Drying-Hydration in Cement Pastes During Curing

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Mater ia ls and S t ructures /Mat6r iaux e t C onst ructions ,Vol . 34 , November 001, pp 557-565

D r y i n g / h y d r a t io n in c e m e n t p a s t e s d u r i n g c u r in g

D . P . B e n t z 1 , K . K . H a n s e n 2, H . D . M a d s e n 2 , F . V a i l & 3 a n d E . J . G r i e s e l 4

(1) B uilding and Fire Research Laboratory, 100 B ureau Drive Sto p 86 21, Na tiona l Institute of Standards and Technology, Gaithersburg,

M D 2 08 9 9- 8 62 1 U S A

(2) Technical University of Denm ark, Lyng by, D enm ark

(3) Centre Scientifique et Technique du Bdtimen t, Ma rne-la-Vall&, France

(4) University of Cape To wn, South Africa

Pap er received:August 21 , 2000; Paperaccepted:March 23 , 2001

A B S T R A C T R I~ S U M I~

As concre te cures in the f i e ld , the re i s a cons tant co m -

pe t i t i on f o r t he mi x i ng w a t e r be t w e e n e va po r a t i on a nd

hydr a t i on p r oc e ss es . U n de r s t a nd i ng t he me c ha n i s ms o fw a t e r m ove m e n t i n t he d r y i ng / hyd r a ti ng c e me n t pa st e is

cri t ical for des igning curing sys tems and special ized ren-

der in g m ate r ia l s , a s we l l a s for s e lec t ing repa i r mate r i a ls

a nd me t hodo l og i e s . I n t h is w or k , X - r a y a bs o rp t ion m e a -

suremen ts indica te tha t f resh cem ent pas te dr i es uni form ly

t h r oughou t i t s t h i c kne s s , a s oppos e d t o e xh i b i t i ng t he

s ha r p d r y i ng f r on t obs e r ve d f o r mos t po r ous ma t e r i a l s .

F u r t h e r m or e , i n l a ye r e d c om pos i t e c e me n t pas te s pe c i-

mens , wa te r a lways f lows f rom the coarse r -pore l ayer to

t he f i ne r one , bo t h w h e n c oa r se r po r e s a re p r oduc e d by

us i ng a n i nc r ea s e d w a t e r - t o - c e m e n t r a t io ( w /c ) a nd w he n

they a re present due to us ing a cem ent wi th a coarser pa r -

t ic le s ize dis t r ibut ion at a constant w / c . Converse ly , no

c l e a r d i f f e r e n t i a l w a t e r move me n t i s obs e r ve d be t w e e n

l aye rs o f c e me n t pas te a n d m or t a r o f t he s a me nom i na l

w/c . Based on th e resul ts of these exper iments , dry in g has

b e e n i n tr o d u ce d in to t he N I S T C E M H Y D 3 D c e m e n t

h y d r a t i o n a n d m i c r o s t r u c t u r e d e v e l o p m e n t m o d e l , b y

e mpt y i ng t he l a r ge s t w a t e r - f i l l e d po r e s p r e s e n t a t a ny

depth in the mode l spec imen a t a use>spec i f i ed (dry ing)

r at e. W i t h th is a dd i ti on , t he C E M H Y D 3D mo de l p r o -

duces result s in goo d agreem ent wi th exper im enta l obser -

v a t io n s o f b o t h t h e d r y i n g p r o f i le s a n d t h e h y d r a t i o n

kine t i cs of th in ce m ent pas te specimens.

Lors de la cure du ciment s ur chantier, la r&ction d 'hydrata-

tion est en constante cornp6tition avec I evaporatzon. La compre-

hension des m& anismes de mouvements d 'eau au se in de la p a t een cours de se 'chage/kydratation est essentie lle non seulemen t en

vue de d& erminer des m(thodes de cure adapt~es et de d6velopper

des mor tiers spe 'c iaux, rnais ~galernent en rue de s&ctionner les

bons produits e t les bonnes rn~tkodologies de r(paration. D an s ce

travail, les mesu res d 'absorption des rayons X mettent en & i-

dence que la pa te de c imen t s& he de a fon un i forme sur rou te son

6pa isseur, et non se lon un fron t marqu6 comme on peu t courarn-

me nt l 'observer dans le s ma t& iaux poreux . D e p lus , c lans le cas

de pa te de c imen t b i -coucke , le s mouvements d 'eau se on t tou-

jours de la couche contenant les pores de taiUe la plu s impor tante

vers celle contenant les pores les plus. fins, que les pores grossiers

soient d us a un rapport e /c 6.1ev6ou a une granu lorn&ie grossi&e

des part icu les de dm ent . A l ' inverse , aucun mo uvem ent d 'eau

n'est observ~ entre couches de p a t e de c imen t e t de mort ie r a rap-

port e /c constant. S ur la base de ces r&ultats, le s&hage a (t~

introduit dans le mo dule d 'kydratation et de d&eloppern ent de la

m i cr os tr uc tu re d u c i m e n t C E M H Y D 3 D d & e l op p d p a r l e

N IS T . Les pores sa tur6s d 'eau de ta i l le s le s p lus importan tes

son t a insi v id6s se lon un e c in~t ique f i x ( e par l 'op6ra teur , cec i

quelle que soit leur position dans l 'bpaisseur de l'6chantillon.

C E M H Y D 3 D g 6 n~ r e a in s i de s r & u lt at s p r & en ta n t u n e b o n n e

corr61ation avec les observations exp&imen tales, en terme de pro-

ills de s6chage et de cin~tiques d 'kydratation, r& lis&s sur des

pates de ciment de aible ~paisseur.

1 . I N T R O D U C T I O N

T o a c h i e ve op t i ma l pe r f o r ma nc e f r om c e me n t - ba s e d

ma t e r ia l s i n t he f i e l d , p r ope r c u r i ng c ond i t i ons m us t be

ma i n t a i ne d t h r o ugh ou t t he f i r s t f e w w e e ks o f t he i r li fe

[1 , 2] . Un for tuna te ly , du e to a lack of qua l i ty cont ro l in

t he f i e l d , c onc r e t e s a nd mor t a r s o f t e n e xpe r i e nc e c o n -

s i d e ra b l e d r y i n g b e f o r e t h e c e m e n t p a s t e m a t r i x h a s

un de r g on e " c ompl e te " hyd r a t i on . F o r som e ma t e r i a ls ,

s u c h a s t h e t h i n c e m e n t / p o l y m e r c o m p o s i t e m o r t a r s

u s e d a s r e nd e r i ng m a t e ri a ls , l a te r a dd i t i on o f l i qu i d w a t e r

t o t he d r i e d c ompos i t e i s no t a b l e t o " r e i n i t i a t e " t he

cem en t hyd ra t ion process [ 3] . For o thers , th i s re - in i t i a -

t i on o f hyd r a t i on c a use s s i gn i f ic a n t i n t e r na l da m a ge t o

1359-5997/01 9 ILILEM 5 5 7



M a t e r i a l s a n d S t r u c t u r e s / M a t 6 r i a u x e t C o n s tr u c t io n s , Vol. 34 , November2001

t he m i c r os t r uc t u r e a nd a l os s o f me c ha n i c a l p r ope r t ie s

[ 4 ]. B a si c unde r s t a nd i ngs o f w a t e r mo ve m e n t a nd t he

k i n e ti c s o f b o t h h y d r a t io n a n d d r y i n g i n c e m e n t - b a s e d

ma t e r ia l s ar e t hus ne e d e d t o p r od uc e s y st ems w i t h a de -

qua te pe r form ance . Thi s appli es to f reshly cast cem ent i -

t ious s y st ems , c u r i ng c o mp oun ds , a nd r e pa i r ma t e r ia l s.

I n t h i s p a p e r , e x p e r i m e n t a l a n d c o m p u t e r m o d e l i n gstudies are appl ied to e l u c i d a t i n g a n u n d e r s ta n d i n g o f t h e

b a si c m e c h a n i s m s o f w a t e r m o v e m e n t i n f r e sh c e m e n t

pas tes and m or ta rs .

F o r t he e xpe r i m e n t a l s tud ie s, u s e w a s ma de o f an X -

r a y e nv i r onm e n t a l c ha mbe r t ha t ha s r e c e n t ly be e n c on -

s t r u c te d a t t h e T e c h n i c a l U n i v e r s i t y o f D e n m a r k t o

e x a m i n e b u i l d i n g m a t er ia l s e x p o s e d t o v a r i o u s d r y i n g

e nv i r onme n t s [ 5 ] . B o t h r e l a t i ve humi d i t y a nd a i r t e m-

p e r a t u re c a n b e c o n t r o l le d w i t h i n t h e c h a m b e r , w h i c h

a l so s e r ves as a s h i e ld f r om t he X - r a y s ou r c e . I n t h i s

s t u d y , t h e X - r a y e n v i r o n m e n t a l c h a m b e r w a s u s e d t o

m o n i t o r w a t e r m o v e m e n t d u r i n g t h e d r y i n g o f sm a l l

c e m e n t pas te a nd mo r t a r s pe c i me ns . B e c a us e t he X - r a yabsorpt ion i s prop or t iona l to the d ens i ty of the m ate r ia l s

t h r oug h w h i c h t he X - r a ys a re pa ss ing , a h i gh w a t e r - t o -

ce m en t rat io (w/c) less den se pas te w il l absorb less X- rays

tha n a low er w/c pas te . Similarly, a pas te that has drie d

ou t wi l l absorb l es s X-rays than the same paste un de r i ts

i n i ti a l s a tu r a t e d c o n d i t i o n s , a l l o w i n g a r a p i d n o n -

de s t r uc t i ve qua n t i f i c a t i on o f t he m o i s t u r e d i s t r i bu t i on

w i t h i n t he d r y i ng / hyd r a t i ng s amp l e.

I n a dd i t i on t o t he e xpe r i me n t a l s t ud i e s , t he N I S T

t h r e e -d i m e n s i o n a l c e m e n t h y d r a t io n a n d m i c r o s t ru c t u r e

d e v e l o p m e n t c o m p u t e r m o d e l [6 , 7 ], C E M H Y D 3 D , h as

be e n e x t e nde d t o d i r e c tl y c ons i de r d r y i ng a nd t he c ons e -que nc e s o f e mp t y ( as oppos e d t o w a t e r - fi l le d ) po r os i t y

on mi c r os t r uc t u r e de ve l opme n t a nd hyd r a t i on k i ne t i c s .

B a s e d o n t h e e x p e r i m e n t a l o b s e r v a t i o n s , a p p r o p r i a t e

a l go r it hms w e r e de ve l ope d f o r s imu l a t i ng the d r y i ng o f

f re sh c e m e n t pa st e a nd t he e x t e n de d c ode w a s a pp l i ed t o

s i mu l a t ing s e ver al o f t he s y st ems o n w h i c h e x pe r i me n t a l

me a s u r e m e n t s w e r e m a de [ 8 , 9 ].

2 . E X P E R I M E N T A L P R O C E D U R E

D e t a il s o f t he X - r a y e nv i r onm e n t a l c ha mbe r a r e p r o -vided in Refs . [5] and [8] . The X-ray sys tem cons i s t s of

a n X - r a y s ou r c e , a de t e c t o r , a nd a pos i t i on i ng t a b l e t o

move t he s ou r c e a nd de t e c t o r r e l a t i ve t o t he s pe c i me n

be i ng e va l ua te d . T h e d e t e c t o r u s es a n N a I c r y s ta l a nd

m e a s u r e s t h e p h o t o n c o u n t f o r e a ch o f 2 56 d i s cr e t e

e n e r g y c h an n e l s. T h e e n t i re s y s te m is c o m p u t e r c o n -

t r o ll e d , s o t ha t t he u s e r m a y s e t up a g r i d o f s pe c i me n

point s to be eva lua ted a t pe r iodic in te rval s.

For th i s s tudy, three d i f fe rent cements were used: 1)

a n A S T M T yp e I / I I o r d i na r y po r tl a nd c e m e n t , i ss ued a s

C e m e n t 1 3 3 i n J u n e 1 9 99 b y th e C e m e n t a n d C o n c r e t e

R e f e r e nc e L a bo r a t o r y a t N I S T [ 10 ] , 2 ) a c oar se l ow C 3A

c o n t e n t c e m e n t g r o u n d t o a f in e n e ss o f 2 5 4 m 2 / k g(med ian par t ic l e d iam ete r of about 25 ~ tm) [11] , and 3) an

u l t r a - f i n e c e m e n t g r o u n d t o a f i n e n e ss o f 6 5 4 m 2 / k g

( me d i a n pa rt ic l e d ia me t e r o f 5 b tm) . C e m e n t 133 has a

po t e n t i al B og ue c o mpo s i t ion o f 58 .6% C 3S , 14 .8% C 2 8 ,

10 . 6% C 3A , a nd 7 . 5% C 4A F ba s e d on ox i de a na l y s i s ,

w i t h a B l a ine f ine ne s s o f a bou t 350 m 2 / kg [ 10 ] . T he l ow

C 3 A c e m e n t h a s a B o g u e c o m p o s i t i o n o f 5 9 .0 % C 3 S,

25 . 9% C 2S , 0 . 6% C 3A , a nd 14 .2% C 4A F , as de t e r m i ne d

by qua n t i t at ive m i c ros c opy , w i t h h e mi hy d r a t e a dde d a t a

ma s s f r a c t i on o f 0 . 05 . F o r t he u l t r a - f i ne c e me n t , t he

B o g u e c o m p o s i t i o n ( vi a q u a n t i t a ti v e m i c r o s c o p y ) is

73 . 5% C 3S , 16 .5% C 28 , 7 . 1% C 3A , a nd 2 . 5% C 4A F ,onc e a ga i n w i t h a 5 . 0% by ma s s he mi hyd r a t e a dd i t i on .

Smal l (50 g to 100 g) s amples of cem en t pas tes and m or-

t ar s w e r e p r e pa r e d i n g l a s s be a ke r s a nd m i xe d by h a nd

us i ng a s pa t u la fo r tw o t o t h r e e m i nu t e s . I n ge ne r a l ,

c e m e n t pas te s a nd mor t a r s o f w / c -- 0 . 3 a nd 0 . 45 w e r e

prepared .T h e f l e sh c e m e n t pa st es w e r e p l a c e d i n e i t he r s ma ll

i nve r t e d L e go 1 b l oc ks ( a c om m on c h i ld r e n 's t oy b u i l d i ng

b l o c k ) o r l a r g e r p a r a l l e l e p i p e d c u v e t t e s , w h i c h w e r e

e i the r l e f t op en o r s ea led w i th a cap . Th e L ego b locks

a nd p l e x ig la ss c uve t t e s w e r e c hos e n a s s amp l e ho l de r sdue t o : 1 ) t he i r l ow a bs o r p t i on o f X - r a ys [ 9 ] , 2 ) t he i r

inh eren t s t ackabil ity (a llows repea table p lacem ent o f the

b l oc k w i t h i n t he X - r a y c ha mbe r ) , 3 ) t he e as e w i t h w h i c h

they can be sea led by adding a cap , and 4) the ease wi th

w h i c h t h e y c a n be f i ll e d w i t h a le ve l vo l um e o f the v i s -

c o u s c e m e n t p a st e . T h e c u v e t t e s h a v e th e a d d i t io n a l

a dva n t a ge o f be ing t r a ns pa r en t , s o t ha t t he d r y i ng be ha v -

i o r m a y be d i r e c tl y in f e r r e d f r om c ha nge s in t he " b r igh t -

nes s" of the sample. Th e bas ic exp er im enta l s e tup , i llus -

t r a t e d f o r t h e c o m p o s i t e ( l a y e re d ) c e m e n t p a s te

s p e c i m e n s t o b e d e s c r i b e d b e l o w i n t h e R e s u l t s a n d

Discuss ion sec t ion , i s show n in F ig . 1 .E a c h b l oc k w a s l a be l e d a n d w e i gh e d ( t o -+ 0 .01 g )

be f o r e t he c e me n t pa st e w a s a dde d . T h e m a ss es o f t he

c e m e n t pa s t e - f il l ed b l oc ks w e r e d e t e r m i ne d i n it ia l ly a nd

p e r i o d ic a l ly th r o u g h o u t t h e e x p o s u r e p e r io d . T h e

b l oc ks w e r e l oc a t e d s e que n t i a ll y on a ho l de r ( a n i nve r t e d

Detector 1

1 .0 mmd i a m e t e rp i n h o l e

# 1 # 2 # 3 # 4

TI X - ra y

source

A i r - 230(2Cap Cap

5 0 % R H

l ' ' , i , .! i 9

/ ~ t w , ~ - ~ .. 4 . 3 w / c ~ . 4 5 w , o = 0 .3

# 1 # 2 # 3 # 4

L e g o b locks

F i g. 1 - E x p e r i m e n t a l se tup for th e f ir s t l aye r e d c e m e nt pas te s

e x p e r i m e n t . I m a g e o n t h e l e f t s h o w s a h o r i z o n t a l v i e w o f t h e

s e t u p w i t h s a m p l e s n u m b e r e d f r o m 1 t o 4 a n d i m a g e o n t h e r i g h t

i s a v e r t i c a l v i e w [ 8 ] .

(1 ) Cer ta in comm erc ia l equ ip me nt i s iden t i fied in th i s pap er to spec i fy the

exper imenta l procedure . I n no case does such iden t i f ica t ion imp ly endorsem ent

by the Nat io na l Ins t i tu te o f S tandards an d T echnology , nor does i t ind ica te

tha t the produc ts are nece ssar i ly the b e s t ava i lab le or the purpose .

5 5 8



B e n t z , H a n s e n , M a d s e n , V a l iS e , G r i e s e l

8

r

3 0 0 0

2 0 0 0

1 0 0 0

~ o l i d - w / c : : = 0 . 3~ d a s h e d w ~ c = 0 , 4 5

0 6 4 1 2 8 1 9 2 2 5 6

C h a n n e l n u m b e r

F i g . 2 - M e a s u r e d s p e c t r a f o r f r e s h c e m e n t p a s t e s [8 ] . E a c h c h a n -

n e l n u m b e r c o r re s p on d s t o a d i s c r e t e e n e r g y l e v e l .

L e go ba s e e l e me n t ) p l a c e d a t a f ixe d l oc a t i on w i t h i n t heX-r ay chambe r . Af te r spec if ic pe r iods of exposu re , the

X - r a y s y s t e m w a s u s e d t o s c a n ve r t i c a l l y ( i n 0 . 2 mm

i nc r e me n t s ) a d i st a nc e o f e i t he r 8 m m ( L egos) o r 20 m m

(cuvet tes ) a long the cent ra l y-axi s of each b lock . A f ive

s e c ond c ou n t t i me w a s u s e d at e a c h l oc a t i on t o i mpr ove

the s igna l to noi se ra t io . Assu min g a Poi s son proces s ,

t h e r e l at iv e s t a n d a r d u n c e r t a i n t y i n t h e s u m o f th e

c oun t s ob t a i ne d f o r c ha nne l s 50 t o 150 s hou l d be on t he

o r d e r o f 3 0 0 c o u n t s o r 0 . 4 % ( f or a s u m o f 7 0 , 0 0 0

coun t s , a typica l va lue for the cem en t pas te spec imens ) .

E xa m pl e s o f s pe c tr a me a s u r e d f o r t w o o f t he f r e s h

cem ent pastes a re provided in F ig . 2 . Th e smal l peak a tc ha nne l 200 i s f r om a n i n t e r na l C oba l t s ou r c e u s e d f o r

sys tem ca l ibra tion . Th e den ser w/c = 0 .3 cem ent paste is

s e e n t o a bs o r b m or e o f t he X - r a ys a s i nd i c a t e d b y i ts

lower count va lues a t a l l channe l s up to 150. Based on

t h e p a t t e r n o b s e r v e d i n F i g. 2 , t h e s u m o f c o u n t s

be t w e e n c ha nne l s 50 a nd 150 w a s s el e c te d as the de pe n -

de n t va r i a b l e t o be a na l yz e d a s be i ng r e p r e s e n t a t i ve o f

t h e d e n s i t y ( a n d w a t e r c o n t e n t ) o f th e c e m e n t p a s te .

T h i s va l ue w a s no r ma l i z e d by d i v i d i ng i t by t he r a ti o o f

t he c o un t s a c h i e ve d i n f r ee a i r a t e a c h me a s u r i ng t i me t o

t he c oun t s a c h i e ve d i n f r e e a i r f o r t he f i r s t me a s u r i ng

t i m e ( to a c c o u n t f o r i n h e r e n t v a r i a b i li ty i n t h e X - r a ys ou r c e ) . I n the g r a phs tha t fo l l ow , t he s e no r m a l i z e d

c o u n t s h a v e b e e n d i v i d e d b y o n e t h o u s a n d p r o d u c i n g

va l ue s ge ne r a l ly be t w e e n 70 a nd 140 . I n s om e c a s es , d i f -

fe rent i a l dens i ty (count s ) prof i l es have been produced by

subtract ing the 3 h absorpt ion values at each spat ial loca-

t ion f ro m a l l subseq uent readings a t the s am e loca t ion , to

h i g h l i g h t t h e c h a n g e s o c c u r r i n g a f t e r t h e i n i t i a l

s e t t ing/ se t tl ing of the c em en t pas te i s com ple te .

3 . C O M P U T E R M O D E L I N G - C E M H Y D 3 D

T o a da pt t he N I S T C E M H Y D 3 D c e m e n t h y d r at io na n d m i c r o s t r u c t u r e d e v e l o p m e n t c o m p u t e r m o d e l t o

e xa m i n i ng d r y i ng / hyd r a t i on i n f r es h c e m e n t pa st es , s e v -

e r al e n h a n c e m e n t s t o t h e m o s t r e c e n t d o c u m e n t e d v e r -

s i on o f th e c o d e [7 ] w e r e r e q u i r e d . T h e s e i n c l u d e d a

m o d i f i c a t i o n o f t h e m o d e l b o u n d a r y c o n d i t i o n s , a n

e x t e n s i o n o f t h e m o d e l p h y s ic a l d i m e n s i o n s , a n d t h e

a dd i t i on o f s ub r ou t i ne s t o i m p l e m e n t t he a c t ual c r e a ti on

o f e mp t y po r os i t y due t o t he d r y i ng .

T h e p r e vi o u s v er si o ns o f C E M H Y D 3 D t yp ic a ll y

s imula te the hy dra t ion o f a 100 p ixe l x 100 p ixe l x 100

p i xe l c ub i c vo l ume w i t h pe r i od i c bounda r i e s a l ong a l l

f a ce s o f t h e v o l u m e [ 7] . T o e x a m i n e d r y i n g i n t h i n

c e me n t pa s t e s , t he pe r i od i c bounda r y c ond i t i ons a t t he

t o p a n d b o t t o m s ur fa ce s a re r e m o v e d a n d m o v e m e n t o f

m od e l d i f fus ing spec ies ac ros s these faces is prohib i t ed .

F u r t h e r m o r e , w h e n g e n e r a t i n g t h e s t a rt in g 3 - D

mi c r os t r uc t u r e , du r i ng pa r t i c l e p l a c e me n t , no pa r t i c l e s

a r e a l l o w e d t o p r o t r u d e a c ro s s th e t o p a n d b o t t o m

hydr a t i on vo l um e s ur fa c es . T o s i mu l a t e a pp r ox i ma t e l y

t he c om pl e t e t h i c kne ss o f t he s pe c i me ns s t ud ie d e xpe r i -

me n t a l l y , t he mode l d i me ns i ons a r e e x t e nde d t o s t udy

micros t ruc tures e i the r 1000 p ixe l s or 4000 p ixe l s th ick .

W i t h e a c h p i xe l 1 t i m i n d i me ns i on , t he s e mode l s s y s -t e ms ar e t hus e i the r i m m o r 4 m m t h i c k .

O r i g i na ll y , i t h a d be e n p l a nne d t o s i mu l a t e the d r y -

ing proces s of a hydra t ing c em ent pas te b y " in truding" a

d r y i ng f r on t f r om t he t op e xpos e d s u rf ac e a t a u s e > s pe c -

i f i ed ra te . Ho we ver , the in i ti a l X-r ay absorpt ion result s

( to be p r e s e n t e d be l ow ) i nd i c a t e d t ha t , i n s te a d o f p r o -

c e e d i ng a s a s ha r p i n t r ud i ng f r on t , t he d r y i ng a c t ua l l y

o c c u r s re l a ti v e ly u n i f o r m l y t h r o u g h o u t t h e s p e c i m e n

thickness . I t appears tha t the l a rgest pores every wh ere

e m p t y f i rs t , f o l lo w e d b y t h e n e x t l a r g e st , et c . T h i s

be ha v i o r c a n be c onve n i e n t l y m ode l e d i n t he s a me w a y

tha t s e l f -des icca t ion due to chemica l shr inkage had pre -v io u sl y b e e n im p l e m e n t e d in t h e C E M H Y D 3 D m o d e l

[6 , 12 , 1 3] . To as ses s the s ize of the "pores" in the 3-D

micros t ruc ture , a d ig i t i zed spher ica l t empla te (d iamete r

= 13 p ixe l s ) i s cente red a t each p ixe l , and the number of

und er ly ing p ixe ls as s igned to b e wate r - f i l l ed poros i ty o r

p r e v i ous l y e mp t i e d po r os i ty is de t e r mi ne d . T he s e va l ue s

a re then sor t ed f rom la rges t to smal les t and the l a rges t

po r e s e mp t i e d f i r s t a t a r a t e de t e r mi ne d by a u s e r - s up -

pl i ed dry in g ( ra te) da taf il e. W hi le th i s a lgor i thm i s re l a -

t i ve l y s l ow i n i mp l e me n t a t i on f o r a 100 x 100 x 4000

mic ros t ruc tu re , i t cou ld eas ily be para l l e l ized to obta in a

f as te r tu r na r oun d t i me on a mu l t i - p r oc e s s o r c ompu t e r .

4 . R ES UL TS A N D D I S C U S S I O N

I n t he f ir s t e xpe r i me n t , c e m e n t pas te s o f w / c = 0 . 3

a nd 0 . 45 w e r e p r e pa r e d a nd i m me d i a t e l y p l a c e d in t he i r

b l oc k mo l ds . W hi l e t he s e b loc ks w e r e e xpos e d t o a va r i -

e t y o f d r y i ng c o nd i t i ons [ 8 ], he r e w e s hal l f oc us on t he

s et o f b loc ks t ha t w a s i mme d i a t e l y e xpos e d t o t he c ha m -

b e r e n v i r o n m e n t o f 5 0 % R H a n d 2 3 ~ F o r t h es e s p e c-

i m e n s , X - r a y m e a s u r e m e n t s w e r e p e r f o r m e d o v e r a

p e r i o d o f 7 d .

Af te r th e in i t i a l s e t tl ing ( set ting) o f the c em en t paste ,surpr is ingly , the d ry ing prof i l es a re observed to ch ange

i n a r e la t ive l y un i f o r m m a nn e r t h r o ugh ou t t he t h i c kne ss

(depth) of the spec imen , as sho wn in F ig . 3. Thi s i s in

5 5 9



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s p e c i m e n s . B o t t o m o f c e m e n t p a s t e s p e c i m e n i s a t p o s i t i o n 28 .5

m m ( i n s y s t e m c o o r d i n a t e s ) a n d t o p e x p o s e d s u r f a c e is a t p o s i t i o n

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" [ ~ i :: d - 0. i5 o vi r 0 .3 :: open: : :: ::o 0 . 6 . . . . . ~ , \ . . . . . . . ! . . . . . . . ~ = 0 . 3 i / ~ e r ~ 0 . 4 5 ! / : / i S e h l . . . . . . . . i . . . . . . . . : " . . . . . . .

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F i g . 5 - R e l a t i v e w a t e r c o n t e n t v s . t i m e f o r e x p o s e d l a y e r e d

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F i g . 4 - D i f f e r e n t i a l d e n s i t y p r o f il e s ( r e l a t iv e t o 3 h ) f o r l a y e r e d

c e m e n t p a s te ( 0.3 o v e r 0 . 4 5) o p e n t o t h e c h a m b e r e n v i r o n m e n t .

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Fig . 6 - D if fe ren t ia l dens i ty p rof i les ( re la t ive to 3 h ) fo r layere d

cem en t pas te (0.45 over 0 .3) sea led w i th a cap .

c o n t r a s t t o t h e i n w a r d p r o g r e s s i o n o f a r e la t i v e ly s h a r p

d r y i n g f r o n t o f t e n o b s e r v e d i n p o r o u s m a t e r i a ls . W h i l e

t h e s e c e m e n t p as t e sp e c i m e n s a re o n l y 4 m m t o 5 m m

t h i c k , s i m i l a r r e s u l t s h a v e r e c e n t l y b e e n o b t a i n e d f o r

c e m e n t - b a s e d s y st e m s 50 m m t h i c k u s in g m a g n e t i c r e so -n a n c e i m a g i n g [ 1 4 ] . F o r h a r d e n e d c o n c r e t e , S e l i h e t a l .

[ 1 5 ] h a v e o b s e r v e d r e l a t i v e l y u n i f o r m d r y i n g f o r a f e w

d a ys , f o ll o w e d b y t h e d e v e l o p m e n t o f a s ig n i f i c a n t m o i s -

t u r e g r a d i e n t o r i g i n a t i n g a t t h e d r y i n g su r f ac e . I n t h e

f r e s h c e m e n t p a s t e , t h e c o m b i n a t i o n o f a r e la t i ve l y w i d e

p o r e s i z e d i s t r i b u t i o n a n d a h i g h p e r m e a b i l i t y a l l o w s a

r a p i d r e a r r a n g e m e n t o f i n t e r n a l w a t e r d u e t o c a p i l la r y

f o rc e s , so t h a t t h e l a rg e s t p o r e s t h r o u g h o u t t h e s p e c i m e n

t h i c k n e s s e m p t y f ir s t . T h i s r e su l t s i n a f a i rl y u n i f o r m

d r y i n g t h r o u g h o u t t h e s p e c i m e n t h i c k n es s . T h e s ca le

o v e r w h i c h t h i s m e c h a n i s m o p e r a t e s i n f r e s h c o n c r e t e i s

y e t t o b e d e t e r m i n e d , b u t t h e r e s u lt s o f C o u s s o t [ 14 ]

su g g es t th a t i t i s a t l eas t 5 0 r am, s imi la r to th e d ep th o f

t h e s t e e l r e i n f o r c e m e n t i n e x p o s e d c o n c r e t e m e m b e r s .

I n F i g . 3 , i t c a n a ls o b e n o t e d t h a t w h i l e t h e r e i s a 1 m m

d i a m e t e r p i n h o l e ( c o l l im a t o r ) i n f r o n t o f t h e X - r a y

d e t e c t o r , t h e s pa ti al r e s o l u t i o n o f t h e s y s t e m i n p r a c t i c e i s

s i g n i fi c a n t ly h i g h e r t h a n t h i s a s t h e s h a r p i n t e r fa c e s p r e -

s e n t a t t h e t o p s a n d b o t t o m s o f t h e s p e c i m e n s s e e m t o b e

r e s ol v a b le t o t h e 0 . 2 m m s p a c i n g u s e d i n s a m p l i n g .

E v e n m o r e i n t e r e s t i n g a r e t h e r e s u l t s f o r l a y e r e d

c o m p o s i t e s p e c i m e n s . F o r e x a m p l e , F i g .4 s h o w s t h e d i f -

f e r en t ia l d en s i ty p roEf les fo r a lay ered co m p o s i te co n s is t -

i n g o f a l a y e r o f w / c = 0 . 3 c e m e n t p a s t e o v e r a l a y er o f

w / c = 0 . 4 5 c e m e n t p a s te . E v e n t h o u g h i t is t h e 0 . 3

c e m e n t p a s te t h a t i s e x p o s e d t o t h e d r y i n g e n v i r o n m e n t ,

t h e 0 . 4 5 p a s t e " d r i e s o u t " f i r s t ( b e t w e e n 3 h a n d 7 h ) .

O n l y a t l a t e r t i m e s , a f t e r t h e 0 . 4 5 p a s t e h a s n e a r l y

r e a c h e d c o m p l e t e d r y i n g , d o e s t h e 0 . 3 p a s t e e x p e r i e n c e

s ig n i f ican t wa te r lo ss . T h e cap i l la ry fo r ces p r ese n t in th e

f r e s h c e m e n t p a s t e ar e e a s il y a b l e t o r e d i s t r i b u t e t h e

w a t e r f r o m t h e l o w e r 0 . 4 5 p a s t e l a y e r t o t h e 0 . 3 p a s t e

lay er r es t in g o n to p o f i t . T h e r esu l t s in F ig . 4 a r e co n s is -

t e n t w i t h t h e m a s s lo ss m e a s u r e m e n t s s h o w n i n F i g . 5 ,w i t h a la r ge a m o u n t o f w a t e r l os s o c c u r r i n g d u r i n g t h e

f i r s t 1 0 h to 2 0 h o f ex p o su re , an d v e ry l i t tl e th e r eaf te r .

T h i s r e a r r a n g e m e n t o f c a p i ll a ry w a t e r is e v e n

5 6 0



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F i g . 7 - D i f f e r e n t i a l d e n s i t y p r o f i l e s ( r e l a t iv e t o 3 h ) f o r l a y e r e d

w / c = 0 . 45 c e m e n t p a s t e (2 5 g m o v e r 5 Ix m ) o p e n t o t h e c h a m b e r

e n v i r o n m e n t . I n t h i s ca s e , t h e 2 5 ~ tm c e m e n t p a s t e i s d i r e c t l y

e x p o s e d t o t h e e n v i r o n m e n t , w i t h i ts t o p s u r fa c e a t p o s i t io n 4 1 m m .

11 0 | , ! , ! ' ! , ! , ! , ! , ! , |

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F i g . 8 - D i f f e r e n t i a l d e n s i t y p r o f i l e s ( r e l a ti v e t o 3 h ) f o r l a y e r e d

w / c = 0 .4 5 c e m e n t p a s t e ( 5 p m o v e r 2 5 p .m ) o p e n t o t h e c h a m b e r

e n v i r o n m e n t . I n t h is c a s e, t h e 5 b t m c e m e n t p a s t e i s d i r e c t l y e x p o s e d

t o t h e e n v i r o n m e n t , w i t h i ts t o p s u r fa c e a t p o s i t io n 4 1 m m .

o b s e r v e d i n a c a p p e d s p e c i m e n w h i c h l o s e s v e r y l i t t l e

c a p i ll a ry w a t e r ( o n l y a b o u t 1 0 % o v e r t h e c o u r s e o f 8 0 h

- - - s ee F ig . 5 ) , a s sh o wn in F ig . 6. He re , th e m o re d e n se

p a s te a t t h e b o t t o m o f t h e s p e c i m e n i s se e n t o f u r t h e ri n c r e a s e i n d e n s i t y ( n e g a t i v e v a l u e s o n t h e d i f f e r e n t i a l

d e n s i t y p l ot ) a t t h e e x p e n s e o f t h e l es s d e n s e p a s te i n t h e

t o p h a l f o f t h e s p e c i m e n . I n t h i s c a s e , a s u b s t a n t i a l

m o v e m e n t o f w a t e r i s o b s e r v e d to o c c u r b e t w e e n 3 4 h

a n d 4 6 h a s t h e f i n e r p o r e s t r u c t u r e o f t h e w / c = 0 .3 p as te

i m b i b e s w a t e r f r o m t h e w / c = 0 . 4 5 p a s t e t o r e p l a c e t h a t

" lo st " d u e t o c h e m i c a l s h r i n k a g e / h y d r a t i o n .

D i f f e r e n c e s i n t h e i n i ti a l p o r e s iz e d i s t r i b u t i o n o f t h e

f r e s h c e m e n t p a s t e c a n a l s o b e p r o d u c e d a t a c o n s t a n t

w / c b y u s i n g c e m e n t s g r o u n d t o d i f f e r e n t f i n e n e s s e s

( P S D s ) . I n g e n e r a l , c oa r s e r c e m e n t p a r ti c l e s i m p l y

c o a r s e r " p o re s " b e t w e e n t h e m [ 1 1 ] . F i gs . 7 a n d 8 s h o w

t h e r e s u l ts o b t a i n e d f o r b o t h c o n f i g u r a t i o n s o f w / c = 0 . 4 5

l a y e r ed c o m p o s i t e s o f t h e " 5 B m " a n d " 25 b t m " c e m e n t s .

R e g a r d l e ss o f w h i c h p a st e is e x p o s e d t o t h e c h a m b e r

e n v i r o n m e n t , t h e 2 5 } . tm c e m e n t p a s t e i s a lw a y s s e e n t o

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A - i6 0 h i :: :: ! ~ :: i i ::

i i i i i i i i ii :: p a s t e : i :: m o t { a t

, ~ , ~ i , ~ i I ~ , 1 I2 9 3 0 3 1 3 2 3 a 3 4 3 5 3 6 3 ? 3 8 3


F i g . 9 - D i f f e r e n t i a l d e n s i t y p r o f i l e s ( r e l a t i v e t o 3 h ) f o r l a y e r e d

w / c = 0 .4 5 m o r t a r o v e r c e m e n t p a s t e o p e n t o t h e c h a m b e r e n v i -

r o n m e n t . I n th i s c a s e, t h e m o r t a r i s d i r e c t l y e x p o s e d t o t h e e n v i -

r o n m e n t , w i t h i ts to p s u r f a c e at p o s i ti o n 3 8 .5 m m .

d r y o u t f i r s t (3 h t o 7 h ) , w h i l e t h e d i f fe r e n t ia l c o u n t s f o r

t h e 5 l x m c e m e n t p a s t e r e m a i n n e a r z e r o ( i n d i c a t i n g n o

lo ss o f wate r ) . O n c e ag a in , th e cap i l la ry fo r ces d r aw th e

w a t e r f r o m t h e c o a r s e r p o r e s to t h e f i n e r o n e s . W i t h i n

each lay er , h o wev er , th e d ry in g i s s t i l l o b serv ed to o ccu r

u n i f o r m l y , w i t h n o i n d i c a t i o n o f a s h a rp d r y i n g f r o n t.

S i m i l a r e x p e r i m e n t a l o b s e r v a t i o n s u s i n g m a g n e t i c r e s o -

n a n c e i m a g i n g h av e b e e n m a d e b y C o u s s o t et al. [1 6 ] fo r

c o m p o s i t e s c o n s i s t i n g o f la y er s o f p a c k e d g l as s b e a d s o f

d i f f e r e n t s iz e s, o n c e a g a i n w i t h e q u i v a l e n t p o r o s i t i e s i n

eac h layer.

C o m p o s i t e s p e c i m e n s c o n s i s t in g o f a la y e r o f c e m e n t

p a s te o v e r a l a y e r o f m o r t a r a n d v i c e v e rs a ( a t a c o n s t a n tw/c) wer e al so in v es t ig a ted . I n th ese cases , th e d ry i n g

w a s r e la t i v e ly u n i f o r m t h r o u g h o u t b o t h l a ye rs s i m u l t a -

n e o u s l y w i t h n o c l e a r i n d i c a t i o n o f d i f f e r e n ti a l w a t e r

m o v e m e n t b e t w e e n t h e l ay e rs , as s h o w n i n F i g . 9 f o r a

w / c = 0 .4 5 c o m p o s i t e . I n t h i s ca s e, t h e c o a r s e r p o re s t h a t

m o s t l i k e l y e xi s t i n t h e i n t e rf a c i a l t r a n s i ti o n z o n e s ( I T Z s )

s u r r o u n d i n g t h e s a n d p a rt i c le s i n t h e m o r t a r a r e b a l a n c e d

b y t h e f i n e r p o r e s t r u c t u re o f th e b u l k c e m e n t p a s te i n

t h e m o r t a r [ 17 ]. I f t h e w / c i s i n c r e a s e d i n t h e I T Z s i n

t h e m o r t a r , i t m u s t a l s o b e d e c r e a s e d i n t h e b u l k p a s t e

r e l a ti v e t o t h e n o m i n a l w / c [ 1 7 ]. T h u s , m o s t l i k el y ,

w a t e r m o v e s lo c a l ly f r o m t h e I T Z t o t h e b u l k p a s ter e g io n s w i t h i n t h e m o r t a r ( w h i c h w o u l d n o t b e

d e t e c t a b l e u s i n g t h e X - r a y a b s o r p t i o n m e a s u r e m e n t s ) ,

w i t h l it tl e d e te c ta b le b u l k m o v e m e n t f r o m t h e c e m e n t

p a s t e t o t h e m o r t a r o r v i c e v e r sa . I t s h o u l d b e n o t e d ,

h o w e v e r , t h a t t h e o b s e r v e d X - r a y a b s o r p t i o n r e s u lt s

w o u l d a ls o be c o n s i st e n t w i t h t h e m o r t a r b e i n g a s i m p l e

t w o - p h a s e ( p as te a n d s a n d) c o m p o s i t e w i t h n o d i s t i n -

g u i s h a b le I T Z r e g i o ns . I n t h is c as e , t h e c e m e n t p a s te

w o u l d b e n o m i n a l l y id e n t i c a l i n t h e p a s te a n d m o r t a r

s p e c i m e n s a n d n o d i f f e re n t ia l w a t e r m o v e m e n t d u e t o

c a p i ll a ry f o r ce s w o u l d b e e x p e c t e d .

Sev era l o f th e sy stems s tu d ied ex p er imen ta l ly we re a l so

s i m u l a te d u si n g t h e n e w v e r s io n o f t h e C E M H Y D 3 D

m o d el . Fo r th ese s imu la t io n s , th e d ry in g r a tes we re ch o -

s e n t o a p p r o x i m a t e l y m a t c h t h o s e o b s e r v e d e x p e r i m e n t a l l y

(F ig . 5 ). Af te r each t im e o f h y d ra t io n , th e r esu lt s were sp a-

5 6 1



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. . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . .

2 0 0 0 3 0 0 0 4000

P o s i t i o n ( ~ m )

Fig. 10 - S i m u l a t i o n r e s u l t s f o r " d r ie d" p o r o s i t y f r a c t i o n v s . d e p t h

f o r l a y e r e d c o m p o s i t e s p e c i m e n . P o s i t i o n s 0 t o 2 00 0 c o r r e s p o n d

t o t h e w / c = 0 .4 5 c e m e n t p a s t e l a y e r w h i l e p o s i t i o n s 2 0 00 t o 4 0 00

a r e t h e w / c = 0. 3 p a s te l a y e r t h a t i s d i r e c t l y e x p o s e d t o t h e d r y i n g

e n v i r o n m e n t .

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w/c=o 45

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P o s i t i o n ( ~ m )

Fig. 12 - S i m u l a t i o n r e s u l ts f o r d e g r e e o f h y d r a t i o n v s . d e p t h f o r

l a y e r e d c o m p o s i t e s p e c i m e n a f t er 7 0 h o f c u r i n g . P o s i t i o n s 0 t o

2 0 00 c o r r e s p o n d t o t h e w / c = 0. 45 c e m e n t p a s t e l a y e r w h i l e p o s i -

t ions 2000 to 4000 a re the w /c = 0 .3 pas te layer tha t i s d i rec t ly

e x p o se d t o t h e d r y i n g e n v i r o n m e n t .

o

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0 .6

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1 0 0 0 2000 3000 4 0 0 0

P o s i t i o n ( ~ m )

F i g . 1 1 - S i m u l a t i o n r e s u lt s f o r p o r o s i t y f r a c t i o n s v s . d e p t h f o r

l a y e r e d c o m p o s i t e s p e c i m e n a f te r 70 h o f c u r i n g . P o s i t i o n s 0 t o

2 0 00 c o r r e s p o n d t o t h e w / c = 0 .4 5 c e m e n t p a s t e l a y e r w h i l e p o s i -

t ions 2000 to 4000 a re the w /c = 0 .3 pas te layer tha t i s d i rec t ly

e x p o s e d t o t h e d r y i n g e n v i r o n m e n t . " I n i t ia l w a t e r - f il l e d " i n d i -

c a t e s t h e i n i t i a l d i s t r i b u t i o n o f c a p i l l a r y p o r o s i t y i n t h e f r e s h ( 0 h )

c e m e n t p a s t e . " W a t e r - f i ll e d " i n d i c a t e s t h e r e m a i n i n g w a t e r -f i U e d

p o r o s i t y ( a f t e r70

h o f c u r i n g ) . " D ri ed " i n d i c a te s e m p t y p o r o s i t ycrea ted du e to d ry ing , wh i le "Se l f-des icca ted" ind ica tes em pty

p o r o s i t y c r e a t e d d u e t o s e l f - d e s i c c a t io n a n d c h e m i c a l s h r i n k a g e

d u r i n g h y d r a t i o n .

d a l ly av erag ed o v er each co n se cu t iv e 2 0 0 -p ix e l th ick lay er

to o b ta in d a ta w i th a sp a t ia l r eso lu t io n th e same as th a t o f

th e ex p er im en ta l d a ta, n am ely 0 .2 r am. F ig s. 10 an d 1 1

s h o w s i m u l a t i o n r es u lt s f o r p o r o s i t y w i t h i n a 4 m m t h i c k

c o m p o s i t e s p e c i m e n c o n s i s t i n g o f a 2 n a n : t h i c k l ay e r o f

w/c = 0 . 4 5 c e m e n t p a st e c o v e r e d b y a 2 m m t h i c k la y er o f

w/c = 0 .3 cem en t p as te. Th e s im i la r ity b e tw een th e r esul ts

i n F i g . 1 0 a n d t h e i r e x p e r i m e n t a l c o u n t e r p a r t s h o w n i n

F ig . 4 can b e c lea rly o b serv ed . Becau se th e f r esh cem en t

p as te h as a h ig h p erm eab i l i ty an d th e cap il la ry fo r ces a r e

r e l a ti v e ly l a rg e ( l e a d i n g t o a r a p i d r e a r r a n g e m e n t o f t h e

wate r ) , we can su ccess fu l ly mo d e l th e d ry in g p ro cess b y

s i m p l y e m p t y i n g t h e l a r g e s t p o r e s i n t h e m i c r o s t r u c t u r e

f i rs t w i t h o u t d i r e ct c o n s i d e r a t i o n o f t h e k i n e t ic s o f t h e

w a t e r m o v e m e n t .

F o r t h e s i m u l a t e d s y s t e m , l it t le d r y i n g i s o b s e r v e d t o

o c c u r w i t h i n t h e w / c = 0 . 3 c e m e n t p a s t e l a y e r f o r t h e

f i rs t 7 h o f e x p o s u r e , as in s t e a d , w a t e r i s d r a w n f r o m t h e

w / c = 0 . 4 5 c e m e n t p a s te la y e r u n d e r n e a t h i t . A f t e r 7 0 h ,

a s s h o w n i n F i g . 1 1 , w h i l e m u c h m o r e w a t e r h a s d r i e d

o u t f r o m t h e w / c = 0 . 4 5 p a s t e l ay e r, t h e e m p t y p o r o s i t y

d u e t o s e l f - d e s i c c a t i o n i s m u c h l e s s a n d q u i t e s i m i l a rw i t h i n t h e t w o la ye rs . W i t h t h e C E M H Y D 3 D m o d e l ,

i n f o r m a t i o n o n t h e d e g r e e o f h y d ra t i o n o f th e c e m e n t

p as te i s a l so r ead i ly av a ilable . F ig . 1 2 sh o ws th e d e g ree o f

h y d r a t i o n o b t a i n e d a f t e r 7 2 h o f h y d r a t i o n a s a f u n c t i o n

o f d e p t h . B e c a u s e i t r e t a in s w a t e r - f i l l e d p o r o s i t y f o r a

l o n g e r t i m e p e r i o d , t h e w / c = 0 . 3 c e m e n t p a s t e l a y e r i s

o b s e r v e d t o a c h i e v e a s u b s t a n t i a l l y h i g h e r d e g r e e o f

h y d r a t i o n t h a n t h e w / c = 0 . 4 5 c e m e n t p as t e. R e s u l t s f o r

e a r li e r h y d r a t i o n t i m e s s u c h a s 3 h ( n o t s h o w n ) i n d i c a t e a

m u c h m o r e u n i f o r m d e g r e e o f h y d r a t i o n w i t h d e p t h , as

t h e i n i t i a l d r y i n g a n d e m p t y i n g o f th e l a rg e s t p o r e s h a s

m i n i m a l e f fe c t s o n t h e i n i t i a l h y d r a t i o n r a t e [ 12 , 1 3 ] .T h e c o m p u t e r s im u l a t i o n s w e r e a l so a p p l ie d t o s i m u -

l a t in g t h e d r y i n g / h y d r a t i o n b e h a v i o r o f t h e s y st em s w i t h

d i f f e r en t PSD s . Re su l t s f o r d ry in g p ro f i les a r e p ro v id ed in

F i g s . 1 3 a n d 1 4 , w h i l e t h o s e f o r s i m u l a t e d d e g r e e o f

h y d r a t i o n a re g i v e n i n F i g . 1 5. T h e s i m u l a t e d d r y i n g p r o -

f il es c l o s e ly m i m i c t h e i r e x p e r i m e n t a l c o u n t e r p a r t s h o w n

in F ig . 8 . O n c e ag a in , wa te r i s s een to b e r em o v e d p re f e r -

e n t ia l ly d u r i n g d r y i n g f r o m t h e c o a rs e r p o r e s iz e d i s t r ib u -

t io n m icro s t ru c tu r e [ I n th i s case , th e d i f f e r en ces in d eg ree

o f h y d r a t i o n b e t w e e n t h e t w o l a ye rs ( F i g . 1 5 ) a r e e v e n

m o r e s t r i k i n g a s t h e 5 g m c e m e n t a c hi ev e s a m u c h h i g h e r

d e g re e o f h y d r a t io n t h a n t h e 2 5 ~ m c e m e n t , d u e t o b o t h

i t s a b i l i t y t o r e t a i n w a t e r a n d t o i t s i n h e r e n t l y h i g h e r

h y d ra t io n r a te (b ecau se o f i ts smal ler s ize cem en t p ar tic les

a n d h i g h e r c o n t e n ts o f C 3S a n d C 3 A .

W h i l e t h e m o d e l a n d e x p e r i m e n t a l r e s u l t s c o m p a r e

5 6 2



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1 0 0 0 3 0 0 0 4 0 0 00 0 0

P o s i t i o n ( / z m )

Fig. 13 - S i m u l a t i o n r e s u l t s f o r " d ri ed " p o r o s i t y f r a c t i o n v s . d e p t h

f o r l a y e r e d c o m p o s i t e s p e c i m e n . P o s i t i o n s 0 t o 2 0 00 c o r r e s p o n d

t o t h e c o a r s e r 2 5 t i m c e m e n t p a s t e l a y e r w h i l e p o s i t i o n s 20 0 0 t o

4 0 00 a r e t h e f i n e r 5 p m p a s t e l ay e r t h a t i s d i r e c t l y e x p o s e d t o t h e

d r y i n g e n v i r o n m e n t .

0 , 6 i

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. . . . . . . . . . . . . . . . . . . r . . . . . . . . . . . . . . . . . . . . . < r . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . .

25 Izm 5 /~m

. .. . .. .. .. .. .. . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . .. .. .. .. .. . .. .. .. ..

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1 0 0 0 2 0 0 0 3 0 0 0

P o s i t i o n ( ~ m )

4 0 0 0

F i g . 1 5 - S i m u l a t i o n r e s u lt s f o r d e g r e e o f h y d r a t i o n v s . d e p t h f o r

l a y e r e d c o m p o s i t e s p e c i m e n a f t e r 60 .2 h o f cur ing . Po s i t ions 0 to

2 0 00 c o r r e s p o n d t o t h e w / c = 0. 45 c e m e n t p a s t e l a y e r w h i l e p o s i -

t ions 2000 to 4000 a r e t h e w/c = 0 .3 p a s t e l a y e r t h a t i s d i r e c t l y

e x p o s e d t o t h e d r y i n g e n v i r o n m e n t .

0 . 6 4 , ! , I , ! ,

o . 5 6 - - ~ - - % ~ .; ~ : : 0 - I n i t . w a t e r - f i l l e d

i i O - S e l f - d b s i c c a t e d0 . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' ! ~ 1 " ] ) r i e d . . . . . . . . . . . . . . . . . . . . . . . .

T .- a O . 3 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " ': . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . .

0 .24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

o . t 6 . . . . . . . . . . . . . . . . . . . ~ - / i - , i i . . . . . . . . . . . . . . . . . . . . . . . . . . 5 - ~ i i i i - i . . . . . . . . . . . . . . . . . . . . . . .

0 . 0 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 , ~ ^ ~ ^ ^ : , ^ ^ ; ^ ~ i

0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0

P o s i t i o n ( m m )

Fig. 14 - S i m u l a t i o n r e s u l ts f o r p o r o s i t y f r a c t i o n s v s . d e p t h f o r

l a y e r e d c o m p o s i t e s p e c i m e n s a f t e r 6 0 .2 h o f c u r i n g . P o s i t i o n s 0

t o 2 0 00 c o r r e s p o n d t o t h e w / c = 0. 45 c e m e n t p a s t e l a y e r w h i l e

pos i t ion s 2000 to 4000 a re the w /c = 0 .3 pas te layer tha t i s d i rec t ly

e x p o s ed t o t h e d r y i n g e n v i r o n m e n t . S y m b o l l ab e ls a re t h e s a m e

a s t h o s e d e f i n e d i n t h e c a p t i o n o f F i g. 1 1 .

f av o ra b ly , t w o p h e n o m e n a t h a t a re n o t i n c l u d e d i n t h e

m o d e l r e su lt s s h o w n a b o v e a r e t h e s e t t li n g o f th e c e m e n tpa r t i c le s (b l eed ing) p r io r t o t he se t t i ng o f the pas t e and

c a r b o n a t i o n r e a c t i o n s b e t w e e n t h e c a l c i u m h y d r o x i d e

p r o d u c e d d u r i n g c e m e n t h y d r a t i o n a n d C O 2 p r e s e n t in

t h e e n v i r o n m e n t . T h e s e r e a c t io n s p r o c e e d r a p i d ly a t

i n t e r m e d i a t e r e la t iv e h u m i d i t i e s [1 8 ]. P r e l i m i n a r y

e f for t s t o i nc lude these two e f fec t s i n t he mode l d ry ing

c o d e s a p p e ar p r o m i s i n g . F o r e x a m p l e , s i m u l a ti o n s o f a

1 .2 m m th i ck cem ent pas te ( in i ti a l w/c = 0 . 6 ) u n d e r g o -

i n g e v a p o r a t i o n / s e t t l i n g / c a r b o n a t i o n

h a v e b e e n c o n d u c t e d t o c o m p a r e t o

ava il able expe r im enta l da t a [3 , 4 ] . I t

h a s b e e n a s s u m e d t h a t a f t e r

s e t t l i n g / e v a p o r a t i o n , t h e f i n a l t h i c k -n e s s o f t h e p a st e i s 1 .0 m m . D r y i n g

a n d c a r b o n a t i o n r a te s w e r e t a k e n f r o m

ava il able exp e r im enta l da t a [3 , 4 ] , and % P o r o s i t y

% A n h y d r o u s So l id s

i t was a ssum ed tha t a l l ca rbo na t ion re su lt s i n t h e fo rm a-

t i o n o f t h e c a l c it e f o r m o f c a l c iu m c a r b o n a t e . S i n c e t h e

mo la r vo lum e of ca lc i t e (36 .93 cm3/m ol ) is g rea t e r t han

t h a t o f c a l c iu m h y d r o x i d e ( 3 3 .0 8 c m 3 / m o l ) a n d t h e r e a c -

t ion occu rs on a 1 :1 mo la r bas i s, ex t ra ca l c ium ca rbo na te

pixels are (probabi l ist ica l ly, p = 0.1164) generated a t the

r e a c t i o n s i t e s t o m a i n t a i n t h e a p p r o p r i a t e v o l u m e s t o i -

c h i o m e t r y fo r t h e c a r b o n a t i o n r e a c t io n . A l s o , t h e w a t e r

r e le a s e d f r o m t h e c a l c i u m h y d r o x i d e d u r i n g c a r b o n a t i o n

i s m a d e a v ai la b le f o r f u r t h e r h y d r a t i o n o f th e c e m e n t .A c o m p a r i s o n o f th e e x p e r i m e n t a l s c a n n i n g e le c t r o n

m icros cop y/ im age analysis (SEM/IA) resul ts for capi l lary

p o r o s i t y a n d t h e r a t io o f a n h y d r o u s c e m e n t t o s o l i d

m a t e r i a l ( a n h y d r o u s c e m e n t + h y d r a t i o n p r o d u c ts ) [ 4] t o

t h o s e b a s e d o n t h e s i m u l a ti o n s o f v a r y i n g c o m p l e x i t y a r e

prov ided in Tab le 1. I t i s obse rv ed tha t t he bes t quan t i -

t at iv e a g r e e m e n t is o b s e r v e d w h e n b o t h s e t tl in g a n d c a r -

b o n a t i o n a re i n c l u d e d i n t h e m o d e l h y d r a t i o n / d r y i n g

c o d e s . T h u s , t h e d e v e l o p m e n t a n d e v a l u a ti o n o f t h e s e

e n h a n c e d c o d e s w i l l b e t h e s u b j e c t o f f u tu r e r es e ar ch .

F i n a ll y , F ig s . 1 6 a n d 1 7 p r o v i d e t h r e e - d i m e n s i o n a l

m i c r o s t r u c t u r e s f r o m t h e t o p s u r f a c e a n d a n i n t e r i o r

100 p ixe l x 100 p ixe l x 100 p ixe l sec t ion of a s imula t ed

I m m t h i c k w / c = 0 . 6 m i c r o s t r u c t u r e ( n o s e t tl in g o r c a r -

b o n a t i o n ) . I n t h e s e f i g u re s , e m p t y p o r o s i t y is w h i t e ,

w a t e r - f i l l e d p o r o s i t y i s d a r k g r e y , a n d t h e u n h y d r a t e d

c e m e n t p a r ti c le s a re l i g h t g r e y . I n c o m p a r i n g t h e t w o

i m a g e s , o n e c a n d e f i n i t e l y o b s e rv e t h e p r e f e re n t i al d r y -

ing a t t he immedia t e t op sur face (due to the l a rge r pores

p r e s e n t t h e r e ) a n d a m o r e " u n i fo r m " d r y i n g w i t h i n t h e

T a b l e 1 - H y d r a t i o n m o d e l s o f v a r y i n g c o m p l e x i t y

Ex p e r im e n t a l

( S E M / I A )

H y d r a t io n +

Evaporat ion

H y d r a t io n +

Evaporat ion+ Set t l ing

H y d r a t io n +

Ev a p o r a t io n +Se t t l in g /C a r b o n a t io n

4 0 . 3 8 . 4 8 . 4 3 .

4 0 . 5 0 . 4 4 . 4 6 .

5 6 3



Materials and Structures/Mat6riaux et Constructions,Vol. 34 , November2001

Fig. 16 - Th ree -di me nsi ona l image (100 pixels x 100 pixels x 100

pixels) of top surface of cement paste (w/c - 0.60) microstruc ture

after 2.9 h of hydra t ion/dry ing. Dried (emp ty) porosi ty is white,

water-f i l led porosi ty is dark grey, and unhy drate d cemen t part i -

cles are ligh t grey.

Fig. 17 - Th ree -dim ens ion al image (100 pixels x 100 pixels x 100

pixels) of middle sect ion of cement paste (w/c - 0.60) microstruc-

ture after 2.9 h of hydra t ion/dry ing. Dried (emp ty) porosi ty is

white, water-f i l led porosi ty is dark grey, and un hydra ted cem ent

particles are light grey.

interior of the specimen, as the larger pores at all thick-

nesses are the first to empty during the evaporation

process. The emp tying of these pores at the top surfaceis not observable using the current X -ray equ ipment due

to limitations in spatial resolution, 0.1 mm experimen-

tally vs. about 0.015 mm in Fig. 16.

The practical implications of these experimental and

computer modeling studies are potentially quite signifi-

cant. Tw o areas wher e correct understandings of water

mo vem ent are crit ical are the cu ring of high perfor-

mance concrete and the application of repair materials

[8]. Care must be taken wh en applying a curing me m-

brane (or compound) to concrete to ensure that the

pores in the curing layer are coarser than those in the

fresh concrete . If this is not the case, the so-caUed cur-ing layer will remain saturated to the casual observer

while it is continuously drawing water from the c oncrete

und erne ath it. Obviously, this will result in concrete

that does not achieve its potential hydration, strength

development, or durability even though "the surface

appears to remain saturated." These same principles of

differential water movement due to capillary forces also

apply to the use of saturated lightweight fine aggregates

to supply internal curing water for high performance

concrete s [19, 20]. In the area of repair materials, the

differential water mo ve me nt betwe en the substrate and

the repair material are critical to both the proper hydra-

tion o f the repair material and the development o f anadequate bond betwee n the two layers.

C O N C L U S I O N S

Experimental and computer simulation studies havebeen applied to examining the drying/hydration behavior

of thin cement paste and mortar specimens at sub-mil-

lime ter resolution. In general, the studies indicat e that:

1) thin c em en t pas te samples of "uniform"

microstructure dry out relatively uniformly throughout

their thicknesses as opposed to exhibitin g the sharp d ry-

ing front encoun tered in o ther porous materials,

2) in composite layered specimens, movement of

water from "coarser" pore layers to the "finer" ones is

observed regardless o f wh eth er the coarser pores are due

to a locally highe r w/c or to the use o f a coarser cem ent,

3) no clear differential mov eme nt of water could beobserved betwe en c emen t paste and mortar layers of the

same nominal w/c,

4) the NIST CEMHYD3D cement hydration model

can be been extended to include drying (and carbona-

tion) as well as hydration and in general the a greement

between experimental and computer model results is

quite good, and

5) the computer simulations particularly indicate that

both m ois ture conte nt and degree of hydrat ion are

affected by drying and large differences in achieved

degree of hydration were observed between the layers in

the tw o-layer composites exam ined in this study.

These results have numerous implications for designand performance of cem ent-based m aterials, particularly

the curing of high-performance concrete and the appli-

cation o f repair materials to existing structures.

5 6 4



B e n t z , H a n s e n , M a d s e n , V a l iS e , G r ie s e l

ACKNOWLEDGEMENTS

Some o f the measurements presented here w ere per-formed during the summ er of 1999, whe n D. P . Bentzwas a visiting professor at the De par tme nt o f StructuralEngineering and M ateria ls , DTU, funded by the K nud

Hojgaard Foundation. Th e authors wo uld like to thankDr. Claus-Jochen Haecker of Dyckerho ff Ze m ent for

supplying the cements ground to the two extremes offineness. D. P. Ben tz would also l ike to acknowledgefruitful discussions with Dr. Daniel Q uenard of C entreS c i e n t if i q u e e t T e c h n i q u e d u B ~ t i m e n t , G r e n o b l e ,

France and to thank his wife, Jane t Carey-Ben tz, for theidea of using the Lego blocks as sample holders. Partialfunding for th is research was provided by the NISTPartnership for High Performance Conc rete Technology(PHPCT) program.

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Drying-Hydration in Cement Pastes During Curing