Materials and S tructures/Mat6riaux et C onstructions, Vol. 34 , November 001, pp 557-565 Drying/hydration in cement pastes during curing D. P. Bentz 1, K. K. Hansen 2, H. D. Madsen 2, F. Vail& 3 and E.J. Griesel 4 (1) B uilding and Fire Research Laboratory, 100 B ureau Drive Sto p 86 21, Nationa l Institute of St andards and Technology, Gaithe rsburg, MD 20899-8621 USA (2) Technical University of Denm ark, Lyngby, D enmark (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; Paper accepted: March 23, 2001 A B S T R A C T R I~ S U M I~ As concrete cures in the field, there is a constant com- petition for the mixing water between evaporation and hydration processes. Understanding the mechanisms of water movement in the drying/hydrating cement paste is critical for designing curing systems and specialized ren- dering materials, as well as for selecting repair materials and methodologies. In this work, X-ray absorption mea- surements indicate that fresh cement paste dries uniformly throughout its thickness, as opposed to exhibiting the sharp drying front observed for most porous materials. Furthermore, in layered composite cement paste speci- mens, water always flows from the coarser-pore layer to the finer one, both when coarser pores are produced by using an increased water-to-cement ratio (w/c) and when they are present due to using a cement with a coarser par- ticle size distribution at a constant w/c. Conversely, no clear differential water movement is observed between layers of cement paste and mortar of the same nominal w/c. Based on the results of these experiments, drying has been introduced into the NIST CEMHYD3D cement hydration and microstructure development model, by emptying the largest water-filled pores present at any depth in the model specimen at a use>specified (drying) rate. With this addition, the CEM HYD3D model pro- duces results in good agreem ent with experimental obser- vations of both the drying profiles and the hydration kinetics of thin cement paste specimens. Lors de la cure du ciment sur chantier, la r&ction d'hydrata- tion est en constante cornp6tit ion avec I evaporatzon. La compre- hension des m& anismes de mouvements d'eau au sein de la pate en cours de se'chage/kydratation est essentielle non seulement en vue de d& erminer des m(thodes de cure adapt~es et de d6velopper des mortiers spe'ciaux, rnais ~galernent en rue de s&ctionner les bons produits et les bonnes rn~tkodologies de r(paration. D ans ce travail, les mesures d'absorption des rayons X mettent en & i- dence que la pate de ciment s&he de afon uniforme sur route son 6paisseur, et non selon un front marqu6 comme on peut courarn- ment l'observer dans les mat&iaux poreux. D e plus, clans le cas de pate de ciment bi-coucke, les mouvements d'eau se ont tou- jours de la couche contenant les pores de taiUe la plus importante vers celle contenant les pores les plus.fins, que les pores grossiers soient dus a un rapport e/c 6.1ev6 ou a une granulorn&ie grossi&e des particules de dment. A l'inverse, aucun mouvement d'eau n'est observ~ entre couches de pate de ciment et de mortier a rap- port e/c constant. Sur la base de ces r&ultats, le s&hage a (t~ introduit dans le module d'kydratation et de d&eloppernent de la microstructure du ciment CEMHYD3D d&eloppd par le NIST. Les pores satur6s d'eau de tailles les plus importantes sont ainsi vid6s selon une cin~tique fix(e par l'op6rateur, ceci quelle que soit leur position dans l'bpaisseur de l'6chantillon. CEMHYD3 D g6n~re ainsi des r&ultats pr&entant une bonne corr61ati on 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. INTRODUCTION To achieve optimal performance from cement-based materials in the field, proper curing conditions must be maintained throughout the first few weeks of their life [1, 2]. Unfortunately, due to a lack of quality control in the field, concretes and mortars often experience con- siderable drying before the cement paste matrix has undergone "complete" hydration. For some materials, such as the thin cement/polymer composite mortars used as rendering materials, later addition of liquid water to the dried composite is not able to "reinitiate" the cem ent hydration process [3]. For others, this re-initia- tion of hydration causes significant internal damage to 1359-5997/01 9 ILILEM 557
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8/2/2019 Drying-Hydration in Cement Pastes During Curing
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
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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
8/2/2019 Drying-Hydration in Cement Pastes During Curing
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
8/2/2019 Drying-Hydration in Cement Pastes During Curing
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
8/2/2019 Drying-Hydration in Cement Pastes During Curing
B e n t ] ., H a n s e n , M a d s e n , V a l l 6 e , G r ie s e l
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' I ' I ' I '3 8 3 9 4 0 4 1
D i s t a n c e ( m m )
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 | , ! , ! ' ! , ! , ! , ! , ! , |
i " : ' , i i , /~,' i'~ ~ . . . . ~ . ,
~ / . . . . . - /
; : ' . ' . : : ; I [
3 3 3 4 3 5 3 6 : 37 3 8 3 9 4 0 4 1
D i s t a n c e ( m m )
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
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
D i s t a n c e ( m m )
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-
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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
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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 . . . . . . . . . . . . . . . . . . . . . . . .
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 .
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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.
REFERENCES
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[2] Griesel, E. J . , 'T he influence o f controlled environmental cond i-t i o n s o n t h e p o t e n t i a l d u r a b i l i t y o f c o n c r e t e ' , M . S . T h e s i s ,
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