KR102357971B1 - Curing method of cement comprising alkali activated slag - Google Patents

Abstract

A method for curing a cement composition according to an embodiment of the present invention comprises the steps of: preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator; A first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more; and a second curing step of curing the firstly cured cement composition in a carbon dioxide atmosphere of 3 to 4 bar.

Description

Curing method of a cement composition comprising alkali activated slag {CURING METHOD OF CEMENT COMPRISING ALKALI ACTIVATED SLAG}

One embodiment of the present invention relates to a curing method of a cement composition containing alkali activated slag. More specifically, an embodiment of the present invention relates to a curing method of a cement composition comprising alkali activated slag using carbon dioxide.

Greenhouse gas emissions, including carbon dioxide, are one of the causes of global warming, which increases the frequency and extent of global natural disasters. Portland cement's cement clinker production accounts for about ₄ % of global CO2 emissions (2017 statistics) and is considered a serious problem in the construction industry.

The use of CO ₂ to accelerate the curing of cement-based materials is known as one of the methods to lower the concentration of CO ₂ in the atmosphere. Calcium silicate, such as alite and belite of Portland cement, is spontaneously carbonated to produce mainly calcium carbonate (CaCO ₃ ). Since CaCO ₃ is deposited in the pores of the cast mortar and concrete during the CO ₂ curing process, the strength of the cement-based material cured at an early stage of CO ₂ is rapidly increased. The densification of the pores refines the microstructure, including the cement paste matrix and interfacial transition zone, to obtain higher strength in a shorter time.

Also, it is known how to reduce CO ₂ emissions by using alternative cement to replace Portland cement. Calcium sulfoaluminate cement has the advantage that the calcination process can be performed at a low temperature. Compared with the calcination of Portland cement, the calcination temperature is reduced by about 100 to 200 °C, which contributes to a reduction in CO ₂ emission. Also, alkali activated binders containing geopolymers synthesized from industrial by-products such as blast furnace slag and fly ash are promising alternatives. Blast furnace slag also contains a large amount of calcium silicate. Hydration is susceptible to carbonization, potentially requiring alkaline activators. The alkali activator allows calcium to dissolve from the slag particles, and calcium participates in the formation of the CSH gel, which contributes to the development of strength.

An embodiment of the present invention is to provide a method for curing a cement composition containing alkali activated slag. More specifically, an embodiment of the present invention is to provide a curing method of a cement composition containing alkali activated slag utilizing carbon dioxide.

A method for curing a cement composition according to an embodiment of the present invention comprises the steps of: preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator; A first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more; and a second curing step of curing the firstly cured cement composition in a carbon dioxide atmosphere of 3 to 4 bar.

A method for curing a cement composition according to another embodiment of the present invention comprises the steps of: preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator; A first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more; and a second curing step of curing the firstly cured cement composition in an atmosphere containing carbon dioxide at a concentration of 10 to 20%.

After the second curing step, a third curing step of curing in an atmosphere of relative humidity of 75% or more may be further included.

The fine powder of blast furnace slag may include SiO ₂ : 30 to 33 wt%, Al ₂ O ₃ : 12 to 14 wt%, CaO: 43 to 48 wt%.

The alkali activator may include NaOH and Na ₂ SiO ₃ .

The weight ratio of the alkali activator to the fine blast furnace slag powder (alkali activator/fine blast furnace slag powder) may be 0.4 to 0.45.

The first curing step and the second curing step may be performed at a temperature of 21 to 25 °C, respectively.

The first curing step may be performed for 0.5 to 2 hours.

The second curing step may be performed for 3 to 168 hours.

The second curing step may be performed in an atmosphere of 90% by volume or more of carbon dioxide.

The second curing step may be performed under a pressure of 3 to 4 bar.

The second curing step may be performed in an atmosphere of 65 to 75% relative humidity.

After the second curing step, the cement composition may contain 1.4 to 8% by weight of calcite.

After the second curing step, the cement composition may include amorphous calcium carbonate.

According to an embodiment of the present invention, it is possible to contribute to the environment by removing carbon dioxide, the main culprit of the greenhouse effect, during the curing process.

According to one embodiment of the present invention, it is possible to shorten the time taken for curing.

According to an embodiment of the present invention, the strength of the cured cement can be improved.

1 is an XRD pattern of blast furnace slag fine powder.
2 is a table summarizing the curing method and compressive strength measurement time of each sample in Examples.
3 to 5 are graphs of compressive strength after 4 hours, 24 hours, and 7 days, respectively.
6 is a graph of pressure loss measured in a pressure vessel during CO ₂ curing.
7 is a graph showing the improvement of compressive strength of samples taken from additional moisture curing after CO ₂ curing.
8 to 12 are XRD patterns of samples.
13 to 17 are TG and derivative TG (DTG) curves of alkali activated slag paste before and after CO ₂ curing.

The terms first, second and third etc. are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. As used herein, the meaning of "comprising" specifies a particular characteristic, region, integer, step, operation, element and/or component, and the presence or absence of another characteristic, region, integer, step, operation, element and/or component It does not exclude additions.

Although not defined otherwise, all terms including technical terms and scientific terms used herein have the same meaning as those commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, are not interpreted in an ideal or very formal meaning.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art can easily implement them. However, the present invention may be embodied in several different forms and is not limited to the embodiments described herein.

A method for curing a cement composition according to an embodiment of the present invention comprises the steps of: preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator; A first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more; and a second curing step of curing the firstly cured cement composition in a carbon dioxide atmosphere of 3 to 4 bar.

A method for curing a cement composition according to another embodiment of the present invention comprises the steps of: preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator; A first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more; and a second curing step of curing the firstly cured cement composition in an atmosphere containing carbon dioxide at a concentration of 10 to 20%.

Hereinafter, the curing method of the cement composition will be described in detail for each step.

First, a cement composition is prepared.

The cement composition includes fine powder of blast furnace slag and an alkali activator. As a cement composition, fine powder of blast furnace slag and an alkali activator are widely known, and an appropriate one can be selected and used.

The fine powder of blast furnace slag serves to generate CSH hydrate that contributes to strength development in the cement composition. The fine powder of blast furnace slag may include SiO ₂ : 30 to 33 wt%, Al ₂ O ₃ : 12 to 14 wt%, CaO: 43 to 48 wt%. The fine blast furnace slag powder may further include various oxides in addition to the aforementioned SiO ₂ , Al ₂ O ₃ , and CaO. For example, at least one of Na ₂ O, K ₂ O, MgO, MnO, TiO ₂ , SO ₃ , P ₂ O ₅ , Fe ₂ O ₃ , BaO, ZrO ₂ , V ₂ O ₅ , SrO and Y ₂ O ₃ . may further include. They may further include 5 wt% or less, respectively. More specifically, the fine powder of blast furnace slag may include SiO ₂ : 31 to 32 wt%, Al ₂ O ₃ : 13 to 14 wt%, CaO: 45 to 48 wt%.

The alkali activator serves to dissolve ions such as Ca and Al from fine powder particles of blast furnace slag in the cement composition. The alkali activator may include NaOH and Na ₂ SiO ₃ .

In this case, NaOH may exist in the form of an aqueous solution dissolved in water. Specifically, it may be added in the form of an aqueous solution of 1 to 10 M.

Na ₂ SiO ₃ may be added in a liquid form. The weight ratio of Na ₂ SiO ₃ to NaOH in the alkali activator (Na ₂ SiO ₃ /NaOH) may be 0.5 to 2.0. When an appropriate ratio is added, the fine powder of blast furnace slag can be properly alkali-activated. More specifically, the weight ratio of Na ₂ SiO ₃ to NaOH in the alkali activator (Na ₂ SiO ₃ /NaOH) may be 0.75 to 1.5. At this time, the weight with respect to NaOH is calculated as the weight of the whole aqueous solution.

The weight ratio of the alkali activator to the fine powder of the blast furnace slag may be 0.42 to 0.45. Through the above-mentioned weight ratio, the fine powder of blast furnace slag can be appropriately alkali-activated. More specifically, the weight ratio of the alkali activator to the fine powder of the blast furnace slag may be 0.43 to 0.44.

The cement composition thus prepared can be poured into a mold.

Next, the cement composition is cured in an atmosphere of relative humidity of 75% or more. By performing primary curing in a high-humidity atmosphere before secondary curing in a carbon dioxide atmosphere, the specimen can be demolded from the mold in a short time. If the humidity in the first curing step is too low, demolding may be difficult in a short time, and also, drying shrinkage may occur. More specifically, the relative humidity of the first curing step may be 80 to 85%.

The first curing step may be carried out at a temperature of 21 to 25 ℃. By being able to be cured in the above temperature range, high temperature curing for alkali activation is not required. More specifically, it may be carried out in the range of 23 to 25 ℃.

The first curing step may be performed for 0.5 to 2 hours. By curing for the time described above, it may be possible to remove the specimen from the mold within a short time. More specifically, it may be carried out for 0.5 to 1.5 hours.

After the first curing step, the mold may be removed from the cured cement composition.

Next, the first-cured cement composition is second-cured in a carbon dioxide atmosphere.

In one embodiment of the present invention, it is cured in a carbon dioxide atmosphere of high pressure of 3 to 4 bar, and in another embodiment of the present invention, carbon dioxide is cured in a carbon dioxide atmosphere containing 10 to 20% concentration. As such, by curing in a carbon dioxide atmosphere, the cement composition absorbs some carbon dioxide during the curing process, thereby removing carbon dioxide, the main culprit of the greenhouse effect.

First, an embodiment of curing in a high-pressure carbon dioxide atmosphere of 3 to 4 bar will be described. By curing in a high-pressure carbon dioxide atmosphere, the strength can be improved in a short time, and a large amount of carbon dioxide can be absorbed. More specifically, it can be cured in a carbon dioxide atmosphere of 3.5 to 4 Bar. When the pressure is lowered by carbon dioxide absorption, carbon dioxide may be added to keep it constant.

The second curing step may be performed at a temperature of 21 to 25 °C. By curing in the above temperature range, it is possible to continuously maintain the temperature of the first curing step. More specifically, it may be carried out in the range of 23 to 25 ℃.

The second curing step may be performed for 3 to 168 hours. By curing for a long time, carbon dioxide absorption may be increased, but the strength of the cured cement may be reduced. Specifically, it can be cured for 3 to 24 hours.

In the second curing step, it may be carried out in an atmosphere of 90% by volume or more of carbon dioxide. By curing in such a pure carbon dioxide atmosphere, it is possible to increase the amount of carbon dioxide absorption. More specifically, it may be carried out in an atmosphere of 99 to 99.9% by volume of carbon dioxide.

Next, another embodiment of curing carbon dioxide in an atmosphere containing 10 to 20% concentration will be described. By curing in the atmosphere described above, the strength of the cement composition can be improved. When exhaust gases are used without a CO2 capture system, the maximum applicable concentration of 20% can be utilized. More specifically, it may be cured in an atmosphere containing 15 to 20% by volume of carbon dioxide.

The second curing step may be performed at a temperature of 21 to 25 °C. By curing in the above temperature range, it is possible to continuously maintain the temperature of the first curing step. More specifically, it may be carried out in the range of 23 to 25 ℃.

The second curing step may be performed for 3 to 168 hours. By curing for a long time, the strength of the cured cement can be improved. Specifically, it can be cured for 24 to 180 hours.

The second curing step may be performed under a pressure of 3 to 4 bar. By curing under the above-mentioned pressure, there is an advantage in that the pressure decrease with time is reflected. More specifically, it may be carried out under a pressure of 3.5 to 4 bar.

The second curing step may be performed in an atmosphere of 65 to 75% relative humidity. Unlike an embodiment of the present invention, strength can be further improved in an appropriate humid atmosphere. More specifically, the second curing step may be performed in an atmosphere of 70 to 75% relative humidity.

As described above, by secondary curing in a carbon dioxide atmosphere in one embodiment and another embodiment of the present invention, the cement composition may contain 1.4 to 8% by weight of calcite. When calcite is properly formed, it can contribute to strength improvement.

In particular, in the case of another embodiment of the present invention, amorphous calcium carbonate is formed during the secondary curing process, which can further contribute to strength improvement.

After the second curing step, a third curing step of curing in an atmosphere of relative humidity of 75% or more may be further included. In the tertiary curing step, by supplying moisture, calcium silicate can be hydrated, thereby further improving strength. More specifically, the relative humidity of the third curing step may be 75 to 85%.

The third curing step may be performed for 20 to 144 hours.

Hereinafter, the present invention will be described in more detail through examples. However, these examples are only for illustrating the present invention, and the present invention is not limited thereto.

Preparation Example: Preparation of Blast Furnace Slag Fine Powder

A fine powder of blast furnace slag (ground-granulated blast-furnace slag, GGBFS, Hyundai Steel) was prepared. The components of the fine blast furnace slag powder were analyzed using an X-ray fluorescence analyzer, and are summarized in Table 1. The unit in Table 1 is weight %. Other included BaO, ZrO ₂ , V ₂ O ₅ , SrO and Y ₂ O ₃ .

SiO ₂ Al ₂ O ₃ CaO Na ₂ O K ₂ O MgO MnO TiO ₂ SO ₃ P ₂ O ₅ Fe ₂ O ₃ Others 30.76 13.26 47.97 0.23 0.54 3.06 0.52 0.87 1.81 0.01 0.61 0.25

As summarized in Table 1, it can be confirmed that lime and silica are the main components of the fine powder of blast furnace slag. 1 shows the XRD pattern of the fine powder of blast furnace slag. It was confirmed that it was consistent with the general pattern of fine powder of blast furnace slag.

Example

The alkali activator was prepared by mixing NaOH deionized water with a concentration of 5M and liquid Na ₂ SiO ₃ . The mixing ratio of NaOH deionization and Na ₂ SiO ₃ was 1.0 by weight. The ratio of the fine blast furnace slag powder and the alkali activator was mixed in a weight ratio of 0.4.

Table 2 below shows the total mixing ratio.

binder (g) Activator (g) Activator/Binder
(weight ratio) Blast Furnace Slag Fine Powder 5M NaOH Na ₂ SiO ₃ 2,400 480 480 0.4

After mixing with a planetary mixer for 4 minutes, the paste was cast into a 25 mm square mold. Each sample was first cured in a mold at 85% relative humidity in air at 25° C. for 1 hour. Thereafter, the mold was removed and the initial pressure was adjusted between 3 and 4 bar, and 99.9% by volume of pure CO ₂ was put into a pressure vessel. Curing in a CO ₂ pressure vessel for 3 hours, 23 hours, and 167 hours, respectively. Samples cured for 167 hours were injected with CO ₂ in the middle. The pressure in the CO ₂ pressure vessel was measured using a digital manometer.

After secondary curing in high-pressure CO ₂ , the third curing was performed in air at 85% relative humidity and 25°C. Samples cured in a CO ₂ pressure vessel for 3 hours, 23 hours, and 167 hours, respectively, were named CO2P-T1, CO2P-T2, and CO2P-T3, respectively.

For comparison, after the first curing, a comparative example (Control) was prepared in which the first curing was performed in air at 85% relative humidity and 25° C. for 1 hour.

In addition, after the first curing, 20 vol% CO ₂ concentration, 70% relative humidity and 25 ℃ was cured for 167 hours. It was named CO2-HC.

Each sample was measured for compressive strength after 4 hours, 24 hours and 7 days after mixing. The compressive strength was obtained by measuring the average of the measured values by measuring each of four samples. The measured compressive strength is summarized in Table 3 below.

After measuring the compressive strength of each, the crushed specimen was finely pulverized and then immersed in isopropanol to stop hydration. After vacuum drying, XRD and TG analysis were applied to the powder samples. XRD measurements were performed using a high-power X-ray diffractometer (Rigaku Corp.) with an incident beam of Cu Kα radiation for a 2θ scanning range of 5-60°. The XRD pattern of the sample was analyzed with ICD (International Center for Diffraction Data) PDF-2 database and Inorganic Crystal Structure Database (ICSD). TG measurements were performed in nitrogen gas at a heating rate of 10 °C/min from room temperature to 950 °C using an SDT Q600 thermal analyzer (TA Instruments, Inc.).

In FIG. 2, the curing method and compressive strength measurement time of each sample are briefly indicated.

note Compressive strength (standard deviation), MPa 4 h 24 h 7 days Control 14.78 (1.33) 65.56 (1.88) 85.77 (0.88) CO2P-T1 18.27 (1.1) 55.72 (1.14) 80.62 (2.99) CO2P-T2 - 53.47 (2.09) 69.11 (2.6) CO2P-T3 - - 61.96 (0.48) CO2-HC 24.81 (1.96) 81.77 (2.86) 111.89 (1.88)

As shown in Table 3 and Figure 3, it can be confirmed that the compressive strength of CO2P-T1 and CO2-HC increases compared to Control after 4 hours. However, as shown in Table 3, Figures 4 and 5, when the curing time in high-pressure CO ₂ is long, it can be confirmed that the strength is rather inferior compared to the control. On the other hand, it can be seen that the curing in CO ₂ 20% by volume (CO2-HC) has excellent compressive strength regardless of the curing time.

6 shows the pressure loss measured in the pressure vessel during the CO ₂ curing. The CO ₂ pressure decreased over time, while the initial pressure was controlled to 3 to 4 bar as described above. CO ₂ was consumed over time and was not depleted. For CO2P-T3, additional CO ₂ was fed after approximately 24 hours.

Continuous moisture curing after CO ₂ curing results in hydration of calcium silicate, further increasing the strength of the cement material.

7 shows the degree of improvement in compressive strength of samples taken from additional moisture curing after CO ₂ curing. First, when CO2-HC samples were consistently treated at 70% relative humidity (RH), it can be estimated that carbonation and hydration proceeded simultaneously. The CO2-HC sample achieved the highest strength of 111.89 MPa after 7 days, whereas the control sample reached 85.77 MPa after 7 days.

The strength increased through additional hydration of the samples cured in high pressure CO ₂ (CO2P-T1), but the increase was lower than that of the control samples. The intensity of the control sample exceeded that of CO2P-T1 at 24 h and 7 days, but the intensity at 4 h was lower than that immediately after CO ₂ curing.

As shown in FIG. 7 , it was confirmed again that long-term CO ₂ curing (CO2P-T2 for 23 hours and CO2P-T3 for 167 hours) in high-pressure CO ₂ was not effective. Their compressive strength was lower than that of the control sample. Additional moisture curing for the CO2P-T2 samples was also ineffective.

8 to 12 show the XRD patterns of all samples.

It can be confirmed that calcite, calcium silicate hydrate (CSH), hydrotalcite and calcium alumina silicate hydrate (CASH) are present in NaOH/Na ₂ SiO ₃ -activated slag.

13 to 17 show TG and derivative TG (DTG) curves of alkali activated slag paste before and after CO ₂ curing. As clearly identified in the DTG curve, the first weight loss below 200 °C represents the loss of bound water by dehydration of CSH (50-200 °C) and CSH(I) (90-110 °C). Weight loss in the range of 600 to 700 °C indicates the presence of calcite

The amount of calcite can be roughly estimated based on weight loss in the range of 600 to 700 °C. The calcite concentration in each sample is summarized in Table 4 below.

note Curing time (wt%) 4 h 24 h 7 days Control 1.33% 1.51% 2.09% CO2P-T1 1.43% 2.90% 4.07% CO2P-T2 - 3.64% 3.93% CO2P-T3 - - 2.76% CO2-HC 8.37% 6.96% 2.27%

On the other hand, in the case of CO2P-T2 and CO2P-T3 samples ( FIGS. 15 and 16 , respectively), weight loss was observed in the range of 520 to 580 °C, and it was confirmed that it was related to the decomposition of vaterite. The difference in strength development according to the curing time in the sample (FIG. 7) appears to be due to the formation of vaterite (metastable CaCO ₃ ). Among the samples cured in high pressure CO ₂ , samples for 23 hours or 167 hours showed the presence of vaterite, except for the samples cured for 4 hours CO ₂ . Therefore, it indicates that the curing time of CO ₂ affects the formation of polymorphism of CaCO ₃ . As shown in FIG. 15 , the additional curing reduced the DTG peak of vaterite and increased the peak of calcite on day 7 . Vaterite can readily recrystallize to calcite when exposed to moisture.

The CO2-HC sample after 24 hours also showed a weight loss in the range of 450 to 550 °C. The weight loss in the range of 450 to 550 °C appears to be due to the decomposition of amorphous calcium carbonate (CaCO ₃ xH ₂ O), but not vaterite. This can be confirmed from the peak of vaterite detected in the XRD results shown in FIG. 12 . Amorphous CaCO ₃ appears to be decomposed at a temperature between 245 °C and 645 °C.

The present invention is not limited to the embodiments, but can be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can use other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that this may be practiced. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (14)

preparing a cement composition comprising fine powder of blast furnace slag and an alkali activator;
a first curing step of curing the cement composition in an atmosphere of relative humidity of 75% or more;
a second curing step of curing the first-cured cement composition in an atmosphere containing carbon dioxide and having a total atmospheric pressure of 3 to 4 bar; and
3rd curing step of curing in an atmosphere of relative humidity of 75% or more;
including,
The second curing step is a curing method of a cement composition that is performed in an atmosphere of 90% by volume or more of carbon dioxide. delete delete According to claim 1,
The blast furnace slag fine powder is SiO ₂ : 30 to 33% by weight, Al ₂ O ₃ : 12 to 14% by weight, CaO: A curing method of a cement composition comprising 43 to 48% by weight. According to claim 1,
The alkali activator is a curing method of a cement composition comprising NaOH and Na ₂ SiO ₃ . The method of claim 1,
The weight ratio of the alkali activator to the fine powder of the blast furnace slag is 0.42 to 0.45. According to claim 1,
The first curing step and the second curing step are each carried out at a temperature of 21 to 25 ℃ curing method of the cement composition. The method of claim 1,
The first curing step is a curing method of the cement composition is carried out for 0.5 to 2 hours. According to claim 1,
The second curing step is a curing method of the cement composition is performed for 3 to 168 hours. delete delete delete The method of claim 1,
After the second curing step, the cement composition curing method of the cement composition comprising 1.4 to 8% by weight of calcite. delete

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