Process for preparing solidified material containing coal ash

Abstract: A process for preparing a solidified material containing a coal ash comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, a second step of molding the mixture to obtain a molded article, and a third step of subjecting the molded article obtained in the second step to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. In the first step, 40 to 95 parts by weight of the coal ash is mixed with 60 to 5 parts by weight of the calcium compound, and the coal ash to be used has a bulk density of 0.8 g/cm³ or more, an average particle diameter of 5 to 40 µm and an aluminum content of 35% by weight or less in terms of Al₂O₃. By the use of a blowing agent or the like, it is also possible to obtain a porous light-weight solid. In consequence, there can be obtained the solid containing the coal ash and having a high mixing ratio of the coal ash, an excellent dimensional stability to water, a high strength and a decreased unevenness of the strength, and this kind of solid can be applied to many fields as building materials, construction materials, artificial aggregates and the like. Furthermore, a laminated solid having not only the above-mentioned characteristics but also weight reduction properties can also be obtained, and this solid can be suitably applied to building materials such as panels, blocks, bricks and boards for accoustic use, artificial light-weight aggregates and the like. Accordingly, the coal ash which is an industrial waste can be effectively utilized in large quantities, so that the conventional hard and troublesome problem of treating the coal ash can be solved or relieved.

Description: BACKGROUND OF THE INVENTION (i) Field of the Invention The present invention relates to a process for preparing a solidified material containing a coal ash as a main component which can be suitably used as building materials such as panels for building, construction materials such as roadbed materials and crushed stones, artificial aggregates, and the like. (ii) Description of the Related Art In thermal power plants and various factories utilizing coal as an energy source, a coal ash such as a fly ash is produced in large quantities. Its amount is about 4,000,000 tons a year in Japan, and only about 40% of such a large amount of the coal ash has been effectively utilized and its rest, i.e., about 60% of the coal ash has presently been disposed of. However, it is not always easy to secure a disposal place for the coal ash, and owing to the compensation of fishery rights and the establishment of a recycle law, to find the disposal place is more and more difficult. Therefore, the effective utilization of the coal ash which will further increase in the future is an urgent theme. Heretofore, it has been suggested that the coal ash is utilized as one of inorganic building materials, as disclosed in Japanese Patent Application Laid-open Nos. 17247/1988 and 305044/1992. As a peculiar example, it has been suggested that the coal ash is utilized as one material for artificial fish gathering places, as shown in a publication "Construction of an artificial submarine mountain with a coal ash", Nippon Kogyo Shinbun K.K., Jan. 26, 1993. Of these suggestions, in the case that the coal ash is utilized as a construction material such as the material for the artificial fish gathering places and as the building material, the use of a large amount of the coal ash can be expected. By the way, in the case that the above-mentioned coal ash is utilized as a construction material such as the material for the artificial fish gathering places or as the building material, the coal ash is usually mixed with cement and then used in either case. However, since a hydration reaction is utilized, the mixing ratio of the coal ash is not always high, though the coal ash is used as the main material. If it is intended to heighten the mixing ratio of the coal ash up to 40% by weight or more, there is a problem that the strength of an obtained solid deteriorates, and for this reason, the utilization of the coal ash is at most 40% by weight. Furthermore, in this case, a large amount of the cement is used, and so there is another problem that the dimensional stability of the solid in water is poor. This can be understood to be due to a fact that alite (3CaO·SiO 2 ), belight (2CaO·SiO 2 ) and the like are contained in a cement clinker, and the hydration reaction is carried out between these components and the coal ash to produce a calcium silicate hydrate gel (C-S-H) and calcium hydroxide, but since the rate of this hydration reaction is slow, the unreacted alight, belight and the like remain in large quantities in the solid. In addition, a large amount of calcium hydroxide is produced by the hydration reaction, but this hydrate is considered to have a bad influence on the dimensional stability. Here, "the dimensional stability of the solid in water is poor" means that a volume change of the solid in dry-wet states is large. A technique can also be contrived in which a composition of the coal ash and the cement is cured in an autoclave, but when the 40% by weight or more of the coal ash is mixed, there is a problem that the strength of the solid cannot be sufficiently exerted even by the autoclave curing. Furthermore, in the case of the building material, weight reduction is required in addition to the enhancement of the strength and the improvement of the water resistance, depending upon a use purpose. Generally, for the sake of the weight reduction, the building material has been prepared by curing a cement slurry containing a blowing agent in an autoclave, and the thus obtained material is excellent in fire resistance, heat insulating properties and sound insulating properties, and it is further light-weight. Thus, this kind of material has been partially utilized as an autoclaved light weight concrete (ALC) in panels for building of houses and the like. Such a coal ash has been also partially utilized as the light-weight building material in the ALC and the like. However, its mixing ratio is not always high, and when the mixing ratio of the coal ash is heightened up to 40% by weight or more, a problem that the strength of the light-weight solid deteriorates is present. In addition, it also has a drawback that the dimensional stability to water is poor. With regard to the utilization of the coal ash solid as the ALC, since the conventional ALC has a large porosity, there is a problem that a reinforcement material such as iron reinforcing rods is necessary in the case that the coal ash is used in, for example, a floor material which requires strength. Thus, it has been desired to develop the ALC having the sufficient strength even without using any reinforcement material. In view of the above-mentioned problems of the conventional techniques, the present invention has been made, and an object of the present invention is to provide a solidified material containing a coal ash in a high mixing ratio and having a good dimensional stability to water, a high strength and a slight unevenness. Another object of the present invention is to provide a solid having weight reduction properties in addition to the above-mentioned characteristics. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article; the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm; an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 ; in the first step, 40 to 95 parts by weight of the coal ash being mixed with 60 to 5 parts by weight of the calcium compound; and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. Furthermore, according to a second aspect of the present invention, there is provided a process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article; the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm; an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 ; in the first step, 40 to 90 parts by weight of the coal ash being mixed with 60 to 10 parts by weight of the calcium compound, a blowing agent or a foaming agent and water; and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. According to the third aspect of the present invention, there is provided a process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article; the coal ash having a density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm; an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 ; in the first step, 40 to 90 parts by weight of the coal ash being mixed with 60 to 10 parts by weight of the calcium compound and water, and air bubbles being blown into this mixture; and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. According to the fourth aspect of the present invention, there is provided a process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article; the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm; an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 ; in the first step, 10 to 90 parts by weight of the coal ash being mixed with 90 to 10 parts by weight of the calcium compound, a blowing agent or a foaming agent and water to obtain two or more mixtures which are different from each other in at least one selected from the group consisting of the kinds and amounts of coal ash, calcium compound, blowing agent or foaming agent, and water; in the second step, the two or more mixtures being molded in a laminate state; and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. According to the fifth aspect of the present invention, there is provided a process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article; the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm; an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 ; in the first step, 10 to 90 parts by weight of the coal ash being mixed with 90 to 10 parts by weight of the calcium compound and water to obtain two or more mixtures which are different from each other in at least one selected from the group consisting of the kinds and amounts of coal ash, calcium compound and water, and a predetermined amount of air bubbles being blown into each of the two or more mixtures; in the second step, the two or more mixtures being molded in a laminate state; and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. In the process for preparing the solidified material containing the coal ash of the present invention, the molded article is preferably cured at 30 to 100°C, prior to the third step. Furthermore, as the above-mentioned calcium compound, there are preferably used one or more calcium compounds selected from the group consisting of calcium oxide, calcium hydroxide, calcium carbonate and calcium silicate. Here, the above-mentioned bulk density is a value measured in accordance with the procedure of JIS Z 2504. DETAILED DESCRIPTION OF THE INVENTION A process for preparing a solidified material containing a coal ash of the present invention comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, a second step of molding the mixture to obtain a molded article, and a third step of subjecting the molded article to a hydrothermal treatment. At a point of time when the coal ash and the calcium compound are mixed and then molded, the respective fine particles are mutually dispersed, and particularly in the case that a water content is suitable, they are mutually closely adhere. By the subsequent hydrothermal treatment, SiO 2 component and the like in the coal ash are reacted with a calcium component of a calcium compound such as CaO and water to mainly produce tobermorite (5CaO·6SiO 2 ·5H 2 O) and a calcium silicate hydrate gel (a C-S-H gel) between the respective fine particles and on these particles. As a result, the solid becomes a hardened material having a small porosity and containing the tobermorite crystals as a main component, the C-S-H gel, the coal ash particles and the like which mutually strongly adhere to each other. Furthermore, when a blowing agent such as a metallic aluminum powder and water are added at the time of mixing the coal ash with the calcium compound, the SiO 2 component in the coal ash, the calcium component in the calcium compound and water are reacted with each other to produce the calcium silicate hydrate gel (the C-S-H gel) and calcium hydroxide, and this calcium hydroxide, the calcium compound, water and the blowing agent are reacted to generate a gas (usually hydrogen), whereby air bubbles are formed in the mixture. In consequence, a soft structure containing water and many pores can be obtained. In this structure, it can be presumed that the C-S-H gel and the remaining coal ash particles constitute its skeleton. Even if a foaming agent is used in place of the blowing agent, a similar structure can be obtained. By the subsequent hydrothermal treatment, the C-S-H gel is reacted with the remaining coal ash particles to produce the tobermorite crystals, with the result that a porous light-weight hardened material (a light-weight solidified material) can be obtained. The porosity of the solid can be controlled to a predetermined value by suitably changing the kind and amount of blowing agent or foaming agent, whereby the strength and the weight reduction state of the solid can be adjusted. In addition, the kind and amount of coal ash and calcium compound and the amount of water to be added can also be changed in consideration of the strength of the solid and the pouring properties of the solid at the time of molding. Several kinds of mixtures can be prepared which are different from each other in the kind or amount of blowing agent or foaming agent, the kind or amount of coal ash, the kind or amount of calcium compound and the amount of water, and these mixtures can then be molded into a laminate to obtain a strong and light-weight solid (a light-weight multi-layer solid). In the preparation process of the present invention, preferable is the employment of the coal ash having a bulk density of 0.8 g/cm 3 or more, preferably 1.0 g/cm 3 or more. If the bulk density is less than 0.8 g/cm 3 , the strength of the solid cannot be sufficiently exerted and the unevenness of the strength increases. The average particle diameter of the coal ash is preferably in the range of 5 to 40 µm, more preferably 10 to 30 µm. If the average particle diameter is less than 5 µm, the sufficient strength cannot be exerted, and if it is more than 40 µm, the strength of the solid deteriorates and the unevenness of the strength increases. These reasons are not definite, but it can be presumed that the coal ash contributes as an aggregate to the strength of the solid and most of the coal ash particles having the small bulk density are hollow, which leads to the deterioration of strength and the increase in the unevenness of the strength. The coal ash having an average particle diameter less than 5 µm scarcely remains as the coal ash particles after an autoclave curing step which is the hydrothermal treatment, so that the coal ash does not contribute as the aggregate any more and so it does not lead to the improvement effect of the strength. On the other hand, the coal ash having an average particle diameter of 40 µm remains as the coal ash particles even after the autoclave curing, but it can be supposed that the particle diameter of the remaining coal ash is too large and hence the coal ash conversely functions as a defect. The particle diameter distribution of the coal ash is preferably such that 60% of the particles are present in the range of 1/4 to 4 times of the average particle diameter in order to improve the strength and to decrease the unevenness of the strength. As the coal ash which can be used, a fly ash is preferable in point of strength, but bottom ash can also be used after subjected to a grinding treatment. With regard to the composition of the coal ash, an aluminum content is preferably 35% by weight or less in terms of Al 2 O 3 , more preferably 30% by weight or less. If the aluminum content is in excess of 35% by weight, the strength of the solid may deteriorate and unevenness of the strength may increase inconveniently. This can be considered to be due to that hydrogarnet is liable to be formed in the solid by the autoclave curing, and the formed hydrogarnet functions as a defect. With regard to the other components in the coal ash, an Si content is preferably in the range of 30 to 80% by weight in terms of SiO 2 , and an Fe content is preferably 15% by weight or less in terms of Fe 2 O 3 from the viewpoint of the strength. Moreover, even if an unburned carbon content is 5% by weight or more, there is no problem at all. As the coal ash, there may be used not only the fine coal ash powder which can usually be formed but also the coal ash which can be formed by a combustion power generation system using a fluidized bed under atmospheric pressure or increased pressure. It is stated above that the mixing ratio of the coal ash should be within the range of 40 to 95 parts by weight, more preferably 70 to 90 parts by weight. However, in the case that air bubbles are formed in the solid by the use of the blowing agent, the foaming agent or the like to obtain the light-weight solid, it is desirable that the mixing ratio of the coal ash should be within the range of 40 to 90 parts by weight, more preferably 50 to 80 parts by weight. Furthermore, in the case that the light-weight multi-layer solid is prepared, it is stated that the mixing ratio of the coal ash should be within the range of 10 to 90 parts by weight, more preferably 30 to 80 parts by weight. If the mixing ratio of the coal ash is less than the lower limit of the above-mentioned range, the strength and the dimensional stability of the solid is badly affected, and if it is more than the upper limit of the range, solidification is difficult, so that a strong state as the solid cannot be maintained. Examples of the calcium compound which can be employed in the present invention include calcium oxide, calcium hydroxide, calcium silicate and calcium carbonate. These compounds can be used singly or a combination of two or more thereof. In view of the strength characteristics, the calcium compound is preferably used in the form of particles having a particle diameter of 500 µm or less. Here, examples of the calcium silicate include alight (3CaO·SiO 2 ), belight (2CaO·SiO 2 ) and the like, and various kinds of portland cements such as common portland cements containing these silicates can also be used. Among these compounds, calcium oxide, calcium hydroxide and calcium silicate are suitable from the view point of a functional effect. Additionally, an ash formed by burning shells contains calcium oxide, and so it can be used in the present invention. However, calcium sulfate such as gypsum is not suitable for the present invention, because a product other than the tobermorite will be formed. When the light-weight solid or the light-weight multi-layer solid is intended, an aluminum powder, a calcium powder or a lime chloride powder can be used as the blowing agent, but the employment of the aluminum powder is preferable. In view of the strength improvement, the amount of the blowing agent to be added is preferably in the range of 0.01 to 1% by weight based on the weight of the solid, and the particle diameter of the blowing agent is preferably in the range of 1 to 300 µm. Examples of the foaming agent which can be used in the present invention include detergents (anionic surface active agents such as sodium lauryl sulfate, alkylaryl sulfonates such as sodium dodecylbenzenesulfonate, alkylnaphthalenesulfonates and nonylphenoxydiethoxyethylsulfonates, and nonionic surface active agents such as alkylarylsulfonic acid, naphthalene alkylsulfonate, polyoxyethylene alkyl ethers, polyoxyethylene lauryl ether, fatty acid diethanol amides, polyoxyethylene lanolin alcohol ether and polyoxyethylene lanolin fatty acid esters), vegetable and animal glues, saponin, modified glue naphthalene butylsulfonate, sodium isopropylsulfonate, calcium chloride, alumina cement, other surface active agents (polyoxyethylene alkylamine ethers and the like), and protein derivatives such as gelatin and casein. The employment of the anionic or the nonionic surface active agent is preferable. The amount of the foaming agent to be used is preferably in the range of 0.01 to 1% by weight. In order to mold the mixture obtained in the first step into a predetermined shape, it is preferable from the viewpoints of the strength improvement and moldability of the solid to add a predetermined amount of water to the mixture. The thus prepared mixture is then molded into the predetermined shape by press molding, extrusion, casting, compaction molding or centrifugal compaction molding. The amount of water to be added is preferably in the range of 1 to 25% by weight for the press molding, 15 to 30% by weight for the extrusion, 20 to 45% by weight for the casting, and 15 to 40% by weight for the compaction molding with respect to weight of the solid. It is preferable to add 20 to 100% by weight of water, preferably warm water at 40°C or more to the solid, because the strength of the solid can be improved and the air bubbles can be uniformly dispersed. In this case, the molding is preferably carried out by the casting. In the case that the foaming agent is used in place of the blowing agent, the coal ash is mixed with the calcium compound, and 20 to 100% by weight of water, preferably warm water at 40°C is added to the solid to prepare the mixture (a slurry). Next, the foaming agent is added to this slurry, and the mixture is then vigorously stirred to generate air bubbles (a mix foam method). Alternatively, fine air bubbles previously formed by the use of the foaming agent may be introduced into the slurry prepared as described above (a prefoam method). In preparing the mixture, a binder and admixtures (a water reducing agent, a water retaining agent, a waterproofing agent, a superplasticizer, a shrinkage reducing agent and the like) may be added, and for the sake of the improvement of the strength of the solid, the adjustment of specific gravity and the decrease of cost, there can be added a siliceous sand, an igneous rock, a blast furnace sludge, a pearlite, scraps of the ALC, a glass fiber, a fiber and a pulp. Examples of the binder include methyl cellulose, ethyl cellulose, carboxylmethyl cellulose, hydroxyethyl cellulose and polyvinyl alcohol. Examples of the admixtures include alkylarylsulfonic acid, a high condensate of formalin naphthalenesulfonate, a co-condensate of naphthalenesulfonic acid and ligninsulfonic acid, an alkylarylsulfonate polymer, an alkylarylsulfonate, a condensate of diethylnaphthalene and formalin, a co-condensate of an olefin and maleic anhydride, a polycarbonate and a modified methylol melamine condensate. In the case that the light-weight solid or the light-weight multi-layer solid is formed by the use of the blowing agent, a foam stabilizer and the surface active agent may be added. In the case of the casting, the compaction molding or the like, the solid can be reinforced by casting the slurry into a frame in which a reinforcement such as iron reinforcing rods has previously been set up. In the case of the casting, the compaction molding or the press molding, slurries and particles having different components can be divided into several portions and then cast into the frame, thereby obtaining the solid having a laminate structure which comprises many layers of different components. In consequence, the surface of the solid can be colored, and so the thus colored solid can be applied to pedestrian plates for roads, building materials and the like in which appearance is regarded as important. If the amount of an aggregate or the porosity is continuously changed, the solid having the inclination of the strength can be obtained. The light-weight multi-layer solid comprising many layers having different porosities can be prepared by pouring the slurries having different kinds and different amounts of blowing agent into the mold in turn. A pouring time of each slurry should be within 2 hours from the viewpoints of the strength. In consideration of the strength and a water absorption, the slurry which is poured to the outer surface portion of the mold (the upper surface portion or bottom surface portion of the mold) preferably contains neither the blowing agent nor the foaming agent. It is preferable in point of the strength that the porosity distribution of the solid continuously changes, and hence regulation is preferably made so that the amounts of the blowing agent or the foaming agent in the respective slurries may continuously change. The distribution of the porosity can be mainly classified into 4 types, i.e., (1) a type in which the porosity between two layers of the solid changes in the state of a low level → a high level, (2) a type in which the porosity between the two layers changes in the state of a low level → a high level → a low level, (3) a type in which the porosity between the two layers changes in the state of a high level → a low level → a high level, and (4) a mixed type of the above-mentioned (1), (2) and (3). The above-mentioned type (2) is not suitable for an outdoor use, because the dimensional stability and the strength are slightly poorer than any other type. In addition, the solid having some layers in a direction vertical to the bottom surface can also be prepared by putting some dividers in the frame. In the case that the blend ratio of the coal ash to the calcium compound is changed every layer, it is preferable in point of the strength to continuously change the blend ratio. As described above, the soft structure having different porosities, pore diameters or pore shapes in some portions thereof in the state of the foamed solid can be obtained by molding a plurality of slurries having different porosities and components into a laminate. It can be presumed that in this structure, the C-S-H gel and the remaining coal ash particles form a skeleton. The strength of the thus foamed structure can be enhanced by the crystalline tobermorite produced by the reaction of the C-S-H gel with the remaining coal ash particles in a subsequent autoclave curing step. Finally, many pores are dispersed in a matrix containing the tobermorite as a main component, the coal ash particles and the C-S-H gel to obtain the structure in which the porosity, the pore diameter or the pore shape is different in its respective portions. In the case of the casting or the compaction molding, during pouring the slurry into the mold, vibration can be given to the mold so as to inhibit the introduction of air bubbles by the pouring, which is preferable to improve the strength and to maintain the shape of a molded article. With regard to the vibration, amplitude is preferably in the range of 0.1 to 5 mm, and oscillation frequency is preferably in the range of 500 to 50 Hz. In the process for preparing the solid according to the present invention, the molded article which has been obtained by the above-mentioned procedure is subjected to a hydrothermal treatment, but as this hydrothermal treatment, an autoclave treatment is usually carried out. It is necessary that the treatment of the temperature by the autoclave is 120°C or more, more preferably 130 to 250°C. A treatment time is preferably 2 hours or more, more preferably 5 hours or more. When the temperature of saturated vapor is 120°C, an absolute pressure is about 2 kg/cm 2 . In the case that the treatment temperature is lower than 120°C, the strength of the solid cannot be sufficiently exerted, and the dimensional stability to water also deteriorates. This can be considered to be due to a fact that the tobermorite having the strength improvement effect is not sufficiently produced and calcium hydroxide and calcium silicate which have a bad influence on the dimensional stability remains in large quantities. In this connection, the C-S-H gel also seems to take part in the deterioration of the dimensional stability. That is to say, at a treatment temperature of less than 120°C, the production of the tobermorite is insufficient, and instead the amount of the remaining C-S-H gel increases. A water content in the C-S-H gel depends largely upon whether it is in a dry or a wet state, and thus it can be presumed that the volume of the solid largely varies with the water content, so that the dimensional stability deteriorates. The reason why the dimensional stability tends to deteriorate in the case of the low mixing ratio of the coal ash can be considered to be due to a fact that calcium hydroxide, calcium silicate and the C-S-H gel remain in large quantities in the solid. It is not preferable that the autoclave treatment is carried out while the molded article is immersed in water. When the blowing agent or the like is used, it is preferable in points of the strength and the dimensional stability that the autoclave treatment is carried out after the completion of the foaming. In particular, when calcium carbonate is used as the calcium compound, the strength and particularly a freeze-thaw resistance can be improved by doing the autoclave treatment in a condition where an alkali is contained. This reason is not definite, but it can be supposed that the reaction of the coal ash with the calcium compound is accelerated in the presence of the alkali, so that a reaction product such as the tobermorite tightly adheres. Examples of the alkali include hydroxides of alkali metals (NaOH, KOH and the like), carbonates of the alkali metals (Na 2 CO 3 and the like) and ammonia. Of these compounds, the hydroxides of the alkali metals, ammonia and the carbonates of the alkali metals are preferable in this order from the viewpoint of a functional effect. Among them, the hydroxides of the alkali metals are preferable, and above all, sodium hydroxide is more preferable. An alkali content is preferably in the range of 0.001 to 20% by weight, more preferably 0.1 to 10% by weight with respect to the weight of the coal ash in the molded article. By adjusting the alkali content in this range, the solid which is excellent in strength, dimensional stability and freeze-thaw resistance can be obtained. The alkali can be introduced into the molded article by a means of adding an alkali powder or an aqueous alkali solution to the above-mentioned mixture or a means of impregnating the molded article with the aqueous alkali solution, and the latter means is more preferable. Prior to the hydrothermal treatment, the molded article can be cured at 30 to 100°C, preferably 40 to 90°C for 2 hours or more, preferably 5 hours or more, which is preferable to improve the strength of the solid. Furthermore, the hydrothermal treatment can be advantageously given to the solid removed from the mold. When the blowing agent or the foaming agent is used, a curing time is preferably set to 1 hour or more, more preferably 3 hours or more. In this case, a time taken until a semi-plastic state (which is such a solidification state that the article can be removed from the mold and can be cut with a piano wire) has been reached can be shortened, and so there is an advantage that the pores can be uniformly dispersed. Furthermore, when the blowing agent is used, the foaming can be promptly completed. As the curing technique, there are wet curing, fresh water curing, spray curing, film curing and the like, but the wet curing and the fresh water curing are preferable. Since the coal ash solid obtained by the process of the present invention has a high compressive strength, for example, 400 kg/cm 2 or more and the decreased unevenness of the strength, the solid can be applied to concrete articles such as concrete panels and concrete stakes in which the high strength is required. Furthermore, the coal ash solid has the good dimensional stability to water, it is possible to use the same in a watering place. Therefore, when the coal ash solid obtained by the process of the present invention is ground into a predetermined particle diameter by a roll crusher, a jaw crusher or the like which is usually used, the ground solid can be utilized in large quantities in many fields as aggregates such as panels for building, construction materials such as roadbed materials and crushed stones, artificial aggregates and the like. Thus, the utilization of a large amount of the coal ash is possible. In addition, the light-weight multi-layer solid obtained by the process of the present invention can be utilized as construction materials such as water-permeable pedestrian plates. In the case that the blowing agent or the foaming agent is used, the solid can possess a structure in which many pores are dispersed in a matrix containing the tobermorite as a main component, the coal ash particles and the C-S-H gel. The thus obtained solid has the high strength and the small unevenness of the strength, though it contains many pores therein. The solid is also excellent in dimensional stability to water and is so light-weight that an absolute dry bulk density is 1.0 or less in the case of the light-weight solid and 1.2 or less in the case of the light-weight multi-layer solid. The solid containing the air bubbles has excellent sound absorbing properties, sound insulating properties, heat insulating properties and non-combustibility. In general, the acoustic material which is excellent in sound absorbing properties has a high porosity and so it is low in strength (e.g., compressive strength = about 10 kg/cm 2 ). Accordingly, the solid has been presently placed in a support such as an iron frame to compensate the lack of the strength. The light-weight multi-layer solid of the present invention exerts the excellent sound absorbing properties and has the high strength, and therefore it can be used without the support. Thus, the light-weight multi-layer solid can be used to manufacture an article simultaneously having two mechanisms of a wall material and a sound absorbing material. Therefore, the number of steps for the manufacture process can be remarkably reduced, which is advantageous in point of cost. In order to use the solid as the absorbing plate, the absolute dry bulk density of the solid is preferably in the range of 0.2 to 0.5. The pores in the solid have an average pore diameter of 100 to 1000 µm, and the respective pores preferably communicate with each other. The communication of the pores can be achieved by setting the absolute dry bulk density of the solid to 0.2 to 0.5 and by adding the following surface active agent together with the blowing agent or the foaming agent to the slurry. Examples of the surface active agent include polyoxyethylene alkyl ethers and their salts, polyoxyethylene nonylphenyl ethers and their salts, alkylbenzenesulfonates, fatty acid diethanol amides, alkylsulfates, sulfonates, soaps, water glass, resins and saponin. Furthermore, it is preferable to add an air bubble stabilizer such as polyvinyl alcohol or methyl cellulose. As is apparent from the aforesaid description, according to the present invention, a solid containing a coal ash can be obtained which has a high mixing ratio of the coal ash, a good dimensional stability to water, high strength, and the limited unevenness of the strength. Thus, the solid of the present invention can be applied to many fields as building materials, construction materials, artificial aggregates and the like. Furthermore, a laminated solid having the above-mentioned characteristics, weight reduction properties, a different pore distribution and the like can also be obtained, and this kind of laminated solid can be suitably applied to building materials such as panels, blocks, bricks and acoustic plates, artificial light-weight aggregates and the like. In consequence, the coal ash which is an industrial waste can be effectively utilized in large quantities by the present invention, so that the conventional hard and troublesome problem of treating the coal ash can be solved or relieved. Hence, the effect of the present invention is extremely large. Next, the present invention will be described in more detail with reference to examples, but the scope of the present invention should not be limited to these examples. Examples 1 to 16 Fly ashes as coal ashes having a bulk density of 0.8 to 1.4 g/cm 3 and an average particle diameter of 5 to 40 µm (components: 30-80% by weight of SiO 2 , 18-35% by weight of Al 2 O 3 and 15% by weight or less of Fe 2 O 3 ) were used, and 4 kinds of calcium oxide, calcium hydroxide, calcium carbonate and common portland cement having an average particle diameter of 1 to 100 µm were used as calcium compounds. These materials were mixed in various weight ratios, and 10 to 30% by weight of water and a water reducing agent were further added thereto, thereby obtaining various kinds of mixtures. Each mixture was subjected to press molding or casting to obtain molded articles having a disc shape (diameter = 120 mm and thickness = 50 mm). In the case of the press molding, the articles were molded under a press pressure of 50 kg/cm 2 , and curing was then carried out at 60°C for 20 hours in a wet condition. In the case of the casting, a slurry was cast into a mold, and the curing was then carried out at 60°C for 20 hours in a wet condition. Next, the thus molded articles were subjected to an autoclave treatment at 120 to 270°C for 20 hours to prepare solids containing the coal ashes. Table 1 shows bulk densities and average particle diameters of the coal ashes, aluminum contents in the coal ashes, kinds of calcium compounds, blend amounts of the coal ashes and the calcium compounds, and temperatures of the autoclave treatment. In this connection, in Examples 1 and 13, the curing in the wet condition was not done. For the thus obtained solids, appearance was observed, and compressive strength and dimensional stability to water were also measured. Table 1 shows the results of these tests. The tests were carried out by the following procedures. With regard to the appearance of the respective solids, the state of damages such as cracks and the state of shape retention were observed, and the solids having the good appearance were judged to be ○, and the solids having the bad appearance were done to be X. The compressive strength was measured by setting metal plates having a diameter of 50 mm to the upper and lower surfaces of each disc, and then applying a certain pressure to the metal plates in accordance with JIS A 1108. That is to say, an average of the measured values of 10 samples was regarded as the compressive strength, and its standard deviation was regarded as unevenness. The dimensional stability was measured in accordance with JIS A 5418. That is to say, samples for the measurement having a size of 40 × 50 × 100 mm were cut out from the obtained solids, and these samples were placed in a drying oven and then dried at 60°C for 24 hours. Afterward, the samples were cooled to ordinary temperature in a desiccator and then immersed in water at ordinary temperature for 24 hours. Next, a dimensional change of each sample was measured and expressed in percent. Example Material Component (wt%) Coal Ash Amount of Coal Ash (wt%) Bulk Density (g/cm 3 ) Average Particle Diameter (µm) Amount of Al 2 O 3 (wt%) 1 95 1.1 13 21 2 90 1.3 22 30 3 85 0.8 18 26 4 85 1.4 20 23 5 80 1.0 5 29 6 80 1.1 10 30 7 70 1.2 40 18 8 70 1.1 16 18 9 75 1.0 29 35 10 75 1.1 27 25 11 70 1.3 12 21 12 70 1.3 10 20 13 80 1.1 30 24 14 80 1.1 30 23 15 40 1.3 16 20 16 70 1.2 27 22 Example Material Component (wt%) Autoclave Curing Temp. (°C) Ca Compound (kind and wt%) 1 2 (a) 3 (b) 160 2 10 (a) 180 3 15 (b) 180 4 15 (b) 180 5 20 (c) 160 6 20 (c) 160 7 15 (a) 15 (b) 140 8 15 (a) 15 (b) 140 9 20 (b) 5 (c) 150 10 20 (b) 5 (c) 150 11 20 (c) 10 (d) 140 12 20 (c) 10 (a) 140 13 20 (b) 270 14 20 (b) 200 15 30 (a) 30 (b) 140 16 30 (b) 120 Note: In the item of Ca Compound, (a) means common portland cement, (b) means CaO, and (c) means gypsum. Characteristics Example Appearance Compressive Strength (kg/cm 2 ) Dimensional Stability (%) Compressive Strength Unevenness 1 ○ 401 15 0.016 2 ○ 590 19 0.010 3 ○ 532 25 0.010 4 ○ 683 21 0.009 5 ○ 598 20 0.011 6 ○ 737 25 0.008 7 ○ 715 29 0.013 8 ○ 806 25 0.011 9 ○ 659 29 0.014 10 ○ 774 25 0.013 11 ○ 682 29 0.016 12 ○ 761 20 0.012 13 ○ 588 20 0.019 14 ○ 701 22 0.010 15 ○ 696 21 0.017 16 ○ 510 19 0.015 Examples 17 to 35 Slurries for molding were prepared under conditions that the mixing ratio of coal ashes was set to 40-90% by weight, the mixing ratio of calcium compounds was set to 60-10% by weight, and after the mixing of a blowing agent, water was added thereto in an amount of 20 to 100% by weight based on the weight of a solid content. Fly ashes having a bulk density of 0.8 to 1.4 g/cm 3 and an average particle diameter of 5 to 40 µm (components: 30-80% by weight of SiO 2 , 19-35% by weight of Al 2 O 3 and 15% by weight or less of Fe 2 O 3 ) were used as the coal ashes, and 4 kinds of calcium oxide, calcium hydroxide, calcium carbonate and common portland cement were used as the calcium compounds. Furthermore, as blowing agents, an aluminum powder and a calcium powder were used in an amount of 0.01 to 1% by weight based on the weight of the solid content. Each slurry was poured into a mold, and then foamed and cured at 40 to 90°C for 20 hours in a wet state, followed by solidification. However, in Examples 26, 30 and 31, the foaming was made at ordinary temperature without the curing. The thus obtained solids were then subjected to an autoclave treatment at 120 to 270°C for 20 hours to prepare light-weight solids containing the coal ashes. Table 2 shows bulk densities and average particle diameters of the coal ashes used in the respective examples, aluminum contents in the coal ashes, kinds of calcium compounds, blend amounts of the coal ashes and the calcium compounds, and temperatures of the autoclave treatment. For the thus obtained solids (40 × 40 × 160 mm), appearance was observed, and compressive strength, absolute dry bulk density and dimensional stability to water were also measured. Table 2 shows the results of these tests. The tests were carried out by the following procedures. The appearance was judged by the same procedure as in Examples 1 to 16. The compressive strength was measured by setting metal plates having a diameter of 50 mm to the upper and lower surfaces of each rectangular parallelepiped, and then applying a certain pressure to the metal plates in accordance with JIS A 1108, as in Examples 1 to 16. The dimensional stability was measured in accordance with the procedure of JIS A 5418. That is to say, samples were placed in a drying oven and then dried at 60°C for 24 hours, and afterward, the samples were cooled to ordinary temperature in a desiccator and then allowed to absorb water at 20°C for 3 days. After the measurement of the dimension, the samples were allowed to stand at a temperature of 20°C and a relative humidity of 60%, and when a water content in each sample was 40% or less, the dimension of the sample was measured. On the basis of the measured dimension values of each sample before and after the water absorption, a dimensional change of each sample was expressed in percent. The absolute dry bulk density of the solids was measured in accordance with JIS A 5416. Example Material Component (wt%) Coal Ash Amount of Coal Ash (wt%) Bulk Density (g/cm 3 ) Average Particle Diameter (µm) Amount of Al 2 O 3 (wt%) 17 90 1.2 15 24 18 80 1.3 10 25 19 80 0.8 11 20 20 80 1.4 13 21 21 50 1.0 5 30 22 50 1.0 15 29 23 70 1.2 40 30 24 70 1.3 25 28 25 70 1.0 24 35 26 80 0.9 10 24 27 60 1.3 30 30 28 60 1.3 29 28 29 70 1.2 26 29 30 80 1.0 11 24 31 50 1.3 7 33 32 40 1.3 16 30 33 55 1.3 18 28 34 65 1.1 22 19 35 65 1.1 24 20 Example Material Component (wt%) Autoclave Curing Temp. (°C) Characteristics Ca Compound (kind and wt%) Appearance 17 10 (a) 130 ○ 18 20 (b) 130 ○ 19 10 (a) 10 (c) 200 ○ 20 10 (a) 10 (c) 200 ○ 21 40 (a) 10 (b) 180 ○ 22 40 (a) 10 (b) 180 ○ 23 10 (a) 10 (b) 10 (c) 150 ○ 24 10 (a) 10 (b) 10 (c) 150 ○ 25 30 (c) 150 ○ 26 20 (a) 130 ○ 27 20 (a) 20 (d) 160 ○ 28 20 (a) 20 (b) 160 ○ 29 30 (b) 270 ○ 30 20 (a) 160 ○ 31 20 (a) 30 (c) 160 ○ 32 30 (a) 30 (b) 180 ○ 33 25 (a) 20 (b) 180 ○ 34 20 (a) 15 (b) 120 ○ 35 20 (a) 15 (b) 180 ○ Note: In the item of Ca Compound, (a) means common portland cement, (b) means CaO, and (c) means gypsum. Example Characteristics Absolute Dry Bulk Density Compressive Strength (kg/cm 2 ) Dimensional Stability (%) Compressive Strength Unevenness 17 1.00 230 26 0.015 18 0.90 232 22 0.008 19 0.81 164 18 0.011 20 0.79 187 16 0.012 21 0.35 41 5 0.026 22 0.36 53 4 0.024 23 0.74 144 17 0.016 24 0.72 166 12 0.014 25 0.60 100 12 0.019 26 0.85 194 22 0.013 27 0.50 71 8 0.021 28 0.51 83 6 0.017 29 0.73 142 12 0.016 30 1.00 263 29 0.013 31 0.63 99 12 0.019 32 0.30 35 3 0.029 33 0.30 39 3 0.023 34 0.45 58 5 0.026 35 0.46 70 6 0.018 Examples 36 to 49 In each example, 3 kinds of slurries were obtained in which mixing ratios of a coal ash to calcium compounds and kinds of blowing agents were different. Each slurry was obtained by mixing the coal ash with the calcium compound, further mixing a blowing agent in an amount of 0 to 0.7% by weight based on the weight of a solid content, and then adding water in an amount of 20 to 100% by weight based on the weight of the solid content. A fly ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm (components: 30-80% by weight of SiO 2 , 35% by weight or less of Al 2 O 3 and 15% by weight or less of Fe 2 O 3 ) was used as the coal ash, and 4 kinds of calcium oxide, calcium hydroxide, calcium carbonate and common portland cement were used as the calcium compounds. Furthermore, as the blowing agents, an aluminum powder and a calcium powder were used. Each of the 3 kinds of slurries was poured into a mold (bottom surface = 90 × 150 mm) to form 3 layers, and foaming and curing were then carried out at 40 to 90°C for 20 hours in a wet state, followed by solidification. In Examples 36 to 45, the thickness of the respective layers was set to 30 mm, and in Examples 46 and 47, the thicknesses of the first, second and third layers were set to 10 mm, 70 mm and 10 mm, respectively. Moreover, in Examples 48 and 49, the thicknesses of the first, second and third layers were set to 20 mm, 50 mm and 20 mm, respectively. In Examples 44 and 49, the curing was not made prior to the molding, and the foaming was carried out at ordinary temperature. The molded solids were subjected to an autoclave treatment at a temperature of 100°C or more for 20 hours to prepare light-weight multi-layer solids containing the coal ash and having 3 layers. Tables 3 and 4 show the kinds of calcium compounds, the blend amounts of the coal ash and the calcium compounds, and the amounts of the blowing agent to be added every layer. In addition, the temperatures of the autoclave treatment are also shown. For the thus obtained solids (90 × 90 × 150 mm), appearance was observed, and compressive strength and absolute dry bulk density were also measured. Tables 3 and 4 show the results of these tests. The tests were carried out by the following procedures. With regard to the appearance of the respective solids, judgement was made by the same procedure as in Examples 1 to 16. The compressive strength was measured by setting metal plates of 90 × 90 mm to the upper and lower surfaces of each rectangular parallelepiped, and then applying a certain pressure to the metal plates in accordance with JIS A 1108, as in Examples 1 to 16. The absolute dry bulk density of the solids were measured in accordance with JIS A 5416. Example Material Component (wt%) Autoclave Curing Temp. (°C) Ca Compund (kind and wt%) Coal Ash Blowing Agent 36 45 (a) 45 (b) 10 0.00 130 45 (a) 45 (b) 10 0.20 130 45 (a) 45 (b) 10 0.35 130 37 40 (a) 30 (b) 30 0.00 130 40 (a) 30 (b) 30 0.05 130 40 (a) 30 (b) 30 0.19 130 38 50 (a) 50 0.00 150 30 (a) 20 (c) 50 0.07 150 20 (a) 30 (c) 50 0.30 150 39 20 (a) 80 0.00 180 20 (a) 80 0.07 180 20 (a) 80 0.27 180 40 10 (a) 90 0.00 200 10 (b) 90 0.07 200 10 (b) 90 0.25 200 41 40 (b) 30 (d) 30 0.00 150 40 (b) 30 (d) 30 0.30 150 40 (b) 30 (d) 30 0.70 150 42 30 (a) 20 (c) 50 0.00 150 30 (a) 30 (b) 40 0.05 150 30 (a) 40 (b) 30 0.20 150 43 30 (a) 70 0.00 120 30 (b) 70 0.04 120 30 (b) 70 0.26 120 Note 1: In the item of Ca Compound, (a) means common portland cement, (b) means CaO, (c) means Ca(OH) 2 , and (d) means CaCO 3 . Note 2: In the item of Material Component, the order of pouring is denoted by upper, middle and lower steps. Example Characteristics Appearance Absolute Dry Bulk Density Compressive Strength (kg/cm 2 ) Compressive Strength Unevenness 36 ○ 1.0 310 32 37 ○ 1.2 473 29 38 ○ 1.0 482 31 39 ○ 0.9 360 25 40 ○ 0.9 250 29 41 ○ 0.9 304 32 42 ○ 1.1 498 30 43 ○ 1.1 264 28 Example Material Component (wt%) Autoclave Curing Temp. (°C) Ca Compound (kind and wt%) Coal Ash Blowing Agent 44 50 (a) 50 0.00 250 30 (a) 20 (b) 50 0.07 250 20 (a) 30 (b) 50 0.30 250 45 50 (a) 50 0.00 150 30 (a) 20 (b) 50 0.07 150 20 (a) 30 (b) 50 0.30 150 46 60 (a) 40 0.00 180 30 (a) 30 (b) 40 0.30 180 60 (a) 40 0.00 180 47 40 (a) 60 0.00 200 25 (b) 25 (c) 50 0.19 200 40 (a) 60 0.00 200 48 20 (a) 20 (b) 60 0.20 160 20 (a) 20 (b) 60 0.00 160 20 (a) 20 (b) 60 0.20 160 49 20 (a) 20 (b) 60 0.20 180 20 (a) 20 (b) 60 0.00 180 20 (a) 20 (b) 60 0.20 180 Note 1: In the item of Ca Compound, (a) means common portland cement, (b) means CaO and (c) means Ca(OH) 2 . Note 2: In the item of Material Component, the order of pouring is denoted by upper, middle and lower steps. Example Characteristics Appearance Absolute Dry Bulk Density Compressive Strength (kg/cm 2 ) Compressive Strength Unevenness 44 ○ 1.0 313 33 45 ○ 0.9 325 35 46 ○ 0.6 421 26 47 ○ 0.7 433 27 48 ○ 1.1 500 46 49 ○ 1.0 354 34 Comparative Examples 1 to 9 Fly ashes having a bulk density of 0.5 to 1.2 g/cm 3 and an average particle diameter of 1 to 60 µm (components: 30-80% by weight of SiO 2 , 18-40% by weight of Al 2 O 3 and 15% by weight or less of Fe 2 O 3 ) were used as coal ashes, and 4 kinds of calcium oxide, calcium hydroxide, calcium carbonate and common portland cement having an average particle diameter of 1 to 100 µm were used as calcium compounds, and they were mixed in various weight ratios. Next, 10 to 30% by weight of water and a water reducing agent were added and mixed to obtain various kinds of mixtures. These mixtures were used in the same manner as in Examples 1 to 16, thereby preparing solids containing the coal ashes. Table 5 shows bulk densities and average particle diameters of the coal ashes used in the respective comparative examples, aluminum contents in the coal ashes, kinds of calcium compounds, blend amounts of the coal ashes and the calcium compounds, and temperatures of the autoclave treatment. In Comparative Example 24, the solid was cured at 60°C for 20 hours without doing the autoclave curing. For the thus obtained solids, appearance was observed, and compressive strength and dimensional stability in water were also measured. Table 5 shows the results of these tests. The tests were carried out by the same procedures as in Examples 1 to 16. Comp. Example Material Component (wt%) Coal Ash Amount of Coal Ash (wt%) Bulk Density (g/cm 3 ) Average Particle Diameter (µm) Amount of Al 2 O 3 (wt%) 1 100 1.1 13 21 2 85 0.5 20 23 3 80 1.2 1 24 4 80 1.1 60 30 5 70 1.1 16 40 6 70 1.2 16 18 7 80 1.1 30 24 8 80 1.1 30 23 9 30 0.8 16 20 Comp. Example Material Component (wt%) Autoclave Curing Temp. (°C) Ca Compound (kind and wt%) 1 - 180 2 15 (b) 180 3 20 (c) 160 4 20 (c) 160 5 15 (a) 15 (b) 140 6 30 (e) 140 7 20 (b) 100 8 20 (b) Vapor Curing 9 70 (a) 140 Note: In the item of Ca Compound, (a) means common portland cement, (b) means CaO, and (c) means gypsum. Comp. Example Characteristics Appearance Compressive Strength (kg/cm 2 ) Dimensional Stability (%) Compressive Strength Unevenness 1 X - - - 2 ○ 227 50 0.019 3 ○ 299 30 0.014 4 ○ 306 51 0.016 5 ○ 360 61 0.018 6 X - - - 7 ○ 195 31 0.031 8 X - - - 9 ○ 441 45 0.040 Comparative Examples 10 to 18 Slurries for molding were prepared under conditions that the mixing ratio of a coal ash was set to 30-100% by weight, the mixing ratio of a calcium compound was set to 70-0% by weight, and after the mixing of a blowing agent, water was added thereto in an amount of 20 to 100% by weight based on the weight of a solid content. Fly ashes having a bulk density of 0.5 to 1.4 g/cm 3 and an average particle diameter of 1 to 60 µm (components: 30-80% by weight of SiO 2 , 24-40% by weight of Al 2 O 3 and 15% by weight or less of Fe 2 O 3 ) were used as the coal ashes, and 3 kinds of calcium oxide, common portland cement and gypsum were used as the calcium compounds. These slurries were used in the same manner as in Examples 17 to 35, thereby preparing solids containing the coal ashes. In Comparative Example 17, however, vapor curing was carried out without doing an autoclave treatment. Table 6 shows bulk densities and average particle diameters of the coal ashes used in the respective comparative examples, aluminum contents in the coal ashes, kinds of calcium compounds, blend amounts of the coal ashes and the calcium compounds, and temperatures of the autoclave treatment. For the thus obtained solids, appearance was observed, and compressive strength and dimensional stability in water were also measured. Table 6 shows the results of these tests. The tests were carried out by the same procedures as in Examples 17 to 35. Comp. Example Material Component (wt%) Coal Ash Amount of Coal Ash (wt%) Bulk Density (g/cm 3 ) Average Particle Diameter (µm) Amount of Al 2 O 3 (wt%) 10 100 1.3 11 24 11 60 0.5 28 27 12 55 1.4 1 29 13 50 1.0 60 29 14 60 1.2 29 40 15 80 1.2 11 24 16 50 1.0 14 28 17 80 1.3 11 26 18 30 1.1 30 27 Comp. Example Material Component (wt%) Autoclave Curing Temp. (°C) Characteristics Ca Compound (kind and wt%) Appearance 10 - 130 X 11 20 (a) 20 (b) 160 ○ 12 25 (a) 20 (b) 180 ○ 13 40 (a) 10 (b) 180 ○ 14 20 (a) 20 (b) 160 ○ 15 20 (e) 130 X 16 40 (a) 10 (b) 100 ○ 17 20 (b) Vapor Curing ○ 18 40 (a) 30 (b) 160 ○ Note: In the item of Ca Compound, (a) means common portland cement, (b) means CaO, and (c) means gypsum. Comp. Example Characteristics Absolute Dry Bulk Density Compressive Strength (kg/cm 2 ) Dimensional Stability (%) Compressive Strength Unevenness 10 - - - - 11 0.50 29 10 0.023 12 0.28 5 1 0.028 13 0.35 4 2 0.026 14 0.49 28 11 0.031 15 - - - - 16 0.36 5 1 0.059 17 0.88 30 6 0.020 18 0.48 9 2 0.060 Comparative Examples 19 to 23 Solids containing a coal ash were prepared by the same procedure as in Examples 36 to 49 under conditions that the weight ratio of the coal ash to a calcium compound was set to 70-80% by weight, and 3 kinds of calcium oxide, common portland cement and gypsum were used as the calcium compounds. In Comparative Example 19 to 22, the thickness of each layer was 30 mm, and in Comparative Example 23, a single layer having a thickness of 90 mm was formed. Furthermore, in Comparative Example 22, vapor curing was carried out for 20 hours without doing an autoclave treatment. Table 7 shows the kinds of calcium compounds, blend amounts of the coal ash and the calcium compounds, and amounts of a blowing agent to be used every layer. In addition, temperatures of the autoclave treatment are also shown. For the thus obtained solids (90 × 90 × 150 mm), appearance was observed, and compressive strength and absolute dry bulk densities were also measured. Table 7 shows the results of these tests. The tests were carried out by the same procedures as in Examples 36 to 49. Example Material Component (wt%) Autoclave Curing Temp. (°C) Ca Compound (kind and wt%) Coal Ash Blowing Agent 19 0 100 0.00 180 0 100 0.20 180 0 100 0.35 180 20 50 (e) 50 0.00 150 50 (e) 50 0.10 150 50 (e) 50 0.30 150 21 20 (a) 80 0.00 100 10 (a) 10 (b) 80 0.07 100 10 (a) 10 (b) 80 0.27 100 22 30 (a) 70 0.00 Vapor Curing 30 (a) 70 0.15 Vapor Curing 30 (a) 70 0.40 Vapor Curing 23 15 (a) 15 (b) 70 0.08 180 Note 1: In the item of Ca Compound, (a) means common portland cement, (b) means CaO and (e) means gypsum. Note 2: In the item of Material Component, the order of pouring is denoted by upper, middle and lower steps. Example Characteristics Appearance Absolute Dry Bulk Density Compressive Strength (kg/cm 2 ) Compressive Strength Unevenness 19 X - - - 20 X - - - 21 ○ 0.9 177 51 22 X - - - 23 ○ 0.9 198 30 As is apparent from the examples and the comparative examples, a coal ash solid prepared by the process of the present invention has an excellent dimensional stability and a high strength. Furthermore, a light-weight solid and a light-weight multi-layer solid prepared by the process of the present invention additionally have the merit of weight reduction. Incidentally, a light-weight solid and a light-weight multi-layer solid prepared by using a foaming agent or by introducing previously formed air bubbles into a slurry can have the same structure as in the case of using a blowing agent and can also possess similar features.

Claims: A process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article, the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm, an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 , in the first step, 40 to 95 parts by weight of the coal ash being mixed with 60 to 5 parts by weight of the calcium compound, and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. A process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article, the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm, an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 , in the first step, 40 to 90 parts by weight of the coal ash being mixed with 60 to 10 parts by weight of the calcium compound, one selected from the group consisting of a blowing agent and a foaming agent, and water, and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. A process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article, the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm, an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 , in the first step, 40 to 90 parts by weight of the coal ash being mixed with 60 to 10 parts by weight of the calcium compound and water, and air bubbles being blown imto this mixture, and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. A process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article, the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm, an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 , in the first step, 10 to 90 parts by weight of the coal ash being mixed with 90 to 10 parts by weight of the calcium compound, one selected from the group consisting of a blowing agent and a foaming agent, and water to obtain two or more mixtures which are different from each other in at least one selected from the group consisting of the kinds and amounts of coal ash, calcium compound, blowing agent or foaming agent, and water, in the second step, the two or more mixtures being molded in a laminate state, and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. A process for preparing a solidified material containing a coal ash which comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, and a second step of molding the mixture to obtain a molded article, the coal ash having a bulk density of 0.8 g/cm 3 or more and an average particle diameter of 5 to 40 µm, an aluminum content in the coal ash being 35% by weight or less in terms of Al 2 O 3 , in the first step, 10 to 90 parts by weight of the coal ash being mixed with 90 to 10 parts by weight of the calcium compound and water to obtain two or more mixtures which are different from each other in at least one selected from the group consisting of the kinds and amounts of coal ash, calcium compound and water, and a predetermined amount of air bubbles is blown into each of the two or more mixtures, in the second step, the two or more mixtures being molded in a laminate state, and after the second step, a third step being carried out in which the molded article obtained in the second step is subjected to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. The process for preparing a solidified material containing a coal ash according to any one of Claims 1 to 5 wherein the molded article obtained in the second step is cured at 30 to 100°C, prior to the third step. The process for preparing a solidified material containing a coal ash according to Claims 1 to 5 wherein the calcium compound is one or any combination of calcium compounds selected from the group consisting of calcium oxide, calcium hydroxide, calcium carbonate and calcium silicate.

Inventors: IMAI OSAMU, SEIKE SHOJI, YOSHIZAWA JUNKO, IMAI, OSAMU, SEIKE, SHOJI, YOSHIZAWA, JUNKO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: C04B18
IPC codes: C04B18, C04B28