The flow of helium into the microspaces of magnesium oxysulfate cement paste

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The flow of helium into the microspaces of magnesium oxysulfate

cement paste

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NATIONAL

RESEARCH COUNCIL OF

CANADA

OONSEIL NATIONAL

DE

RECHERCHES DU CANADA

FLOW OF HELIUM INTO THE MICROSPACES

OF MAGNESIUM OXYSULFATE

CEMENT PASTE

by

J. J. Beaudoin and R. F. Feldman

9

Reprinted from

CEMENTANDCONCRETERESEARCH

Vol. 7, September 1977

DBR Paper No. 729 Division of Building Research

Price 25 cents Ol-rAWA

65366

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CEMENT and CONCRETE RESEARCH. Vol. 7, pp. 585-596, 1977. Pergamon Press, Inc. P r i n t e d i n t h e U n i t e d States.

THE FLOW OF HELIUM INTO THE MICROSPACES OF MAGNESIUM OXYSULFATE CEMENT PASTE

J.J. Beaudoin and R.F. Feldman

Materials Section, Division of Building Research National Research Council of Canada, Ottawa, Canada.

(Communicated by F. H. Wittmann) (Received J u l y 14, 1977) ABSTRACT

Helium flow measurements were made on samples of magnesium oxysulfate cement paste initially conditioned at 11% R.H. and dried in stages by combinations of pumping and heating. Physical changes in the

microstructure of the cement paste due to chemical decomposition and water removal were monitored. It was concluded that helium can penetrate the microspaces of magnesium oxysulfate cement paste which form during removal of structural water. It appears that helium also enters discrete pores (unfilled with water) that become accessible when the cement paste is heated to 200°C. Mercury (at 407 MPa) can intrude into those microspaces not initially accessible to helium. Collapse of structure occurs when water is removed from this system. An estimate of the density of the water removed up to 15% weight change was 1.21

+

0.07 g/ec.

Des mesures du flux d'h6lium ont 6t6 pratiqu6es sur des 6chantillons de. pBte de ciment d'oxysulfate de magn6sium conditionngs initialement

2 11% HR et s6ch6s par 6tapes en op6rations d'ensemble de pompage et

de chauffage. Des changements physiques dans la microstructure de la pste de ciment caus6s par la d6composition chimique et l'enlsvement de l'eau ont 6t6 observ6s. I1 a 6t6 conclu que l'h6lium peut p6n6trer dans les vides microscopiques de la pzte de ciment d'oxysulfate de magn6sium q11i se produisent en enlevant l'eau structurale. I1 apparait que l'h6lium entre aussi dans les petits pores (vides d'eau) qui deviennent accessibles lorsque la pzte de ciment est chauff6e ii 200"~. Le mercure (2 407 MPa) peut s'introduire dans ces vides microscopiques qui ne sont pas au d6part accessibles 5 l'h6lium. L'effondrement des structures se produit lorsque l'eau est retir6e de ce systsme. Une 6valuation de la densit6 de l'eau enlev6e jusqu'ii 15% du changement de poids 6tait 1.21

+

0.07 ~ / c c .

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J.

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K . F .

Feldman

Introduction

Sore1 was the first to recognize the potential use of magnesium oxysulfate

cement as a binder (1). This cement. has good fire resistance and insulation

properties and is less sensitive to high temperatures than oxychloride. It is used particularly in prefabricated structural units that require processing at elevated temperatures. Recent proposals have discussed the use of this cement for nuclear applications as it is a more efficient neutron captor than portland cement. Magnesium oxysulfate is obtained by the reaction of an aqueous

solution of magnesium sulfate (MgS04.7H20) with magnesium oxide.

Depending on the conditions of formation, four types of oxysulfate

complexes are known to form (2,3). These are: 5Mg(OH)2.MgS04.3H20 (or

.

2H20), 3Mg (OH) 2. MgS04. 8H20, Mg (OH) 2. MgS04. 5H2O and Mg (OH) 2. 2MgS04. 3H20;

however, only 3Mg(OH) 2.MgS04. 8H20 appears to be stable below about 3S°C (3). It has been shown that porosity is an important property in strength determinations of oxysulfate cement (4); however, there appears to be no published information elucidating the nature of the pore spaces in this system and how they are affected by the instability of the solid phase during drying. Recently, a new technique, employing the diffusion of helium gas, has been shown to be a useful tool in studying changes during drying in the solid phase

of hydrated portland cement (5). This technique provides the means of obser-

ving changes in solid volume due to drying as well as the capability of measur ing quantitatively the creation of space due to the decomposition of complex hydrates, i.e., space resulting from the removal of hydrate water.

Helium diffusion methods have detected collapse of structure on removal of structural water in hydrated portland cement past.e as well as aging phenomena; this structural water had an important role in the mechanical behaviour of the solid material. These methods provided new information that was valuable in the evolution of a model for hydrated portland cement paste. It was thought that the helium diffusion technique could also be used to elucidate the pore structure of magnesium oxysulfate paste; this study is a report of the results obtained from the application of this technique to the oxysulfate system.

Experimental Materials

(a) Magnesium Oxide. The MgO powder was supplied by Basic Chemicals, Cleveland, Ohio. It had the following characteristics: N2 surface area,

20m2/~; active CaO 1.5%; ignition loss 4%; fraction passing through 200 mesh,

98%. The material satisfied the requirements of ASTM C-275-61.

(b) Magnesium Sulfate Solution. Aqueous solutions were prepared by mixing MgS04.7H20, as supplied by Anachemia, in a dry form with distilled water. A partially saturated solution having specific gravity 1.18 and a saturated

solution with specific gravity 1.303 were prepared. Preparation of Specimens

Series I - Cement paste samples were prepared at MgS04.7H20/Mg0 solution-solid

ratios of 0.72 and 1.07 using the partially saturated MgS04.7H20 solution. Specimens having a solution-solid ratio of 0.72 were cast in cube molds and

cured at 50% R.H. for 2 weeks. Discs 3.2 cm in diameter, 1.3 mm thick, were

cut from 3.2 cm cores obtained from the cube samples. The discs were stored at 11% R.H. for periods up to 3 months. Specimens having a soln/solid ratio of 1.07 were cast in 3.2 cm diameter rubes thzt were rotated during the

hydration process to prevent segregation. After demolding, the samples were

sliced into discs 1.3 mm thick and then conditioned at 11% R.H. Series I

samples had surface areas as determined by N2 adsorption of 15.7 m2/g (soh/

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V o l . 7, No. 5 587 HELIUM FLOW, MAGNESIUM OXYSULFATE CEMENT

Series I1 - Cement paste samples were prepared at a soln/solid. ratio of

0.72 using, in contrast to Series I, the saturated Mg.SO4.7H20 solution.

Series I1 samples had a surface area as determined by N2 adsorption of 5.2 m2/g. Disc samples were prepared as described previously for Series I samples. The discs were conditioned at 11% R.H. as with Series I, except that they were placed at 32% R.H. for approximately 3 days prier to the helium diffusion experiments. As Series I and Series I1 samples represent two distinctly different preparations, results are not dirkctly comparable; however, it was felt that placing Series I1 samples at 32% R.H. prior to drying would generate some preliminary information regarding volume change characteristics resulting from re-wetting this system.

Procedures Helium Comparison Pycnometry and Helium Flow

The apparatus and procedure for helium flow measurements are described in detail in previous papers (5,6,7). At the end of 60 h of helium flow, the density of the material is measured. The technique uses the gas laws and the assumption of ideal gas behaviour to calculate the solid volume of a sample. Small spaces or "poresw, which cause a delayed flow of helium into the sample, are, in the first instance, regarded as part of the solid.

General Procedure

The helium flow and density were measured at different moisture contents,

starting from the 11% R.H. condition for Series

I

samples and 32% R.H.condition

for Series 11 samp2es. Moisture was then removed incrementally by evacuation alone and later by evacuation and heating at increasing temperatures for different periods of time. This was done in a separate vacuum vessel and the sample was then transferred to the pycnometer. Helium was allowed to flow into the sample for 60 h at 2 atm. After the flow was terminated, the pressure was reduced to 1 atm and recompressed to 2 atm for solid volume determinations. Mercury Porosimetry

Pore size distributions for Series

I

samples were determined using an

Aminco porosimeter (407 MPa). The samples were conditioned at 11% R.H. prior

to testing.

Differential Thermal Analysis

Differential thermograms were obtained using the 990 Dupont thermal

analysis system. Twenty mg of the material were used for each run. The rate of heating was 20°C/min (T = 150 s/in.) and the sensitivity was AT =

5 mcal/s/in.

Results Helium Flow as a Function of Time

Helium inflow versus time curves for Series I samples (soln/solid

ratio = 0.72) are presented in Fig. 1 (weight loss up to 9.93%). The results

for the soln/solid ratio = 1.07 are qualitatively similar and are not

presented. The curves in Fig. 1 are of three types and are numbered for

reference in the text. The curve for the sample-at 11% R.H. and the subsequent 3 curves up to a weight loss of 2.6% show a very rapid helium flow for approximately the first hour. After approximately 10 h, the flow rate is negligible and one can assume that helium is no longer flowing into the sample. This is

the first type of curve designated Type

I.

At 3% weight loss, there is even

more rapid flow during the first hour, the flow continues throughout subsequent hours and equilibrium is not attained even after 60 h. This type of curve is

designated Type 11. Curves numbered 5 , 6, 8, 9, 10, 12, 13, 14, 16, 17 and 18

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5 88

J.

J. Beaudoin,

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F. Feldman

Vol. 7, No.

5

c u r v e s a f t e r 4 h w i t h a p p r e c i a b l e flow c o n t i n u i n g even a f t e r 60 h . T h i s c u r v e i s d e s i g n a t e d Type I11 a s a r e c u r v e s 11, 1 5 , 19, 20, 21, 22, 23, 24, 25 and 26.

Some s p e c i f i c f e a t u r e s of t h e helium flow v e r s u s time c u r v e s a r e of i n t e r e s t . A t 4.39% weight l o s s , Type I 1 flow o c c u r s ; however, t h e t o t a l flow d u r i n g t h e f i r s t hour i s much l a r g e r t h a n any o f t h e p r e c e d i n g Type I 1 c u r v e s . A t 4.53% weight l o s s , t h e t o t a l flow i s a g a i n g r e a t l y reduced b u t t h e flow curve remains Type 1 1 . A t 4.87% weight l o s s , Type I11 flow o c c u r s and e q u i l i b - rium i s n o t achieved even a f t e r 60 h . The n e x t t h r e e c u r v e s up t o weight l o s s 5.57% a r e Type I 1 and t h e flow (curve 14) i s f u r t h e r reduced. The n e x t c u r v e a t 5.96% weight l o s s (Type 111) h a s g r e a t l y reduced flow ( j u s t s l i g h t l y g r e a t e r t h a n t h e flow f o r t h e f i r s t two c u r v e s ) b u t t h e curve c u t s a c r o s s t h e f i r s t two c u r v e s a f t e r 27 h and u n l i k e Type I , t h e r e remains low b u t d i s c e r n i b l e flow a f t e r 60 h . There i s a l s o a d e c r e a s e i n flow r a t e w i t h r e s p e c t t o Type 1 c u r v e s i n t h e f i r s t hour. The f o u r c u r v e s s t a r t i n g a t weight l o s s 8.05% a r e Type I11 and e q u i l i b r i u m i s n o t reached a f t e r 60 h . A t weight l o s s = 9.52%, t h e flow i s a g a i n g r e a t l y reduced and i s Type 111. The f i n a l two c u r v e s , a t 9 . 8 3 and 9.93% weight l o s s , r e s p e c t i v e l y , show t h e l a r g e s t flow r a t e a t any time and a r e a l s o Type 111. The flow c u r v e s f o r S e r i e s I , s o l n / s o l i d = 1 . 0 7 , a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e f o r s o l n / s o l i d = 0.72 and a r e n o t p r e s e n t e d . The flow c u r v e s f o r S e r i e s I 1 samples a r e p r e s e n t e d i n Fig. 2, and a l t h o u g h t h e r e a r e q u a l i t a t i v e S E R I E S I s i m i l a r i t i e s t o t h o s e c u r v e s f o r S e r i e s I samples, Type I c u r v e s

1 . 0 were n o t observed. Crossover o f

c u r v e s throughout t h e t e s t sequence was minimal f o r S e r i e s I 1 samples.

TIME, h FIG. 1 Helium i n f l o w v e r s u s time f o r o x y s u l f a t e cement p a s t e S e r i e s I , s o l n / s o l i d = 0.72 ( s e e Table I f o r weight change d a t a ) . The c h a r a c t e r o f t h e helium flow v e r s u s time c u r v e was observed t o be i n f l u e n c e d by d i s s o c i a t i o n of h y d r a t e s and w a t e r removal a s a r e s u l t of t e m p e r a t u r e i n c r e a s e s and pumping. The sequence of t h e s e o p e r a t i o n s i s given i n Table I w i t h t h e weight changes f o r each of t h e c u r v e s o f Fig. 1.

E f f e c t o f Weieht Loss and Temperature on t h e Volume of Helium Flow To i l l u s t r a t e how t h e helium flow v a r i e s w i t h m o i s t u r e c o n t e n t , t h e volume t h a t flowed i n t o t h e sample a t 48 h was p l o t t e d a s a f u n c t i o n of m o i s t u r e removed from t h e 11% R . H . c o n d i t i o n f o r S e r i e s I samples and from t h e 32% R . H .

c o n d i t i o n f o r S e r i e s I 1 samples. The p l o t s f o r s o l n / s o l i d = 0.72 a r e shown i n F i g . 3 . The numerical s u b s c r i p t s f o r t h e p l o t p o i n t s correspond t o t h e c u r v e s numbered i n F i g s . 1 and 2 and Table I . For S e r i e s I samples, t h e r e i s an i n c r e a s e i n flow from 1.60 cc/100 g a t 11% R.H. up t o 4 . 9 cc/100 g a t approximately 4% weight l o s s , where

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V o l .

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589

HELIUM FLOW, MAGNESIUM OXYSULFATE CEMENT

t h e r e i s a small d e c r e a s e i n flow followed by a l a r g e i n c r e a s e t o 6 . 1 c c / 100 g a t 4.39% weight l o s s . The n e x t s i x p o i n t s up t o weight l o s s o f 5.96% i n d i c a t e a r e d u c t i o n i n flow t o approximately 2.5 cc/100 g ; t h e addi- t i o n a l weight l o s s i s achieved by pum- p i n g and h e a t i n g from 50°C t o 100°C. F u r t h e r h e a t i n g t o 150°C 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 flow t o 5.80 c c / 100 g a t 8.05% weight l o s s . I t i s noteworthy t h a t DTA r e s u l t s (Fig. 4) show two low- temperature endo- t h e r m i c peaks a t approximately 100°C and 150°C correspon- d i n g t o t h e l o s s o f f o u r and t h r e e mole- c u l e s o f w a t e r , r e s p e c t i v e l y , from 3 Mg (OH) 2 . MgS04. 8H20 ( 3 ) . TIME, h FIG. 2 Helium i n f l o w v e r s u s time f o r o x y s u l f a t e cement p a s t e S e r i e s 11, s o l n / s o l i d = 0.72 A d d i t i o n a l pumping and h e a t i n g t o 200°C r e s u l t e d i n a l a r g e d e c r e a s e i n flow t o 2.6 cc/100 g a t a weight l o s s o f 9.51%. F u r t h e r h e a t i n g a t 200°C r e s u l t e d i n extremely l a r g e i n c r e a s e s i n flow, t h e l a t t e r being g r e a t e r t h a n 8.5 cc/100 g a t a weight l o s s o f 9.93%. R e s u l t s f o r S e r i e s I , s o l n / s o l i d = 1.07, a r e q u a l i t a t i v e l y s i m i l a r and a r e n o t p r e s e n t e d .

For S e r i e s I 1 samples, t h e helium flow v e r s u s weight l o s s curve i s p l o t t e d w i t h v a l u e s o b t a i n e d by removal of w a t e r from t h e sample i n e q u i l i b r i u m w i t h 32% R . H . , which i n c l u d e s t h e removal of adsorbed w a t e r i n e x c e s s o f a mono-

\ l a y e r a s w e l l a s s t r u c t u r a l w a t e r . I t i s n o t e d t h a t t h e changes i n helium flow

w i t h weight l o s s observed f o r t h e curve f o r S e r i e s I 1 samples f o l l o w a s i m i l a r p a t t e r n t o t h o s e observed f o r S e r i e s I samples, b u t t h e magnitude of flow i s

much l e s s f o r S e r i e s I 1 samples up t o 9% weight lbss. T h i s may n o t be r e a d i l y a p p a r e n t a s t h e peaks f o r S e r i e s I 1 samples a r e s l i g h t l y d i s p l a c e d along t h e weight change a x i s ; d r y i n g from 32% R.H. may p a r t i a l l y e x p l a i n t h i s . I t i s n o t expected t h a t t h e removal o f t h e e x t r a sorbed w a t e r due t o 32% R.H. condition- i n g would r e s u l t i n s i m i l a r behaviour f o r b o t h S e r i e s I and S e r i e s I 1 samples because t h e y a r e two d i s t i n c t l y d i f f e r e n t p r e p a r a t i o n s ; S e r i e s I1 was p r e p a r e d w i t h a s a t u r a t e d s a l t s o l u t i o n and S e r i e s I was made w i t h p a r t i a l l y s a t u r a t e d s a l t s o l u t i o n .

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Vol. 7, No.

5 J. J. Beaudoin,

R. F. Feldman

FIG. 3 Helium i n f l o w a t 48 h v e r s u s weight l o s s f o r o x y s u l f a t e cement p a s t e There a r e s e v e r a l f l u c t u a t i o n s of i n c r e a s i n g and d e c r e a s i n g flow f o r S e r i e s I 1 samples a s f o r S e r i e s I samples b u t d i f f e r e n c e s between maxima and minima a r e l e s s . Heating

t o 200°C, however, y i e l d s an 11.59%

weight l o s s and r e s u l t s i n an extremely l a r g e i n c r e a s e i n flow

( s i m i l a r t o t h a t observed w i t h S e r i e s I samples).

T o t a l Space Vacated by Water Versus Degree o f Drying

I n t h e helium pycnometric t e c h n i q u e , one measures a " s o l i d volume" o f t h e sample by u s i n g t h e i n s t a n t a n e o u s v a l u e o b t a i n e d by

compressing t h e gas t o 2 atm i n t h e

sample c y l i n d e r . No helium flow i s

t a k e n i n t o account. One may t h e n

p l o t a c u r v e of change i n " s o l i d volume" (AV) w i t h weight l o s s a s

shown i n F i g . 5 . (This weight l o s s

i s due t o t h e w a t e r removed i n i n c r e m e n t a l d r y i n g from t h e s t a r - t i n g c o n d i t i o n

-

11% R . H . f o r S e r i e s I and 32% R . H . f o r S e r i e s 11.) In F i g . 3 , " t o t a l inflow" a t 48 h h a s been p l o t t e d a g a i n s t weight l o s s . The term " t o t a l inflow" r e f e r s t o t h e time-dependent helium flow i n t o t h e micro-

s p a c e s . Removal o f m o i s t u r e

I

r e s u l t s i n a n e g a t i v e change

-

i n " s o l i d volume" and a

_ - _ - - -

A

-

/ change i n helium flow (AD).

1 _{The a l g e b r a i c sum o f t h e s e}

f two, AV-AD, can be r e g a r d e d

1 _{a s t h e space v a c a t e d by water,} I I i f it i s assumed t h a t l i t t l e helium e n t e r e d t h e small \ 1 \ J s p a c e s v a c a t e d by water d u r i n g

compression and measurement of d e n s i t y , and i f t h e flow c u r v e s i n d i c a t e e q u i l i b r i u m I h a s been o b t a i n e d . I t i s a l s o assumed t h a t t h e helium I I I I b I I I I does n o t i n t e r a c t w i t h t h e I 50 1 0 0 1 5 0 Z O O 2 5 0 300 350 4 0 0 4511 XI s i a s u r f a c e o r o t h e r body f o r c e s T E M P E R A T U R E m~ and i s a t 2 atm w i t h i n a l l t h e s m a l l s p a c e s t h a t w a t e r h a s v a c a t e d ; t h i s f o l l o w s

FIG. 4 _{from t h e o t h e r assumptions.}

D i f f e r e n t i a l thermogram f o r I n F i g . 6 , AV-AD i s

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V o l . 7, N o . 5

H E L I U M FLOW, MAGNESIUM OXYSULFATE CEMENT

TABLE I (AW) f o r S e r i e s I samples ( s o l n /

weight c h a n g e due to Removal of Water s o l i d = 0.72, 1.07) and S e r i e s 11 from Magnesium Oxysulfate Cement Paste _{samples ( s o l n / s o l i d}

= 0 . 7 2 ) . For

Curve N O . series I, S o l n I S o l i d = 0.72 S e r i e s I p a s t e s , t h e p a r a m e t e r ,

(Fig. 1) Welght Change, % T r r r r m e n r _AV-AD, _{a p p e a r s l i n e a r up t o about}

I 15% weight l o s s . The p l o t p o i n t s

2 1.54 1 h pumpplng f o r s o l n / s o l i d = 0.72, w i t h

3 2.17 I h pumping

4 2.61 1 h pumping numerical s u b s c r i p t s 7 , 9 and

5 3.02 1 h B SODC 19-26, have v a l u e s of AV-AD which

6 3.80 1 h @ 50-c f a l l below t h e l i n e d e s c r i b e d by

4.00 I h pumping most of t h e d a t a and a c t u a l l y

8 4.22 1 h purnplng

9 4 . 3 9 I h pumplng form a s e p a r a t e l i n e having a

10 4 5 3 I h punplng v e r y s t e e p s l o p e . The p l o t

1 1 4 - 8 7 I h B 50°C p o i n t s f o r s o l n / s o l i d = 1.07

1 2 5 13 2 h n 50-c having numerical s u b s c r i p t s 15-21

13 5.32 I h d 75°C

14 5.57 I h B 75°C a t weight l o s s e s l a r g e r t h a n 15%

IS 5.96 I h pumping @ 1 0 0 ~ ~ a l s o f a l l below t h e l i n e and l i e 1 6 6 . 5 1 I h pumping B I O O - C along y e t a n o t h e r l i n e having a 17 6.73 1 h pumping b IOO-C v e r y s t e e p s l o p e . The r e a s o n f o r

18 7 7 4 I h pumping Id 150°C

19 8 0 5

,

@ L S O D C t h i s w i l l be d i s c u s s e d l a t e r .

2 0 9 3 1 2 h pumping "75'C

2 1 9.35 I h pumping @ 175°C For S e r i e s I 1 p a s t e s t h e r e 2 2 9.49 I h punping @ 17SDC i s a l i n e a r r e l a t i o n s h i p between

2 3 9 5 2 I h Pump,ng @ 2 0 0 ' ~ AV-AD and weight change (AW) . u p

24 9 . 5 1 I h p u m p i n g B 2 0 0 ' ~ t o a weight l o s s of approximately

I 25 9.83 1 h pumping @ 200CC

2 6 9 . 9 3 I h pumping b 200-c 8 . 5 % where t h e r e i s a change i n

s l o p e . The r e l a t i o n s h i p between

AV-AD and weight change a t h i g h e r v a l u e s of weight l o s s i s a l s o l i n e a r b u t h a s a s t e e p e r s l o p e s i m i l a r t o t h a t observed f o r S e r i e s I samples. The

AV-AD l i n e s f o r S e r i e s I 1 samples a r e d i s p l a c e d t o t h e r i g h t of t h e l i n e f o r S e r i e s I samples. A l i n e a r r e g r e s s i o n a n a l y s i s was a p p l i e d t o t h e AV-AD v e r s u s AW d a t a f o r -I

-

S e r i e s I and I 1 samples w i t h t h e e x c l u - a. D 0 s i o n o f t h e p l o t p o i n t s w i t h s u b s c r i p t s z .n - - mentioned p r e v i o u s l y . The s t r a i g h t l i n e f o r S e r i e s I samples i s > a - I -

I

AV-AD = -1.401 (20.261)-0.827 (+0.049)AW, CO<AW<l51 S O L N , S O L I O D R Y I N G \a (1) S E R I E S I 1 1 % 0 % R H 1 1 - The 90% confidence l i m i t s f o r i n t e r c e p t S E R I E S II

.

0 7 2 3 2 % 0 % RH and s l o p e a r e given i n b r a c k e t s . I f a l l

t h e assumptions a r e c o r r e c t and helium f i l l e d a l l t h e s p a c e t h a t t h e w a t e r 17 L v a c a t e d , t h e i n v e r s e of t h e s l o p e g i v e s t h e d e n s i t y of t h e w a t e r removed. T h i s 0 2 4 6 8 1 0 1 2 14 1fi 18 v a l u e comes t o 1.209 +0.07 f o r S e r i e s I W E I G H T L O S S % samples a s d e s c r i b e d by E q . ( I ) , and FIG. 5 a p p e a r s t o s u p p o r t t h e assumptions; _{r e s u l t s of d e n s i t y f o r some a d s o r b a t e s}

Change i n s o l i d volume v e r s u s weight up t o 30% above t h e i r bulk v a l u e s a r e l o s s f o r o x y s u l f a t e cement p a s t e r e c o r d e d i n t h e l i t e r a t u r e ( 8 ) .

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Vol.

7, No. 5

J. J. Beaudoin, R. F. Feldman

However, the line for Series I, soln/solid = 0.72, drawn through plot

points 18-26 and the line for soln/solid = 1.07 through plot points 16-21

appear not to result from the formation of space created by removal of water because the steep slope of these lines results in a density for water which is far too low.

The two lines for Series I1 samples are as follows:

AV-AD = -0.156 (20.487)-0.432 (+0.084) A w , (O<~W<8.51 (2)

and

AV-AD = 11.436 (21.72)-1.75 (20.159) AW, (8.5<~W<12.51 (3)

The inverse slope of the line given by Eq. (2) is 2.310, which is too high to be accounted for by the removal of water from the microspaces alone. The

inverse slope of the line given by Eq.

( 5 )

is 0.571, which is too low to be

accounted for by the removal of water from the microspaces alone.

The curves for AV versus AW as presented in Fig. 5 indicate a change in AV with respect to AW, which is much larger for Series I than for Series I1 samples.

Density by Helium Comparison Pycnometer

Density calculations were made with the helium flow volume included as

pore volume, and the results were plotted versus weight loss (Fig. 7). The

density versus weight loss curve is

I I I I I I I I

17\;41

I ' I I ' _- _{weight loss curve (Fig.}similar in shape to the helium flow versus _3). _{In general, the}

6 '.\

!

.,

density (Series I samples) increases from

-2",, '\

O:

,

-5 \ about 2.09 g/cc to 2.23 g/cc at 4% weight

L 2,?,04 8 . : .

, lPR.12

-

loss and after a decrease to 2.16 g/cc at

- 4

-

,:\

'.

I 3

-

4.25% weight loss increases to 2.29 at 4.4%

'* 3

.I6

-

b!pI$.,~,

\

-

weight loss. There follows a general

itlB,fK 11+* decrease to approximately 2.16 g/cc at 5.1%

- 6 -

d;':5.'$, \in,,

-

weight loss and subsequently a general

..

c \ \ - increase to 2.39 g/cc at 8.05% weight loss.

4

A;&

m

0 - E L Inn ::b - The density decreases to 2.26 g/cc at 9.6%

0

4 1 \

-

I \ weight loss and increases to greater than

U

,

<I:\~)

U , 0 2 4 _I \ \

' \

-

2.60 g/cc at approximately 10% weight loss.

- 1 0 - lZ22 +Q

d 6 2 , - Density was also calculated on the basis of

a - . 1 9 : 8 , - weight loss alone (assuming a water density

qA\f::

I a n of 1.20 g/cc) and plotted in Fig. 7.

- 1 2 - I

I l:IL,, -

I '1;2:h - In general, the solid densities of

-

; .24 +I:' Series

I

samples determined from the helium

-14 - \

I

\,;

flow results are similar to those calculated

I

- 0 2 6

- from weight loss alone. However, densities

- 1 6 - O R Y I N G

\-

determined by helium periodically deviate

I 0 , , 2 RH

\

from the calculated curve at several weight

-

o 1 . 0 7 losses, i.e., densities by helium give higher

-18 - S E R I E S ZI

-

0 . 7 2 32%-0% R H

,,;

values. This can be attributed to trapped

-

: : : : I : : \ : ,I )>~44~7 (empty) space (due to irregular collapse of

I I I I I r I I I I I I I I I I 1 , structure) becoming accessible to helium, or

0 2 4 6 8 10 12 1 4 16 18 discrete empty spaces opening up and suddenly

becoming accessible to helium. The densities W E I G H T L O S S , %

by helium for Series I1 samples all lie below

FIG.

6 the calculated line (based on weight loss

only). This can be attributed to the Parameter AV-AD versus weight inability of helium to penetrate spaces on loss for oxysulfate cement paste removal of water from this system.

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V o l . 7 , N o . 5 593 H E L I U M FLOW, MAGNESIUM OXYSULFATE CEMENT

2 I 1 1 1 1 1 1 1 1 1 1 1 _{In general the densities}

I O L N l , 0, l 2 { o S E R I E S I 11% R H - 0 % R H for Series I 1 samples are

S O L I D S E R I E S U 32% R H - 0 % R H

---

C A L C U L A T E D D E N S I T Y O N lower than Series I samples at

2 4 B A S I S O F W E I G H T L O S S the same weight loss. After

A L O N E A S S U M I N G

W A T E R . I 20 g ~ ~ c small fluctuations to a low

-

e

value of approximately 2.03

u 2 3 g/cc at 4.5% weight loss,

1 S t R l f l I I there is a general increase up

z-

-

", Z to 2.55 g/cc at 11.6% weight loss. z 2 P Mercury Porosimetry Mercury porosimetry results (intrusion pressure 407 MPa) for Series I samples conditioned at 11% R.H. gave

I I I I I values for total porosity

2 0

i

I

1

7 s P L O I I 12 which exceeded those deter-

W E I G H T L O S S 5 mined by helium pycnometry at

FIG. 7 11% R.H. This was surprising

because for most other sys- Density calculated with helium flow tems, such as porous glass, included as "pore" volume versus weight loss the porosity measured by

helium is equal to or greater

1 than that measured by mercury.

It was thought that this discrepancy might be due to the penetration of mercury into those spaces which became accessible to helium only after heating to 200°C

(see plot points 24-26 (Series I) Fig. 3). The contribution to total porosity that these spaces provide is calculated from flow curves (A(AD) /VT) obtained at weight changes subsequent to heating to 200°C. Results are presented in

Table 11.

TABLE I1

Comparison of Hg Porosity and the Porosity of Oxysulfate Cement

~

A (AD) [Fig. 61

,

He + A (AD)

Soln/Solid He Porosity,

v~

Hg Porosity,

Porosity, %

% % %

The data in Table I1 indicate that most of the space which mercury enters can be accounted for by the sum of the instantaneous helium porosity and the space which becomes accessible to helium after heating to 200°C. For

Series I samples, it is apparent that continued heating at 200°C opens up spaces that become accessible to helium.

Discussion

It has been observed that helium enters the pores of porous Vycor glass instantaneously and that no time-dependent flow takes place. The pores con- tained in Vycor glass have an average pore radius of approximately 301 and hence it is likely that helium, observed in the present experimenss, flowed

i

into space with openings having least dimension much less than 30A.

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594

J. J. Beaudoin, R. F. Feldman Vol. 7, No.

5

As referred to earlier, the helium molecule can give indications of what changes are occurring in a solid matrix during change in its chemical nature

(e.g., decomposition) or during change in its physical nature (e.g., collapse of structure). The reason is that the helium molecule is very small and inert. If holes are created into which helium can enter, this will be indicated; if regions are formed from which helium is excluded, this should also be indicated. Values of density can be computed with some knowledge of what these values mean.

The following mechanisms are suggested to be operative during exposure of the magnesium oxysulphate to helium, and it is felt that they explain the changing character of the time-dependent helium flow and thus the changes to the solid as water is withdrawn:

(1) Space containing bulk or adsorbed water empties completely and fills with he 1 ium.

(2) Space formed by decomposition of magnesium oxysulfate complexes fills with helium.

( 3 ) New unfilled (with water) spaces open and fill with helium.

(4) Previously water-filled space is incompletely filled with helium, possibly

because of structural collapse (which may involve a change in dimension of pore entrances).

Mechanisms (1) and (2) seem to be applicable to the data described by the

regression line for Series

I

samples having an inverse slope = 1.209. This

value represents the density of the water removed and is in line with measurements made on expanding and non-expanding clays for water contents of less than one monolayer (9).

Mechanism (3) can account for large increases in flow and solid density

with small changes in weight, e.g., large increases in flow (Fig. 1, curves 9,

19-26, soln/solid = 0.72) and density when the system is heated to 200°C. This

can also account for unrealistically low values for density of water obtained from a plot of AV-AD versus AW.

Mechanism (4) can account for unrealistically high values for water density obtained from a plot of AV-AD versus AW. It is also possible that some previously accessible (to helium) microspace may simultaneously become inaccessible as diffusion paths may disappear with structural collapse. It is apparent that a microporous material, which contains microspace with entrances having least dimension sufficiently small so as to regulate the inflow of helium, will present difficulty in the measurement of its solid density. The

inability of a fluid to enter the microspaces in this system would necessarily result in a value for solid density that is too low. The ability of helium to enter microspaces vacated by water enables determination of solid density that includes structural water remaining in the microspaces.

Increasing temperature probably affects the kinetics of structural collapse associated with decreased flow and removal of bulk, adsorbed and structural water. The two low-temperature DTA peaks reach a maximum at approximately 100°C and 150°C. However, the peaks are broad and there is gradual dehydration of the oxysulfate complexes and MgS04. 7H20 up to 300°C. The microspaces which are formed by the decomposition of these hydrates appear to be accessible to helium because the density value for water 1.209 g/cc obtained from the plot of AV-AD versus AW is within limits that would be expected for this type of water. For Series I samples, increased flow to 4.5% weight loss followed by decreased flow to 6% weight loss results from pumping and heating in stages up to approximately 100°C.

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V o l . 7, No. 5 595 HELIUM FLOW, MAGNESIUM OXYSULFATE CEMENT

The second cycle of increased and decreased flow up to a weight loss of 9.5% involves pumping and heating in stages from l00'C to 175'~. It seems likely that this cyclic behaviour is related to the gradual decomposition of the oxysulfate complexes.

Mercury intrusion experiments in this study give somewhat anomalous results; calculations, where one assumes a structurally stable matrix (to pressure) and the validity of the Kelvin equation, predict the filling of relatively large pores at 407 MPa ( ~ 3 0 A depending on the contact angle). It seems likely that the above assumptions do not hold for this system and that structural damage or alteration occurs to the matrix such that mercury penet- rates into space that was not immediately accessible to helium. Internal isolated space may be opened up by differential pressure. In this case,

calculations from the data based on conventional assumptions may not be valid but the results (total porosity) (Table 11) support the above conclusion.

Conclusions

The helium-flow technique can follow physical changes in the microstructure of magnesium oxysulfate cement paste, which are due to chemical decomposition and water removal, and establish the physical nature of the structure formed

.

I 1. There are discrete, unfilled (with water) microspaces that are not

initially accessible to helium. Removal of moisture and application of increased temperature opens up diffusion paths to these microspaces

1 allowing the inflow of helium.

2. The density of the water removed during drying from 11% R.H. was determined to be 1.209 k0.07 g/cc, which is in accord with results for adsorbed water reported in the literature.

3. When water is removed from this system (Series I), structural collapse occurs, i.e., as a result of solid volume changes due to drying, the microspace is reduced as solid surfaces come closer together.

4. Hg (at 407 MPa) can intrude into those microspaces not initially accessible to helium; this is probably the result of some structural damage or

alteration to the system resulting from large differential pressures that open up internal, isolated microspace.

Acknowledeement

The authors wish to thank J. Wood for his fine work in performing the experiments. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

References 1. S. Sorel, Compt. Rend. 65, 102 (1867).

2. T. Demediuk and W. Cole, Aust. J. Chem. 10, 287, (1957).

_-

3. E.S. Newman, J. Res., Nat. Bur. Stand. A, 68A, 645, (1964).

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

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

4,

1 (1973). 6. R.F. Feldman, Cem. Concr. Res.

1,

285 (1971). 7. R.F. Feldman, Cem. Concr. Res., 2, 123 (1972).

_-

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596 Vol. 7, No. 5

J. J. Beaudoin, R. F. Feldman

e

8. S. Brunauer, The Adsorption of Gases and Vapors, p.420. Princeton University Press, Princeton (1943).

9. R.T. Martin, Proc. Ninth Nat. Conf. Clays and Clay Minerals (Lafayette) 1960, 28 (1962).

b