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cement ratio concretes
CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 969-978, 1985. Printed 0008/8846/85 $3.00+OO. Copyright (c) 1985 Pergamon Press, Ltd.
in the USA
STUDIES ON CHEMICAL RESISTANCE OF LOW WATER/CEMENT RATIO CONCRETES
Civil
P.K. Mehta Engineering Department, University of California, Berkeley, CA 97420
(Communicated by J.P. Skalny) (Received June 25, 1985) ABSTRACT Solutions containing m i n e r a l acids, and c e r t a i n o r g a n i c a c i d s and s a l t s are h i g h l y c o r r o s i v e to portland cement concrete. Since permeability is t h e key factor governing the rate of deterioration, it is c u s t o m a r y to u s e a l o w water-cement ratio in m a k i n g concretes or concrete overlays required to r e s i s t c o r r o s i v e action of aggressive chemical solutions. Pozzolanic admixtures are o f t e n u s e d to p r o v i d e additional protection against acidic attack. Highly reactive pozzolanie admixtures, s u c h as condensed silica fume, which rapidly react with calcium hydroxide and reduce both the a l k a l i n i t y and p e r m e a b i l i t y of c o n c r e t e are n o w being used for improving durability. During the last two decades, latex admixtures have also found widespread application. The polymeric constituents of a l a t e x s e e m to coat the alkaline hydration products of p o r t l a n d cement, thus protecting them from attack by aggressive solutions. An experimental study was undertaken to evaluate the relative chemical resistance of low water-cement ratio concretes, containing either a styrene-butadiene l a t e x or a s i l i c a fume admixture, to the f o l l o w i n g solutions; 1% HCI, 1% H ~ S O 4, 1% l a c t i c a c i d , 5% a c e t i c a c i d , 5% a m m o n l u m sulfate, a n d 5% s o d i u m sulfate. T i m e t a k e n to r e g i s t e r 25 p e r c e n t weight loss by f u l l y s u b m e r g e d concrete specimens was used as a c r i t e r i o n for failure. F r o m t h e d a t a it appears that, except for the ammonium sulfate solution, the concrete containing the silica fume generally showed better resistance to c h e m i c a l a t t a c k t h a n o t h e r c o n c r e t e types. 969
970
Vol. P.K.
15, No. 6
Mehta
S u i t a b l y p r o p o r t i o n e d , w e l l c o m p a c t e d , and a d e q u a t e l y cured portland cement concrete has a history of satisfactory long-term performance under most natural and i n d u s t r i a l c o n d i t i o n s . S i n c e the h y d r a t i o n p r o d u c t s of p o r t l a n d c e m e n t c o n t a i n c a l c i u m c o m p o u n d s w h i c h are decomposed by a c i d i c solutions, various protective measures are often necessary to p r o l o n g the service l i f e of c o n c r e t e s e x p o s e d to a c i d i c e n v i r o n m e n t s . M i n e r a l acids, viz., HC1, HN03, and H2S04, are h i g h l y c o r r o s i v e to c o n c r e t e b e c a u s e t M e y f o r m s o l u b l e c a l c i u m salts that are leached away. This increases the p o r o s i t y and l o w e r s the s t r e n g t h of the m a t e r i a l . High molecular w e i g h t o r g a n i c a c i d s are not as c o r r o s i v e as mineral acids, but acetic and lactic acids, which are found in m a n y food products, are very corrosive to concrete. P e r m e a b l e c o n c r e t e s m a d e w i t h h i g h C3A (>5%) p o r t l a n d cement c a n be d a m a g e d by e x p a n s i o n an~ cracking when e x p o s e d to c o n c e n t r a t e d s o l u t i o n s of s o d i u m , potassium, or m a g n e s i u m sulfate. Among the sulfates, ammonium sulfate solution is t h e m o s t c o r r o s i v e because it n o t o n l y r e a c t s w i t h p o r t l a n d c e m e n t h y d r a t i o n p r o d u c t s to form gypsum, but also can dissolve the gypsum coating. C h l o r i d e s and d e i c e r c h e m i c a l s are u s u a l l y not h a r m f u l to p l a i n c o n c r e t e b u t c a n be i n s t r u m e n t a l in e x p a n s i o n and c r a c k i n g of c o n c r e t e c o n t a i n i n g e m b e d d e d steel. Permeability is g e n e r a l l y the key f a c t o r g o v e r n i n g the r a t e of d e t e r i o r a t i o n of c o n c r e t e e x p o s e d to a g g r e s s i v e chemical solutions. When permeability is l o w , the decomposition of the c e m e n t i t i o u s m a t t e r is l i m i t e d to the surface; otherwise the process of d e c o m p o s i t i o n s p r e a d s i n t o the i n t e r i o r . The r a t e of d e t e r i o r a t i o n is accelerated if t h e p r o d u c t s of chemical decomposition are washed a w a y by a s o l u t i o n in m o t i o n , or w h e n t h e c o n c r e t e s u r f a c e is e x p o s e d to s o m e k i n d of m e c h a n i c a l attrition, as f o r e x a m p l e heavy vehicular traffic on concrete floors. Thus, it i s t h e g r a d u a l loss of material rather t h a n t h e l o s s of s t r e n g t h or e l a s t i c m o d u l u s w h i c h b e c o m e s the p r i m a r y c a u s e of f a i l u r e . When highly corrosive chemicals are present in t h e environment, one way to p r o t e c t the concrete is to provide a 20-50 mm overlay of a v e r y l o w w a t e r / c e m e n t ratio concrete mixture. Water-reducing and airentraining, admixtures are commonly u s e d to a c h i e v e water/cement ratios of t h e o r d e r of 0.3. Pozzolanic a d m i x t u r e s are h i g h l y e f f e c t i v e in r e d u c i n g not o n l y the permeability of c o n c r e t e by the p o r e r e f i n e m e n t process (I-3) but also the calcium hydroxide hydrated cement paste, the calcium h i g h l y v u l n e r a b l e to a c i d a t t a c k .
content hydroxide
of t h e being
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971 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
The objective of t h e r e s e a r c h reported h e r e w a s to evaluate the chemical durability of l o w w a t e r / c e m e n t ratio concretes containing a latex or a p o z z o l a n i c admixture. Materials
and
Procedure
The m i x p r o p o r t i o n s and p r o p e r t i e s of c o n c r e t e m i x t u r e s investigated in t h i s s t u d y are s h o w n in T a b l e s 1 and 2, respectively. Using a Type I/II portland cement (7% C~A), a c l e a n sand, and a g l a c i a l g r a v e l (12 m m m a x i m u m size), t h r e e t y p e s of c o n c r e t e m i x t u r e s w e r e made. One type c o n t a i n e d a s t y r e n e - b u t a d i n e l a t e x w h i c h w a s in the form of a water emulsion. T h e l a t e x w a s a d d e d i n an amount which corresponded to 16% s o l i d l a t e x by w e i g h t of c e m e n t . The net water/cement r a t i o of t h e c o n c r e t e batch was 0.35. The presence of s u r f a c t a n t s in the l a t e x e m u l s i o n w a s the r e a s o n for 5.6 to air e n t r a i n m e n t and high consistency (225 mm slump) of t h e c o n c r e t e mixture.
Concrete
Mix
TABLE I Proportions,
Type
kg/m 3
of
LMC
Concrete SFMC
LWC
Portland cement Fine aggregate Coarse aggregate Water
391 1013 676 137
404 887 887 153
488 876 876 161
W/C
0.35
0.33
0.33
Ratio
Note:
LMC Contains 16% s o l i d l a t e x a n d 1 5 % s i l i c a f u m e by wt. o f c e m e n t TABLE Properties
of
contains
2 Concrete
Type
S l u m p , mm Air, % Average compressive s t r e n g t h of c y l i n d e r s b e f o r e i m m e r s i o n in solutions, MPa
SFMC
of
Concrete
LMC
SFMC
LWC
225 5.6
250 4.1
18 4.9
47
74
64
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Vol. 15, No. () P.K. Mehta
The second concrete was made with a proprietary product containing silica fume, water, and waterreducing admixtures. The addition of t h i s product accounted for 15% s i l i c a f u m e (also c a l l e d m i c r o s i l i c a ) by w e i g h t of c e m e n t in t h e c o n c r e t e mixture. The net r a t i o b e t w e e n w a t e r to c e m e n t i t i o u s materials (cement ÷ silica fume) was 0.33. In s p i t e of the low water c o n t e n t , a b o u t 250 m m c o n c r e t e s l u m p w a s o b t a i n e d ; also, the concrete mixture w a s f o u n d to h a v e e n t r a i n e d 4.1% air. A third concrete mixture was made for reference purpose. The r e f e r e n c e c o n c r e t e had w a t e r / c e m e n t ratio of t h e s a m e o r d e r as t h e o t h e r t w o c o n c r e t e s . B o t h an air-entraining admixture and a w a t e r - r e d u c i n g admixture were u s e d in t h i s c o n c r e t e to b r i n g d o w n the water/ c e m e n t r a t i o to 0.33. The c o n c r e t e , h o w e v e r , was quite s t i f f (18 m m s l u m p ) but c a s t a b l e . Cylindrincal specimens, 45 m m d i a m e t e r by 90 m m long, were c a s t in p l a s t i c m o l d s . A f t e r 24 h o u r s of c o n c r e t e p l a c e m e n t , the m o l d s w e r e s t r i p p e d . For the n e x t 6 d a y s the silica-fume modified concretes (SFMC) and the reference low water/cement ratio concrete (LWC) were cured in w a t e r , whereas the latex modified concrete (LMC) was cured in laboratory a i r ( 5 0 % RH, 7 0 + 3 F), as per latex manufacturer's recommendation. After the i n i t i a l 1 - w e e k p e r i o d , all c o n c r e t e s p e c i m e n s w e r e c u r e d in a i r ( 5 0 % RH, 7 0 + 3 F) f o r a b o u t s i x w e e k s ; then they w e r e i m m e r s e d in v a r i o u s c h e m i c a l s o l u t i o n s . The following six solutions were selected for immersion of c o n c r e t e cylinders at 20 C ( 6 8 F): 1% HCI, 1% H 2 S 0 4 , 1% l a c t i c a c i d , 5% a c e t i c a c i d , 5% a m m o n i u m sulfate, a n d 5% s o d i u m sullfate. The immersion tanks were fitted with magnetic stirrers which kept the s o l u t i o n s in a s t a t e of c o n s t a n t m o t i o n . T h i s h e l p e d to accelerate the chemical attack by p r e v e n t i n g the deposition of r e l a t i v e l y i n s o l u b l e r e a c t i o n p r o d u c t s on or n e a r the c o n c r e t e s u r f a c e . For the s a m e r e a s o n , o n c e every week the spent solutions were replaced with freshly made solutions. There a r e no s t a n d a r d criteria for evaluation of resistance of c o n c r e t e exposed to c o r r o s i v e chemical fluids. E x p e r i e n c e s h o w s t h a t w e i g h t l o s s of s p e c i m e n s after removal of the weak reaction product from the s u r f a c e of c o n c r e t e w i t h a s t e e l - w i r e b r u s h is the m o s t convenient way of simulating the industrial floors subjected to c o r r o s i v e f l u i d s and m e c h a n i c a l attrition. The number of days needed to r e g i s t e r a 2 5 % l o s s in weight is g e n e r a l l y accepted as t h e f a i l u r e c r i t e r i o n . Therefore, at t h e t i m e of s o l u t i o n change, i.e., o n c e every week, specimens were wlre-brushed, washed, and weighed. Three specimens of e a c h c o n c r e t e in a g i v e n environmental c o n d i t i o n w e r e m o n i t o r e d for w e i g h t loss.
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973 CHEMICAL RESISTANCE, ADMIXTURES,
LOW W/C, CONCRETES
A n o t h e r o b j e c t of t h i s i n v e s t i g a t i o n w a s to d e t e r m i n e the chemical resistance of concretes at a warmer temperature of e x p o s u r e ; therefore a duplicate s e t of immersion tanks containing the s p e c i m e n s w e r e s t o r e d in a room which was maintained at a c o n s t a n t t e m p e r a t u r e of 45 C. R e su i t s
and
~
m
i
~
Average weight loss from three specimens of e a c h c o n c r e t e t y p e s t o r e d in d i f f e r e n t s o l u t i o n s at 20 C are s h o w n in Fig. 1-5. The d a t a for the f a i l u r e c r i t e r i o n , are t a b u l a t e d in T a b l e 3. TABLE Time
for
25
Percent
Solution 1% HCI 5% a c e t i c I% l a c t i c I%H2S04
acid acid
5%
(NH4)2SO 4
5%
Na2so 4
*20%
weight
no loss
at
3
Weight
Loss
20
C,
Days
LMC
SFMC
LWC
39 75 115 *
63 168 182 *
39 48 110 105
*
120
120
loss
the
at
end
no of
the
loss
test
no l o s s
(182
The hydrochloric acid solution was most corrosive. B o t h the L W C r e f e r e n c e
days)
f o u n d to be c o n c r e t e and
the the
tJ E ! G H T
10S
Lwc [ ]
100
,.c O SFMC A
95 90
Fig.
i
W e i g h t L o s s of Concrete E x p o s e d to I% HCI
O F
85
O
80
R I G I M A L
?5
Q
55
E I
50
G H T
70 65 60
e
I
I
I
14
28
4~
i~
56
I
I
?e
84
~i
I
I
I
I
I
98 112 128 14e 154 168 18a
TZME OF ZMMERSZOM ( d a M s )
974
VoT.
15, N<'. 6
Mehta
P.K.
LMC lost 25% weight in 39 d a y s , w h e r e a s 63 d a y s w e r e required f o r t h e S F M C to r e g i s t e r the same amount of weight loss. F r o m Fig. i, it m a y be o b s e r v e d that the reference LWC and the LMC lost 50% weight in 60 a n d 95 days, respectively, but the SFMC lost only 45% weight during the test period (182 days). This clearly established the s u p e r i o r i t y of the l a t t e r in the I% HCI solution. F r o m Fig. 2 a n d 3, it c a n be s e e n t h a t t h e a c e t i c a n d lactic acid solutions were somewhat less corrosive than the h y d r o c h l o r i c acid; however the g e n e r a l t e n d e n c y of t h e S F M C b e i n g a b l e to r e s i s t the acid attacks better than either the LMC or the reference concrete is obvious. In 5% a c e t i c acid solution, the reference concrete and the LMC met the failure criterion in 48 days, respectively; it t o o k 1 6 8 d a y s f o r t h e S F M C to register the weight loss required for the failure criterion. In l a c t i c a c i d s o l u t i o n , b o t h the r e f e r e n c e concrete a n d L M C f a i l e d in 1 1 0 - 1 1 5 days, whereas SFMC required 182 days. T h e I% H 2 S O 4 s o l u t i o n w a s , in g e n e r a l , f o u n d to be less corrosive than HCI, acetic, and lactic acid solutions (Fig. 4). T h i s w a s d u e to t h e f o r m a t i o n of l a r g e a m o u n t s of g y p s u m w h i c h c r y s t a l l i z e d in the p o r e s of t h e s p e c i m e n s near the surface. X-ray diffraction analyses of the d e t e r i o r a t e d concrete f r o m the s u r f a c e of s p e c i m e n s immersed in s u l f u r i c a c i d or a m m o n i u m s u l f a t e s h o w e d l a r g e a m o u n t s of g y p s u m p r e s e n t (Fig. 6). The reference L W C f a i l e d in 1 0 5 d a y s , b u t b o t h t h e L M C and S F M C S h o w e d o n l y a b o u t 20% l o s s in w e i g h t d u r i n g the 182-days test period.
10S
LWC
100
LMC~
95
SFMC ./~
9e
8s
Fig.
8e
?s
2
W e i g h t L o s s of Concrete Exposed to 5% A c e t i c A c i d
70 6s 6e
%%
ss
"%%
Se e
14
2B
i
t
i
i
42
56
?e
84
i
i
i
%~s
I
i
98 1 1 2 1 2 6 14@ 1s4 1 6 B 1 8 2
TIME OF IMMERSION ( d a B s )
Vol. 15, No. 6
975 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
LWC [ ]
lOO
• .~c <~
g5
SFMC
Fig.
ge
0 F
85
0
80
R I
?5
G 1[
?0
N A
65
L
60
U E I G H T
55
3
W e i g h t L o s s of Concrete Exposed to I% L a c t i c Acid
50 0
I
I
I
I
I
I
I
14
:=8
4~
S6
70
84
98
TIME
OF
t
t
t
118 126140
IMMERSION
m
m
1 5 4 1 6 8 1 8 ;=
(di~l)
Surprisingly, the 5% ( N H 4 ) 2 S O 4 s o l u t i o n p r o v e d to be more aggressive t h a n t h e 1% H 2 S O 4 (Fig. 5). Both SFMC and the reference LWC registered 25% weight l o s s in a b o u t 120 d a y s ; h o w e v e r the LMC showed only 20% weight l o s s in 1 8 2 d a y s . It a p p e a r s that ammonium salts are a b l e to d e c o m p o s e the c a l c i u m s i l i c a t e h y d r a t e , w h i c h is the principal s o l i d p h a s e in h y d r a t e d portland cement pastes. That SFMC contains relatively more calcium s i l i c a t e h y d r a t e due to the p o z z o l a n i c reaction between the s i l i c a f u m e and c a l c i u m h y d r o x i d e was, t h e r e f o r e , of
105
10o 9s
~l.
~'II
SFMC A
ge 85
Fig. 4 Weight L o s s of Concrete Exposed to I% H 2 S O 4
80
75
IL.
70 65 60
55 50 0
14
28
42
56
70
84
g8 1121~6
TZffiE OF IffiMERSZON
140 154 168 182
(dl~8)
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Mehta
l i t t l e h e l p in p r o v i d i n g additional protection against attack from the ammonium salt solution. On t h e o t h e r hand, thin coating of l a t e x on the cement hydration products seemed to be m o r e e f f e c t i v e in r e t a r d i n g the ammonium s u l f a t e attack.
10S
G
ge F
8':5
0 R
Be 7S
"~'---o.
"'~.-
•
Fig.
"~'" ""~ ....... I
GI
7e
N
6s
~
"
'
8
-
~
~-IL
-
5
Weight Loss of Concrete Exposed to 5% ( N H 4 ) 2 S O 4
-
60
SS S0
e
I
I
i
I
14
28
42
SG
!
?@
!
84
i
!
I
!
I
!
98 1 1 N 1 ~ 6 1 4 @ 1 5 4 1 6 8 1 8 2
TIME OF IMMERSION (daM:)
GYPSUM
(d=7.56)
I II
I 12
I 13
I 14
GYPSUM (d:4.27) & QUARTZ (d=4.26)
CALCIUM HYDROXIDE (d 4.9 )
1 15
I 16
1 17
18
I 19
I 20
21
Cu Ka, DEGREES 28 Fig. X-Ray Diffraction Specimen After
6
Analysis Exposure
of a Deteriorated to S u l f u r i c Acid
6
Vol. 15, No. 6
977 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
N o n e of the c o n c r e t e m i x t u r e s s h o w e d any w e i g h t l o s s in six months of i m m e r s i o n on exposure to 5% s o d i u m sulfate solution. A l t h o u g h the p o r t l a n d c e m e n t u s e d for making the concretes contained 7% p o t e n t i a l C 3 A , it seems that in the range of 0.33 to 0.35 w a t e r / c e m e n t r a t i o s the p e r m e a b i l i t y of c o n c r e t e w a s too l o w to c a u s e any deterioration by sulfate attack. Photographs showing the typical appearance of s p e c i m e n s a f t e r 182 d a y s of e x p o s u r e to H 2 S O 4 a n d H C I s o l u t i o n s a r e s h o w n in Figs. 7. The d a t a for the i m m e r s i o n t e s t s at 45 C were not significantly different from t h e 20 C immersion data, and are t h e r e f o r e not i n c l u d e d in this report.
LMC
LWC
Fig.
SFMC
7
T y p i c a l A p p e a r a n c e of S p e c i m e n A f t e r S i x M o n t h s of E x p o s u r e to S u l f u r i c A c i d or H y d r o c h l o r i c Acid Solutions It s h o u l d be p o i n t e d out that, in p r a c t i c e , the durability of concrete structures exposed to the chemical solutions investigated in t h i s s t u d y s h o u l d be m u c h b e t t e r t h a n i n d i c a t e d by the t e s t d a t a g i v e n here. First, because the chemical solutions of high concentrations used in the test are not commonly encountered in f o o d and m o s t o t h e r i n d u s t r i e s . Second, the c o n c r e t e s p e c i m e n s in the t e s t w e r e f u l l y s u b m e r g e d , whereas in the industrial practice the structural element s u c h as a f l o o r is u s u a l l y exposed to a t t a c k f r o m one s i d e only. Third, the i m m e r s i o n solutions were
978
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P.K. Mehta
k e p t in a s t a t e of c o n s t a n t m o t i o n , and w e r e f r e q u e n t l y replaced, with fresh brushing of t h e s p e c i m e n s once every week; this removed the reaction products and almost continuously forced new material to c o m e i n t o c o n t a c t w i t h the a g g r e s s i v e solution.
AQknowled~ement The research was sponsored by E l k e m C h e m i c a l s , Inc. L. T r e s c o n y , A. M o r , a n d C. C h a n a s s i s t e d in c a r r y i n g out the e x p e r i m e n t a l work. The p a p e r w a s p r e s e n t e d at a symposium on c o n c r e t e technology in M o n t e r r e y , Mexico, M a r c h 1985.
References 1.
D. Manmohan and P.K. Mehta, "Influence of Possolanic, Slag, and Chemical Admixtures on Pore Size Distribution and Permeability of H a r d e n e d Cement Paste," Cement, Concrete, and Aggregates, V.3, No. i, pp. 6 3 - 6 7 , S u m m e r 1981.
2.
R. F e l d m a n , "Pore Structure Formation During Hydration of F l y Ash and Slag Cement Blends," Proceedings, Symposium on Fly A s h I n c o r p o r a t i o n in Cement and Concrete, Materials Research Society, Editor: S. D i a m o n d , pp. 1 2 4 - 1 3 3 , 1981.
.
E. J. S e l l e v o l d , D.H. B a g e r , E.K. J e n s e n , a n d T. K n u d s e n , " S i l i c a F u m e C e m e n t P a s t e s - H y d r a t i o n and Pore S t r u c t u r e , " P r o c e e d i n g s , Nordisk Mini Seminar on S i l i c a in C o n c r e t e , 32 pages, ( D e c e m b e r 1981).
in the USA
STUDIES ON CHEMICAL RESISTANCE OF LOW WATER/CEMENT RATIO CONCRETES
Civil
P.K. Mehta Engineering Department, University of California, Berkeley, CA 97420
(Communicated by J.P. Skalny) (Received June 25, 1985) ABSTRACT Solutions containing m i n e r a l acids, and c e r t a i n o r g a n i c a c i d s and s a l t s are h i g h l y c o r r o s i v e to portland cement concrete. Since permeability is t h e key factor governing the rate of deterioration, it is c u s t o m a r y to u s e a l o w water-cement ratio in m a k i n g concretes or concrete overlays required to r e s i s t c o r r o s i v e action of aggressive chemical solutions. Pozzolanic admixtures are o f t e n u s e d to p r o v i d e additional protection against acidic attack. Highly reactive pozzolanie admixtures, s u c h as condensed silica fume, which rapidly react with calcium hydroxide and reduce both the a l k a l i n i t y and p e r m e a b i l i t y of c o n c r e t e are n o w being used for improving durability. During the last two decades, latex admixtures have also found widespread application. The polymeric constituents of a l a t e x s e e m to coat the alkaline hydration products of p o r t l a n d cement, thus protecting them from attack by aggressive solutions. An experimental study was undertaken to evaluate the relative chemical resistance of low water-cement ratio concretes, containing either a styrene-butadiene l a t e x or a s i l i c a fume admixture, to the f o l l o w i n g solutions; 1% HCI, 1% H ~ S O 4, 1% l a c t i c a c i d , 5% a c e t i c a c i d , 5% a m m o n l u m sulfate, a n d 5% s o d i u m sulfate. T i m e t a k e n to r e g i s t e r 25 p e r c e n t weight loss by f u l l y s u b m e r g e d concrete specimens was used as a c r i t e r i o n for failure. F r o m t h e d a t a it appears that, except for the ammonium sulfate solution, the concrete containing the silica fume generally showed better resistance to c h e m i c a l a t t a c k t h a n o t h e r c o n c r e t e types. 969
970
Vol. P.K.
15, No. 6
Mehta
S u i t a b l y p r o p o r t i o n e d , w e l l c o m p a c t e d , and a d e q u a t e l y cured portland cement concrete has a history of satisfactory long-term performance under most natural and i n d u s t r i a l c o n d i t i o n s . S i n c e the h y d r a t i o n p r o d u c t s of p o r t l a n d c e m e n t c o n t a i n c a l c i u m c o m p o u n d s w h i c h are decomposed by a c i d i c solutions, various protective measures are often necessary to p r o l o n g the service l i f e of c o n c r e t e s e x p o s e d to a c i d i c e n v i r o n m e n t s . M i n e r a l acids, viz., HC1, HN03, and H2S04, are h i g h l y c o r r o s i v e to c o n c r e t e b e c a u s e t M e y f o r m s o l u b l e c a l c i u m salts that are leached away. This increases the p o r o s i t y and l o w e r s the s t r e n g t h of the m a t e r i a l . High molecular w e i g h t o r g a n i c a c i d s are not as c o r r o s i v e as mineral acids, but acetic and lactic acids, which are found in m a n y food products, are very corrosive to concrete. P e r m e a b l e c o n c r e t e s m a d e w i t h h i g h C3A (>5%) p o r t l a n d cement c a n be d a m a g e d by e x p a n s i o n an~ cracking when e x p o s e d to c o n c e n t r a t e d s o l u t i o n s of s o d i u m , potassium, or m a g n e s i u m sulfate. Among the sulfates, ammonium sulfate solution is t h e m o s t c o r r o s i v e because it n o t o n l y r e a c t s w i t h p o r t l a n d c e m e n t h y d r a t i o n p r o d u c t s to form gypsum, but also can dissolve the gypsum coating. C h l o r i d e s and d e i c e r c h e m i c a l s are u s u a l l y not h a r m f u l to p l a i n c o n c r e t e b u t c a n be i n s t r u m e n t a l in e x p a n s i o n and c r a c k i n g of c o n c r e t e c o n t a i n i n g e m b e d d e d steel. Permeability is g e n e r a l l y the key f a c t o r g o v e r n i n g the r a t e of d e t e r i o r a t i o n of c o n c r e t e e x p o s e d to a g g r e s s i v e chemical solutions. When permeability is l o w , the decomposition of the c e m e n t i t i o u s m a t t e r is l i m i t e d to the surface; otherwise the process of d e c o m p o s i t i o n s p r e a d s i n t o the i n t e r i o r . The r a t e of d e t e r i o r a t i o n is accelerated if t h e p r o d u c t s of chemical decomposition are washed a w a y by a s o l u t i o n in m o t i o n , or w h e n t h e c o n c r e t e s u r f a c e is e x p o s e d to s o m e k i n d of m e c h a n i c a l attrition, as f o r e x a m p l e heavy vehicular traffic on concrete floors. Thus, it i s t h e g r a d u a l loss of material rather t h a n t h e l o s s of s t r e n g t h or e l a s t i c m o d u l u s w h i c h b e c o m e s the p r i m a r y c a u s e of f a i l u r e . When highly corrosive chemicals are present in t h e environment, one way to p r o t e c t the concrete is to provide a 20-50 mm overlay of a v e r y l o w w a t e r / c e m e n t ratio concrete mixture. Water-reducing and airentraining, admixtures are commonly u s e d to a c h i e v e water/cement ratios of t h e o r d e r of 0.3. Pozzolanic a d m i x t u r e s are h i g h l y e f f e c t i v e in r e d u c i n g not o n l y the permeability of c o n c r e t e by the p o r e r e f i n e m e n t process (I-3) but also the calcium hydroxide hydrated cement paste, the calcium h i g h l y v u l n e r a b l e to a c i d a t t a c k .
content hydroxide
of t h e being
Vol.
15, No. 6
971 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
The objective of t h e r e s e a r c h reported h e r e w a s to evaluate the chemical durability of l o w w a t e r / c e m e n t ratio concretes containing a latex or a p o z z o l a n i c admixture. Materials
and
Procedure
The m i x p r o p o r t i o n s and p r o p e r t i e s of c o n c r e t e m i x t u r e s investigated in t h i s s t u d y are s h o w n in T a b l e s 1 and 2, respectively. Using a Type I/II portland cement (7% C~A), a c l e a n sand, and a g l a c i a l g r a v e l (12 m m m a x i m u m size), t h r e e t y p e s of c o n c r e t e m i x t u r e s w e r e made. One type c o n t a i n e d a s t y r e n e - b u t a d i n e l a t e x w h i c h w a s in the form of a water emulsion. T h e l a t e x w a s a d d e d i n an amount which corresponded to 16% s o l i d l a t e x by w e i g h t of c e m e n t . The net water/cement r a t i o of t h e c o n c r e t e batch was 0.35. The presence of s u r f a c t a n t s in the l a t e x e m u l s i o n w a s the r e a s o n for 5.6 to air e n t r a i n m e n t and high consistency (225 mm slump) of t h e c o n c r e t e mixture.
Concrete
Mix
TABLE I Proportions,
Type
kg/m 3
of
LMC
Concrete SFMC
LWC
Portland cement Fine aggregate Coarse aggregate Water
391 1013 676 137
404 887 887 153
488 876 876 161
W/C
0.35
0.33
0.33
Ratio
Note:
LMC Contains 16% s o l i d l a t e x a n d 1 5 % s i l i c a f u m e by wt. o f c e m e n t TABLE Properties
of
contains
2 Concrete
Type
S l u m p , mm Air, % Average compressive s t r e n g t h of c y l i n d e r s b e f o r e i m m e r s i o n in solutions, MPa
SFMC
of
Concrete
LMC
SFMC
LWC
225 5.6
250 4.1
18 4.9
47
74
64
972
Vol. 15, No. () P.K. Mehta
The second concrete was made with a proprietary product containing silica fume, water, and waterreducing admixtures. The addition of t h i s product accounted for 15% s i l i c a f u m e (also c a l l e d m i c r o s i l i c a ) by w e i g h t of c e m e n t in t h e c o n c r e t e mixture. The net r a t i o b e t w e e n w a t e r to c e m e n t i t i o u s materials (cement ÷ silica fume) was 0.33. In s p i t e of the low water c o n t e n t , a b o u t 250 m m c o n c r e t e s l u m p w a s o b t a i n e d ; also, the concrete mixture w a s f o u n d to h a v e e n t r a i n e d 4.1% air. A third concrete mixture was made for reference purpose. The r e f e r e n c e c o n c r e t e had w a t e r / c e m e n t ratio of t h e s a m e o r d e r as t h e o t h e r t w o c o n c r e t e s . B o t h an air-entraining admixture and a w a t e r - r e d u c i n g admixture were u s e d in t h i s c o n c r e t e to b r i n g d o w n the water/ c e m e n t r a t i o to 0.33. The c o n c r e t e , h o w e v e r , was quite s t i f f (18 m m s l u m p ) but c a s t a b l e . Cylindrincal specimens, 45 m m d i a m e t e r by 90 m m long, were c a s t in p l a s t i c m o l d s . A f t e r 24 h o u r s of c o n c r e t e p l a c e m e n t , the m o l d s w e r e s t r i p p e d . For the n e x t 6 d a y s the silica-fume modified concretes (SFMC) and the reference low water/cement ratio concrete (LWC) were cured in w a t e r , whereas the latex modified concrete (LMC) was cured in laboratory a i r ( 5 0 % RH, 7 0 + 3 F), as per latex manufacturer's recommendation. After the i n i t i a l 1 - w e e k p e r i o d , all c o n c r e t e s p e c i m e n s w e r e c u r e d in a i r ( 5 0 % RH, 7 0 + 3 F) f o r a b o u t s i x w e e k s ; then they w e r e i m m e r s e d in v a r i o u s c h e m i c a l s o l u t i o n s . The following six solutions were selected for immersion of c o n c r e t e cylinders at 20 C ( 6 8 F): 1% HCI, 1% H 2 S 0 4 , 1% l a c t i c a c i d , 5% a c e t i c a c i d , 5% a m m o n i u m sulfate, a n d 5% s o d i u m sullfate. The immersion tanks were fitted with magnetic stirrers which kept the s o l u t i o n s in a s t a t e of c o n s t a n t m o t i o n . T h i s h e l p e d to accelerate the chemical attack by p r e v e n t i n g the deposition of r e l a t i v e l y i n s o l u b l e r e a c t i o n p r o d u c t s on or n e a r the c o n c r e t e s u r f a c e . For the s a m e r e a s o n , o n c e every week the spent solutions were replaced with freshly made solutions. There a r e no s t a n d a r d criteria for evaluation of resistance of c o n c r e t e exposed to c o r r o s i v e chemical fluids. E x p e r i e n c e s h o w s t h a t w e i g h t l o s s of s p e c i m e n s after removal of the weak reaction product from the s u r f a c e of c o n c r e t e w i t h a s t e e l - w i r e b r u s h is the m o s t convenient way of simulating the industrial floors subjected to c o r r o s i v e f l u i d s and m e c h a n i c a l attrition. The number of days needed to r e g i s t e r a 2 5 % l o s s in weight is g e n e r a l l y accepted as t h e f a i l u r e c r i t e r i o n . Therefore, at t h e t i m e of s o l u t i o n change, i.e., o n c e every week, specimens were wlre-brushed, washed, and weighed. Three specimens of e a c h c o n c r e t e in a g i v e n environmental c o n d i t i o n w e r e m o n i t o r e d for w e i g h t loss.
Vol.
15, No. 6
973 CHEMICAL RESISTANCE, ADMIXTURES,
LOW W/C, CONCRETES
A n o t h e r o b j e c t of t h i s i n v e s t i g a t i o n w a s to d e t e r m i n e the chemical resistance of concretes at a warmer temperature of e x p o s u r e ; therefore a duplicate s e t of immersion tanks containing the s p e c i m e n s w e r e s t o r e d in a room which was maintained at a c o n s t a n t t e m p e r a t u r e of 45 C. R e su i t s
and
~
m
i
~
Average weight loss from three specimens of e a c h c o n c r e t e t y p e s t o r e d in d i f f e r e n t s o l u t i o n s at 20 C are s h o w n in Fig. 1-5. The d a t a for the f a i l u r e c r i t e r i o n , are t a b u l a t e d in T a b l e 3. TABLE Time
for
25
Percent
Solution 1% HCI 5% a c e t i c I% l a c t i c I%H2S04
acid acid
5%
(NH4)2SO 4
5%
Na2so 4
*20%
weight
no loss
at
3
Weight
Loss
20
C,
Days
LMC
SFMC
LWC
39 75 115 *
63 168 182 *
39 48 110 105
*
120
120
loss
the
at
end
no of
the
loss
test
no l o s s
(182
The hydrochloric acid solution was most corrosive. B o t h the L W C r e f e r e n c e
days)
f o u n d to be c o n c r e t e and
the the
tJ E ! G H T
10S
Lwc [ ]
100
,.c O SFMC A
95 90
Fig.
i
W e i g h t L o s s of Concrete E x p o s e d to I% HCI
O F
85
O
80
R I G I M A L
?5
Q
55
E I
50
G H T
70 65 60
e
I
I
I
14
28
4~
i~
56
I
I
?e
84
~i
I
I
I
I
I
98 112 128 14e 154 168 18a
TZME OF ZMMERSZOM ( d a M s )
974
VoT.
15, N<'. 6
Mehta
P.K.
LMC lost 25% weight in 39 d a y s , w h e r e a s 63 d a y s w e r e required f o r t h e S F M C to r e g i s t e r the same amount of weight loss. F r o m Fig. i, it m a y be o b s e r v e d that the reference LWC and the LMC lost 50% weight in 60 a n d 95 days, respectively, but the SFMC lost only 45% weight during the test period (182 days). This clearly established the s u p e r i o r i t y of the l a t t e r in the I% HCI solution. F r o m Fig. 2 a n d 3, it c a n be s e e n t h a t t h e a c e t i c a n d lactic acid solutions were somewhat less corrosive than the h y d r o c h l o r i c acid; however the g e n e r a l t e n d e n c y of t h e S F M C b e i n g a b l e to r e s i s t the acid attacks better than either the LMC or the reference concrete is obvious. In 5% a c e t i c acid solution, the reference concrete and the LMC met the failure criterion in 48 days, respectively; it t o o k 1 6 8 d a y s f o r t h e S F M C to register the weight loss required for the failure criterion. In l a c t i c a c i d s o l u t i o n , b o t h the r e f e r e n c e concrete a n d L M C f a i l e d in 1 1 0 - 1 1 5 days, whereas SFMC required 182 days. T h e I% H 2 S O 4 s o l u t i o n w a s , in g e n e r a l , f o u n d to be less corrosive than HCI, acetic, and lactic acid solutions (Fig. 4). T h i s w a s d u e to t h e f o r m a t i o n of l a r g e a m o u n t s of g y p s u m w h i c h c r y s t a l l i z e d in the p o r e s of t h e s p e c i m e n s near the surface. X-ray diffraction analyses of the d e t e r i o r a t e d concrete f r o m the s u r f a c e of s p e c i m e n s immersed in s u l f u r i c a c i d or a m m o n i u m s u l f a t e s h o w e d l a r g e a m o u n t s of g y p s u m p r e s e n t (Fig. 6). The reference L W C f a i l e d in 1 0 5 d a y s , b u t b o t h t h e L M C and S F M C S h o w e d o n l y a b o u t 20% l o s s in w e i g h t d u r i n g the 182-days test period.
10S
LWC
100
LMC~
95
SFMC ./~
9e
8s
Fig.
8e
?s
2
W e i g h t L o s s of Concrete Exposed to 5% A c e t i c A c i d
70 6s 6e
%%
ss
"%%
Se e
14
2B
i
t
i
i
42
56
?e
84
i
i
i
%~s
I
i
98 1 1 2 1 2 6 14@ 1s4 1 6 B 1 8 2
TIME OF IMMERSION ( d a B s )
Vol. 15, No. 6
975 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
LWC [ ]
lOO
• .~c <~
g5
SFMC
Fig.
ge
0 F
85
0
80
R I
?5
G 1[
?0
N A
65
L
60
U E I G H T
55
3
W e i g h t L o s s of Concrete Exposed to I% L a c t i c Acid
50 0
I
I
I
I
I
I
I
14
:=8
4~
S6
70
84
98
TIME
OF
t
t
t
118 126140
IMMERSION
m
m
1 5 4 1 6 8 1 8 ;=
(di~l)
Surprisingly, the 5% ( N H 4 ) 2 S O 4 s o l u t i o n p r o v e d to be more aggressive t h a n t h e 1% H 2 S O 4 (Fig. 5). Both SFMC and the reference LWC registered 25% weight l o s s in a b o u t 120 d a y s ; h o w e v e r the LMC showed only 20% weight l o s s in 1 8 2 d a y s . It a p p e a r s that ammonium salts are a b l e to d e c o m p o s e the c a l c i u m s i l i c a t e h y d r a t e , w h i c h is the principal s o l i d p h a s e in h y d r a t e d portland cement pastes. That SFMC contains relatively more calcium s i l i c a t e h y d r a t e due to the p o z z o l a n i c reaction between the s i l i c a f u m e and c a l c i u m h y d r o x i d e was, t h e r e f o r e , of
105
10o 9s
~l.
~'II
SFMC A
ge 85
Fig. 4 Weight L o s s of Concrete Exposed to I% H 2 S O 4
80
75
IL.
70 65 60
55 50 0
14
28
42
56
70
84
g8 1121~6
TZffiE OF IffiMERSZON
140 154 168 182
(dl~8)
976
Vol. P.K.
15,
No.
Mehta
l i t t l e h e l p in p r o v i d i n g additional protection against attack from the ammonium salt solution. On t h e o t h e r hand, thin coating of l a t e x on the cement hydration products seemed to be m o r e e f f e c t i v e in r e t a r d i n g the ammonium s u l f a t e attack.
10S
G
ge F
8':5
0 R
Be 7S
"~'---o.
"'~.-
•
Fig.
"~'" ""~ ....... I
GI
7e
N
6s
~
"
'
8
-
~
~-IL
-
5
Weight Loss of Concrete Exposed to 5% ( N H 4 ) 2 S O 4
-
60
SS S0
e
I
I
i
I
14
28
42
SG
!
?@
!
84
i
!
I
!
I
!
98 1 1 N 1 ~ 6 1 4 @ 1 5 4 1 6 8 1 8 2
TIME OF IMMERSION (daM:)
GYPSUM
(d=7.56)
I II
I 12
I 13
I 14
GYPSUM (d:4.27) & QUARTZ (d=4.26)
CALCIUM HYDROXIDE (d 4.9 )
1 15
I 16
1 17
18
I 19
I 20
21
Cu Ka, DEGREES 28 Fig. X-Ray Diffraction Specimen After
6
Analysis Exposure
of a Deteriorated to S u l f u r i c Acid
6
Vol. 15, No. 6
977 CHEMICAL RESISTANCE, ADMIXTURES, LOW W/C, CONCRETES
N o n e of the c o n c r e t e m i x t u r e s s h o w e d any w e i g h t l o s s in six months of i m m e r s i o n on exposure to 5% s o d i u m sulfate solution. A l t h o u g h the p o r t l a n d c e m e n t u s e d for making the concretes contained 7% p o t e n t i a l C 3 A , it seems that in the range of 0.33 to 0.35 w a t e r / c e m e n t r a t i o s the p e r m e a b i l i t y of c o n c r e t e w a s too l o w to c a u s e any deterioration by sulfate attack. Photographs showing the typical appearance of s p e c i m e n s a f t e r 182 d a y s of e x p o s u r e to H 2 S O 4 a n d H C I s o l u t i o n s a r e s h o w n in Figs. 7. The d a t a for the i m m e r s i o n t e s t s at 45 C were not significantly different from t h e 20 C immersion data, and are t h e r e f o r e not i n c l u d e d in this report.
LMC
LWC
Fig.
SFMC
7
T y p i c a l A p p e a r a n c e of S p e c i m e n A f t e r S i x M o n t h s of E x p o s u r e to S u l f u r i c A c i d or H y d r o c h l o r i c Acid Solutions It s h o u l d be p o i n t e d out that, in p r a c t i c e , the durability of concrete structures exposed to the chemical solutions investigated in t h i s s t u d y s h o u l d be m u c h b e t t e r t h a n i n d i c a t e d by the t e s t d a t a g i v e n here. First, because the chemical solutions of high concentrations used in the test are not commonly encountered in f o o d and m o s t o t h e r i n d u s t r i e s . Second, the c o n c r e t e s p e c i m e n s in the t e s t w e r e f u l l y s u b m e r g e d , whereas in the industrial practice the structural element s u c h as a f l o o r is u s u a l l y exposed to a t t a c k f r o m one s i d e only. Third, the i m m e r s i o n solutions were
978
Vol.
15, No. 6
P.K. Mehta
k e p t in a s t a t e of c o n s t a n t m o t i o n , and w e r e f r e q u e n t l y replaced, with fresh brushing of t h e s p e c i m e n s once every week; this removed the reaction products and almost continuously forced new material to c o m e i n t o c o n t a c t w i t h the a g g r e s s i v e solution.
AQknowled~ement The research was sponsored by E l k e m C h e m i c a l s , Inc. L. T r e s c o n y , A. M o r , a n d C. C h a n a s s i s t e d in c a r r y i n g out the e x p e r i m e n t a l work. The p a p e r w a s p r e s e n t e d at a symposium on c o n c r e t e technology in M o n t e r r e y , Mexico, M a r c h 1985.
References 1.
D. Manmohan and P.K. Mehta, "Influence of Possolanic, Slag, and Chemical Admixtures on Pore Size Distribution and Permeability of H a r d e n e d Cement Paste," Cement, Concrete, and Aggregates, V.3, No. i, pp. 6 3 - 6 7 , S u m m e r 1981.
2.
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