The significance of helium diffusion measurements in studying the removal of structural water in inorganic hydrated systems

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The significance of helium diffusion measurements in studying the

removal of structural water in inorganic hydrated systems

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c *

_a

NATlONAL

RESEARCH COUNCIL

OF

RESEARCH CO~NCR

OONSElL NATIONAL DE RECHERCHES

DU CANADA

THE SIGNIFICANCE OF HELIUM DIFFUSION MEASUREMENTS IN

STUDYING THE REMOVAL OF STRUCTURAL WATER

IN INORGANIC HYDRATED SYSTEMS

by

J. J. Peaudoin and R. F. Feldman

Reprinted from

CEMENT AND CONCRETE RESEARCH

Vol.

8,

No.

2,

March 1978

pp. 223-232

DBR Paper No.

754 Division of Building Research

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CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 223-232, 1978. Pergamon Press, I n c . Printed i n the United States.

THE SIGNIFICANCE OF HELIUM DIFFUSION MEASUREMENTS I N STUDYING THE REMOVAL OF STRUCTURAT. WATER

I N INORGANIC HYDRATED SYSTEMS

J . J . Beaudoin and R. F. Feldman

M a t e r i a l s S e c t i o n , D i v i s i o n o f B u i l d i n g Research N a t i o n a l Research Council of Canada, Ottawa, Canada

(Communicated by J . Skalny) (Received Dec. 28, 1977) ABSTRACT

Helium d i f f u s i o n s t u d i e s w e r e made o n s i x d i f f e r e n t i n o r g a n i c h y d r a t e d systems: p o r t l a n d cement, magnesium o x y s u l f a t e , magnesium o x y c h l o r i d e , t r i c a l c i u m s i l i c a t e , gypsum and s e l e c t e d m o l e c u l a r s i e v e s . The h e l i u m i n t a k e v e r s u s t i m e c u r v e s o b t a i n e d a f t e r s t r u c t u r a l w a t e r was removed i n c r e m e n t a l l y , s t a r t i n g a t 11% RH, p r o v i d e d d a t a t h a t e n a b l e d a n a s s e s s m e n t o f t h e c h a n g i n g c h a r a c t e r of t h e s o l i d p h a s e . Changes i n s o l i d volume and s o l i d d e n s i t y were monitored a s w a t e r was removed from t h e s e s y s t e m s . F a c t o r s i n £ lw n c i n g physico-chemical p r o c e s s e s such a s s t r u c t u r a l c o l l a p s e due t o removal of i n t e r l a y e r o r composi- t i o n a l w a t e r a r e d i s c u s s e d w i t h r e f e r e n c e t o t h e s y s t e m s s t u d i e d . T h i s comparative s t u d y a l s o p r o v i d e d a f u r t h e r e l u c i d a t i o n o f t h e f a c t o r s t h a t a f f e c t t h e r e l o c a t i o n o f t h e w a t e r molecule i n t o t h e s t r u c t u r e

.

Des 6 t u d e s de l a d i f f u s i o n de l ' h 6 l i u m o n t S t 6 f a i t e s s u r s i x d i f f 6 r e n t e s c o m p o s i t i o n s i n o r g a n i q u e s hydra t 6 e s : l e ciment P o r t l a n d , l ' o x y s u l f a t e de magngsium, l ' o x y c h l o r u r e de magnEsium, l e s i l i c a t e t r i c a l c i u m , l e gypse e t d e s tamis m o l 6 c u l a i r e s c h o i s i s . La p r i s e dfh61ium c o n t r e l e s c o u r b e s de temps o b t e n u e s , a p r z s que l ' e a u

s t r u c t u r a l e p a r a c c r o i s s e m e n t a i t 6tE enlevEe P p a r t i r d'une h u m i d i t 6 r e l a t i v e de 11%, a p r o d u i t d e s donn6es q u i p e r m e t t e n t une E v a l u a t i o n d e s p r o p r i 6 t 6 s c h a n g e a n t e s de l a phase s o l i d e . Les changements de volume e t d e . d e n s i t E du s o l i d e o n t 6 t 6 o b s e r v 6 s _P mesure qu'on r 6 d u i s a i t l a t e n e u r e n e a u de c e s c o m p o s i t i o n s . Les f a c t e u r s i n f l u e n g a n t l e s proc6dEs physic-chimiques t e l que l ' e f f o n d r e m e n t s t r u c t u r a l _P c a u s e de l ' e n l e v e m e n t de l ' e a u _P l ' i n t e r c o u c h e ou de l ' e a u composante, s o n t d i s c u t 6 s e n f a i s a n t r 6 f 6 r e n c e aux c o m p o s i t i o n s 6 t u d i E e s . C e t t e 6 t u d e c o m p a r a t i v e permet de t i r e r a u c l a i r l e s f a c t e u r s q u i i n f l u e n c e n t l a r e l o c a l i s a t i o n de l ' e a u m o l 6 c u l a i r e dans l a c o m p o s i t i o n .

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V o l . 8, No. 2

J. J. Beaudoin, R. F. Feldman

I n t r o d u c t i o n

Helium d i f f u s i o n s t u d i e s on h y d r a t e d p o r t l a n d cement p a s t e have

d e m o n s t r a t e d t h a t c h a n g e s t o t h e s o l i d phase o f t h i s system, which o c c u r when w a t e r i s removed, can b e s y s t e m a t i c a l l y f o l l o w e d ( 1 ) . Helium i n t a k e v e r s u s

time c u r v e s f o r samples h a v i n g d i f f e r e n t m o i s t u r e c o n t e n t s p r o v i d e d t h e b a s i c d a t a f o r a n a s s e s s m e n t o f t h e s e c h a n g e s . The d a t a was p r e s e n t e d i n s e v e r a l ways: c u r v e s were p l o t t e d f o r t o t a l helium i n t a k e (when t h e flow r a t e

a p p r o a c h e s z e r o ) v e r s u s w e i g h t change, s o l i d d e n s i t y v e r s u s w e i g h t change, and s o l i d volume v e r s u s w e i g h t change. I n f e r e n c e s r e g a r d i n g m i c r o s t r u c t u r a l change were drawn from t h e shape o f each c u r v e . I n t h e c a l c u l a t i o n o f s o l i d d e n s i t y , i n i t i a l s o l i d volume, measured w i t h i n 15 s a f t e r h e l i u m was a d m i t t e d t o t h e sample, and t o t a l helium i n t a k e were t a k e n i n t o a c c o u n t . A l t e r n a t i v e l y , t h e change i n s o l i d d e n s i t y on removal o f w a t e r from t h e system was c a l c u l a t e d by u s i n g a v a l u e o f 1 . 2 7 g / c c o b t a i n e d from h e l i u m flow d a t a f o r t h e d e n s i t y o f t h e w a t e r removed. Comparison o f t h e s o l i d d e n s i t y v e r s u s w e i g h t change curve p l o t t e d f o r e a c h method o f d e n s i t y measurement p r o v i d e d a means o f

d e t e r m i n i n g t h e e x t e n t t o which h e l i u m was a b l e t o f i l l s p a c e v a c a t e d b y water. Helium i n t a k e v e r s u s time c u r v e s f o r r e w e t t e d specimens o f d r i e d p o r t l a n d cement p a s t e were a n a l y z e d s i m i l a r l y and c o n c l u s i o n s drawn r e g a r d i n g micro- s t r u c t u r a l changes (1)

.

The r e l a t i o n s h i p between t h e w e i g h t change (AW) and t h e a l g e b r a i c sum o f t h e i n i t i a l s o l i d volume change (AV) and t h e t o t a l h e l i u m i n t a k e , ( A D ) p r o v i d e d a means o f e s t i m a t i n g t h e d e n s i t y o f t h e w a t e r removed a s w e l l a s t h e e x t e n t t o which h e l i u m e n t e r e d t h e a v a i l a b l e m i c r o s p a c e .

I t was t h o u g h t t h a t p r e s e n t i n g t h e h e l i u m flow d a t a a s d e s c r i b e d h e l p e d t o e l u c i d a t e t h e p r o c e s s e s i n v o l v e d i n t h e d r y i n g a n d w e t t i n g o f h y d r a t e d p o r t l a n d cement and t h a t t h i s a n a l y s i s would b e a p p l i c a b l e t o s t u d i e s o f o t h e r h y d r a t e d s y s t e m s . The purpose o f t h i s s t u d y was t o a s s e s s t h e s i g n i f i c a n c e o f t h e h e l i u m d i f f u s i o n t e c h n i q u e - a s p r e v i o u s l y a p p l i e d t o t h e h y d r a t e d p o r t - l a n d cement s y s t e m s - and t o examine f a c t o r s i n f l u e n c i n g t h e physico-chemical p r o c e s s e s o p e r a t i v e i n s e v e r a l d i f f e r e n t h y d r a t e d systems when s t r u c t u r a l w a t e r i s removed. S i x h y d r a t e d systems were s t u d i e d .

Experimental

The p r e p a r a t i o n d e t a i l s o f t h e s i x systems s t u d i e d have b e e n r e p o r t e d e l s e w h e r e ( 2 - 5 ) . Samples f o r a l l s y s t e m s were d i s c s 3 . 2 cm i n d i a m e t e r , 1 . 3 mm t h i c k . Four o f t h e systems were p a s t e h y d r a t e d and two systems were powders t h a t were compacted. A l l systems were c o n d i t i o n e d t o 11% RH p r i o r t o d r y i n g . Drying was c a r r i e d o u t i n vacuo by pumping o r c o m b i n a t i o n s o f pumping and h e a t i n g .

The s i x s y s t e m s i n v e s t i g a t e d were a s f o l l o w s :

(1) Magnesium o x y s u l f a t e p a s t e . Tho s e t s o f samples were p r e p a r e d h a v i n g s o l u t i o n / s o l i d = 0 . 7 2 and 1 . 0 7 u s i n g a MgS04. 7H20 s o l u t i o n h a v i n g s p e c i f i c g r a v i t y = 1 . 1 8 . A t h i r d s e t o f samples was p r e p a r e d w i t h s o l u t i o n / s o l i d = 0 . 7 2 u s i n g MgS04.7H20 s o l u t i o n h a v i n g s p e c i f i c g r a v i t y

= 1 . 3 3 . A l l samples were c u r e d a t 50% RH f o r 2 weeks and s u b s e q u e n t l y c u r e d a t 11% RH f o r 3 months.

(2) C3S p a s t e . Two s e t s o f p a s t e samples were p r e p a r e d h a v i n g water/cem&t

= 0 . 5 a n d 0 . 8 . The p a s t e was h y d r a t e d i n t h e wet s t a t e f o r 2 . 5 y e a r s . (3) P o r t l a n d cement p a s t e . Samples were p r e p a r e d a t f o u r w a t e r l c e m e n t r a t i o s

o f 0 . 4 , 0 . 6 , 0 . 8 , and 1 . 0 . The samples were h y d r a t e d i n w a t e r f o r 2.5 y e a r s .

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Vol. 8, No. 2

HELIUM FLOW, STRUCTURAL WATER, INORGANIC HYDRATES

(4) CaS04 .2H?O. Samples were p r e p a r e d by compacting r e a g e n t gracie gypsum a t 680 MPa.

(5) Magnesium o x y c h l o r i d e p a s t e . Samples were p r e p a r e d a t MgC12.6H20/Mg0 s o l u t i o n / s o l i d r a t i o s o f 0 . 8 6 . Samples were cured a t 50% RH f o r 10 months.

(6) Molecular s i e v e s . Samples were p r e p a r e d from molecular s i e v e s s u p p l i e d b Linde Co., USA. Two s e t s o f samples were p r e p a r e d f o r e a c h o f t h e

B:

3

,

5W and l0A molecular s i e v e s .

Helium Comparison Pycnometry and Helium Flow Data

The a p p a r a t u s and procedure f o r helium flow measurements a r e d e s c r i b e d i n d e t a i l i n p r e v i o u s p a p e r s ( 1 - 3 ) . The technique u s e s t h e gas laws and t h e assumption of i d e a l gas b e h a v i o u r t o c a l c u l a t e t h e s o l i d volume of a sample. Small s p a c e s o r "pores" i n t o which helium does n o t i n s t a n t a n e o u s l y flow a r e , i n t h e f i r s t i n s t a n c e , r e g a r d e d a s p a r t o f t h e s o l i d . The t o t a l helium i n t a k e (when helium flow r a t e approaches zero

-

u s u a l l y about 48 h) i s used i n

d e n s i t y c a l c u l a t i o n s .

General Procedure

Helium i n t a k e a s a f u n c t i o n o f time and d e n s i t y were measured a t d i f f e r e n t m o i s t u r e c o n t e n t s beginning w i t h t h e 11% RH c o n d i t i o n . Water was removed i n c r e m e n t a l l y by e v a c u a t i o n a l o n e and l a t e r by e v a c u a t i o n and h e a t i n g a t i n c r e a s i n g t e m p e r a t u r e s f o r d i f f e r e n t p e r i o d s of t i m e . T h i s was done i n a s e p a r a t e vacuum v e s s e l and t h e sample was then t r a n s f e r r e d t o t h e pycnometer.

R e s u l t s

Helium I n t a k e v e r s u s Time Curves

Typical helium i n t a k e c u r v e s f o r p o r t l a n d cement p a s t e a r e p r e s e n t e d i n F i g . l ( a ) . Helium i n t a k e i n c r e a s e s a s w a t e r i s removed ( c u r v e s 1 and 2 ) . As more w a t e r i s removed, t h e i n i t i a l flow r a t e i s l e s s b u t t o t a l i n t a k e i s i n c r e a s e d (curve 3 ) . F u r t h e r removal o f w a t e r r e s u l t s i n a d e c r e a s e d flow r a t e a s w e l l a s d e c r e a s e d t o t a l i n t a k e ( c u r v e s 4 and 5 ) . Helium i n t a k e v e r s u s time c u r v e s f o r molecular s i e v e samples a r e p r e s e n t e d i n F i g . l ( b ) . A l l

samples were h e a t e d f o r a minimum of 20 h. T o t a l i n t a k e i n c r e a s e d w i t h o s i e v e s i z e and t h e r a t e o f flow was s i g n i f i c a n t a f t e r 144 h f o r t h e 5A and 10A s amp1 e s

.

A1 1 systems s t u d i e d had e x p o n e n t i a l t y p e flow c u r v e s ; however, t h e i n i t i a l flow r a t e and t o t a l i n t a k e , a s w e l l a s t h e number and sequence of flow b e h a v i o u r s , v a r i e d w i t h m a t e r i a l and m o i s t u r e c o n t e n t . The helium i n t a k e v e r s u s time curves f o r t h e o t h e r systems s t u d i e d a r e n o t p r e s e n t e d .

Total Helium I n t a k e v e r s u s Weight Change

Helium i n t a k e a f t e r 48 h v e r s u s weight change i s p l o t t e d i n F i g . 2 f o r oxysul f a t e , C3S and h y d r a t e d p o r t l a n d cement p a s t e s . I n c r e a s e d t o t a l i n t a k e was observed f o r C3S and h y d r a t e d p o r t l a n d cement p a s t e s a s w a t e r was removed; a f t e r r e a c h i n g a maximum, t h e t o t a l i n t a k e d e c r e a s e d u n t i l a l l w a t e r was removed.

Two maxima were observed when t h e t o t a l helium i n t a k e v e r s u s w e i g h t change d a t a was p l o t t e d f o r o x y s u l f a t e p a s t e ; t h e s e maxima o c c u r r e d a f t e r w a t e r was removed by h e a t i n g i n vacuo a t approximately 100 and 150°C. L a r g e s t t o t a l i n t a k e was observed when o x y s u l f a t e p a s t e was h e a t e d t o 200°C.

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V o l . 8, No. 2 J. J. Beaudoin, R. F. Feldman 1 0 2 0 30 40 T I M E . h 0.800 1 . 6 0 0 2 . 4 0 0 3 . 2 0 0

-

4 . 0 0 0 - 1. 3 1 H E A T E D 3,OAYS p = 2 . 3 0 7

-

2. 3A H E A T E D 1 D A Y p ; 2 . 5 7 7 4. 5 A H E A T E O 2 4 H O U R S 5 . 6 0 0 p = 2 . 6 0 9 5 . 1 0 1 H E A T E D 4 8 H O U R S p = 2 . 4 6 5 6. 1 0 1 HEATEO 4 8 H O U R S p = 2 . 4 6 6 6 . 4 0 0 0 24 4 8 7 2 96 ₁₂₀ 144 T I M E . h Fig. l ( a ) F i g . 1 (b)

Helium i n t a k e v e r s u s time f o r h y d r a t e d Helium i n t a k e v e r s u s time f o r p o r t l a n d cement p a s t e molecular s i e v e s

1 1 1 1 7 1 1 1 1 1 1 1 T o t a l he1 ium i n t a k e v e r s u s weight

8 - M A G N E S I U M O X Y S U L F A T E C P

1 -

change p l o t s f o r b o t h magnesium oxy-

7

-

.

S O L I S O L I D = 0. 7 2

-

c h l o r i d e and CaS04. 2H20 systems

-

1 1 % - 0 % R H _- e x h i b i t e d two o r m r e maxima; t h e r e s u l t s a r e n o t p r e s e n t e d . I n t e r -

-

p r e t a t i o n o f t o t a l helium i n t a k e 4 -

-

v e r s u s weight change p l o t s w i l l b e d i s c u s s e d l a t e r .

-

Density Measurements Y 1 1 1 1 1 ~ ~ ~ ~ ~ ~ ~ Y S o l i d d e n s i t y v e r s u s weight change c u r v e s f o r CaS04. 2H20, C3S, 1 1 % - 0 % R H C 3 S P A S T E magnesium o x y s u l f a t e and o x y c h l o r i d e W I S = 0 . 5 0

i

p a s t e s a r e p r e s e n t e d i n F i g . 3 . #d S o l i d d e n s i t y v a l u e s were determined I 1

-1 by two methods: ( a ) from w e i g h t and

a

+ s o l i d volume measurements, t h e

P O R T L A N D l a t t e r c o r r e c t e d f o r t h e amount o f C E M E N T P A S T E helium t h a t had d i f f u s e d i n t o t h e

WIS = 0 . 4 0 system; (b) by a c c o u n t i n g f o r volume 2 changes, u s i n g a value f o r t h e

1

?z?2J

d e n s i t y o f t h e F i g . w a t e r 2 removed.

0 2 4 6 8 1 0 1 2 _{T o t a l helium i n t a k e v e r s u s weight} W E I G H T L O S S , % l o s s f o r s e l e c t e d h y d r a t e d systems

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No. 2

227

HELIUM FLOW, STRUCTURAL WATER, INORGANIC HYDRATES

l . C a S O q 2 H 2 0 2. C - 5 P A S T L ; W l S * 0.50 -#'#

t '

3 . M A G . U X Y S . P A S T E S O L I S O L I D ' 0. 72 : S G

.

1. I 8 4 MAC. 0 X Y C H L . P A S T E C A L C U L A T E D

-

W E I G H T L O S S . %

CaS04. 2H20. There i s good agreement F i g . 3 between t 6 e two methods up t o

Changes i n d e n s i t y due t o removal o f approximately 15% weight l o s s , and s t r u c t u r a l w a t e r from s e l e c t e d t h e d e n s i t y i n c r e a s e s from a n h y d r a t e d systems i n i t i a l v a l u e o f 2.32 g/cc t o 2.68 g / c c . From 15 t o 19% weight l o s s t h e r e i s a d e c r e a s e i n d e n s i t y t o 2.62 g / c c a s measured by method ( a ) followed by a r a p i d i n c r e a s e i n d e n s i t y t o 2.75 g/cc a s an a d d i t i o n a l small amount o f w a t e r i s removed (19.0 t o 19.8% weight l o s s ) . The d e c r e a s e i n d e n s i t y up t o 19% weight l o s s i s accompanied b y a d e c r e a s e i n t h e amount of helium i n t a k e , and t h e subsequent i n c r e a s e i n d e n s i t y up t o 19.8% weight l o s s i s accompanied by

a

l a r g e i n c r e a s e i n helium i n t a k e . The d e n s i t y a s

c a l c u l a t e d by method (b) c o n t i n u e s t o i n c r e a s e over t h e whole weight l o s s range; t h e v a l u e o f d e n s i t y measured by method (a) approaches t h e v a l u e c a l c u l a t e d by method ( b ) a t 19.8% weight l o s s .

C3S p a s t e . Density v a l u e s o b t a i n e d by b o t h methods a r e s i m i l a r up t o a b o u t 5% weight l o s s and i n c r e a s e from 2.30 g/cc t o 2.39 g / c c . A s more w a t e r i s removed from t h e sy'stem, t h e d e n s i t y measured by method ( a ) d e c r e a s e s t o 2.32 g / c c a t 7% weight l o s s . T h i s d e c r e a s e i n d e n s i t y i s a l s o accompanied by a d e c r e a s e i n helium i n t a k e . The d e n s i t y c a l c u l a t e d i n method (b) c o n t i n u e s t o i n c r e a s e t o 2.43 g/cc a t 7% weight l o s s .

Magnesium o x y s u l f a t e p a s t e . Density v a l u e s measured by method ( a ) a r e l a r g e r : t h a n t h o s e c a l c u l a t e d by method (b) a t weight change i n t e r v a l s of 3 t o 5% and 7.5 t o 9 . 6 % . I n each i n t e r v a l t h e d e n s i t y v a l u e (method ( a ) ) r e a c h e s a maximum and t h e n d e c r e a s e s t o t h e v a l u e o b t a i n e d by method (b)

.

F u r t h e r removal o f w a t e r t o 10% w e i g h t change r e s u l t s i n a l a r g e i n c r e a s e i n d e n s i t y (method ( a ) )

.

I n c r e a s e s and d e c r e a s e s i n d e n s i t y a r e accompanied by i n c r e a s e s and d e c r e a s e s i n helium i n t a k e .

Magnesium o x y c h l o r i d e p a s t e . Density v a l u e s determined b y b o t h methods a r e s i m i l a r up t o 8 % weight l o s s . Bewteen 8 t o 10% and 10 t o 11.5% weight l o s s , d e n s i t y v a l u e s (method ( a ) ) r i s e t o a maximum and t h e n d e c r e a s e .

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F. Feldman

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From 1 1 . 5 t o 14% weight l o s s , t h e d e n s i t y v a l u e s i n c r e a s e t o a maximum v a l u e of 2 . 2 0 g / c c . I n c r e a s e s i n d e n s i t y v a l u e s a r e accompanied by i n c r e a s e s i n heliwn i n t a k e .

T o t a l Space Vacated by Water v e r s u s Degree o f Drying

An e s t i m a t e o f t h e space v a c a t e d by w a t e r on d r y i n g samples was provided by c a l c u l a t i o n o f t h e parameter AV-AD. In F i g . 4 , AV-AD was p l o t t e d a g a i n s t weight change f o r a l l systems s t u d i e d . The c u r v e s f o r a l l systems were

i n i t i a l l y l i n e a r . The i n v e r s e s l o p e of t h e l i n e provided an e s t i m a t e o f t h e d e n s i t y o f w a t e r removed f o r each system; t h e w a t e r d e n s i t y v a l u e s a r e r e c o r d e d i n Table I .

TABLE I

Water D e n s i t i e s Determined from Slope o f A V - A D v e r s u s AW P l o t

Density o f Water ( g / c c ) M a t e r i a l Magnesium o x y s u l f a t e p a s t e CaS04. 2H20 C3S p a s t e P o r t l a n d cement p a s t e Magnesium o x y c h l o r i d e p a s t e

-

L

-

A

-

m o 0 - 8

-

d

-

_"_"

-

- 1 0 - a a

-

1 1 % - 0 % R H > - 1 2

-

4

-

1 . M A G . O X Y S P A Z T F . Z n l l z n l In = n 7 7 2. C j S P A S T E : W I S = 0 . 8 0 3. H P C : W I S - 0 . 4 0 - 1 . 0 0

-

5 . M A G . O X Y C H L . P A S T E : S O L I S O L I D = 0 . 8 6

-

6 . M A G . O X Y S . P A S T E : S O L l S O L l D = 0 . 7 2 S G = 1 . 1 8

-

7 . M A G . O X Y S . P A S T E : S O L I S O L I D = 1 . 0 7 - -20 1 1 [ i l l 1 l l l l l l l l l l l l 0 2 4 6 8 1 0 1 2 1 4 1 6 18 2 0 W E I G H T L O S S . % Fig. 4

Parameter AV-AD v e r s u s weight

l o s s f o r s e l e c t e d h y d r a t e d systems Curves 1, 6 and 7 i n F i g . 4 ( d a t a p l o t t e d f o r magnesium o x y s u l f a t e p a s t e s ) s u b s e q u e n t l y had a change i n s l o p e ; t h e s l o p e f o r c u r v e s 6 and 7 i n c r e a s e d s i g n i f i c a n t l y f o r small changes in'sample w e i g h t . The s l o p e f o r curve 1 i n c r e a s e d s i g n i f i - c a n t l y o v e r a g r e a t e r range o f w e i g h t change. Curves 2 and 3 f o r C3S and p o r t l a n d cement p a s t e s had a s i g n i f i c a n t d e c r e a s e i n s l o p e a f t e r 6 and 7% weighs l o s s , r e s p e c t i v e l y . The s l o p e o f c u r v e s 4 and

5 (CaSO4. 2H20 and magnesium

o x y c h l o r i d e p a s t e s ) remained c o n s t a n t t o approximately 15% weight change. The i n t e r p r e t a - t i o n o f t h e s e c u r v e s w i l l b e d i s c u s s e d l a t e r . A t g r e a t e r weight changes (15 t o 19.8%) t h e s l o p e o f t h e AV-AD v e r s u s weight change curve f o r CaS04. 2H20 d e c r e a s e d s i g n i f i - c a n t l y . A t 1 9 . 8 % weight change, A V - A D changed v e r y

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HELIUM FLOW, STRUCTURAL WATER, INORGANIC HYDRATES

r a p i d l y o v e r a small weight- change i n t e r v a l a s t h e s l o p e became v e r y s t e e p . These r e s u l t s f o r t h e CaS04. 2H20 system w i l l be d i s c u s s e d l a t e r .

Discussion

T h i s s t u d y has provided n e c e s s a r y d a t a f o r an assessment o f t h e s i g n i f i c a n c e o f t h e helium d i f f u s i o n technique t o monitor m i c r o s t r u c t u r a l changes r e s u l t i n g from t h e removal o f w a t e r from s e v e r a l d i f f e r e n t hydrated cement systems. The d a t a has been p r e s e n t e d i n s e v e r a l ways:

(1) Helium i n t a k e v e r s u s time c u r v e s - Changes i n q u a n t i t y and r a t e o f flow a s w a t e r was removed i n increments from t h e s e systems p r o v i d e d a means o f d e t e c t i n g changes t h a t occur t o t h e s o l i d phase. d u r i n i d r y i n g . The s y s t e m a t i c i n c r e a s e i n t o t a l i n t a k e t o a maximum v a l u e followed by a d e c r e a s e i n t h e amount o f t o t a l i n t a k e f o r t h e C3S system h a s been

a t t r i b u t e d t o s t r u c t u r a l c o l l a p s e due t o removal o f i n t e r l a y e r w a t e r ( 3 ) .

S i m i l a r o b s e r v a t i o n s f o r t h e magnesium o x y s u l f a t e system have been

a t t r i b u t e d t o s t r u c t u r a l c o l l a p s e r e s u l t i n g from t h e thermal decomposition o f o x y s u l f a t e complexes a t a p p r o x i m a t e l y 100 and 1 5 0 " ~ . The curves f o r t h e molecular s i e v e s i n d i c a t e t h a t t h e r a t e and amount o f flow a r e c o n t r o l l e d by t h e s i z e o f e n t r a n c e s t o a v a i l a b l e microspace. P r o c e s s e s t h a t a l t e r t h e dimensions o f t h e s e e n t r a n c e s , such a s s t r u c t u r a l

c o l l a p s e , probably r e s u l t i n changing t h e c h a r a c t e r o f t h e helium i n t a k e v e r s u s time c u r v e . P r o c e s s e s t h a t r e s u l t i n t h e opening o f d i f f u s i o n p a t h s t o p r e v i o u s l y d i s c r e t e o r t r a p p e d empty space can r e s u l t i n l a r g e i n c r e a s e s i n r a t e and amount o f

helium i n t a k e w i t h v e r y l i t t l e weight change. T h i s type o f flow b e h a v i o u r a p p e a r s t o o c c u r f o r t h e magnesium ~ x ~ s u l f a t e system. When CaS04.2H20 was h e a t e d i n vacuo a t 200°C, t h e r e was a t r a n s f o r m a t i o n from s o l u b l e t o i n s o l u b l e a n h y d r i t e . T h i s t r a n s f o r m a t i o n p r o c e s s was accompanied by a l a r g e i n c r e a s e i n flow r a t e and amount o f helium i n t a k e . The s t r u c t u r e a p p a r e n t l y opens up and helium becomes a c c e s s i b l e t o p r e v i o u s l y trapped s p a c e . This w i l l b e d i s c u s s e d i n d e t a i l e l s e w h e r e .

T o t a l helium i n t a k e v e r s u s weight change c u r v e s

-

For systems s t u d i e d , i t was observed t h a t helium i n t a k e i n c r e a s e s t o a maximum v a l u e and t h e n d e c r e a s e s a s more w a t e r i s removed from t h e system. This d e c r e a s e i n i n t a k e a s more w a t e r i s removed i s a t t r i b u t e d t o s t r u c t u r a l c o l l a p s e . The s t r u c t u r a l c o l l a p s e a s d i s c u s s e d above may b e due t o t h e removal o f i n t e r l a y e r w a t e r ( p o r t l a n d cement and C3S p a s t e systems) o r t o thermal-decomposi- t i o n o f complex h y d r a t e s (magnesium o x y s u l f a t e and o x y c h l o r i d e s y s t e m s ) . Schematic W E I G H T L O S S , % F i g . 5 o f parameter A V - A D v e r s u s weight l o s s

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(3) D e n s i t y v e r s u s w e i g h t change c u r v e s

-

D e n s i t i e s o f t h e s o l i d phase were d e t e r m i n e d by t a k i n g i n t o a c c o u n t t h e s v a c e i n t o which h e l i u m had

-

d i f f u s e d . Where d e n s i t i e s d e t e r m i n e d from w e i g h t a n d volume measurements were l a r g e r t h a n t h o s e d e n s i t i e s c a l c u l a t e d b y assuming a v a l u e f o r t h e d e n s i t y o f t h e w a t e r removed, i t was a p p a r e n t t h a t helium e n t e r e d empty s p a c e s t h a t were p r e s e n t i n i t i a l l y b u t had b e e n i n a c c e s s i b l e . These may have b e e n t r a p p e d o r d i s c r e t e s p a c e s t o which d i f f u s i o n p a t h s opened up a s a r e s u l t o f i n t e r n a l c r a c k i n g due t o h e a t i n g o r t h e d r y i n g p r o c e s s . (4) A V - A D v e r s u s w e i g h t change c u r v e s - A s c h e m a t i c o f AV-AD v e r s u s w e i g h t

change i s p l o t t e d i n F i g . 5. Curve 0 - 4 i s a s t r a i g h t l i n e . The i n v e r s e s i o p e o f t h i s l i n e g i v e s a v a l u e f o r t h e d e n s i t y o f w a t e r removed from t h e system s t u d i e d . With t h e e x c e p t i o n o f t h e magnesium o x y c h l o r i d e cement system, t h e v a l u e s o b t a i n e d i n t h i s s t u d y were r e a l i s t i c and s i m i l a r t o v a l u e s f o r a d s o r b e d w a t e r i n o t h e r s y s t e m s ( 6 ) . I t i s p r o b a b l e t h a t h e l i u m e n t e r e d a l l t h e s p a c e made a c c e s s i b l e b y t h e

removal o f w a t e r . F u r t h e r d r y i n g o f C3S, p o r t l a n d cement and CaS04. 2H20 p a s t e s r e s u l t e d i n s m a l l e r changes o f A V - A D w i t h change i n w e i g h t such a s i n d i c a t e d b y t h e segment o f t h e c u r v e ( 4 - 5 ) . Helium c a n n o t e n t e r a l l s p a c e v a c a t e d b y w a t e r ; t h i s i s i n t e r p r e t e d a s a r e s u l t of s t r u c t u r a l c o l l a p s e . Large i n c r e a s e s i n AV-AD w i t h small w e i g h t c h a n g e s (segments 4-6 a n d 1 - 3 ) a r e p r o b a b l y due t o d i s c r e t e empty p o r e s o r t r a p p e d s p a c e becoming a c c e s s i b l e t o h e l i u m ; t h i s phenomena was o b s e r v e d f o r magnesium o x y s u l f a t e and o x y c h l o r i d e s y s t e m s . Curve 0 - 1 - 2 h a s a n i n v e r s e s l o p e which g i v e s an u n r e a l i s t i c a l l y h i g h v a l u e f o r t h e d e n s i t y o f w a t e r removed. I t i s a p p a r e n t t h a t h e l i u m i s n o t e n t e r i n g a l l t h e s p a c e v a c a t e d by w a t e r .

The f o l l o w i n g i s a sunnnary o f t h e p o s s i b l e mechanisms t h a t a r e o p e r a t i v e d u r i n g e x p o s u r e o f i n o r g a n i c h y d r a t e d systems t o h e l i u m a f t e r w a t e r h a s b e e n removed i n c r e m e n t a l l y from t h e s e systems:

(1) S p a c e v a c a t e d by b u l k , a d s o r b e d o r i n t e r l a y e r w a t e r i s c o m p l e t e l y f i l l e d w i t h h e l i u m .

(2) S p a c e formed b y d e c o m p o s i t i o n o f h y d r a t e s i s f i l l e d w i t h helium. (3) New, u n f i l l e d ( w i t h w a t e r ) s p a c e s a r e opened and f i l l e d w i t h helium. (4) P r e v i o u s l y w a t e r - f i l l e d s p a c e i s i n c o m p l e t e l y f i l l e d w i t h h e l i u m (5) Combinations o f t h e s e mechanisms.

C o n c l u s i o n s

The helium-flow t e c h n i q u e is a u s e f u l t o o l i n cements r e s e a r c h b e c a u s e i t p e r m i t s e v a l u a t i o n o f t h e f o l l o w i n g :

( a ) changes i n s o l i d volume a n d d e n s i t y o f c e m e n t i t i o u s systems

r e s u l t i n g from removal o f s t r u c t u r a l w a t e r b e c a u s e o f ( i ) d r y i n g a n d ( i i ) decomposition due t o h e a t i n g ; (b) d e n s i t y o f s t r u c t u r a l w a t e r removed on d r y i n g ; ( c ) f a c t o r s i n f l u e n c i n g p h y s i c o - c h e m i c a l p r o c e s s e s s u c h a s s t r u c t u r a l c o l l a p s e due t o removal o f i n t e r l a y e r o r c o m p o s i t i o n a l w a t e r , o r o t h e r t y p e s o f d e c o m p o s i t i o n ; (d) f a c t o r s which a f f e c t t h e r e l o c a t i o n o f w a t e r m o l e c u l e s i n t o t h e s t r u c t u r e , such a s r e e n t r y o f i n t e r l a y e r w a t e r o r r e h y d r a t i o n on w e t t i n g .

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V o l . 8, No. 2 231 HELIUM FLOW, STRUCTURAL WATER, INORGANIC HYDRATES

Acknow 1 edgement

The a u t h o r s would l i k e t o acknowledge t h e v a l u a b l e a s s i s t a n c e o f J . Wood f o r p e r f o r m i n g most o f t h e e x p e r i m e n t s .

T h i s p a p e r i s a c o n t r i b u t i o n from t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Council o f Canada a n d p u b l i s h e d w i t h t h e a p p r o v a l o f t h e D i r e c t o r o f t h e D i v i s i o n .

R e f e r e n c e s

1. R.F. Feldman, Cem.Concr. Res. - 4 , 1 ( 1 9 7 3 ) .

2 . R.F. Feldman, Cem. Concr. Res.

1,

285 (1971)

.

3. R.F. Feldman, Cem. C o n c r . Res. _-2 , 1 2 3 ( 1 9 7 2 ) .

4 . J . J . Beaudoin and V.S. Ramachandran, Cem. Concr. Res.

5,

617 ( 1 9 7 5 ) .

5 . J . J . Beaudoin and R.F. Feldman, Cem. Concr. Res.

z,

585 ( 1 9 7 7 ) .

6 . S . Brunauer, The A d s o r p t i o n o f Gases and Vapors, p . 4 2 0 . P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n ( 1 9 4 3 ) .