Study of the Variations in Volume of Plasters During and After Hardening

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PREFACE

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Amongst t h e b u i l d i n g m a t j e r i a l s u n d e r a c t i v e s t u d y i n t h e D i v i s i o n o f i3uiI.ding S e s e a r c h l.s p l a s t e r , filt,~iough In u s e f o r such s 1 . m ~ p e r i o d a s a sta.n<larr! b u i l d l n g m a t e r i a l , t h e r e 1 s s t i l l much t h a t i s n o t p r n p e r l y known a.bout p l a s t e r and

I t s properties and tk,c D i v i s i o n has i n hand some s i ~ n l f l c a n t ; i n v e s t i g a t i o n s . I n t.kie c o u r s e o f t h e s e , c a r e f u l s t u d y i s r e g u l a r l y made of a v a l . l a . b l e

l i t e r a t u r e i n t h e c o u r s e o f which t h i s p a p e r has

beell reviebretl.

A s t u d y of t h e d i m e n s i o n a l change:: l.n s e t t i n g gypsum p l a s t e r and t h e mech~rnisrn r e s p o n s i b l e f o r

such chani:es, c o m p r i s e s one of tk.e p r o j e c t s o f t h e M a t e r i a l s : S e c t i o n of t h e D i v i s i o n o f i 3 u l l d i n g

R e s e a r c h , I n view o f t h e c o n f u s i o n o f tllought c h a r a c t e r i z i n g much of t h e p u b l i s h e d work i n ' t h i s f i e l d , t h e p r e s e n t a u t h o r i t a t i v e and comprehensive paper. i s b e l i e v e d t o b e a most v h l u a b l e con1;rlbution t o t h e l i t e r a t u r e .

The D i v i s i o n 1 s Inclebted t o M r . D.A. S - L r l c l a i ~ ~ o f t h e T r a n s l a t i o n s S e c t i o n o f t h e N a t i o n a l R e s e a r c h C o u n c i l f o r p r e p a r i n g t h i s translation.

O t t a w a ,

T i t l e :

NATIONAL RESEARCH COUNCIL OF CANADA

Technical T r a n s l a t i o n TT-689

Study of t h e v a r i a t i o n s i n volume of p l a s t e r s d u r i n g and a f t e r hardening*

&

(Etude d e s v a r i a t i o n s de volume de p l g t r e s pendant e t a p r g s l e u r p r i s e )

Author : Louis Chassevent

Reference : Revue des , ~ a t g r i a . u x de C o n s t r u c t i o n e t de Travaux P u b l i c s , E d i t i o n C , ( 4 0 5 ) : 188-194; ( 4 0 6 ) : 214-224; (407) : 267-272 and ( 4 0 8 ) : 304-308, 1949. T r a n s l a t o r : D.A

.

S i n c l a i r

,

T r a n s l a t i o n s S e c t i o n , N.R. C. L i b r a r y T r a n s l a t e d with permission

*

P u b l i c a t i o n of Centre d l g t u d e s e t d e Recherches de l t I n d u s t r i e d e s L i a n t s Hydrauliques

STUDY OF THE VARIATIONS I N VOLUME O F PLASTERS

DUH 11\83 AND AFTER HARDENING'

Purpose of t h e Study

T h i s study h a s a l r e a d y been t h e s u b J e c t of communications t o t h e ~ c a d k m i e d e s s c i e n c e s ' ' ) . I thought t h a t i t might be

u s e f u l t o supplement t h e s e communications, which were n e c e s s a r i l y very b r i e f , w i t h a more d e t a i l e d e x p o s i t i o n , paying p a r t i c u l a r a t t e n t i o n t o c e r t a i n p o i n t s which might i n t e r e s t t h o s e who

manufacture p l a s t e r s a s w e l l as t h o s e who u s e o r experiment w i t h h y d r a u l i c b i n d i n g a g e n t s

.

In t h i s study t h e f o l l o w i n g two alms have been pursued:

(1) T h e measurement of v a r i a t i o n s i n volume of p a s t e s of p l a s t e r and w a t e r d u r i n g and a f t e r t h e i r hardening.

( 2 ) T h e s e p a r a t e s t u d y of each of t h e v a r i o u s f a c t o r s i n o r d e r t o be a b l e t o i n f l u e n c e t h e s e v a r i a t i o n s of volume

m e t h o d i c a l l y .

T h e s e a i m s were chosen f o r t h e f o l l o w l n g r e a s o n s :

It i s w e l l knovm t h a t p l a s t e r p a s t e s undergo v a r i a t i o n s of volume d u r i n g and a f t e r hardening j u s t l i k e a l l p a s t e s made w i t h

*

Paper p r e s e n t e d a t t h e T h i r t y - f i r s t I n d u s t r i a l Chemistry Conference a t B r u s s e l s ,

15th

September 1949.

h y d r a u l i c binding a g e n t s . However, one f r e q u e n t l y h e a r s very d i f f e r e n t opinions about t h e magnitude, t h e d i r e c t i o n and t h e causes of t h e v a r i a t i o n s of volume. A s Messrs. Cocagne and Manley r e c e n t l y put it "Everybody s a y s t h a t p l a s t e r heaves,

but no one says how and t o w h a t e x t e n t

."

( 2 ) I t h e r e f o r e thought

it would be i n t e r e s t i n g t o study t h o s e phenomena which e x e r t an e f f e c t on c e r t a i n p r o p e r t i e s of p l a s t e r s c o n s t r u c t i o n s , 0.g.

t h e adherence of p l a s t e r c o a t ~ n g s t o o t h e r m a t e r i a l s , t h e

p r e s s u r e s e x e r t e d by t h e c o a t i n g s , j o i n t s and p l a s t e r products

>n door and window frames, the behaviour of s e a l i n g a g e n t s and t h e accuracy of reproduction by means of p l a s t e r moulds.

Furthermore, t h e study of t h e v a r i o u s f a c t o r s a f f e c t i n g t h e v a r i a t i o n s of volume of p l a s t e r s has caused me t o look i n t o t h e means of surmounting t h e d i f f i c u l t i e s

,

which a r e a l s o encountered

i n s t u d i e s of t h e expansion and shrinkage of o t h e r h y d r a u l i c binding a g e n t s . Many more shrinkage and expansion measurements have indeed been c a r r i e d out on c e r t a i n i n d u s t r i a l binding

a g e n t s such as t h e s i l i c e o u s cements, t h a n on p l a s t e r s , enabling b u i l d e r s t o be informed concerning t h e p r o p e r t i e s of t h e cement which t h e y employ. However, i f we compare t h e r e s u l t s obtained by v a r i o u s a u t h o r s , a s LfHermite has r e c e n t l y done, it i s very d i f f i c u l t t o d r a w e x a c t conclusions about t h e p a r t played by each of t h e v a r i o u s f a c t o r s which have a c t e d , i n g e n e r a l

simultaneously, on t h e r e s u l t s of t h e measurements. N e v e r t h e l e s s ,

it would be very u s e f u l t o know t h e e f f e c t of each of t h e

f a c t o r s , not only i n o r d e r t o be a b l e t o i n f l u e n c e t h e s w e l l i n g and shrinkage of t h e b i n d i n g a g e n t s s y s t e m a t i c a l l y during manu- f a c t u r e , but a l s o i n o r d e r t o be more a c c u r a t e l y informed

concerning t h e s t r u c t u r e of t h e bird ing a g e n t s during and a f t e r

4 4 )

t h e i r r e a c t i o n with t h e water. The r e s u l t s obtained by P e r r e during h i s r e c e n t r e s e a r c h i n t o t h e manufacture of non-shrinking

cements and expansive cements have shown t h e Importance of such s t u d i e s ,

What t h e n a r e t h e d i f f i c u l t i e s whlch must b e overcome b e f o r e t h e aims o f t h i s s t u d y can be achieved?

(1) I t i s n e c e s s a r y t o a v o i d t h e c o n f u s i o n s which f r e - q u e n t l y o c c u r between t h e d i f f e r e n t volume v a r i a t i o n s due t o

d i f f e r e n t c a u s e s , I s h a l l l a t e r r e c a l l c e r t a i n g e n e r a l con- s i d e r a t i o n s ,

( 2 ) The v a r i a t i o n s of volume r e s u l t i n g from t h e chemical h y d r a t i o n r e a c t i o n must be c a r e f u l l y s e p a r a t e d from t h o s e due t o p h y s i c a l phenomena such as v a r i a t i o n s o f t e m p e r a t u r e , d r y i n g and moistening. I t h e r e f o r e measure t h e v a r i a t i o n s accompanying h y d r a t i o n a t c o n s t a n t t e m p e r a t u r e w i t h avoidance of d r y i n g on t h e one hand, and v a r i a t i o n s o f p h y s i c a l o r i g i n on t h e o t h e r hand, u s i n g f u l l y h y d r a t e d b i n d i n g a g e n t s .

( 3 )

Volume v a r i a t i o n s o f chemical o r i g i n a r e g e n e r a l l y s t u d i e d d u r i n g o n l y a p a r t o f t h e h y d r a t i o n o f t h e b i n d i n g a g e n t s , a p a r t , moreover, which I s o n l y r a r e l y s p e c i f i e d . k t

t h e b e g i n n i n g of h y d r a t i o n t h e samples a r e t o o f r a g i l e t o p e r m i t measurement o f t h e i r v a r i a t i o n s i n volume by t h e u s u a l methods. T h i s d i f f i c u l t y i s p a r t i c u l a r l y s e r i o u s w i t h the p l a s t e r p a s t e s , which a r e g e n e r a l l y very f l u i d and which then h a r d e n very

r a p i d l y . A s a consequence I d e c i d e d :

( a ) t o c o n s t r u c t a n a p p a r a t u s f o r s t u d y i n g t h e v a r i a t i o n s of volume from t h e b e g i n n i n g of h y d r a t i o n ;

( b ) t o s t u d y t h e h y d r a t i o n of t h e p a s t e s , c a l o r i m e t r i c a l l y when t h e h y d r a t i o n i s very f a s t , o r g r a v i m e t r l c a l l y when t h e

h y d r a t i o n i s slow.

( 4 ) It 1s Impossible t o s e p s r a t e t h e e f f e c t o f each f a c t o r on s w e l l i n g and s h r i n k a g e by u s i n g , a s h a s normally been done,

i n d u s t r i a l p r o d u c t s of complex and p o o r l y d e f i n e d composition.

I t h e r e f o r e examined t h e volume v a r i a t i o n s accompanying t h e h y d r a t i o n of e a c h of t h e two b a s i c components of p l a s t e r s , and t h e n of m i x t u r e s of t h e s e two components i n well-defined

p r o p o r t i o n s . I s h a l l now t a k e up i n o r d e r : c e r t a i n g e n e r a l c o n s i d e r a t i o n s ; t h e volume v a r i a t i o n accompanying t h e h y d r a t i o n of p l a s t e r s of v a r i o u s composition (moulding p l a s t e r s , c o n s t r u c t i o n a l p l a s t e r s , p l a s t e r s c o n t a i n i n g a l u m ) ; volume v a r i a t i o n s o f p h y s i c a l o r i g i n . 11, G e n e r a l C o n s i d e r a t i o n s

During an2 a f t e r t h e i r h y d r a t i o n t h e p l a s t e r p a s t e s undergo d i f f e r e n t v a r i a t i o n s of volume due t o t h e e f f e c t s of physical

and chemical phenomena.

1, Volume V a r i a t i o n s of Chemtcal O r i ~ S n

During chemical r e a c t i o n of t h e p l a s t e r w i t h t h e mixing water t h e f o l l o w i n g occur: f i r s t , v a r i a t i o n s i n t h e a b s o l u t e volumes o f t h e s o l i d and of t h e w a t e r ; second, v a r i a t i o n s o f t h e a p p a r e n t volume of t h e compok:ltfon which becomes c o h e r e n t .

The a b s : ~ l u t e volume o f t h e s o l i d i s t h e sum of t h e volumes of a l l t h e g r a i n s c o n t a i n e d i n t h e composition i n q u e s t i o n .

employed i n t h e p r e p a r a t i o n of t h e p a s t e .

I n t h e c o u r s e of chemical r e a c t i o n of t h e p l a s t e r with t h e water t h e following occurs:

(a) A v a r i a t i o n of t h e a b s o l u t e volume of t h e s o l i d owing t o t.he f a c t t h a t t h e gypsun1 formed by h y d r a t i o n occupies a d i f f e r - e n t volume from t h a t initially occupied by t h e p l a s t e r ;

( b ) A decrease in t h e a b s o l u t e volurne of t h e water em- ployed f o r mixing owing t o t h e f a c t t h a t p a r t of t h i s water com5lnes w i t h the p l a s t e r t o form gypsum,

These v a r i a t i o n s of a b s o l u t e volume may be c a l c u l a t e d o r

measured very s i n p l y , as w i l l be i n d i c a t e d f u r t h e r on,

B. V a r i a t i o n s of Apparent Volurfi~ D u e t o C h e m L ~ g I a

cause^

The apparent volume of t h e p a s t e o r t h e hardened mass i s t h e sum of t h e a b s o l u t e s o l i d volilrne and t h e spaces between t h e s o l i d g r a i n s , l e e . , t h e pores.

Before t h e s t a r t of t h e h y d r a t i o n of t h e binding a g e n t , t h e volume of t h e pores i s equal t o t h e volume of t h e water of mix- t u r e when t h e p a s t e c o n t a i n s no a i r bubbl3s

-

as i s g e n e r a l l y t h e case with p l a s t e r s . The apparent vohlme of t h i s p a s t e i s

t h e n equal t o t h e sum of t h e a b s o l u t e volumes of t h e p l a s t e r and of t h e water of mixture.

O n h y d r a t i o n o f t h i s p a s t e a porous m R S S i s formed, t h e volume of t h e pores depending on t h e following t h r e e f a c t o r s :

(1) The q u a n t i t y of wa-ter of m i x t u r e , which a t t h e p r e s e n t time i s much g r e a t e r t h a n 1s needed f o r h y d r a t i o n of t h e p l a s t e r , T h i s g r e a t excess of w a t e r of mlxture i s t h e main reason f o r t h e

p o r o s i t y of t h e p l a s t e r products.

( 2 ) The r e d u c t i o n of t h e t o t a l a b s o l u t e volume due t o combination of t h e p l s s t e r with p a r t of t h e water mixture.

( 3 )

Expansion ~ f t h e s k e l e t o n formed by h y d r a t i o n of t h e p l a s t e r .

A f t e r h y d r a t i o n t h e pores of t h e coherent mass a r e f i l l e d

( a ) by t h e excess water of mixture which h a s not combined w i t h t h e p l . s s t e r and (b), depending on t h e c o n s e r v s t l o n c o n d i t i o n s f o r t h i s mass, on t h e a i r o r water which may have p e n e t r a t e d t h e porous mass so a s t o f i l l t h e pores r e s u l t i n g from t h e h y d r a t i o n

and from t h e expansion of t h e skeleton.

The s k e l e t o n expansion accounts only f o r a small part of t h e apparent vol.~rne of t h e compositions obtained by h y d r a t i o n of t h e p l a s t e r . It t h u s r e p r e s e n t s only 1 t o 2% of t h e pore volume of a composition obtained by h y d r a t i o n of semi-hydrat ed p l a s t e r , mixed i n a proportion of 145 gm, t o 100 gm, of water,

Although t h i s v a r i a t i o n i n t h e apparent volume I s small, it

i s n e v e r t h e l e s s of g r e a t p r s c t i c a l and s c i e n t i f i c i n t e r e s t .

I n c o n s t r u c t i o n a l work it r e s u l t s i n s t r a i n s and deformations which a r e of i n t e r e s t t o b u i l d e r s , I n a d d i t i o n , I t depends on a number of f a c t o r s which i n f l u e n c e t h e s t r u c t u r e of t h e hydrated b i n d i n g agent and which a r e of i n t e r e s t t o r e s e a r c h workers

-

e s p e c i a l l y t h e form, t h e dimensions and t h e network a n d t h e

adhesion of t h e c r y s t a l s forming t h e s k e l e t o n of t h e hardened m%ss.

I t i s t h u s necessary t o measure t h e v a r i a t i o n s of t h e

apparent volume accompanying t h e h y d r a t i o n of t h e p l a s t e r s , s i n c e t h e s e v a r i a t i o n s cannot be c a l c u l a t e d a p r i o r l .

2, V a r i 3 t l o n s of A n ? > : , n ~ ~ t V o l ~

Due t o PhyslceX Causes

-

---

The porous and c o h e r e n t cornposXtions o b t a i n e d by r e a c t i o n of t h e p l a s t e r s w i t h w a t e r a l s o uadergo, l i k e a l l composltlons formed by h y d r a t i o n of b i n d i n g a g e n t s , v a r i a t i o n s of a p p a r e n t volume due t o t h e e f f e c t s of v a r i o u s p h y s i c a l phenomena such as:

V a r i a t i o n s of t e m p e r a t u r e

-

thermal expansion; D e s i c c a t i o n

-

s h r i n k a g e due t o d r y i n g ;

R e h u ~ n i d i f i c a t i o n

-

expansion due t o moistening,

There i s a l s o sometimes a n a c c i d e n t a l d r y i n g shrinltage o f t h e non-coherent mass o b t a i n e d a f t e r

"

r e b e a t i n s ' ' of t h e p l a s t e r p a s t e , What happens, In f a c t , i s t h a t t h e p l a s t e r e r s do n o t u s e r a g i d l y enough a l l t h e p a s t e which t h e y p r e p a r e , They t h e n b e a t t h e remaining p a s t e vigorously with t h e t r o w e l a f t e r i t h a s a l - ready begun t o s e t , and o f t e n a d d w a t e r i n o r d e r t o f a c i l i t a t e t h e r e s o f t e n i n g of t h l s " r e b e a t e n " p l a s t e r m a s s , When t h i s r e - b e a t e n p l a s t e r i s employed a f t e r h y d r a t i o n i t no l o n g e r h a r d e n s , The composition remains s o f t and, when d r y i n g I n a i r , undergoes

s h r i n k a g e and c r a c k s , just l i k e a l l p a s t e s made from i n e r t

m a t e r i a l s , T h i s d r y l n g s!~rini+:age o f p a s t e s from i n e r t m a t e r i a l s

I s w e l l known, It doc2s n o t f a l l w i t h i n t h e scope of t h i s s t u d y , which i s concerned only w i t h t'ns f o r m a t i o n of c o h e r e n t masses by h y d r a t i o n of t h e p l a s t e r s ,

Varhnt1or.s i n tb.s flbso7 i.?.tr: Vol.r.?!"e of tb.8 So'.l.d and t h e LiquiA

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;I:? t h e C h e m ! c.?,?" 13R?2~JJs-~ of P I . . a s t e r 1.~1- th i d a t

e-c

These v a r i a t i o n s a r e I n t e r e s t i n g from t h e f o l l o ~ ~ i n g p o i n t s o f view: (1) f o r calcul-it4.1ag t h e volume occupied by t h e h y d r a t e d

b i n d i n g a g e n t i n t h e c o m p o s i t i o n a f t e r i t h a s become c o h e r e n t ; ( 2 ) f o r d e t s r m i n i n g t h e r a t e of r e a c t i o n o f t h e p l a s t e r w i t h t h e w a t e r . The v a r i a t i o n s i n a b s o l u t e volume a r e p r o p o r t i o n a l t o t h e amount o f h y d r a t e d p l a s t e r .

C a l . c u l a t i o n of t h e A b s o l u t e Vo3.~1me V a r i a t ioi11~_of t h e Soxid a n d t h e L i q u i d R e s u l t i n ; ? from CovpJete H y d r a t i o n o f t h e P l a s t e r s

It i s v e r y e a s y t o c a l c u l a t e t h e a b s o l u t e volume v a r i a t i o n s r e s u l t i n g from complete h y d r a t i o n o f t h e two e s s e n t i a l p l a s t e r components

-

t h e s e m i h y d r a t e and t h e i n s o l u b l e a n h y d r i t e *

.

A c t u a l l y t h e s e two h y d r a u l i c components, on r e a c t i o n w i t h the water, g i v e o n l y a s i n g l e h y d r a t e d p r o d u c t , namely gypsum, whose c o m p o s i t i o n and s p e c i f i c d e n s i t y i s v e r y w e l l known. The r a p i d t r a n s f o r m a t i o n on c o n t a c t w i t h t h e w a t e r o f t h e s e m i h y d r a t e o f s p e c i f i c d e n s i t y 2.75 i n t o gypsum o f d e n s i t y 2 . 3 2 - s u l t s i n t h e f o l l o w i n g v a r i a t i o n s : Weight S p e c i f i c d e n s i t i e s A b s o l u t e volumes CaS04 1/2H20 ( s o l i d )

+

j/2H20 ( l i q u i d ) = CaSO, 2H20 ( s o l i d ) 1 4 5 gm. 27 gma 172 gm. 74.10 c c . 74.10 CC.

*

Some p l a s t e r b s a l s o c o n t a i n s o l u b l e a n h y d r l t e . T h i s s u b s t a n c e

i s immediately tramsformed i n t o s e m i h y d r a t e when I t coRes i n t o c o n t a c t w i t h t h e w a t e r of rnixtu.re. C o n s e q u e n t l y , from t h e p o i n t o f view of t h e a b s o l u t e and a p p a r e n t volurne v a r l a . t i o n s , t h e

s o l u b l e a n h y d r i t e b e h a v e s l i k e t h e sernihydrs.tes d u r i n g t h e t r z n s - f o r m a t i o n o f t h e p a s t e i n t o a c o h e r e n t mass.

The slow h y d r a t i o n of t h e i n s o l u b l e a n h y d r i t e o f d e n s i t y 2 - 9 0 g i v e s r i s e t o t h e f o l l o w i n g v a r 1 a . t i o n s : CaSO ( s o l i d )

+

2H 0 ( l i q u i d ) = CaSO 2H 0 ( s o l i d ) 4 2 4 2 Weight

136

gm. 36 1 7 2 gm. S p e c i f i c 2 0 9 0 d e n s i t i e s A b s o l u t e 46.95 cc. volumes

.

_ _ L - 36 c c . , 82.-95 cc. 74.1 cc. 74.1 CC. E x a m i n a t i o n of t h e s e f i g u r e s shows t h a t : (1) The a b s o l u t e volume o f t h e s o l i d i s g r e a t l y i n c r e a s e d by c o m p l e t e h y d r s t i o n o f t h e p l a s t e r s . T h i s i n c r e a s e i s 40% o f t h e i n i t i a l volume w i t h t h e s e n ~ i h y d r a t e d p l a s t e r and

577:

w i t h i n s o l u b l e a n h y d r i t e . ( 2 ) D e s p i t e t h i s v e r y g r e a t i n c r e a s e i n t h e c o u r s e o f h y d r a t i o n t h e a b s o l u t e volume o f t h e s o l i d i s s l i g h t l y l e s s t h a n t h e sum o f t h e i n i t i a l volumes o f water and p l a s t e r which h a v e combined t o g i v e s o l i d gypsum. T h i s d i f f e r e n c e i s :

3.9

c c . f o r 1 0 0 gm. of s e m i h y d r a t e o f d e n s i t y 2.75;

6.5

c c . f o r 100 gm. o f i n s o l u b l e a n h y d r i t e o f d e n s i t y 2.90. T h e r e f o r e :

(a) The mass o b t a i n e d by h y d r a t i o n o f t h e p l a s t e r i s s t i l l very p o r o u s , even when t h e p l a s t e r i s mixed w i t h a q u a n t i t y of w a t e r e x a c t l y s u f f i c i e n t f o r c o m p l e t e h y d r a t i o n . T h e r e f o r e ,

when I had t r i e d p r e v i o u s l y t o prepare p l a s t e r products which would be a s compact a s p o s s i b l e ( 7 ) , I o b t a i n e d products whose d e n s i t y was no g r e a t e r t h a n 2.18, a f i g u r e c o n s i d e r a b l y lower t h a n t h e s p e c i f i c d e n s i t y 2.32 of t h e gypsum. The products were approxima.tely tell times a s s t r o n g as t h o s e o b t a i n e d by o r d i n a r y methods b u t they were s t i l l porous.

( b ) When t h e p l a s t e r i n t h e u s u a l way i s nixed with a g r e a t d e a l of water t h e formation of pores r e s u l t i n g from t h e d e c r e a s e of t h e t o t a l a b s o l u t e volume of t h e s o l i d and t h e l i q u i d i n t h e course of h y d r a t i o n i s much l e s s t h a n that r e s u l t i n g from t h e excess water of mixture. Thus, when mixing 1 0 0 gm, of semi- h y d r a t e with 60 cc. of w a t e r we g e t

3.9

cc. pore volume due t o t h e d e c r e a s e of a b s o l u t e v o l u m and 41.4 cc. of pores due t o ' t h e excess water of mixture,

( c ) By recovering t h e p l a s t e r from t h e e x c e s s water i t i s p o s s i b l e , on observing t h e v a r i a t i o n s of a b s o l u t e volume of t h e whole, e i t h e r t o determine t h e temperatures between which t h e v a r i o u s p l a s t e r components a r e s t a b l e , a s has been done by v a n l t

off'^),

o r t o observe t h e h y d r a t i o n a t c o n s t a n t temperature by Le C h a t e l i e r ' s method invented i n 1927 i n t h e course of h i s study

( 6 )

on aluminous cements

.

Determination of t h e Course of Hydration o f P l a s t e r s by Measuring t h e Decrease i n t h e T o t a l Absolute

Volumes of t h e S o l l d and of t h e L i s u i d

L e C h a t e l i e r ' s d i l a t o m e t r i c method h a s been a p p l i e d f o r t h e p a s t 20 y e a r s by numerous r e s e a r c h workers i n o r d e r t o study t h e h y d r a t i o n of t h e binding a g e n t s . T h i s method, indeed, h a s t h e advantage of being very simple. A weighed q u a n t i t y of weakened binding a g e n t i s introduced i n t o t h e water i n a t e s t tube o r i n a

small b o t t l e w i t h a graduated g l a s s t u b e a t t h e t o p , then a n e x c e s s of water i s added. It i s t h c n merely necessary t o maintain t h e

z p p a r a t u s a t a c o n s t a n t temperature and t o measure t h e f a l l i n t h e water l e v e l i n t h e graduated t u b e a s a f u n c t i o n of t h e time i n o r d e r t o determine t h e r e d u c t i o n i n t h e t o t a l a b s o l u t e

volumes of t h e s o l i d and l i q u i d r e s u l t i n g from h y d r a t i o n of t h e b l n d i n g a g e n t , Thus it i s v e r y easy t o follow t h e v a r i a t i o n s of t h e a b s o l u t e volumes i n t h e course of h y d r a t i o n of aluminous and s i l i c e o u s cements. However, it i s d i f f i c u l t t o c a l c u l a . t e t h e q u a n t i t y of hydrated cement from t h e r e s u l t s of t h e s e measure- ments. I n f a c t , t h e compositions and s p e c i f i c d e n s i t i e s of t h e products formed by h y d r a t i o n of t h e s e cements a r e n o t a c c u r a t e l y known. Furthermcre, t h e r e a r e g e n e r z l l y s e v e r a l chemf c a l r e a c t i o n s going on a t t h e same t i m e , t h u s coniplicating s t i l l f u r t h e r t h e i n t e r p r e t a t i o n of t h e r e s u l t s * .

The a p p l i c a t i o n of Le C h a t e l i e r ' s d i l a t o m e t r i c method does n o t encounter t h e same d i f f i c u l t i e s w i t h p l a s t e r a s it does w i t h t h e s i l i c e o u s and aluminous cements, The i n t e r p r e t a t i o n of t h e r e s u l t s i s much e a s i e r , b u t t h e execution of t h e measurenients i s

more d i f f i c u l t .

A s a m a t t e r of f a c t , t h e p l a s t e r p a s t e s contained not more t h a n two h y d r a u l i c components, t h e semihydrate and t h e i n s o l u b l e a n h y d r i t e , which do n o t undergo h y d r a t i o n simultaneously, The

h y d r a t i o n o f t h e semihydrate i s complete two hours a f t e r t h e mixing, whereas t h a t of t h e i n s o l u b l e a n h y d r i t e i s n e g l i g i b l e ,

*

Some a u t h o r s have s i m p l i f i e d t h i s calculatl.on by assuming t h a t t h e quc.ntity of water f l x e d by t h e 'oincling agent i s equal t o t h e rceasured volurnc decrezse, This i r \ _ t e r p r b e t s t i o n , which was

a ~ a i n 111ad.e a t t h e preceding Chenical I n d u s t r i a l Consress

,

i s CI~UIC~..C iraaccurate, The f f x a t i o n of 27 gin, of w a t e r by t h e semi- h y d l - a t e d p l a . a t e r of denslP;y 2,75 r e s u l t s i n a decrease of only

Consequently t h e observed decrease i n a b s o l u t e volume during t h i s period corresponds t o t h e h y d r a t i o n of t h e semihydrate, A f t e r t h i s a second volume decrease t a k e s p l a c e which goes on very slowly f o r weeks o r months and which corresponds t o

h y d r a t i o n of t h e i n s o l u b l e a n h y d r l t e ,

P i l a t o m e t r i c Measurement of t h e Hvdration Rate of t h e

Semi

-

h y d r a t e

Le C h a t e l i e r l s method has been modified f o r t h e f o l l o w i n g reasons 3

(1) The r a p i d i t y with which t h i s p l a s t e r s e t s l e a v e s l i t t l e

time f o r f i l l i n g t h e d i l a t o m e t e r , e l i m i n a t i n g a l l t h e a i r bubbles from t h e t h i c k p a s t e and b r i n g i n g t h e a p p a r a t u s t o a very d e f i n i t e temperature b e f o r e t h e s t a r t of h y d r a t i o n ;

( 2 ) The c o n s i d e r a b l e , r a p i d l i b e r a t i o n of h e a t due t o t h e h y d r a t i o n of t h i s p l a s t e r t r a n s f o r m s t h e d i l a t o m e t e r i n t o a thermometer and t h u s f a l s i f i e s t h e measurements;

3

The presence of c a p i l l a r y channels f i l l e d with air i n c e r t a i n semihydrate g r a i n s f a l s i f i e s t h e s p e c i f i c d e n s i t y and a b s o l u t e volume v a r i a t i o n measurements,

Thus, i n o r d e r t o follow t h e h y d r a t i o n of t h e semihydrate c o r r e c t l y by d i l a t o m e t r l c means i t i s necessary:

(1) To immerse t h e d i l a t o m e t e r i n a l a r g e q u a n t i t y of w a t e r kept a t a c o n s t a n t temperature;

( 2 ) To i n t r o d u c e only a small q u a n t i t y of semihydrated p l a s t e r i n t o t h e d i l a t o m e t e r ;

3

To measure only s m a l l d e c r e a s e s of volume.

It i s t h u s necessary t o use very f r a g i l e p r e c i s i o n a p p a r a t u s e s . Because of t h e s e m o d i f i c a t i o n s Le C h a t e l l e r r s method l o s e s i t s

advantage of s i m p l i c i t y . I n t h e p r e s e n t s t u d y , t h e r e f o r e , I

p r e f e r r e d t h e c a l o r i m e t r i c method t o t h e d i l a t o m e t r l c f o r 8tudyin.g t h e r a p i d hydraation of t h e semihydrate.

D i l a t o m e t r i c Measurement of t h e Bate of Hydration of t h e I n s o l u b l e Anhydrite

The d i l a t o m e t r i c method of L e C h a t e l i e r i s very s u i t a b l e f o r studying t h e slow h y d r a t i o n of t h e i n s o l u b l e a n h y d r i t e . T h i s ~ l b s t a n c e i s a n e s s e n t i a l component of t h e s p e c i a l p l a s t e r s which a r e burned a t high t e m p e r a t u r e s , e.g. t h e alumed p l a s t e r s a s well as t h e cements of n s t u m l a n h y d r i t e , and which we a l s o encounter i n t h e French c o n s t r u c t i o n p l a s t e r s .

I have t h e r e f o r e had t h e opportunity of applying t h i s d i - l a t o m e t r i c method on v a r i o u s occasions over t h e l a s t 20 y e a r s and have followed t h e following procedure: A t u b e graduated i n 1 0 t h ~ and 2 0 t h ~ of a cubic c e n t i m e t r e i s secured by means of a wax-covered rubber s t o p p e r i n t h e neck of a 250 cc. b o t t l e of t h i c k g l a s s . A

known q u a n t i t y of p l a s t e r (200 o r 300 gm.) i s d i l u t e d with w a t e r and t h e n poured i n t o t h e tube. Water i s t h e n added u n t i l i t s

l e v e l comes t o about t h e c e n t r e of t h e graduated tube. The a i r

bubbles a r e c a r e f u l l y e l i m i n a t e d from t h e b o t t l e . The graduated tube i s c l o s e d by means of a Bunsen valve o r by a s t o p p e r p i e r c e d by a c a p i l l a r y i n o r d e r t o avoid evaporation of t h e water. The b o t t l e i s t h e n kept a t a c o n s t a n t temperature. It Is now necessary

only t o n o t e t h e v a r i a t i o n s of volume as a f u n c t i o n of t h e time. Fig. 1 shows some of t h e r e s u l t s obtained by following t h e

h y d r a t i o n of t h e a n h y d r i t e d i l a t o m e t r i c a l l y when t h e gypsum i s

b u r n t a t 600

-

800°, a s well as t h e a n h y d r i t e contained i n a p l a s t e r obtained by burning chunks of t h e gypsum i n a n abutment f u r c a c e .

study the hydration of t h e insoluble anhydrite, However, it

has t h e disadvantage t h a t sometimes t h e container breaks, owing t o t h e expansion of t h e p l a s t e r , even when it i s made of t h i c k g l a s s , I thus preferred a gravimetric method, which I have

previously d e s c r i b e d ( 8 ) , t o study t h e hydration of insoluble an- hydrite.

To sum up, the study of t h e v a r i a t i o n s of absolute volume r e s u l t i n g from t h e chemical r e a c t i o n of t h e p l a s t e r s with t h e water has shown t h a t :

The absolute volume of t h e s o l i d increases g r e a t l y during hydration (from 40 t o 57% depending on t h e composition of the p l a s t e r s ) .

The absolute volume of t h e s o l i d formed by hydration i s

7

t o 105 l e s s than t h e t o t a l i n i t i a l volume of t h e p l a s t e r and t h e water combined.

The r a t e of hydration of t h e insoluble .anhydrite m y e a s i l y be determined by measuring t h e v a r i a t i o n s of absolute volume of

t h e t o t a l s o l i d and l i q u i d by Le C h a t e l i e r l s d i l a t o m e t r i c method.

8. Variations of A ~ ~ a r e n t Volume Accom~anvina th e C h e r n i a

Reaction of Hydration of t h e P l a s t e r s

1, Order of Mamitu&

The chemical r e a c t i o n of t h e p l a s t e r with water i s ac- companied by an increase i n t h e apparent volume of t h e mass as

i t becomes coherent. The growth of t h e gypsum c r y s t a l s due t o hydration of the p l a s t e r r e s u l t s i n an expansion of t h e skeleton

of t h e porous mass, This increase i n volume accompanying t h e hydration of t h e p l a s t e r s i s generally between 0.3 and 1.5$ of

t h e apparent volume of t h e p l a s t e r p a s t e , Accordingly, It i s

much smaller t h a n t h e a b s o l ~ x t e s o l i d volume v a r i a t i o n s s t u d i e d i n t h e preceding s e c t i o n . It modifies t h e p o r o s i t y of t h e

p l a s t e r products b u t l i t t l e , s i n c e t h e s e products a r e g e n e r a l l y very porous owing t o t h e f a c t t h a t a t t h e present t i m e p l a s t e r I s mixed with a g r e a t d e a l of water,

2, Methods Employed

It w a s necessary t o employ d i f f e r e n t methods t o s t u d y the time dependence of t h e h y d r a t i o n of t h e p l a s t e r s on t h e one hand, and of t h e apparent volume v a r i a t i o n s accompanying t h e h y d r a t i o n on t h e o t h e r ,

neasurement of t h e Rate

of

H ~ d r a t i o n of t h e P l a s t e r s

I s t u d i e d t h e h y d r a t i o n of p l a s t e r s by two methods, which I have described previously :'

(1) The r a p i d h y d r a t i o n of semihydrated p l a s t e r s by c a l o r i - metry, using J o l i b o l s and Chasseventl s thermal

p r i someter* (11).

( 2 ) Slow hydration of t h e i n s o l u b l e a n h y d r i t e , by

(8

gravimetry

.

Determiaation of t h e Awwrent Volume V a r i a t i o n s

Following t h e two methods c u r r e n t l y employed i n t h i s study

I measured t h e l e n g t h v a r i a t i o n s of samples. It i s t h e r e f o r e necessary t o multiply t h e r e s u l t s of the measurements i n d i c a t e d

f u r t h e r on by t h r e e i n o r d e r t o convert from v a r i a t i o n s of l e n g t h t o v a r i a t i o n s of apparent volume,

I n a l l c a s e s where I was a b l e t o work with samples which h a d

a l r e a d y become q u i t e s t r o n g s o t h a t t h e y could b e handled without deformation, I uoed t h e a p p a r a t u s c o n s t r u c t e d by A m s l s r f o r t h e expansion and shrinkage t e s t i n g of cements, This a p p a r a t u s i s shown i n Fig. 2.

The l e n g t h v a r i a t i o n s were measured i n t h i s a p p a r a t u s on p r i s m a t i c samples 16 x 4 x 4 cm. These samples a r e prepared i n metal moulds, each end of which p r e s e n t s a h o l e i n t h e prism a x i s . i n t o t h e s e h o l s s were i n s e r t e d small metal p l u g s , t h e rounded

ends of which a c t a s abutments f o r t h e l e n g t h measurements of t h e samples, The mould i s now f i l l e d w i t h a p a s t e of t h e binding a g e n t and t h e sample i s removed from t h e mould when it has ac-

q u i r e d s u f f i c i e n t s t r e n g t h t o be handlsd. The v a r i a t i o n s of l e n g t h a r e t h e n measured as a f u n c t i o n of t h e time, t h e sample b e i n g

placed v e r t i c a l l y i n such a way t h a t one of t h e metal plugs r e s t s on a n abutment s i t u a t e d a t t h e bottom of a metal frame. A reading cornparator graduated i n 1 0 0 t h ~ of mm, i s mounted a t t h e t o p of t h i s frame, I t s f e e l e r r e s t s on t h e second metal plug of t h e sample. A s t a n d a r d c y l i n d e r permits c o r r e c t i o n of t h e thermal expansions of t h e metal base. I used t h i s very simple and very rugged a p p a r a t u s t o determine t h e apparent volume v a r i a t ions r e s u l t i n g e i t h e r from t h e slow h y d r a t i o n of t h e i n s o l u b l e an-

h y d r i t e contained i n t h e c o n s t r u c t i o n a l p l a s t e r o r from t h e a c t i o n of p h y s i c a l phenomena on t h e completely hydrated p l a s t e r masses

-

shrinkage due t o d r y i n g , s w e l l i n g due t o remoistening and thermal expansion,

However, t h e Amsler a p p a r a t u s does not permit t h e study of t h e volume v a r i a t i o n s while t h e p a s t e of b i n d i n g agent i s s t i l l s o f t . T h i s disadvantage i s much more s e r i o u s with p l a s t e r s t h a n w i t h cements. T h i s i s because t h e p l a s t e r s a r e mixed w i t h a g r e a t

d e a l of water, e s p e c i a l l y i n France where c o n s t r u c t i o n a l p l a s t e r s a r e n o t employed i n t h e same way as t h e y a r e i n v a r i o u s f o r e i g n

c o w t r i s s . Very l i q u i d p a s t e s a r e prepared which then harden w i t h i n a f e n minutes. Thus, when t h e Amsler a p p a r a t u s o r s i m i l a r ones c u r r e n t l y b e i n g used f o r t e s t s on cement a r e employed,

measurements of l e n g t h can only be undertaken a f t e r t h e h y d r a t i o n of t h e p l a s t e r i s a l r e a d y f a r advanced.

I n o r d e r t o b e g i n t h e n,sasurements b e f o r e most of t h e p l a s t e r i s h y d r a t e d , Gibson and Johnston developed a n a p p a r a t u s c a l l e d a n "extensometer"

(9).

T h i s a p p a r a t u s i s shown i n Fig.

3.

The a p p a r a t u s comprises a h o r i z o n t a l mould 10 cm. l o n g of t r i a n g u l a r c r o s s s e c t i o n w i t h a 6 cm. b a s e and a h e i g h t of

2.5

cm, One end of t h e mould i s f i x e d while t h e o t h e r end moves with t h e l e n g t h v a r i a t i o n s of t h e sample and a c t s on t h e f e e l e r of a

micrometer d i a l . T h i s a p p a r a t u s i s used i n England f o r o f f i c i a l t e s t s on t h e expansion of p l a s t e r s ( 1 0 )

.

However, t h e use of t h i s a p p a r a t u s p r e s e n t s c e r t a i n d i f f i c u l t i e s f o r t h e e x a c t d e t e r m i n a t i o n of a p p a r e n t volume v a r i a t i o n s from t h e beginning of t h e r e a c t i o n of t h e p l a s t e r w i t h water. The c o n t r a c t i o n which o c c u r s a t t h e beginning of t h e r e a c t i o n of p l a s t e r w i t h w a t e r , and which i s sub-

s e q u e n t l y d e s c r i b e d i n t h i s paper, p a s s e s unnoticed when t h i s

a p p a r a t u s i s used, A c t u a l l y t h i s c o n t r a c t i o n r e s u l t s i n a v i r t u a l s e t t l e m e n t of t h e s t i l l deformsble p a s t e while t h e measurements a r e b e i n g made h o r i z o n t a l l y w i t h t h e extensometer. Furthermore,

a l l a p p a r a t u s e s with f e e l e r s e x e r t some p r e s s u r e on t h e f l u i d p l a s t e r p a s t e . T h e r e f o r e , t h e v a r i a t i o n of volume does n o t show u n t i l t h e h y d r a t i o n h a s a l r e a d y been going on f o r a c e r t a i n l e n g t h of t ime

,

depending on t h e i n i t la1 c o n s i s t e n c y of t h e p a s t e and on t h e p r e s s u r e e x e r t e d by t h e f e e l e r s p r i n g . F i n a l l y , d u r i n g t h e extensometer t e s t s , t h e sample, which h a s c o n s i d e r a b l e volume, h e a t s up more and more as t h e tempo of r e a c t i o n of t h e p l a s t e r

i n c r e a s e s . The thermal esipansl ons a r e then superimposed on t h e expansions due t o h y d r a t i o n .

t h e volume v a r i a t i o n s of p l a s t e r p a s t e s from t h e beginning of t h e r e a c t i o n of t h e p l a s t e r with t h e water, even while t h e p a s t e s a r e s t ill very f l u i d .

To hold t h e p a s t e I f i r s t t r i e d u s i n g v e r t i c a l c y l i n d e r s of very t h i n copper which had been greased on t h e i n s i d e and were s l i t a l o n g a g e n e t r i x . The s l i t w a s f i l l e d i n w i t h p l a s t i c o r e l a s t i c m a t e r i a l and t h e l e n g t h measurements were made a l o n g t h e c y l i n d e r a x i s . Then I t r i e d rubber c o n t a i n e r s f o r t h e p l a s t e r p a s t e ( g l o v e f i n g e r s ) which were placed i n a water d i l a t o m e t e r , I d i d n o t o b t a i n e n t i r e l y s a t i s f a c t o r y r e s u l t s i n t h i s way because of l e a k s between t h e p a s t e and t h e water and because of a i r

g e t t i n g i n and v a r i a t i o n s of temperature i n t h e d i l a t o m e t e r , I

was f i n a l l y a b l e t o avoid t h e s e d i f f i c u l t i e s by u s i n g an a p p a r a t u s c o n s t r u c t e d e s p e c i a l l y a t my r e q u e s t by M. F o u r e t i e r , T h i s

a p p a r a t u s i s shown s c h e m a t i c a l l y i n Fig. 4 and a photograph of it a p p e a r s i n Fig.

5,

The p l a s t e r p a s t e i s poured i n t o a v e r t i c a l c y l i n d e r made o f t h i n rubber 100 ma. high and 20 mm,

i n

diameter ( a b i c y c l e i m e r t u b e ) . The bottom of t h e c y l i n d e r i s closed by a rubber cork t r a v e r s e d by a g l a s s rod, upper end of which i s fused t o a g l a s s d i s c 20 am, i n diameter, while i t s lower, rounded end r e s t s

i n

a c o n i c a l h o l e , A small b r a s s d i s c i s placed a t t h e t o p of t h e p a s t e , suspended from a small p e r p e n d i c u l a r arm with an a x i s t o which a m i r r o r i s a t t a c h e d , The volume v a r i a t i o n s of t h e p a s t e d i s p l a c e t h e metal d i s c and as a consequence move t h e image of a f i x e d source of l i g h t r e f l e c t e d by t h e m i r r o r , The l e n g t h

v a r i a t i o n s of 100 mm. samples can t h u s be measured, by t h e d i s -

placements of t h e image, w i t h a n accuracy of two one-thousandths of a a m , without t h e n e c e s s i t y of e x e r t i n g any p r e s s u r e on t h e p l a s t e r p a s t e which i s c a p a b l e of deforming i t , During t h e e n t i r e experiment t h e rubber c y l i n d e r c o n t a i n i n g t h e p a s t e i s kept a t a c o n s t a n t temperature i n t h e water. I n a d d i t i o n t o t h i s t h e

apparatus i s covered i n order t o prevent evaporation of t h e water of the paste a t t h e upper p a r t of t h e cylinder,

These d i f f e r e n t methods enabled m e t o study t h e e f f e c t s of d i f f e r e n t f a c t o r s on t h e swelling accompanying t h e hydration of p l a s t e r s of d i f f e r e n t composition, e.g, moulding p l a s t e r s (semi- hydrated p l a s t e r s ) , p l a s t e r mortars, constructional p l a s t e r s and alumed p l a s t e r s .

I was g r e a t l y aided i n p a r t of t h i s work by M. Roland ~ Q c a r t , who made a g r e a t many measurements,

3,

Moulding P l a s t e r s . a s t e r s , Varlatlons

i n A ~ p a r e n t Volume accompany in^ t h e i r Hydratlon

Raw Materia&

The p l a s t e r s used i n t h e s e experiments were made from gypsum obtained In t h e P a r i s b a s i n ( t h e f i r s t composition, t h e qu8rry of ~ o r m e i l l e s - e n - ~ a r i s i s )

.

This stone, which was extracted i n such a way a s t o avoid including any a r g i l l a c e o u s t h r e a d s , comprised 94

-

+ 2% gypsum. The r e s t was calcium carbonate

(3.5

t o 4.8%) and sand (1 t o

1.5%).

This stone contains no n a t u r a l anhydrite

o r a l k a l i n e chlorides, After pulverization i t was burned e i t h e r i n a pot o r on t h e autoclave and was then crushed again t o t h e following g r a i n s i z e s :

Grain Diameter*

from 0

-

25p from

25

-

50p over

5 @ ~

Pot-burned p l a s t e r 65.5% 17.5% 17%

Autoclave-burned p l a s t e r 48.3%

32.5%

19%

These p l a s t e r s , c o n s i s t i n g mainly of semihydrated calcium sul- phate*, when mixed with water produce p a s t e s which harden quickly. Hydration i s completed i n approximately one hour. I followed

t h e v a r i a t i o n s of apparent volume of t h e s e p l a s t e r p a s t e s w i t h

t h e a i d of t h e o p t i c a l d i l a t o m e t e r a l r e a d y described and a t t h e same time I followed t h e hydration by means of t h e thermal

prisometer.

These measurements revealed t h e e f f e c t of t h e following f a c t o r s on t h e v a r i a t i o n s i n apparent volume:

(1) The presence of a f i l m of water on t h e s u r f a c e of t h e p a s t e during hydration;

( 2 ) The proportions;

3

Various a d d i t i o n s .

E f f e c t of t h e Presence of a Film of Water on t h e Surface of

t h e Paste h r i n n Hydration

I w a s s u r p r i s e d t o obtained with t h e same p l a s t e r , mixed

w i t h a f i x e d proportion of water, expansions which sometimes vary by more t h a n one t o two, depending on t h e c o n d i t i o n s of use and

conservation of t h e paste.

When t h e p a s t e i s poured j u s t b e f o r e t h e start of hydration of t h e p l a s t e r , so as t o avoid t h e formation of a f i l m of water on t h e s u r f a c e of t h e p a s t e by d e c a n t a t i o n , a c o n t r a c t i o n of t h e p a s t e , which i s s t i l l s o f t , t a k e s p l a c e during t h e f i r s t moments of

hydration. This c o n t r a c t i o n i s caused by t h e r e d u c t i o n of t h e

*

Some moulding p l a s t e r s and some agglomerate p l a s t e r s c o n t a i n a

few percent of i n s o l u b l e a n h y d r i t e i n a d d i t i o n t o t h e semi- hydrate. A s a r e s u l t t h e r a t h e r l a r g e , r a p i d expansion due t o t h e hydration of t h e semihydrate i s followed by a small -ex- pansion t a k i n g s e v e r a l days, which i s due t o t h e hydration of

t o t a l a b s o l u t e volumes of t h e s o l i d and l i q u i d a s a r e s u l t of hydrztion. The p a s t e , which i s

a t

f i r s t l i q u i d o r s o f t , s e t t l e s , I n t h e next s t a g e thickening counteracts t h e settlement. Voids form i n t h e paste which r e s u l t i n a suction. A i r then p e n e t r a t e s t h e mass t o f i l l t h e pores formed by hydration. A s a r e s u l t the c o n t r a c t i o n c o n t r i b u t e s an ever smaller share of the t o t a l ab- s o l u t e volumes of t h e s o l i d axd l i q u i d as the thickening of t h e p a s t e progresses. This i s c l e a r from a comparison of curves I and

111 In Fig, 6 , representing the v a r i a t i o n s of apparent volume and t h e reduction i n t h e t o t a l a b s o l u t e volume of t h e s o l i d s and

l i q u i d s , r e s p e c t i v e l y , i n one and t h e same p l a s t e r ,

This c o n t r a c t i o n i s followed by an expansion. The l a t t e r

begins t o show only a f t e r t h e beginning of s e t t i n g P , a s determined

w i t h t h e k n i f e according t o t h e AFNOR method, Curves I of Fig.

6 and

7

t h u s show t h a t t h e expansion begins t o appear only a t t h e following times: 20 min. a f t e r mixing a very l i q u i d p a s t e (one p a r t water t o 1.2 p a r t s p l a s t e r ) , i n which t h e beginning of s e t t i n g

P i s a t 11 min.; 18 min, a f t e r mixing a t h i c k p a s t e (one part

water t o 2.2 p a r t s p l a s t e r ) , where t h e beginning of s e t t i n g i s

11 min,

The expansion then continues u n t i l hydration of t h e semi- hydrated p l a s t e r i s complete,

I obtained very d i f f e r e n t r e s u l t s from t h e s e with p a s t e s

which were covered with a f i l m of water during t h e e n t i r e d u r a t i o n of the hydra.tion process, e i t h e r from a considerable decantation of t h e p a s t e before s e t t i n g o r from water added t o i t s s u r f a c e ,

The c o n t r a c t i o n a t t h e beginning of hydration i s then very s l i g h t , The water p e n e t r a t e s t h e p a s t e f o r t h e purpose of f i l l i n g t h e voids r e s u l t i n g from hydration, This reduces t h e settlement, Expansion then appears d e f i n i t e l y a t t h e beginning of s e t t i n g , as determined with t h e k n i f e , Thls point a t which s e t t i n g begins

corresponds t o hydrations of between

5%

f o r t h i c k p a s t e s and

1 0 $ f o r very f l u i d p a s t e s ,

Thus, when t h e p a s t e i s covered with water t h e expansion b e g i n s t o show as soon as a small q u a n t i t y of t h e p l a s t e r i s

hydrated, whereas when t h e r e i s no water on t h e s u r f a c e of t h e p a s t e t h e expansion b e g i n s only when a considerable q u a n t i t y of t h e p a s t e h a s a l r e a d y been hydrated. A s a consequence, t h e ex- pansion of a p a s t e covered w i t h water i s considerably g r e a t e r t h a n t h a t of a p a s t e n o t so covered, as w i l l be apparent from a com- p a r i s o n of curves I and I1 of Fig,

6

and 7. Thus, t h e expansion r e s u l t i n g from complete hydration of t h e semihydrated p l a s t e r is:

(1) For a very f l u i d p a s t e mixed i n t h e proportion of

120/100, 3.05 mm./metre when t h e paste i s covered with a f i l m of water during t h e e n t i r e hydration process, and 1.6 mm. w i t h p a s t e not covered by water.

( 2 ) For a t h i c k p a s t e mixed i n t h e proportion of 220/100,

4.15

mm. and 3.05 mm.

F i n a l l y , I observed c o n t r a c t i o n s and expansions between t h e

extreme values obtained i n t h e two preceding c a s e s , when the

p l a s t e r p a s t e w a s covered only by f i l m of water which w a s in- s u f f i c i e n t t o f i l l a l l t h e voids produced by hydration.

This e f f e c t on t h e v a r i a t i o n s of apparent volume accompany- i n g t h e hydration of t h e p l a s t e r s , produced by t h e presence of a

f i l m of water on t h e s u r f a c e of t h e p a s t e , may be of i n t e r e s t f o r t h e use of p l a s t e r s a s well as f o r t h e t e s t i n g of cements.

I n most moulding shops t h e p l a s t e r p a s t e i s poured only when it begins t o thicken. Thus, g e n e r a l l y speaking, no f i l m of water

i s formed on t h e s u r f a c e of t h e paste between t h e times of pour- ing and s e t t i n g , These c o n d i t i o n s a r e t h o s e of t h e f i r s t case

examined previously. F i r s t a s u c t i o n and a s l i g h t s e t t l e m e n t occur which tend t o make t h e p a s t e adhere t o t h e w a l l s of t h e mould, This i s followed by t h e expansion r e p r e s e n t e d by curves I i n Fig,

6

and

7 .

I n t h e manufacture of v a r i o u s p l a s t e r p l a t e s and s q u a r e s , very f l u i d p a s t e s a r e poured i n t o t h e mould b e f o r e t h i c k e n i n g begins, We t h e n come n e a r e r and n e a r e r t o curves I1

of Fig,

6

and

7

as t h e d e c a n t a t i o n of t h e p a s t e p r i o r t o s e t t i n g becomes more and more thorough,

It i s probable t h a t t h e presence of t h e f i l m of water on t h e s u r f a c e of t h e p a s t e must have a n e f f e c t not only on t h e con-

t r a c t i o n s and expansions of p l a s t e r s as has been determined, b u t a l s o on t h o s e of cements. Apparently a t t h e p r e s e n t time no one a t t a c h e s much importance t o t h i s e f f e c t of a f i l m of water on t h e measurements of cements, A s a consequence, very d i f f e r e n t r e s u l t s have been obtained by d i f f e r e n t a u t h o r s using pure cement p a s t e s ,

as has been shown r e c e n t l y by L1Hermite, Lucas, f o r example, ob- served no c o n t r a c t i o n s , a p p a r e n t l y owing t o t h e f a c t t h a t he used methods which d i d not permit measurements t o be made from t h e beginning of h y d r a t i o n of t h e binding a g e n t s , Dutron, on t h e o t h e r hand, employed a more r e f i n e d method t h a n Lucas and, with v a r i o u s cements mixed i n t o pure p a s t e s , observed c o n t r a c t i o n s similar t o t h o s e which I observed w i t h p l a s t e r s . It a p p e a r s , moreover, according t o t h e r e s u l t s o b t a i n e d with p l a s t e r s , t h a t t h e c o n t r a c t i o n s of cement mentioned by Dutron would have been even g r e a t e r i f t h i s a u t h o r had c a r r i e d out measurements w i t h a n a p p a r a t u s e x e r t i n g no p r e s s u r e on t h e p a s t e s n o t covered w i t h w a t e r and contained i n v e r t i c a l and e l a s t i c moulds,

E f f e c t of t h e Prowortions of P l a s t e r s and Water of Mixture on t h e Y a r i a t l o n s of A n ~ a r e n t Volume A c c o m ~ a n ~ i n ~ H v d r a t l o ~

P a s t e s were prepared c o n t a i n i n g f o r each 100 gm. of w a t e r 50 t o 180 gm. of semihydrated p l a s t e * b u r n t i n t h e pot and 180 t o

260 gm. of semihydrated p l a s t e r b u r n t i n t h e a u t o c l a v e .

During t h e h y d r a t i o n of t h e s e p l a s t e r s t h e i r maximum ex- pansions ( p a s t e s covered with w a t e r ) and t h e i r minimum expansions

( p a s t e s not covered with w a t e r ) were s t u d i e d ,

The expansions r e s u l t i n g from complete h y d r a t i o n of t h e s e p a s t e s a r e r e p r e s e n t e d i n Fig, 8. Examination of t h i s f i g u r e

shows t h e following:

(1) The expansions i n c r e a s e g r e a t l y when t h e weight r a t i o of p l a s t e r t o water of mixture i s increased. T h i s r e s u l t has

a l r e a d y been observed with a l l t h e h y d r a u l i c binding a g e n t s , and i n p a r t i c u l a r f o r p l a s t e r s by Gibson and ~ o h n s t o n ( ~ ' . The

i n c r e a s e from 1.2 t o 2.4 i n t h e weight r a t i o of p l a s t e r t o water of mixture r e s u l t s i n a n i n c r e a s e i n t h e maximum expansion from

3

t o 4.5 mm. p e r metre, and i n t h e minimum expansion from

1.3

t o

3.5

mm. p e r metre,

( 2 ) The d i f f e r e n c e between t h e maximum expansion ( p a s t e covered with w a t e r ) and t h e minimum expansion ( p a s t e n o t covered with w a t e r ) d e c r e a s e s as t h e r a t i o of p l a s t e r t o water i s i n c r e a s e d ,

i . e , , as t h i c k e r and t h i c k e r p a s t e s a r e employed.

( 3 )

When t h e p a s t e i s covered with water t h e maximum ex- pansion r e s u l t i n g from complete h y d r a t i o n of t h e p l a s t e r i s ap-

proximately p r o p o r t i o n a l t o t h e q u a n t i t y of hydrated p l a s t e r p e r u n i t volume of t h e hardened mass. T h i s expansion i s

3

mm, p e r metre f o r a p a s t e c o n t a i n i n g , a f t e r complete h y d r s t i o n , 100 gm. of gypsum p e r l i t r e , i.e., f o r a mass of d e n s i t y equal t o u n i t y a f t e r

drying. The comparison of t h e courses of expansion and h y d r s t i o n of one of t h e same p l a s t e r a g a i n l e a d s t o t h e conclusion t h a t t h e expansion i s approximately p r o p o r t i o n a l t o t h e q u a n t i t y of

gypsum formed by h y d r a t i o n of t h e p l s s t e r . Thus, Fig. 9 repre- s e n t s , s s f u n c t i o n s of t h e tirne, t h e h y d r s t i o n measured c a l o r i - m e t r i c a l l y by rneans of' t h e thermal p r i s o m a t e r , and a l s o t h e

v a r i a t i o n of apparent volume w i t h d i f f e r e n t p l a s t e r s ,

Curves 1 a r e t h o s e of a moulding p l a s t e r from t h e P a r i s a r e a , mixed i n a p r o p o r t i o n of 150/100. The measurement w a s made with t h e o p t i c a l d i l a t o a e t e r ,

Curves 2 show a n o t h e r moulding p l a s t e r from t h e P a r i s r s g i o n mixed i n a proportion o r 160/100,

Curves

3:

A J u r a moulding p l a s t e r mixed i n a p r o p o r t i o n of 160/100. The expansion i s measured by v a r i a t i o n of volume of

a plugged rubber c o n t a i n e r f i l l e d with t h e paste. The i n i t i a l c o a t r s c t i o n was a c c e n t u a t e d by t h i s a p p a r a t u s , which does n o t permit any a i r t o p e n e t r a t e i n t o t h e pores formed by hydration.

Examination of t h e curves shows t h a t from t h e moment t h e expansion begins t o show, t h e curves r e p r e s e n t i n g h y d r a t i o n and t h o s e r e p r e s e n t i n g expansion have t h e same shape.

The E f f e c t of t h e GYB-

The p l a s t e r s o f t e n c o n t a i n a small amount o f gypsum due e i t h e r t o incomplete burning o r t o a n a d d i t i o n made t o t h e b u r n t product i n o r d e r t o speed up I t s s e t t i n g , I n o r d e r t o study t h e e f f e c t of gypsum on t h e expansion of t h e p l a s t e r s I added some n a t u r a l s a c c h a r o i d a l gypsum t o semihydrate c o n t a i n i n g no gypsum

( s t a r t of s e t t i n g according t o t h e k n i f e t e s t

-

29 min,), This n a t u r a l gypsum w a s very f i n e l y ground u n t i l it no l o n g e r l e f t

any r e s i d u e on a screen containing 10,000 meshes/sq. cm, The e f f e c t of t h i s gypsum i s not merely t o s t i m u l a t e t h e s e t t i n g g r e a t l y , as i s well knorm, but a l s o t o i n c r e a s e considerably t h e expansion due t o complete h y d r a t i o n of t h e p l a s t e r , a s t h e r e s u l t s

s e t down i n Tables A and B show.

(1) The s t a r t of s e t t i n g i s d i s p l a c e d from 29 t o

5

min. and t h e expansions a r e i n c r e a s e d approxi.mately

50%.

( 2 ) The expansion i n c r e a s e i s n o t p r o p o r t i o n a l t o t h e amount o f gypsum added t o t h e p l a s t e r .

The i n c r e a s e , which i s very l a r g e a t f i r s t f o r small a d d i t i o n s , t e n d s t o r e a c h a maximum a s t h e q u a n t i t y of gypsum added i s

increased. T h i s i s probably due t o t h e f a c t t h a t t h e a c t i o n of t h e gypsrim i s l i m i t e d by t h e r a t e of d i s s o l u t i o n of t h e p l a s t e r ,

The e f f e c t of t h e gypsum on t h e expansion of t h e p l a s t e r nay be explained as f o l l o w s m When t h e p l a s t e r c o n t a i n s no gypsum t h e c r y s t a l s formed by h y d r a t i o n of t h e p l a s t e r a r e d e p o s i t e d i n t h e pores of t h e p a s t e , b u t when gypsum i s p r e s e n t c r y s t a l l i z a t i o n

s t a r t s around t h e gypsum n u c l e i , t h e f i x e d p o s i t i o n of which

i n

t h e p a s t e r e s u l t s i n an i n c r e a s e of expansion.

E f f e c t of Various Chemical Products Added t o t h e P l a s t e r

A number of p r o d u c t s , when added i n s m a l l q u a n t i t i e s t o

t h e p l a s t e r o r t o t h e w a t e r of mixture, have a v e r y decided e f f e c t on t h e expansion and s e t t i n g p r o c e s s e s ,

The experiments of t h e present a u t h o r i n t h i s regard

i n

g e n e r a l have confirmed t h e r e s u l t s p r e v i o u s l y o b t a i n e d by Johnston and Gibson, who s t u d i e d t h e e f f e c t of almost a l l t h e s o l u b l e mineral s a l t s on t h e expansion of moulding p l a s t e r s . T h e r e f o r e , I s h a l l i n d i c a t e below only t h o s e r e s u l t s which were

o b t a i n e d w i t h products used from time t o time i n t h e manufacture of s p e c i a l p l a s t e r s f o r t h e purpose of a c c e l e r a t i n g o r . r e t a r d i n g t h e i r s e t t i n g . These a r e , s u l p h a t e of potassium, borax, s i z e s , potassium c i t r a t e and sodium hexametaphosphate.

A small percentage of potassium sulphate is added from time to time to certain plasters used by dentists and surgeons.

Potassium sulphate greatly speeds up the setting and harden-

ing of the plasters, as

I

had already indicated in a study pub-

lished in 1.~26'~). This latter study was completed by carrying out expansion measurements and measurements of mechanical

strength, in addition to hydration measurements.

Similar results were obtained regardless of whether the potassium sulphate was dissolved in the water of mixture or was added in the solid state after being finely ground.

Table C shows the results obtained by adding 1 and 2% potassium sulphate to moulding plaster which had been burnt in the pot.

Examination of this table reveals the following:

(1) The addition of 2 % potassium sulphate results in: (a) displacement of the start of setting from

29 to

4.5

mln.;

(b) a 50% decrease in expansions due to complete hydration of the semihydrate plaster;

(c) about a 20% reduction in the bending strength.

(2) In manufacturing special, quick-setting plasters It is

by no means unimportant whether gypsum or potassium sulphate Is used. These two substances have opposite effects on the expansion processes. At first sight this may seem surprising, since both substances contain sulphate which activate the setting. Potassium sulphate, however, acts in the dissolved state, whereas gypsum does not. The effect of potassium sulphate on the expansion is