Deterioration of Brick Masonry Caused by Acid Rain - American

17 Deterioration of Brick Masonry Caused by Acid Rain 1

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A. E. Charola and L. Lazzarini 1

Metropolitan Museum of Art, New York, NY 10028 Laboratorio Scientifico della Misericordia, 30121 Venice, Italy

Downloaded by UNIV OF LEEDS on August 25, 2016 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch017

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Several different mechanisms are operant in the dete rioration of brick masonry through the action of acid rain. The bricks are susceptible to acid rain through the selective dissolution of their glassy phase. The mortar is affected mainly by the reaction of its cal careous components. The soluble salts resulting from these reactions, in solution with rain water or con densed moisture, will migrate through the porous ma trix of the masonry. In the places where the water evaporates the salts will be deposited. Repeated dissolution and recrystallization of these salts leads to the mechanical disruption of the ma sonry structure. Since the salts will concentrate in the more porous material, either the brick or the mortar will be more seriously affected, depending on their relative porosity.

Do we build a house to last forever? The Epic of Gilgamesh (1) (originated about third millenium BC) D e t e r i o r a t i o n of b r i c k masonry i s a problem t h a t has w o r r i e d man e v e r s i n c e the f i r s t b r i c k w a l l was c o n s t r u c t e d . I t i s i n t e r e s t i n g to remember t h a t a l r e a d y by the f i r s t c e n t u r y Α.Ό. b r i c k s were c o n s i d e r e d more r e s i s t a n t t o d e t e r i o r a t i o n than m a r b l e . , w h i c h had an e s t i m a t e d u s e f u l l i f e t i m e o f about 80 y e a r s ( 2 ) . Today, the problem i s s t i l l i n s t u d y , i f o n l y due t o the i n c r e a s e d number of masonry s t r u c t u r e s . The d e t e r i o r a t i o n of b r i c k masonry i s a complex problem, i n w h i c h two v a r i a b l e i n g r e d i e n t s , b r i c k and m o r t a r , make up the whole. As each component can have a l a r g e v a r i a t i o n i n c o m p o s i t i o n and s t r u c t u r e , when b o t h a r e combined i n a w a l l , the number of v a r i a t i o n s t h a t r e s u l t i s g i v e n by a l l the p o s s i b l e c o m b i n a t i o n s of the two components. Both m a t e r i a l s are more o r l e s s s u s c e p t i b l e t o a t t a c k by a c i d r a i n , but the o v e r a l l d e t e r i o r a t i o n i s not a s i m p l e c h e m i c a l a t t a c k 0097-6156/ 86/ 0318-0250S06.00/ 0 © 1986 American Chemical Society

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

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on the i n d i v i d u a l m a t e r i a l s . Other phenomena, such as the c r y s t a l l i z a t i o n of the s a l t s formed by t h i s a t t a c k , can compound the e f f e c t , and i n some i n s t a n c e s c o n t r i b u t e s i g n i f i c a n t l y t o the d e t e r i o r a t i o n of b r i c k masonry.


Brick B r i c k s a r e ceramic b o d i e s manufactured from c l a y s , molded and f i r e d . The n a t u r e and q u a l i t y of the b r i c k s w i l l depend on a l l the above mentioned parametes: the c l a y s used i n the m a n u f a c t u r e , the m o l d i n g t e c h n i q u e u s e d , the f i r i n g temperature and the r e s i d e n c e time i n the f i r i n g k i l n o r time a t temperature ( 3 ) . B r i c k s produced a t h i g h e r temperatures and l o n g e r t i m e s a t temperature w i l l have a more v i t r e o u s m a t r i x . A l s o , t h e i r hardness and s t r e n g t h w i l l i n c r e a s e and the water a b s o r p t i o n c a p a c i t y w i l l d e c r e a s e as t h e pore s i z e d i s t r i b u t i o n s h i f t s towards s m a l l e r p o r e s . T h i s l a s t f e a t u r e i s i m p o r t a n t as the d e t e r i o r a t i o n of b r i c k s i s u s u a l l y r e l a t e d t o the amount of w a t e r t h a t can be absorbed by them. The p r e s e n c e of a l a r g e number of s m a l l e r pores (9), a second s u r f a c e c o n t r o l l e d r e a c t i o n t h a t d i s s o l v e s the l e a c h e d l a y e r can o c c u r s i m u l t a n e o u s l y ( 7 ) . A l l s i l i c a glasses are p a r t i c u l a r l y s u s c e p t i b l e to d i s s o l u t i o n by s o l u t i o n s above pH 9 due t o the n u c l e o p h i l i c a t t a c k of the hydroxy1 i o n on the s i l i c o n - o x y g e n bond ( 7 ) . T h i s i s r e l e v a n t i n our p a r t i c u l a r d i s c u s s i o n on b r i c k s , as t h e s e a r e s e t i n masonry w i t h m o r t a r , from w h i c h l i m e may be l e a c h e d out such t h a t s o l u t i o n s r e a c h i n g the n e i g h b o u r i n g pores c o u l d w e l l r e a c h pH 11 o r 12. In the case of d i l u t e a c i d s o l u t i o n s (pH >2), the a l k a l i s and b a s i c o x i d e s a r e d i s s o l v e d p r e f e r e n t i a l l y , but the amount of s i l i c a d i s s o l v e d i s l e s s than t h a t removed by water a l o n e 9). T h i s can be e x p l a i n e d by the f a c t t h a t the a c i d n e u t r a l i z e s the l e a c h e d a l k a l i s , so t h a t the pH does not r i s e t o h i g h e r v a l u e s where s i l i c a d i s s o l u t i o n becomes i m p o r t a n t . The p r e f e r e n t i a l l e a c h i n g produces an a l k a l i - d e p l e t e d l a y e r t h a t can be t w i c e as t h i c k as the one o b t a i n e d by n e u t r a l w a t e r ( 1 0 ) . Because of the replacement of the a l k a l i i o n s by the s m a l l e r hydrogen i o n s , s t r e s s e s w i l l be i n d u c e d i n t h i s l a y e r which can cause i t t o c r a c k . F u r t h e r s h r i n k a g e can a l s o o c c u r i f t h i s h y d r a t e d s i l i c a l a y e r l o s e s water (10,11). T h i s t y p e of d e t e r i o r a t i o n was observed i n the g l a s s y m a t r i x of a b r i c k exposed t o c o n c e n t r a t e d s u l f u r i c a c i d where the l e a c h e d l a y e r crumbled away e x p o s i n g the more a c i d r e s i s t a n t m i n e r a l s (12) . S u l f u r i c a c i d s o l u t i o n s formed from a t m o s p h e r i c c o n s t i t u e n t s were e s t i m a t e d t o i n c r e a s e t e n f o l d the r a t e o f a t t a c k on b r i c k s ( 1 3 ) . Gaseous S 0 i n a water vapour s a t u r a t e d atmosphere was found t o 2


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d e t e r i o r a t e b r i c k s by i n c r e a s i n g t h e i r p o r o s i t y , m a i n l y i n r e s p e c t t o the l a r g e r pores ( 1 4 ) . N i t r i c and h y d r o c h l o r i c a c i d were found t o be even more a g g r e s s i v e t o g l a s s than s u l f u r i c a c i d ( 1 5 ) . I t c o u l d t h e r e f o r e be e x p e c t e d t h a t n i t r o g e n o x i d e s would be more a g g r e s s i v e than s u l f u r oxides w i t h respect to b r i c k d e t e r i o r a t i o n . Another important f a c t o r i n the d i s s o l u t i o n of g l a s s i s the r a t i o of s u r f a c e a r e a exposed t o volume of l e a c h i n g s o l u t i o n . The h i g h e r t h i s r a t i o the h i g h e r the amount of s i l i c a t e d i s s o l v e d though the amount of a l k a l i i o n e x t r a c t e d does not appear to be q u i t e so a f f e c t e d (7,10). C o n s i d e r i n g the p a r t i c u l a r case of b r i c k s exposed t o a c i d r a i n , the d i s s o l u t i o n of the g l a s s y m a t r i x w i l l o c c u r i n the f i l m s of w a t e r t h a t wet the exposed s u r f a c e and the i n n e r w a l l s of the pores of the b r i c k . In e i t h e r case the SA/V r a t i o s w i l l be v e r y h i g h thus a c c e l e r a t i n g d i s s o l u t i o n . Because of the c o n s t a n t renewal of the w e t t i n g s o l u t i o n the d i s s o l u t i o n w i l l be even more i m p o r t a n t on the exposed s u r f a c e f i l m ( 1 6 ) . I t s h o u l d a l s o be p o i n t e d out t h a t the d i s s o l u t i o n e f f e c t of water r u n - o f f may be more important than the pH of the s o l u t i o n . T h i s has been shown t o be the case even f o r a c i d s u s c e p t i b l e c a l c a r e o u s m a t e r i a l s . The r u n - o f f e f f e c t was more i m p o r t a n t than the a c t u a l pH of the s o l u t i o n , f o r pHs above 3 ( 1 7 ) . The appearance of the s u r f a c e of 16th c e n t u r y b r i c k s i n V e n i c e , I t a l y , exposed t o r a i n water r u n - o f f i s shown i n F i g u r e 1. S u r f a c e e r o s i o n , as can be seen, i s f a i r l y a p p r e c i a b l e , and e s p e c i a l l y n o t i c e a b l e i s the uneveness i n the e r o s i o n due t o d i f f e r e n c e i n v i t r i f i c a t i o n . These b r i c k s , and even o l d e r ones (12th c e n t u r y ) have been s t u d i e d because they have r e s i s t e d r a i n w a t e r r u n - o f f f o r seven c e n t u r i e s and h i g h a i r p o l l u t i o n f o r the p a s t h a l f c e n t u r y , the l a t t e r due t o the p r o x i m i t y of V e n i c e t o the r e f i n e r y a t P o r t o Marghera. SEM e x a m i n a t i o n of t h e s e b r i c k samples show i n d e t a i l the r e s u l t of the p r o l o n g e d exposure t o r a i n - w a t e r r u n - o f f ( F i g u r e 2 ) . The eroded s u r f a c e i s apparent even a t low m a g n i f i c a t i o n . Nodules of h i g h l y v i t r i f i e d m a t e r i a l can be seen as w e l l as the "channels" carved around them by the r u n n i n g w a t e r . F i g u r e 3 shows the d e t a i l a t h i g h e r m a g n i f i c a t i o n of the r e s i d u a l p l a t y m i n e r a l s w h i c h were exposed as the g l a s s y phase eroded away. F i g u r e s 2 and 3 are of l o c a l V e n e t i a n b r i c k s f a b r i c a t e d i n the 12th century. These b r i c k s are f o r h i s t o r i c a l reasons called " a l t i n e l l e " and have a f a i r l y u n i f o r m s i z e of a p p r o x i m a t e l y 17 cm χ 7 cm χ 5 cm ( 1 8 ) . T a b l e I g i v e s a t y p i c a l c h e m i c a l a n a l y s i s f o r the m a t r i x of the b r i c k and f o r the more r e s i s t a n t , v i t r i f i e d n o d u l e s . Table I .

C h e m i c a l A n a l y s i s of 12th c e n t u r y V e n e t i a n brick

Si0 %w/w

2

A1 0 %w/w 2

3

Fe 0 %w/w 2

3

"altinella"

MgO %w/w

CaO %w/w

Na 0 %w/w

K0 %w/w

Ti0 %w/w

2

2

Matrix

87.10

6.94

1.31

0.13

0.24

0.28

0.74

0.56

Nodules

71.43

7.14

15.42

0.13

0.36

0.20

0.75

0.46


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CHAROLA A N D LAZZARINI

Deterioration of Brick Masonry

F i g u r e 1. S u r f a c e o f a 16th c e n t u r y V e n e t i a n b r i c k eroded by r a i n water r u n - o f f . A h i g h l y - v i t r i f i e d nodule r e s i s t s d e t e r i o r a t i o n b e t t e r than t h e l e s s v i t r i f i e d m a t r i x .

F i g u r e 2. SEM photomicrograph o f an eroded s u r f a c e o f a 12th c e n t u r y b r i c k , " a l t i n e l l a " , from V e n i c e . The q u a r t z g r a i n s a r e exposed as t h e more s u s c e p t i b l e m a t r i x d e t e r i o r a t e s f a s t e r .

F i g u r e 3. SEM photomicrograph showing i n d e t a i l t h e m a t r i x o f the " a l t i n e l l a " b r i c k . P l a t y agglomerates s t a r t t o be exposed as t h e g l a s s y m a t r i x i s d i s s o l v e d away by t h e a c i d r a i n .


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As can be seen, the n o d u l e s have a h i g h e r F e 0 c o n t e n t w h i c h has the double f u n c t i o n of i n c r e a s i n g the f u s i b i l i t y of the c l a y , g i v i n g r i s e to a l e s s porous m a t r i x , and p r o d u c i n g a c h e m i c a l l y more r e s i s t a n t g l a s s y phase. I t i s most i n t e r e s t i n g t h a t these b r i c k s , even though they were f i r e d at r e l a t i v e l y low temperatures (850-900°C) and are not h i g h l y v i t r i f i e d , are more d u r a b l e than any modern b r i c k s ( 1 8 ) . T h i s can be explained by the f a c t t h a t these b r i c k s are v e r y homogeneous throughout and l a c k i n g the h i g h l y v i t r i f i e d outer s k i n which characterizes modern b r i c k s . T h e i r pore s i z e d i s t r i b u t i o n i s c e n t e r e d around l a r g e r pore s i z e s (1-2 ym) thus a v o i d i n g the c r i t i c a l s m a l l e r range (

Deterioration of Brick Masonry Caused by Acid Rain - American

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