JPS5830258B2 - Method for preventing iron corrosion caused by chlorides in hardened cement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing corrosion of iron caused by the presence of chlorides in hardened cement.

-Recently, corrosion of reinforcing steel in hardened cement has become a major problem.

One of the problems is thought to be due to the fact that coastal sand containing corrosive inorganic components such as sodium chloride has been increasingly used as fine aggregate due to the shortage of river sand as fine aggregate.

However, the reinforcing bars are corroded by chlorine ions and the like in the hardened cement body, and if the corrosion progresses further, cracks and ruptures occur in the hardened cement body, and as a result, reinforcement or renewal of the structure is often required.

By the way, methods for preventing the corrosion of reinforcing bars due to chlorides include: 1) Washing coastal sand as fine aggregate with water, 2) Adding an inhibitor to suppress corrosion, 3) Galvanizing the surface of reinforcing bars, 4) Thickening the concrete. Methods such as increasing the number of tiles and finishing the surface layer with tiles were taken.

However, washing the fine aggregate described above requires a large amount of water at one time, and there are also problems with wastewater treatment after washing.

In addition, it is known that the addition of alkali metal nitrites or the combined addition of nitrites and phosphates are used as agents (inhibitors) to suppress the corrosive effects of chlorides on reinforcing steel. It has to be used in a relatively large amount, and has high solubility in water, so there are concerns about pollution and toxicity.

In addition, conventional products had the disadvantage of a relatively short shelf life of only a few years.

Furthermore, in the method of applying galvanization to the surface of the reinforcing bars, corrosion-resistant reinforcing bars can be obtained, but if electrolytic corrosion occurs in the galvanizing, the adhesion between the reinforcing bars and concrete will be significantly impaired. was there.

Furthermore, the methods of increasing the thickness of the concrete or finishing the surface layer with tiles are very expensive and are not economical methods.

The present invention aims to eliminate or improve the various drawbacks of the prior art as described above, and provides an effective method for preventing corrosion of iron by chlorides in hardened cement.

That is, the present invention provides a cement composition characterized by adding at least one of fine powders of lead zinc oxide, lead cyanamide, red lead, and calcium leadate to a fresh concrete composition when producing a hardened cement body. This invention relates to a method for preventing corrosion of iron due to chlorides in a hardened body.

As is well known, concrete exhibits strong alkalinity immediately after construction, but its pH gradually decreases over time.
H tends to decrease.

Further, iron corrosion on the alkaline side is usually extremely low compared to the neutral or acidic side, but iron corrosion becomes a serious problem even in alkaline conditions when chlorine ions are present.

However, as shown in the present invention, if at least one specific lead compound is dispersed in concrete in the form of fine powder, moisture in the concrete or moisture penetrating from the outside can
Gradually the lead compounds dissolve and then react with the chloride ions to form harmless lead chloride.

The present inventors have found that the production of lead chloride is particularly noticeable in the neutral to acidic range where iron corrosion is likely to occur.

Furthermore, corrosion of iron usually occurs when in the presence of near-neutral water, one part of the iron becomes an anode and another part becomes a cathode, creating a potential difference between the two and forming an electrode battery. is known to occur.

However, when lead ions are present therein, the hydrogen overvoltage becomes large and the corrosion reaction does not proceed.

Therefore, at least one specific lead compound used in the present invention not only prevents normal corrosion of iron by increasing the hydrogen overvoltage, but also fixes chlorine ions in the hardened cement. This is a novel method that prevents corrosion of iron in hardened cement by the synergistic action of both.

The fine powder of the lead compound used in the present invention preferably has an average particle size of o, oi~50μ, particularly preferably O15~50μ.
It is at least one type of fine powder of lead zinc oxide, lead cyanamide, red lead, and calcium lead acid, each having a size of about 20 μm.

The present inventors put a large amount of various lead compounds into a beaker, then poured seawater into the beaker, and then immersed a polished steel plate to test the degree of corrosion (visual observation and corrosion protection potential measurement).

As a result, it was found that lead zinc oxide was found to be most effective in preventing corrosion of polished steel sheets in seawater, followed by lead cyanamide and calcium leadate.

That is, from these results, it is clear that the addition of the lead compound used in the present invention is effective in preventing corrosion of reinforcing bars in hardened cement with a high salt concentration.

In the present invention, the amount of the lead compound added depends on the chloride concentration in the concrete, but the amount of lead compound added depends on the chloride concentration in the concrete.
The amount is in the range of 0.05 to 5 weight, preferably 0.1 to 3 weight, based on the total of sand and cement.

Conventionally, it was empirically known that if the salt concentration in concrete was 0.1% or less, there would be no actual damage, but according to the present invention, fine powder of lead compounds in an amount equivalent to chloride is added to concrete. It has been found that if it is allowed to exist, it is possible to suppress the salinity to this level.

On the other hand, if a large amount of lead oxide is mixed into the hardened cement, the strength of concrete tends to decrease significantly, which is not preferable.

Therefore, in the present invention, it is desirable that the fine powder of the lead compound be used in an amount of up to 5% by weight relative to the concrete as described above.

Further, the solubility of lead compounds in water is usually very low.

Therefore, when carrying out the present invention, it is necessary to make the lead compound into a fine powder and uniformly disperse it in the hardened cement body.

Furthermore, if a lead compound with too large a particle size is mixed into the hardened concrete, not only will it not react uniformly with chlorine ions, but most of the generated chlorine ions will not be able to be immobilized. It is desirable to maintain it.

One of the features of the present invention is that the lead compound has low solubility in water.

That is, unlike conventional inhibitors, there is no risk of the inhibitor leaking out together with water when it is in the fresh concrete state, and therefore there is no pollution problem associated with it.

In addition, even if lead acid ions are formed in an alkaline solution and dissolved in water, they will not flow out as an aqueous solution because they will react with carbon dioxide gas in the water or air and become insoluble.

As mentioned above, the method of the present invention differs from the conventionally known method of protecting iron in the hardened cement by adding an inhibitor. It reacts directly with lead compounds, regardless of whether they are ions, and directly removes corrosive factors in hardened cement.

Therefore, unlike organic inhibitors, there is no need to add large amounts, and the strength of concrete will not be impaired.

Furthermore, it has the advantage that the effect of removing chlorine ions lasts for a very long time.

Hereinafter, the details of the present invention will be explained with reference to Examples.

"Department" or "section" is indicated by "part by weight" or "part by weight."

Examples and Comparative Examples Gravel 59 parts, No. 6 silica sand 29 parts, cement 12 parts, salt 2
part, size 0.5-1m71! cut wire iron powder 1
Add 0 parts of lead compound pre-moistened with water (average charcoal particle size 3 to 5μ) in the amount shown in Table 1, and finally knead with an appropriate amount of water to form a ready-mixed concrete composition. I got it.

The composition was filled into a 5x5x15G:rrL plywood mold and left at room temperature for 7 days to obtain a test specimen.

After visually determining the curing properties of the concrete when the plywood formwork was removed, the specimens were subjected to a salt spray test (5000 hours).

The results are shown in Table 2.

From the above comparative test results table, it is clear that the method of the present invention has sufficient concrete hardening properties and also exhibits excellent resistance to corrosion of iron by chlorine ions (including penetration from the outside). I understand.

In other words, it is clear that the lead compound of the present invention prevents corrosion of iron by immobilizing chlorine ions and increasing hydrogen overvoltage in the hardened cement body.

Claims (1)

[Claims] 1. In preparing the hardened cement, at least one of fine powders of lead zinc oxide, lead cyanamide, red lead, and calcium leadate is added to the fresh concrete composition, A method for preventing iron corrosion caused by chlorides in hardened cement. 2. The fine powder has a content of 0.2 to the concrete composition.
A method for preventing corrosion of iron by chloride in a hardened cement body as claimed in claim 1, characterized in that a weight ratio of 0.05 to 5.0 is added.