Description of the Prior Art Fly ash, a waste product resulting from the combustion of coal, is generated in an enormous amount every year. Thus, its effective disposal has been a worldwide issue.

One disposal method is to utilize the fly ash as an additive for cement after it is filtered to reduce the amount of unfired carbon from 6-10 % by weight to 3 % by weight or less. However, this filtration technique costs a great deal of money. Further, there is a tendency for users to hesitate to employ the additive because the amount of the unfired carbon is not constant in the fly ash. For these reasons, the technique is not effective in disposing of fly ash.

Alternatively, fly ash can be solidified for use as a construction material without filtration. The solidification of fly ash is conventionally carried out, as follows: 1) by adding a gypsum binder, such as slaked lime, quick lime and anhydrous gypsum; 2) by adding a cement binder; and 3) by adding alkali.

Of these, the addition of gypsum binder is disclosed in Korean Pat. Laid-Open Publication No. 85-3373 and Japanese Pat. Laid-Open Publication No. Sho 56-93555.

This method is advantageous in easily curing fly ash by using slaked lime and quick lime etc,. But, the hardened bodies obtained lack in water resistance and strength so that they cannot be used as general construction materials.

The addition of cement binders are described in Korean Pat. Laid-Open Publication Nos. 86-1754 and 87-3951 and Japanese Pat. Laid-Open Publication No. Sho. 56-61883.

The molded bodies obtained by adding the cement binders show good strength and water resistance but, since fly ash has a tendency to hinder the curing, there are problems in that the workability with the cement binders is seriously poor and fly ash can only be used to a certain extent.

The curing of fly ash by alkali addition is disclosed in Korean Pat. Publication No. 93-5252 and Korean Pat.

Appln. No. 91-19795, both to the present inventor.

According to these patents, the hardened bodies produced are far superior in strength and water resistance.

However, their production cost is relatively high, making it difficult for them to be generally used as construction materials. Further, there is indicated a problem in that strong alkali makes an inferior working environment.

Besides, fly ash may be used as a soil conditioner because it contains large amounts of the trace elements necessary for the growth of plants, such as Ca, Mg and Fe, in addition to SiO2 and is alkaline with an acidity of 8- 10.

It is reported in many documents that when fly ash is applied at a proper amount to soil, the yield of crops

is improved, as representatively described in an article entitled "Study on the utilization of fly ash as soil conditioner", yielded on May, 1994 by Korean Electronic Current Technology Institute. Based on research for various crops including rice, corn, beans and apples for three years, the article asserts that fertilizing using manure with fly ash at a proper amount results in improving the yield of the crops at an extent of about 20 %.

However, there are the following two problems in applying fly ash to soil, as mentioned in the article.

First, since fly ash is very fine, it is scattered in the wind, which contaminates the surrounding air.

During a dry period, the fly ash is blown away.

In addition, three years after being placed in the soil, the fly ash is partially cured by itself, deteriorating water- and air penetration of soil.

SUMMARY OF THE INVENTION Therefore, it is an objective of the present invention to overcome the above problems encountered in prior arts and to provide an economically favorable method for producing hardened bodies with fly ash which are useful as construction materials for general use and are neither blown away by wind nor cured spontaneously so that they can be used as an effective soil conditioner.

In accordance with the present invention, the above objective could be accomplished by a provision of a method for producing hardened bodies with fly ash, which comprises mixing fly ash and sea water or 3 wt% aqueous NaCl solution (hereinafter referred generally to as "sea water") in a ratio of 72:28 to 55:45 and sintering the

mixture at a temperature of 650 to 1,200"C.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objectives and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: Fig. l(a) is a photograph showing turf grown in humus soil added with the hardened bodies according to the present invention; and Fig. l(b) is a photograph showing turf grown in an ordinary humus soil.

DETAILED DESCRIPTION OF THE INVENTION Most fly ash in the current generation is buried in ash ponds near thermoelectric power plants. For its burial, sea water is pumped up and mixed with fly ash and the mixture is carried through pipes into the ash ponds.

The present invention is economically very favorable because the mixture of fly ash and sea water carried, as it is, is utilized without addition of any binder or additive. What is necessary is to maintain the ratio of fly ash to sea water in a range of 72:28 to 55:45. For example, if the amount of sea water is less than the range, the hardened bodies of the present invention are difficult to mold and are short in strength. On the other hand, if the amount of sea water is more than the range, excess salt may remain in the resulting hardened bodies.

Instead of sea water, aqueous NaCl solution may be used as mentioned above. In this case, it is possible to

raise the concentration of salt to saturation but its absolute concentration in the mixture must be about 3 % by weight. For example, when the salt is 9 % by weight in concentration, the aqueous solution should be used one third-fold.

After being molded into desirable shapes, the mixture is sintered at a temperature of about 650 to 1,200"C, to produce hardened bodies. They are found to be very light in addition to being superior in water resistance and strength. They are relatively light with a gravity ranging from 1.2 to 2.0. In order to lighten the hardened bodies, well-known foaming methods can be applied. For example, the mixture is added with a little amount of a CMC aqueous solution and aluminum powder, expansion aged, and sintered to produce ultra light hardened bodies with a gravity of about 0.3 to 0.8. For easier foaming, sodium hydroxide may be added at an amount of 3 % by weight.

When being applied for construction materials, the foamed hardened bodies may be used as light aggregates.

For agriculture, they may used as carriers which absorb the effective ingredients of agricultural chemicals and manures and slowly release them. In this case, the effective ingredients of the agricultural chemicals or manures are dissolved in water and then, impregnated in the hardened bodies which are, in turn, dried.

In order to confer higher strength and water resistance to the hardened bodies, waste glass powder may be added at an amount of 10 weight parts or less based on 100 weight parts of the mixture of fly ash and sea water and preferably at an amount of 2 to 7 weight parts. The addition of waste glass powder may reduce the amount of sea water without lessening the desired strength.

A better understanding of the present invention may

be obtained in light of following examples which are set forth to illustrate, but are not to be construed to limit, the present invention.

EXAMPLES I THROUGH IV AND COMPARATIVE EXAMPLES I AND II Fly ash and sea water were mixed together as indicated in Table 1 below. The resulting mixtures were placed in a mold with a dimension of 5cm x 5cm x 5 cm, compression molded under a pressure of 25 kg/cm2 and dried. Thereafter, the molded bodies are sintered at a temperature of 1,050"C for 30 min. in a furnace and then, slowly cooled, to produce hardened bodies.

The hardened bodies show physical properties as shown in Table 2 below.

TABLE 1 Examples Fly Ash Sea Water (% by weight) C. Exmpl I 75 25 Example I 72 28 Example II 70 30 Example III 60 40 Example IV 55 45 C. Exmpl II 50 50

TABLE 2 Compression Unit Weight Examples Strength (kg/cm2) (kg/cm3) Note C. Exmpl I 40 1350 not mixed Example I 80 1320 Example II 95 1320 Example III 130 1280 Example IV 145 1260 C. Exampl II 65 1255 not molded Taken together, the data suggested in Tables 1 and 2 show that, when the amount of sea water is below 28 % by weight, the mixing is not well accomplished, which leads the obtained bodies to be lacking in strength while, when the amount of sea water is above 45 % by weight, the mixtures are so lacking in viscosity that they are difficult to mold.

EXAMPLE V To 100 weight parts of a mixture of fly ash and sea water having the same composition as Example III, 10 weight parts of 2 % CMC aqueous solution, 0.005 weight parts of aluminum powder and 0.5 weight parts of sodium hydroxide were added. Then, the resulting mixture was molded into a desirable shape, foamed and dried.

Sintering was carried out in the same manner as Example 3, to produce a hardened body. It was found to be 0.4 in gravity and 105 kg/cm2 in compression strength.

EXAMPLE VI A mixture of fly ash and sea water having the same composition as Example III was sintered in a rotary kiln whose inside temperature was 700"C, to produce globular hardened bodies with an average diameter of 2.5 mm. They were added to ordinary humus soil at an amount of 20 % by weight and turf was grown in the resulting soil for six months. For a control, turf was grown in the ordinary humus soil alone.

To test the turfs for viability in the absence of water, watering was not executed for ten days. About 15 % of the sample turf was viable whereas only 3 % of the control survived.

Fig. l(a) is a photograph after the surviving sample turf was re-grown by watering while Fig. l(b) is for the surviving control turf.

When germination rates were compared, the turf of Fig. l(a) was higher than that of Fig. l(b) by about 30 %. For growth speed, the turf of Fig. l(a) was better than that of Fig. l(b) by about 50 %.

According to this example, the hardened bodies of the present invention have an effect of promoting the germination and the growth of herbage when being used in soil at proper amounts. In particular, the hardened bodies contribute a great deal of viability to herbage during a draught by virtue of their high ability to absorb water.

As described hereinbefore, the hardened bodies according to the present invention can be obtained at low cost and are superior in lightness, water resistance and strength so that they can be used as construction materials for general uses. In addition, the rich

minerals in the fly ash are those necessary to grow plants, making the hardened bodies play a role of soil conditioner. Therefore, the enormous amount of fly ash generated every year can be effectively reused according to the present invention. Thus, the ash pond whose construction is indispensably required for the construction of thermoelectric power plants and requires an immense cost can be little or not constructed.

The present invention has been described in an illustrative manner, and it is to be understood the terminology used is intended to be in the nature of description rather than of limitation.

Many modifications and variations of the present invention are possible in light of the above teachings.

Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.