Glass-water interactions

Journal of Non-Crystalline Solids 102 (1988) 1-10 North-Holland, Amsterdam

1

Section 1. Water and glass, durability, and transport phenomena GLASS-WATER INTERACTIONS H. SCHOLZE Keesburgstra/3e 22, D-8700 Wfirzburg, FRG

The interactions between glass and water are many-sided and the results of such interactions depend on many parameters. It is well known that the stability of glass in water depends on the composition of the glass. Amongst the further parameters the temperature plays a specific role because primarily the temperature determines whether the interaction takes place with water vapour or with liquid water. In general the durability of glass in water is very high. The good condition of the surface of ancient glasses, exposed to water for a long time, proves this very impressively. However, this impression usually does not show that complicated interactions between glass and water occur yielding a remarkable change of the structure and microstructure of the glass surfaces. These phenomena shall be discussed in detail. These interactions are influenced by the compounds dissolved in the water. This must be taken into consideration especially when judging the long-term behaviour of glass in a limited aqueous volume. This problem is important for some practical cases. It was found that the corrosion of glass continues even after the dissolved components have reached their saturation, however, with a lower rate. Two practical examples shall be discussed, the behaviour of HLW-containing glass in a stock and the behaviour of glass fibers in human tissues.

1. Introduction The task of preparing a survey of the above topic in a few pages raises some problems, especially due to the various types of interactions and the large number of publications. Considering the existence of some review papers by various experts the following text represents more personal views on earlier research, observed discrepancies, unsolved problems, and some recent results. Reflecting on the theme of this conference "Stability of Glasses", it can be concluded that the chemical stability plays a very important role in the history, development and use of glass. It is obvious that ancient people after the discovery of the production of glass some thousand years ago continued this development only after the decisive practical observation that glass is stable against attack by water. Many antique glasses in many museums prove this, and some natural glasses also appear to be in good condition after millions of years. The author's own interest in the system glass-water started in the early fifties after observing a discrepancy between the two well-known 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

statements of this time about the behaviour of glass in water: (1) the leached amounts of alkali are proportional to the square root of time, which means diffusion and therefore a smooth curve of the concentration profile on the one hand, and (2) a gel-like layer is formed, which means a steep increase in the concentration profile on the other hand.

2. Classical experiments The possibility of applying IR spectroscopy to glass yielded not only the experimental observation of the remarkable difference between the near-IR spectra of vitreous silica and soda-lime glasses but also turned out to be a good method for looking at the leaching process, especially by use of glass foils as samples. The first results of these observations were published 25 years ago [1]. These experiments were performed with glasses of the system N a 2 0 - C a O - S i O 2 and with a relatively high volume V of solution compared with the surface area SA of the glass in order to avoid a counteraction of the reaction products, Further,

2

H. Scholze / Glass-water interactions

acid conditions were chosen. Under these simple conditions, which can be termed classical conditions, it was found that (1) both the leached amount of Na ÷ and the intensity of the O H band at 3 ttm increases with ~-, and (2) an exchange occurs by replacing 1 N a ÷ ion by 1 H ÷ ion accompanied with ½ H 2 0 molecule on the average. It was observed later [2] that, in the IR spectra of leached glasses, a band occurs at about 6.1/~m, typical for molecular water. This band is proportional to the 3 /~m band, and quantitative comparison shows that nearly all of the "water" in the leached layer is present as molecular H20. This is an important observation. But there were further remarkable observations, e.g., that the N a 2 0 concentration profiles of the leached layers are by no means simple but show a certain level of N a ÷ ions, in the case of N a 2 0 - C a O - S i O 2 glasses, before reaching the bulk concentration by a steep increase. In addition this steep concentration increase moves with V~- into the glass. Some publications with similar observations can be found in the journals. (To save space, citations are limited with preference to more recent papers. Additional citations are to be found in papers with more survey character [3-8].) Another limitation is necessary: mainly experimental observations shall be reported and discussed; the mathematical and theoretical treatment is left in other hands. Chiefly the behaviour of soda-lime glasses is discussed in order to consider fundamental reactions. However, it should be taken into consideration that these reactions can depend, sometimes very sensitively, on the chemical composition and other parameters. But even the soda-lime glasses raise some problems or questions, as the following observations show.

tinctly less than 3, which has also been found by several other authors. But this ratio is not constant; it depends on the glass composition. In glasses with large cations, e.g., lead crystal glasses, the H : K ratio approaches 3 [9]; Gottardi et al. [10] also observe an increase of this ratio with increasing size of the cation. There are some hints that the amount of water molecules which accompany the protons depends on the space available in the glass structure. This statement results in another statement: the chemical durability is better the fewer water molecules can enter the glass structure, i.e., the tighter the glass structure is. This is reasonable considering the problems of movement of the bare proton. It can be assumed that the movement of the protons is made easier the more H 2 0 molecules are present to form intermediate H3 O+ ions, as is also described by Ernsberger [11].

2.1. The ratio leached cation: entered water

2.3. Rearrangement of the structure of the leached

With the word "water" is meant the sum of all H-bearing groups independent of the state of bonding. It is tempting to look for a ratio H : N a = 3, which would mean an ion exchange N a + - H 3 0 ÷. However, this ratio is the exception; usually the experimentally determined ratio is dis-

2.2. Alkali level in the leached layer The existence of a certain level of alkali ions in the leached layer was measured by several authors; however, it is scarcely discussed. If this alkali level is mentioned, then it is assumed that these alkali ions are not mobile, either due to stronger bonding or due to the structural inclusion of these ions in the glass network preventing their movement. These assumptions suppose a stable situation, which is in reality not the case. If a very thin foil is leached for such a long time that the leaching profiles meet in the middle of the foil, then the alkali level decreases to zero, as fig. 1 (from [5]) shows. That means that all alkali ions are mobile. The observed alkali level may be caused by the necessity of the maintenance of a certain cation concentration in the gel layer in order to promote the ion exchange.

layer The experimental observation of the high portion of molecular water instead of S i - O H groups was surprising. This shows not only a condensation reaction according to -Si-OH + HO-Si = ~ -Si-O-Si- + H20,

H. Scholze / Glass-water interactions 1,0

(

composition

E e U

g 0,5

u

o Z

0,0

o

°r

I

t =16d

~

Ttoc)

2O No20 6 C~O

I

time of leoching

o .<~-2 [1 - ~xp (-V2tl ]

(mot.%} % S;O2

I

3

I I /

0,8

Z,5

"6

\

~

distance from surface (pro)

Fig. 1. Na profile in 24/~na thick foils of soda-lime glass after leaching in 0.1 N HC1 at 80 o C and various times.

but also means a remarkable rearrangement of the structure of the network, since the distance between both Si atoms becomes smaller after condensation. This rearrangement is not limited to their neighbourhood, but seems to include larger regions, since a swelling of 5.5 vol.% was measured [2]. Zhdanov [12] also discussed the effect of rearrangement. The solution of this problem was provided by the observation of Tomozawa and Capella [13] that the leached layer shows a phase separation. This has been confirmed by Bunker et al. [14]. However, the question of the role of the alkali level is still open. Where is this alkali to be found? Perhaps there is a connection with the remark by Bunker et al. that " t h e mierostructures are consistent with colloid growth in a medium having a p H of 9-11 regardless of the solution pH". There is another proof of the rearrangement of the network, namely the leaching experiments with the heavy oxygen isotopes 180 or ~70. Baer and Pederson et al. [15,16], Bunker [17], and recently also March [18] found after such experiments that 180 or 170 have been incorporated into the network in such an amount that probably all oxygen atoms have taken part in this rearrangement. The following reactions may be responsible for this behaviour: - S i - O - S i - + H2180 ~ - S i - O H + H a 8 0 - S i - , - S i - O H + H180-Si - ~ ---Si-180-Si -- + HEO.

O~ 25

c o

'-t-

0,4

O

2

4

6

time of exchange (h} foil {H20} ÷ D20 ~

V2

foil (020) + H20t

Fig. 2. Kinetics of water exchange of leached foils of N a z O - C a O - S i O z glass (20-6-74 mol%).

The latter reaction corresponds to the above-mentioned condensation reaction, and confirms this reaction. This high mobility during the leaching process raises the question about the behaviour during storage after the leaching. Does the exchange of oxygen also occur in the leached layer without liquid water in the neighbourhood? The answer would be very interesting in connection with some properties of leached glasses. 2.4. Behaviour of water in the leached layer Some authors, experimenting with isotopes, report a loss of D and a80 during storage of the leached samples, e.g., Baer et al. [15] have written that " . . . significant decreases in both D and a80 occurred if specimens were exposed to air for as little as 24 h". The same effect was observed some years earlier after leaching a soda-lime glass in 0.1 N DC1 and storing in normal room atmosphere. Fig. 2 shows the results of some systematic experiments, in this case the entering of D20 from a DzO-containing atmosphere into a normal, leached, soda-lime glass. Exchange takes place, and it is relatively fast. Fig. 3 shows the progress of the exchange followed by means of the extinction of the O H band at 3 /~m in dependence on the square root of time. It shows moreover that

4

H. Scholze / Glass-water interactions

/

0./*],~ ]

,.,,,,,.,,.,......~. ~ D=a.lO-12cm2/s ~ °

°'31

~o

"o,~o~.

"~...._ ~D2Ootos-H2OAtm.

~'--..~ D20Gtas__ H20f t(~ssig

"

20 \,

16.

u m

12.

% 'T

0,21

i,,,/

%.

n2, Gtas~ 2

Atm.

0

I 'II "

CI 4.

0'1t i/

~.~

D= o'12cm2/-is 2'1 lJO

oV 0

.

50 - -

150 200 250 q-r (sU2)~ Fig. 3. Variation of the OH band at 3 /~m of leached foils of Na20-CaO-SiO 2 glass (20-6-74) mol%) due to H20-D20 exchange. 100

the rate of exchange is independent of the aggregation state of the water in the surroundings. But there is a small isotopic effect, since the entering of H 2 0 is a little faster than that of D20. This exchange behaviour fulfils very well the ideal diffusion equation. Hence it follows that the H 2 0 (or D20 ) molecules possess a high mobility throughout the whole leached layer. However, this is valid only for the exchange, not for the escape into a dry atmosphere or into vacuum. Fig. 4 shows the results of such experiments. After a certain time, e.g., 10 4 S = 2.8 h, the exchange H 2 0 ~ D20 amounts to about 50 to 80%, accord-

I00"

0

o

>C

"

/

60-

\ ct~,~xj

8 P 40-

• x./"

x~.X~ ~ ~

x/X

x/

x/x

0

x x~

x...x....x~200"C

(x

01 10

ing to fig. 3, but the escape into vacuum amounts, according to fig. 4, to only about 20%, both at room temperature. This very different behaviour of the water in the leached layer has to be considered by thinking about reactions of this water inside and outside of the leached layer. The above-mentioned exchange experiments were performed with D as isotope. Recently March [18] followed the exchange behaviour of the leached layer on a soda-lime glass by tracing with D~80 and measuring the 180 concentration after various times by the nuclear reaction method. Fig. 5 shows the calculated diffusion coefficients. Again, the exchange is fast; however, it is recognizable that the exchange is faster at shorter times. The interpretation may be that at first, the oxygen of the water molecule exchanges, and later, the oxygen of the network, but with a lower rate.

2.5. Isotope effects As has already been shown, the exchange H 2 0

25"C

xx~

x/x°

,,7/~•

x: weight o: IR

No.20-Co.0-SiO.~ 20:6:%-gias~

O.x~" x/X / 9"x'x~xR~x'~" 10 3

Fig. 5. Diffusion coefficient D of ]80 for the exchange of water of a leached layer of soda-lime glass in D 2180 at room temperature after March [18].

against D20 is accompanied by an isotope effect.

x /x ~*"

|0 2

30

~,x....x~.x.x..-..---100"C x?.,.~x

/ ~ / x o'/ 20"

2JO t[h]

10 t'

6d'60~C~n/10 He[ I0 S

10 6

time Is)

Fig. 4. Time and temperature dependence of the weight loss in vacuum of leached foils of N a 2 0 - C a O - S i O 2 glass (20-6-74 mol%).

However, the leaching and the dissolution of the network also show that the rate of these reactions is lower in DC1 or D20 than in HC1 or H20. Though Richter et al. [19] could not find this effect by short-time leaching of a soda-lime glass, Pederson [20] confirmed a decrease of the leached amounts of about 30% by leaching of Na 2O . 3SiO 2 glass in D20 instead of H 2 0 , and March [18] by leaching of a soda-lime glass in 1 N D2SO4.

H. Scholze / Glass-water interactions

These experimental observations show that the proton determines the rate of the leaching process. Pederson proposes a very plausible interpretation assuming that an activated complex is formed consisting of a - S i - O - S i = bridge accompanied with one proton (at the oxygen) and one H 2 0 molecule (at one silicon). The rate-determining step is then the breaking of the O - H bond resulting in two remaining - S i - O H groups. This hydrolysis allows the hydronium ions to penetrate and thus to exchange with sodium. This also explains that the location of the main occurrence is the interface between bulk glass and gel layer.

5

Glass: S i 0 2 - N a 2 0 - C a O : 7 4 - 16 - 1 0 t4ol% 100h - 80°C 100

400

[~.--*

g

c~

[]--*

'-

c

_c co~

o

200

.--®

50

~®

.-®

c° o_~ ~,.~

__.~® I 1

I 3 Concentration

F 5 in m o l / I

Fig. 6. Leaching of soda-lime glass in various RCI solutions: dependence of the weight loss on the kind and concentration of R; analysed SiO 2 content in the solution.

2.6. Intermediate summary The hitherto-mentioned observations result in a many-sided picture showing that the leaching process, as the most important aspect concerning the chemical stability of glasses, is a very complicated process, and is especially characterized by the leached layer, which changes its structure and composition. There are several subprocesses, and therefore it is plausible that the theoretical description needs the corresponding number of parameters. A simple sketch of a model was recently proposed together with Conradt [21].

3. Further observations Until now the reported experiments were mostly performed in such a manner that the solution was constant in its composition, i.e., there was no influence of any reaction products. That demands a high ratio S A / V . However the stability of glass is interesting not only in pure water or pure solutions, but also in ordinary water, containing various salts. Considering only the fundamental experiments, the following observations are both of primary importance: in the acid region leaching is prevailing, and is independent of the p H from 1 to 7, while in the basic region the dissolution of the network prevails. The independence of the pH in the acid region gives rise to the supposition that the influence of other dissolved components also may be low.

The last inference is not confirmed in practice. On the contrary, there are many observations showing greater or lesser changes of the corrosion rates by the presence of salts or other inorganic compounds. Some examples are discussed in [8]; they show a many-sided and often confusing picture. This means, that an explanation of the various observations is only rarely possible. It is, however, possible, to trace out some main phenomena. The first one is the forming of a protective layer by which the corrosion rate is decreased. This phenomenon occurs relatively often under basic conditions because of the low solubility of some silicates or hydroxides in this region. However, only rarely is the protective layer tight enough to guarantee a durable protection. The next phenomenon has an opposite effect, namely an increase of the corrosion rate by an increase of the dissolution of the SiO 2 network. This effect occurs by the presence of some salts or metal cations. A simple explanation is not possible, since the reverse effect is also reported depending on the kind of compounds and on the composition of the glass. Fig. 6 shows some results with alkali chloride solutions. Under the quoted conditions it is remarkable that the weight loss is increased very distinctly by increasing the NaCI concentration to 1 mol/1. Higher concentrations do not show a stronger effect; however, KC1 is much more effective, while the LiCI solution is less effective. The weight loss is mainly due to the dissolution of the network, since the SiO2 portion

6

H. Scholze / Glass-water interactions

of the weight loss corresponds to the o~iginal glass composition. Finally, a further phenomenon shall be mentioned, which is also observed during leaching experiments with solutions containing other cations than in the glass. Besides the leaching of the cations of the glass and eventually the dissolution of the network, an ion exchange takes place. In the case of soda-lime glasses, it was observed that, with KCl-containing leaching solutions, a different Na concentration profile occurs, with the N a concentration equal to zero directly in the glass surface. This also proves that all the N a ÷ ions are mobile. Another effect is noticed if organic compounds are present in the solutions. It is well known that such compounds, which are able to form chelates or complex compounds with silicon or other elements of the glass, promote the leaching rates. Newton [22] mentions some relevant papers in his survey.

4. Applications Many parameters determine the chemical stability of glass. Unfortunately, very rarely in the publications is all the necessary information specified which is important, not only for the comparison of various experiments, but also for the improvement of our knowledge of the behaviour of glass. At present we have to notice that the chemical stability of glass is determined by a great multiplicity of effects and parameters. This is no reason for despondency but rather a stimulus for further research. Already it is possible to solve certain problems; however, generally two conditions have to be considered: (1) a minimum number of experiments is necessary in order to show the right way; (2) for the evaluations and discussions a feeling not only for mathematics but also for chemistry is desirable.

4.1. Long-term behaviour of nuclear waste glass It is well known that the final storage of radioactive waste shall be performed using glasses containing the radio-element oxides as glass compo-

nents. One plan is that these waste forms are to be stored in rock-salt formations, in order to isolate the waste from the biosphere. It is important to achieve a high safety standard which will remain valid for a long term, even under extreme conditions that are improbable but scientifically thinkable. Therefore a lot of experiments have been performed, and many publications exist. Again, it is impossible to mention all these papers in this survey; however, it shall be mentioned that all the above-discussed influences also play a role in this field with various degrees of importance. There are some parameters which will be somewhat different. For example, one reckons with a limited solutions volume, which may influence the corrosion rate remarkably, as investigated, e.g., by Shade and Strachan [23]. Further, the aggressive solutions contain many components; in the case of rock-salt storage, it is the so-called quinary Q solution. It has been shown in section 2.7 of this paper that the presence of alkali chlorides increases the corrosion rate; however, with Q solution the reverse effect can be observed. There are several reasons which may cause this effect, e.g., the formation of protective layers on the surfaces and the reaching of the saturation of compounds with low solubility. The first effect, protective layers, is obtained only rarely, unfortunately. But the solubility effects seem to play an important role, and their influence has been discussed at times, e.g., by Maurer et al. [24]. Theoretically, the dissolution of a solid stops when saturation has been reached in the solution. Grambow [25] especially has argued with this concept, remarking that the glassy state involves a certain residual affinity Ao~ which is responsible for the long-term corrosion after reaching the thermodynamic saturation limit. To a first approximation, the time dependence of the weight loss q (per surface area) can be represented by the simple equation q = at ~ with the constants a and /3. Conradt et al. [26] discussed the validity and applicability of this equation. For long-term considerations, it is better to refer to the initial rate of corrosion r0. It has been shown for many different glasses, that the

H. Scholze / Glass-water interactions

long-term rate r~ may be represented by an equation of the form r~ = (1 - B)ro(1 - e x p [ A o J R T ] ) , where B represents the fraction of surface sites possibly passivated by a re-adsorption process (with 0 < B < 1) and A~ represents the abovementioned residual affinity (with a small negative value). For instance, B = 0, A~ = 1000 J / ( K mol) and T = 150 ° C lead to r~ = 0.15r 0. Hitherto available experiments show that this factor lies between 0.02 and 0.18.

4.2. In-vitro investigations on siliceous fibres Another problem has arisen in recent times in connection with the use of siliceous fibres for replacing asbestos. Since asbestos fibres were identified as carcinogenic agents, other fibres also have been presumed to have a similar potential. It has been demonstrated that, not only the geometry of mineral and glass fibres, but also the duration of fibres in-vivo are important factors. The latter factor is determined by the chemical durability of the respective fibres. Because there was a need for more corresponding data, a comparative in-vitro study on a set of natural and m a n - m a d e mineral fibres in a simulated biological fluid was performed, with the aim being to establish a quantitative durability classification [27]. For this study 13 different siliceous fibre materials were selected representing a broad variety. Table 1 gives a survey of the whole set of materials, their chemical composition and a characterization in terms of their important geometrical properties and chemical behaviour. The fibres were exposed to a simulated extracellular fluid derived from Gamble's solution, also containing organic compounds, in a flow cell at 37 ° C and at a constant p H value of (7.6 _+ 0.2). Si, B and K were analyzed in the fluid. Silicon is suitable for describing the network dissolution and can be used to calculate the reduction of the fibre diameter to a residual value. Boron and potassium describe the selective leaching of the remaining fibres. Figs. 7a and b present the relative amounts

7

of dissolved fibres (ratios of dissolved matter to initial fibre mass) based on silicon data. A logarithmic plot was chosen in order to cover the whole range. This evaluation reveals remarkable differences between the fibres. There is a group of fibres with apparently low chemical durability which dissolve within four months to more than 50%. On the other hand, there is a group with high durability with a loss of 5% or less, main refractory and natural fibres. Fig. 7c presents results based on K and B analysis. The comparison with figs. 7a + b shows the extent of matrix dissolution or matrix alteration. For example, the superfine glass fibre (no. 4) shows a strong selective leaching, whereas the diabase wool (no. 5) dissolves almost congruently. The mass loss per initial mass is insufficient for a complete understanding, because it is influenced by the temporal decrease of the fibre surface areas, which is responsible for the temporal decrease of the mass losses. Therefore it was necessary to take into consideration the decrease of the fibre diameter. Fig. 8 shows the modified results, now as network dissolution velocity. Most of the fibres show the theoretically demanded constancy of this velocity, i.e., their independence of the radius or of the time. Fig. 8 presents the corrected durability hierarchy of the investigated fibre materials. A comparison with fig. 7 shows that the straight forward evaluation gives a fair estimate for m a n y cases; however, for several cases the exact evaluation yields major corrections of the hierarchy. For example, the E and superfine glass fibres (nos. 2 and 4) are much more durable than directly concluded from silica release. The relatively fast dissolution of the investigated silica fibre is also remarkable (no. 10). It might be of interest to calculate the time t required for a fibre to dissolve completely. These values for t for diameters of 1 /~m are listed in table 1, too. Since diameters of 1 /~m are extremely unlikely for the natural fibres, the respective period is referred to a diameter equal to five times the average diameter. These values show that all investigated m a n - m a d e siliceous fibres with diameters less than 1/~m are dissolved completely in 6.5 y or less. These times are clearly shorter

8

H. Scholze / Glass-water interactions

Table 1 Properties of the investigated fibre materials Chemical composition: in wt%. Geometry: ds0 is the 50% abundance diameter (values in brackets were calculated from BET surface data). Chemical durability: network dissolution velocity o in n m / d a y and lifetime 7 in years for a fibre with d = 1 /.tm (values in brackets were calculated for d = 5d50 ) No. 1 2 3 4 5 6 7 8 9 10 11 12 13 a b c d

Material glass wool 475, JM 104 glass wool E, JM 104 glass wool, T E L glass wool, super fine diabase wool basalt wool slag wool refractory, Fiberfrax R refractory, Fiberfrax H refractory, silica chrysotile, U I C C standard krokydolite, U I C C standard erionite

SiO2 ( + TiO 2)

B203 a

57.9 54.9 63.8 63.1 49.5 49.4 40.9 51.8 46.7 100.0

10.7 8.0 4.8 5.0 nd nd nd nd nd nd

A1203 ( + Fe203)

CaO

5.9 14.4 4.0 3.9 10.2 26.2 c 16.5 48.0 53.0 nd

3.0 17.4 7.0 7.0 14.8 10.7 34.9 nd nd nd

nd 4.7 3.0 3.0 12.0 9.3 4.4 nd nd nd

13.0 0.4 17.3 16.8 3.2 4.3 1.5 0.1 0.1 nd

nd

43.6

nd

nd 5.8 b

nd nd

43.4 b

nd

1.5

51.4 b 56.4 b

nd nd

45.0 d 16.0 b

a

MgO

Na20 + K 20

a

5.8 4.9

b

d5 °

v

?

a

0.41 0.47 3.5 0.38 4.0 4.9 4.8 1.85 1.85 0.77

0.9 0.21 3.45 1.4 1.14 1.11 0.69 0.27 0.28 1.1

(0.074)

0.005

( -- 100)

(0.17)

0.011 0.0002

( = 110) ( ~ 170)

(0.005)

1.7 6.5 0.4 1.0 1.2 1.2 2.0 5.0 4.9 1.2

nd not determined, Calculated from ideal stoichiometry, Fe203 content = 12.5%, Fe203 content = 43.2%.

Q.

b.

102

102

C.

102

~~4 ~'I0

") ;

10~

.c_

101 /

101

Ja" l ~ / "

i -z

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I/1 u'l

o

E

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/

5

l

2

? ~'~

2

2

i0-I

10-1 0

100

Y

100

10-I 0 time

IO0 in d a y s

0

IO0

Fig. 7. Time dependence of the relative amounts (mass ratios) of matter released from fiber bundles; nos. refer to table 1. (a) Network dissolution based on silicon data for glass and mineral wools. (b) Like (a), however for refractory and natural fibers. (c) Network dissolution plus selective leaching based on boron (no. 2) and potassium (the rest) data.

H. Scholze / Glass-water interactions

a,

"7

sufficient solubility i n a simulated fluid of h u m a n tissues o n the other h a n d . This s o u n d s like a c o n t r a d i c t i o n , b u t this is n o t the case, considering that a certain b e h a v i o u r d e p e n d s on a whole set of parameters. Some of t h e m have been m e n t i o n e d above in sections 2 a n d 3. The e x p e r i m e n t a l o b s e r v a t i o n s indicated above d e m o n s t r a t e that the reaction between glass a n d water or a q u e o u s solutions is n o t so simple as t h o u g h t i n the last decade. Some new knowledge could have b e e n o b t a i n e d recently, b u t the mechan i s m s are often u n k n o w n . It is the task for the future to throw light u p o n these mechanisms, in order to u n d e r s t a n d better the g l a s s - w a t e r interactions.

b.

10~

10~

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10o

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10 -1

10 -2

,10-2

10 -3

10-3

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o

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9

o

References

c

5

2

2

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10-~ 0

100

O time

100

in d G y s

Fig. 8. Time dependence of the network dissolution velocity v; nos. refer to table 1; a: for glass and mineral wools b: for refractory and natural fibers.

t h a n the discussed latency period of a b o u t 20 y for the a p p e a r a n c e of cancer after the first exposure to asbestos. It is to be assumed that this relatively high dissolution rate is caused b y the c o m p o s i t i o n of the fluid used, especially b y its c o n t e n t of organic c o m p o u n d s . But nevertheless, this beh a v i o u r of commercial glass fibres shows that a certain solubility of glass u n d e r special c o n d i t i o n s also has its good side, which has b e e n k n o w n for some time for special glasses.

5. Final remarks This survey shows that glass exhibits m a n y o u t s t a n d i n g properties i n contact with water. Thus, e.g., it shows a very good stability i n a highly c o n c e n t r a t e d salt solution on the one hand, a n d a

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H. Scholze / Glass-water interactions

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