Significance of microhardness of porous inorganic materials

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Cement and Concrete Research, 2, 6, pp. 717-729, 1972-11

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Significance of microhardness of porous inorganic materials

Sereda, P. J.

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CEMENT and CONCRETE RESEARCH. Vol , 2, pp. 71 7-729, 1972. 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 .

SIGNIFICANCE O F MICROHARDNESS O F POROUS INORGANIC MATERIALS

P. J. S e r e d a

Division of Building R e s e a r c h , National R e s e a r c h Council of Canada,

Ottawa, C a n a d a

(Communicated by

R.

E. P h i l l e o )

ABSTRACT

M i c r o h a r d n e s s w a s m e a s u r e d of p o r o u s and n o n p o r o u s s a m p l e s of h a l i t e and s e l e n i t e and i t h a s b e e n shown t h a t the r e l a t i o n s h i p b e - tween m i c r o h a r d n e s s and p o r o s i t y m a y b e e x p r e s s e d by the e m - p e r i c a l r e l a t i o n H = H e - b p . T h e s i g n i f i c a n c e of t h e m i c r o h a r d -

0

n e s s m e a s u r e m e n t s a s applied t o p o r o u s m a t e r i a l s i s d i s c u s s e d - -

f r o m the standpoint of t h e d i s t u r b e d zone of c r y s t a l s o r m i c r o - u n i t s , including t h e p r o b l e m of "work h a r d e n i n g " when p o w d e r e d s a m p l e s a r e c o m p a c t e d . T h e e f f e c t of m o r p h o l o g y on m i c r o h a r d n e s s i s a l s o d i s c u s s e d with r e g a r d t o a s e r i e s of g y p s u m s a m p l e s . R e f e r e n c e i s m a d e t o the effect of humidity on m i c r o h a r d n e s s of p o r o u s g l a s s and t h e conclusion d r a w n t h a t t h e m i c r o m e c h a n i c s of f a i l u r e i n m i c r o h a r d n e s s a r e s i m i l a r t o f l e x u r e s t r e n g t h . SOMMAIR E On a m e s u r B l a m i c r o d u r e t k d l e c h a n t i l l o n s p o r e u x e t non p o r e u x d ' h a l i t e e t d e s k l k n i t e e t on a o b s e r v 6 que l a r e l a t i o n e n t r e l a m i c r o d u r e t k e t l a p o r o s i t k p e u t e t r e r e p r k s e n t g e p a r l a r e l a t i o n e m p i r i q u e H = ~ ~ e - ~ p . L 1 a u t e u r Btudie l1 i m p o r t a n c e d e s m e s u r e s d e m i c r o d u r e t g appliqukes a u x m a t B r i a u x p o r e u x , d u point d e v u e d e l a zone t r o u b l k e d e c r i s t a u x e t d e m i c r o - G l e m e n t s , y c o m - p r i s l e p r o b l k m e d e ' ~ l l k c r o u i s s a g e ~ l o r s q u e l e s gchantillons pul- v b r i s e s s o n t c o m p a c t k s . L 1 a u t e u r Btudie Bgalement 1' effet d e l a m o r p h o l o g i e s u r l a m i c r o d u r e t e , p o u r c e q u i r e g a r d e une s e r i e d l e c h a n t i l l o n s d e g y p s e . 11 m e n t i o n n e l f e f f e t d e l l h u m i d i t k s u r l a m i c r o d u r e t k d u v e r r e p o r e u x e t c o n c l u t que l a m i c r o m 6 c a n i q u e d e r u i n e d a n s l a m i c r o d u r e t B e s t s e m b l a b l e

B

l a f o r c e d e flexion.

MICROHARDNESS, P O R O S I T Y , M A T E R I A L S

V o l . 2, No. 6

Introduction

Indentation h a r d n e s s h a s b e e n u s e d a s a m e a s u r e of s t r e n g t h of

m e t a l s ( l ) , and a s a m e a s u r e of the s t r e n g t h of nonporous, c r y s t a l l i n e r o c k s ( 2 , 3 ) . I t h a s b e e n shown t h a t m i c r o h a r d n e s s of m e t a l s and r o c k s e x p r e s s e d

a s load p e r unit a r e a i s equal t o t h r e e t i m e s the yield s t r e n g t h . This, i n p a r t , h a s s a t i s f i e d t h e need f o r a n o n - d e s t r u c t i v e t e s t of a n i m p o r t a n t m e c h a n i c a l p r o p e r t y and one t h a t c a n b e applied t o s u r f a c e l a y e r s o r m i c r o - a r e a s .

The application of m i c r o h a r d n e s s m e a s u r e m e n t s t o i n o r g a n i c p o r o u s m a t e r i a l s was b e g u n i n 1966 a t t h e Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, and a n u m b e r of p a p e r s (4, 5, 6) r e p o r t r e s u l t s of s u c h m e a s u r e m e n t s involving g y p s u m p l a s t e r and h y d r a t e d p o r t l a n d c e m e n t .

T h e p r e s e n t p a p e r p r e s e n t s a s u m m a r y of t h e d e v e l o p m e n t w o r k r e - l a t i n g t o t h e application oSi t h i s technique t o i n o r g a n i c p o r o u s m a t e r i a l s and a d i s c u s s i o n of i t s m e r i t s and i t s l i m i t a t i o n s . I t will not d e a l with t h e cone p e n e t r a t i o n technique developed by R e h b i n d e r and h i s c o - w o r k e r s in USSR, although i t i s s i m i l a r t o t h e m i c r o h a r d n e s s technique i n s o m e r e s p e c t s . T h i s s u b j e c t will b e d e a l t with i n a s e p a r a t e publication.

M a t e r i a l s

N a t u r a l s e l e n i t e s i n g l e c r y s t a l s w e r e obtained f r o m Kingdon Mine, F i t z r o y H a r b o u r , O n t a r i o ( c o u r t e s y H. M. Woodrooff e ) ; n a t u r a l h a l i t e w a s ob- tained a s a c o r e f r o m a S a s k a t c h e w a n s a l t d e p o s i t ( c o u r t e s y R. Hanson,

S a s k a t c h e w a n P o w e r C o m m i s s i o n ) ; p o t t e r y p l a s t e r and

a

h e m i h y d r a t e s w e r e obtained a s c o m m e r c i a l p r o d u c t s ( c o u r t e s y U.S. Gypsum). A n a l y s e s of s a m p l e s showed i m p u r i t i e s below 0. 01%.

E q u i p m e n t

Both Tukon and L e i t z m i c r o h a r d n e s s t e s t i n g m a c h i n e s w e r e used, and both Knoop and V i c k e r s i n d e n t e r s . F o r t h e s a m p l e s under s t u d y both i n d e n t e r s g a v e s i m i l a r v a l u e s of m i c r o h a r d n e s s .

T o m e a s u r e t h e diagonal of t h e indentation of a s a m p l e of a p o r o u s m a t e r i a l i t w a s n e c e s s a r y t o modify t h e lighting s o t h a t a b e a m of l i g h t p a s s e d a l m o s t p a r a l l e l t o t h e indented s u r f a c e and highlighted t h e b o u n d a r y of t h e i n -

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6

MICROHARDNESS, POROSITY, MATERIALS

d e n t a t i o n . T h e d i a g o n a l w a s m e a s u r e d b y m e a n s of t h e o p t i c a l s y s t e m p r o - v i d e d with t h e m i c r o h a r d n e s s i n s t r u m e n t s . M i c r o h a r d n e s s m e a s u r e m e n t s w e r e m a d e in e i t h e r a r o o m c o n d i t i o n e d t o 50% RH o r a gloved b o x w h e r e r e l a t i v e h u m i d i t y w a s m a i n t a i n e d a t d i f - f e r e n t l e v e l s . C o m p a c t s of p o w d e r e d s a m p l e s w e r e p r e p a r e d b y t h e t e c h n i q u e of d r y c o m p a c t i o n i n a s t e e l m o l d ( 4 ) . E x a m i n a t i o n of m i c r o s t r u c t u r e u n d e r t h e a p e x of a n i n d e n t a t i o n w a s m a d e with SEM ( C a m b r i d g e S t e r e o s c a n ) on a f r a c t u r e d s u r f a c e . What D o e s M i c r o h a r d n e s s M e a s u r e ? T h e q u e s t i o n of what m i c r o h a r d n e s s m e a s u r e s h a s b e e n d i s c u s s e d f u l l y with r e g a r d t o n o n p o r o u s m a t e r i a l s (1, 7, 8). A r e v i e w i n 1966 of t h i s t e c h n i q u e of t e s t i n g by ASTM C o m m i t t e e E - 4 , S u b s e c t i o n V, l e d t o t h e c o n - c l u s i o n t h a t h a r d n e s s i s s t i l l r e l a t i v e l y undefined a n d l i t t l e u n d e r stood. It is v a l i d t o a s k t h e b a s i c q u e s t i o n with r e g a r d t o t h e a p p l i c a t i ~ n of t h e t e c h n i q u e t o p o r o u s m a t e r i a l s s u c h a s h y d r a t e d c e m e n t and g y p s u m p l a s t e r . F o r n o n p o r o u s m a t e r i a l s it h a s b e e n shown t h a t m i c r o h a r d n e s s is i n - d e p e n d e n t of l o a d ( 7 , 9 ) . A s i m i l a r c o n c l u s i o n w a s r e a c h e d f o r p o r o u s m a - t e r i a l s b a s e d on m e a s u r e m e n t s of c o m p a c t e d s a l t . A twenty -f old v a r i a t i o n i n l o a d d i d n o t p r o d u c e a v a r i a t i o n i n t h e v a l u e s of m i c r o h a r d n e s s beyond t h e n o r m a l s c a t t e r within t h e a c c u r a c y l i m i t s of t h e m e a s u r e m e n t . P o r o u s m a t e r i a l s t e n d t o h a v e a h e t e r o g e n e o u s m i c r o s t r u c t u r e i n - volving a wide r a n g e of p a r t i c l e s i z e s and p o r e s i z e s . If m i c r o h a r d n e s s m e a s u r e m e n t i s t o b e r e p r e s e n t a t i v e of t h e bulk m a t e r i a l i t m u s t , f o r t h i s r e a s o n , i n c l u d e a s t a t i s t i c a l l y s i g n i f i c a n t n u m b e r of p a r t i c l e s and p o r e s . F o r n o n p o r o u s m a t e r i a l s A t k i n s e t a1 (8) c o n c l u d e d t h a t t h e d e f o r m e d zone r e s e m b l e s r a d i a l c o m p r e s s i o n and t h a t t h e e l a s t i c / p l a s t i c b o u n d a r y i s a p p r o x i m a t e l y a h e m i s p h e r e a n d p r a c t i c a l l y unaffected b y t h e d e t a i l e d s h a p e of t h e i n d e n t a t i o n i t s e l f . A s s u m i n g t h a t t h i s c o n c l u s i o n i s a p p l i c a b l e t o t h e p o r o u s s y s t e m s , t h e d e p t h t o which t h e m i c r o s t r u c t u r e i s d i s t u r b e d a t t h e a p e x of t h e i n d e n t a t i o n should b e t h e r a d i u s of t h e h e m i s p h e r e of t h e d e f o r m - ed zone ( F i g . l a ) .

MICROHARDNESS, POROSITY, MATERIALS

V o l . 2, No.

6

FIG. l a

M i c r o h a r d n e s s indentation and d e f o r m e d zone

( 1 ) M i c r o g r a p h of- undisturbed m i c r o s t r u c t u r e outside deformed zone. ( 2 ) M i c r o g r a p h of d i s t u r b e d m i c r o s t r u c t u r e i n d e f o r m e d zone. FIG. l b

SEM m i c r o g r a p h s of gypsum p l a s t e r below the a p e x of V i c k e r t s m i c r o h a r d n e s s indent

A c a s t gypsum s a m p l e of 52% p o r o s i t y examined by SEM h a s r e - vealed t h a t this depth of d i s t o r t i o n and f r a c t u r e of t h e c r y s t a l s under the a p e x of an indentation i s roughly equivalent t o the length of the diagonal of t h e indentation using the V i c k e r s i n d e n t e r . F i g u r e 1 b shows the m i c r o s t r u c - t u r e of the deformed and undeformed zones.

F r o m t h e s e o b s e r v a t i o n s it m a y b e concluded that a n indentation of 400 y m (diagonal of V i c k e r s indentation), t h e volume of the d e f o r m e d zone,

6 3

i s contained by a h e m i s p h e r e of r a d i u s 400 y m o r 134 x 10 y m

.

F o r t h i s 3

s a m p l e i t w a s e s t i m a t e d that t h e r e i s about 0. 018 c r y s t a l / y m

,

s o t h a t t h e total n u m b e r of c r y s t a l s affected i s about 2 . 4 million f o r the one t e s t .

Vol.

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MICROHARDNESS, POROSITY, MATERIALS

than those in the gypsum sampl'e analysed above. If these observations a r e applicable to the hydrated cement system, then an indentation produced by microhardness measurement would, f o r the s a m e porosity, involve a l a r g e r number of micro-units and should be valid a s representing a statistically significant sample. This cannot be easily verified by any d i r e c t observations in the cement system.

According to King and Tabor (9) microhardness indentation on a non- porous m a t e r i a l produces a plastic deformation that can be observed a s a barrel-shaped indentation resulting f r o m a piling -up a t the c e n t r e of the pyramidal faces. This piling-up i s not observed with a porous m a t e r i a l be- cause the displaced m a t e r i a l can be accommodated in the p o r e s . Thus the indentation of a porous m a t e r i a l m u s t involve, wholly or in part, the p r o c e s s of breaking of bonds between particles a s well a s fracturing of particles a c - companied by densification, a s occurs in compaction of powders. As porosity d e c r e a s e s this p r o c e s s m u s t be proportionately replaced by one of plastic deformation.

Indirect evidence f r o m changes in microhardness with changes in relative humidity (to b e discussed in a l a t e r section) indicates that m i c r o - hardness of a porous m a t e r i a l i s representative of i t s flexure strength. Data on Young's modulus and hardness of hydrated cement, Soroka and Sereda

( 6 ) ,

have been combined and replotted in Fig. 2 to show the relation between these m e a s u r e m e n t s . The r e s u l t s suggest that these properties a r e r e - lated f o r s i m i l a r samples but their significance i s not apparent a t present.

FIG. 2

Relation between hardness and Young' s modulus for hydrated

cement paste and compacts.

80 60 - D A T A - o C O M P A C T S I

,

30 - Ln = ;;1 20 - Y" 15 - Ln S 10 - I 4 8 6 -4 - 4 6 8 10 15 20 30 E X 10-4. K C P E R S Q C M

722

M I C R O H A R D N E S S , P O R O S I T Y , M A T E R I A L S

V o l . 2,

No.

6

Effect of P o r o s i t y

Strength and Young's modulus of v a r i o u s m a t e r i a l s have been r e l a t e d t o their porosity by a n e m p i r i c a l relation (10-14). A s i m i l a r e m p i r i c a l r e - lation has been found f o r m i c r o h a r d n e s s and porosity of compacts of a num- b e r of c r y s t a l l i n e powdered m a t e r i a l s a s well a s hydrated gypsum and hy- d r a t e d cement (4,

6).

w h e r e

H and Ho a r e the m i c r o h a r d n e s s of the porous and nonporous s a m - p l e s of a given m a t e r i a l , respectively,

b = a n e m p i r i c a l constant, and p = s a m p l e porosity.

The basic c o r r e l a t i o n was c a r r i e d out using l a r g e single c r y s t a l s of halite and selenite. M i c r o h a r d n e s s of the single c r y s t a l s of halite was m e a s u r e d by m e a n s of the Knoop indenter. A value f o r Knoop Hardness

2

Number (KKN) of 19.5 k g / m m was obtained that a g r e e s reasonably well with values of 20.4 to 22.8, published by Kurichina (15), and a value of 17 to 18, r e p o r t e d by King and Tabor

(9).

The m i c r o h a r d n e s s of the selenite single c r y s t a l v a r i e d g r e a t l y on different f a c e s and depended on the orientation of the Knoop indenter. At a p a r t i c u l a r orientation of the indenter on c e r t a i n f a c e s i t was not possible to m e a s u r e m i c r o h a r d n e s s because of the tendency of the c r y s t a l t o cleave.

The m i c r o h a r d n e s s of s e l e n i t e single c r y s t a l s was a l s o v e r y sensitive t o r e l a t i v e humidity. The values i n c r e a s e d considerably when the r e l a t i v e hu- midity condition was changed f r o m 5 0 to 0%.

A number of t h e s e single c r y s t a l s w e r e c r u s h e d in a m o r t a r and p e s t l e to p a s s the 200-mesh sieve; the powder was then compacted a t differ- ent p r e s s u r e s i n a mold to f o r m compacts of varying porosity 3.17 c m i n d i a m e t e r and about 1.3 rnm thick. M i c r o h a r d n e s s of e a c h compact was d e - t e r m i n e d using the Knoop indenter. All m e a s u r e m e n t s w e r e m a d e i n a i r a t 50% r e l a t i v e humidity. Ten v a l u e s along the d i a m e t e r w e r e taken and a v e r - age values used t o plot the l o g a r i t h m of m i c r o h a r d n e s s v s porosity. F i g u r e

V o l . 2 , No. 6 7 2 3 MICROHARDNESS, POROSITY, IIATERIALS

P O R O S I T Y . P E R C E N T

1 0 0 I I

3 shows the s t r a i g h t - l i n e relation confirming the validity of equation (1). The

8 0

value f o r the constant, b, was found t o b e 6.3 f o r halite and 9.3 f o r selenite.

I I _{I I}

-

0 S E L E N I T E

E f f e c t of Compaction

Nonporous compacted halite is m o r e than twice a s h a r d a s the single

6 0 H A L I T E 0 H A L I T E I A N N E A L E D I H A L I T E I S I N G L E C R Y S T A L 1 0 Y Ln Ln 1 0 - FIG. 3 n 2 M i c r o h a r d n e s s v s porosity O

c r y s t a l . This can b e attributed to I1work hardening1' and reduction of g r a i n

6

of compacts. - I

4

2

1

size, both effects produced by compaction a t n o r m a l t e m p e r a t u r e . B r a c e (3)

-

I I I I I I

observed work hardening brought about by s u r f a c e rubbing of a single c r y s t a l ,

1 0 20 3 0

with the resulting doubling of m i c r o h a r d n e s s . He a l s o produced compacts having porosity of 0. 50/0, and by annealing obtained n o r m a l values of h a r d n e s s . S i m i l a r r e s u l t s w e r e obtained a t DBR/NRC when the s e r i e s of halite c o m - pacts of various porosities w e r e annealed by heating t o 600" C and cooling a t a r a t e of 0. 6 " c / m i n . The r e s u l t s a r e shown on Fig. 3. King and Tabor

(9)

demonstrated the effect of s t r a i n upon m i c r o h a r d n e s s of a nonporous rock L

s a l t when they obtained a value of 27 k g / m m f o r a highly s t r a i n e d specimen f o r which the n o r m a l value was found to b e 17.2. This i s confirmed in the p r e s e n t work where a single c r y s t a l was s t r e s s e d i n a compaction mold to

100, 000 p s i and the m i c r o h a r d n e s s doubled.

F o r a nonporous compact of s e l e n i t e the extrapolated (H ) m i c r o h a r d -

MICROHARDNESS, P O R O S I T Y , M A T E R I A L S

V o l . 2 , N o . 6

n e s s has a single value that cannot be compared to the r a n g e of values ob- tained f o r a single c r y s t a l . The hardening effect i s even m o r e d r a m a t i c in t h i s c a s e than i n halite b e c a u s e the h a r d n e s s of the single c r y s t a l a t a par -

ticular orientation of the diamond point i s s o low that i t cannot be m e a s u r e d . In the compact, the direction of e a s y f r a c t u r e d i s a p p e a r s b e c a u s e of the

s m a l l s i z e of g r a i n s and their random orientation; i t i s not possible to anneal the compacts by heating. I t was thought the exposure to high humidity would r e l i e v e s t r e s s , but this did not happen. In fact, exposure to humidities of 5070 for a period of 10 days caused a considerable i n c r e a s e in m i c r o h a r d n e s s , due possibly to aging ( r ecrystallization), and resulted in strengthening a t points of contact between c r y s t a l s .

P r e v i o u s work by Soroka and S e r e d a

( 6 )

h a s shown that in hydrated cement the relation of h a r d n e s s to porosity i s the s a m e for cement p a s t e a s for compacts of bottle hydrated cement. It i s apparent that "work hardening" does not occur i n this system, possibly owing to the f a c t that the m a t e r i a l i s poorly c r y s t a l l i z e d and in p a r t amorphous.

Effect of Morphology

Ridge and Surkevicius (16) have shown that different c r y s t a l habits of gypsum can be obtained by the use of c e r t a i n a d m i x t u r e s that r e t a r d or a c - c e l e r a t e s e t of the hemihydrate. I t was of i n t e r e s t to t e s t whether t h e s e changes in habit would be detected a s changes i n m i c r o h a r d n e s s .

T h r e e modifications of the gypsum c r y s t a l s w e r e obtained by h y d r a - ting pottery p l a s t e r in a rotating polyethylene bottle a t a g / w r a t i o of 1: 10, with admixtures of calcium a c e t a t e 0. 7q0, gelatin 0.570 and without admixture.

The t e m p e r a t u r e was controlled a t 77 OF. The c r y s t a l s w e r e d r i e d a t 3070 RH and compacted a t different p r e s s u r e s to provide a r a n g e of porosity.

These compacts w e r e m e a s u r e d f o r m i c r o h a r d n e s s and the r e s u l t s a r e plotted on Fig. 4 with r e s u l t s f o r compacts of selenite. The r e s u l t s show c l e a r l y that m i c r o h a r d n e s s v s p o r o s i t y c u r v e s a r e d i s t i n c t f o r each preparation, but the significance of this finding i s not yet evident. I t i s antici- pated that the c r y s t a l habit giving the highest m i c r o h a r d n e s s c u r v e r e p r e -

V o l . 2 , No. 6 7 2 5 MICROHARDNESS, POROSITY, M A T E R I A L S P O R O S I T Y . P E U C E N l FIG. 4 P O R O S I T Y . P E R C E N l FIG. 5 M i c r o h a r d n e s s v s porosity f o r c o m p a c t s of different s a m p l e s of gypsum. M i c r o h a r d n e s s v s porosity of compacts of hemihydrates.

Again, this i s not c e r t a i n , in view of the possibility of work hardening due to c ompac tion.

As a n example of a v e r y g r e a t change in m i c r o h a r d n e s s r e s u l t i n g f r o m changes in morphology, Fig. 5 shows the r e s u l t s of m i c r o h a r d n e s s of two s a m p l e s of hemihydrate compacts. The two s a m p l e s have been identified a s p o t t e r y p l a s t e r and B - B a s e . According t o DTA a n a l y s e s , using the method

shown by Holdridge (17), the p o t t e r y p l a s t e r i s m o s t l y

B

modification and B - B a s e i s a modification. I t should be noted that the B - B a s e m a t e r i a l c o n s t i - t u t e s the only exception t o d a t e w h e r e data do not yield a s e m i - l o g a r i t h m i c r e l a t i o n with porosity.

Effect of humiditv

Evidence h a s been p r e s e n t e d (18, 19) t h a t a d s o r b e d water and other p o l a r s p e c i e s c a n have a c o n s i d e r a b l e effect upon m i c r o h a r d n e s s , p a r t i c u - l a r l y on indentation c r e e p . In the work now presented, involving inorganic

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non-metallic m a t e r i a l s , it i s a significant f a c t o r with r e s p e c t to the value of h a r d n e s s and a s a n aging f a c t o r i n c e r t a i n porous m a t e r i a l s . Because of the l a t t e r i t i s often difficult to s e p a r a t e the effect of humidity on m i c r o h a r d n e s s because, i n conditioning, the sample undergoes aging. Comparison of m i c r o - h a r d n e s s values a t different humidities i s t h e r e f o r e not valid for c e r t a i n m a - t e r i a l s .

Feldman and S e r e d a (20) have r e p o r t e d the effect of different humid- i t i e s on m i c r o h a r d n e s s of porous g l a s s ( s e e Fig.

6).

The effects of humidity on m i c r o h a r d n e s s of porous g l a s s and on f l e x u r e strength of portland cement p a s t e show the s a m e c h a r a c t e r i s t i c s , suggesting that m i c r o h a r d n e s s i s r e p - r e s e n t a t i v e of the f l e x u r e strength of a porous m a t e r i a l . Additional data a r e r e q u i r e d t o confirm or r e j e c t this apparent relation.

In s o m e nonporous m a t e r i a l s i t a p p e a r s that humidity can have a l a r g e effect on m i c r o h a r d n e s s . A l a r g e single c r y s t a l of selenite was tested f o r m i c r o h a r d n e s s a t 0 and 50% RH conditions. T h e Knoop m i c r o h a r d n e s s v a l u e s obtained on the 010 cleaved f a c e ranged f r o m 13.5 t o 31.5 kg/rnmL f o r 0% RH (depending on t h e orientation of the indenter) and f r o m 8. 1 t o 9.5 k g / m m 2 f o r 50% condition. FIG. 6 1 . o _{I I I I I I I} I . o -0-+-POROUS S I L I C A G L A S S ( P = 2 4 % r = 3 0 A ) 0 . 9 - 0 . 9 \ \

T

P O R T L A N D C E M E N T PASTE ( P = 3 2 % r Z 2 0 A ) ( r e f . 21) \ \ 0

- -

- F U S E D Q U A R T Z ( r e f . 2 2 )

-

0 . 8 r 0 . 7

-

\ I

Effect of humidity on m i c r o h a r d n e s s and s t r e n g t h

0 . 6 0 . 5 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 R E L A T I V E H U M I D I T Y , PER C E N T

-

-

-

L? T- I I I I I I I I I 0 . 6 - 0 . 5

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S u m m a r y and Conclusions

1. I t a p p e a r s t h a t sufficient work h a s b e e n c a r r i e d out t o s u p p o r t t h e conclusion t h a t m i c r o h a r d n e s s m e a s u r e m e n t s of p o r o u s m a t e r i a l s p r o v i d e meaningful i n f o r m a t i o n about m e c h a n i c a l behaviour and t h a t t h e s e m e a s u r e - m e n t s a r e a s significant f o r p o r o u s m a t e r i a l s a s they a r e f o r nonporous m a t e r i a l s . I t i s c l e a r t h a t d i f f e r e n c e s i n p o r o s i t y and morphology of t h e s a m p l e c a n b e r e a d i l y o b s e r v e d .

2 . M i c r o h a r d n e s s m e a s u r e m e n t s of p o r o u s m a t e r i a l s a p p e a r to yield the

s a m e c h a r a c t e r i s t i c c u r v e a s f l e x u r e s t r e n g t h when i t i s r e l a t e d t o humidity, indicating t h a t the m i c r o - m e c h a n i c s of f a i l u r e a r e s i m i l a r i n both c a s e s .

3 . A r e l a t i o n e x i s t s between m i c r o h a r d n e s s and m o d u l u s of e l a s t i c i t y f o r c e m e n t p a s t e . S i m i l a r l y , t h e r e i s a r e l a t i o n between compacting p r e s s u r e and m i c r o h a r d n e s s when dealing with c o m p a c t s of powdered m a t e r i a l s . Work should b e done to d e t e r m i n e whether a r e l a t i o n e x i s t s between c o m - p r e s s i v e s t r e n g t h and m i c r o h a r d n e s s .

4. M i c r o h a r d n e s s m e a s u r e m e n t i s a n o n d e s t r u c t i v e t e s t of m e c h a n i c a l behaviour and a s s u c h o f f e r s a useful tool f o r studying t h e effect of v a r i o u s m i c r o s t r u c t u r a l c h a n g e s i n a m a t e r i a l with c h a n g e s i n conditions of e x p o s u r e . I t i s believed t h a t indentation c r e e p m e a s u r e m e n t c a n p r o v i d e i n f o r m a t i o n on c r e e p behaviour of m a t e r i a l s .

Acknowledgements

The c r y s t a l s of s e l e n i t e w e r e f r o m the Kingdon Mine, F i t z r oy H a r b o u r , Ontario, and w e r e obtained by M r . H. M . Woodrooffe, Chief, M i n e r a l P r o c e s s i n g Division, D e p a r t m e n t of Mines, E n e r g y and R e s o u r c e s ; the h a l i t e s a m p l e w a s obtained f r o m a c o r e taken f r o m a Saskatchewan s a l t deposit, obtained through t h e c o u r t e s y of M r . R. Hanson, S a s k a t c h e w a n P o w e r Corp. The a u t h o r i s g r a t e f u l f o r t h e s e s a m p l e s without which the w o r k could not h a v e b e e n done.

T h e a u t h o r a l s o w i s h e s to acknowledge the c o n t r i b u t i o n of M e s s r s . D. E . Kennedy, E. G. Quinn and N. D a r b y f o r a s s i s t i n g with the e x p e r i - m e n t a l work.

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T h i s p a p e r i s a contribution f r o m the Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, and i s published with the a p p r o v a l of the D i r e c t o r of the Division.

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16 ( 4 ) 5 0 4 - 5 0 6 (1950) 16. M. J. Ridge and H. S u r k e v i c i u s , Aust. J. Appl. Sci. 11, (3) 385-39 8

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64 ( 4 ) 211-231 (1965). 18. J. H. Westbrook, and P. J. J o r g e n s e n , Arner. M i n e r a l . 53, 1899-1909

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19. R.E. Hanneman, and J.H. Westbrook, Phil. Mag.

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( 1 5 1 ) 89-99 (1968). 20. R . F . F e l d m a n and P. J. S e r e d a , Engrg. J.

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