Porosity of Ancient Egyptian Mortars

F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam

693

POROSITY OF ANCIENT EGYPTIAN MORTARS

J . R A G A I l , K.S.W.

SING',

and

M. YATES2

'The American U n i v e r s i t y i n C a i r o , Egypt 2Brunel, The U n i v e r s i t y o f West London, Uxbridge, England

SUMMARY Mercury p o r o s i m e t r i c s t u d i e s were c a r r i e d o u t on A n c i e n t E g y p t i a n m o r t a r s e x t r a c t e d f r o m t h e Sphinx body c o r e and N o r t h s i d e c h e s t as w e l l as f r o m t h e K h a f r a V a l l e y temple. Such s t u d i e s were complemented b y t h e a p p l i c a t i o n o f Xr a y d i f f r a c t i o n and simultaneous t h e r m o g r a v i m e t r i c and d i f f e r e n t i a l thermal a n a l y s i s , The r e s u l t s o b t a i n e d suggest t h a t A n c i e n t E g y p t i a n m o r t a r s a r e c h a r a c t e r i z e d b y two s e t s o f pores w i t h i n t h e gypsum m a t r i x . The l a r g e r nores ( o f w i d t h '~1-15,um) appear t o be l o c a t e d between t h e CaS04 g r a n u l e s , whereas t h e s m a l l e r pores ( o f w i d t h < l o 0 nm) a r e p r o b a b l y formed as a r e s u l t o f t h e removal o f w a t e r f r o m w i t h i n t h e g r a n u l e s ( i . e . f r o m between and w i t h i n t h e i n d i v i d u a l crystals) ! INTRODUCTION I n h i s p i o n e e r i n g work on A n c i e n t E g y p t i a n M o r t a r s , A l f r e d Lucas ( r e f . 1 ) concludes t h a t t h e m o r t a r used i n A n c i e n t Egypt, b e f o r e Graeco-Roman times, was m a i n l y gypsum.

Lucas d i f f e r e n t i a t e s between two t y p e s o f gypsum i n Egypt.

One

poor q u a l i t y t y p e u s u a l l y a s s o c i a t e d w i t h v a r i o u s p r o p o r t i o n s o f CaC03 and sand Lucas

and a b e t t e r q u a l i t y gypsum i n which t h e CaS04.2H20 phase predominates.

c o n f i r m s , t h a t no i n s t a n c e of l i m e m o r t a r i s known t o have been used i n A n c i e n t Egypt b e f o r e t h e t i m e of Ptolemy I (323-283 B.C.).

Furthermore, gypsum was p r e -

f e r r e d r a t h e r t h a n l i m e because o f t h e s c a r c i t y o f f u e l . It i s w e l l known t h a t c h e m i c a l l y , gypsum i s c a l c i u m s u l f a t e d i h y d r a t e

"CaS04.2H20" which on h e a t i n g t o a t e m p e r a t u r e o f about 13OoC l o s e s t h r e e f o u r t h s o f i t s w a t e r and becomes t h e hemihydrate CaS04.3H20 ( P l a s t e r o f P a r i s ) . 13OoC (CaS04)2.H20 + 3H20. When p l a s t e r o f P a r i s i s Thus 2CaS04.2H20 --------+ made i n t o a p a s t e w i t h w a t e r i t s e t s t o a s o l i d mass i n a few m i n u t e s . The s e t t i n g i s due t o t h e r e f o r m a t i o n of t h e d i h y d r a t e i n t h e f o r m o f s m a l l c r y s t a l s which f i t c l o s e l y t o g e t h e r and produce a s o l i d mass. It i s g e n e r a l l y b e l i e v e d (refs.2,3)

t h a t the strength o f a given mortar o r

cement i s r e l a t e d t o t h e p o r e s i z e d i s t r i b u t i o n as opposed t o t h e volume f r a c t i o n s o f t h e pores.

The p o r e s i z e d i s t r i b u t i o n i s governed b y t h e n a t u r e o f t h e

s t a r t i n g m a t e r i a l s and b y t h e mode o f m i x i n g and compacting ( r e f . 4 ) . M i k h a i l and Malek ( r e f . 5 ) , u s i n g n i t r o g e n a d s o r p t i o n , have s t u d i e d modern E g y p t i a n gypsum m o r t a r s and have i d e n t i f i e d i n t h e s e systems two p o r e s i z e s

694

w.

8 and 70 Mercury porosimetric studies were n o t carried o u t on such systems, b u t Mikhail and Malek drew attention t o the similarity of corresponding to 10

the microstructures of hardened gypsum and Portland cement pastes. Since mercury porosimetry has been used t o a considerable extent i n the study of hardened Portland cement, i t seemed appropriate t o apply i t in the assessment of porosity of Ancient Egyptian Mortars. X-ray diffraction and simultaneous thermogravimetry and differential thermal analysis were used as additional techniques in the present study. EXPERIMENTAL Materials The SB1-SB5 samples refer t o mortars extracted from the Sphinx body core. The SN sample refers to a mortar from the Sphinx North Side Chest and the KV1KV3 samples r e f e r t o mortars extracted from the Khafra Valley Temple. A detailed account of the extraction s i t e s of these nine mortars suggests that sample KV1 i s l i k e l y to be the oldest one. The compositions of the mortars as determined by X-ray diffraction are reported in Table I . The thermal studies revealed t h a t the gypsum samples containing c a l c i t e exhibited a number of endotherms in the ranges 2O-12O0C, 120-170°C, 390-585'C and 637-850°C. These endotherms are attributed respectively t o the loss of i n t e r s t i t i a l H20, t o the transformation of the gypsum t o the hemihydrate and t o y-CaS04, followed by dehydration and the loss of structural water, and f i n a l l y t o the decomposition of CaC03 t o CaO. Thermogravimetric r e s u l t s indicated a f i r s t weight loss ranging from 22% t o 28% f o r most of the samples. Such a loss encompassed the two i n i t i a l DTA endotherms. Sample KV1 exhibited only 11% i n i t i a l weight loss followed by two very sharp additional losses of %2%around 350°C and 40OoC. We a t t r i b u t e the l a t t e r t o the formation of the so-called "soluble anhydrites" ( r e f . 6 ) . The present r e s u l t s accord very well with previously reported X-ray and thermal studies carried o u t on different s e r i e s of mortars (refs.6-9). Techniques Mercury intrusion-extrusion measurements were made w i t h a Carlo-Erba 225 pressure porosimeter. The X-ray diffraction patterns were obtained by means of a General Electric X-ray diffraction u n i t , model XRD-6, using Ni f i l t e r e d CuKa radiation. The thermal studies were carried out on a Stanton Redcroft STA-780, simultaneous thermal analyzer s e r i e s designed t o give simultaneous differential thermal analysis (DTA) and differential thermogravimetry ( D T G ) .

695

RESULTS Unheated Mortars Fig. 1 gives Hg intrusion-extrusion plots for six representative samples.

Fig. 1 . Mercury intrusion-extrusion plots for some representative mortars.

It

696

will be noted t h a t in a l l cases the intrusion-extrusion curves e x h i b i t considerable h y s t e r e s i s . With the exception of sample KV1 t h i s hysteresis is largel y confined t o the region of pressure below about 20-30 bars. Furthermore, a l l o f these intrusion curves e x h i b i t points of i n f l e c t i o n a t around 100-200 bars.

The corresponding extrusion curves t h e n remain almost horizontal over a very wide range of pressure leading t o l a r g e q u a n t i t i e s of mercury entrapment a t a pressure of 1 bar. A s t r i k i n g f e a t u r e i s t h a t the amount of entrapped mercury corresponds q u i t e closely t o t h e i n f l e c t i o n point a t 100-200 bars. In view of these findings we have subdivided the amount of mercury intruded i n t o two stages ' I ' and '11'. Stage ' I ' i s characterized by l a r g e r pores ( o f width -1-15 urn)

whereas stage I 1 corresponds t o smaller pores ( o f width < 100 nm) . As indicated in Fig. 1 , i t i s evident t h a t sample KV1 behaves q u i t e d i f f e r e n t l y . Fig. 2 gives t h e mercury intrusion-extrusion p l o t s f o r mortars KV1, KV2 and

697

SB2 as w e l l as f o r t h e i r h e a t - t r e a t e d p r o d u c t s ( h e a t i n g i n a i r r e s p e c t i v e l y a t 2OO0C and 4OO0C f o r 3 h o u r s ) .

I t i s seen t h a t f o r samples KV2 and SB2, h e a t t r e a t m e n t a t 200°C and 4OO0C l e a d s t o a g r a d u a l decrease i n t h e p o r e volume

c o r r e s p o n d i n g t o s t a g e 'I'o f t h e mercury p e n e t r a t i o n , whereas t h e p o r e volume i n stage '11' i n i t i a l l y i n c r e a s e s a t 200°C ( t o g e t h e r w i t h an enlargement o f t h e pore r a d i i ) , f o l l o w e d b y a s m a l l decrease a t 40OoC.

As shown i n F i g . 2, t h e shape o f t h e mercury i n t r u s i o n - e x t r u s i o n p l o t s f o r t h e heated m o r t a r s KV2 and SB2 t o some e x t e n t resemble t h a t o f t h e o r i g i n a l m o r t a r K V l . GENERAL DISCUSSION The r e s u l t s r e p o r t e d h e r e suggest t h a t A n c i e n t E g y p t i a n m o r t a r s may be c h a r a c t e r i z e d by two s e t s o f p o r e s ' I ' and ' 1 1 ' w i t h i n t h e gypsum m a t r i x . Indeed t h e shape and e x t e n t o f t h e mercury i n t r u s i o n - e x t r u s i o n p l o t s F i g . 1, determined on t h e unheated m o r t a r s i n d i c a t e a f i r s t s t a g e 'I'o f t h e mercury p e n e t r a t i o n which corresponds t o t h e f i l l i n g o f l a r g e t h r e e dimensional c a v i t i e s connected by narrow t h r o a t s o r channels. ween t h e CaS04 g r a n u l e s .

Such pores appear t o be l o c a t e d b e t -

The p l o t s a l s o i n d i c a t e a second s t a g e ' 1 1 ' o f t h e

mercury p e n e t r a t i o n which seems t o correspond t o t h e f i l l i n g up o f s m a l l e r pores p r o b a b l y a r i s i n g f r o m t h e removal o f w a t e r f r o m w i t h i n t h e CaS04 g r a n u l e s . Hammond and Withrow ( r e f . 10) have shown t h a t l a r g e lumps o f CaS04.2H20 r e t a i n t h e i r o r i g i n a l shape and c r y s t a l s t r u c t u r e even a f t e r complete d e h y d r a t i o n . Such r e s u l t s suggest t h a t i n t h e p r e s e n t s t u d y pores r e l a t i n g t o s t a g e ' I ' o f t h e p e n e t r a t i o n r e s u l t from t h e s t a b i l i t y o f t h e gypsum phase, i . e . a r e i n t e r granular i n nature,

Outgassing t h e m o r t a r samples p r i o r t o c a r r y i n g o u t t h e

mercury i n t r u s i o n measurements seems, on t h e o t h e r hand t o be r e s p o n s i b l e f o r t h e s m a l l e r pores ( s t a g e '11' o f t h e p e n e t r a t i o n ) t h r o u g h t h e s t e a d y removal o f i n t e r s t i t i a l w a t e r . An enlargement o f t h e l a t t e r pores t o g e t h e r w i t h an i n creased p o r e volume would f o l l o w t h r o u g h t h e subsequent removal o f t h e w a t e r of c r y s t a l 1 iz a t i o n . An i n t e r e s t i n g comparison w i t h t h e work o f M i k h a i l and Malek ( r e f . 5 ) c o u l d be drawn a t t h i s p o i n t . As a l r e a d y mentioned e a r l i e r these a u t h o r s have i d e n t i f i e d two s e t s o f p o r e s i z e s i n modern E g y p t i a n hardened gypsum pastes. s m a l l e r s e t ' S ' w i t h an average h y d r a u l i c r a d i u s o f 10

8

One

was assumed t o c o n s t i -

t u t e t h e i n t e r s p a c e s between t h e c r y s t a l l i n e p a r t i c l e s whereas a l a r g e r s e t of pores ' L ' w i t h an average r a d i u s o f 70

fl -

80

8

was a t t r i b u t e d t o t h e w a t e r It i s there-

f i l l e d spaces i n t h e pastes which were i n t r a p a r t i c u l a t e i n n a t u r e .

f o r e q u i t e obvious t h a t t h e o r i g i n o f m i c r o p o r o s i t y as s t u d i e d i n t h e work o f M i k h a i l and Malek f o l l o w s q u i t e a d i f f e r e n t t r e n d t h a n t h a t observed i n o u r p r e s e n t s t u d y i n which much l a r g e r pores a r e i n v o l v e d . The d i f f e r e n c e observed i n t h e h e a t - t r e a t e d samples SB2 and KV2 a r e c o n s i s H e a t - t r e a t m e n t of t h e s e m o r t a r s a t 200°C

t e n t w i t h our present i n t e r p r e t a t i o n .

698

seems t o lead, as i n d i c a t e d i n F i g . 2, t o a general c o n t r a c t i o n o f t h e CaS04 s t r u c t u r e t o g e t h e r w i t h an enlargement of the s m a l l e r pore r a d i i . The former effect may be a t t r i b u t e d t o a c e r t a i n degree o f s i n t e r i n g , whereas t h e l a t t e r e f f e c t r e s u l t s from t h e removal o f t h e water o f C r y s t a l l i z a t i o n . The thermal studies have confirmed t h a t b o t h the i n t e r s t i t i a l and a good p r o p o r t i o n of the water o f c r y s t a l l i z a t i o n are removed a t 200°C. It i s noteworthy t h a t samples SB4 and SB5 as w e l l as KV1 and SN1 although

seemingly i d e n t i c a l i n terms o f component minerals and composition (Table I ) vary considerably i n terms o f ranges o f i n t r u s i o n pore r a d i i and volumes.

,

A

probable f a c t o r c o n t r o l l i n g pore s i z e i n these mortars i s the change i n t h e water content o f CaS04 gH20 when the l a t t e r i s made i n t o a paste. TABLE I Composition o f t h e mortar samples as determined by X-ray d i f f r a c t i o n . ~

~~

Sample SB1

~

~

Components

SB2

Gypsum (A), CaC03 (D), Q u a r t z ( D ) Gypsum (A), Q u a r t z (D)

SB3

Gypsum (A),

CaC03 (B), Q u a r t z (D)

SB4

Gypsum ( A ) ,

CaC03 (C), Q u a r t z (D)

SB5

Gypsum ( A ) ,

CaC03 (C), Q u a r t z (D)

SN1

CaC03(A), Gypsum (B) , Quartz (C)

KV 1

CaC03(A), Gypsum (B),

KV2

CaC03(A), Gypsum ( C ) Gypsum (A), CaC03 (B), Quartz ( C )

KV3

Quartz ( C )

A = Major component, B,C and D r e f e r t o components i n o r d e r o f decreasing amounts as i n d i c a t e d by the r e l a t i v e i n t e n s i t i e s o f the X-ray d i f f r a c t i o n peaks. X-ray d i f f r a c t i o n s t u d i e s on KV1 and on o t h e r s i m i l a r systems ( r e f s . 5,8) have i n d i c a t e d the unusual prevalence o f t h e y-CaS04 phase u n t i l temperatures as h i g h as 40OoC. The thermal s t u d i e s i n d i c a t e t h a t t h i s m o r t a r i n p a r t i c u l a r behaved q u i t e d i f f e r e n t l y e x h i b i t i n g a very small amount o f water o f c r y s t a l l i z a t i o n and very d i s t i n c t i v e sharp endothems corresponding t o the s o - c a l l e d " s o l u b l e anhydri tes". I t i s t h e r e f o r e l i k e l y t h a t ageing e f f e c t s may have l e d t o t h e removal o f t h e i n t e r s t i t i a l water f o l l o w e d by t h e slow d i f f u s i o n and e l i m i n a t i o n o f p a r t o f t h e

water o f c r y s t a l l i z a t i o n .

This would account f o r t h e observation o f a marked

h y s t e r e s i s i n stage '11' o f t h e p e n e t r a t i o n i n t h e unheated mortar KV1. Thermog r a v i m e t r i c r e s u l t s corroborate such an i n t e r p r e t a t i o n since a l l the h e a t - t r e a t ed mortars a t 200°C e x h i b i t e d a g r e a t e r i n i t i a l weight l o s s ( 2 2 %

-

28%) as w e l l

as an increase i n t h e I 1 pore volumes (Table 11) as compared w i t h m o r t a r KV1

699

where o n l y 11% weight l o s s was observed w i t h no increase i n pore volume.

In

keeping w i t h such an i n t e r p r e t a t i o n Okhotonikov e t a1 ( r e f . 11) have shown t h a t t h e k i n e t i c s o f dehydration o f CaS04.

nH20 i n t h e (010) d i r e c t i o n a r e d i f f u s i o n

c o n t r o l l e d i n which water molecules d i f f u s e through the s o l i d p r o d u c t CaS04. TABLE I1 Representative ranges o f i n t r u s i o n r a d i i and pore volumes corresponding t o stages 'I'and ' 1 1 ' o f mercury p e n e t r a t i o n f o r some unheated and heated mortars. Sample

r p ranges i n nm I I1

unheated SB1 SB4 SB5 KV1 SB2 heated KV1/200°C, KV1/400°C, SB2/200°C, SB2/400°C,

3 3 3 3

3 Ranges o f pore volumes A V (mm ) P I 11

8000-40 4000-40 15000-40 30000-200 8000-40

40-5 40-5 40-5 200-5 40-5

200 200 100 120 200

30 30 30 110 30

h r s . 30000-700 h r s . 15000-200 h r s . 8000-700 h r s . 8000-700

700-5 200-5 700-5 700-5

80 40 160 55

50 40 55 45

REFERENCES 1

A. Lucas and J.R. H a r r i s , Ancient Eqyptian M a t e r i a l s and I n d u s t r i e s , -- .

Edward A r n o l d Ltd., 1962: 2 J.D. B i r s h a l l , A.J. Howard and K. Kendall, Nature, 289 (1981) 388. 3 K. Kendall. A.J. Howard and J.D. B i r s h a l l , P h i l . Trans. Roy. SOC. Lond. , 310 (1983)-139. 4 R. Malinowski, Cem. Conc. Res., 1 (1971) 531. 5 R.Sh. M i k h a i l and R . I . A . Malek, J . Appl. Chem. Biotech., 21 (1971) 277. 6 H.Y. Ghorab, J . Ragai and A. Antar, Cem. Concr. Res., 16 (1986) 813. 7 J. Ragai, Cem. Concr. Res., 18 (1989) 9. 8 J. Ragai, Cem. Concr. Res., 19 (1988) 179. 10 W.A. Hamnond and J.R. Withrow, Ind. Eng. Chem., 25 (1933) 1112. 11 V.B. Okhotnikov, B . I . Yakobson and N.Z. Lyakhov, Ser. Khim. Nauk, 1 (1985) 23.