Materials Degradation Caused by Acid Rain - American Chemical

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Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch019

Effects of Atmospheric Exposure on Roughening, Recession, and Chemical Alteration of Marble and Limestone Sample Surfaces in the Eastern United States 1

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C. A. Youngdahl and B. R. Doe 1

Argonne National Laboratory, Argonne, IL 60439 U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 22092

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Marble and limestone surfaces were exposed to atmospheric conditions at four eastern U.S. sites and were monitored for changes in surface chemistry, surface roughness/recession, and weight. The effect of acid deposition, to which calcareous materials are especially sensitive, was of particular interest. Results are described for the first year of testing, and aspects of a preliminary equation to relate damage to environmental factors are discussed. Thus far, findings support that acid deposition substantially damages marble and limestone surfaces. Improvements in methods to reduce uncertainties in the quantitative contributions of important chemical species are outlined for the ongoing effort. The d e t e r i o r a t i o n o f marble and l i m e s t o n e exposed b o t h t o anthropog e n i c a c i d d e p o s i t i o n from the environment and t o n a t u r a l w e a t h e r i n g i s b e i n g a s s e s s e d as one o f the major a c t i v i t i e s o f the M a t e r i a l s E f f e c t s Task Group o f the N a t i o n a l A c i d P r e c i p i t a t i o n Assessment Program (NAPAP). There i s much concern f o r the c a l c a r e o u s stone m a t e r i a l s because o f t h e i r w i d e s p r e a d use as the e x t e r i o r s t r u c t u r e of commercial, i n s t i t u t i o n a l , and p r i v a t e b u i l d i n g s as w e l l as i n v a l u e d monuments and memorials. These c a l c i u m carbonate m a t e r i a l s a r e e s p e c i a l l y s e n s i t i v e t o an a c i d environment. Much o f the e n v i r o n m e n t a l damage t o the stone r e s u l t s i n s u r f a c e m a t e r i a l l o s s , u s u a l l y by r e a c t i o n and d i s s o l u t i o n p r o c e s s e s , b u t a l s o by a c c u m u l a t i o n o f s u r f a c e and s u b s u r f a c e r e a c t i o n p r o d u c t s t h a t s u b s e q u e n t l y s p a l l away t o g e t h e r w i t h some u n r e a c t e d stone (1_) . An example o f m a t e r i a l l o s s i s shown i n F i g u r e 1: l o s s o f engraved d e t a i l was n e a r l y complete on the marble tombstone a f t e r 65 y e a r s o f exposure a t A r l i n g t o n N a t i o n a l Cemetery. The r a t e o f damage i s enhanced a t the edges and c o r n e r s o f e n g r a v i n g and s c u l p t u r e , where the s u r f a c e a r e a i s i n c r e a s e d by c a r v i n g such f e a t u r e s . The e n v i r o n m e n t a l damage t o these stone m a t e r i a l s has been t h e s u b j e c t o f s e v e r a l p r e v i o u s s t u d i e s ( 2 - 6 ) . The p a s t s t u d i e s have been more l i m i t e d i n scope, e.g., they have been concerned w i t h a This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

In Materials Degradation Caused by Acid Rain; Baboian, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Chemical Alteration of Marble and Limestone


YOUNGDAHL AND DOE

F i g u r e 1. Tombstones of s i m i l a r v a r i e t i e s of marble. (a) unexposed t o a t m o s p h e r i c c o n d i t i o n s and (b) exposed f o r 65 y e a r s a t A r l i n g t o n N a t i o n a l Cemetery, A r l i n g t o n , VA. Comparison o f engraved i n s c r i p t i o n s shows the e x t e n t of m a t e r i a l d i s s o l u t i o n caused by exposure (names of i n d i v i d u a l s were m a n u a l l y removed).


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s i n g l e e n v i r o n m e n t a l p o l l u t a n t , have measured o n l y one mode o f m a t e r i a l s damage, o r have been o f s h o r t d u r a t i o n . The p r e v i o u s s t u d i e s o f t e n have been concerned w i t h damage t o e x i s t i n g s t r u c t u r e s f o r w h i c h e n v i r o n m e n t a l and maintenance h i s t o r i e s a r e d i f f i c u l t t o e s t a b l i s h . T h e r e f o r e NAPAP began an e x p e r i m e n t a l s t u d y t h a t i s r e l a t i v e l y comprehensive w i t h r e s p e c t t o (1) m o n i t o r i n g o f e n v i r o n m e n t a l f a c t o r s throughout t h e d u r a t i o n o f stone e x p o s u r e , (2) damage modes and mechanisms i n a v a r i e t y o f g e o g r a p h i c l o c a t i o n s , and (3) time o f s t o n e exposure — up t o t e n y e a r s i n t h e s c h e d u l e d p l a n . Synergism among damaging agents a l s o can be c o m p r e h e n s i v e l y i n v e s t i gated i n t h e f i e l d t e s t s , w h i c h form t h e f o c u s o f the NAPAP s t u d y o f stone d e t e r i o r a t i o n . The f i e l d program o f the M a t e r i a l s E f f e c t s Group c u r r e n t l y e x poses w e l l - c h a r a c t e r i z e d specimens o f marble and l i m e s t o n e , as w e l l as m e t a l s and o t h e r m a t e r i a l s , t o outdoor c o n d i t i o n s a t s i t e s t h a t are i n s t r u m e n t e d t o m o n i t o r m e t e o r o l o g i c a l v a r i a b l e s , r a i n c h e m i s t r y , and a i r q u a l i t y f a c t o r s . F u r t h e r d e s c r i p t i o n s o f t h e e n v i r o n m e n t a l c o n d i t i o n s a t t h e s i t e s a r e g i v e n by F l i n n and by Reddy e t a l . i n t h i s volume. Stone samples have been i n s t a l l e d a t s i t e s near R a l e i g h , NC; C h e s t e r , N J ; Newcomb, NY; and i n Washington, DC, and have been m o n i t o r e d f o r one y e a r . The damage t o stone exposed a t t h e f i e l d t e s t s i t e s i s a s s e s s e d by s e v e r a l methods. T h i s paper d e s c r i b e s t h e m o n i t o r i n g o f s u r f a c e roughness, r e c e s s i o n , and c h e m i c a l a l t e r a t i o n as w e l l as stone s p e c i men weight changes. Chemical a n a l y s e s o f r a i n w a t e r and o f r u n o f f from r e l a t i v e l y l a r g e specimens i n s l a b form a r e b e i n g conducted by M. Reddy, and t h e r e s u l t s a r e d i s c u s s e d elsewhere i n t h i s volume. T e s t s o f changes i n c o l o r and m i n e r a l o g i c a l s t a t e and o f s u l f a t i o n d e t e c t e d by i n f r a r e d s e n s i n g a r e a l s o b e i n g made and w i l l be r e p o r t e d i n f u t u r e by t h e p e r t i n e n t i n v e s t i g a t o r s . A l t h o u g h t h e r e s u l t s r e p o r t e d below a r e n o t y e t s u f f i c i e n t i n themselves t o produce a m a t e r i a l s damage f u n c t i o n ( i . e . , an e q u a t i o n r e l a t i n g damage t o one o r more e n v i r o n m e n t a l f a c t o r s ) , they can be used t o h e l p e v a l u a t e and perhaps m o d i f y f u n c t i o n s t h a t have been proposed on o t h e r bases. T h i s paper t e n t a t i v e l y c o n c l u d e s t h a t stone m a t e r i a l i s l o s t from s k y w a r d - f a c i n g s u r f a c e s a t a r a t e e q u i v a l e n t t o about 15 micrometers o f s u r f a c e r e c e s s i o n p e r y e a r a t t h e f i e l d t e s t s i t e s and t h a t an approximate agreement w i t h the r e s u l t s o f R e d d y s r u n o f f c h e m i s t r y i s found where comparison i s now p o s s i b l e , i . e . , a t s i t e s h a v i n g low ambient c o n c e n t r a t i o n s o f s u l f u r d i o x i d e . 1

Experimental Procedures Shelburne Marble from Vermont, used f o r example i n t h e J e f f e r s o n M e m o r i a l , and Salem Limestone from I n d i a n a , as used i n the N a t i o n a l C a t h e d r a l , were o b t a i n e d as sample s t o c k m a t e r i a l s . Each o f t h e two stone s t o c k s was s e l e c t e d as a m o n o l i t h i c b l o c k and was c u t i n t o s l a b s measuring 610 χ 305 χ 51 mm, under t h e s u p e r v i s i o n o f NAPAP p e r s o n n e l ( 7 - 8 ) . S e l e c t e d s l a b s were used t o f a b r i c a t e b r i q u e t t e s , w h i c h were employed f o r s e v e r a l o f t h e damage s t u d i e s . Briquette dimensions a r e shown i n F i g u r e 2. S u r f a c e f i n i s h e s t y p i c a l o f t h o s e p r o v i d e d on s t o n e used f o r e x t e r i o r s o f b u i l d i n g s were employed on what would become t h e skyward s u r f a c e s o f s l a b s and b r i q u e t t e s : an 8 0 - g r i t ground s u r f a c e was produced on m a r b l e , and a "smooth p l a n a r



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f i n i s h " was used on l i m e s t o n e . A l l s l a b s and b r i q u e t t e s were l a b e l e d s y s t e m a t i c a l l y t o p r e s e r v e i n f o r m a t i o n on t h e i r r e l a t i v e l o c a t i o n s and o r i e n t a t i o n s i n t h e source b l o c k s : t h e g e o l o g i c a l b e d d i n g p l a n e s of t h e stone a r e p a r a l l e l t o t h e broad f a c e s o f b r i q u e t t e s , and t h e more r e c e n t sedimentary l a y e r s a r e n e a r e r t h e upper f a c e s . Specimens were w a t e r - r i n s e d , a l l o w e d t o d r y , and s t o r e d i n p o l y e t h y l e n e envel o p e s u n t i l use. Samples were i n d i v i d u a l l y p r o t e c t e d by p l a s t i c bubblewrap d u r i n g t r a n s p o r t a t i o n t o and from t h e f i e l d s i t e s . Exposure r a c k s f o r stone samples a t f i e l d s i t e s ( F i g u r e 3) a r e designed t o support t h e b r i q u e t t e s a t a 30° angle t o t h e h o r i z o n t a l , i n c l i n e d toward t h e s o u t h . The r a c k and wedge-shaped sample d e s i g n s ensure t h a t t h e samples a r e s e c u r e l y p r o t e c t e d from d i s p l a c e m e n t ( e . g . , by w i n d ) , w h i l e a v o i d i n g b i n d i n g f o r c e s and p r o v i d i n g a p r a c t i c a l minimum o f c o n t a c t between specimens and t h e L u c i t e r a c k mat e r i a l ( 9 ) . Rack l o c a t i o n s a r e i n secure a r e a s , and f r e q u e n t i n s p e c t i o n s a r e made and r e p o r t e d by s i t e o p e r a t o r s , who b r u s h away i n t e r f e r i n g m a t e r i a l such as b i r d d r o p p i n g s and who a l s o p e r i o d i c a l l y remove and r e p l a c e specimens a c c o r d i n g t o a S i t e Management P l a n ( 9 ) . Samples removed a r e s e a l e d , padded, and s h i p p e d by ground t r a n s p o r t a t i o n to p a r t i c i p a t i n g laboratories for s p e c i f i c tests. S u r f a c e Chemistry. The schedule o f sample exposure d u r i n g t h e f i r s t y e a r o f t h e t e s t program was as f o l l o w s : f o r each three-month p e r i o d , b r i q u e t t e s were exposed i n r a c k s and then r e p l a c e d w i t h f r e s h samples f o r t h e f o l l o w i n g three-month p e r i o d . An a d d i t i o n a l s e t o f b r i q u e t t e s was exposed f o r t h e e n t i r e y e a r , and o t h e r samples p r e s e n t i n t h e r a c k s a r e p a r t o f a p l a n n e d m u l t i y e a r exposure program. Q u a r t e r l y w i t h d r a w a l s f o r c h e m i c a l a n a l y s i s a r e t o be made f o r two y e a r s , a f t e r w h i c h a c u m u l a t i v e annual frequency i s t o be m a i n t a i n e d . One s e t o f samples i s c o n t i n u o u s l y exposed f o r p o s t e r i t y o r f o r unf o r e s e e n developments. Three samples o f each stone type a r e employed i n each c y c l e o f t e s t s . The p l a n i s s u b j e c t t o change i n t h e l i g h t of a n a l y s i s r e s u l t s as they a r e produced. The s u r f a c e c h e m i s t r y a n a l y s i s p r o t o c o l u t i l i z e s methods dev e l o p e d f o r t h e s u b j e c t program (10). L a y e r s 0.25 mm t h i c k a r e s e q u e n t i a l l y removed from t h e stone b r i q u e t t e s a t t h e s u r f a c e l o c a t i o n s i l l u s t r a t e d i n F i g u r e 2. The p r o c e s s o f removal, shown i n F i g u r e 4, produces powder samples w e i g h i n g a p p r o x i m a t e l y one gram p e r l a y e r . Weighed p o r t i o n s o f each powdered l a y e r sample a r e s u b j e c t e d t o an e x t r a c t i o n p r o c e d u r e t o r e c o v e r t h e l e a c h a b l e anions f o r a n a l y s i s by i o n chromatography ( I C ) . A s e p a r a t e p o r t i o n o f each powder can be d i s s o l v e d and a n a l y z e d by i n d u c t i v e l y c o u p l e d plasma s p e c t r o s c o p y ( I C P ) ; however, because t h e ICP a n a l y s e s have so f a r i n d i c a t e d l i t t l e c a t i o n change a t t h e stone s u r f a c e s , t h i s type o f a n a l y s i s i s c u r r e n t l y b e i n g more s e l e c t i v e l y a p p l i e d . The exposed and a n a l y z e d b r i q u e t t e s , as w e l l as r e s i d u a l powders and s o l u t i o n s , a r e a r c h i v e d f o r f u r t h e r s t u d y , i f needed. S u r f a c e Roughening/Recession. Two approaches t o measurement o f s u r f a c e r e c e s s i o n were e x p l o r e d . One method a p p l i e d a c l a s s i c a l form of o p t i c a l i n t e r f e r o m e t r y t o o p t i c a l l y smooth samples o f stone i n an attempt t o measure r e s u l t s a f t e r b r i e f exposure o f t h e s u r f a c e s t o r a i n ; t h i s approach was o n l y p a r t i a l l y s u c c e s s f u l and w i l l be o n l y



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F i g u r e 2. Dimensions o f wedge-shaped b r i q u e t t e s employed as specimens o f marble and l i m e s t o n e . A l s o shown a r e l o c a t i o n s o f areas sampled i n l a y e r s t o a s s e s s depth o f c h e m i c a l a l t e r a t i o n caused by a t m o s p h e r i c exposure.

F i g u r e 3. Stone b r i q u e t t e s a r r a y e d f o r exposure t o a t m o s p h e r i c a c i d d e p o s i t i o n and w e a t h e r i n g a t f i e l d t e s t s i t e . (a) s i d e and (b) f r o n t v i e w s .



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b r i e f l y summarized. A second method employed l a s e r - h o l o g r a p h i c moire p r o f i l i n g , developed by Dr. C. S c i a m m a r e l l a o f t h e I l l i n o i s I n s t i t u t e of Technology ( I I T ) and ANL (11-12), and was s u c c e s s f u l l y a p p l i e d even t o t h e r e l a t i v e l y unsmooth, commercial s u r f a c e s , f o r t e s t s i n which a s u b s t a n t i a l exposure t o r a i n i s e x p e r i e n c e d by samples. In the f i r s t approach noted above, t h e p r e p a r e d stone specimen s u r f a c e s were p r o t e c t e d o v e r a p o r t i o n o f t h e i r areas by means o f a P a r a f i l m M s e a l w h i c h was h e l d i n p l a c e by a p l a t e o f i n e r t m a t e r i a l . The a s s e m b l i e s were then exposed b r i e f l y i n the North C a r o l i n a t e s t r a c k u n t i l 38 mm o f r a i n (pH 4) had o c c u r r e d d u r i n g exposure. The a s s e m b l i e s were r e t u r n e d t o t h e l a b o r a t o r y , and t h e samples were e x amined i n a Twyman-Green i n t e r f e r o m e t e r i n an attempt t o measure t h e h e i g h t o f t h e s t e p expected between p r o t e c t e d and u n p r o t e c t e d a r e a s . Some v a r i a t i o n s i n e l e v a t i o n were observed, b u t r e s u l t s were ext r e m e l y i r r e g u l a r (an average h e i g h t o f 0.25 micrometers was r o u g h l y e s t i m a t e d f o r m a r b l e , and l i m e s t o n e seemed u n a f f e c t e d ) . F o r a f o l l o w i n g t e s t , t h e sample d e s i g n was m o d i f i e d t o p r o v i d e two p r o t e c t e d zones on each s a m p l e d upper s u r f a c e , s e p a r a t e d by a b r o a d and i n i t i a l l y f l a t zone t o be exposed t o r a i n . I t was planned t o employ a F i z e a u i n t e r f e r o m e t e r t o measure changes i n s u r f a c e p r o f i l e s r a t h e r than s t e p h e i g h t s ; however, f o l l o w i n g a p r o l o n g e d exposure o f assemb l i e s a t t h e t e s t s i t e t o accumulate 38 mm o f r a i n e x p o s u r e , i t was found t h a t a s u b s t a n t i a l upward bowing o f t h e upper s u r f a c e o f b o t h marble and l i m e s t o n e had o c c u r r e d . T h i s e f f e c t was n o t r e a d i l y a c commodated i n d a t a r e d u c t i o n . The d i m e n s i o n a l i n s t a b i l i t y o f t h e m a t e r i a l has n o t y e t been overcome t o enable r e c e s s i o n measurements of such h i g h s e n s i t i v i t y ( 1 3 ) . The d e s i g n o f sample a s s e m b l i e s used f o r the l a s e r - m o i r e approach i s i l l u s t r a t e d i n F i g u r e s 5 and 6. P o r t i o n s o f t h e t e s t s u r f a c e a r e p r o t e c t e d by T e f l o n b a r s t h a t have machined r e c e s s e s t o a v o i d c o n t a c t w i t h r e f e r e n c e (unexposed) areas o f the sample. F o r some samples, the T e f l o n i s supplemented w i t h P a r a f i l m M g a s k e t s . N y l o n f a s t e n e r s are used where r e q u i r e d n e a r specimens, and s t a i n l e s s s t e e l screws f a s t e n t o g e t h e r t h e p a r t s o f t h e L u c i t e h o l d e r s . A l t h o u g h some e x change o f l i q u i d water between u n p r o t e c t e d and p r o t e c t e d zones i s u n a v o i d a b l e f o r porous samples h a v i n g unsmooth s u r f a c e s , the l i q u i d i s uncontaminated by m a t e r i a l s o f t h e h o l d e r s and may be e q u i l i b r a t e d w i t h t h e specimens; no d i f f i c u l t y has been apparent. Annual and p o s t e r i t y s e t s o f l a s e r - m o i r e specimens o f b o t h stone types have been p r o v i d e d f o r a l l f o u r t e s t s i t e s . One y e a r o f e x posure has been accumulated a t the N o r t h C a r o l i n a and New J e r s e y s i t e s , and s e v e r a l months exposure a t t h e New York and D i s t r i c t o f Columbia s i t e s . Samples r e t u r n e d t o t h e l a b o r a t o r y a f t e r exposure a r e measured by micrometer p r i o r t o l a s e r - m o i r e e v a l u a t i o n ; micrometer measurements are r e p r o d u c i b l e o n l y t o w i t h i n 13 m i c r o m e t e r s because s u r f a c e roughness produces an u n u s u a l l y h i g h s e n s i t i v i t y o f measurements t o p l a c e ment o f t h e micrometer. F o l l o w i n g l a s e r - m o i r e measurements, t h e samples have i n some i n s t a n c e s been checked by an e l e c t r o n i c d i a l gage p r o f i l i n g t e c h n i q u e . Because t h e r e i s r i s k o f damage t o samples by t h e s t y l u s o f t h e gage, i t s use i s m i n i m i z e d ; t h u s , t h e s t a t i s t i c s are fewer than d e s i r e d and f a r l e s s comprehensive than those o f t h e c o m p l e t e l y n o n d e s t r u c t i v e l a s e r - m o i r e method. Samples a r e r e t u r n e d to t e s t s i t e s f o r c o n t i n u e d exposure a f t e r measurements. 1


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F i g u r e 4. Stone specimen mounted i n s h a p i n g machine f o r s u r f a c e sampling. Tungsten c a r b i d e t o o l b i t r e c i p r o c a t i n g h o r i z o n t a l l y p r e c i s e l y removes l a y e r s o f s t o n e , p r o d u c i n g powders f o r c h e m i c a l analysis.

F i g u r e 5. S e c t i o n s o f b r i q u e t t e s i n L u c i t e h o l d e r s f o r atmosp h e r i c exposure i n t e s t r a c k s p r i o r t o measurements o f stone s u r f a c e roughness and r e c e s s i o n . T e f l o n b a r s p r o t e c t p o r t i o n s o f s u r f a c e s f o r r e f e r e n c e i n measurements by l a s e r h o l o g r a p h i c moire p r o f i l i n g method.

1 1

EXPOSED MASK SKYWARD MASK 27 SURFACE ~I4* — 2 2 — -H4—

SIDE SURFACE

54

FRONT SURFACE

F i g u r e 6.

DIMENSIONS : mm

Dimension drawing o f stone specimens shown i n F i g u r e 5.



19.

YOUNGDAHL A N D DOE


273

The l a s e r - m o i r e method employs a helium-neon source t o produce a double exposure hologram from w h i c h an i n t e r f e r o g r a m i s l a t e r made. The a n g l e o f i n c i d e n c e o f t h e l i g h t on t h e specimen i s changed between t h e two exposures by an a c c u r a t e l y measured amount. O p t i c a l f i l t e r i n g i s used t o e l i m i n a t e l i g h t randomly s c a t t e r e d from t h e rough sample s u r f a c e , p r o v i d i n g a f i n a l p r o f i l e o f the average s u r f a c e (12). The amount o f sample area c o n s i d e r e d can be v a r i e d by changing the m a g n i f i c a t i o n used, and t h i s c a p a b i l i t y i s u t i l i z e d i n roughness m o n i t o r i n g . F o r r e c e s s i o n measurements, t h e e n t i r e upper s u r f a c e o f the sample i s p r o f i l e d by e v a l u a t i n g phase changes o f t h e i n t e r f e r e n c e f r i n g e s as a f u n c t i o n o f l o c a t i o n . I n e v a l u a t i o n s o f b o t h roughness and r e c e s s i o n , t h e i n t e r f e r o g r a m s a r e d i g i t i z e d , and d a t a r e d u c t i o n i s performed a u t o m a t i c a l l y by computer r o u t i n e s dev e l o p e d e s p e c i a l l y f o r t h i s program. D i m e n s i o n a l i n s t a b i l i t y o f t h e stone m a t e r i a l s on a m i c r o s c a l e , mentioned above, has been o b s e r v e d by t h e l a s e r - m o i r e method as w e l l as by t h e F i z e a u and e l e c t r o n i c d i a l gage methods. However, the d i f f i c u l t y has n o t been apparent i n many o f t h e samples, and t h e r e l a t i v e e f f e c t o f t h e i n s t a b i l i t y may d i m i n i s h as t h e magnitudes o f roughness and r e c e s s i o n i n c r e a s e w i t h exposure time. Weight Change. B r i q u e t t e s used f o r m o n i t o r i n g w e i g h t changes were p r e p a r e d w i t h rounded edges and c o r n e r s i n o r d e r t o m i n i m i z e e r r o r s caused by i n a d v e r t e n t damage d u r i n g h a n d l i n g . A f t e r f a b r i c a t i o n , the samples were r i n s e d under f l o w i n g d i s t i l l e d water as s u r f a c e powder was removed w i t h a s o f t b r i s t l e b r u s h and then p l a c e d i n a 45°C d r y i n g oven f o r one week. B r i q u e t t e s were then a l l o w e d t o e q u i l i b r a t e w i t h a i r ( a t 22°C and

Materials Degradation Caused by Acid Rain - American Chemical

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