adding gypsum to fly ash - shondeep shorkar

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CEM ENT and CONCRET E RESEAR CH, Vol. 21, pp. 1137-1147, 1991. Pr inted in the USA.

0008-8846/91. $3.00+00. Copyright (c) 1991 Pergamon Press plc.

MICROSTRUCTURAL STUDY OF GYPSUM ACTIVATED FLY ASHHYDRATION IN CEMENT PASTE

Xu Aimin

Division of Building MaterialsChalmers University of Technology, Grteborg, Sweden

Shondeep L. SarkarDepartment of Civil Engineering, Universit6 de Sherbrooke, Qurbec, Canada

(Communicated by G.G. Litvan)

(Received M ay 30, 1991)

ABSTRACTThe addition of 3 to 6% gypsum to a low alkali, low C3 A cement blended with low(30 %) and high (60%) volume of a Class F fly ash (FA) led to a distinct increase instrength in comparision to the blends without additional gypsum. This is discussed interms of the reaction of the FA with cement and gypsum. The role of FA alkalies isalso described. The investigation included scanning electron microscopy/energydispersive X-ray analysis (SEM/EDXA) to study the microstructural development ofFA replacement pastes with progressive hydration, and X-ray diffraction (XRD) formineralogical identification of hydrated phases.

Introduction

Fly ash, particularly the Class F-FA, as a partial replacement of cement has been widely used inconcrete for over half a century [1] because of its beneficial effect in lowering the heat of hydrationin mass concrete, which in turn reduces the thermal expansion. Sulfate resistance of concrete alsoincreases when Class F-FA is incorporated [2]. Lately, the use of high volume FA concrete hasgained increasing attention due to energy conservation, economic and ecologic considerations, andthe accumulated experience gained in using this product [3, 4]. It has been reported that thestrength of concrete containing Class F-FA is generally low at early age, and even at late age insome cases [5]. This is attributed to the slow hydration and incomplete pozzolanic reaction of FAresulting in a porous matrix [6].

Although in a FA blended cement system the latter can act as an activator, the relatively lowpercentage of strength gain associated with it strongly indicates the need for an additionalcomponent to activate the hydrolysis ability of low calcium FA. The possibility of FA activationmainly lies in breaking down its glass phases. Fraay et al. [7] observed that in presence of limeaqueor, the pH value required to dissolve the alumina and silica is about 13.3 or higher. The usualway of achieving a high pH by the addition of NaOH in FA, however, supresses the solubility of

CH [7] due to the common ion effect, thus reducing the Ca 2+ concentration in the solution. Thiscan also lead to some uncertainty in the hydration process: substitution of sodium for calcium in

1137



1138 Xu Aimin and S.L. Sarkar Vol. 21, No. 6

C-S-H can occur [8]. It is also well known that alkalies in cement increase the early strength at theexpense of the later age strength [9].

Among the different ways of FA activation reviewed by Uchikawa [10], the addition of excessgypsum was found to be suitable for FA-cement blends. The principle underlying activation bygypsum is based on the ability of the sulfate ions to react with the alumina, the latter being one ofthe principal components in most Class F-FAs. This results in dissociation of the glass structure.The known benefit of using low calcium FA in enhancing the sulfate resistance of concreteindicates that the reaction between sulfate and FA may also lead to a denser structure [ 11].Research by Huang [12] on the properties of a FA cement (FA content 20% by weight of thecement) containing 4% (by weight of the cement) additional gypsum has shown that the 28 daystrength increases significantly, and the pores in the paste are filled with ettringite (AFt) andmonosulfoaluminate hydrate (AFm). Similar results by others were reported elsewhere [6]. Thisled the authors to investigate the possibility of using a much higher FA replacement ratio (up to

60% by wt.) in order to enhance still further the activation properties.

The primary objective of the present work was to study this effect in terms of microstructuraldevelopment between ages 1 and 90 day, together with mineralogical identification of the reactionproducts. Since a cement high in C3A can generate ample calcium aluminosulfates on its own(during hydration), and reportedly FA enhances the hydration of C3A in FA-cement blends [6], avery low C3A cement was deliberately chosen to reduce this effect, and its alkali content was alsolow in order to amplify the role of FA alkalies.

Experimental

Materials

A cement low in alkalies and C3A, corresponding to ASTM Type V cement, and a Class F-FAcombustioned from bituminous coal were used (Table 1). The gypsum added was 98.5% pure.

Table 1 Chemical compositions of the cement and fly ash (wt %)

CaO SiO2 A1 203 Fe203 M~O SO3 K20Cement 64.2 22.3 3.46 4.82 0.84 2.06 0.69 0.03Fly ash 7.2 47.3 22.8 9.31 4.00 1.00 2.60*

Na20

* As the total alkalies.

The cement of 289 m2/kg Blaine surface area and mean particle size of 24 gm (determined by thesuspension gravity method) was slightly finer than the FA whose corresponding properties were

230 m2/kg and 33 I.tm. The ignition loss of the cement and FA was 2.46% and 3.50%respectively. The mineralogical composition of the cement was 61.7% C3S, 17.4% C2S, 14.7%C4AF and only 1.0% C3A. The FA contained 55% glass, the rest being composed of crystallinephases, namely 10% quartz, 10% mullite, 9% hematite, 4% periclase, and less than 1% arcanite,thenardite and anhydrite (determined semi-quantitatively by XRD).

SamNes

Five different proportions of cement - FA - gypsum blends were mixed at the weight ratio ofcement:FA:gypsum = 100:0:0 (control), 70:30:0 (low volume replacement, no additional gypsum),40:60:0 (high volume replacement, no additional gypsum), 70:30:3 (low, with gypsum) and40:60:6 (high, with gypsum). The water-to-binder (cement, FA and gypsum) ratio of all the mixeswas kept constant at 0.465. High gypsum : FA weight ratio (1:10) was used in this series ofexperiments to provide adequate amount of sulfate ions to react with the alumina, but the totalsulfate content in the binder was within the limit of the sulfate resistance requirement (4.5% SO3,ASTM C150).

Mortar samples for compressive strength test were proportioned at sand (standard quartz sand,max. diameter 2 mm)-to-binder ratio 2.5:1. Prisms (40 x 40 × 160 mm) were prepared according



Vol. 21, No. 6 GYPSUM-ACYIVATED FLY ASH, CEMENT PASTE, MICROSTRUCTURE 1139

to ASTM C 348, and water cured after demolding. The compressive strength was tested at ages 1,3, 7 and 28 day on the specimens left after flexural strength test. Paste samples were specificallyused for microstructural examination and phase identification to avoid the peak interferencenormally induced by the sand fraction (quartz and feldspar minerals) when analyzing mortarsamples by XRD. The same water-to-binder ratio and curing condition was used for the pastes,and these samples were studied at ages 2 hr, 1, 3, 7, 28 and 90 day.

M~thods

The compressive strength was tested according to ASTM C 349 standard. Morphology, phaseassociation, and elemental composition of hydration products were studied on fractured specimensby a JEOL JSM-840A SEM/Link 100085 EDXA. Phase identification and semi quantitativeanalysis of the hydration rate were carded out using a Rigaku D-Max XRD (Cu Kct radiation, 40kV, 35 mA).

R~sults

Compressive ~gength

Figure 1 shows the compressive strength development of the different mortars used in this study.It is obvious that replacement of cement by FA gives lower strength at ages 1 to 28 day incomparision to the plain cement mortar. Nevertheless, the strength of the low volume FA mortarwith 3% gypsum shows a distinct increase in strength by 28 day that is nearly as high as thecontrol sample (38.7 MPa compared to 40.8 MPa). This is followed by the low volume FA mixwithout gypsum (34.9 MPa at 28 day). The strength of the high volume FA replacement mortars

remain low at ages 1 to 28 day. However, the strength of the mortar with 6% gypsum can be seento increase rapidly from 7 day onwards, and surpasses that of the one without gypsum at 28 day.

1 3 . .

v

t- -

t - -

5 0

C'F :G

1 0 0 : 0 : 0

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7 0 : 3 0 : 0

3 0

4 0 : 6 0 : 6

2 0 4 0 : 6 0 : 0

lO

0 •. . . . . . . ! , • • , • , ,

1 3 7 28Time (day)

Fig. 1 Compressive strength development of mortar samples, where C=cement, F=FA andG=gypsum.

Rate of hydration from X-rav diffraction analysis

The XRD patterns of the samples with high FA content and gypsum at ages 8 hr to 90 day areshown in Figure 2 as a typical illustration (for all the samples examined in this work). It can beseen that with the progress of hydration, cement components and gypsum (in the pastes containingadditional gypsum) decrease, whereas hydration products, such as CH, C-S-H, C4AH13, AFt andAFro begin to form. It should be pointed out that AFt develops at a rather slow rate and onlybecomes clearly distinguishable at 90 day, while AFm peaks still remain very weak despite the fact



1140 Xu Aim in and S.L . Sarkar Vo l. 21, No. 6

t h a t g y p s u m d i m i n i s h e s s i g n i f i c a n t l y b y 7 d a y ( F i g . 3 ) . T h i s i s i n c o n t r a s t t o t h e f i n d i n g s o fB u d n i k o v [ 13 ] o n t h e r e a ct io n b e t w e e n C 3 A a n d g y p s u m , w h e r e t h e f o r m a t i o n o f A F t s ta rt edi n st a n tl y . I n t h e p r e s e n t s t u d y , i t w a s o b s e r v e d t h a t t h e h i g h F A p a s t e p r o d u c e s s i g n i f ic a n t l y m o r eA F t a t 9 0 d a y t h a n th e l o w F A p a s t e , a l t h o u g h t h e g y p s u m : F A r a t i o w a s c o n s t a n t f o r b o th . O n t heo t h e r h a n d , th e h i g h F A p a s te w i t h o u t g y p s u m p r o d u c e s m u c h m o r e c a l c i u m a l u m i n o - h y d r a t ep h a s e t h a n th a t w i t h g y p s u m a t 9 0 d a y .

C l o s e e x a m i n a t i o n o f t h e c e m e n t a n d F A m i n e r a l p e a k s i n th e X R D p a t t e r n s s h o w e d t h a t f r o m 8 ht h e c e m e n t c o m p o n e n t p e a k s ( b e t w e e n 2 9 ° a n d 3 5 ° 2 0 ) d e c r e a s e i n h e i g h t b u t i n c r e a s e in w i d t h ,i n d ic a ti n g t h e i r g r a d u a l c o n v e r s i o n t o C - S - H , t o g e t h e r w i t h a s m a l l re d u c t i o n i n p e a k h e i g h t o f F Aquar tz and mu l l i t e (25 ° to 27 ° 20).

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F i g . 2 . X R D p a t t er n s f o r t h e p a s t e c o n t a i n i n g h i g h F A a n d g y p s u m f r o m 2 h t o 9 0 d a y .

T h e h y d r a t i o n k i n e t i c s c a n b e b e t t e r u n d e r s t o o d f r o m F i g u r e 3 i n w h i c h t h e c a l c u l a te d i n t e g r a t e di n te n s i ti e s o f s e le c t e d C H a n d g y p s u m p e a k s a r e p l o t te d a s a f u n c t i o n o f h y d r a t i o n t im e . I t c l e a r lys h o w s t h a t C H i n c r e a s e s r a p i d l y b e t w e e n 8 h a n d 1 d a y . A s a m a t t e r o f i n t e re s t , a t 1 d a y t h e F Ap a s te s ( e s p e c ia l ly th e o n e s w i th g y p s u m ) s h o w C H c o n t e n t h i g h e r th a n t h e " h y p o t h e t i c a l" v a l u e( CHpla in m ix x C / ( C+ FA) b l ended m i x, w he r e C= cem en t ) . I t c an a l s o be s een t ha t eve n a t 8 h ( c l ea r l ys h o w n b y th e C H ( 0 0 1 ) p e a k ), t h e C H i n th e l o w F A p a s te i s n e a rl y a s m u c h a s i n t h e c o n tr o l.T h i s is in a g r e e m e n t w i t h t h e r e s u l ts o f L a r b i a n d B i j e n [1 4 ] . T h o u g h t h e ( 0 0 1 ) p e a k i n t e n s i t ys h o w s a d i s t in c t in c r e a s e f r o m 1 t o 7 d a y , t h e ( 1 0 1 ) i n t en s i t y d o e s n o t i n c r e a s e a s m u c h . F r o m 7d a y , h o w e v e r , b o t h t h e p e a k s b e g i n t o d e c r e a s e , a n d a t 9 0 d a y t h e C H c o n t e n t i n t h e p a s t e s w i t ha d d i ti o n a l g y p s u m i s m u c h l o w e r t h a n t h o s e w i t h o u t t h e g y p s u m . I t i s a ls o e v i d e n t t h a t s u b s ta n t ia la m o u n t o f g y p s u m i s c o n s u m e d b y t h e t i m e C H r e a c h e s i t s m a x i m u m l e v e l . T h o u g h g y p s u mc o n t e n t b e g i n s t o d e c r e a s e r a p i d l y f r o m 1 d a y , i t s c o n s u m p t i o n is f a s t e r in t h e l o w v o l u m e F A

p a st e. T h e a m o u n t o f g y p s u m a n d C H d e t e r m i n e d b y X R D w a s c o n f i r m e d f r o m d i f fe r e n ti a lt he r m a l ana l ys is , t hough t he r e s u l ts o f t he l a t te r have no t been p r e s en t e d he r e .

M i c ro s m a ct ur e fr o m S E M / E D X A

F r o m S E M m o s t o f t h e F A w e r e f o u n d t o b e s m o o t h , r o u n d p a r t ic l e s , w i t h a s m a l l f ra c t i o n o fc e n o s p h e r e s , a n d o n l y a f e w i r r e g u l a r s h a p e d g r a in s . I n g e n e r a l , t h e p a r t ic l e s c o n t a i n r a t h e r h i g hK , A 1 a n d S i a s d e t e c t e d b y E D X A . T h i s i s in a c c o r d a n c e w i t h t h e c h e m i c a l a n a l y s is r e s u l ts ( T a b l e1 ). Som e pa r t ic l e s wi t h r ou gh s u r f ace t ex t u r e a r e r i ch in Fe .



Vol. 21, No. 6 GY PSU M-A CTIV AT EDLY ASH , CEMENTPASTE,MICROSTRUCTURE 1141

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F i g. 3 C H a n d g y p s u m c o n t e n t in t h e p a s te s a t d i ff e r e n t a g e s a s d e t e r m i n e d b y X R D .

T h e m i c r o s t r u c t u r e o f t h e p a s te s a m p l e s s h o w t h a t th e c e m e n t a s o p p o s e d t o F A , d e v e l o p sh y d r a t i o n s y m p t o m s w i t h i n 2 h ( F i g . 4 ). T h o u g h v e r y s l i g h t r e a c t i o n , o r s u r f a c e d i s s o l u t i o n ( th a tg i v e s a m i l d e t c h i n g e f f e c t ) o n s o m e F A p a r t i c l e s w e r e o b s e r v e d , m o s t o f t h e m s t il l r e t a in t h e i rs m o o t h s u r f a c e i n d i c a t in g t h e n o n r e a c t i v i t y o f t h e F A a t s u c h a n e a r l y a g e . U n d i s s o l v e d p l a t yc r ys t a l s o f gyp sum w er e a l so r ead i l y i den t i fi ab l e in t he pa s te s ( w i t h add i t i ona l gypsu m ) .

F i g .4 . T h e p a s t e w i t h h i g h F A c o n t e n t a n dg y p s u m a t 2 h h y d r a t i o n . I = F A , 2 = c e m e n t ,3 = g y p s u m .

F i g . 5 . C H , p a r t l y h y d r a t e d c e m e n t g r a i n sa n d C H c o v e r e d F A in t h e p a s t e w i t h lo wF A w i t h o u t a d d i t io n a l g y p s u m a t 1 d a y .I = F A , 2 = c e m e n t , 3 = C H .

A t 1 d a y , t y p i c a l c e m e n t h y d r a t i o n p r o d u c t s , C H , C - S - H , a n d H a d l e y g r a in s t r u c t u r e a p p e a r i n th ep a s te s t o g e t h e r w i t h s h o r t a c ic u l a r a n d n e e d l e - l ik e c r y s t a ls o n c e m e n t a n d F A p a r t ic l e s ( F i g .5 ) . T h en e e d l e s, t h o u g h s u s p e c t e d t o b e e t t r in g i te , a r e t o o f i n e t o b e a n a l y z e d b y E D X A , a n d t h e l at te r d o e s

n o t a p p e a r in t h e X R D p a t t e rn . T h e n e e d l e - l i k e c ry s t a ls , h o w e v e r , r a r e l y d e v e l o p a t t h i s a g e in t h ep a s te s w i t h h i g h F A a n d a d d i t io n a l g y p s u m , w h i c h o n t h e o t h e r h a n d e x h i b i t s h o rt a c ic u l a r g r o w t hc o m p o s e d o f C a , S i , A 1 a n d S o n t h e g r a in s u r f a c e ( F ig . 6 ). I ts m o r p h o l o g y c l o s e l y re s e m b l e sT y p e I C - S - H . T h i s h y d r a t i o n p r o d u c t is m u c h t h i c k e r b u t l e ss d e n s e t h an i n it s c o u n t e r p a r tw i t h o u t g y p s u m . I t c a n b e s e e n f r o m F i g u r e 6 t h a t t h e r e a re u n i f o r m l y d i s tr i b u t ed c a p i l la r ie s , 1 t o 3I .tm i n s i z e be t w e en t he g r a i ns m ak i ng t he pa s t e r a t he r po r ous . S om e p l a t y c r ys t a l s i n t h is f i gu r e a r eA F m p h a s e. A n u m b e r o f g y p s u m c r y s t a l s h a v e n o t c o m p l e t e l y d i s s o l v e d , b u t d e v e l o p s u r f a ce. p re c ip i ta te s o f s h o r t a c i c u l a r c r y s t a l s ( F i g . 7 ). I d e n t i f i c a t io n o f F A p a r t i c l e s b y E D X A s h o w s a ni n c r e a s e m C a c o n t e n t , w h i c h m a y b e a t t r i b u t e d t o C H o r C - S - H f o r m a t i o n , o r a c o m b i n a t i o n



1142 Xu Airnin and S.L. Sarkar Vol. 21, No. 6

t he reo f . D iam ond [15 ] c l a ims the f ib rou s fea tu re on the FA pa r t i c l e s a t e a r ly ag e hyd ra t ion to bep a r t l y C H d e p o s it s . C o m p a r e d to t h e ( 1 0 1 ) C H p e a k i n t e n s i t y , t h e h i g h e r ( 0 0 1 ) i n t e n s i t y s t ro n g l y

sugges t s p re fe r red o r i en ta tion o f CH c rys ta l s , such a s those on the FA su r face s .

F i g .6 . T h e p a s t e w i th h i g h F A a n d g y p s u ma t 1 day . 1= F A , 2 = c e m e n t, 3 = A F m .

F i g . 7 . P r e c i p i t a t e s o n a g y p s u m c r y s t a l i nt h e h i g h v o l u m e F A p a s t e c o n t a i n i n ga d d i ti o n a l g y p s u m a t 1 d a y .

At 7 day , the mic ro s t ruc tu re o f the con t ro l and the low F A pas te s i s ve ry s im i la r , i .e . , i t becom esm u c h d e n s e r w i t h p r o n o u n c e d f o r m a t i o n o f n e e d l e - l i k e c ry s t a l s, r e t i c u l a r a n d a c i c u l a r fe a t ur e s ,r e s e m b l i n g T y p e I a n d T y p e I I C - S - H ( F i g . 8 ) . A t t h i s a g e , u n r e a c t e d - t o - s l i g h t l y e t c h e d F Ap a r t i c l e s e m b e d d e d i n m a s s i v e t a b u l a r C H o b s e r v e d b y o t h e r r e s e a r c h e r s [ 1 6 ] a r e c o m m o n ,e s p e c i a ll y in t h e p a s te s w i th o u t a d d i ti o n a l g y p s u m . F i g u r e 9 s h o w s t h a t in t h e l o w F A a n d g y p s u mpas te , in add i t ion to the th ickenn ing o f the FA su r face coa t ing , abun dan t ne ed le - l ike c rys ta ls fo rm.T h i s c o i n c i d es w i t h th e c o n s u m p t i o n o f m o s t o f t h e g y p s u m ( as s e e n f r o m X R D ) . T h e s m o o t he x p o s e d p a r ts o f s o m e o f t h e F A p a r t ic l e s ( F i g .1 0 ) s u g g e s t s th a t t h e F A h a s n o t y e t e n t e r ed i n to a nac t ive s t a te o f r eac t ion . The re l i c t s truc tu re o f gyp sum c ry s ta l s w i th d i s t inc t need le fo rm a t ion wasc l e a r l y id e n t i f ia b l e ( F ig . 10 ) . E D X A s h o w s t h a t t h e re i s a s i g n i f i c a n t a m o u n t o f S i i n t h e t h ic k e s tp a r t o f t h e s e n e e d l e s , w i t h C a : S i : A I : S = 1 : 0 . 7 6 : 0 . 3 6 : 0 . 5 . T h i s i s a p p r o x i m a t e l y t h e e t t r i n g i t ecom pos i t ion , excep t fo r the S i .

A t age 28 day the re i s ev idence o f fu r the r hydra t ion in the p la in cem en t pa s te ; the hydra ted g ra ins

a re i n t e r c o n n e c t e d b y t h e o u t g r o w t h s f o r m i n g a c o n t i n u o u s s t ru c t u r e ( F ig . 1 1 ). T h e h i g h F A p a s tewi th no add i t iona l gyp sum a l so exh ib i t s a c icu la r f ea tu re s , excep t tha t the f ib re s a re m uch longe r, a si s s h o w n i n F i g u r e 1 2. A n u m b e r o f F A p a r t ic l e s a p p e a r t o h a v e r e a c t e d a l m o s t c o m p l e t e l y w i t hh y d r a t i o n p r o d u c t s s u c h a s A F t f i l l in g t h e i n n e r s p a c e . E D X A a r o u n d F A s h o w s p r o d u c t

F ig .8 . P la in cem en t pa s te mic ros t ruc tu re a t7 d a y .

F i g . 9 T h e p a s te w i t h l o w F A a n d g y p s u ma t 7 d a y , s h o w i n g s u r f a c i a l g r o w t h o n F A ,and abund an t need le - l ike c rys ta l s .



Vol. 21, No. 6 GYPSUM-ACTIVATED FLY ASH, CEMENT PASTE, MICROSTRUCTURE 1143

Fig.10. Coated FA and relict structure ofgypsum in the low FA and gypsum paste.I=FA, 2=gypsum.

Fig.11. Control paste at 28 day. AbundantC-S-H forming a continous structure.

Fig.12. The paste with high FA but nogypsum at 28 day.

Fig.13. AFt filling the interior of a FAparticle (high FA and gypsum paste) at age28 day.

composition of approximately Ca:Si:AI:S = 1.00:1.03:0.63:0.50. Figure 13 shows such a particle,

with profuse well-crystallized AFt roZts (inside the sphere) as the replacement of the original FA.At this stage the AFt becomes identifiable in the XRD pattern of this paste. The dendritic mullitecrystals on the surface of FA is similar to that observed by Tenoutasse and Marion [17]. Thisindicates that it is the inert crystalline phases on the surface layer that retain the spheric shape of FAduring hydration. The additional gypsum in the paste (with gypsum) was observed to havehydrated as much as the low FA paste at 7 day.

At 90 day, more cenospheric structures and ruptured sections of FA particles become visible. Thismay be due to an increase in bonding between FA particles and the surounding paste at this age,which results in through-fracturing during sample preparation. The inward growing hydrationproducts become vivid in the pastes (Fig. 14), and the complete structure resembles "geode". Thereticular structure inside the FA is uniformly composed of Ca-Si-A1-S, whereas the wails of FAparticles contain less Ca, but higher K and A1 compared to the inner products. It is interesting tonote that dissolution of surfacial alkalies, particularly K20, observed by others [17] was marginalin this FA, even at 90 day. At this age, the paste becomes very dense. Different types of C-S-Hand several FA particles in a highly advanced hydration state can be seen in Figure 15. Thoughsome unreacted FA particles still remain, their amount is significantly lower than at the early age. Itwas difficult to estimate the number of small FA panicles that underwent inner diffusion inducedhydration, mainly observed in the larger particles (20-40 p.m) (Fig 15). The pastes develop auniformly interlinked dense structure partly contributed by the reacted FA, among which manybeen completely consumed. The space between the hydration products is smaller than 0.5 ~tm



1144 Xu Aimin and S.L. Sarkar Vol. 21, No. 6

F i g . 1 4 . I n n e r h y d r a t i o n p r o d u c t s i n t h e l o wF A p a s te ( n o g y p s u m ) a t 9 0 d a y . F i g . 1 5. D e n s e p a s t e i n t h e h i g h F A - g y p s u mp a s t e a t 9 0 d a y . F r a c t u r e d F A s a r e f i l l e dw i t h h y d r a t i o n p r o d u c t s .

F i g . 1 6 . H i g h e r m a g n i f i c a t i o n s h o w s

an i n t e r l i nked s t r uc t u r e .

i n d i c a t in g a m u c h l e s s p o r o u s s t r u c t u r e ( F i g 1 6 ) c o m p a r e d t o t h a t a t e a r l y a g e s . T h e A F m p h a s et houg h no t tha t e a s i l y de t ec t ab l e in a l l, cou l d occas i ona l l y be s een i n t he po r e s o f the h i gh F A pas t e

w i t h o u t a d d i ti o n a l g y p s u m .

D i scus s i on

T h i s s t u d y i n d i ca t e s t h a t t h e re a c t i o n o f F A w i t h g y p s u m p o s i t i v e l y p la y s a n i m p o r t a n t r o le ind e n s i f y i n g t h e m i c r o s t r u c t u r e , d e s p i t e t h e l o w e r c e m e n t c o n t e n t in t h e s e F A - r e p l a c e d c e m e n t i t i o u ss y s t e m s . T h i s l e a d s t o a n i n c r e a s e in s t r e n g t h a c c o r d i n g t o t h e k n o w n d e p e n d e n c e o f s t re n g t h o n

t h e p o r o s i t y a n d p o r e s t r u c t u r e o f a c e m e n t i t io u s m a t r ix . T h e r e a c t i o n o f g y p s u m i n t h e c e m e n t - F A

p a s te c a n b e d i v i d e d i n t o t w o s t a g es .

I t i s p o s t u l a t e d t h a t a t t h e e a r l y a g e , g y p s u m i s n o t th a t e f f e c t i v e a n a c t i v a t o r u n t i l i t r e a c h e s t h ed i s s o lv e d s ta t e, w h i c h o c c u r s o n l y w h e n C H f o r m a t i o n h a s s lo w e d d o w n . T h i s c a n b e e x p l a i n e d

b y t h e s o lu b i li ty p r o d u c t s o f C H a n d C a S O 4 , w h i c h a r e 4 .6 8 × 1 0- 6 and 7 . 10 × 10 -5 r e sp ec t i v e l y

[ 1 7 ]. F r o m t h is i t c a n b e d e r i v e d t h a t t h e C a 2 + c o n c e n t r a t i o n a t w h i c h t h e p r e c i p i t a t io n o c c u r s i s

l o w e r ( 0 . 0 0 8 4 m o l /1 ) f o r g y p s u m t h a n C H ( 0 . 0 1 6 7 m o l /1 ). T h u s , t h e C a 2 + i o n s r e l e a s e d f r o m t h e

c e m e n t c o m b i n e w i t h t h e S O 4 2- t o f o r m g y p s u m ( u n l e s s A I 3 + i s r e a d i l y a v a i l a b l e t o f o r m c a c i u ma l u m i n o s u l f a t e p h a se s ) . T h e p r e c i p i t a t i o n o f C H o c c u r s w h e n t h e n e c c e s s a r y s u p e r s a tu r a t io n l e v e l

i s e s t a b l i s h e d , a n d c o n t i n u e s u n t i l th e s u p p l y o f C a 2 + b e c o m e s d e p l e t e d . T h i s r a p i d f o r m a t i o n o fC H , h o w e v e r , o c c u r s o n l y a t t h e v e r y e a r l y a g e (u p t o 1 d a y ) w h e n t h e c e m e n t a c ts a s t h e p r in c i p a l

s u p p l i e r o f C a 2 + f r o m t h e h y d r a t i o n o f t h e a li te c o m p o n e n t . A t t h e e n d o f t h is p e r i o d , g y p s u m

r a p i d l y d i s s o l v e s t o b a l a n c e t h e O H - i o n s . T h u s t h e S O 4 + 2 c o n c e n t r a t i o n i n th e s o l u t io n i n c r e a s e s ,m a k i n g t h e c o n d i t i o n s u i t a b le f o r r e a c t i o n w i th t h e a l u m i n a p h a s e i n t h e F A t o f o r m a l u m i n o su l fa t e .



V o l . 2 1 , N o . 6 G Y P S U M - A C T I V A T E D F L Y A S H , C E M E N T P A S T E , M I C R O S T R U C T U R E 1 14 5

G y p s u m d i s s o lu t io n s h ow n i n F i g u r e 3 , w h i c h o c c u r s s i m u t a n e o u s l y w i t h C H f o r m a t i o n a n d isa c c e le r a te d w h e n t h e r a te o f C H p r o d u c t i o n b e c o m e s s l o w e r c o n f i r m s t h e a b o v e s t a te m e n t . T h en o n h y d r a u l i c n a t u r e o f g y p s u m m e r e l y m a k e s i t a f i l le r m a t e r i a l a t th e e a r l y a g e , a n d d o e s n o tcon t r ibu te towards s treng th . I t i s the cem en t com pon en t wh ich g ives s t ren g th a t the ea r ly age . Th i sa ls o e x p la i n s th e l o w 1 d a y s t r e n g t h o f t h e h i g h v o l u m e F A s y s t e m s ( w i t h l e s s e r ce m e n t ) .

The a lka li e s in th i s FA a re re t a ined in the g la s s phase , and do no t pa r t i c ipa te in the i r so lub le fo rm int h e h y d r a t i o n p r o c e s s u n ti l t h e b r e a k d o w n o f t h e g l a ss s t ru c t u r e . T h i s i s i n a g r e e m e n t w i t h B e r r ya n d H e m m i n g s ' f i n d in g s , t h o u g h a n u m b e r o f o t h e r r e s e a r c h e r s n o t e d t h e p r o m p t d i s s o l u t i o n o fK 2 0 f r o m t h e F A s u rf a ce [ 18 ] . I t m u s t b e p o i n te d o u t t h a t F A i s i n d e e d a c o m p l e x m a t e r i a l w h o s ehydra t ion p rope r t ie s depend on s eve ra l f a c to r s , such a s the ty pe o f coa l , com bus t io n o r ga s i f i c a tiont e m p e r a tu r e , a n d t h e r at e o f c o o l i n g , e t c . E v e n i n t h e s a m e b a t c h o f F A , d i f f e r e n t p ar t ic l e s b e h a v ed i f fe ren t ly , and i t is com m on to f in d s eve ra l types o f r eac ted and un reac ted F A pa r t ic l e s in the s ame

spec imen . Neve r th le s s , i t i s pos s ib le fo r the a lka l ie s on the su r face o f F A to inc rea se the loca l OH-

concen t ra t ion , wh ich in tu rn sup re s se s the so lub i l i ty o f CH [14] in th i s zon e (acco rd ing to the we l lk n o w n c o m m o n i o n e f fe c t ), c a u s i n g e a r l y p r ec i p i t at i o n o f C H o n t h e F A s u r f ac e . T h i s p r o v i d e snuc lea t ion cen te rs fo r more an d f ine r c rys ta l l i z a t ion on the su r face , a s s een f rom the dense l aye r inF igure 6 , com pr i s ing minu te c rys ta l s .

B e f o r e d i s c u s s in g t h e r o le o f g y p s u m a t la t e r a g e s , it s h o u l d b e n o t e d t h a t a t t h e e a r ly a g e t h o u g hm o r e C H i s p r o d u c e d i n t h e p a s t e w i t h F A a n d g y p s u m t h a n t h a t w i t h o u t a d d i t i o n a l g y p s u m , l es s

o f i t i s r e t a i n e d a t th e l a te r a g e . T h e m e c h a n i s m r e l a t e d t o t h i s m a y b e 6 C a ( O H ) 2 + 3 SO 4 2+ +2Al (OH)3 ~ E t t r ing it e , a s sugges ted by Be r ry e t a l . [19 ], i . e. , t he fo rm a t ion o f e t t r ing i te consume sm o r e C H . I t i s s e e n th a t b e t w e e n 3 a n d 2 8 d a y , t h o u g h t h e a m o u n t o f g y p s u m d e c r e a s e ss i g n i f i c an t l y , t h e e x p e c t e d A F t p h a s e f o r m a t i o n i s v e r y l i tt le a s p e r X R D a n a l y s is . S E M

e x a m i n a t i o n , h o w e v e r , s h o w s s e v e r a l F A p a r t i c l e s , e s p e c i a l l y t h o s e p a s t e s w i t h a d d i t i o n a lg y p s u m , a re f i l l ed w i t h A F t - l i k e p ha s e s. S i n c e th e c e m e n t u s e d w a s e x t r e m e l y l o w i n C 3 A , a n d t h eg y p s u m d o s e n o t s h o w a n y e v i d e n c e o f a c t iv e r e a c ti o n w i t h F A a t a n e a r l y a g e , i t i s p o s s i b le t h a tt h e n o n a v a i l a b i l i ty o f s o m e o f t h e e s s e n t ia l c o m p o n e n t s r e s t r ic t s t h e f o r m a t i o n o f t h e A F t p h a se .

On ce the g la s s phase b reaks dow n a t a l a t e r age , su f f i c i en t quan t i ty o f A13+ f rom the FA, O H- f rom

r e d i ss o l u ti o n o f C H a n d S O 4 - f r o m d i s s o lv e d g y p s u m b e c o m e r e a d i l y a v a i la b l e f o r i t to f o r m .T h u s , t h i s a ls o r e su l t s in h i g h e r a m o u n t o f A F t f o r m a t i o n i n t h e h i g h F A p a s te w i t h g y p s u m .C-S-H i s a l so capab le o f inco rpo ra t ing a num ber o f fo re ign ions in i t s l a t t ic e [20 ]. I t i s pos s ib le tha ts o m e o f t h e s e c o n s t i t u e n t i o n s e n t e r t h e C - S - H g e l l a t t i c e , r a t h e r t h a n f o r m i n g c r y s t a l l i n esu l foa lum ina te p roduc t s .

W i t h p r o g r e s s i v e h y d r a t i o n , m o r e f r a c t u re d F A p a r t i c l e s b e g i n t o a p p e a r , w h i c h c a n b e e x p l a in e din te rms o f an inc rea se in bo nd in g be tw een FA and the su r round in g pas te . I t i s obv ious tha t un le s sthe bond ing s t reng th exceed s the s t reng th o f the FA she l l, t he pa r t i c l e s w i l l r em a in unc racked , and

t h e s a m p l e w i l l f ra c t u r e a l o n g t h e w e a k e r z o n e , i .e ., b e t w e e n t h e F A s u r f a c e a n d i ts c o a t in g . T h eth rough - f rac tu r ing s een f rom age 28 d ay a l so g ive s a be t t e r chan ce to obse rve the inne r pa rt s o f FApart ic les more c lear ly .

L a t e r a g e ( h e re i t s i g n i fi e s t h e a g e w h e n l a r g e a m o u n t o f t h e g y p s u m h a s d i ss o l v e d ) b r in g s a b o u t a

d i f f u s i o n c o n tr o l l e d r ea c t io n i n v o l v i n g t h e d i f f u s i o n o f S O 42 - f r o m t h e d i s s o l v e d g y p s u m a n d C a2+f r o m C H i n t o t h e F A a n d t h e s u b s e q u e n t d is s o c i a t io n o f a l u m i n a a n d s i l ic a t e f r o m t h e F A g l a ssp h a s e i n t o t h e s o l u t i o n . T h i s r e a c t i o n o c c u r s b o t h i n s i d e a n d f a r a w a y f r o m F A p a r t i c l e s . T h e

coex i s t anc e o f SO42- and a lum ina phase in a s t a te o f hydra t ion fu r the r enha nces the d i s soc ia t ion o fa l u m i n a . T h e f a c t t h a t th e F A p a s t e w i t h o u t a d d i ti o n a l g y p s u m d e v e l o p s m o r e C 4 A H 13 , w h e r e a st h e o n e w i t h g y p s u m h a s m o r e A F t p h a s e s u g g e s t s t h e r e a c t io n m o d e s a r e d i ff e r en t . I t c a n b e s a i dt h a t a m o r e t h o r o u g h F A h y d r a t i o n o c c u r s in t h e p a s t e s w i t h a d d i t i o n a l g y p s u m , g iv i n g r i s e to ad e n s e r m i c r o s t r u c t u r e , a s in d i c a t e d b y t h e i r h i g h e r s tr e n g t h . H o w e v e r , b e t w e e n 1 a n d 2 8 d a y t h ec e m e n t p l a y s a d o m i n a n t r o l e i n e n h a n c i n g t h e s t r en g t h . T h i s i s c l e a r l y d e m o n s t r a t e d b y t h e h i g h e rs t r en g t h o f t h e l o w v o l u m e F A m o r t a r s ( b ot h w i t h a n d w i t h o u t a d d i t i o n a l g y p s u m ) w h i c h c o n t a inh i g h e r f r a c t io n o f c e m e n t .



1 1 4 6 X u A i m i n a n d S . L . S a r k a r V o l . 2 1 , N o . 6 •

The gypsum-FA reaction process is said to involve the depolymerization of the dense glass

structure which facilitates further penetration of ions, such as Ca 2+ and SO42-, accompanied with

the exchange of glass network modifiers, e.g., K ÷ by Ca2÷ [21], leading to a higher pH condition.This in turn can cause an enhanced dissociation of the glass phase. Fraay et al. [7] proved thatwhen FA is surrounded by a porous paste, more outer diffusion of dissociated alumina from theFA will occur resulting in precipitation of the hydration products further away. The relict gypsumstructure containing AFt needles seen in some of these pastes strongly suggests this outer diffusionphenomenon to have occurred in the pastes under study. Since this system contains very low C3A(the total A1203 is only 3.5%, mainly in the C4AF, ref. Table 1), the needle-like AFt inside therelict must have formed from the alumina supplied by the FA.

This study also shows that lime or cement alone may not be sufficient to activate FA, as wasevidenced by the lower strength of the mortars containing only cement and FA. Besides, it was

observed that the FA particles encapsulated in CH often retain their smooth surfaces, even at laterages, e.g., 28 day, thus indicating their lower reactivity. This is in agreement with the findings of

Ghose and Pratt [22]. It can be said that the neccessary OH- concentration for the dissociation ofglass in FA is much higher than the saturation for CH (pH about 12.5).

Conclusions

SEM/EDXA and XRD studies show that the Class F fly ash used in this work, with and withoutadditional gypsum, has different hydration rates in terms of CH consumption. Additional gypsum(up to 6% by weight of the blended cement) accelerates the dissociation of the FA glass phase from7 day, when the precipitation of CH becomes slow. This happens by virtue of gypsum dissolution,which occurs not later than 7 day for the low FA mix, but takes longer for the high FA mix.

In the presence of excess gypsum, the hydration products of the FA and cement are composed ofCa, Si, A1 and S. Compared to ordinary Portland cement hydration, and the results of otherresearchers, the formation of AFt is much slower in these FA mixes. This is attributable to the low

quantity of A13+ available due to the low C3A cement used.

Many FA particles show development of inner hydration products suggesting ionic diffusionthrough the weak points of FA surface.

There is no distinct evidence of prompt dissolution of alkalies from FA surfaces as noted by manyother researchers. In this FA, the alkalies are retained within the glass phase. They, however, mayhave been released during hydration, creating locally high pH environment which lowers the

solubility of CH. This consequently can lead to the nucleation and precipitation of CH on the FAsurface.

The use of additional gypsum oppears to be beneficial in the terms of dissolving the glass phase inFA, but its optimum concentration in relation to the proportion and composition of FA and cementneeds to be carefully studied in order to derive the advantage of higher strength.

Acknqwl~dgements

The authors wish to acknowledge the financial support of Natural Sciences and EngineeringResearch Council of Canada for this work. XA would like to thank the Division of BuildingMaterials, Chalmers University of Technology.

1.

References

E.E. Berry and V.M. Malhotra, Fly Ash in Concrete, Publ. SP 85-3, CANMET, 1986.



Vol. 21, No. 6 GYP SUM -AC TIV ATE DLY ASH, CEMENT PASTE, MICROSTRUCTURE 1147

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D.C. Hughes , Cem. and Concr. Res., Vol. 15, 1985, pp. 1003-1012.S . Huang , Hydration of Fly Ash Cement and Microstructure of Fly Ash Cement Pastes,CBI R esearch fo 2 .81 , S tockho lm , 1981 .P .P . Budn ikov , Proc. Fourth Int. Symp. on the Chem. of Cem., W ashing ton , 2 , pp 469-477, (1960).J .A. Larbi and J .M. Bi jen , MRS Symp. Proc., 178, pp. 127-138, (1990).S . Diamond , D. Ra v ina and J . Lovel l , Cem. and Concr. Res., 113,pp. 297-300, (1980).D . G . M o n t g o m ery an d D . C . Hu g h es an d R .I .T . W i ll i am s , Cem. and Concr. Res., 11, pp.591-603, (1981).CRC Handbook of Chemistry and Physics, 68th ed. , pp . B-207-208, CRC Press 1987.N . Ten o u t a s s e an d A . M . M ar io n , Blended Cements, G. Frohnsdorf f , ed . ASTM STP 897 ,

pp. 65-85, (1986).E.E. Berry , R .T. Hemmings , W.S. Lang ley , and G.G. Care t te , Proc. Third Int. Conf. onFly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Trond heim , i , Pp. 241-273, (1989).W. Richar tz , Zement-Kalk-Gips, Vol. 22 (10), 1969, p. 447.R .T . H em m i n g s an d E . E . B e r ry , MRS Symposium Proc., 11~, pp. 3-38, (1987).A. Ghose and P .L. Pra t t , Effects of Flyash Incorporation in Cement and Concrete,S. Diamond, ed . , Mat . Res . Soc. , pp . 82-91, (1981).

adding gypsum to fly ash - shondeep shorkar

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