JP5270145B2 - Method for producing hardened surface layered cement - Google Patents

Abstract

An apparatus for carbonation curing is provided which can be kept at any desired carbon dioxide concentration exceeding 20%, preferably exceeding 30%. The apparatus for carbonation curing has a shielded space where a cured cement object is placed and shielded from the outside atmospheric environment. The shielded space is equipped with: a gas introduction opening which is connected to a carbon dioxide supply source through a gas flow rate regulation mechanism; and a gas discharge opening which is connected to the outside through a gas inflow prevention mechanism. The gas flow rate regulation mechanism and the gas inflow prevention mechanism can be switched so that the mode of controlling the internal atmosphere can be any of at least a 'gas replacement mode' and a 'stationary mode.' This apparatus is especially suitable for use in the carbonation curing of a cured object formed from a kneaded cement mixture containing low-heat portland cement and ?C2S, the amount of the ?C2S being 30±10 parts by mass per 100 parts by mass of the binder, and having a water/binder ratio of 35% or lower.

Description

The present invention relates to a method of manufacturing a hardened cement body surface is densified using a carbonation curing equipment for performing the carbonation curing of the hardened cement paste using carbon dioxide.

  In the hardened cement typified by concrete and mortar, when carbon dioxide in the atmosphere enters from the surface and carbonate ions react with cement components, the region becomes neutral. This neutralization causes cracking of the hardened cement body, and in reinforced concrete, when the neutralization region becomes deeper, it also causes corrosion of the reinforcing steel. For this reason, it is generally considered that neutralization should be suppressed, and various measures for reducing neutralization have been studied.

On the other hand, the inventors applied forced neutralization (hereinafter referred to as “carbonation”) with carbon dioxide gas to cementitious materials containing γC ₂ S (γ-2CaO · SiO ₂ , also called γ belite). We have been studying structural changes in the surface layer. As a result, it was found that γC ₂ S does not react with water but reacts with carbonate ions, and the reaction product slightly expands to fill the voids of the cement matrix to form a dense structure. Using this method, it is possible to provide a dense and chemically stable layer near the surface of concrete and mortar, and to suppress the leaching of calcium ions from the cement matrix and the entry of chloride ions etc. from the outside. A cement-based material having excellent durability is realized (Patent Documents 1 to 3).

  As a curing device capable of carbonating a hardened cement body, Patent Document 4 describes a device including a container in which the hardened body is stored and sealed.

JP 2006-348465 A JP 2006-182583 A JP 2007-22878 A JP 2006-143531 A

  Conventionally, in a curing room (neutralization curing tank) that performs carbonation curing, it is regarded as a practical limit to maintain the carbon dioxide gas concentration at about 20%. On the other hand, since the curing apparatus of Patent Document 4 performs carbonation curing in a sealed state, curing in a high-concentration gas having a carbon dioxide concentration of 100% (so-called “bag curing”, see FIG. 4) is possible. However, when the carbon dioxide concentration is 100% and when the conventional curing room (neutralization curing tank) is used and the carbon dioxide concentration is 20%, the carbonation depth of the concrete obtained after curing for 4 weeks There is no significant change between the voids (FIG. 4), and the progress of neutralization is equivalent (paragraph 0034). Even if the carbon dioxide gas concentration is 100%, the progress of neutralization is equivalent to the case of 20% because the curing method disclosed in Patent Document 4 is curing in a sealed container (bag curing). It is assumed that this is the cause. That is, a predetermined carbon dioxide gas concentration is maintained while replenishing carbon dioxide in the conventional curing room, but basically no carbon dioxide is replenished in the bag curing. Patent Document 4 does not describe means for keeping the carbon dioxide gas concentration in the sealed container constant during the curing period.

  Up to now, there has been no systematic research report on the carbonation behavior of hardened cement in the carbon dioxide concentration region exceeding 20%, and there remains room for study to develop a hardened hardened cement. . Recently, in order to further consider environmental problems and improve the durability of reinforced concrete, there is an increasing demand for concrete with improved resistance to calcium leaching and ion intrusion.

The present invention has been made in view of such a situation, using a practical carbonation curing equipment can be maintained at any concentration of the carbon dioxide concentration exceeds 20%, the surface layer dense with improved durability than conventional An object of the present invention is to provide a hardened cementitious body.

In order to achieve the above object, the present invention has a shielding space that contains a cemented body and shields it from the external atmospheric environment, and the shielding space has a carbon dioxide gas supply source via a gas flow rate adjusting mechanism. There are a gas inlet connected to the gas outlet and a gas outlet connected to the outside via a gas inflow prevention mechanism. The gas flow rate adjustment mechanism and the gas inflow prevention mechanism set the control mode of the internal atmosphere to at least one of the following X and Y Thus, a carbonation curing facility that can be switched can be used .
Mode X: A mode in which carbon dioxide gas is introduced into the shielded space from the gas inlet and gas in the shielded space is discharged to the outside from the gas outlet (gas replacement mode).
Mode Y: Mode in which the carbon dioxide concentration in the shielded space is controlled to a steady state in a state where the average introduction flow rate of carbon dioxide gas is smaller than that in the gas replacement mode (steady mode)
The gas discharge port can be further connected to the outside via a carbon dioxide trap mechanism.

When this carbonation curing equipment is used, the surface layer portion of the cement hardened body can be carbonated in an atmosphere having a carbon dioxide concentration of 20 to 90% by volume, preferably 30 to 90% by volume. In that case, it was found that the cement matrix in the surface layer portion was greatly densified even in the case of a hardened body of a cement kneaded material having a general composition not containing γC ₂ S. Moreover, in the hardened body of a cement kneaded material having a specific composition containing γC ₂ S based on low heat Portland cement, particularly when carbonized in an atmosphere having a carbon dioxide gas concentration of 50 to 90%, the curing period is relatively short. It became clear that remarkable surface densification can be realized.

As a method for producing such a surface densified cement hardened body, placing the cured product of the cement kneaded product, the closed space where the carbon dioxide concentration is shielded from the atmosphere environment is maintained in the range of 20 exceeds 90% By this, the manufacturing method of the surface layer densified cement hardening body which carbonates the surface layer part of the hardening body is provided.

  More specifically, when carbonizing and curing the hardened body of the cement kneaded material, a gas inlet connected to a carbon dioxide supply source via a gas flow rate adjusting mechanism and a gas outlet connected to the outside via a gas inflow prevention mechanism The cemented body in the shielded space is placed by placing the hardened cement that has been removed in the shielded space, introducing carbon dioxide from the gas inlet to the shielded space, and discharging the gas in the shielded space from the gas outlet. After passing through the mode (gas replacement mode) to raise the carbon dioxide concentration at the position where the cured body is placed to over 20 to 90%, adjust the amount of carbon dioxide introduced from the gas inlet and the amount of gas discharged from the gas outlet. Then, a mode (steady mode) for controlling the carbon dioxide concentration in the shielded space to a steady state with the average introduction flow rate of carbon dioxide being smaller than that in the gas replacement mode is performed. And the method of manufacturing a surface densified cement hardened body to carbonation in an atmosphere of carbon dioxide concentration 20 exceeding 90% of the surface layer portion of the hardened cement is provided. The above “steady state” does not necessarily require the carbon dioxide concentration to be constantly constant, and may vary within a certain range (for example, target concentration (%) ± 10 (%)).

In particular, the kneaded mixture contains low heat Portland cement and γC ₂ S so that the amount of γC ₂ S is 30 ± 10 parts by mass and the water binder ratio is 35% or less with respect to 100 parts by mass of the binder. By setting the carbon dioxide concentration range to a range of 50 to 90% with respect to the cured product, extremely remarkable densification of the surface layer portion is realized. The binder preferably contains 40% by mass or more of low heat Portland cement, and may additionally contain admixtures such as fly ash and silica fume as the binder. The water binder ratio can be managed in the range of 30 ± 5%, for example. As for γC ₂ S, for example, those having a Blaine specific surface area of about 1500 to 8000 cm ² / g can be used. Particularly preferred are those having a Blaine specific surface area of 1500 to 2000 cm ² / g immediately after dusting. The manufacturing method of the surface-densified cement hardening body of this invention can be implemented suitably by using the said carbonation curing equipment.

According to said carbonation curing equipment, carbonation curing can be performed by setting the carbon dioxide gas concentration to an arbitrary concentration exceeding 20%. This makes it possible to study various carbonation behaviors of hardened cement in a high carbon dioxide concentration region, which has not been known so far, and contributes to the development of a new highly durable cement-based material. According to the manufacturing method of the surface densified cement cured product of the present invention, those having a very densified surface layer portion is obtained, and since the carbonation region is formed shallowly, which reinforced concrete high It is extremely useful for durability. Such a remarkable effect by carbonation can be sufficiently realized without increasing the carbon dioxide gas concentration to nearly 100%. Therefore, the carbonation curing equipment of the present invention does not need to be configured with a particularly highly sealed container, It is highly practical in terms of cost.

FIG. 1 schematically illustrates the configuration of a carbonation curing facility that can be suitably used in the practice of the present invention.
The carbonation curing equipment of the present invention has a shielded space 3 that is shielded from the atmospheric environment 1 by the shield 2. The shield 2 is made of a material that can prevent the intrusion of the atmosphere from the atmospheric environment 1 into the shielded space 3 and the outflow of carbon dioxide from the shielded space 3 to the atmospheric environment 1, and the shielded space 3 side has a heat resistance up to about 80 ° C. It is preferable that the material has properties. Moreover, it is desirable to have a structure excellent in heat retention and moisture retention. In the shielded space 3, the hardened cement body 4 that is a living body is accommodated in a demolded state. The cement hardened body 4 is disposed so that the surface that needs to be carbonized is exposed to the gas atmosphere in the shielding space.

  A gas introduction port 11 for introducing carbon dioxide gas is provided in the shielding space 3. The gas inlet 11 is connected to the carbon dioxide supply source 5 by a gas flow path 21. The carbon dioxide gas supply source 5 is a device that can supply carbon dioxide-containing gas at a pressure higher than atmospheric pressure. For example, a tank containing a carbon dioxide gas cylinder or carbon dioxide-containing gas can be adopted, and gas can be supplied as necessary. A pumping mechanism can be built in. A gas flow rate adjusting mechanism 6 for adjusting the flow rate of carbon dioxide gas is interposed between the carbon dioxide gas supply source 5 and the gas inlet 11. As the gas flow rate adjusting mechanism 6, it is preferable to use a valve capable of adjusting the flow rate, but it may be a two-stage valve. Since carbon dioxide is heavier than air, it is desirable that the installation position of the gas inlet 11 be higher than the installation position of the cement hardened body 4.

  Further, a gas exhaust port 12 for exhausting the gas in the space to the outside is provided in the shielded space 3. The gas discharge port 12 is connected to the outside by the gas flow path 22, but the gas inflow prevention mechanism 7 is interposed in the middle. The gas inflow prevention mechanism 7 prevents the outside air from flowing into the shielded space 3 through the gas flow path 22, and for example, a check valve or an on-off valve can be used. When the on-off valve is used, gas inflow can be prevented by controlling the on-off valve to be “closed” when the introduction of the carbon dioxide gas from the gas inlet 11 is stopped. . In order to quickly and efficiently replace the air in the shielded space 3 with carbon dioxide gas in the gas replacement mode described later, it is desirable that the installation position of the gas discharge port 22 be higher than the installation position of the cement hardened body 4.

  A carbon dioxide trap mechanism 8 is interposed in the gas flow path 22 connected from the gas discharge port 12 to the outside as necessary. As the carbon dioxide trap mechanism 8 having a simple structure, for example, a device that allows gas to pass through an aqueous calcium hydroxide solution can be employed.

In this carbon dioxide gas curing facility, it is desirable to provide a carbon dioxide concentration measuring device 13 for measuring the carbon dioxide concentration in the shielded space 3. It is desirable to set the position of the sensor or the detection gas sampling port so that the carbon dioxide gas concentration at the position in the shielding space 3 corresponding to the installation height of the hardened cement body 4 can be monitored. Furthermore, it is desirable to provide a temperature adjuster 14 that controls the temperature of the gas in the shielded space 3, and it is also desirable to provide a humidity adjuster 15 that controls the humidity. The example illustrated in FIG. 1 is a type that takes in the gas in the shielded space 3 and returns the gas adjusted to an appropriate temperature and humidity to the shielded space 3. The temperature adjuster 14 and the humidity adjuster 15 are shown in FIG. Is an integrated temperature / humidity adjuster.

The gas flow rate adjusting mechanism 6 and the gas inflow preventing mechanism 7 can be implemented by switching between the following X and Y modes by operating in cooperation with each other.
Mode X: A mode in which carbon dioxide gas is introduced into the shielded space 3 from the gas inlet 11 and the gas in the shielded space 3 is exhausted to the outside from the gas outlet 12 (gas replacement). mode)
Mode Y: Mode in which the carbon dioxide concentration in the shielded space 3 is controlled to a steady state with the average introduction flow rate of carbon dioxide being smaller than that in the gas replacement mode (steady mode)

  The “gas replacement mode” of mode X is a mode in which the gas in the shielded space 3 is expelled from the gas discharge port 12 by a gas supply pressure larger than the atmospheric pressure generated in the carbon dioxide gas supply source 5 basically. That is, since the internal gas is replaced in a state where the pressure in the shielded space 3 is not significantly different from the atmospheric pressure, this method is used when the carbon dioxide concentration in the shielded space 3 is adjusted within a range of approximately 90% or less. Suitable for Although it is difficult to raise the carbon dioxide concentration to 100%, the object of the present invention can be sufficiently achieved even when the carbon dioxide concentration is about 80% as described later.

  When an on-off valve is used for the gas flow rate adjustment mechanism 6 and a check valve is used for the gas inflow prevention mechanism 7, carbon dioxide gas is introduced by opening the on-off valve of the gas flow rate adjustment mechanism 6. The air existing inside the shielded space 3 is discharged to the outside by pushing the check valve open by itself, and the air inside the shielded space 3 is replaced with carbon dioxide. If the on / off valve of the gas flow rate adjusting mechanism 6 is “closed” when the carbon dioxide gas concentration in the shielded space 3 exceeds the set value, the check valve is also “closed”, and the mode X is changed from the “gas replacement mode”. The mode Y can be shifted to the “steady mode”. Alternatively, by switching the opening of the valve of the gas flow rate adjusting mechanism 6, it is possible to shift to the “steady mode” so that a small amount of carbon dioxide gas is continuously introduced while the check valve is kept “open”. When an on-off valve is used instead of the check valve, the on-off valve of the gas flow rate adjustment mechanism 6 and the on-off valve of the gas inflow prevention mechanism 7 need to be linked. In this case, for example, both use solenoid valves, and both are set to “open” by the gas flow rate control device 31 to set “gas replacement mode”, and then both are closed to introduce and discharge gas. Can be switched to the “steady mode” by stopping the gas flow or by allowing the gas to flow in a state in which the external gas does not flow from the on-off valve of the gas inflow prevention mechanism 7 by reducing the opening degree of both.

  The “steady mode” of mode Y is a process in which the carbonation curing is advanced while the carbon dioxide gas concentration in the shielded space 3 is kept substantially constant. The carbon dioxide gas in the shielded space 3 is consumed by the carbonation reaction of the cement hardened body 4 and inevitably decreases by leaking from the shielded space 3 to the outside. For this reason, if the on-off valves of the gas flow rate adjustment mechanism 6 and the gas inflow prevention mechanism 7 are both left in the “closed” state, the carbon dioxide gas concentration gradually decreases. Therefore, in order to maintain the “steady mode”, carbon dioxide needs to be replenished on the way. The replenishment method includes a method in which the on-off valve of the gas flow rate adjusting mechanism 6 and the on-off valve or check valve of the gas inflow prevention mechanism 7 are simply operated to repeatedly open and close, and the opening of both valves is reduced. And a method of continuously introducing a small amount of carbon dioxide gas. In any case, the gas flow rate adjusting mechanism 6 and the gas inflow preventing mechanism 7 are controlled so that the carbon dioxide concentration in the shielded space 3 is monitored by the carbon dioxide concentration measuring device 13 so that the carbon dioxide concentration within a certain range is maintained. It is desirable to control the operation in conjunction with each other.

Next, the manufacturing method of the surface layer densified cement hardening body using said carbonation curing equipment is demonstrated. First, after placing the cement kneaded material, the mold is removed after a predetermined time, and the demolded cement hardened body 4 (mortar or concrete) in the course of hardening is placed in a place covered with the shield 2. In the case of a precast product, the shielded space 3 is constructed after moving the product into the shield 2 installed in a factory or the like so that the surface requiring carbonation is exposed to the atmospheric gas. That's fine. Also in the case of the cement hardened body 4 placed on site, it is possible to construct the shielding space 3 by devising the installation method of the shielding body 2 (for example, covering the structure with the sheet-like shielding body 2). is there.

After the shielding space 3 is constructed, the “gas replacement mode” is performed. That is, the valve of the gas flow rate adjusting mechanism 6 is set to “open”, and if the on / off valve is used for the gas inflow prevention mechanism 7, the valve is also set to “open”, so that the carbon dioxide is introduced into the shielded space 3 from the gas inlet 11. While introducing gas, the air which existed in the shielding space 3 is expelled outside from the gas discharge port 12 using the carbon dioxide introduction pressure from the carbon dioxide supply source 5. “Gas replacement mode” until the carbon dioxide concentration in the shielded space 3, particularly in the portion where the cemented body 4 is disposed, exceeds a predetermined concentration set in the range of more than 20 to 90%, preferably more than 30 to 90%. To continue. The carbon dioxide concentration can be accurately grasped by monitoring with the carbon dioxide concentration measuring device 13. Note that contain low heat Portland cement and γC ₂ S, γC ₂ S amount is 30 ± 10 parts by mass with respect to the binder 100 parts by weight, subject the cured product of the cement kneaded product water binder ratio is 35% or less In this case, it is possible to form a very dense surface layer portion by setting the carbon dioxide gas concentration in the range of 50% or more.

  After a predetermined carbon dioxide gas concentration is reached, the operation proceeds to the “steady mode”. In order to maintain this mode, it is necessary to replenish carbon dioxide. As described above, the replenishment method is a method in which the on-off valve of the gas flow rate adjusting mechanism 6 and the on-off valve or check valve of the gas inflow prevention mechanism 7 are simply operated so as to repeatedly open / close, or both valves A method in which a small amount of carbon dioxide gas is continuously introduced by narrowing the opening degree may be adopted. Although the carbon dioxide gas concentration may vary somewhat, in order to obtain the desired densified surface layer in a short period of time, it is important to control the carbon dioxide gas concentration necessary for it.

In the “steady mode”, it is desirable to control the temperature and humidity of the atmosphere. The higher the curing temperature, the faster the cement hydration reaction rate, and the higher the carbonation temperature, the higher the rate of reaction with carbonate ions in the case of blending γC ₂ S. Carbonation curing is performed at a high temperature. High strength and high density of the surface layer can be realized at an early stage. Specifically, it is desirable to control the temperature to 25 ° C. or higher, and it is more effective to control it to 40 ° C. or higher. However, since it is often difficult to realize curing at a very high temperature, it is usually performed in a range of 70 ° C. or less. It is desirable to control the humidity to 40-60% RH. Within this range, the carbonation rate tends to be greatest.

  The carbonation curing period may be usually in the range of 7 to 28 days. That is, the “steady mode” is completed within this period, and the carbonation curing can be terminated. According to curing under high-concentration carbon dioxide gas, the surface layer is remarkably densified, so the progress of carbonation is saturated in a relatively short period of time.

Using the materials shown in Table 1, a mortar kneaded material having the composition shown in Table 2 was prepared. γC ₂ S was used as it was immediately after dusting by gradual cooling using an industrial raw material of limestone and silica that was calcined at 1450 ° C. for 20 minutes in an actual kiln. The mortar A in Table 2 is a mortar of JIS R5201-1997, and is a cement 1, standard sand 3, and a water binder ratio of 50% by mass. Mortar B is obtained by adding γC ₂ S in an amount corresponding to 30% by mass of the binder to the above-mentioned mortar A by fine aggregate replacement. Mortar C uses a binder based on low heat Portland cement, and γC ₂ S in an amount corresponding to 30% by mass of the binder is added by fine aggregate replacement to obtain a water binder ratio of 30% by mass. . Note that γC ₂ S is regarded as an inert powder in terms of blending and is not handled as a binder.

For these three types of mortar, the target air amount was 5%. Regarding the mortar flow, mortars A and B did not define target values, and mortar C was 225 ± 40 mm in mortar flow. The mixing method is basically the same for all three types of mortar. Using a 10L mortar mixer, add the binder, γC ₂ S, and water in that order, and add the fine aggregate at low speed for 3 minutes. The mixture was kneaded at a low speed for 2 minutes and then at a medium speed for 2 minutes for a total of 7 minutes to obtain a kneaded product.

  After each kneaded material is placed in a mold, it is subjected to moisture curing (temperature 20 ° C., humidity 90% RH or more) for 1 day, then demolded, and hardened cement made of mortar (40 × 40 × 160 mm) Got. This was cured in water at 20 ° C. for 1 day, followed by carbonation curing for 5 days, for a total curing period of 7 days. Here, the carbonation depth is proportional to the square root of the elapsed time, and it is confirmed that the accuracy is also high, so the carbonation curing period is relatively short for the purpose of grasping the tendency at an early stage. 5 days.

  Carbonation curing was performed by a carbonation curing facility having the configuration shown in FIG. Specifically, a constant temperature and humidity chamber is used as the shield 2, a carbon dioxide cylinder is used as the carbon dioxide supply source, an open / closed two-stage switching type electromagnetic valve is used as the gas flow rate adjusting mechanism 6 and the gas inflow prevention mechanism 7. . The carbon dioxide concentration measuring device 13 is installed so that the gas in the constant temperature and humidity chamber corresponding to the shielded space 3 is collected and the carbon dioxide concentration is constantly measured, and the gas flow rate is adjusted based on the measured value of the carbon dioxide concentration. A gas flow rate control device 31 that controls the solenoid valve of the mechanism 6 and the gas inflow prevention mechanism 7 to be simultaneously switched to “open” or “closed” was installed. Carbonation curing was carried out by placing the cement hardened body in this constant temperature and humidity chamber so that the entire surface of 40 × 160 mm was exposed to carbon dioxide gas. First, in the “gas replacement mode”, the air inside the constant temperature and high humidity tank was replaced with carbon dioxide gas to quickly form a carbonation atmosphere of a predetermined concentration. After that, the “steady mode” was entered, and carbonation was carried out for 5 days. Carbonation curing conditions were based on a temperature of 40 ° C. and a humidity of 50% RH, and for mortar C, a test was performed in which the temperature was varied in the range of 10 to 70 ° C. and the humidity was 30 to 90%. The carbon dioxide concentration was varied in the range of 0-80%. Here, the carbon dioxide gas concentration of 0% is obtained by water curing at 40 ° C. instead of carbonation curing. In the “steady mode”, the carbon dioxide concentration was controlled within a range of a predetermined concentration ± 2% by volume. For example, what is displayed as a carbon dioxide concentration “50%” means that the carbon dioxide concentration was performed under the control of 48 to 52%. About the cement hardening body after carbonation processing, it used for the test which measures the following items.

[Carbonation depth]
With respect to the fracture surface of the 40 × 40 × 160 mm hardened cement body, a region in which vivid reddish purple coloring due to phenolphthalein does not occur is determined as a carbonation region by a method in accordance with JIS A1152-2002. The carbonation depth was measured. FIG. 2 schematically shows the state of coloring with phenolphthalein on the fracture surface and the carbonation depth measurement method. The measurement in the cross section covers only the side surface excluding the placement surface and the bottom surface, and further measures 20 mm near the center excluding 10 mm close to the placement surface and the bottom surface in 0.1 mm units with a caliper at a 5 mm pitch. The average value was adopted.

FIG. 3 shows the relationship between the carbon dioxide concentration and the carbonation depth at a temperature of 40 ° C. and a humidity of 50% RH. In the mortars A and B, the carbonation depth increased in proportion to the power of the carbon dioxide concentration in the region where the carbon dioxide concentration exceeded 20%. On the other hand, in mortar C containing a predetermined amount of γC ₂ S based on low heat Portland cement, when the carbon dioxide gas concentration is increased, the depth of the carbonation region generated in a certain carbonation curing period decreases. It was seen.

Regarding the influence of humidity, it is said that 40-60% RH has the largest carbonation rate based on the results of previous studies at a carbon dioxide concentration of 5-10%, but the same tendency is observed at carbon dioxide concentrations exceeding 20%. It was observed. The same applies to mortars B and C containing γC ₂ S. That is, even in the mortar containing γC ₂ S, it is considered that the carbonation reaction is rate-determined by the supply rate of the carbon dioxide gas from the pore water and the dissolution of the carbon dioxide gas.

[Porosity]
By the mercury intrusion method, the porosity was measured in the vicinity of the surface from the surface of the carbonation region to 2 mm. FIG. 4 shows the relationship between the carbon dioxide concentration and the porosity at a temperature of 40 ° C. and a humidity of 50% RH. In any mortar, when the carbon dioxide gas concentration increased, the porosity decreased and the cement matrix became denser. rC any mortar A typical composition not blended with ₂ S, the carbon dioxide concentration of 80%, be densified to a porosity close to the mortar B blended with rC ₂ S was confirmed. However, with regard to mortar C, a very remarkable densification phenomenon occurred in a high carbon dioxide concentration region exceeding 20%. This mortar C contains γC ₂ S based on low heat Portland cement. However, especially in the region of carbon dioxide concentration of 50% or more, the porosity is remarkably reduced in comparison with mortar B containing γC ₂ S. Since the effect is seen, the specific densification behavior of mortar C is not simply due to the blending of γC ₂ S, but is brought about by an appropriate combination of cement type and water binder ratio Conceivable. Although the mechanism is not yet elucidated at present, it is presumed that the type of hydrate such as the production of a large amount of vaterite has an effect.

Detailed examination using the above carbonation curing equipment, the present inventors have, as seen in the example of mortar C, contain low heat Portland cement and rC ₂ S, rC respect binder 100 parts by ₂ It has been found that a cement kneaded material having an S blending amount of 30 ± 10 parts by mass and a water binder ratio of 35% or less can be markedly densified by carbonation curing particularly at a carbon dioxide concentration of 50% or more. . Moreover, such a hardened body of cement kneaded material has a thin carbonation region as shown in FIG. 3 described above, and carbonation does not proceed deeply into the hardened body. This is considered because the surface layer remarkably densified is formed at an early stage. Therefore, it is considered that the concrete obtained by carbonizing the hardened body of the cement kneaded material under a high carbon dioxide gas concentration can maintain excellent durability for a long time against corrosion of the reinforcing bar. It is considered that the concrete structure can be rationalized by reducing the cover thickness.

The figure which showed typically an example of the structure of the carbonation curing equipment applicable to this invention. The figure which showed typically the measuring method of carbonation depth. The graph which illustrated the relationship between carbon dioxide gas concentration and carbonation depth. The graph which illustrated the relationship between the carbon dioxide gas concentration and the porosity of the cement hardened body surface layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Atmospheric environment 2 Shielding body 3 Shielding space 4 Cement hardening body 5 Carbon dioxide supply source 6 Gas flow rate adjusting mechanism 7 Gas inflow prevention mechanism 11 Gas inlet 12 Gas outlet 13 Carbon dioxide concentration measuring device 14 Temperature adjuster 15 Humidity adjuster 21, 22 Gas flow path 31 Gas flow control device

Claims (2)

A hardened body of cement kneaded material containing low heat Portland cement and γC ₂ S, the blending amount of γC ₂ S is 30 ± 10 parts by weight with respect to 100 parts by weight of the binder, and the water binder ratio is 35% or less. A method for producing a surface-densified cement hardened body in which the surface layer portion of the hardened body is carbonated by placing it in a shielded space that is shielded from and maintained in a carbon dioxide concentration range of 50 to 90%. Containing low thermal portland cement and γC ₂ S, 30 ± 10 parts by weight rC ₂ S amount to the binder 100 parts by weight, carbonation curing of the water-binder ratio of cement kneaded product is less than 35% Curing In this case, the demolded cement cured body is placed in a shielded space having a gas inlet connected to the carbon dioxide supply source via the gas flow rate adjusting mechanism and a gas outlet connected to the outside via the gas inflow prevention mechanism. In addition, carbon dioxide gas is introduced into the shielded space from the gas inlet and the gas in the shielded space is discharged to the outside from the gas outlet, thereby setting the carbon dioxide concentration at the cement hardened body arrangement position in the shielded space to a predetermined concentration of 50 to 90%. After passing through the mode to increase the gas (gas replacement mode), the carbon dioxide gas introduction amount from the gas inlet and the gas discharge amount from the gas outlet are adjusted, and the average introduction flow rate of carbon dioxide is adjusted to the gas By carrying out a mode (steady mode) for controlling the carbon dioxide gas concentration in the shielded space to a steady state in a state smaller than the replacement mode, the surface layer portion of the hardened cement body is in an atmosphere having a carbon dioxide gas concentration of 50 to 90%. A method for producing a hardened surface layer-densified cement to be carbonated.