GB2224025A - Glass-ceramics containing needle-like crystals and method of producing same - Google Patents

Abstract

A glass-ceramics block containing needle-like crystals of at least one compound oxide such as CaSiO3 or Na2Ca3Si6O16 is obtained by mixing a soda-lime glass power with a needle-like crystals precipitation promotor selected from carbonates, hydroxides and nitrates of alkali metals or alkaline earth metals, forming the mixture into a desirably shaped body and heating the shaped body at a temperature not lower than the softening point of the soda-lime glass and not higher than the flow point of the soda-lime glass. The precipitation promotor serves also as a foaming agent. It is also possible to use a powder of a glass containing 10-20 wt% of Li2O as a precipitation promotor for precipitating needle-like crystals of the compound oxide(s) without foaming the glass matrix. In either case the obtained glass-ceramics is high in mechanical strength and thermal shock resistance and useful as a building material.

Description

GLASS-CERAMICS CONTAINING NEEDLE-LIKE CRYSTALS AND METHOD OF PRODUCING SAME This invention relates to glass-ceramics containing needle-like crystals of compound oxides uniformly dispersed in a soda-lime glass matrix, which may be a foamed glass matrix, and method for producing same.

Glass-ceramics according to the invention are high in strength and thermal shock resistance and useful particularly as building materials'.

Soda-lime glasses are easy to vitrify and low in the costs of production and hence have very wide uses.

An important merit of soda-lime glasses is very weak tendency to devitrification during solidification from a melted state. However, there are proposals of forcibly precipitating some crystals from a soda-lime glass by adding a nucleating agent to the glass composition and making a suitable heat treatment with the intention of economically producing a building material afforded with a decorative effect or enhanced in mechanical strength. As to the nucleating agent for soda-lime glasses JP 42-2271 shows using a fluorine compound preferably together with titanium oxide or zirconium oxide, and JP 45-3554 and JP-A 49-69730 show using a metal sulfide such as zinc sulfide (the former) or iron sulfide or manganese sulfide (the latter). In every case a heat treatment for precipitation of crystals is made after the usual glass melting and shaping operations.

Any of the above proposed crystallizing treatment causes precipitation of some needle-like crystals such as wollastonite crystals and/or diopside crystals together with silica crystals such as of cristobalite and/or tridymite. However, precipitaion of these silica crystals is not desirable because in a relatively low heating temperature range these silica crystals exhibit an abnormal change in volume attributed to transition and hence induce strains in or collapse of the glassceramics products. Besides, sophisticated techniques and very careful operations are needed for the preparation and melting of the raw material including a nucleating agent, which is relatively high in specific gravity and in the form of a fine power, for uniformly dispersing the nucleating agent and preventing segregation of same.In another aspect, it is rather unfavorable for productivity to accomplish the crystallization by a heat treatment subsequent to the vitrifying and shaping operations, and there is a possibility of deformation of the shaped glass articles during the crystallizing heat treatment.

As a method of partially crystallizing a soda-lime glass, JP 36-16729 shows producing an opaline glass body by heat treatment of a compact of a fine powder of a soda-lime glass, or a mixture of the glass powder and a binder such as water glass or an alkali metal phosphate, within a devitrifying temperature range. By this method the crystallization by devitrification occurs in the surfaces of the individual particles of the glass powder. Therefore, the crystallization does not lead to appreciable enhancement of the mechanical strength and thermal shock resistance of the obtained glass body.

It is well known to produce heat resistant glassceramics by adding a lithium source and a nucleating agent to the raw material of an aluminosilicate glass, melting the raw material and shaping the vitrified material and then making a heat treatment to cause precipitation of crystals of eucryptite (LiAlSiO4) or spodumene (LiAlSi2O6). Glass-ceramics of this type are used mainly for cooking utensils. However, it is difficult to use these glass-ceramics as building materials because of high costs attributed to the use of a large amount of an expensive lithium compound and the preciseness of the operations utilizing a nucleating agent.

It is an object of the present invention to provide a soda-lime glass base glass-ceramics which contains needle-like crystals of at least one compound oxide and has high mechanical strength and thermal shock resistance.

It is another object of the invention to provide method of producing a glass-ceramics according to the invention.

The present invention provides a glass-ceramics comprising needle-like crystals of at least one compound oxide selected from CaSiO3, Na2Ca3Si6016, Na2CaSi3O8, CaMgSi206, SrSiO3, Ba2Si308 and BaSiO3 dispersed in a soda-lime glass matrix, the needle-like crystals occupying 20-60 vol% or 15-50 wt% of the glass-ceramics and being precipitated by heat treatment of a shaped body of a mixture of a soda-lime glass powder and at least one needle-like crystals precipitation promoting agent selected from carbonates, hydroxides and nitrates of alkali metals and carbonates, hydroxides and nitrates of alkaline earth metals.

In another aspect, the invention provides a method of producing a glass-ceramics comprising needle-like crystals of at least one compound oxide selected from CaSiO3, Na2Ca3Si6O16, Na2CaSi308, CaMgSi2O6, SrSiO3, Ba2Si308 and BaSiO3 dispersed in a soda-lime glass matrix, the method comprising the steps of mixing a soda-lime glass powder with at least one needle-like crystals precipitation promoting agent selected from carbonates, hydroxides and nitrates of alkali metals and carbonates, hydroxides and nitrates of alkaline earth metals, the precipitation promoting agent(s) amounting to 1-10 wt% of the glass powder by calculating as alkali metal or alkaline earth metal oxide(s), forming the mixture into a desirably shaped body and heating the shaped body at a temperature not lower than the softening point of the soda-lime glass and not higher than the flow point of the soda-lime glass for 10 to 60 min to thereby precipitate needle-like crystals of at least one compound oxide in the shaped body.

Since the needle-like crystals precipitation promoting agents employed in this invention serve also as foaming agents, the above method usually gives a foamed glass-ceramics. When a glass-ceramics low in porosity or a foamless glass-ceramics is desired, the desire can be met by compressing the foamed glassceramics during or immediately after the above stated heat treatment.

Besides, as a modification of the above stated glass-ceramics, the invention provides a glass-ceramics comprising needle-like crystals of at least one compound oxide selected from CaSiO3, Na2Ca3Si6O16, Na2CaSi3O8 and Li2SiO3 dispersed in a soda-lime glass matrix, the needle-like crystals occupying 20-60 vol% of the glassceramics and being precipitated by heat treatment of a shaped body of a mixture of a soda-lime glass powder and, as a precipitation promoting agent, a powder of a glass comprising 10-20 wt% of Li 20.

By using a Li2O-containing glass powder as the needle-like crystals precipitation promoting agent it is possible to produce a foamless glass-ceramics body without need of compressing the heated body since in this case the precipitation promoting agent does not act as a foaming agent.

In producing a glass-ceramics according to the invention the needle-like crystals precipitation promoting agent melts, decomposes and reacts with the softened soda-lime glass during the heat treatment, and this agent serves the function of providing an adequately viscous liquid phase which promotes the precipitation of needle-like crystals from the mixture under heat treatment. In this regard, the precipitation promoting agents employed in this invention fundamentally differ from nucleating agents hitherto used, which are hardly meltable and dispersible in the glass as the host material and behave as a sort of foreign matter.

In the case of using a Li2O-containing glass powder as the precipitation promoting agent, a production method according to the invention comprises the steps of mixing a soda-lime glass powder with a powder of a glass containing 10-20 wt% of Li2O such that the resultant mixture contains 1-10 wt of Li2O, forming the mixture into a desirably shaped body and heating the shaped body at a temperature not lower than the softening point of the soda-lime glass and not higher than the flow point of the soda-lime glass for 10 to 60 min.

In this case the precipitation of needle-like crystals by the heat treatment is not accompanied by foaming of the glass body. Since the lithium containing glass powder provides a liquid phase relatively low in viscosity and high in fluidity the lithium component rapidly reacts with the softened soda-lime glass and efficiently promotes precipitation of needle-like crystals of compound oxides. It is easy to obtain a glass-ceramics body of a very dense structure high in strength and low in water absorption. If desired it is possible to produce a foamed glass-ceramics by using a conventional foaming agent such as calcium carbonate together with a Li2O-containing glass powder.

A glass-ceramics according to the invention is an economical material since it can easily be produced by using cullet of a soda-lime glass as the principal material and an inexpensive alkali metal or alkaline earth metal salt as an auxiliary material. Glassceramics according to the invention are sufficiently high in mechanical strength, moisture resistance, heat resistance and thermal shock resistance and accordingly are useful as building materials. Besides, this invention can be embodied in either foamed glassceramics blocks or foamless and dense glass-ceramics blocks.

In this invention a soda-lime glass means, as is usual, a glass essentially consisting of, expressed as oxides, 65-75 wt% of SiO2, 10-20 wt% of Na2O, 5-15 wtX of CaO, 0-5 wt% of K20, 0-5 wt% of MgO and 0-5 wt% of Al203. The glass may further contain small amounts of optional ingredients such as, for example, Fe203, As203, TiO2 and/or CeO2. In industrial practice of the invention it is favorable to use cullet of a soda-lime glass. The soda-lime glass must be pulverized into a relatively fine powder because it is desirable that the glass is large in specific surface area for good reaction with the needle-like crystals precipitation promoting agent. It is preferred to use a soda-lime glass powder not larger than 100 om in particle size.

According to the invention it is essential to add a needle-like crystals precipitation promoting agent (hereinafter, will be referred to simply as a precipitation promoter) to a soda-lime glass powder. The precipitation promotor is selected from carbonates, hydroxides and nitrates of alkali metals and alkaline earth metals, or a glass containing Li2 0 is used as the precipitation promotoer. If desired it is free to jointly use two or more kinds of metal salts selected from the above group. When any of the aforementioned alkali metal or alkaline earth metal salts is used the amount of the precipitation promoter (or the total amount of the precipitation promotors), calculated as the alkali metal or alkaline earth metal oxide(s), should be from 1 to 10 wt% of the glass powder.When a Li2O-containing glass is used the amount of Li2O contained in that glass should be from 1 to 10 wt% of the soda-lime glass powder. If the amount of the promotor is less than 1 wt% the effect of promoting precipitation of needle-like crystals is insufficient.

If the amount of the promotor is more than 10 wt% the precipitation of oxide crystals other than needle-like crystals is promoted, and in the case of an alkali metal salt the glass matrix becomes too rich in alkali and hence inferior in chemical resistance and durability.

When the promotor is an alkali metal salt it is preferable to limit the amount of the promotor to 5 wt% (as oxide) of the glass powder.

The particle size of the promotor is not strictly limited. In the case of an alkali metal or alkaline metal salt it is possible to use commercial products in powder form smaller than 0. D mm in particle size, and it is preferable to use a powder not larger than 100 Am in particle size. In the case of a Li2O-containing glass, the glass may be either a lithium glass produced by melting a mixture of a lithium compound and silica and optional alumina, calcium and/or alkali compounds or a composite glass produced by melting a mixture of a lithium compound and soda-lime glass, aluminosilicate glass or borosilicate glass. It is suitable to use a Li2O-containing glass powder not larger than 100 Am in particle size.

In the case of using an alkali metal salt as a precipitation promotor it is preferable to choose a lithium salt, a sodium salt or a potassium salt. In general lithium salts are higher than sodium salts and potassium salts in the funciton of promoting precipitation of needle-like crystals, and alkali metal carbonates and nitrates are higher than alkali metal hydroxides in that function.

In the case of using an alkaline earth metal salt as a precipitation promotor it is preferable to choose a magnesium salt, a calcium salt, a strontium salt or a barium salt. In general magnesium salts and calcium salts are higher than strontium salts and barium salts in the function of promoting precipitation of needlelike crystals, and there are not great differences in that function between carbonates, hydroxides and nitrates of alkaline earth metals though strontium carbonate and barium carbonate are relatively low in that function because of very high decomposition temperatures. Sulfates and halides of alkaline earth metals hardly promote precipitation of needle-like crystals, and phosphates of alkaline earth metals are unsuitable because of preferentially promoting precipitation of silica crystals unfavorable for the mechanical strength and heat resistance of the glassceramics.

The reasons for the differences in the precipitation promoting effect between the above named alkali metals or alkaline earth metals and between the above named salts of the same metal have not been elucidated yet. Presumably, important factors include diffusion rates of the metal ions and dissociation temperatures of the metal salts.

In general alkaline earth metal salts are more favorable than alkali metal salts. When an alkali metal salt is used it is likely that crystals of an alkali containing oxide relatively low in melting point, such as Na2Ca3Si6016 or Na2CaSi308 precipitate in a considerable amount. In such a case the obtained glassceramics is relatively low in heat resistance and moisture resistance. By comparison, an alkaline earth metal salt tends to precipitate needle-like crystals of compound oxides relatively high in melting point, and an increased content of the alkaline earth metal in the glass matrix is also favorable for heat resistance and moisture resistance of the glass-ceramics.

For mixing a selected precipitation promotor with a soda-lime glass powder, either a dry mixing method or a wet mixing method can be employed. Before the heat treatment for precipitation of crystals the mixture is formed into a block of a desired shape. In the case of a powder mixture a press-shaping method is used. In the case of a sludge-like mixture a casting method is used, and the shaped material is dried.

The shaped body is heated to a temperature not lower than the softening point of the soda-lime glass and not higher than the flow point of the soda-lime glass and maintained at that temperature for 10 to 60 min to thereby cause precipitation of needle-like crystals from the softened glass. As is usual the softening point means a temperature at which the viscosity of the glass reaches 10 65 poise and is about 720 0C in the case of an ordinary soda-lime glass, and the flow point means a temperature at which the viscosity of the glass reaches 10 5'poise and is about 9000C in the case of an ordinary soda-lime glass. If the heating temperature is below the softening point of the glass the precipitation promotor does not well react with the glass.If the heating temperature is above the flow point of the glass the precipitation of crystals decreases because the temperature is close to the vitrifying temperature range. If the duration of the heat treatment is shorter than 10 min the precipitation of needle-like crystals is insufficient. When the duration of the heat treatment exceeds 60 min, sometimes there arises a possibility that the precipitated crystalline phase again reacts with the glass phase to produce another glass phase or a possibility that the precipitation of needle-like crystals is followed by precipitation of silica crystals unfavorable for the heat resistance of the glass-ceramics. It is preferred to carry out the heat treatment at a temperature in the range from 800 to 850 0C for 30 to 60 min.

During the heat treatment the softened glass reacts with the melted and decomposed precipitation promotor, and the precipitation of crystals begins at the grain boundaries of the glass powder or in the reacting glass phase. When the promotor is an alkali metal or alkaline earth metal salt (other than Li2O) a portion of the gases generated by the decomposition of the promotor is trapped in the viscous liquid phase to produce a foaming effect.

Usually the above heat treatment causes precipitation of needle-like crystals of CaSiO3 (wollastonite), Na2Ca3Si6O16 (devitrite) and/or Na2CaSi308. Although there is a tendency of precipitation of silica crystals such as cristobalite and/or tridymite crystals from a soda-lime glass, such tendency is suppressed by the addition of a precipitation promotor according to the invention to soda-lime glass. The needle-like crystals precipitated by the present invention will include CaMgSi206 (diopside) crystals when a magnesium salt is used, SrSiO3 crystals when a strontium salt is used and Ba2Si308 and/or BaSiO3 crystals when a barium salt is used.When a Li2O-containing glass is used as the precipitation promotor, the precipitated needle-like crystals are mainly CaSiO3 crystals and may include Na2Ca3Si6016 crystals and/or Na2CaSi308 crystals. When a relatively large amount of Li2 0 exists in the mixed glass powders, Li2SiO3 (lithium silicate) crystals too precipitate.

In a glass-ceramics according to the invention, the content of the above described needle-like crystals ranges from 20 to 60 vol% or from 15 to 50 wt%. If the content of needle-like crystals is less than 20 vol% or 15 wt% the favorable effects of the needle-like crystals on the mechanical strength and heat resistance of the glass-ceramics are insufficient. If it is intended to increase the content of needle-like crystals to more than 60 vol% or 50 wt% it is inevitable that undesirable silica crystals precipitate together with the desired needle-like crystals.

The above described heat treatment is followed by ordinary annealing. As the result a block of a glassceramics having a finely foamed glass matrix.

To obtain a foamless or only slightly foamed glassceramics block by using an alkali metal or alkaline earth metal carbonate, hydroxide or nitrate, the foamed glass-ceramics block is compressed during or immediately after the heat treatment, i.e. while the foamed glassceramics block is in a hot and softened state. For example, the compression is accomplished by forcing the hot glass-ceramics body to pass through an adequate gap between a pair of rollers. It is optional whether to completely destroy the foams to obtain a dense glassceramics block or to partly destroy the foams to obtain a foamed glass-ceramics block of desirably controlled porosity and apparent specific gravity. The compressing operation is followed by annealing. When a Li2O- containing glass powder is used as the precipitation promotor the compressing operation is unnecessary.

Therefore, by using a Li2O-containing glass powder as the promotor it is easy to produce a multilayer glass body having a foamless and dense glass-ceramics layer on a foamed glass or glass-ceramics layer.

EXAMPLES 1-18 For use in Examples 1 to 18, cullet of a soda-lime glass containing 71 wt% of SiO2, 13 wt% of Na20, 0.8 wt% of K2O, 11 wt% of CaO, 2.1 wt% of MgO and 1.6 wt% of Awl 203 was pulverized and sieved to obtain a glass powder smaller than 75 )tm in particle size. As shown in Table 1, lithium carbonate, lithium hydroxide, potassium carbonate, potassium hydroxide and sodium carbonate were alternately used as needle-like crystals precipitation promoters.

In each example the glass powder and the selected precipitation promoter were mixed in the proportion shown in Table 1 by a dry mixing method or a wet mixing method using 7 wt% of water. In the case of using the dry mixing method the obtained powder mixture was compacted into a rectangular block in a mold under pressure of 20 kg/cm2. In the case of using the wet mixing method the obtained sludge was cast into a mold and then dried to obtain rectangular block. In either case the shaped body was heated in an electric furnace to the temperature shown in Table 1 at a rate of 4000C/hr and maintained at that temperature for the length of time shown in Table 1. After that the heated glass body was left to annealing. By this process a foamed glass block containing needle-like ceramic crystals was obtained in every example.However, in Example 6 the foamed glass body was converted into a dense and substantially foamless glass block by interposing the hot glass block (immediately after making the above described heat treatment) between a pair of steel plates preheated to the temperature shown in Table 1 and compressing the glass block by manually rolling a roll on the upper steel plate thereby destroying the foams in the glass body.

Important characteristics of the glass-ceramics blocks produced in Examples 1-18 were measured by the following methods, and the results are shown in Table 1.

Apparent specific gravity: measured with a picnometer.

Identification of the precipitated crystals: made mainly by X-ray diffraction analysis and observation with microscope and auxiliarily by EPMA analysis.

Quantity of the crystals: a plurality of arbitrary sections of each sample block were observed with microscope to measure the total area of the crystalline phases, and the total volume of the crystals was calculated from an average of the measured areas.

Thermal shock resistance: test pieces (50 mm square and 20 mm thick) were heated at 5000C in an electric furnace and immediately put into water maintained at 100C, and observation was made with the naked eye to give grade "A" to test pieces having no perceptible cracks, grade "B" to test pieces slightly cracked and grade "C" to test pieces seriously cracked or collapsed.

Transverse strength: measured by the three-point bending method.

As shown in Table 1, in Examples 4-11 strip-like crystals of lithium silicate precipitated together with needle-like crystals of wollastonite. However, lithium silicate crystals are low in the coefficient of thermal expansion and high in thermal shock resistance, and hence the coexistence of lithium silicate crystals is not a departure from the present invention.

COMPARATIVE EXAMPLES 1-5 In Comparative Examples 1-3 the glass-ceramics manufacturing process of Examples 1-18 was modified only in the quantity of the precipitation promoter or the duration of the heat treatment as shown in Table 1. In Comparative Example 4 a combination of phosphates (in accordance with JP 36-16729) was used in place of a precipitation promoter according to the invention. That is, the glass powder used in the examples was mixed with 1.5 wt% of 30 wt% aqueous solution of sodium phosphate and 1.5 wt% of 30 wt% aqueous solution of potassium phosphate (0.6 wt% in total, calculated as alkali metal oxides). In Comparative Example 5 the glass powder was press-shaped without adding any precipitation promoter or alternative. The characteristics of the glassceramics blocks produced in Comparative Examples 1-5 were as shown in Table 1.In every case the quantity of the precipitated crystals was too small. The glassceramics of Comparative Examples 2 and 3 were fairly good in thermal shock resistance but were not sufficient in mechanical strength.

TABLE 1 Additive (wt%) Shaping Heating Precipitated Apparent Thermal Transverse Hethod Condition Crystals Specific Shock Strength Gravity Resistance Temp. Time Kind Needle- (kg/cm2) like ( C) (min) Crystals (remarks) (vol%) Example 1 Li2CO3 (1) casting 900 10 W 20 1.0 A Example 2 ditto (2) ditto 800 30 W 20 0.6 A 190 Example 3 ditto (2) pressing 850 30 W 40 0.6 A Example 4 ditto (5) casting 800 10 W, LS 20 0.5 A Example 5 ditto (5) pressing 800 10 W, LS 20 0.5 A Example 6 ditto (5) ditto * 800 10 W, LS 20 2.3 A (foamle@@) Example 7 ditto (10) casting 800 30 W, LS 40 0.7 A 205 Example 8 ditto (10) ditto 850 30 W, LS 50 0.8 A Example 9 ditto (5) pressing 850 10 W, LS 40 0.4 A Example 10 ditto (5) ditto 860 30 W, LS 50 0.8 A Example 11 ditto (10) ditto 850 10 W, LS 50 0.7 A Example 12 LiOH (2) casting 850 30 W 20 0.7 B Example 13 ditto (5) ditto 850 30 W 25 0.6 A 165 Example 14 K2CO3 (2) ditto 900 30 D, W 25 0.7 A Example 15 ditto (5) ditto 850 30 D, W 25 0.8 A 175 Example 16 KOH (5) ditto 900 30 D, W 20 1,0 A Example 17 Na2CO3 (5) pressing 850 30 W, D 30 0.7 A Example 18 Li2CO3 (2) + ditto 850 30 W, D 45 0.6 A K2CO3 (2) Comp. Ex. 1 KOH (0.5) pressing 900 30 D, W 5 1.8 C (cracked) Comp. Ex. 2 Li2CO3 (6) ditto 900 100 W 5 2.0 B (foamless) Comp. Ex. 3 ditto (10) ditto 900 100 W 5 2.1 A 115 Comp. Ex. 4 Na3PO4 + ditto 800 60 W 5 1.9 C (cracked) 105 K3PO4 (0.6) Comp. Ex. 5 none ditto 800 60 W, Cr 5 2.3 C (broke) * After the heat treatmont, compressed with roller. W: wollastonite LS: lithium silicate D: devitrite Cr: cristobalite EXAMPLES 19-42 The soda-lime glass powder used in Examples 1-18 was used also in Examples 19-42.As shown in Table 2, calcium hydroxide, magnesium hydroxide, barium hydroxide, strontium hydroxide, calcium carbonate, magnesium carbonate, calcium nitrate and sodium nitrate were alternately used as needle-like crystals precipitation promoters. The glass-ceramics producing method was as described in Examples 1-18. In most examples the combination of dry mixing and press-shaping was employed, and only in Examples 20, 26 and 32 the combination of wet mixing and cast-shaping was employed.

In Examples 26 and 36, the heat treatment was immediately followed by the compressing operation described with respect to Example 6.

The characteristics of the glass-ceramics blocks produced in Examples 19-42 were measured by the methods described hereinbefore, except that the quantity of the precipitated crystals was measured by the following method. The results are shown in Table 2.

The quantity of the precipitated crystals in each glass-ceramics block was measured by the known quantitative method proposed by Ohlberg and Strickler. That is, the glass-ceramics, the glass itself and the crystalline substance were each subjected to X-ray diffraction analysis at 28 angles of 20-15 . Representing the diffraction intensity of the glass-ceramics by Is, intensity of the crystalline substance by I and the c background intensity of the glass by Ig, the amount of the crystals (wt%) in the glass-ceramics is given by (Ig - IS)/(Ig - Ic). The quantity of the crystals in each sample was determined by preparing calibration curves by this method.As an auxiliary method, arbitrary sections of each sample were observed with microscope.

TABLE 2 Precipitation Heating Precipitated Apparent Thermal Transverse Promoting Condition Crystals Specific Shock Strength Agent (wt%) Gravity Resistance Temp. Time Kind Needle- (kg/cm2) like ( C) (min) Crystals (remarks) wt% (vol%) Example 19 Mg(OH)2 + MgCO3 (5) 800 30 D 20 (17) 0.7 B Example 20 ditto (10) 800 30 D 35 (30) 0.6 A Example 21 CaCO3 (5) 800 30 NCS3 * 25 (25) 0.6 A Example 22 ditto (10) 800 30 NCS3 * 40 (40) 0.5 A 190 Example 23 Mg(OH)2 (2) 800 30 D 21 (17) 1.0 B Example 24 ditto (5) 800 30 D 24 (20) 0.9 A 170 Example 25 ditto (10) 800 30 D 28 (23) 0.7 A Example 26 ditto (10) 850 30 D, W 40 (36) 2.3 A (foamless) Example 27 Ca(OH)2 (10) 750 60 NCS3 21 (21) 0.8 B 155 Example 28 ditto (2) 800 30 NCS3 20 (20) 0.9 B Example 29 ditto (5) 800 30 NCS3 * 25 (25) 0.8 A 175 Example 30 ditto (10) 800 30 NCS3, W 35 (35) 0.7 A Example 31 ditto (10) 850 30 NCS3, W 45 (43) 0.6 A Example 32 ditto (1) 900 10 NCS3, W 21 (20) 1.2 B Example 33 Ba(OH)2 (5) 850 30 B2S3 22 (18) 0.8 B Example 34 Sr(OH)2 (5) 850 30 SS 23 (18) 0.7 A Example 35 Ca(NO3)2 (5) 800 30 NCS3 25 (25) 0.7 A Example 36 ditto (10) 800 30 NCS3 35 (35) 2.3 A (foamless) Example 37 MgCO3 + Ca(OH)2 (5) 800 30 D 23 (19) 0.6 A Example 38 ditto (10) 800 30 D 38 (33) 0.5 A Example 39 NaNO3 (10) 750 60 NCS3 20 (20) 0.7 B Example 40 ditto (5) 800 30 NCS3 23 (23) 0.8 A Example 41 ditto (10) 800 30 NCS3 36 (36) 0.7 A Example 42 ditto (1) 900 10 NCS3, W 21 (20) 1.2 B Dp: diopside NCS3: Na2CaSi3O8 W: wollastonite B2S3: B2Si3O8 SS: SrSiO3 * A very small amount of crystalline CaO was existing.

COMPARATIVE EXAMPLES 6-12 As shown in Table 3, in place of the alkali metal or alkaline earth metal salts used in the foregoing examples, phosphorus pentaoxide, calcium hydrogen phosphate, boric acid, lead oxide, calcium sulfate, calcium chloride and magnesium chloride were alternately added to the glass powder used in the examples. Except the modification in this point the glass-ceramics producing process of Examples 1-42 was repeated. In every case the mixture of the glass powder and the additive was shaped by pressing. The characteristics of the glass-ceramics or glass blocks produced in Comparative Examples 6-12 were measured by the above described methods. The results are shown in Table 3.

In Comparative Examples 6 and 7, silica precipitated as principal crystals. In Comparative Examples 8 and 9 the products were glass blocks containing no crystals. In either of these two cases the additive, boric acid or lead oxide, was effectless for precipitation of crystals and, rather, seemed to have aided vitrification. In Comparative Examples 10-12 the added sulphate or chloride did not decompose by the heat treatment and did not induce precipitation of crystals.

TABLE 3 Additive (wt%) Heating Precipitated Apparent Thermal Transverse Condition Crystals Specific Shock Strength Gravity Resistance Temp. Time Kind Needle- (kg/cm2) like ( C) (min) Crystals (remarks) wt% (vol%) Comp. Ex. 6 P2O5 (5) 800 30 S, NCS3 15 (16) 1.8 C Comp. Ex. 7 CaHPO4 (5) 800 30 S, NCS3 20 (21) 1.8 C Comp. Ex. 8 H3BO3 (5) 800 30 - 1.5 C Comp. Ex. 9 PbO (5) 800 30 - 2.0 C 115 Comp. Ex. 10 CaSO4 (5) 800 30 - 1.5 C (additive remained undecomposed) Comp. Ex. 11 CaCl2 (5) 800 30 - 1.3 C 100 (ditto) Comp. Ex. 12 MgCl2 (5) 800 30 - 1.3 C (ditto) S: SiO2 NCS3: Na2CaSi3O8 EXAMPLES 43-48 The soda-lime glass powder used in Examples 1-18 was used also in Examples 43-48.In any of these examples, the precipitation promotor was a Li2O- containing glass prepared by melting a mixture of cullet of the aforementioned soda-lime glass and a variable amount of lithium hydroxide. As shown in Table 4, the obtained glass contained 10 wt% of 20 wt% of Li2O. It is impracticable to increase the content of Li2 0 to more than 20 wt% because vitrification becomes difficult. In every example the Li2O-containing glass was pulverized to obtain a powder smaller than 75 Xm in particle size.

The soda-lime glass powder and the Li2O-containing glass powder were mixed to obtain a mixture containing a predetermined amount of Li2O, as shown in Table 4. In Examples 44 and 45, calcium carbonate powder was added as a foaming agent. The mixture was compacted into a rectangular block in a mold under pressure of 500 kg/cm2, and the shaped body was heated in an electric furnace at 8000C (Examples 43 and 48) or 8500C (Examples 44-47) for 30 hr, followed by annealing.

The characteristics of the obtained glass-ceramics blocks are shown in Table 4.

The quantity of the precipitated crystals, apparent specific gravity and thermal shock resistance were measured by the methods described hereinbefore, respectively.

The heat resistance was measured by heating test pieces (50 mm square and 20 mm thick) in an electric furnace at 9000C for 60 min and, after natural cooling, examining the degree of deformation of the test pieces and the degree of decrease of the needle-like crystals.

In Table 4, the mark A means little deformation and only a slight decrease of the crystals; the mark "B" means little deformation and a considerable decrease of the crystals; and "C" means considerable deformation and a great decrease of the crystals.

The degree of water absorption was measured by immersing precisely weighed samples in water at normal temperature for 36 hr and then weighing each sample.

Mark "A" was given when the weight increase was less than 1%, mark "B" when the weight increase was not less than 1% but less than 2%, mark "C" when the weight increase was not less than 2% but less than 4%, and mark "D" when the weight increase was more than 4%.

COMPARATIVE EXAMPLES 13 AND 14 In Comparative Example 13 the manufacturing process of Example 44 was modified only in that the content of Li2 0 in the precipitation promotor was decreased to 5 wt%, and in Comparative Example 14 the manufacturing process of Example 47 was modified only in that the proportion of the Li2O-containing glass was greatly increased as shown in Table 4. The characteristics of the obtained glass-ceramics blocks are shown in Table 4. TABLE 4 Precipitation Promotor Foaming Precipitated Glass-ceramics Characteristics Agent Needle-like Li2O Amount of (CaCO3) Crystals Apparent Heat Thermal Degree of Compressive Addition to Specific Resist- Shock Water Strength in Glass Soda-lime Kind vol% Gravity ance Resist- Absorption used as Glaz@, (wt%)* (mainly) ance (kg/cm2) Promotor as Li2O (remarks) (wt%) (wt%) Example 43 10 1 - W 20 2.5 A A A (foamless) Example 44 10 3 0.4 W 25 0.7 A A C 290 Example 45 10 3 0.2 W 25 1.6 A A B 350 Example 46 10 3 - W 25 2.5 A A A (foamless) Example 47 20 5 - W 30 2.5 A A A (foamless) Example 48 20 10 - W, LS 40 2.5 B A A (foamless) Comp. Ex. 13 5 1 0.4 W 10 0.8 A C C Comp. Ex. 14 20 15 0.4 W, LS 50 0.7 C A D * CaCO3/(soda-lime glass + Li2O-containing glass)

Claims (14)

1. CLAIMS 1. A glass-ceramics, comprising needle-like crystals of at least one compound oxide selected from CaSi03, Na2Ca3Si6016, Na2CaSi308, CaMgSi206, SrSiO3, Ba2Si308 and BaSiO3 dispersed in a soda-lime glass matrix, said needle-like crystals occupying 20-60 vol% or 15-50 wt% of the glass-ceramics and being precipitated by heat treatment of a shaped body of a mixture of a soda-lime glass powder and at least one needle-like crystals precipitation promoting agent selected from carbonates, hydroxides and nitrates of alkali metals and carbonates, hydroxides and nitrates of alkaline earth metals.
2. 2. A glass-ceramics according to Claim 1, wherein said soda-lime glass matrix is a foamed glass matrix.
3. 3. A method of producing a glass-ceramics according to Claim 1, the method comprising the steps of: mixing a soda-lime glass powder with at least one needle-like crystals precipitation promoting agent selected from carbonates, hydroxides and nitrates of alkali metals and carbonates, hydroxides and nitrates of alkaline earth metals such that said at least one needle-like crystals precipita-tion promoting agent amounts, by calculating as alkali metal or alkaline earth metal oxide, to 1-10 wt% of said glass powder; forming the mixture of said soda-lime glass powder and said at least one precipitation promoting agent into a desirably shaped body; and heating said shaped body at a temperature not lower than the softening point of said soda-lime glass and not higher than the flow point of said soda-lime glass for 10 to 60 min to thereby precipitate said needle-like crystals.
4. 4. A method according to Claim 3, wherein said at least one precipitation promoting agent is selected from carbonates, hydroxides and nitrates of lithium, sodium and potassium.
5. 5. A method according to Claim 3, wherein said at least one precipitation promoting agent is selected from carbonates, hydroxides and nitrates of calcium, magnesium, strontium and barium.
6. 6. A method according to Claim 3, 4 or 5, wherein said temperature of the heating is in the range from 800 to 8500C.
7. 7. A method according to any of Claims 3 to 6, wherein said soda-lime glass powder is not larger than 100 ssm in particle size.
8. 8. A method according to any of Claims 3 to 7, wherein said at least one precipitation promoting agent is in the form of a powder not larger than 100 ym in particle size.
9. 9. A method according to any of Claims 3 to 8, further comprising the step of compressing said shaped body while said shaped body is in a softened state during or immediately after heating said shaped at said temperature to thereby enhance the density of said shaped body.
10. 10. A glass-ceramics, comprising needle-like crystals of at least one compound oxide selected from CaSiO3, Na2Ca3Si6O16, Na2CaSi308 and Li2SiO3 dispersed in a soda-lime glass matrix, said needle-like crystals occupying 20-60 vol% of the glass-ceramics and being precipitated by heat treatment of a shaped body of a mixture of a soda-lime glass powder and, as a needlelike crystals precipitation promoting agent, a powder of a glass comprising 10-20 wt% of Li2O.
11. 11. A method of producing a glass-ceramics according to Claim 10, the method comprising the steps of: mixing a soda-lime glass powder with needle-like crystals precipitation promoting agent, which is a powder of a glass containing 10-20 wt% of Li2O, such that said Li2O amounts to 1-10 wt% of said soda-lime glass powder; forming the mixture of said soda-lime glass powder and said precipitation promoting agent into a desirably shaped body; and heating said shaped body at a temperature not lower than the softening point of said soda-lime glass and not higher than the flow point of said soda-lime glass for 10 to 60 min to thereby precipitate said needle-like crystals.
12. 12. A method according to Claim 11, wherein both said soda-lime glass powder and said powder of glass containing Li2O are not larger than 100 m in particle size.
13. 13. A method according to Claim 11 or 12, further comprising the step of mixing a foaming agent with said soda-lime glass powder in addition to said precipitation promoting agent.
14. 14. A method of producing a glass-ceramics comprising needle-like crystals of at least one compound oxide dispersed in a soda-lime glass matrix, substantially as hereinbefore described in any of Examples 1 to 48.