball mill, vertical roller mill, or roller press, or the various components may be crushed individually and then mixed using a mixer such as a mechanical mixer.

In the first step, the gypsum content in the cement is adjusted to 0.05 to 1.70 mass% in terms of _SO3 . This adjustment prevents gypsum from inhibiting the hardening-accelerating action of the accelerator in the concrete composition.

The gypsum content in the first step may be adjusted so that the content of the granulated blast furnace slag and the content of the gypsum satisfy the relationship of the following general formula (X). In the following general formula (X), the content of gypsum is the content of gypsum in cement (unit: mass%), expressed as an _SO3 equivalent value. In the following general formula (X), the content of the granulated blast furnace slag is the content (unit: mass%) based on the total amount of cement. In the following general formula (X), A is a constant and may be, for example, 1.5 or less. When the content of the granulated blast furnace slag and the content of the gypsum satisfy the relationship of general formula (X) in which the constant A is a smaller value, the effects of the present disclosure can be more pronounced. For example, the constant A may be 1.5, 1.0, 0.8, or 0.6. For reference, the formula when the constant A is 1.5 is shown below (see formula (1)).
[Gypsum content]≦A−2.0([granulated blast furnace slag content]−60)/100...General formula (X)
[Gypsum content]≦1.5−2.0([granulated blast furnace slag content]−60)/100...Equation (1)

In the second step, cement and an accelerator are mixed. The mixing method may be the same as in the first step, or may be different. Other components may be added in the second step, or in a step other than the first and second steps. Examples of other components include silica powder, other calcium-containing inorganic powders, fly ash, and inorganic minerals containing Si and Al.

[Method for increasing the compressive strength of a concrete composition]
By applying the above findings, it is possible to provide a method for increasing the compressive strength of a composition containing a high proportion of granulated blast furnace slag, etc. The concrete composition obtained by applying this method can exhibit excellent compressive strength both in the early stages and over the long term. One embodiment of a method for increasing the compressive strength of a concrete composition includes: measuring a gypsum content in a composition containing cement that includes an alkali activator and granulated blast furnace slag, the granulated blast furnace slag content being 40.0 to 95.0% by mass; adjusting the gypsum content in the cement to 0.05 to 1.70% by mass, calculated as _SO3 ; measuring a content of an accelerator in the composition and adjusting the content of the accelerator to 0.2 to 10.0 parts by mass per 100 parts by mass of the cement; and blending water, a water-reducing agent, fine aggregate, and coarse aggregate into the composition so that the blending amount of the composition is 250 to 600 kg/ ^m3 , the blending amount of the water relative to the blending amount of the composition is 0.25 to 0.60, and the content of the water-reducing agent is 0.5 to 3.0% by mass, based on 100% by mass of the composition.

In the above-mentioned compressive strength enhancement method, the content of the accelerator is adjusted, so the composition may or may not contain an accelerator.

Although several embodiments have been described above, the present disclosure is in no way limited to the above embodiments. Furthermore, the descriptions of the above embodiments can be mutually applied.

The present disclosure will be explained in more detail below with reference to examples, comparative examples, and reference examples. However, the present disclosure is not limited to the following examples.

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

(cement)
Alkali stimulant: Commonly used ordinary Portland cement was used as the component containing cement clinker, which is an alkaline stimulant. In Table 1, ordinary Portland cement is referred to as OPC. The chemical composition of ordinary Portland cement was measured in accordance with the description in JIS R 5202:2015 "Methods for chemical analysis of cement." The results are shown in Table 1.

Granulated blast furnace slag: Slag A was used for granulated blast furnace slag produced at the same facility. Slag A contained no gypsum, while slag B contained gypsum. Slag C contained gypsum and had a lower basicity than typical slags such as Slag A and Slag B. The chemical composition of each slag was measured in accordance with JIS R 5202:2015, "Methods for Chemical Analysis of Cement." The results are shown in Table 1. The loss on ignition (also referred to as ig. loss) in Table 1 was measured at a heating temperature of 700°C in accordance with the method described in "5.2 For Materials Other Than Blast Furnace Cement and Blast Furnace Slag" in "5. Methods for Determining Ignition Loss" of JIS R 5202:2010.

(blast furnace cement)
In the Reference Examples described below, commonly used blast furnace cement Type B was used instead of the hydraulic composition. In Table 1, blast furnace cement Type B is referred to as BB. The chemical composition of the blast furnace cement was measured in accordance with the description in JIS R 5202:2015 "Methods for chemical analysis of cement." The results are shown in Table 1.

(accelerator)
As the accelerator, an inorganic accelerator was used.
Accelerator A: Calcium nitrite monohydrate (powder) manufactured by Kishida Chemical Co., Ltd. was used.
Accelerator B: A liquid accelerator containing nitrous acid and nitrate, which is a commercially available concrete hardening accelerator conforming to JIS A 6204:2011 "Chemical admixtures for concrete," was used.
Accelerator C: A liquid accelerator containing nitrate, which is a commercially available concrete hardening accelerator conforming to JIS A 6204:2011 "Chemical admixtures for concrete," was used.

(admixture)
As admixtures, a water reducing agent and an air entraining agent were used.
Water reducing agent: "MasterEase 8050" (product name, main component: polycarboxylic acid ether compound) manufactured by Pozzolith Solutions Co., Ltd., which complies with the high-performance AE water reducing agent retardation type (Type I) of JIS A 6204:2011 "Chemical admixtures for concrete," was used.
Air entraining agent: "Master Air 303A" (product name, main component: alkyl ether-based anionic surfactant) manufactured by Pozzolith Solutions Co., Ltd., which complies with JIS A 6204:2011 "Chemical admixtures for concrete," was used.

(fine aggregate)
As the fine aggregate, a mixture of sea sand (surface dry density: 2.57 g/m ³ ) and crushed sand (surface dry density: 2.66 g/m ³ ) in a volume ratio of 1:1 was used.

(coarse aggregate)
As the coarse aggregate, "Crushed Stone 2005" (product name, surface dry density: 2.70 g/m ³ ) manufactured by Yokokawa Crushed Stone Co., Ltd. was used.

[Mixture of hydraulic composition, water, fine aggregate, and coarse aggregate in concrete composition]
The formulations of the hydraulic composition, water, fine aggregate, and coarse aggregate used to prepare a concrete composition containing the hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate are shown in Table 2. In Table 2, in addition to the formulations of each component, the water-cement ratio (value expressed as unit amount of water/unit amount of hydraulic composition), fine aggregate rate (value expressed as unit volume of fine aggregate/unit volume of total aggregate), and coarse aggregate bulk volume are also shown. Note that the Reference Example is an example in which blast furnace cement was used instead of the hydraulic composition according to the present disclosure.

[Composition of concrete composition]
The compositions of the alkali activator, granulated blast furnace slag, accelerator, and admixture in the concrete compositions prepared in Examples 1 to 4, Comparative Examples 1 to 6, and Reference Example are shown in Table 3. Table 3 also lists the air content in fresh concrete, which will be described later.

The concrete compositions of Examples 1 to 3 and Comparative Example 1 use slag A without gypsum addition, and the _SO3 equivalent of the gypsum-equivalent component is 0.59 mass%, but since the blast furnace slag is blended in at 70 mass parts, they can be considered to be compositions equivalent to blast furnace cement Class C. Furthermore, the concrete compositions of Comparative Examples 2 to 5 use slag B with gypsum addition, and the _SO3 equivalent of the gypsum-equivalent component is 1.94 mass%, but since the blast furnace slag is blended in at 70 mass parts, they can be considered to be compositions equivalent to blast furnace cement Class C. Furthermore, the concrete compositions of Example 4 and Comparative Example 6 use slag C with gypsum addition, and the SO3 equivalent of the _gypsum -equivalent component is 0.67 mass%, but since the blast furnace slag is blended in at 70 mass parts, they can be considered to be compositions equivalent to blast furnace cement Class C.

Comparative examples 2 to 5 were designed in accordance with conventional technical knowledge, increasing the amount of gypsum to improve the compressive strength obtained by hardening the concrete composition. Comparative examples 3 to 5 were designed to further improve the compressive strength obtained by hardening the concrete composition by blending an accelerator.

[Evaluation of Concrete Composition: Compressive Strength Test]
For each of the concrete compositions prepared in Examples 1 to 4, Comparative Examples 1 to 6, and Reference Example, compressive strength was measured, compressive strength ratio was determined, and evaluation was performed according to the methods described below. The results are shown in Table 4.

First, the hydraulic composition, fine aggregate, and coarse aggregate were placed in a horizontal biaxial forced mixer to obtain the compositions shown in Tables 2 and 3. The mixture was then dry-mixed for 30 seconds, followed by the addition of water and admixtures, which were then mixed for 120 seconds and allowed to stand for 5 minutes to prepare fresh concrete. The concrete composition was mixed in a constant temperature and humidity chamber maintained at a temperature of 20±2°C and a relative humidity of 60±5%. The air content of the resulting fresh concrete was measured. The results are shown in Table 3. The air content was measured in accordance with the method specified in JIS A 1128, "Test method for air content in concrete by pressure - air chamber pressure method." For all concrete compositions, the air content was well within the range of 4.5±1.0%, confirming that the resulting hardened concrete can be expected to be sufficiently protected against deterioration due to the freezing and thawing action of water (i.e., it is possible to provide hardened concrete with excellent resistance to freezing and thawing).

The fresh concrete obtained as described above was packed into a cylindrical formwork with an inner diameter of 10 cm and a height of 20 cm, stored in a constant temperature and humidity chamber at a temperature of 20±2°C and a relative humidity of 60±5%, and demolded after one day. The concrete was then submerged in a water tank at a temperature of 20±2°C until the concrete reached a predetermined age (7 days or 28 days), yielding a hardened concrete body. The resulting hardened concrete body was used as a test specimen to measure its compressive strength. The compressive strength measurement was performed in accordance with the method described in JIS A 1108:2018, "Testing Method for Compressive Strength of Concrete." Based on the measured compressive strength values, a compressive strength ratio was calculated as a relative value, with the result of Reference Example 1 being set at 1. Based on the calculated compressive strength ratio, evaluation was performed according to the following criteria.
<Evaluation criteria for compressive strength ratio>
A: The compressive strength ratio at 7 days of age is 1.00 or more, and the compressive strength ratio at 28 days of age is 1.00 or more.
B: The compressive strength ratio at 7 days is 1.00 or more, and the compressive strength ratio at 28 days is 0.95 or more and less than 1.00.
C: The compressive strength ratio at 7 days is 0.90 or more and less than 1.00, and the compressive strength ratio at 28 days is 1.00 or more.
D: The compressive strength ratio at 7 days is 0.90 or more and less than 1.00, and the compressive strength ratio at 28 days is 0.95 or more and less than 1.00.
E: The compressive strength ratio at 7 days is less than 0.90, or the compressive strength ratio at 28 days is less than 0.95.

As shown in Table 4, the concrete compositions of Examples 1 to 3, which used slag A as the granulated blast furnace slag, were confirmed to exhibit superior compressive strength compared to the concrete composition of Comparative Example 1, which did not contain an accelerator. The concrete composition of Example 4, which used slag C, which had a lower basicity than slag A, as the granulated blast furnace slag, was also confirmed to exhibit superior compressive strength compared to the concrete composition of Comparative Example 6, which did not contain an accelerator. Furthermore, the concrete compositions of Examples 1 to 3 were confirmed to exhibit compressive strength equal to or greater than that of the concrete composition of the Reference Example, which simulates blast furnace cement type B, which is considered to have superior compressive strength. Furthermore, the results of the concrete compositions of Comparative Examples 2 to 5, which used slag B, which contained gypsum as the granulated blast furnace slag, confirmed that their compressive strength was inferior to that of the concrete compositions of Examples 1 to 3, regardless of whether an accelerator was used or not.

[Evaluation of Concrete Composition: Slump Test]
A slump test was conducted for each of the concrete compositions prepared in Examples 1 to 4 and Comparative Examples 1 to 6 according to the method described below. First, fresh concrete was prepared in the same manner as described in the above-mentioned [Evaluation of Concrete Composition: Compressive Strength Test]. A slump test was conducted for the obtained fresh concrete. The slump test was conducted in accordance with the method described in JIS A 1101 "Concrete Slump Test Method." The results are shown in Tables 5 and 6. Table 5 shows the relationship between the elapsed time after mixing and the slump. Table 6 shows the relationship between the elapsed time after mixing and the slump residual rate, based on the slump immediately after mixing.

As shown in Table 6, the concrete compositions of Examples 1 to 3 and Comparative Example 1, which contain a low amount of gypsum equivalent, were found to have a superior slump retention rate and workability during concrete construction compared to the concrete compositions of Comparative Examples 2 to 5, which contain a high amount of gypsum equivalent.

The present disclosure provides a concrete composition that contains a relatively high proportion of granulated blast furnace slag, has sufficient fluidity for concrete construction, and is capable of exhibiting excellent compressive strength both in the early and long term. The present disclosure also provides a method for increasing the compressive strength of a concrete composition that contains a relatively high proportion of granulated blast furnace slag, both in the early and long term.

Claims (13)

A concrete composition comprising a hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate,
The hydraulic composition contains cement containing an alkali activator and granulated blast furnace slag, and an accelerator,
The content of the granulated blast furnace slag is 40.0 to 95.0 mass% based on the total amount of the cement, the content of gypsum in the cement is 0.05 to 1.70 mass% in terms of _SO3 , and the content of the accelerator is 0.2 to 10.0 parts by mass per 100 parts by mass of the cement,
The content of the hydraulic composition is 250 to 600 kg/ ^m3 ,
the content of the water relative to the content of the hydraulic composition is 0.25 to 0.60;
The content of the water-reducing agent is 0.5 to 3.0% by mass based on 100% by mass of the hydraulic composition;
In the granulated blast furnace slag, the aluminum oxide content is 8 to 14.5% by mass, the silicon dioxide content is 30.0 to 40.0% by mass, the calcium oxide content is 35.0 to 45.0% by mass, and the magnesium oxide content is 4.0 to 10.0% by mass;
The basicity of the granulated blast furnace slag is 1.55 to 1.95,
The accelerator contains a salt having a monovalent anion .
Concrete composition. A concrete composition comprising a hydraulic composition, water, a water-reducing agent, fine aggregate, and coarse aggregate,
The hydraulic composition contains cement containing an alkali activator and granulated blast furnace slag, and an accelerator,
The content of the granulated blast furnace slag is 40.0 to 95.0 mass% based on the total amount of the cement, the content of gypsum in the cement is 0.05 to 1.70 mass% in terms of _SO3 , and the content of the accelerator is 0.2 to 10.0 parts by mass per 100 parts by mass of the cement,
The content of the hydraulic composition is 250 to 600 kg/ ^m3 ,
the content of the water relative to the content of the hydraulic composition is 0.25 to 0.60;
The content of the water-reducing agent is 0.5 to 3.0% by mass based on 100% by mass of the hydraulic composition;
In the granulated blast furnace slag, the aluminum oxide content is 8 to 14.5% by mass, the silicon dioxide content is 30.0 to 40.0% by mass, the calcium oxide content is 35.0 to 45.0% by mass, and the magnesium oxide content is 4.0 to 10.0% by mass;
The basicity of the granulated blast furnace slag is 1.55 to 1.95,
The accelerator contains at least one selected from the group consisting of nitrites, nitrates, and chlorides .
Concrete composition. The concrete composition according to claim 1 or 2 , wherein the water reducing agent comprises a high-range air-entraining water reducing agent. The content of the granulated blast furnace slag and the content of the gypsum satisfy the relationship of the following formula (1):
[Gypsum content]≦1.5−2.0([granulated blast furnace slag content]−60)/100...Equation (1)
The concrete composition according to any one of claims 1 to 3 . 5. The concrete composition according to claim 1 , wherein the accelerator comprises at least one selected from the group consisting of alkali metal salts and alkaline earth metal salts. The concrete composition according to any one of claims 1 to 5, wherein the accelerator contains a calcium salt. The concrete composition according to any one of claims 1 to 6 , wherein the alkali activator contains at least one selected from the group consisting of Portland cement clinker, slaked lime, and quicklime. The concrete composition according to any one of claims 1 to 7 , wherein the gypsum contains at least one selected from the group consisting of gypsum dihydrate and gypsum hemihydrate. The concrete composition according to any one of claims 1 to 8 , wherein the granulated blast furnace slag has a basicity of less than 1.75. The concrete composition according to any one of claims 1 to 8 , wherein the basicity of the granulated blast furnace slag is 1.75 to 1.95. The concrete composition according to any one of claims 1 to 10 , wherein the content of the granulated blast furnace slag is 60.0 to 95.0 mass% based on the total amount of the cement. For a composition containing an alkaline activator and granulated blast furnace slag, wherein the content of the granulated blast furnace slag is 40.0 to 95.0 mass%, a gypsum content in the cement is measured, and the gypsum content in the cement is adjusted to 0.05 to 1.70 mass% in terms of _SO3 ;
measuring the content of the accelerator in the composition and adjusting the content of the accelerator to 0.2 to 10.0 parts by mass per 100 parts by mass of the cement; and
water, a water-reducing agent, fine aggregate, and coarse aggregate are blended into the composition so that the blending amount of the composition is 250 to 600 kg/ ^m3 , the blending amount of the water relative to the blending amount of the composition is 0.25 to 0.60, and the content of the water-reducing agent is 0.5 to 3.0 mass% based on 100 mass% of the composition;
Including,
In the granulated blast furnace slag, the aluminum oxide content is 8 to 14.5% by mass, the silicon dioxide content is 30.0 to 40.0% by mass, the calcium oxide content is 35.0 to 45.0% by mass, and the magnesium oxide content is 4.0 to 10.0% by mass;
The basicity of the granulated blast furnace slag is 1.55 to 1.95,
10. A method for enhancing the compressive strength of a concrete composition , wherein the accelerator comprises a salt having a monovalent anion . For a composition containing an alkaline activator and granulated blast furnace slag, wherein the content of the granulated blast furnace slag is 40.0 to 95.0 mass%, a gypsum content in the cement is measured, and the gypsum content in the cement is adjusted to 0.05 to 1.70 mass% in terms of _SO3 ;
measuring the content of the accelerator in the composition and adjusting the content of the accelerator to 0.2 to 10.0 parts by mass per 100 parts by mass of the cement; and
water, a water-reducing agent, fine aggregate, and coarse aggregate are blended into the composition so that the blending amount of the composition is 250 to 600 kg/ ^m3 , the blending amount of the water relative to the blending amount of the composition is 0.25 to 0.60, and the content of the water-reducing agent is 0.5 to 3.0 mass% based on 100 mass% of the composition;
Including,
In the granulated blast furnace slag, the aluminum oxide content is 8 to 14.5% by mass, the silicon dioxide content is 30.0 to 40.0% by mass, the calcium oxide content is 35.0 to 45.0% by mass, and the magnesium oxide content is 4.0 to 10.0% by mass;
The basicity of the granulated blast furnace slag is 1.55 to 1.95,
A method for increasing the compressive strength of a concrete composition, wherein the accelerator contains at least one selected from the group consisting of nitrites, nitrates, and chlorides .