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Some factors affecting durability of sulphur-impregnated porous
bodies
NATIONAL RESEARCH COUNCIL
OF
CANADA
CONSElL NATIONAL D€ RECHERCHES DU
CANADA
SOME FACTORS AFFECTING DURABILITY OF
.
ISULPHUR-IMPREGNATED POROUS BODIES
R.
F.Jeldrnan a;
d
J.
J.
Beaudoin
,@
%+-
&e+
Reprinted from
CEMENT AND CONCRETE RESEARCH
Vol. 8, No.
3,
May 1978
pp.
273-282
DBR Paper No. 780
Division of Building Research
CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 273-282, 1978. Pergamon Press, I n c . P r i n t e d i n t h e U n i t e d S t a t e s .
SOME FACTORS AFFECTING DURABILITY OF SULPHUR-IMPREGNATED POROUS BODIES
R.F. Feldman and J.J. Beaudoin
Division of Building Research, National Research Council of Canada, Ottawa KIA OR6
(Communicated by F. H. Wittmann) (Received Feb. 6, 1978)
ABSTRACT
The effect of impregnation on long-term durability has been
investigated by exposing 1.3-mm thick specimens of sulphur-impregnated, autoclaved, portland cement-silica mixtures, room-temperature-cured pastes and porous glass to various liquids including water.
All impregnated bodies were permeable to vapours, and matrices with specific surface areas in excess of 20 m2/g and impregnated with sulphur were subject to high internally-generated local stresses when exposed to water and other vapours, leading to possible failure.
I
RESUMEI L'effet d'impregnation'sur la durabilit-6 h long terme a 6t6 6tudi6 en
I exposant des sp6cimens de 1.3 mm d16paisseur de m6langes ciment-silice
Portland imprGgn6s de soufre et autoclav6s, des p2tes 1 tempgrature de la pike, et du verre poreux h divers liquides y compris l'eau.
Tous les corps impr6gn6s 6taient perm6ables 1 la vapeur, et les matrices ayant une surface sp6cifique de plus de 20 m21g et impr6gn6es de soufre ont 6t6 sujettes 5 de hautes contraintes internes lorsqu'elles ont 6t6 expos6es h l'eau et 1 d'autres vapeurs, menant h une possibilit6 de rupture
.
Vol. 8, No. 3
R . F . Feldman, J . J . Beaudoin
New types of composites have been formed by the technique of impregnation
1 Physical properties increased by several hundred per cent when porous
bodies were impregnated with a variety of organic resins, and recently it has been shown that sulphur impregnation can achieve similar results (3-7).
A theoretical study (6) showed that Young's modulus and the microhardness of a completely impregnated composite may be described by an equation similar to that derived from the Reuss model. It involves a knowledge of the properties of the matrix and impregnant as well as determination of the volume fraction of the two components.
Impermeability can also be improved by impregnation, which leads to an increase in corrosion resistance, but little is known about this property on a long-term basis. In fact, no reported work to date has adequately illustrated the effectiveness of an impregnant in improving long-term durability.
Samples already prepared for a previous study (6)
-
very thin, impregnated specimens-
were used for the present investigation of the effects of impreg- nation on long-term durability. They were exposed to either liquid or vapourand any change in dimension or modulus of elasticity was noted. Autoclaved portland cement-silica mixtures of six different compositions, room-temperature- cured hydrated portland cement, and porous glass were impregnated with sulphur. The latter was exposed to methanol, carbon tetrachloride and water; the
portland-cement systems were exposed only to water. Samples
Experimental
Silica-portland cement mixtures having 5, 10, 20, 30, 50 and 65% by weight of silica were prepared and autoclaved at a water-to-cement ratio of 0.45, as was room-temperature cured paste at a water-to-cement ratio of 0.45.
The specimens were first prepared as 32-mm diameter discs, 1.3 mm thick. Prisms approximately 30 x 10 x 1.3 mm were cut from these. The porous glass
(supplied by Corning glass works) in the form of 25-mm diameter tubes of 2-mm wall thickness were also cut in the form of prisms 10 x 30 x 2 mm.
Impregnant and solvents
Sulphur was of reagent grade, containing 3 ppm of H2S. Carbon tetrachloride was reagent grade (Anachemia).
Methanol also was reagent grade (Fisher). These solvents were dried with molecular sieves, Type 4A (Linde Co.)
Procedure Impregnation
Prior to impregnation, which has been described ( 6 ) , the samples were maintained at 128OC in vacuum for 24 h. They were then weighed and placed with solid sulphur in another vacuum vessel and evacuated for 1 112 h. The vessel was heated to 128OC, allowing the sulphur to melt and the sample to become completely immersed. The amount of sulphur that was impregnated was determined by weighing and helium pycnometry. Porous glass samples were heated, prior to impregnation, to 160°C for 40 h in vacuum.
Mercury porosimetry and nitrogen adsorption measurements
Measurements were made on samples heated for 24 h at 105OC under vacuum conditions. Those for surface area determination by nitrogen adsorption were ground to pass 100 mesh. Mercury intrusion was performed to a pressure of 60,000 psi, using an American Instrument Co. porosimeter.
V o l . 8, No. 3
SULFUR, IMPREGNATION, POROUS BODIES
Young's modulus and length change measurements
Young's modulus was measured on 32-mm diameter d i s c s , 1 . 3 mm t h i c k ; one d i s c was used f o r each preparation. The procedure involved exposing each d i s c t o
100% RH i n vacuum and measuring, with time, t h e d e f l e c t i o n of t h e d i s c when loaded a t i t s centre and supported a t t h r e e points located a t t h e circumference of a c i r c l e 25 mm i n diameter. The 30-x 10-mm prisms were used f o r length change; they were mounted on 1-in. gauge length extensometers with s e n s i t i v i t y t o 4 x i n . / i n . These systems were enclosed i n vacuum chambers and exposed t o 100% r e l a t i v e vapour pressure o r immersed i n l i q u i d . The porous g l a s s prisms were exposed t o C C E 4 , CH30H and water i n both l i q u i d and vapour form. The cement systems were exposed t o 100% water vapour pressure only.
Results Porous g l a s s
Samples of porous g l a s s , 29.7% pore volume, were impregnated with molten sulphur. Residual p o r o s i t y a f t e r cooling was 4.40% measured by t h e helium pycnometer. Young's modulus, measured on a d i s c , was 1.46 x
l o 4
MPa before impregnation and 1.72 xlo4
MPa afterwards, a 17.8% increase. Microhardness values were 13.14 xl o 2
and 1 7 . 7 3 . ~l o 2
MPa, r e s p e c t i v e l y , representing an increase of 34.9%.Length change on exposure t o methanol. These r e s u l t s a r e presented on Fig. 1, exposure being i n vapour phase. Expansion of t h e unimpregnated g l a s s was approximately 0.25% and was close t o t h e equilibrium s t a t e a t t h e end of t h e t e s t . The impregnated sample expanded slowly' a t f i r s t , but a f t e r 15 min sur- passed t h e blank,and i n l e s s than 1 h a t t a i n e d an expansion of 2.5%. Severe cracking of t h e specimen occurred j u s t s h o r t of 2 h of exposure.
2 . 5 p l ! l 1 1 1 1
1
I i 2 . 0 - d. 1 i i 0 - 5 I M P R E G N A T E D 0 - B L A N K i I - B L A N K 1.5-
spi
i 2 iLength change on exposure t o carbon t e t r a c h l o r i d e . Samples were exposed t o both vapour and l i q u i d (Fig. 2). Length change occurred immediately a f t e r exposure. For t h e unimpregnated sample it was
r e l a t i v e l y small, approximately 0.04%; f o r t h e impregnated body, it was about 1.5% f o r both vapour and l i q u i d phases. Crack- ing of t h e sample was again observed, but was not of t h e same s e v e r i t y as had occurred with methanol.
Length change on exposure t o water. The length change r e s u l t s f o r exposure t o water i n both vapour and l i q u i d phases. a r e presented on Fig. 3. The blank expanded about 0.3% and a t t a i n e d equilibrium by t h e end of t h e t e s t . Although it expanded more slowly a t f i r s t , t h e impfegnated body surpassed t h e blank a f t e r l e s s than 1 h of exposure. Exposure t o water i n t h e vapour phase produced an expansion of approxi- mately 1.5% a f t e r 1.3 h, with very severe
TIME, h cracking. In t h e l i q u i d phase, expansion FIG. 1 slowed down a f t e r 1 h , but t h e sample
cracked and broke between 8 and 23 h of Length change of porous g l a s s on exposure. The surface of t h e g l a s s a f t e r
exposure t o methanol i n vapour exposure t o water vapour i s shown on phase versus time Fig. 4. The sulphur i s i n an unknown
276 b o l . 8 , No. 3 R.F. Feldman, J . J . Beaudoin
s t a t e and gives t h e impression of being extruded from t h e pores and subsequently agglomerated. Small p i e c e s of g l a s s broken o f f during exposure were examined; they had become completely t r a n s p a r e n t , i n d i c a t i n g t h a t t h e sulphur had been removed from them.
I I I l I I I I 0 - 5 I M P R E G N A T E D 0 - B L A N K
-
-
o-.-- -
d , 1 0 ' 7' os 0 '-
P P f om@ I-
1 I-
d"
s o d dfl--
---
O ' l I I I 2 . 0 1 . 5 C ? 1 0 - s a 0.5 2 . 0 1 I I I I I ) I 0 - S I I C P R I G N A T E D.-
B L A N K-
1.5 L S 1 . D - 4 a 0 . 5 U 1 2 1 4 5 6 1 8 r l 0 1 2 3 4 5 6 7 8 9 TIME, h TIME, h (A) (B) FIG. 2Length change of porous g l a s s on exposure t o carbon t e t r a c h l o r i d e versus time. (A) l i q u i d phase ( B ) vapour phase
_ C d - d - / Y - -
-
,' ,' P )I'
d I-
IP
I d 1-
/-
I I I---,---
+--
+---,
---+---I-- -+---
1 . 5 - I,
I I I I I,
0 - 5 I M P R E G N A T E D.-
B L A N K 1.0-
C w-
a- -
-a 1 . 5 1.0- k 9-
9 l o l I I I I I I I I I I 0 I - f f 0 I +-..-
-
-.-
m- aj
0.5 0 . 5 -&,+-.---,-I-
0 I l I I t l d'
-
P
dJ.
---
-*# .I'# 0 1 2 3 4 5 6 7 8 q - & oC 1 2 3 4 5 6 7'
8 9 TIME, h TIME, h (A) (B) FIG. 3Length change of porous g l a s s on exposure t o water versus time. (A) l i q u i d phase (B) vapour phase
277 SULFUR, IMPREGNATION, POROUS BODIES
FIG. 4
SEM micrographs of fractured glass surface with extruded sulphur after exposure to water vapour
SEM photographs of the sample exposed to methanol also displayed an ill-defined form of sulphur, but for carbon tetrachloride well-crystallized orthorhombic forms were present.
Autoclaved portland cement-silica mixtures and room-temperature-cured paste The length change of impregnated specimens of autoclaved cement-silica mixtures, silica content 5, 10, 20, 30, 50 and 65%, and room-temperature-cured paste on exposure to 100% water vapour are presented on Fig. 5. The results
for the unimpregnated specimens are
2 . 5 I I shown on Fig. 6. For all impregnated
65% s 1 0 2 ' specimens except the 5 and 10% silica,
R O O M T E M P
,
20% expansion increased in rate after a/ short period of exposure. For 5 and
2.0 -
/
10% silica content samples, expansion/
showed no rate increase up to 260 days.!
/
The 20% silica content specimen showedI
/
an increase in rate of expansion after1 . 5
i
Y :30%, 48 h; the 30% after 14
h; the 50% after
-
q. .
4
.
.
I
18 h; and the 65% and room-temperature-
1 ;
~. .
!
1
cured paste after 2 h. The blanks, onL O +
. .
!
I
the other hand, showed a rapid initial1 : I rate of expansion, decreasing with time.
A " T 0 C L A " E . In each case measured expansion was
/
C E M E N T - S I L I C A considerably less for the blanks than P R E P A R A T I O N S - for the impregnated bodies. All theW I S = 0 . 4 5
10%
7 ~
=
7-7=.=7=.=7-7=.== .5 % FIG. 5
I I
o 5 1 0 IS 20 Length change of impregnated,
T I M E , d autoclaved, cement-silica
preparations on exposure to water vapour versus time
V o l . 8, No. 3 R . F . Feldman, J . J . Beaudoin
TIME, d TIME, d, AT 100% R . H .
FIG. 6 FIG. 7
Length change of unimpregnated, autoclaved, cement-silica preparations on exposure to water vapour versus time
The change in Young's modulus on exposure to 100% relative
humidity versus time
impregnated specimens disintegrated except those having 5 and 10% silica
,
content.The change in Young's modulus of the autoclaved samples with exposure to 100% relative humidity is presented on Fig. 7. The results are similar in character to the length change results. The 30, 50 and 65% silica content specimens show large decreases; the 5 and 10% specimens show negligible change. The 20% sample shows only a slight decrease, but after five days the disc warped and the readings are probably unreliable.
Surface area and pore size distribution
The surface areas measured by nitrogen adsorption of the specimens before impremation are shown on Table
I.
Both the 50 and 65% silica content specimens contained unreacted silica after autoclaving, and consequently the specific surface area of the actual reaction products was higher than the values tabulated, since the specimens include the relatively low surface area silica. These results conform
approximately to the observation that the higher the surface area the greater the expansion on exposure (see Fig. 5).
TABLE I Sample, % silica 5 10 2 0 30 5 0 65 Room-temperature-cured paste, w/c 0.5 porous glass
Pore size distribution of all specimens, plotted on a relative scale of intrusion pressure versus pore volume, is presented
on Fig. 8; a pore size value calculated by the Kelvin equation is on the top scale. There is a clear difference between the three groups: 5 and 10% silica, 20 to 65% silica, and room-temperature paste and porous glass. On the basis of these results the samples are grouped as for the surface area results presented on Table I, although for porous glass the mercury penetrated only 60% of its pore volume.
V o l . 8, No. 3
SULFUR, IMPREGNATION, POROUS BODIES
P O R E D I A M E T E R . p m m m o a O - LO O 0 0 d 0 N I N T R U S I O N P R E S S U R E , p s i FIG. 8
Pore s i z e d i s t r i b u t i o n of unimpregnated bodies
Discussion
Measurements made f o r t h i s study show t h a t a l l t h e impregnated matrices can
be r e a d i l y permeated by e i t h e r l i q u i d o r vapour. There a r e several possible
reasons f o r t h i s :
(a) The bond between t h e surface of t h e matrix and t h e sulphur may not be good; measurements of Young's modulus on impregnated porous g l a s s i n d i c a t e t h a t t h i s may indeed be so. A knowledge of t h e modulus of zero p o r o s i t y g l a s s and sulphur enables one t o c a l c u l a t e t h e expected modulus of t h e f u l l y impreg-
nated porous g l a s s by means of t h e Reuss equation (6,7). A value of approxi-
mately 3.2 x
lo4
MPa was c a l c u l a t e d , compared t o t h e measured value of1.72 x
l o 4
MPa. The same reasoning might, however, lead one t o conclude t h a tportland cement systems would y i e l d composites r e s i s t a n t t o exposure t o water, because t h e modulus due t o impregnation d i d increase according t o t h e Reuss
model. The r e s u l t a n t mechanical p r o p e r t i e s on impregnation were i n accord with
those predicted by t h e Reuss equation, which assumes good bonding.
(b) Chemical i n t e r a c t i o n occurs a t t h e i n t e r f a c e , with subsequent chemical
a t t a c k by t h e l i q u i d ; t h e p o s s i b i l i t y exists that sulphur may react with
t h e s i l i c a surface, forming a compound t h a t may i n t u r n r e a c t with water. The
expansion observed with both t h e r e l a t i v e l y i n e r t carbon t e t r a c h l o r i d e and
methanol, however, v i r t u a l l y r u l e s out t h i s p o s s i b i l i t y . When exposed t o each
of t h e f l u i d s , including carbon t e t r a c h l o r i d e , the impregnated specimens of
porous g l a s s expanded t o a much g r e a t e r e x t e n t than t h e r e s p e c t i v e unimpreg-
nated specimens. I t i s concluded t h a t expansion i s due t o a physical
Vol. 8, No. 3 R.F. Feldman, J.J. Beaudoin
The r e s u l t s of t e s t s on specimens of t h e various portland cement mixtures a l s o i n d i c a t e t h i s . A l l t h e specimens conformed approximately t o t h e obser- vation t h a t t h e g r e a t e r t h e s p e c i f i c surface a r e a of t h e matrix m a t e r i a l , t h e g r e a t e r t h e expansion on exposure t o water vapour. The 5 and 10% s i l i c a content specimens with s p e c i f i c surface areas below 20 m2/g were t h e only impregnated specimens t o remain i n t a c t and show expansion with water o r other l i q u i d s not g r e a t l y i n excess of t h e unimpregnated material.
Although it appears t h a t t h e cause of expansion i s physical, t h e r e a r e several possible mechanisms by which s t r e s s might be imposed on t h e body:
(a) During s o l i d i f i c a t i o n , sulphur (within t h e pores) shrinks as much a s 20% ( t h i s was v e r i f i e d by measuring p o r o s i t y before and a f t e r impregnation ( 6 ) ) ; t h i s process may have imposed a s t r e s s on t h e body t h a t was released when t h e body was exposed t o vapour. The p o s s i b i l i t y was investigated by measuring sample dimensions before and a f t e r impregnation. They were a l s o measured during melting and s o l i d i f i c a t i o n of t h e impregnant with heating and cooling of t h e body; both measurements yielded n e g l i g i b l e length changes.
(b) During adsorption of vapour on t h e surface of unimpregnated g l a s s ,
expansion occurred, as shown i n Fig. 3, owing t o t h e decrease of surface- f r e e energy. I f t h e decreased r a t e ofpermeation (owing t o t h e impregnant) of vapours were t o allow l a r g e s t r e s s gradients t o develop, then f a i l u r e might occur. The specimens t e s t e d were very t h i n , however, and t h e i n t e r a c t i o n and thus expansion produced by adsorption of carbon t e t r a c h l o r i d e on unimpregnated porous g l a s s was very small; i t i s t h e r e f o r e u n l i k e l y t h a t l a r g e s t r e s s
gradients would occur. Yet t h e expansion produced by exposing t h e impregnated body t o carbon t e t r a c h l o r i d e was 50 times as l a r g e . There must be some o t h e r mechanism t o explain the, very l a r g e expansion.
(c) In order t o f i l l t h e very small pores e x i s t i n g i n some of t h e matrices studied (Fig. 8), t h e sulphur has t o be i n a very f i n e l y divided s t a t e . I t may be calculated t h a t t h e s o l i d i f i e d sulphur i n t h e impregnated porous g l a s s would have a s p e c i f i c surface i n excess of 500 m21g. I f t h i s surface could be reached by t h e vapour molecules and t h e i n t e r a c t i o n energies were of the normal adsorptive type, then t h e swelling forces created by t h e decrease i n surface-free energy of t h e sulphur could be very high; owing t o an i r r e g u l a r i n t e r f a c i a l boundary, high l o c a l s t r e s s e s a c t i n g a t s p e c i f i c s i t e s along t h e i n t e r f a c e might e x i s t , causing ultimate d e s t r u c t i o n of t h e matrix. This hypotheses i s supported by t h e SEM micrographs (Fig. 4 ) , which show apparent extrusion of t h e sulphur impregnant. Small pieces of broken sample examined a f t e r exposure, do i n f a c t show t h a t t h e sulphur had completely vacated t h e pores. The process requires access by t h e vapour t o some of t h e impregnant surface, adsorption by t h e impregnant, and a'high surface a r e a of matrix.
Sulphur i n t h e c r y s t a l l i n e orthorhombic o r monoclinic form i s not r e a d i l y wetted by water. S o l i d i f i c a t i o n i n 50 diameter pores has l e f t t h e sulphur i n a f i n e l y divided, amorphous form (Fig. 4 ) , however, and one would e . q e c t i t s surface energy t o be q u i t e d i f f e r e n t from t h a t of a c r y s t a l l i n e s t a t e .
Conclusion
1. A l l t h e sulphur-impregnated systems studied were permeable t o water and o t h e r vapours, whether l a r g e increases i n mechanical p r o p e r t i e s were achieved on impregnation o r n o t .
2. Expansion of impregnated bodies on exposure t o vapour i s a physical phenomenon.
3 . Expansion of impregnated bodies on exposure t o vapours i n excess of t h e expansion of unimpregnated bodies i s due t o swelling of t h e high s p e c i f i c
Vol. 8, No. 3 2 8 1 SULPHUR, IMPREGNATION, POROUS BODIES
s u r f a c e a r e a impregnant w i t h i n t h e p o r e s owing t o i t s a d s o r p t i o n of t h e vapour
.
2
4. M a t r i c e s with s p e c i f i c s u r f a c e a r e a s i n excess of 20 m /g and impregnated with s u l p h u r a r e s u b j e c t t o h i g h i n t e r n a l l y - g e n e r a t e d l o c a l s t r e s s e s when exposed t o water and o t h e r vapours, l e a d i n g t o p o s s i b l e f a i l u r e .
I Acknowledgement
The a u t h o r s wish t o thank G . A a r t s f o r h e r v e r y f i n e work i n performing t h e experiments. This paper i s a c o n t r i b u t i o n from t h e Division o f Building Research, National Research Council of Canada, and i s published with t h e approval of t h e D i r e c t o r o f t h e Division.
References
1. J . Gebauer, D.P.H. Hasselman and R . E . Long. Am. Ceram. Soc. Bull.
-
51, 471 (1972).2 . A. Auskern and W. Horne. J . Am. Ceram. Soc.
54,
282 (1971).3 . N . Thaulow. Cem. Concr. Res.
-
4, 269 (1974).4. V.M. Malhotra, K.E. P a i n t e r and J . A . S o l e s . Proceedings, F i r s t I n t e r n . Cong. on Polymers i n Concrete, 1975. C o n s t r u c t i o n P r e s s L t d . , Hornby, Eng.
5. F i r s t I n t e r n . Symposium on New Uses f o r Sulphur and P y r i t e s . Madrid 1976. 6. R.F. Feldman and J . J . Beaudoin. Cem. Concr. Res.
