Variability of the nitrogen surface area of hydrated cement paste

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Cement and Concrete Research, 6, January 1, pp. 139-143, 1976-01-01

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Variability of the nitrogen surface area of hydrated cement paste

Litvan, G. G.

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S e r

THl

N21d

no.

669

c. 2

National Research

Conseil national

EsiG

_{Council Canada}

_{de recherches Canada}

YARLABILITY OF THE NITROGEN SURFACE AREA

I

OF HYDRATED CEMENT PASTE

, .

Reminted from

Cement and Concrete Research

5909s

Vol. 6, 1976

r. I

1

i

APR

14

1976

t

(

NATIONAL RESEARCH COUNCIL ; *

DBR

Paper No. 669

Division of Building Research

CEMENT and CONCRETE RESEARCH. Vol. 6, pp. 139-144, 1976. Pergamon Press, I n c . P r i n t e d i n t h e U n i t e d S t a t e s .

VARIABILITY OF

THE

NITROGEN SURFACE AREA OF HYDRATED CEMENT PASTE

G. G. Litvan

Division of Building Research

National Research Council of Canada, Ottawa

(Communicated by P. J. Sereda) (Received Oct. 24, 1975)

ABSTRACT

The difference between the BET specific surface areas computed from water and nitrogen adsorption isotherms of bottle hydrated cement paste were found to vanish if special techniques for drying the samples were applied. Typical features of the methods were replace- ment of the pore water by methyl alcohol which in turn was substituted by pentane and final drying was achieved by distillation either in vacuo or above the critical temperature. The nitrogen and water surface areas of the specially dried paste are both approximately 200 m2/g. It is suggested that the commonly observed discrepancy in surface areas is due not to deficiencies of the measuring method but

to real differences evoked by the particular drying,technique applied.

La diffsrence entre les aires de surface spgcifiques B.E.T. calculges

2 l'aide des isothermes d'adsorption d'eau et d'azote de la pste de

ciment hydratge dans une bouteille dispara?t lorsque l'on utilise des techniques spgciales de sgchage des gchantillons. Ces mgthodes comportent le remplacement de l'eau interstitielle par de l'alcool mgthylique, lui-msme remplac6 par du pentane, et un sgchage final par distillation sous vide ou bien au-dessus de la tempgrature critique. Les aires de surface de l'eau et de l'azote dans la pzte sgch6e de

facon spgciale sont toutes deux d'environ 200 m2/g.

a auteur

estime

que la divergence communBment observge des aires de surface est le rgsultat non pas de quelque lacune de la mgthode de mesure mais d'un &art r6el dG 2 la technique de sgchage utilis6e.

V o l .

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L i t v a n

The B E T s p e c i f i c s u r f a c e a r e a s of hydrated Portland cement computed from t h e water adsorption isotherm (1,2) a r e much l a r g e r than t h o s e c a l c u l a t e d from t h e n i t r o g e n isotherm (3

-

6 ) . The d i f f e r e n c e s a r e q u i t e s u b s t a n t i a l : although t h e n i t r o g e n a r e a values f a l l i n t h e range between 13 and 84 m2/g (5) t h e water a r e a i s t y p i c a l l y 200 m2/g (1,2). Values obtained from t h e adsorption of oxygen, argon ( 3 ) and o r g a n i c vapors a r e i n l i n e with t h e n i t r o g e n a r e a s : approximately 50 m2/g <or cyclohexane and isopropanol, and between 50 and 114 m2/g f o r methanol (7)'. The s p e c i f i c s u r f a c e a r e a values determined by small-angle x-ray s c a t t e r i n g , an independent p h y s i c a l method, a r e s i m i l a r t o t h a t o f t h e water a r e a f o r t h e d r y m a t e r i a l ( 8 ) . The method makes it p o s s i b l e t o determine t h e s u r f a c e a r e a of a s o l i d a l s o i n t h e wet s t a t e .

The p e r t i n e n t l i t e r a t u r e d e a l i n g with t h e reasons and t h e o r e t i c a l i m p l i c a t i o n s of t h e discrepancy i s e x t e n s i v e (9

-

12) and w i l l n o t be discussed h e r e .

Hunt, Tomes and Blaine showed t h a t s u r f a c e a r e a s a r e a f f e c t e d n o t only by t h e n a t u r e o f t h e a b s o r p t i v e b u t a l s o by t h e l e n g t h of t h e drying p e r i o d , t h e degree of aggregation (13) and t h e r a t e of drying (14). m e s e f i n d i n g s suggest t h a t t h e d i s c r e p a n c i e s i n s u r f a c e a r e a s a r e n o t s o much due t o t h e inadequacies of t h e measuring technique but r a t h e r t o t h e v a r i a b i l i t y of t h e s u r f a c e a r e a caused by changes i n environmental c o n d i t i o n s and t h e n a t u r e of t h e adsorbing vapor. S i m i l a r i t y appears t o e x i s t i n t h i s r e s p e c t between cement and o t h e r h y d r o p h i l i c adsorbents such a s c l a y s ( I S ) , c a s e i n (16), and b a c t e r i a l s p o r e s (17). For t h e last-mentioned substance, Neihof, Thompson and Deitz d e v e l o p e d a d r y i n g method known a s s o l v e n t replacement which g r e a t l y increased t h e n i t r o g e n s u r f a c e a r e a s although t h e y were s t i l l only h a l f a s l a r g e a s t h e a r e a computed from t h e water a d s o r p t i o n isotherm (17).

The aim o f t h e p r e s e n t work was t o i n v e s t i g a t e i f drying of cement p a s t e by s o l v e n t replacement increased i t s n i t r o g e n s u r f a c e a r e a . In

a d d i t i o n , t h e e f f e c t of u s i n g t h e c r i t i c a l p o i n t drying method was explored. For t h i s method, a s t h e name i m p l i e s , t h e s o l v e n t i s removed a t a temperature above i t s c r i t i c a l p o i n t , t h u s avoiding t h e c o l l a p s e of t h e s t r u c t u r e caused by t h e receding meniscus.

Experimental

The cement u s e d i n t h e s e t e s t s w a s h y d r a t e d i n a s i n g l e batch for more than t h r e e y e a r s i n a s e a l e d polyethylene b o t t l e under continuous r o t a t i o n . The s u r f a c e "areas were determined i n a high vacuum volumetric adsorption apparatus. P r i o r t o measurement, pumping through a l i q u i d n i t r o g e n t r a p was continued u n t i l t h e r e s i d u a l p r e s s u r e was l e s s t h a n 1 x 1 0 - l ~ a (lpmm Hg). The s u r f a c e a r e a values a r e expressed i n square metre p e r gram o f i g n i t e d

(1050 OC) cement.

The water s u r f a c e a r e a was determined by t h e ' d e s i c c a t o r method'

( 2 ) and a modification of i t , which c o n s i s t e d of exposing t h e s l u r r y t o t h e approximate r e l a t i v e humidity and a f t e r a minimum o f 133 days of e q u i l i b r a t i o n time, 'D-drying' t h e conditioned sample.

R e s u l t s

A summary of t h e s u r f a c e a r e a v a l u e s and b r i e f d e s c r i p t i o n of t h e sample p r e p a r a t i o n a r e given i n Table I . Sample No. 1 was d r i e d i n a d e s i c c a t o r over Mg(C104)2 f o r 133 days followed by f i n a l pumping a t room temperature. The n i t r o g e n a r e a was found t o be 69.6 m2/g. Sample No. 2 was pumped continuously f o r 39 days a t room temperature and t h e s u r f a c e a r e a was 105.0 m2/g. Pumping time was d r a s t i c a l l y reduced t o 6.5 h when t h e tempera- t u r e of sample (No. 3) was r a i s e d t o 110 OC. The s u r f a c e a r e a was 121.8 m2/g.

V o l . 6 , N o .

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141 SURFACE AREA, BET, NITROGEN, V A R I A B I L I T Y

TABLE I

Nitrogen s u r f a c e a r e a of samples prepared by v a r i o u s techniques.

*

r e f l u x e d

Sample No. 4 was prepared by c e n t r i f u g i n g t h e aqueous s l u r r y f o r

1 h a t 1300 rpm, t h e water decanted and t h e s o l i d washed f i v e times with methyl a l c o h o l during t h e subsequent eight-day p e r i o d . F i n a l l y , t h e alcohol was vacuum d i s t i l l e d . The s u r f a c e a r e a was 196.3, m2/g. Sample No. 5 was d r i e d i n a s i m i l a r manner except t h a t it had been kept under methyl alcohol f o r 108 days, during which t h e l i q u i d was renewed e i g h t times. No s i g n i f i c a n t change i n s u r f a c e a r e a r e s u l t e d (194.0 m2/g). Area, m2/g 69.6 105.0 121.8 196.3 194.0 209.5 219.9 228.1 208.9 188.2 219.2 249.4

Samples Nos. 6 and 7 were d r i e d under s i m i l a r c o n d i t i o n s except t h a t , i n t h e second s t a g e , methyl a l c o h o l was replaced by n-pentane. The s u r f a c e a r e a s were 209.5 and 219.9 m2/g r e s p e c t i v e l y . A f u r t h e r moderate i n c r e a s e of t h e a r e a t o 228.1 m2/g occurred when t h e above procedure included r e f l u x i n g of t h e s o l v e n t s (sample No. 8 ) . Final D-Drying hours t , "C 24 20 40 20 6 5 110 20 50 2 0 40 2 0 50 2 0 50 2 0 40 20 110 2 0 40 2 0 40 2 0 50 Sam- p l e No. 1 2 3 4 5 6 7 8 9 I 10 11 12

Sample No. 9 was s t o r e d i n methyl alcohol f o r 60 days a f t e r which t h e s o l v e n t was replaced by n-pentane. F i n a l l y , t h e s o l v e n t was l e f t t o evaporate a t room temperature. The s u r f a c e a r e a was 208.9 m2/g.

Samples Nos. 10 and 11 were companion t o Nos. 6 and 7. The f i n a l drying s t e p , however, was n o t by vacuum d i s t i l l a t i o n i n t h e s e cases butaby evaporation above t h e c r i t i c a l p r e s s u r e and temperature (205 OC) of pentane. The n i t r o g e n s u r f a c e a r e a of t h e samples were 188.2 and 219.2 m2/g,

r e s p e c t i v e l y .

The h i g h e s t s u r f a c e a r e a v a l u e t o d a t e , 249.4 m2/g, was obtained with sample No. 12 which was s t o r e d i n methyl alcohol f o r 47 days, r e f l u x e d with pentane f o r 16 h and d r i e d by vacuum d i s t i l l a t i o n .

Drying Mode Mg(C1O4)2 133d D-drying 39d Vac. d i s t . Vac. d i s t

.

Vac. d i s t

.

Vac. d i s t . Vac. d i s t

.

Evap. 20 O C C r i t . p o i n t C r i t . p o i n t Vac. d i s t . Storage i n Methyl Alcohol

--

- -

-

..

8 108 8 1 0.8* 69 8 1 44 Time, days i n Pentane

- -

- -

--

- -

--

6 3 0.3* 2 6 3 0.8"

Vol.

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L i t v a n

The w a t e r s u r f a c e a r e a from t h e d e s o r p t i o n branch of t h e i s o t h e r m i s 286 m2/g; from t h e a d s o r p t i o n branch i t i s 182 m 2 / g .

D i s c u s s i o n

The n i t r o g e n s u r f a c e a r e a s o f ID-dried1 samples o f t h e same cement p a s t e were found t o range from 69.6 t o 249.4 m2/g. I n view of t h i s extremely l a r g e v a r i a t i o n , extreme c a u t i o n must be e x e r c i s e d i n t h e u t i l i z a t i o n of a r e a v a l u e s i n t h e o r e t i c a l c o n s i d e r a t i o n s . Admittedly t h e d i s c r e p a n c y was c r e a t e d by t h e d i v e r s i t y of t h e d r y i n g procedure b u t t h e e f f e c t of t h e mode of d r y i n g a s opposed t o t h e f i n a l s t a t e i s n o t g e n e r a l l y r e a l i z e d i n t h e l i t e r a t u r e ; d e t a i l s of sample p r e p a r a t i o n a r e seldom g i v e n . Even i f t h e d r y i n g procedure were s t a n d a r d i z e d t h e o b t a i n e d v a l u e s could be more c h a r a c t e r i s t i c of t h e p r e p a r a t i v e procedure t h a n of t h e p a s t e i t s e l f .

Hunt, Tomes and B l a i n e r e p o r t e d (14) t h a t a r a p i d l y d r i e d p a s t e h a s a l a r g e r n i t r o g e n s u r f a c e a r e a t h a n a s l o w l y d r i e d one. This o b s e r v a t i o n may e x p l a i n t h e p r e s e n t r e s u l t s . Solvent replacement w i t h t h e h i g h l y v o l a t i l e n-pentane and c r i t i c a l - p o i n t d r y i n g methods p r o v i d e p r a c t i c a l means f o r a c h i e v i n g v e r y h i g h d r y i n g r a t e s . The e f f e c t i v e n e s s of t h e s e methods a p p e a r s t o depend p r i m a r i l y on t h e completeness o f t h e f i r s t w a t e r - r e p l a c i n g phase which can be promoted by e l e v a t i n g t h e t e m p e r a t u r e and e x t e n d i n g t h e s t o r a g e p e r i o d . Very probably a r e a v a l u e s h i g h e r t h a n 250 m 2 / g can be achieved by f u r t h e r improvements o f t h e procedure.

The r e s u l t s p r e s e n t e d i n t h i s p a p e r i n d i c a t e t h a t t h e n i t r o g e n s u r f a c e a r e a d r i e d by s p e c i a l t e c h n i q u e became comparable t o t h e water a r e a of c o n v e n t i o n a l l y d r i e d cement samples. In r u n No. 1 3 , cement was p r e p a r e d i n a manner s i m i l a r t o t h a t followed w i t h sample No. 8 and t h e water and n i t r o g e n

a r e a s were determined u s i n g s e p a r a t e p o r t i o n s . The v a l u e s o b t a i n e d were 201 and 214 m2/g r e s p e c t i v e l y , i n d i c a t i n g t h a t t h e d i f f e r e n c e between t h e two a r e a s v a n i s h e s i f t h e sample i s d r i e d by s o l v e n t replacement.

The n i t r o g e n a r e a s o f s e v e r a l samples exceeded t h e w a t e r s u r f a c e a r e a v a l u e , i n c o n t r a s t t o normal e x p e r i e n c e . D i f f e r e n c e s i n s u r f a c e a r e a s o b t a i n e d w i t h d i f f e r e n t a d s o r b a t e s i s u s u a l l y a t t r i b u t e d t o t h e i n a b i l i t y of t h e a d s o r b i n g molecule t o r e a c h t h e e n t i r e s u r f a c e , which i s of c o n s t a n t magnitude. I t may w e l l b e , however, t h a t t h e a d s o r b a t e h a s a d i r e c t e f f e c t on t h e a r e a inasmuch a s a d s o r p t i o n of d i f f e r e n t molecules induce d i f f e r e n t

r e s p o n s e o f t h e s o l i d . E s s e n t i a l l y t h e r e a r e two a l t e r n a t i v e s : 1 ) a r e a l s u r f a c e a r e a does e x i s t , which i s c o n s t a n t and i t s v a l u e can be determined t o a d i f f e r e n t degree o f accuracy by t h e a d s o r p t i o n of molecules, o r 2) t h e s u r f a c e a r e a of t h e s o l i d i s v a r i a b l e , a f u n c t i o n of v a r i o u s parameters and each a d s o r b a t e measures more o r l e s s a c c u r a t e l y t h e e f f e c t i v e a r e a . Although t h e f i r s t a l t e r n a t i v e has been accepted g e n e r a l l y , t h e second a p p e a r s t o be a d i s t i n c t p o s s i b i l i t y .

Acknowledgement

The a u t h o r i s i n d e b t e d t o R . E . Myers f o r c a r r y i n g o u t t h e experimental work and f o r many h e l p f u l s u g g e s t i o n s .

T h i s i s a c o n t r i b u t i o n of t h e D i v i s i o n of Building Research,

N a t i o n a l Research Council o f Canada and i s p u b l i s h e d w i t h t h e approval of t h e D i r e c t o r of t h e D i v i s i o n .

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