EP0162917A4

EP0162917A4 - Calcia-aluminosilicate glasses, glass-forming mixtures and methods for producing same.

Info

Publication number: EP0162917A4
Application number: EP19850900313
Authority: EP
Inventors: Durai N Raghavan; Tetsuro Horiuchi
Original assignee: Atlantic Richfield Co
Current assignee: Atlantic Richfield Co
Priority date: 1983-11-23
Filing date: 1984-11-19
Publication date: 1986-08-21
Also published as: EP0162917A1; JPS61500490A; WO1985002393A1

Abstract

Calcia-aluminosilicate glasses and glass-forming mixtures having controlled molar ratios of the predominant oxide components and formed by adding glass-forming and glass-modifying materials to naturally occurring zeolites. Further the invention relates to cementitious bodies reinforced with fibers of the glasses of the invention.

Description

Calcia-Aluminosilicate Glasses, Glass-Forming Mixtures and Methods for Producing Same.

Technical Field

The invention herein relates to alkaline- resistant glasses. While it pertains to glass bodies generally, it has particular pertinence to glasses which are fiberizable.

Background Art The natural mineral zeolites are a group of hydrous alkali metal and/or alkaline earth metal aluminosilicates which have an open three-dimensional crystalline framework. While a large number of individual mineral zeolites are known and have been described in the literature, eleven (11) minerals make up the major group of mineral zeolites: analcime, chabazite, clinoptilolite, erionite, ferrierite, heulandite, laumontite, mordenite, natrolite, phillipsite and wairakite. The chemical and physical properties of these major mineral zeolites, as well as the properties of many of the minor mineral zeolites, are described extensively in Lefond (ed.). Industrial Minerals and Rocks (4th Ed., 1975), pp. 1235-1274; Breck, Zeolite Molecular Sieves (1974), especially Chapter 3; and Mumpton (ed.), Mineralogy and Geology of Natural Zeolites, Vol. 4 (Mineralogical Society of America: November, 1977). These publications also describe the geologic occurrence of the natural mineral zeolites and some industrial and agricultural uses which have been proposed or in which the natural mineral zeolites are now being used commercially. It is important to note that the natural mineral zeolites are an entirely different class of materials from the "synthetic zeolites" which have been widely described in many recent articles and patents. Because there is no universally recognized system for naming the synthetic zeolites, and because some of the synthetic materials exhibit x-ray diffraction patterns which suggest possible similarities in structure with the natural mineral zeolites, some reports in the literature and patents have described certain synthetic zeolites as "synthetic" versions of the natural mineral zeolites. Thus, for instance, certain synthetic zeolites have been described as "synthetic analcime" or "synthetic mordenite" and so forth. As noted in the aforementioned Breck reference, however, this approach is technically unsound and has merely led to confusion between the two (2) otherwise distinct classes of materials: the natural mineral zeolites and synthetic zeolites. While it has been recognized that there are structural similarities between the two groups, it is clear that the natural mineral zeolites constitute a class of materials significantly separate and distinct in structure and properties from the synthetic zeolites. Glasses are vitreous materials composed largely of silica. Because silica is a highly refractory material, however, substantial quantities of soda ash, lime or other fluxing materials are often added to the silica to permit the glass-forming composition to be melted at reasonable temperatures. Small quantities of other materials, usually elemental materials or oxides, are commonly added to glass melts to provide particular properties such as color or chemical resistance to the finished glass. One experiment has been reported in which a clinoptilolite and glass mixture was fired at

800°C (well below the melting point of either) to produce what was described as a porous low density glass composition; see Mumpton, supra, p. 197, referring to Tamura Japanese published application 74/098,817 (1974). Alkaline resistance is provided in some glasses by the inclusion of zirconia and/or titania, such as in AR glasses of Pilkington. Although these materials enhance the alkaline resistance of glass bodies, these are refractory materials which increase the melting point of such glasses. Also, zirconia and titania tend to add cost to the glass inasmuch as these are much more expensive materials than silica, soda, calcia and the usual components of soda lime silica glasses. Although calcia tends to lower the melting point of the glass composition, a general admonition exists in the glass technology against using calcium oxide in quantities greater than about fifteen percent (15%) by weight of the glass body.

Discosure of the Invention

Objects of the Invention: It is an object of the invention to produce fiberizable alkaline-resistant glasses. Another object of the invention is to modify a silica source, e.g., naturally occurring zeolite materials with readily available aluminum and alkaline earth metal compounds, to achieve an optimal ratio of certain components of the resulting glass composition. A further object of the invention is to optimize alkaline resistance and fiberizability by controlling the molar ratio of certain oxides within the glass composition.

Summary of the Invention: The invention herein comprises glass compositions which have outstanding resistance to alkaline environments and, in particular, to glass compositions which are readily fiberizable. Such glass compositions are characterized by a high alkaline earth metal oxide content, a relatively low silica content and a significant alumina content. In particular, these glass compositions are derived from a silica source, e.g., a naturally occurring zeolite, at least one alumina source, preferably an alumina source which is separate from the silica source, and at least one alkaline earth metal oxide source, especially a calcia source, to yield a glass composition predominately of silica, alumina and alkaline earth oxide, especially calcia, wherein the molar ratio of silica plus alumina to calcia plus magnesia falls within the range of about 1.4:1 to about 2.0:1 for certain calcia contents and from about 1.6 to 2.3 for lower calcia contents. Soda, potassia, and boria as well as zirconia and other glass-forming and glass-modifying components may be present in minor amounts. Also included within the scope of the present invention are glass bodies, particularly fibers, formed from the aforesaid glass composition.

Brief Description of the Drawing FIG. 1 is a graph of alkaline resistance and fiberizability plotted against the molar ratio of silica plus alumina to calcia plus magnesia for various glass compositions.

Detailed Description and Best Modes for Carrying Out Invention

The present invention relates to the preparation of alkaline resistant glasses having excellent viscosity and devitrification characteristics especially suitable for fiberization, particularly fiberization by drawing said glasses through dies. The preparation of such glasses involves the combining of glass-forming materials including a silica source, especially silceous materials containing sodium, potassium and/or aluminum components, so that the glasses which preferably contain minor portions of soda and/or potassia have a molar ratio of silica plus alumina to calcia plus magnesia of about 1.4:1 to about 2.0:1 for glasses having in excess of 24% by weight calcia and preferably from about 1.5:1 to about 1.9:1 and especially from about 1.6:1 to about 1.8:1. For glasses containing less than about 24% calcia, said molar ratio is generally from about 1.6 to about 2.3, preferably from about 1.8 to about 2.3, and especially from about 2.0 to about 2.3.

Glasses of this invention may involve preparation from silica sources having one or more of a plurality of other materials such as aluminum, calcium, magnesium, sodium and potassium in various quantities. In one specific embodiment a glass-forming mixture comprising at least one silica source, frequently one containing a minor quantity of alumina, e.g., less than about 5% by weight, at least one separate alumina source, and at least one calcia source such that the glass formed from said mixture comprises about 45% by weight to about 60% by weight of silica, about 2% by weight to about 20% by weight of alumina, about 24% by weight to about 30% by weight of calcia, and about 0% by weight to about 24% by weight of magnesia, and less than 3% by weight of zirconia, provided that the molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.4 to about 2, preferably from about 1.5 to about 1.9 and especially between about 1.6 and about 1.8.

These molar ratios are particularly applicable to glasses containing minor quantities, e.g., up to 5% by weight of soda and/or potassia and more particularly to glasses containing minor quantities of boria, e.g., up to about 5% by weight, and especially to glasses having a combined soda, potassia and boria content of from about 3% to about 10% by weight. Furthermore, the molar ratio is most effective in providing optimum properties, such as fiberizability and alkaline resistance when the CaO + MgO content is from about 25% to about 35% by weight. In most instances, the magnesia content is preferably less than about 10% by weight and especially below about 5% by weight. In another specific embodiment, a glass-forming mixture comprising at least one silica source, at least one alumina source, and at least one calcia source, such that the glass formed from said mixture comprises about 45% by weight to about 60% by weight of silica, about 2% by weight to about 20% by weight of alumina, about 18% by weight to about 22% by weight of calcia, about 0% by weight to about 30% by weight of magnesia, and less than 3% by weight of zirconia, provided that the molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.6 to about 2.3, preferably about 1.8 to about 1.3, and especially from about 2.0 to about 2.3

Boron and zirconium components are absent from naturally occurring mineral zeolites in quantities which have any effect upon the processing characteristics of batch materials or properties of a finished glass article. It is frequently desirable to include boron and/or zirconium components in the glass batch in minor quantities such that the finished glass article has less than about 6% by weight B₂O₃ and less than about 3% by weight ZrO₂. The inclusion of boron compounds in the glass batches of this invention tends to improve fiberizability and reduce melting temperature of the resulting glass while the presence of zirconia in the glass tends to improve alkaline resistance of a glass already possessing outstanding alkaline resistance.

The invention particularly relates to glasses formed from naturally occurring zeolites and especially from glasses wherein such zeolites provided a predominance of the glass-forming components. It is significant, as described hereinafter, that such glasses may be easily and inexpensively formed by melting the glass-forming mixture noted above.

Many naturally occurring zeolite materials, especially those of high alumina content, may be formed into glasses, especially as fibers, under appropriate conditions. The zeolites, as a glass-forming material, have many advantages. Naturally occurring zeolites have already undergone reaction and the various elements are intimately mixed and reacted with one another. Also, the zeolite materials are particularly useful inasmuch as they have a very low sulfur content. In particular, very useful glass bodies may be formed by combining various quantities of alumina and, preferably, an alkaline earth metal component such as calcia or calcia and magnesia combinations with a zeolite of the following compositional range:

Silica - about 60% to about 78%, alumina - about 6% to about 30%, Fe₂O₃ - about 1% to about 3%, calcia - about 0% to about 15%, magnesia - about 0% to about 5%, potassia - about 1% to about 5%, soda - about 1% to about 5%, with the percentage expressed being in weight per cent.

Suitable alumina sources for inclusion in the glass-forming mixture of the present invention include alumina, various clays having a high alumina content such as kaolin, montmorillonite and the like, and aluminum compounds such as aluminum chloride, aluminum sulfate and the like. Suitable silica sources include zeolite materials, pure silica, and various glass cullets having a high silica content.

A glass-forming composition may be readily formed by mixing a calcia source, e.g., a finely ground limestone with a silica source, e.g., a finely ground zeolite material, such as the composition identified above, and a finely ground alumina-forming material in proportion to obtain the above-noted glass composition.

The glass material, upon cooling, exhibits good physical properties, having strengths and other qualities substantially equivalent to a typical soda-lime silicate glass. If the glass has the composition parameters noted above, resistance to alkaline solutions from about ten-fold to twenty-fold better than a typical soda-lime silicate window glass, as well as improved fiberizability, are achieved.

Besides improving fiberizability and durability of the glass in alkaline environments, zeolite-type glasses containing increased alumina and high calcia and/or magnesia loadings have other advantages as well. The addition of alumina-forming materials and calcium and/or magnesium compounds tends to even. out variances in the zeolite composition. Zeolites are naturally occurring materials and are not homogenous or uniform in their composition. Naturally occurring zeolites contain various quantities of alumina and calcia. The alumina content may vary significantly depending upon the mineral type. A few mineral zeolites contain alumina in sufficient quantities to form readily fiberizable glasses. However, most zeolite materials contain low amounts of alumina in differing quantities. Thus, addition of alumina to such zeolites provides a glass-forming composition of substantially uniform composition from batch to batch which has good fiberizability and, through addition of alkaline earth metal components, good alkaline resistance.

Although smaller or larger quantities of zeolite material may be utilized, good results are achieved from a glass-forming composition which has about 35% by weight, or more, of a naturally occurring zeolite. Improved fiberizability is achieved from glass-forming compositions having about 40% or more zeolites unless additional silica, alumina, etc. of the same ratios as exist in the zeolite are included in the batch. Excellent results have been achieved with zeolite compositions of about 50% or more.

Such naturally occurring zeolites may contain high quantities of alumina, but typically contain up to about 10% by weight. Thus, alumina-forming materials are added in quantities of about 0.1% by weight to about 20% by weight, and preferably from about 2% to about 15% by weight, calculated as alumina to the zeolite materials to obtain glasses with improved fiberizability.

As indicated elsewhere herein, alkaline earth metal components are also preferably added to alumina modified zeolite glass-forming compositions. It is, of course, within the scope of the invention to add quantities of silica to achieve uniform batch compositions or to achieve particular silica to alumina ratios. Usually such silica additions are unnecessary but may be made if desired. Additions of soda, potassia and the like may also be made, but since low soda and potassia contents are desired, such additions are usually not practiced. The zeolites contain relatively substantial quantities of water, that is, hydrated materials.

Hydrated crystalline materials generally tend to melt at a lower temperature. Thus, there are further advantages to beginning the glass-forming operation with a prereacted zeolite, rather than initiating it with silica. The melting temperatures of the glasses of this invention come within a range, preferably about 1300ºC to about 1500°C, which permits the drawing of glass fibers through platinum dies. The glass fibers could also be formed by spinning or other techniques. However, formation of continuous strands is best accomplished by drawing through an orifice in a platinum or platinumrhodium body.

Fibers of the glass compositions of this invention are particularly useful inasmuch as they may be used to strengthen bodies which are highly alkaline in nature, for example, cement and plaster. Such fibers may also be used to strengthen organic matrices of various types. Remrorcement or cement with such fibers, however, provides a particularly advantageous use inasmuch as asbestos has been frequently used heretofore for that purpose. Because of various health and/or environmental concerns, the use ot asbestos is diminishing. Continuous strands or mats of glass fibers having the glass compositions described herein effectively reinforce concrete bodies.

EXAMPLE I

Naturally occurring zeolites were finely comminuted, admixed with particulate alumina, calcium carbonate, magnesium carbonate, limestone or dolomite, as indicated, and melted to form glass bodies and fibers. The melting was conducted batch-wise in small crucibles at temperatures of about 1350"C to about 1500ºC depending upon zeolite composition and quantity ot alumina and alkaline earth metal material added.

In Table I, glasses la and Ila exhibit good resistance to alkali attack, but exhibit poor fiberizability and have a limited working temperature range. Glasses lb and lIc, in contrast, have compositions within the scope of the present invention and have improved fiberizability and working range while maintaining good resistance to alkali attack.

Glasses Illb and IIIc of Table II have compositions within tne scope of the present invention and exhibit progressively improving fiberizing and working properties. While some diminishment of alkali resistance was experienced with glasses Illb and IIIc, the alkali resistance is still very good.

EXAMPLE II

Glass-formmg materials were finely comminuted, admixed with particulate additives as identified in the following tables (expressed in percent by weight) and melted to form glass bodies and fibers. The melting was conducted batch-wise in small crucibles at temperatures of about 1250ºC to about 1500ºC depending upon batch composition and quantity of additives.

The glasses set forth in Table III were prepared from silica, alumina, calcium carbonate, boria and magnesium carbonate. A zeolite material was not present in the batch. These glasses were prepared from traditional glass-forming materials in order to examine the fiber-forming, alkaline resistance and other properties of the resulting glasses.

Glass IVc has a composition within the scope of the present invention and exhibits very good properties. Its melting point was reasonably low while its alkaline resistance was very good. Fibers were formed without difficulty.

Glasses Va and Vb exhibited excellent alkaline resistance; however, fiberizability was rated fair and good respectively. Glass Vc exhibited an improved working range over Glasses Va and Vb, although the alkaline resistance was lower.

Glasses VIa through VId exhibited excellent alkaline resistance and melting temperatures. Glasses VId and VIe exhibited substantial fiberizability and working temperature range although their alkaline resistance was less than Glasses VIa through VIc. Glass VIa exhibits an unacceptably reduced working range.

The glasses identified in Tables I, II, IV and V were prepared from a zeolite having the following composition: SiO₂ 82.8 percent by weight

AI₂O₃ 8.4 percent by weight Fe₂O₃ 0.3 percent by weight CaO 0.9 percent by weight

MgO 0.6 percent by weight K₂O 2.9 percent by weight

Na₂O 4.1 percent by weight

Minor variations in the composition may occur from batch to batch of the zeolite.

Very minor quantities of other elements, for example boron, manganese, zirconium, titanium, vanadium, antimony, barium, in combined form, may be present in such a naturally occurring zeolite. The quantities of such materials generally are individually below about 0.1% by weight and are usually less than 0.01% by weight and frequently present in amounts less than 0.005% by weight. Various other materials, especially those having metallic elements, may be found in trace amounts in the zeolite material.

In forming the alkaline resistant glasses of this invention, it is preferred, if starting with a zeolite material, to have such zeolite material present at least about 35% by weight of the glass batch mixture. If the zeolite provides substantially all the silica component for the resulting glass, then quantities of about 40% to about 50% by weight or more of the zeolite may be utilized in the glass batch mixture. The glass batch mixture may contain a minor quantity of a boria-forming ingredient, such as borax, colemanite, sassolite, ulexite and the like. Various naturally occurring borosilicate materials or borosilicate or boroaluminosilicate glass cullet, of course, may be utilized to provide the boron component in the glass batch. The boron-containing component is usually present in the glass batch in quantities of up to about 6% by weight and is typically present in sufficient quantities to provide a boria content in the resultant glass of from about 0.1% to about 6% by weight and preferably from about 1% to about 5% by weight and especially preferred from about 1% to about 4% by weight. A plot of fiberizability and alkaline resistance for various glass compositions containing about 24% to about 38% by weight calcium oxide is illustrated in FIG. 1. Glass compositions identified by molar ratio of silica plus alumina to calcia plus magnesia are plotted along the abscissa. A value of the above-noted molar ratio of less than about 1.4:1 is considered for the purposes of the invention as representing a lower fiberizability limit; i.e. a value of 4 on the fiberizability scale. A fiberizability value of at least about 7, at a molar ratio of about 1.5, is preferably present in alkaline resistant glasses while a fiberizability of about 9, at a molar ratio of about 1.6, is especially desirable.

Although alkaline resistance appears to diminish substantially proportionately as the above-noted molar ratio increases, the fiberizability appears not to have a linear relationship to the molar ratio. Fiberizability increases significantly at molar ratio values above about 1.4. Optimum alkaline resistance is achieved at molar ratio values below 1.8. It is especially significant in the instant invention to control the molar ratios within the aforesaid ranges since the presence of soda and potassia may result in the crystallization of various undesirable crystals during fiberizing of the glass. However, the presence of sodium and potassium compounds in at least some of the batch materials utilized in the instant invention requires proportioning of the batch materials to provide a glass composition having molar ratios of silica plus alumina to calcia plus magnesia within the ranges set forth herein.

Industrial Applicability The outstanding tolerance to alkaline environments render these glasses, especially in fiber form, as excellent reinforcement materials for concrete, plaster and other inorganic matrices of an alkaline nature. This is especially significant inasmuch as asbestos, which has been a standard extender as reinforcement material in cement and concrete bodies, is considered undesirable because of the health hazard it may present.

Glass fibers formed from glasses of this invention have particular utility as a reinforcement material for cementatious bodies, e.g., of cement and concrete. Cementatious bodies exhibit enhanced strength when such bodies are reinforced with a minor amount of glass fiber, preferably from about 1% to about 10% by weight, and more preferably about 1.5% to about 7.5% by weight glass fibers of the type described herein. The fibers are included in cementatious bodies in sufficient amount to enhance the strength of such bodies.

The glasses of this invention have excellent resistance to moisture degradation and do not degrade or deteriorate during normal or extended storage periods. Although embodiments of the instant invention have been described as having significant loadings of calcia, it is to be recognized that at least minor substitutions of other alkaline earth metal oxides in lieu of calcia may be made. For example, magnesium compounds, particularly magnesium carbonate may be substituted for at least some of the calcium carbonate in preparing a batch for melting into an alkaline-resistant glass. Similarly, barium and strontium compounds may be substituted as well as beryllium compounds, many of which are naturally occurring materials found in the same geographic regions as zeolites.

The oxides of alkaline earth metal elements are not considered glass formers, which is a term applied to elements having a valence greater than three, e.g. silicon, boron, and phosphorous, which may form three-dimensional networks with their oxides, namely, silica, boric oxide, and phosphorous. Alkaline earth metal elements, being divalent, are more tightly bound in a glass than are alkali metal elements.

Sources of alkaline earth metals to form oxides in the glasses of this invention are as follows:

Alkaline Earth Metal Compound Source

Calcium Carbonate Limestone

Marble Chalk

Magnesium Carbonate Dolomite

Magnesium Silicate Serpentine

Barium Carbonate Wetherite

Strontium Carbonate Strontianite

Beryllium Aluminum Silicate Beryl

Sources of calcium and magnesium carbonates are generally more plentiful and cheaper than sources of barium, strontium or beryllium compounds. Also, beryllium metal is considered toxic, although beryllium oxides bound within a glass body are not hazardous.

Glasses of this invention preferably having less than about 24% by weight CaO exhibit excellent fiberizability but with somewhat diminished alkaline resistance in comparison to glasses having more calcia. The lower calcia content glasses are more useful in alkaline environments than many fiberglass materials having much lower alkaline resistance. Also, certain sizings and coatings upon such fiberglass materials may further enhance the alkaline resistance of the glasses of this invention.

It is noteworthy that the zeolite-derived glasses of this invention have good working properties and strength in addition to outstanding alkaline resistance. These glasses may be used in any form, e.g. containers, sheets, fibers and the like, and especially for any use in which transparency or colorlessness are not required. The glasses may be used as flakes, bubbles (microspheres), fibers and the like to reinforce organic or inorganic matrices, especially cement, plaster and the like.

Claims

1. A glass-forming mixture comprising at least one silica source, at least one alumina source, and at least one calcia source such that the glass formed from said mixture comprises about 45% by weight to about 60% by weight of silica, about 2% by weight to about 20% by weight of alumina, about 24% by weight to about 30% by weight of calcia, about 0% by weight to about 24% by weight of magnesia, and less than 3% by weight of zirconia; provided that the molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.4 to about 2.

2. A glass-forming mixture of Claim 1 wherein said molar ratio is in tne range of about 1.5 to about

1.9.

3. A method of forming a glass comprising melting the glass-forming mixture of Claim 1.

4. A method of forming a glass comprising melting the glass-forming mixture of Claim 2.

5. A glass-forming mixture comprising at least one silica source, at least one alumina source and at least one calcia source, such that the glass formed trom said mixture comprises about 45% by weignt to about 60% by weight of silica, about 2% by weight to about 20% by weignt of alumina, about 18% by weignt to about 22% by weight of calcia, about 0% by weight to about 30% by weignt or magnesia and less than 3% by weight of zirconia; provided that the molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.6 to about 2.3.

6. The glass-forming mixture of Claim 5 wherein said molar ratio is in the range of about 1.8 to about 2.3.

7. A method of forming a glass comprising melting the glass-forming mixture of Claim 5.

8. A method of forming a glass comprising melting the glass-forming mixture of Claim 6.

9. A glass composition comprising about 45% by weight to about 60% by weight of silica, about 2% by weight to about 20% by weight of alumina, about 18% by weignt to about 22% by weight of calcia, about 0% by weight to about 30% by weight of magnesia and less than 3% by weight of zirconia; provided that tne molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.6 to about 2.3.

10. The glass composition of Claim 9 wherein said molar ratio is in tne range of about 1.8 to about 2.3.

11. The glass-forming mixture of Claim 1 wherein said molar ratio is in the range of about 1.6 to about 1.8.

12. A method of forming a glass comprising melting the glass-forming mixture of Claim 11.

13. The glass-forming mixture of Claim 5 wherein said molar ratio is in the range of about 2.0 to about 2.3.

14. A method of forming a glass comprising melting the glass-forming mixture of Claim 13.

15. The glass composition of Claim 9 wherein said molar ratio is in the range of about 2.0 to about 2.3.

16. The mixture of Claim 1 wherein said silica source comprises a naturally occurring zeolite.

17. The mixture of Claim 2 wherein said silica source comprises a naturally occurring zeolite.

18. The mixture of Claim 5 wherein said silica source comprises a naturally occurring zeolite.

19. The mixture of Claim 6 wherein said silica source comprises a naturally occurring zeolite.

20. A cementatious body containing a minor amount ot fibers of glass having the glass composition set forth in Claim 9.

21. A cementatious body containing a minor amount ot fibers of glass having tne glass composition set forth in Claim 20.

22. A cementatious body containing a minor amount ot fibers of glass having a composition of the glass outlined in Claim 5.

23. A cementatious body containing a minor amount of fibers of glass having a composition of the glass outlined in Claim 6.

24. A glass composition which comprises about 45% by weignt to about 60% by weight of silica, about 2% by weight to about 20% by weight of alumina, about 24% by weight to about 30% by weight of calcia, about 0% by weight to about 24% by weight of magnesia, and less than 3% by weight ot zirconia; provided that the molar ratio of silica plus alumina to calcia plus magnesia in said glass is in the range of about 1.4 to about 2.

25. The glass composition of Claim 24 wherein the molar ratio is about 1.5 to about 1.9.

26. The glass composition of Claim 24 wherein the molar ratio is about 1.6 to about 1.8.