JPS6227698A - Crack preventive method of cement structure - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for preventing cracks in cement structures or cement solidified bodies, and is particularly suitable for preventing cracks in cement structures or cement solidified bodies, and is particularly suitable for preventing cracks in cement from radioactive waste containing sulfate ions generated from nuclear power plants. This invention relates to a method for preventing cracking of a solidified body.

[Background of the invention]

Currently, radioactive waste generated from nuclear facilities is reduced in volume and then solidified and stored either intermediately or on land.

Here, we will specifically discuss the conventional solidification methods that have been technically established for concentrated waste liquid (mainly consisting of sodium sulfate or sodium borate) or ion exchange resins generated from nuclear power plants. Consider the problem you are having.

In recent years, concentrated waste liquid C (main component Na2so4), which is the main waste generated from BWR power plants, and slurry of used ion exchange resin have been dried and powdered to remove water, which accounts for most of the volume of radioactive waste. A method has been implemented in which this is pelletized and then solidified with a solidifying agent. According to this method, it has been confirmed that the volume can be reduced to about 1/8 compared to the conventional method of directly solidifying waste liquid or slurry with cement. Further, a method of solidifying waste by homogeneously mixing it with a solidifying material in the form of dry powder is also being considered.

Typical solidification methods include asphalt solidification, plastic solidification, and inorganic material solidification.

In plastic solidification, a thermosetting resin is used as a solidifying material, but if even water from flit is mixed into the thermosetting resin, it will not be able to exhibit the desired performance as a solidifying material. This is because if moisture is present during solidification, the curing accelerator (such as cobalt naphthenate) in the thermosetting resin will be decomposed and the thermosetting resin will not harden. This is because it exists as a liquid (liquid).

On the other hand, used ion exchange resin or sulfuric acid) IJum (
Even with careful drying, moisture may not be completely removed from Na25O4). Therefore, when solidifying plastics, used ion exchange resin or Na
If 2SO4 and thermosetting resin are mixed and solidified, it will not be possible to create a solidified product with high strength.
The current situation is that the powder dried in a centrifugal thin film dryer must be measured with a moisture content measuring device such as a neutron moisture meter, and the moisture content must be thoroughly controlled.

In asphalt solidification, the waste powder and asphalt are mixed and heated to remove moisture and solidify, so the above-mentioned moisture management is not necessary, but since asphalt has thermoplasticity.

There is a problem of fluidization at 40-50°C. It also has the disadvantage of poor fire resistance. For these reasons, it is difficult to say that the asphalt solidified body is a stable solidified body.

For solidification using inorganic materials (for example, cement solidification, vitrification, etc.), storage and disposal on land, this is a desirable method because the solidified material has good compatibility with soil and rocks and has high fire resistance. Currently, solidification methods using cement or sodium silicate (water glass, etc.) as solidification agents are being considered. The technology for pellet solidification and homogeneous solidification using cement glass (cement + alkaline silicate solution) has been established, and the same applies to cement. However, in the solidification method using inorganic materials, in the case of waste containing sulfate ions, using a hydraulic solidification material such as cement tends to cause cracks, and the solidification is not stable for a long time. Cannot be created. This is because sulfate ions act within the solidified cement, producing a compound called ettringite that has a large volumetric expansion, causing the pellet to expand due to moisture.

Due to this drawback, currently only about 10% by weight of sodium sulfate is filled in solidified cement.

Furthermore, this problem of cracks in the solidified material also applies to the solidified material that is currently being intermediately stored as cement solidified material.
There is a high risk that this will occur in the near future.

However, at present, there are no effective measures to prevent cracking of these solidified cement bodies.

[Purpose of the invention]

An object of the present invention is to provide a method for preventing the occurrence of cracks even when a cement solidified body or a cement structure contains sulfate ions, and to provide a method for solidifying radioactive waste by solidifying it with a hydraulic solidifying agent, especially cement. The purpose is to prevent the expansion and cracking of cement structures or other cement structures and to improve their soundness.

[Summary of the invention]

The method for preventing cracks in a cement structure according to the present invention is characterized by maintaining the cement structure at a temperature of 44° C. or higher and lower than 100° C. for a predetermined period of time during or after the hardening and curing period of the cement structure.

The method of the present invention can be applied not only to cement solidified radioactive waste but also to general cement concrete structures. It is used in a broad sense, including.

Furthermore, the crack prevention method of the present invention can be applied not only to cement structures being manufactured, but also to existing cement structures to prevent the occurrence of cracks.

That is, by once holding an existing cement solidified body at a temperature of 44° C. or higher for a predetermined period of time, it is possible to prevent the occurrence of cracks and to reform it into a cement solidified body that is reliable over a long period of time.

The present invention will be explained below. The components of cement, commonly called Portland cement, are a mixture of chemical species as shown in Table 1. When cement is solidified, non-hydrates and hydrates listed in Table 1 may be mixed in the solidified product.
However, essentially the hardening of cement proceeds through a water utilization reaction that creates hydrates. What should be noted here is that the system contains 4 to 8% by weight of aluminic acid (At203).
This is the source of forming ettringite, which will be described later.

Next, FIG. 3 schematically shows the hardening mechanism of cement. When water is added to cement, cement dal is produced, and when this is examined in detail using a scanning electron microscope (SEM), it can be seen that it is made up of hydrates with several distinctive shapes.

Calcium silicate hydrate, which is comprehensively represented by the symbol C-8-H and has various compositions and crystallinities, is the most abundant in terms of quantity. To,
It can be classified into fibrous, two-dimensional rope-like, three-dimensional isometric, and minute internal water bodies. That is, unwatered kneelite particles (32 in Fig. 3) exist and are densely packed in a three-dimensional equidimensional shape C-5.
-H (33 in Figure 3), coarsely distributed fibrous C-8-H
(34 in Figure 3) grows at the same time as ettringite (
35) in Figure 3 is observed to grow. In addition, monosulfate hydrate (36 in Figure 3) is present, and the outline of the cement paste includes the capillary space (37 in Figure 3). Although the growth of C-8-H is largely related to cement hardening and strength development, it is the formation of ettringite shown at 35 in FIG. 3 that is related to cement deterioration and the appearance of cracks. The capillary space (37 in Figure 3) is
As the hydration reaction progresses, the voids are filled with hydrates, ie, cement glue, so there is no problem.

Ettringite was formerly called Cementobacillus.
This is the cause of expansion and failure of concrete. On the other hand, cement has the property of shrinking during hydration and hardening, and this property is one of the causes of cracking, which is concrete's biggest drawback. Therefore, efforts are being made to develop non-shrinkage cement by making good use of ETOIJNGA-f). However, when processing radioactive waste containing sulfate ions, which is the main target of the present invention, sulfate ions are present at a much higher concentration than in normal cases, so it is rather difficult to treat radioactive waste containing sulfate ions. Cracking caused by expansion failure due to the formation of carbonite has become an important problem.

Below, we will explain why the formation of ettringite causes a large volumetric expansion by taking up a chemical reaction.

It is known that IJ guides are produced by the following chemical reaction.

3CaO-At20. +30a(OH)2+3Na2S
o4+31H20-+ 3CaO"At203'3Ca
S04'31H20+6NaOH(1) (ettringite) That is, 31-hydrate salt of ethrin is produced from calcium salt and aluminic acid. This substance contains three molecules of water of crystallization and exhibits an extremely large expansion in volume.

Therefore, the present inventors focused on aluminic acid and ettringite among the various components of cement, and performed equilibrium calculations of various chemical reactions using a combinator.

The results obtained are shown in FIG. According to this, the temperature is 44
The results show that at temperatures above 0.9°C (if the present invention is not applied) the formation of ettringite, which is thought to exist with a solidified body width of about 5% by weight, can be suppressed. In addition, a calculation result was obtained that at temperatures above 44°C, decomposition of ethrin as shown in formula (2) progresses.

3CaO'At203'3CaSO4'31H20-3
0aS04'2H20+2At(OH)3+3Ca(O
H)2+19H20(2) Figure 5 shows CaO, ht2o3
(Part of the cement components) This is a summary of the results of equilibrium calculations of the reaction system leading to each assumed chemical species. Under normal solidification conditions (25° C. or lower), the chemical reaction proceeds as shown in Equation 3.

3CaO・At20342110(OH) →3CaO
'At203'3CaSO4'31)L, 0→4CaO
-At203-19H20(3) However, if the system is kept at 44℃ or higher for a certain period of time (approximately 1 to 3 months) during the solidification period, 2A
tO(01() species to ettringite(3CaO'At
203'3CaSO4'31H20) stable 4
It was found that the transition to CaO・At203・19H20 occurred.

Also, 4CaO・At203・19H2 from ettringite
The change to 0 is an irreversible reaction.4CaO”At203・
19H20 does not change to ettringite. This is because the final product species 4CaO.At203.19H20 is chemically and thermodynamically more stable than ettringite. According to the calculation results, the generation of schedlingite can be prevented by maintaining the system at the cement solidification curing temperature of 44° C. or higher for a certain period (about 1 to 3 months).

Next, we will discuss the applicable temperature range and effects when restoring a cement solidified body using the present invention.

Conventionally, when radioactive waste containing sulfate ions, such as sodium sulfate, is solidified with cement, the filling rate of the waste is kept below 10% by weight in order to prevent volumetric expansion and destruction due to the formation of ettringite. According to the results of preliminary experiments, it is possible to fill up to 40 weight units using the present invention. Figure 6 shows the effect calculated assuming that the amount of radioactive waste containing sulfate ions is 40% by weight and 8%. Where the solidification curing temperature is 100°C, it is thought that the water added during kneading will evaporate as water vapor if operated in an open system, so the temperature should be set at 44°C or higher.
The applicable temperature range is below 0°C.

According to the basic principles of the present invention, by adding simple equipment to existing equipment, radioactive waste containing sulfate ions, which is difficult to solidify with cement, can be solidified.
Alternatively, it is possible to improve the soundness of cement solidified bodies that are estimated to develop cracks in the future, or to restore the soundness of cement structures that may come into contact with solutions containing high concentrations of sulfate ions (such as seawater). It is possible.

[Embodiments of the invention]

Example 1 In this example, sulfate ion-containing waste liquid (for example, sodium sulfate concentrated waste liquid, etc.) generated from a nuclear power plant is converted into a helette and solidified.

FIG. 1 shows a system diagram of a processing system used in this embodiment. Concentrated waste liquid (hereinafter abbreviated as waste liquid) 1 containing sodium sulfate as its main component is discharged from a nuclear power plant, and is introduced into a dry powder manufacturing machine 3 via a pulp 21 to be turned into dry powder. . The dry powder (hereinafter abbreviated as powder 6) is temporarily stored in a powder storage tank 4. Next, a predetermined amount of the pulp is introduced into the pelletizer 5 via the pulp 2□. The pelletized material is put into a drum 10.

Therefore, at the same time that cement is introduced into the drum from the tank 6 through the pulp 23, water is introduced into the drum from the tank 7 through the pulp 24. It doesn't matter if the water in tank 7 is heated beforehand and becomes hot water. Further, the cement and water in tanks 6 and 7 may be mixed in advance. In the drum, the gaps between the pellets 8 are filled with cement 4-st 9. Beret 8 and cement covering
The drum filled in the drum 9 is conveyed to a belt conveyor 11 and is conveyed to a heating constant temperature 4g12.

The heating constant temperature bath 12 includes a heater 13 and blades 15 rotated by an electric motor 14, and is maintained at a temperature of 44° C. or more and less than 100° C. When left in this heating constant temperature bath 12 for at least 3 weeks, At203 in cement yeast becomes
The stable compound 40aO' does not form ettringite.
All change to At203/19H20 (shown in Figure 5). After a predetermined period of time (approximately 1 to 3 months), the solidified material is transported from the heating constant temperature bath 12 through the outlet 16 to a temporary storage warehouse or land storage facility.

Although this embodiment has been described as starting from the waste liquid 1, for cement solidified bodies or cement structures that have already exploded, the process shown within the broken line in FIG.
The effects of the present invention can be expected by simply carrying out the step of temporarily storing or transferring to a land storage facility after holding it for a certain period of time.

Example 2 In this example, similar to Example 1, waste liquid generated from a nuclear power plant is converted into a long-term stable homogeneous cement solidified body. FIG. 2 shows the processing system of this embodiment. The waste liquid is stored in the tank 17 and introduced into the dry powder manufacturing machine 19 via the pulp 181. The powder thus pulverized is temporarily stored in a tank 20. Next, a predetermined amount of water is directly introduced into the drum 25 from the tank 20 via the valve 182. Subsequently, predetermined amounts of cement and water are introduced into the drum 25 from the tank 21 via the pulp 183 and from the tank 22 via the pulp 184, respectively. A stirring blade 24 rotated by an electric motor 23 is introduced into the drum 25, and the mixture is mixed evenly. The powder from the tank 20, the cement from the tank 21, and the water from the tank 22 may be introduced into the drum 25 sequentially, or may be introduced simultaneously. After the kneading by stirring is completed, the rotating blades 24 are removed and a uniform cement paste 26 is produced. The solidified body produced by K as described above is conveyed to a heating constant temperature bath 30 by a belt conveyor 27.

The heating constant temperature bath 30 has a rotary blade 24 rotated by a heater 29 and an electric motor 28, and the temperature is quickly raised to a predetermined temperature and then kept constant.

The temperature is maintained at 44°C or higher and less than 100°C. When left in this heating constant temperature bath 30 for at least 3 weeks, At2o3 in the cement yeast becomes ethrin.
Shown in the figure. ) all change. After a predetermined period of time (1 to 3 months), the solidified material is heated and kept at a constant temperature [30] and is transported to a temporary storage facility or a land storage facility through an outlet 31.

K, which is applied to cement solidified bodies that have already been homogeneously solidified with cement, can be expected to maintain soundness for a long period of time by going through the process surrounded by the broken line in Figure 2, as in Example 1. Can be converted into solidified cement.

By employing the systems of Examples 1 and 2 above, sulfate ion-containing waste discharged from nuclear power plants can be efficiently and stably treated and disposed of using cement.

In Examples 1 and 2, we mainly described the solidification of concentrated waste liquid (mainly composed of sodium sulfate) generated from nuclear power plants, where the effects of the present invention can be expected most, but the type of radioactive waste to be filled is concentrated. It is not limited to waste liquid, and any number of people may use it as long as it can be solidified with cement. Furthermore, it is also possible to implement the present invention using a concrete system that does not contain radioactive waste, as long as the durability of the cement structure is to be increased.

In Examples 1 and 2, currently commercially available Portland cement is used, which contains 5 to 8 g of aluminic acid (At203), which is related to ettringite production. Aluminic acid is an important compound in making cement, but if a cement with a lower aluminic acid content is used, curing at 44°C, which is the curing condition of at least 3 weeks at 44°C or more in Example IK, is necessary. It is possible to shorten the period.

In addition, if cement solidified radioactive waste is stored underground or disposed of, the temperature at the time of storage should be 44°C.
By selecting a location where the temperature is above ℃, a storage device with special heating equipment is not required.

〔Effect of the invention〕

According to the present invention, it is possible to prevent the occurrence of cracks even in cement structures containing sulfate ions. If this is applied, for example, to a solidified cement of radioactive waste containing sulfate ions, it is possible to obtain a solidified cement of high efficiency, high filling rate, and excellent durability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a radioactive liquid treatment and solidification system according to one embodiment of the present invention, and FIG. 2 is a diagram showing a radioactive liquid waste treatment and solidification system according to another embodiment of the present invention.
Figure 3 is a diagram showing a uniform solidification system, Figure 3 is a diagram schematically showing a scanning electron microscope image of hardened cement paste, Figure 4 is a diagram showing the reaction equilibrium calculation with special attention to the ettringite formation process K in cement components. Figure 5 is a flow diagram that simply shows the process of producing each component based on the equilibrium calculation results of the cement paste hardening process. Figure 6 is the applicable solidification curing temperature range in the present invention. This is a diagram showing the estimated value. 5. Explanation of symbols 1... Radioactive waste (e.g. sodium sulfate bath) storage tank, 3... Dry powder manufacturing machine, 4... Dry powder storage tank, 5... Pellet granulator, 6... Cement tank, 7... Water tank, 8... Radioactive waste, 9... Cement yeast, 10... Drum can, 12... Heating constant temperature bath, 1
3-1. Heater, Part 6...Heating constant temperature chamber outlet,
17...Radioactive waste (e.g. sodium sulfate solution)
Storage tank, 19... Dry powder manufacturing machine, 20... Dry powder storage tank, 21... Cement tank, 22... Water tank, 23... Electric motor, 24...
- Rotating blade for stirring, 25... Drum can, 26... Cement paste, 29... Heater, 30... Heating constant temperature bath, 31... Heating constant temperature bath outlet, 32... Unhydrated knee light Particles, 33...Densely packed three-dimensional uniform size C-8-H13
4... Roughly distributed L ;/c fi fa C-8-
H135...Ettringite, 36...Monosulfate hydrate, 37...Capillary space. Temperature (0C) (): Molecular weight

Claims (1)

[Claims] 1. A method for preventing cracks in a cement structure, which comprises maintaining the temperature of the cement structure at 44°C or higher and lower than 100°C for a predetermined period during or after the hardening and curing period of the cement structure. 2. The method for preventing cracks in a cement structure according to claim 1, wherein the cement structure is a cement solidified body containing radioactive waste containing sulfate ions.