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Interaction of chemical admixtures in the cement-water system
Ramachandran, V. S.
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Ser TH1
N21d National Research Conseil national
0 1442 Council Canada
de
recherches Canadac. 2 BLDG institute for - lnstitut de Research in recherche en Construction construction
Inieraction of Chemical Admixtures
in the
Cemenf
-
Water
System
by V.S. Ramachandran
A N A L Y Z E D
Appeared in
"Research on the Manufacture and Use of Cements" Proceedings of the Engineering Foundation Conference Henniker, New Hampshire, July 28-August 2, 1985
p. 121 -131
(IRC Paper No. 1441)
Reprinted with permission
Price $2.00 NRCC 27412 1 NRC
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CISTII R C
L I B R A R Y3
1987
11 peut se produire de l'adsorption, de la chimieorption et des rhctions chimiques entre les adjuvants et les hydratants du ciment. Des modifications physiques et chimiques oat lieu au moment de l'addition au s y s t h e ciment-eau d'adjuvsnts tels les accblbrateurs, les retardateurs de prise, les plastifiants rgducteurs dteau et les superplastlfiants. I1 est possible d'expliquer par des ef f ets d'interaction le dcaoisme dlaction des adjuvants et les variations de la d e u n d e en eau, de la viscosit6, de la prise, de la perte par affaissement, du retrait, de la cingtique d'hydratation, de la microstructure, de la resistance et de la durabilitg du ciment frais et du ciment durci. - - >
..
S e r N a t i o n a l Research Council Canada. I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n no.1441
Paper c. 2 R ~ a c h a n d r a n , V. S. BLDG I n t e r a c t i o n o f chemical a d m i w + , , - - -INTERACTION OF CHEMICAL ADMIXTURES I N THE CEMENT
-
WATER SYSTEM V.S. Ramachandran*ABSTRACT
Physical a d s o r p t i on, chemi s o r p t i on and chemical r e a c t i o n s may occur between t h e admixtures and t h e h y d r a t i n g conponents o f cements. Physical and chemical changes occur when admixtures such as
accelerators, r e t a r d e r s , water reducers, and superpl a s t i c i zers a r e added t o t h e cement
-
water system. Mechanisms o f t h e a c t i o n of admixtures, changes i n water demand, v i s c o s i t y , s e t t i n g , slump loss, shrinkage, k i n e t i c s o f hydration, m i c r o s t r u c t u r e , s t r e n g t h anddurabi 1 i t y o f f r e s h and hardened cement pastes can be explained by t h e i n t e r a c t i o n e f f e c t s .
1. INTRODUCTION
Chemical admixtures c o n f e r c e r t a i n benefi c i a1 e f f e c t s on concrete, i n c l u d i n g reduced water r e q u i rement, increased workabi 1 i t y , c o n t r o l 1 ed s e t t i n g and hardening, improved s t r e n g t h and b e t t e r d u r a b i l i t y .
Many approaches have been adopted t o i n v e s t i g a t e t h e r o l e of chemical admixtures. One approach i s t o determine t h e s t a t e of t h e admixture i n concrete a t d i f f e r e n t times o f curing. The admixture may remain i n a f r e e s t a t e as a s o l i d o r i n s o l u t i o n , o r i n t e r a c t a t t h e surface o r chemically combine w i t h t h e c o n s t i t u e n t s o f cement o r cement paste. The t y p e and e x t e n t o f t h e i n t e r a c t i o n may i n f l u e n c e t h e
physi co-chemi c a l and mechanical p r o p e r t i e s o f cement paste. I n t h i s paper an attempt i s made t o discuss t h e p o s s i b l e i n t e r a c t i o n s o f d i f f e r e n t types o f chemical admixtures i n t h e cement
-
water system, w i t h p a r t i c u l a r reference t o t h e changes i n physi co-mechani c a l p r o p e r t i e s .DISCUSSION
2.1 Accelerators
Many i norgani c and organi c compounds such as c h l orides, f l u o r i d e s
,
carbonates, s i 1 icates, a1 umi nates, borates, n i t r i t e s , t h i o s u l f a t e s ,t r i ethanol ami ne, d i e t h a n o l ami ne, and formates have been advocated f o r use as accelerators.
*V.S. Ramachandran, Bui 1 ding M a t e r i a l s Section, D i v i s i o n of Bui l d i n g Research, National Research Counci 1 Canada, Ottawa, Canada
2.1.1 Cal c i um c h l o r i d e
Calcium c h l o r i d e i s perhaps t h e most e f f i c i e n t and economical accelerator. I n t h e h y d r a t i o n o f C3S, t h e r e i s evidence t h a t calcium c h l o r i d e e x i s t s i n d i f f e r e n t s t a t e s i n t h e C S paste. Based on thermal a n a l y s i s and 1 eachi ng studies, Ramachandran 11) has concluded t h a t , depending on t h e t i m e o f hydration, t h e c h l o r i d e may e x i s t i n a f r e e form ( e x t r a c t a b l e by e t h y l a l c o h o l ), i n c o r p o r a t e d s t r o n g l y i n t o t h e
C-S-H phase (unleachabl e w i t h water), chemi sorbed o r i n i n t e r l ayer
p o s i t i o n s (leachable w i t h water). At 168 h about 20% o f t h e added
c h l o r i d e i s i n c o r p o r a t e d s t r o n g l y i n t o t h e C-S-H phase. The r e s t i s i n a chemisorbed s t a t e o r i n t h e i n t e r l a y e r s ( e a s i l y leachable w i t h
water). These r e s u l t s may have i m p l i c a t i o n s i n e x p l a i n i n g t h e mechanism o f t h e a c c e l e r a t i o n a c t i o n , mi c r o s t r u c t u r a l development, differences i n t h e CaO/Si02 r a t i o o f C-S-H products, c o r r o s i o n p o t e n t i a1 o f c h l o r i d e s and i n t r i n s i c p r o p e r t i e s o f p o r t l a n d cement. Cal c i um c h l o r i de accelerates t h e r e a c t i o n between C3A and gypsum, and
monochl o r o a l umi nate i s formed a f t e r a1 1 gypsum i s consumed (2). Using
l e a c h i n g and pressure techniques i t has been found t h a t more c h l o r i d e i s immobilized by t h e C 3 A
+
gypsum m i x t u r e t h a n by C3S o r p o r t l a n d cement (3).2.1.2 Triethanolamine
T r i ethanol ami ne i s termed an accelerator. T r i ethanol ami ne acts,
however, as a r e t a r d e r o f C3S h y d r a t i o n ( 4 ) . An examination of t h e thermal behavior o f C3S hydrated t o d i f f e r e n t periods i n t h e presence
of t r i ethanolami ne reveal s t h e development o f exothermal peaks t h a t
c o u l d be a t t r i b u t e d t o t h e decompositon o f a complex of t h e amine w i t h
t h e hydrated products o f C3S. This complex may be responsible f o r
e a r l y r e t a r d a t i o n and p o s s i b l y f o r t h e h i g h e r C/S r a t i o of t h e C-S-H product. T r i ethanol ami ne a c c e l e r a t e s t h e r e a c t i o n between C3A and gypsum (5). The c a t i o n - a c t i v e ami ne may r e a c t w i t h A1 and Ca i o n s on
t h e surface o f C3A. I n cements, a t a dosage o f 0.1 o r 0.5%
t r i e t h a n o l a m i ne, s e t t i n g occurs r a p i d l y w i t h i n about 2-6 m i n. ; t h i s i s a t t r i b u t e d t o t h e a c c e l e r a t e d f o r m a t i o n o f e t t r i n g i t e and C3A-hydration products (5).
2.1.3 Formates
Calcium formate i s used as a non-chloride accelerator. I n
cements, more e t t r i n g i t e i s formed w i t h formate t h a n w i t h calcium c h l o r i d e (6). The increased f o r m a t i o n o f e t t r i n g i t e i n t h e presence of Ca-formate i s a t t r i b u t e d t o t h e f o r m a t i o n o f t h e complex
C3Ae3Ca(HC02)2e30H20 a t o r d i n a r y temperatures, analogous t o e t t r i n g i t e . Enhanced formation o f e t t r i n g i t e i n t h e presence o f Ca-acetate and Ca-propi onate i s a1 so e x p l a i n e d by t h e i r a b i 1 i t y t o form hexagonal
prism-type phases w i t h C3A (7). Calcium formate accelerates t h e
h y d r a t i o n o f C S. An e x p l a n a t i o n i s t h a t formate i o n s i n t e r f e r e o r i n t e r a c t w i t h t h e p r o t e c t i v e l a y e r normally formed on t h e C3S
surface (8).
2.1.4 Other a c c e l e r a t o r s
Sodium carbonate decreases t h e s e t t i n g t i m e o f cement by 2-4 hours b u t a f t e r 10-12 hours t h e h y d r a t i o n i s r e t a r d e d due t o t h e
p r e c i p i t a t i o n o f CaCO, by t h e r e a c t i o n o f Na C03 w i t h lime. The p r e c i p i t a t i on o c c u r r i n g w i t h i n t h e pores o f $he product decreases permeabi 1 i t y (9).
Oxalic a c i d may a l s o a c t as an a c c c e l e r a t o r by decreasing t h e s e t t i n g t i m e o f cement by 43% and i n c r e a s i n g s t r e n g t h s by 12% (10). Strength development i s explained by t h e f o r m a t i o n o f Ca-oxalate, although t h e amount formed may be small.
Although calcium c h l o r i d e i s a comparatively simple molecule
compared t o o t h e r accelerators, i t s a c c e l e r a t i n g mechanism has n o t been resolved. A t l e a s t t w e l v e t h e o r i e s have been proposed f o r i t s
a c t i o n (11). It can, t h e r e f o r e , be appreciated t h a t t h e r o l e of more complex compounds i n v o l v i ng organi c compounds i s n o t easi l y r e s o l ved.
2.2 Retarders
Organic compounds such as u n r e f i n e d Na, Ca o r NH, s a l t s of
1 i gnosulfoni c acids, hydroxy-carboxyl i c acids, carbohydrates and
i n o r g a n i c compounds (oxides o f Pb and Zn, phosphates, Mg s a l t s , f l u o r a t e s and borates) a c t as r e t a r d e r s .
2.2.1 Sugars
Theories o f t h e r e t a r d i n g e f f e c t a r e based on adsorption, p r e c i p i t a t i o n , compl e x a t i on o r nucleation. I n a1 1 these processes, i n t e r a c t i o n s a r e i n v o l ved.
According t o Milestone, sugar and sugar acids adsorb on t o ca2+ i o n s on t h e h y d r a t i n g C3S s u r f a c e and poison t h e C-S-H n u c l e a t i n g
s i t e s (12). Adsorption o f glucose, f o r example, r e s u l t s i n t h e zeta
p o t e n t i a l becoming -ve from t h e t v e values. Poisoning of. CH n u c l e i by
a d s o r p t i o n o f sugars i s a l s o envisaged (13).
According t o t h e p r e c i p i t a t i o n theory, a d d i t i o n o f mono- o r
polysaccharides increases t h e c o n c e n t r a t i o n o f Ca, A1 and Fe. Sugars
may combine w i t h them t o form i n s o l u b l e metal organic complexes on t h e cement g r a i n s and r e t a r d h y d r a t i o n (14). Not a l l sugars r e t a r d cement
t o t h e same extent. Non-reduci ng sugars, f o r example, c o n t a i n i n g
f i ve-membered r i n g s (sucrose, r a f f i nose) a r e t h e b e s t retarders.
Although no s u c r o s e - s i l i c a t e o r Ca-complex has been detected w i t h these
sugars, i t i s suggested t h a t a h a l f s a l t formed by attachment o f Ca
and OH groups t o t h e f i ve-membered r i n g may poison Ca(0H) and C-S-H
n u c l e i (15). Retardation o f C,A conversion t o t h e c u b i c
AH,
i sexplained by t h e f o r m a t i o n o f an i n t e r l a y e r complex o f t h e hexagonal aluminate h y d r a t e w i t h t h e organic compounds (16,17).
It appears t h a t no s i n g l e t h e o r y can be a p p l i e d t o t h e behavior of a l l sugars on a l l cement components under a l l c o n d i t i o n s o f hydration. Adsorption need n o t occur o n l y on unhydrated o r hydrated surfaces.
Some sugars a c c e l e r a t e t h e i n i t i a l f o r m a t i o n o f e t t i i n g i t e i n t h e
C3A
-
gypsum-
H20 system and others r e t a r d it. Sucrose i s anaccelerator, whereas r a f f i nose and t rehal ose a r e retarders. When
sucrose i s used, i t i s assumed t h a t adsorption occurs on t h e anhydrous
surface, p r e v e n t i n g t h e formation o f an impermeable l a y e r of
e t t r i n g i t e (18). Why t h i s does n o t occur w i t h o t h e r sugars needs t o be examined.
The p r e c i p i t a t i o n t h e o r y cannot be a p p l i e d f o r a l l cases. For example, t h e s t a b i 1 i t y constants o f Ca-compl exes o f v a r i ous compounds do n o t bear c o r r e l a t i o n w i t h t h e i r potency as retarders. A study of many complexes has shown t h a t some a r e r e t a r d e r s and o t h e r s a r e not. The poisoning of Ca(0H) n u c l e i by i t s e l f may n o t always cause
r e t a r d a t i o n . A1 though S a ( 0 ~ ) growth can be m o d i f i e d by i n c o r p o r a t i o n of dyes, i t does n o t show any r e t a r d i n g c h a r a c t e r i s t i c s (18). A
n u c l e a t i o n e f f e c t o f C-S-H should a l s o be considered. A combination of
two mechanisms may occur and i t i s n o t easy t o separate t h e occurrence
of two mechanisms. For example, EDTA r e t a r d s t h e h y d r a t i o n of C3S.
The r e t a r d i n g a c t i o n may be explained by t h e p r e c i p i t a t e d g e l c o a t i n g of C-S-H on t h e h y d r a t i n g C3S, consequent on t h e f o r m a t i o n o f a complex
between EDTA and Ca(OH)2 i n t h e s o l u t i o n phase (19). The i d e a t h a t
r e t a r d i n g agents should c o n t a i n t h e a-hydroxy carbonyl group c o u l d be
questioned. According t o Daugherty and Kowalewski (20), organic
compounds w i t h two o r more (OH) groups a r e necessary f o r t h e
r e t a r d a t i o n o f C3A hydration. Hydroxycarboxyl i c a c i d s
Adsorption s t u d i e s have been c a r r i e d out o f hydroxycarboxyl i c
a c i d s such as s a l i c y l i c a c i d on cement and cement components (21-23). Only a small amount o f a d s o r p t i o n occurs on t h e unhydrated phases
compared t o t h a t on hydrated products o f cement minerals. A complex of
s a l i c y l i c a c i d w i t h A1 may be responsible f o r t h e r e t a r d a t i o n of
h y d r a t i o n of C3A. Simi 1 a r complexes may form w i t h t h e C,AF component
i n cement. The hydroxycarboxyl i c a c i ds a c t more e f f i c i e n t l y on cements
c o n t a i n i n g low a l k a l i and C3A contents. At lower C3A contents, s m a l l e r
amounts of r e t a r d e r a r e adsorbed, l e a v i n g l a r g e r amounts of t h e
admixture t o a f f e c t t h e C3S component. A1 k a l i s may a f f e c t d i s s o l u t i o n and i n t e r a c t i o n reactions.
E a r l y h y d r a t i o n o f C3A
+
g+
CH may be accelerated and t h e l a t e r r e a c t i o n i n v o l v i n g t h e conversion o f e t t r i ngi t e t o monosulfoal umi nate may be retarded by c i t r i c a c i d (24). A c c e l e r a t i o n i n t h e i n i t i a l stages may be due t o p r e f e r e n t i a l a d s o r p t i o n o f c i t r i c acid, which promotes h y d r o l y s i s o f C3A t o hexagonal phases. The l a t e r r e t a r d a t i o n may be due t o t h e f o r m a t i o n o f a complex between c i t r i c a c i d andmonosulfate. Calcium c i t r a t e formed by r e a c t i o n o f l i m e and CaC03
( i m p u r i t i e s ) w i t h c i t r i c a c i d may h i n d e r t h e development o f n u c l e i i n t h e h y d r a t i o n of p l a s t e r o f p a r i s .
C o r r e l a t i o n between s o l u b i 1 i t y and r e t a r d i n g behavior i s n o t
always possible. S o l u b i l i t y o f Ca s a l t o f o x a l i c a c i d i s 5 x 10-6M and
C3A hydration, although i t i s t h e l e s s s o l u b l e o f t h e two, b u t g l u c o n i c a c i d i s more s o l u b l e and i s a good retarder.
2.2.3 L i gnosulfonates
The re1 a t i ve r e t a r d i n g e f f e c t s o f sugars and 1 i gnosul fonates a r e n o t easy t o resolve. The h y d r a t i n g C3A adsorbs i r r e v e r s i b l y
s u b s t a n t i a l amounts o f l i g n o s u l f o n a t e and hence i t i s p o s s i b l e t h a t pure 1 i gnosulfonate c o n t r i b u t e s t o t h e r e t a r d i n g e f f e c t (25).
Thermograms show t h a t both commerci a1 and sugar-f r e e 1 i gnosul fonates a r e e q u a l l y e f f e c t i v e i n r e t a r d i n g t h e h y d r a t i o n o f cement (26). Both t h e comrnerci a1 and pure 1 i gnosulfonates r e t a r d s e t t i n g times.
Accordi ng t o o t h e r i n v e s t i gations, e i t h e r t h e sugar-f ree 1 i gnosulfonate i s a poor r e t a r d e r o r i t i s i n e r t i n i t s a c t i o n on cement. The
disagreement may be due t o t h e d i f f i c u l t y o f p r e p a r i n g pure 1 ig n o s u l f o n a t e and t h e d i f f e r e n c e s i n molecular weights of 1 ig n o s u l f o n a t e s used.
2.2.4 I n o r g a n i c r e t a r d e r s
Many i n o r g a n i c s a l t s r e t a r d t h e h y d r a t i o n o f cement. These s a l t s : form i n s o l u b l e hydroxides i n a l k a l i n e s o l u t i o n and may form a c o a t i n g on t h e cement p a r t i c l e s . There i s evidence t h a t i n o r g a n i c compounds form complexes w i t h t h e h y d r a t i n g cement products. Zinc oxide i s known t o be a good r e t a r d e r o f cement hydration. Zinc o x i d e r e t a r d s t h e h y d r a t i o n o f C S and does n o t i n f l u e n c e t h e h y d r a t i o n o f C3A + gypsum mixture. The Bormati on o f c a l c i urn hydroxyzincate (Ca[Zn (OH) H,O],) by t h e r e a c t i o n o f Ca(OH)2 w i t h ZnO has been confirmed (27).
~t
ana d d i t i o n o f 10% Zn e q u i v a l e n t o f hydroxyzincate, 3.3% Zn may be
i n c o r p o r a t e d i n t o t h e C-S-H phase. The r e t a r d a t i o n e f f e c t o f Pb(N0 ) 2
i s a t t r i b u t e d t o t h e very r a p i d p r e c i p i t a t i o n o f a p r o t e c t i v e P ~ ( o H ~ * on t h e cement g r a i n s (28). Most phosphates r e t a r d s e t t i n g . The
adsorption o f phosphate i o n s a t t h e surface o f t h e c l i n k e r phase o r on t h e f i r s t h y d r a t i o n product i s thought t o r e s u l t i n t h e p r e c i p i t a t i o n o f Ca-phosphates.
2.3 Water Reducers
Water reducers c o n s i s t o f Ca, Na o r NH, s a l t s o f l i g n o s u l f o n i c acids, Na, NH, o r t r i ethanol ami ne s a l t s o f hydroxycarboxyl i c a c i d s and carbohydrates. L i gnosul fonates c o n t a i n i n g (OH), (COOH) and (SO H) groups a r e more widely used t h a n others. Hydroxycarboxyl i c a c i i s , such as c i t r i c acid, t a r t a r i c acid, s a l i c y l i c acid, heptoni c acid, sacchari c a c i d and g l u c o n i c acid, c o n t a i n (OH) and (COOH) groups. Gluconic
a c i d-based admi x t u r e s a r e used extensi v e l y
.
Carbohydrates i n c l ude glucose, sucrose o r hydroxyl ated polymers obtained by p a r t i a l h y d r o l y s i s o f saccharides. The r o l e o f water reducers (normal, a c c e l e r a t i n g o r r e t a r d i n g ) i n terms o f t h e i r e f f e c t on h y d r a t i o n o f cement i s s i m i l a r t o t h a t o f retarders, a c c e l e r a t o r s ands u p e r p l a s t i c i zers. The e f f e c t o f some a c c e l e r a t i n g and r e t a r d i n g
admixtures has a1 ready been discussed. I n t h i s s e c t i o n t h e i n t e r a c t i o n o f 1 i gnosul fonates w i t h cement w i 11 be emphasized.
The p l a s t i c i z i n g a c t i o n o f water reducers i s r e l a t e d t o t h e i r adsorption and d i s p e r s i n g e f f e c t s i n t h e cement
-
water system. I n o t h e r words, some s o r t of i n t e r a c t i o n seems t o be involved.2.3.1 T r i c a l c i um a1 umi nate
The h y d r a t i o n o f t r i c a l c i u m aluminate i s r e t a r d e d by
1 i gnosul f onate. The i n t e r a c t i on between 1 i gnosul f o n a t e and h y d r a t i n g
C3A can be s t u d i e d by a d s o r p t i o n experiments. Adsorpti on isotherms
cannot be obtained i n t h e C3A
-
l i g n o s u l f o n a t e-
H20 system becauseh y d r a t i o n of C3A occurs d u r i n g t h e measurements, e s p e c i a l l y a t low
admixture concentrations. It i s p o s s i b l e t o determi ne t h e a d s o r p t i o n
-desorpti on isotherm i n t h e system hexagonal a1 umi n a t e
-
l i g n o s u l f o n a t e
-
H20. Scanning loops i n t h e isotherms show completei r r e v e r s i b i l i t y , i n d i c a t i n g a complex formation. Increase i n t h e c-axi s spacing o f t h e hexagonal phase c o n t a i n i ng 1 i gnosul fonate i s caused by t h e formation o f an i n t e r l a y e r complex. The i n t e r l a y e r
complex would impede t h e conversion o f t h e hexagonal phase t o t h e c u b i c
phase. I n a non-aqueous phase, C A does n o t adsorb any l i g n o s u l f o n a t e .
The r e t a r d i n g e f f e c t seems t o be h e t o t h e r e a c t i o n of h y d r a t i n g CIA
and lignosulfonate. Jawed e t a l . (29) observed more f l u i d i t y i n cement
paste c o n t a i n i n g a m i x t u r e o f 1 ig n o s u l f o n a t e and Na2C03 than when each
of them was added separately. They proposed t h a t an i o n i c complex
occurred between 1 ignosulfonate and CO$-and t h a t I t was more a n i o n i c
than l i g n o s u l f o n a t e and, hence, acted as a b e t t e r dispersant.
The conversion o f e t t r i n g i t e t o monosulfoaluminate i s r e t a r d e d by
adding a water reducer t o C3A
+
gypsum. One o f t h e suggestedmechanisms of t h e plasticizing a c t i o n may be r e l a t e d t o a lower water
demand caused by t h e r e t a r d a t i o n o f e t t r i n g i t e f o r m a t i o n and a decrease i n t h e i n t e r l o c k i n g o f t h e e t t r i n g i t e p a r t i c l e s . This observation has t o be confirmed by a more e x t e n s i v e i n v e s t i g a t i o n .
T r i c a l c i um s i 1 i c a t e
The h y d r a t i o n o f t r i c a l c i u m s i l i c a t e i s r e t a r d e d by
1 i gnosulfonate. Apparent adsorption-desorption i sotherms of t h e
C3S
-
l i g n o s u l f o n a t e-
H20 system, d i s p e r s i o n and h y d r a t i o n e f f e c t s can be explained (30). There i s o n l y p a r t i a l r e v e r s i b i l i t y d u r i n gdesorption, i n d i c a t i n g t h e e x i s t e n c e of a s t r o n g l y bound surface complex i n v o l v i n g C S, l i g n o s u l f o n a t e and H 0. Such a complex may cause r e t a r d a t i o n o f h y d r a t i o n o f C,S. ~ h a 8 t h e hydrated CIS paste adsorbs i r r e v e r s i b l y s u b s t a n t i a l amounts o f 1 ig n o s u l f o n a t e may be concluded from a d s o r p t i o n isotherms on t h e completely hydrated C3S. Even i n t h e non-aqueous medium, t h e hydrated C3S, u n l i k e t h e anhydrous
C S phase, adsorbs calcium lignosulfonate. These r e s u l t s suggest t h a t
tRe r e t a r d i n g e f f e c t i nvoi ves a r e a c t i o n o f 1 i gnosulfonate w i t h t h e h y d r a t i n g C3S surface.
2.3.3 T r i c a l c i u m aluminate
-
t r i c a l c i u m s i l i c a t eThe C3A phase adsorbs l a r g e r amounts o f l i g n o s u l f o n a t e than C3S
when exposed t o aqueous s o l u t i o n s o f 1 i gnosulfonate. Therefore, i n
h y d r a t i n g cements, C3A may a c t as a s i n k f o r l i g n o s u l f o n a t e . When
l i g n o s u l f o n a t e i s added a few minutes a f t e r water has come i n t o contact w i t h cement, t h e h y d r a t i o n o f C S i s retarded more s t r o n g l y . This would mean t h a t t h e hydrated C
A
phase adsorbs l e s s l i g n o s u l f o n a t e and leaves l a r g e r amounts o f t h e a i m i x t u r e i n t h e s o l u t i o n phase f o rr e t a r d a t i o n o f C3S hydration.
The e f f e c t o f l i g n o s u l f o n a t e on cement depends n o t o n l y on t h e amounts o f C3A and C3S b u t a l s o on t h e a l k a l i s , t h e SO,, t h e p a r t i c l e
s i z e o f cement, etc. Depending on these f a c t o r s and others, e a r l y s e t
may be r e t a r d e d o r accelerated b u t t h e f i n a l s e t i s g e n e r a l l y retarded.
The e a r l y a c c e l e r a t i o n o f s e t i s promoted i n cements w i t h h i g h e r aluminate/SO r a t i o s . At e a r l y times, due t o t h e a d s o r p t i o n of
l i g n o s u l f o n a a e on C3S, Ca(OH), i s n o t released and t h e r a t e of f o r m a t i o n o f e t t r i n q i t e i s increased. This i m p l i e s t h a t t h e
C3A + gypsum r e a c t i o n t o f o r m e t t r i n g i t e i s f a s t e r t h a n t h a t c o n t a i n i n g
Ca (OH),
.
The r o l e o f t h e ani oni c 1 i gnosulfonate molecule as a water reducer
i s apparently r e l a t e d t o i t s a d s o r p t i o n on t h e cement components. The, amount o f adsorption, t h e r a t e and t y p e o f adsorption may p l a y a r o l e i n t h e s e t t i n g , d i spersi b i l i t y , m i c r o s t r u c t u r e , shrinkage, d u r a b i l i t y and o t h e r p r o p e r t i e s o f cement. The e f f e c t o f c a t i o n s and t h e
molecular weight o f 1 i gnosulfonate on cement p r o p e r t i e s has n o t been
explored f u l ly .
2.4 Superpl a s t i c i z e r s
Most superpl a s t i c i z e r s a r e based on s u l f o n a t e d me1 ami ne
formaldehyde (SMF)
,
s u l f o n a t e d naphthalene formaldehyde (SNF) andmodified 1 i gnosul f onates. The a c t i o n o f water reducers i nvol ves
a d s o r p t i o n and d i s p e r s i o n i n t h e cement
-
water system. S i m i l a r l y , e f f e c t i v e s u r f a c e i n t e r a c t i o n and d i s p e r s i o n o f cement occur when s u p e r p l a s t i c i z e r s a r e used. A study o f t h e r a t e and amount of adsorption o f superpl a s t i c i zers on cement and cement compounds has provided some i n f o r m a t i on on t h e r h e o l o g i c a l,
s e t t i n g and h y d r a t i o n c h a r a c t e r i s t i c s .2.4.1 T r i c a l c i urn a1 umi n a t e
The h y d r a t i o n o f C,A i n t h e presence o f SMF i n d i c a t e s t h a t
h y d r a t i o n i s retarded. Adsorption o f SMF on C3A occurs as soon as t h e
s o l u t i o n comes i n t o c o n t a c t w i t h i t (31). The amount and r a t e of
a d s o r p t i o n on t h e C A phase f a r exceed those on C3S o r p o r t l a n d cement.
I n a non-aqueous meaium, adsorption i s n i 1 on C,A b u t small amounts of
SMF a r e adsorbed by t h e hexagonal phase. Adsorption i s i r r e v e r s i b l e ,
i n d i c a t i n g t h a t a chemical i n t e r a c t i o n occurs between t h e h y d r a t i n g C3A and SMF. Thus, t h e r e t a r d a t i o n o f C3A may be explained by s t r o n g adsorption o f SMF on t h e h y d r a t i n g C3A surface.
The reported r e s u l t s on t h e r a t e o f h y d r a t i o n o f C3A
+
gypsum m i x t u r e c o n t a i n i n g s u p e r p l a s t i c i z e r s a r e c o n t r a d i c t o r y . Adsorption ofperiods. Desorption experiments show t h a t SMF i s i r r e v e r s i b l y
adsorbed. A surface chemical o r chemical i n t e r a c t i o n seems t o occur
between t h e h y d r a t i n g C3A o r C3A
+
gypsum m i x t u r e w i t h SMF. The enhanced d i s p e r s i o n e f f e c t o f s u p e r p l a s t i c i z e r added a few minutes a f t e r m i x i n g water i s added t o concrete can be explained as o u t l i n e d under 2.3.3.T r i c a l c i um s i 1 i c a t e
Hydration o f C3S i s r e t a r d e d by SMF. I r r e v e r s i b l e a d s o r p t i o n i s i n d i c a t e d i n t h e system c o n t a i n i n g C S
-
SMF-
H20. T h i s may i n v o l v e i n t e r a c t i o n between h y d r a t i n g C3S an8 superpl a s t r c i zer. Thei n t e r a c t i o n s of s u p e r p l a s t i c i z e r s w i t h h y d r a t i n g C3S, C A and
C3A
+
gypsum components cannot be d i r e c t l y a p p l i e d t o t t e cementbehaviour. The i n t e r a c t i o n w i t h i n t h e cement system i s much more
complex because, i n a d d i t i o n t o i n t e r f e r i n g e f f e c t s o f t h e s i l i c a t e s and aluminates, t h e a l k a l i s and SO3 a l s o p l a y an important r o l e .
2.4.3 Cement
The amount o f adsorption o f SMF on cement v a r i e s w i t h l e n g t h of .
exposure t o t h e s o l u t i o n . W i t h i n a few seconds t h e r e i s a steep
increase i n adsorption due t o t h e C3A
-
C,AF components i n t h e cement.F u r t h e r a d s o r p t i o n does n o t occur f o r about f o u r t o f i v e hours, a f t e r
which i t increases continuously. Adsorption beyond about f i v e hours i s
mainly due t o t h e h y d r a t i n g C3S component. These r e s u l t s suggest t h a t a d s o r p t i o n and i n t e r a c t i o n o f SMF w i t h t h e cement components a r e
i n v o l v e d i n t h e d i spersion o f cement and t h e r e t a r d a t i o n of cement h y d r a t i on.
The amount of a d s o r p t i o n o f s u p e r p l a s t i c i z e r on cement can be r e l a t e d t o workabi 1 i t y . With t h e SNF s u p e r p l a s t i c i z e r , t h e m i n i slump
values increase as t h e amount o f a d s o r p t i o n increases (32). The
a d s o r p t i o n values o f SNF on t h r e e t y p e s o f cement decrease as f o l l o w s : Type I 1 1
>
Type I>
Type I 1 (33). The C3A/S03 r a t i o s i n t h e cementsf o l l o w t h e same trend. That a d s o r p t i o n i s dependent on C3A content
becomes c l e a r : f o r t h e same w o r k a b i l i t y , a h i g h e r dosage of s u p e r p l a s t i c i z e r i s r e q u i r e d f o r Type I t h a n f o r Type V cement.
Zeta p o t e n t i a l development i n suspensions o f cement, a l i t e , C A
and Ca(OH)2 c o n t a i n i n g s u p e r p l a s t i c i zers has been studied. ~ e n e r a f l ~ , both a d s o r p t i o n and zeta p o t e n t i a l values increase as t h e c o n c e n t r a t i o n of s u p e r p l a s t i c i z e r added t o cement i s increased (34).
The mechanism o f r e t a r d a t i o n o f h y d r a t i o n and even d i s p e r s i o n may n o t be e n t i r e l y due t o t h e a d s o r p t i o n e f f e c t o f t h e anions. I n SNF s u p e r p l a s t i c i z e r c o n t a i n i n g NH,, Co, Mn, L i and Ni cations, t h e t i m e
f o r t h e development o f maximum heat i s 12.7 h w i t h NH, and o n l y 9.25 h
w i t h Ni (35). The r e l a t i v e r o l e s o f c a t i o n s i n t h e s u p e r p l a s t i c i z e r s a r e n o t w e l l understood.
Hi gher-than-normal workabi 1 i t y o f concrete c o n t a i n i n g a
s u p e r p l a s t i c i z e r i s maintained f o r about 30-60 min., a f t e r which t h e slump value decreases. I n t h e p e r i o d d u r i n g which slump l o s s i s
occurring, t h e C3A phase r e a c t s w i t h gypsum. The e x t e n t of r e a c t i o n of C3A and gypsum and t h e c r y s t a l l i n e form o f t h e product c o u l d have an important e f f e c t on t h e w o r k a b i l i t y o f concrete. A d d i t i o n o f
superpl a s t i c i z e r enhances t h e i n i t i a1 r e a c t i o n between C3A and gypsum. A1 k a l i s a1 so enhance t h i s reaction. According t o H a t t o r i (36),
coagulation o f t h e p a r t i c l e s p l a y s a more important r o l e t h a n t h e chemical bonding i n slump losses. Experiments on t h e CjS
-
SMF-
H0
system have shown t h a t r a p i d l o s s o f slump occurs i n t h ~ s system ( 3 f ) . Thus t h e c o n t r i b u t i o n o f t h e C3S phase should a l s o be t a k e n i n t o account. A1 1 these mechanisms i n v o l v e some s o r t o f i n t e r a c t i o n w i t h t h e s u p e r p l a s t i c i zer.
One o f t h e methods o f m a i n t a i n i n g t h e slump i n s u p e r p l a s t i c i z e d concrete i s t o add r e t a r d e r s such as calcium gluconate t o cement (38).
It i s p o s s i b l e t h a t t h e r e t a r d e r , although n o t i n t e r f e r i n g w i t h d i s p e r s i o n caused by t h e s u p e r p l a s t i c i z e r , adsorbs on t h e cement components, a f f e c t i n g t h e chemical o r p h y s i c a l processes t h a t cause agglomeration o r i n t e r l o c k i n g o f t h e cement p a r t i c l e s .
3. CONCLUSIONS
Chemical admixtures i n aqueous s o l u t i o n s o r i n t h e s o l i d form may i n t e r a c t w i t h t h e h y d r a t i n g cement compounds i n t h e cement
-
water-
admixture system. Physical,
chemical and mechanical p r o p e r t i e s o f t h e cement paste a r e a f f e c t e d t o d i f f e r e n t e x t e n t s by these i n t e r a c t i o n s .I n many instances
,
conclusions on i n t e r a c t i o n processes a r e s p e c u l a t i v e because t h e y a r e based on i n d i r e c t evidence. A study of i n t e r a c t i o n s o c c u r r i n g i n systems c o n t a i n i n g more than one admixture becomes even more complex. Even t h e so-cal l e d s i n g l e component commercial admixture may c o n t a i n small amounts o f chemical t h a t c o u l d i n t e r f e r e w i t h t h ei n t e r a c t i o n s o f t h e main component. D i sagreement i n r e p o r t e d r e s u l t s on i n t e r a c t i o n s i n cement systems may be t r a c e d t o t h e v a r i a b i l i t y i n t h e c h a r a c t e r s t i cs o f t h e s t a r t i n g m a t e r i a l s , methods o f curing,
t e s t i n g methods and i n t e r p r e t a t i o n .
P r a c t i c a l consequences o f an understanding o f t h e cement-admi x t u r e
i n t e r a c t i ons i nclude: development o f methods t o e s t i m a t e admi x t u r e s i n f r e s h and hardened concrete, understanding o f t h e c o m p a t i b i l i t y between two o r more admixtures i n concrete, p r e d i c t a b i l i t y o f p r o p e r t i e s of concrete, t r o u b l e shooting, p r o d u c t i o n o f b e t t e r concrete, development o f new t y p e s o f admixtures, standards development, and b e t t e r
u t i l i z a t i o n o f waste and marginal m a t e r i a l s i n concrete.
4. ACKNOWLEDGEMENT
This paper i s a c o n t r i b u t i o n o f t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Counci 1 Canada.
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T h i s p a p e r i s b e i n g d i s t r i b u t e d i n r e p r i n t f o r m by t h e I n s t f t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t of b u i l d i n g p r a c t i c e and r e s e a r c h p u b l i c a t i o n s a v a i l a b l e from t h e I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e ~ u b l i c a t i k n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l Research C o u n c i l of C a n a d a , O t t a w a , O n t a r i o , KlA 0R6. Ce document e s t d i s t r i b u E s o u s forme d e t i r l - 3 - p a r t p a r 1 ' I n s t i t u t de r e c h e r c h e e n c o n s t r u c t i o n . On p e u t o b t e n i r une l i s t e d e s p u b l i c a t i o n s de l t I n s t i t u t p o r t a n t s u t l e s t e c h n i q u e s ou les r e c h e r c h e s e n m a t i 2 r e de b l t i m e n t e n f c r i v a n t