- Email: [email protected]
A contribution to the problem of cement-aggregate bond
CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 801-808, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.
A CONTRIBUTION TO THE PROBLEM OF CEMENT-AGGREGATEBOND R. Zimbelmann Otto-Graf-lnstitut, S t u t t g a r t , F.R. Germany (Communicated by F. Locher) (Received March 22; in f i n a l form A p r i l 30, 1985) ABSTRACT Research on the s t r u c t u r e and bond between cement stone and aggregates shows t h a t the s t r u c t u r e of the contact zone c l e a r l y di#fers from that of the p l a i n cement stone. The main c o n s t i t u e n t of the contact zone seems to b e c a l c i u m hydroxide which is also mainly responsible for the cement-aggregate bond. Introduction According to the most recent studies, the bond between aggregates and cement stone has an important influence on the behaviour of concrete under compressive stress. D i f f e r e n t authors have t r i e d to i n v e s t i g a t e or at l e a s t d i s cuss the influence of bond on compressive strength of concrete on the one hand, and on the other to clear up the determining factors of bond. In a l i t e r a t u r e survey L e s l i e Struble, et a l . ( I ) reported on tests carried out on t h i s subj e c t . As t h i s r e p o r t shows the t e s t r e s u l t s concerning the basis of bond as well as the influence of bond on the compressive strength of concrete are not uniform and in some cases c o n t r a d i c t o r y . Several p u b l i c a t i o n s say t h a t the bond between cement stone and aggregates depends on three d i f f e r e n t mechanisms. These are the mechanical indentation of the hydration products of cement with the rough surface of aggregate stone covered with f i n e cracks; the e p i t a x i s growth of hydration products at the stone surface; and the bond by chemical reaction between the cement paste or, in a hardened s t a t u s , the cement stone, and the c o n s t i t u a n t s of the aggregate grain. In the beginning the bond strength of cement stone at the aggregate surface is e s s e n t i a l l y smaller than the t e n s i l e strength of the cement stone. The r e l a t i o n of the two strength values is about 1:2, where an influence of d i f f e r ent aggregate materials can be determined. These bond problems have been studied thoroughly by FMPA S t u t t g a r t (OttoG r a f - l n s t i t u t ) over the l a s t few years. At the moment some long-term tests are s t i l l being c a r r i e d out. Hereafter a s h o r t r e p o r t about the tests and t h e i r essential r e s u l t s is given and the possible influence of the bond on the compressive strength of concrete is discussed. 801
802
Vol. 15, No. 5 R. Zimbelmann
Performance of the tests By means of tests, the bond strength between the aggregates and the cement stone, on the one hand, and the s t r u c t u r a l , chemical and mineralogical composition of the fracture surfaces in the contact zone resulting from bond strength tests, on the other hand, were determined. The l a t t e r investigations were carried through by a scanning electron mciroscope (SEM) with an EDAX-system and by a x-ray d i f f r a c t i o n device. Pure crystals of quartz, calc spar and feldspar which were smoothed and polished on one crystal face, limestone - cement s t o n e specimens with a polished and onduloted plastic a fracture-rough surface, gran~ube i t with a polished surface and f l i n t s without cracks sorted aggregate out from the usual aggregates were used as aggregates for the bond tests. The cements used were a commercial Portland cement PZ 35 F and a blast furnace cement HOZ 35 L. Normally the water/cement r a t i o of the cement FIG. 1 paste was 0.35 and with a smallSample for testing the bond strength er number of specimens used for between aggregate and cement stone. the SEM-investigations of 0.40, 0.5 and 0.70. -
-
W a X
The specimens were fabricated by f i x i n g an ondulated p l a s t i c tube with wax to the aggregate surface and by f i l l i n g the fresh cement into the mould ( F i g . l ) . Until testing these specimens were stored in metal boxes above a water bath, covered with humid cloths hanging into the water bath The~tensile strength of the cement stone with w/c = 0.35 a f t e r 28 days was 4.8 N/mm~ for Portland cement and 5.0 N/mm2 for blast furnace cement. Fig, 2 represents the development of tensile strength until an age of about 80 days. 6 E z5 c
c~
-
•~
FIG.
3
2
Development of tensile and bond strength of the portland cement and the blastfurnace slag cement used in the tests. w/c = 0.35
c
2 c
2
L,
6 8 10 20 time in days
40 60 80 100
Vol. 15, No. 5
803 BOND, CEMENT, AGGREGATE, CONTACT ZONE
Test Results Bond tests The bond strength was determined with specimens up to 80 days old. I t became evident, as reported by Hsu (2) that the bond strength is c l o s e l y connected with the type of aggregate stone (Figs. 3 and 4). Neglecting for the moment the r e s u l t s obtained with limestone, the bond strength curves run almost p a r a l l e l in the period concerned. This means t h a t there is a s p e c i f i c bond energy between every stone and the cement stone adhering to i t s surface which doesn't change at l e a s t in the period observed. From t h i s follows t h a t to the stones and minerals used in these t e s t s , no chemical reaction could have happened in a humid status of the specimens which has a remarkable influence on the bond strength. In t h i s case, considering the d i f f e r e n t reactions of the cement stone with the various aggregates, the curves c o u l d n ' t have moved in such a p a r a l l e l way. An exception are the curves for limestone aggregates. In the f i r s t 14 days these curves with Portland cement run almost p a r a l l e l to the curves of other stones. A f t e r I0 days, owing to an e s s e n t i a l l y greater increase of the bond s t r e n g t h , the curves d i f f e r more and more from the other curves. In the cases in which the aggregate surface was polished and a mechanical indentation i s , t h e r e f o r e , excluded, the bond strength of the b l a s t furnace cement stone l i e s e s s e n t i a l l y underneath the values achieved for Portland cement. A phenomenon which cannot appear in the curves represented (because the period considered is about 80 days) is the f a c t t h a t at an age above 80 to I00 days, the bond strength doesn't increase anymore but seems to decrease unexpectedly, with the except i o n of the limestone aggregates. I t has to be stated E E t h a t owing to t h e i r humid Z storage during which water 3 - c drops appear at the surface, tthe specimens c o u l d n ' t s h r i n k during the t e s t period. That D7 c means the decrease of bond i_ strength cannot r e s u l t from (,'1 the action of shrinkage stresses. ~2 I
,,..
In conclusion, the r e s u l t s of bond tests show that in the period concerned, chemical reactions on aggregates other than limestone d o n ' t exert any d i s cernable i n f l u e n c e on the bond
:
%,
o
,
c o
~,oo
%_
I
,.-
71 o
I Fig. 3 Tensile strength and bond strength of cement stone and cement mortar with portland cement and d i f f e r e n t aggregates by Shu (2).
'
7o c 0
0,2 5 O,35 c emen t
stone
0.45
L
0.55
mortar1:21 tensile
0,65
mortar1:3 J
strength
bond strength to n o .
m o O
0,75 w/c
limestone granite sandstone
804
Vol. 15, No. 5 R. Zimbelmann
l Z, E
mestone, polished mestone, cracked
1.2
Z rc-
10 uartz,polished uartz, gravel alcite,polished .:ldspar, polished ranite, polished
08 o3
06
c 0
OZ. 02 0 1
2
4
6 8 10 20 time in days
~0
60 80100
FIG. 4 Bond s t r e n g t h between p o r t l a n d cement stone and d i f f e r e n t aggregates with coarse or polished surface, w/c - 0.35. s t r e n g t h and t h a t the adhesion of the cement stone to the aggregates is essenti a l l y due to physical forces (surface f o r c e s ) and the mechanical i n d e n t a t i o n . As f a r as limestone as an aggregate is concerned, an a d d i t i o n a l adhesion growth can be stated which is e i t h e r due to chemical r e a c t i o n s or to e p i t a x i s growth; but on b e h a l f o f the curve run, e p i t a x i s growth is more probable than a chemical r e a c t i o n .
J
cGkOB~~ min.
I
I
I
I
10
1
24
7
hours
I
28 days
t i m e of h y d r a t i o n
FIG. 5 Formation of the h y d r a t i o n phases in p o r t l a n d cement h y d r a t i o n , by Richartz (3).
Vol. 15, No. 5
805 BOND, CEMENT, AGGREGATE, CONTACT ZONE
Structural investigations The i n v e s t i g a t i o n s of the contact zone are carried out with specimens of d i f f e r e n t ages in order to be able to pursue the development from the beginning. The age of the specimens investigated varied from I0 minutes to 80 days and in some cases even more. According to these i n v e s t i g a t i o n s the formation of the contact zone, e s p e c i a l l y at a young age, can be seen in d i r e c t c o r r e l a t i o n with the hydration course of Portland cement represented by Richartz (3). Figure 5 shows the schematic representation by Richartz. Analogous to the cement hydrat i o n , the development of the contact zone can be described according to our present knowledge as follows: A few minutes a f t e r mixing the cement with water, e t t r i n g i t e needles appear at the aggregate surface (Fig. 6). They are enveloped by calcium hydroxide c r y s
FIG. 6 Network of e t t r i n g i t e c r y s t a l s on the aggregate surface.
FIG. 7 Nearly closed layer of f i n e calciumhydroxide c r y s t a l s on the aggregate surface (contact l a y e r ) .
FIG. 8 Great panel-shaped hexagonal c r y s t a l s of calcium-hydroxide, anchored in the intermediate and t r a n s i t i o n area.
FIG. 9 View into the transition area, consisting of different hydrates. On the right and on the l e f t side of the picture the contact layer of calciumhydroxide is detached from the aggregate surface.
806
Vol. 15, No. 5 R. Zimbelmann
t a l s which with t h e i r (O01)-plane, run perpendicular to the aggregate surface. I t seems as i f above the aggregate surface, at which e t t r i n g i t e c r y s t a l s have p r e c i p i t a t e d , a one-crystal layer of calcium hydroxide begins to develop ( F i g . 7 ) . The growth of these calcium hydroxide c r y s t a l s is terminated a f t e r about 16 hrs. This layer is called contact layer as i t c o n s t i t u t e s a contact between the cement stone and the aggregate. This layer is about 2 to 3 ~m t h i c k . On the side of t h i s contact layer opposite to the aggregate, r e l a t i v e l y big panel-shaped c r y s t a l s of calcium hydroxide began to develop. Their (O01)-plane is oriented almost perpendicular to the aggregate surface (Fig. 8). These panel c r y s t a l s which have a diameter from I0 ~m to about 30 um are anchored in the socalled t r a n s i t i o n area. The t r a n s i t i o n area consists of a loose u n i t of hydrates of the neighboring cement grains. I t consolidates with the progressing hydration owing to the growth of hydrates and reaches in i t s " f i n a l phase" a p o r o s i t y of about 50% (Fig. 9). Besides t h a t , complexes of small calcium hydroxide c r y s t a l s s e t t l e in the intermediate layer which with a decreasing distance of the aggregate surface, lose t h e i r o r i e n t a t i o n to the contact layer. The (O01)-plane of these c r y s t a l s is mainly p a r a l l e l to the aggregate surface. The intermediate layer is about 20 um t h i c k ; i t turns over i n t o the dense cement stone in the outer I0 um region in which the s t r u c t u r e consolidates. The mainly needle- and rod-shaped hydrates in the intermediate layer rub against the contact layer and, as the f r a c t u r e p i c t u r e shows, adhere to the cont a c t layer. The overwhelming part of the bond forces might be transmitted by the big hexagonal calcium hydroxide panel c r y s t a l s (Fig. I 0 ) . The whole area of the-contact zone is shown schematically in Fig. I I . With increasing age, cracks appear in the contact layer. Owing to these cracks new calcium hydroxide panels appear which are oriented perpendicular to the aggregate surface (Fig. 12), and with great p r o b a b i l i t y separate the contact layer from the aggregate surface. This means that in some regions adhesion between aggregates and the contact layer is eliminated by the c r y s t a l l i z a t i o n pressure. This event is probably responsible for the fact that with increasing age (80 days and o l d e r ) , adhesion of the specimens decreases, contrary to a l l espectations, although strength of the cement stone increases. S i t u a t i o n of the f r a c t u r e As the contact layer is f u l l y realized at a very young age and the i n t e r mediate layer has only a few p a r t i c l e s which c o n s t i t u t e a connection between the contact layer and the cement stone, adhesion ruptures appear at a young age e s p e c i a l l y in the intermediate layer.
FIG. I0 Hexagonal calcium hydroxide c r y s t a l , anchored in the intermediate area. On the upper face, the adhering pull out of the contact layer is to be seen.
Vol. 15, No. 5
807 BOND, CEMENT, AGGREGATE, CONTACT ZONE
~o Co{OH) 2 crystels CSH
c N
5 "~
§
.~-
o
--
+
ettringite 20 .urn Ca(OH) 2 panel crystal
ettringite Ca(OH) 2 crystals
FIG. I I Model of the contact zone between cement stone and aggregate With increasing hydration the intermediate layer is overlapped by the panel-shaped calcium hydroxide c r y s t a l s and the CSH. The bond between the s o l i d cement stone and the contact layer is improved and the f r a c t u r e changes mainly over i n t o the adhesion surface between the contact layer and the aggregates; t h a t is to say the proper adhesion surface. At a higher age, the adhesive strength may decrease as the contact layer is removed from the aggregate surface by new production of calcium hydroxide. Influences on the Formation of the Contact Zone The tests described above were s l i g h t l y d i v e r s i f i e d in order to be able to state in which way the formation of the contact zone is influenced by a change
FIG. 12 Cracks in the contact layer and new calcium-hydroxide c r y s t a l s in these cracks.
808
Vol. 15, No. 5 R. Zimbelmann
of environmental c o n d i t i o n s . By an increase of the water/cement r a t i o from 0.35 to 0.40 and O.50, an increase of the porous intermediate layer and of the transi t i o n area was obtained, but no change of the thickness and s t r u c t u r e of the contact layer could be determined. Changing the p o s i t i o n or o r i e n t a t i o n of the contact l a y e r , no change of the contact layer was determined. As expected the intermediate layer was t h i c k e r with the aggregate placed above the cement paste than with the cement paste app l i e d on the aggregate. At a v e r t i c a l o r i e n t a t i o n of the aggregate l a y e r , no change could be determined in comparison to placing the cement paste on the aggregate. Using b l a s t furnace cement, the contact zone had the same s t r u c t u r e as for Portland cement, but the thickness of the layers, e s p e c i a l l y of the contact layer, was e s s e n t i a l l y smaller than f o r Portland cement. Summary cement paste was applied in a mould on d i f f e r e n t minerals and stones with one polished surface. By bond tests the bond strength of the cement stone was pursued u n t i l an age of 80 days. I t could be determined that the curves of bond strength for the d i f f e r e n t stones used as aggregates, with the exception of the d u r a t i o n , run p a r a l l e l to each other. From t h i s , i t can be deduced that the adhesion of these minerals and stones depends (besides mechanical indentation in an unpolished status) mainly on the e f f e c t of physical forces; an influence of chemical connections could not be determined. As for limestone, the development of the bond strength d i f f e r s e s s e n t i a l l y from that of other aggregates. Chemical procedures could possible play an important r o l e , but the main reason for t h i s behavior can be assumed to be in the formation of e p i t a x i s growths. The chemical and s t r u c t u r a l composition of cement stone was determined by electron microscope and x-ray i n v e s t i g a t i o n s on f r a c t u r e faces of specimens tested in the bond t e s t , where the p o s i t i o n of the bond f r a c t u r e was l o c a l i z e d . The contact zone shows a m u l t i - l a y e r s t r u c t u r e in which the contact l a y e r , that is to say the layer adhering to the aggregate surface, is composed of calcium hydroxide with e t t r i n g i t e c r y s t a l s s i t u a t e d at the aggregate surface. This follows d i r e c t l y to the cement stone, a very porous layer c o n s i s t i n g mainly of needle-and panel-shaped hydrates of the cement, the so-called intermediate layer which by a t r a n s i t i o n area and, under decrease of the p o r o s i t y , changes over i n to the firm and dense cement stone. The t r a n s f e r of load from the cement stone to the aggregate is mainly done by big panel-shaped calcium hydroxide c r y s t a l s and, to a smaller part, by the CSH pressing i n t o the contact layer. In the early stage, rupture occurs to the formation of the contact zone in the s t i l l weak intermediate layer and, with the increasing h y d r a t i o n , in the proper adhesion surface. At a higher age adhesion seems to become weaker owing to f u r t h e r c r y s t a l l i z a t i o n procedures at the cont a c t layer. References I. 2. 3.
L. Struble and J. Skalny, A Review of the Cement-Aggregate Bond, Cem. Concr. Res. I0, 277-286 (1980). T.T.C-:--Hsu, et a l , Microcracking of Plain Concrete and the Shape of the Stress-Strain~Curve, ACI Proc. 60, 209-224 (1963). W. Richartz, Uber die Gefuge- und Festigkeitsentwicklung des Zementsteins, Betontechnische Berichte 1969, pp. 67-83, Betonverlag GmbH, DUsseldorf.
A CONTRIBUTION TO THE PROBLEM OF CEMENT-AGGREGATEBOND R. Zimbelmann Otto-Graf-lnstitut, S t u t t g a r t , F.R. Germany (Communicated by F. Locher) (Received March 22; in f i n a l form A p r i l 30, 1985) ABSTRACT Research on the s t r u c t u r e and bond between cement stone and aggregates shows t h a t the s t r u c t u r e of the contact zone c l e a r l y di#fers from that of the p l a i n cement stone. The main c o n s t i t u e n t of the contact zone seems to b e c a l c i u m hydroxide which is also mainly responsible for the cement-aggregate bond. Introduction According to the most recent studies, the bond between aggregates and cement stone has an important influence on the behaviour of concrete under compressive stress. D i f f e r e n t authors have t r i e d to i n v e s t i g a t e or at l e a s t d i s cuss the influence of bond on compressive strength of concrete on the one hand, and on the other to clear up the determining factors of bond. In a l i t e r a t u r e survey L e s l i e Struble, et a l . ( I ) reported on tests carried out on t h i s subj e c t . As t h i s r e p o r t shows the t e s t r e s u l t s concerning the basis of bond as well as the influence of bond on the compressive strength of concrete are not uniform and in some cases c o n t r a d i c t o r y . Several p u b l i c a t i o n s say t h a t the bond between cement stone and aggregates depends on three d i f f e r e n t mechanisms. These are the mechanical indentation of the hydration products of cement with the rough surface of aggregate stone covered with f i n e cracks; the e p i t a x i s growth of hydration products at the stone surface; and the bond by chemical reaction between the cement paste or, in a hardened s t a t u s , the cement stone, and the c o n s t i t u a n t s of the aggregate grain. In the beginning the bond strength of cement stone at the aggregate surface is e s s e n t i a l l y smaller than the t e n s i l e strength of the cement stone. The r e l a t i o n of the two strength values is about 1:2, where an influence of d i f f e r ent aggregate materials can be determined. These bond problems have been studied thoroughly by FMPA S t u t t g a r t (OttoG r a f - l n s t i t u t ) over the l a s t few years. At the moment some long-term tests are s t i l l being c a r r i e d out. Hereafter a s h o r t r e p o r t about the tests and t h e i r essential r e s u l t s is given and the possible influence of the bond on the compressive strength of concrete is discussed. 801
802
Vol. 15, No. 5 R. Zimbelmann
Performance of the tests By means of tests, the bond strength between the aggregates and the cement stone, on the one hand, and the s t r u c t u r a l , chemical and mineralogical composition of the fracture surfaces in the contact zone resulting from bond strength tests, on the other hand, were determined. The l a t t e r investigations were carried through by a scanning electron mciroscope (SEM) with an EDAX-system and by a x-ray d i f f r a c t i o n device. Pure crystals of quartz, calc spar and feldspar which were smoothed and polished on one crystal face, limestone - cement s t o n e specimens with a polished and onduloted plastic a fracture-rough surface, gran~ube i t with a polished surface and f l i n t s without cracks sorted aggregate out from the usual aggregates were used as aggregates for the bond tests. The cements used were a commercial Portland cement PZ 35 F and a blast furnace cement HOZ 35 L. Normally the water/cement r a t i o of the cement FIG. 1 paste was 0.35 and with a smallSample for testing the bond strength er number of specimens used for between aggregate and cement stone. the SEM-investigations of 0.40, 0.5 and 0.70. -
-
W a X
The specimens were fabricated by f i x i n g an ondulated p l a s t i c tube with wax to the aggregate surface and by f i l l i n g the fresh cement into the mould ( F i g . l ) . Until testing these specimens were stored in metal boxes above a water bath, covered with humid cloths hanging into the water bath The~tensile strength of the cement stone with w/c = 0.35 a f t e r 28 days was 4.8 N/mm~ for Portland cement and 5.0 N/mm2 for blast furnace cement. Fig, 2 represents the development of tensile strength until an age of about 80 days. 6 E z5 c
c~
-
•~
FIG.
3
2
Development of tensile and bond strength of the portland cement and the blastfurnace slag cement used in the tests. w/c = 0.35
c
2 c
2
L,
6 8 10 20 time in days
40 60 80 100
Vol. 15, No. 5
803 BOND, CEMENT, AGGREGATE, CONTACT ZONE
Test Results Bond tests The bond strength was determined with specimens up to 80 days old. I t became evident, as reported by Hsu (2) that the bond strength is c l o s e l y connected with the type of aggregate stone (Figs. 3 and 4). Neglecting for the moment the r e s u l t s obtained with limestone, the bond strength curves run almost p a r a l l e l in the period concerned. This means t h a t there is a s p e c i f i c bond energy between every stone and the cement stone adhering to i t s surface which doesn't change at l e a s t in the period observed. From t h i s follows t h a t to the stones and minerals used in these t e s t s , no chemical reaction could have happened in a humid status of the specimens which has a remarkable influence on the bond strength. In t h i s case, considering the d i f f e r e n t reactions of the cement stone with the various aggregates, the curves c o u l d n ' t have moved in such a p a r a l l e l way. An exception are the curves for limestone aggregates. In the f i r s t 14 days these curves with Portland cement run almost p a r a l l e l to the curves of other stones. A f t e r I0 days, owing to an e s s e n t i a l l y greater increase of the bond s t r e n g t h , the curves d i f f e r more and more from the other curves. In the cases in which the aggregate surface was polished and a mechanical indentation i s , t h e r e f o r e , excluded, the bond strength of the b l a s t furnace cement stone l i e s e s s e n t i a l l y underneath the values achieved for Portland cement. A phenomenon which cannot appear in the curves represented (because the period considered is about 80 days) is the f a c t t h a t at an age above 80 to I00 days, the bond strength doesn't increase anymore but seems to decrease unexpectedly, with the except i o n of the limestone aggregates. I t has to be stated E E t h a t owing to t h e i r humid Z storage during which water 3 - c drops appear at the surface, tthe specimens c o u l d n ' t s h r i n k during the t e s t period. That D7 c means the decrease of bond i_ strength cannot r e s u l t from (,'1 the action of shrinkage stresses. ~2 I
,,..
In conclusion, the r e s u l t s of bond tests show that in the period concerned, chemical reactions on aggregates other than limestone d o n ' t exert any d i s cernable i n f l u e n c e on the bond
:
%,
o
,
c o
~,oo
%_
I
,.-
71 o
I Fig. 3 Tensile strength and bond strength of cement stone and cement mortar with portland cement and d i f f e r e n t aggregates by Shu (2).
'
7o c 0
0,2 5 O,35 c emen t
stone
0.45
L
0.55
mortar1:21 tensile
0,65
mortar1:3 J
strength
bond strength to n o .
m o O
0,75 w/c
limestone granite sandstone
804
Vol. 15, No. 5 R. Zimbelmann
l Z, E
mestone, polished mestone, cracked
1.2
Z rc-
10 uartz,polished uartz, gravel alcite,polished .:ldspar, polished ranite, polished
08 o3
06
c 0
OZ. 02 0 1
2
4
6 8 10 20 time in days
~0
60 80100
FIG. 4 Bond s t r e n g t h between p o r t l a n d cement stone and d i f f e r e n t aggregates with coarse or polished surface, w/c - 0.35. s t r e n g t h and t h a t the adhesion of the cement stone to the aggregates is essenti a l l y due to physical forces (surface f o r c e s ) and the mechanical i n d e n t a t i o n . As f a r as limestone as an aggregate is concerned, an a d d i t i o n a l adhesion growth can be stated which is e i t h e r due to chemical r e a c t i o n s or to e p i t a x i s growth; but on b e h a l f o f the curve run, e p i t a x i s growth is more probable than a chemical r e a c t i o n .
J
cGkOB~~ min.
I
I
I
I
10
1
24
7
hours
I
28 days
t i m e of h y d r a t i o n
FIG. 5 Formation of the h y d r a t i o n phases in p o r t l a n d cement h y d r a t i o n , by Richartz (3).
Vol. 15, No. 5
805 BOND, CEMENT, AGGREGATE, CONTACT ZONE
Structural investigations The i n v e s t i g a t i o n s of the contact zone are carried out with specimens of d i f f e r e n t ages in order to be able to pursue the development from the beginning. The age of the specimens investigated varied from I0 minutes to 80 days and in some cases even more. According to these i n v e s t i g a t i o n s the formation of the contact zone, e s p e c i a l l y at a young age, can be seen in d i r e c t c o r r e l a t i o n with the hydration course of Portland cement represented by Richartz (3). Figure 5 shows the schematic representation by Richartz. Analogous to the cement hydrat i o n , the development of the contact zone can be described according to our present knowledge as follows: A few minutes a f t e r mixing the cement with water, e t t r i n g i t e needles appear at the aggregate surface (Fig. 6). They are enveloped by calcium hydroxide c r y s
FIG. 6 Network of e t t r i n g i t e c r y s t a l s on the aggregate surface.
FIG. 7 Nearly closed layer of f i n e calciumhydroxide c r y s t a l s on the aggregate surface (contact l a y e r ) .
FIG. 8 Great panel-shaped hexagonal c r y s t a l s of calcium-hydroxide, anchored in the intermediate and t r a n s i t i o n area.
FIG. 9 View into the transition area, consisting of different hydrates. On the right and on the l e f t side of the picture the contact layer of calciumhydroxide is detached from the aggregate surface.
806
Vol. 15, No. 5 R. Zimbelmann
t a l s which with t h e i r (O01)-plane, run perpendicular to the aggregate surface. I t seems as i f above the aggregate surface, at which e t t r i n g i t e c r y s t a l s have p r e c i p i t a t e d , a one-crystal layer of calcium hydroxide begins to develop ( F i g . 7 ) . The growth of these calcium hydroxide c r y s t a l s is terminated a f t e r about 16 hrs. This layer is called contact layer as i t c o n s t i t u t e s a contact between the cement stone and the aggregate. This layer is about 2 to 3 ~m t h i c k . On the side of t h i s contact layer opposite to the aggregate, r e l a t i v e l y big panel-shaped c r y s t a l s of calcium hydroxide began to develop. Their (O01)-plane is oriented almost perpendicular to the aggregate surface (Fig. 8). These panel c r y s t a l s which have a diameter from I0 ~m to about 30 um are anchored in the socalled t r a n s i t i o n area. The t r a n s i t i o n area consists of a loose u n i t of hydrates of the neighboring cement grains. I t consolidates with the progressing hydration owing to the growth of hydrates and reaches in i t s " f i n a l phase" a p o r o s i t y of about 50% (Fig. 9). Besides t h a t , complexes of small calcium hydroxide c r y s t a l s s e t t l e in the intermediate layer which with a decreasing distance of the aggregate surface, lose t h e i r o r i e n t a t i o n to the contact layer. The (O01)-plane of these c r y s t a l s is mainly p a r a l l e l to the aggregate surface. The intermediate layer is about 20 um t h i c k ; i t turns over i n t o the dense cement stone in the outer I0 um region in which the s t r u c t u r e consolidates. The mainly needle- and rod-shaped hydrates in the intermediate layer rub against the contact layer and, as the f r a c t u r e p i c t u r e shows, adhere to the cont a c t layer. The overwhelming part of the bond forces might be transmitted by the big hexagonal calcium hydroxide panel c r y s t a l s (Fig. I 0 ) . The whole area of the-contact zone is shown schematically in Fig. I I . With increasing age, cracks appear in the contact layer. Owing to these cracks new calcium hydroxide panels appear which are oriented perpendicular to the aggregate surface (Fig. 12), and with great p r o b a b i l i t y separate the contact layer from the aggregate surface. This means that in some regions adhesion between aggregates and the contact layer is eliminated by the c r y s t a l l i z a t i o n pressure. This event is probably responsible for the fact that with increasing age (80 days and o l d e r ) , adhesion of the specimens decreases, contrary to a l l espectations, although strength of the cement stone increases. S i t u a t i o n of the f r a c t u r e As the contact layer is f u l l y realized at a very young age and the i n t e r mediate layer has only a few p a r t i c l e s which c o n s t i t u t e a connection between the contact layer and the cement stone, adhesion ruptures appear at a young age e s p e c i a l l y in the intermediate layer.
FIG. I0 Hexagonal calcium hydroxide c r y s t a l , anchored in the intermediate area. On the upper face, the adhering pull out of the contact layer is to be seen.
Vol. 15, No. 5
807 BOND, CEMENT, AGGREGATE, CONTACT ZONE
~o Co{OH) 2 crystels CSH
c N
5 "~
§
.~-
o
--
+
ettringite 20 .urn Ca(OH) 2 panel crystal
ettringite Ca(OH) 2 crystals
FIG. I I Model of the contact zone between cement stone and aggregate With increasing hydration the intermediate layer is overlapped by the panel-shaped calcium hydroxide c r y s t a l s and the CSH. The bond between the s o l i d cement stone and the contact layer is improved and the f r a c t u r e changes mainly over i n t o the adhesion surface between the contact layer and the aggregates; t h a t is to say the proper adhesion surface. At a higher age, the adhesive strength may decrease as the contact layer is removed from the aggregate surface by new production of calcium hydroxide. Influences on the Formation of the Contact Zone The tests described above were s l i g h t l y d i v e r s i f i e d in order to be able to state in which way the formation of the contact zone is influenced by a change
FIG. 12 Cracks in the contact layer and new calcium-hydroxide c r y s t a l s in these cracks.
808
Vol. 15, No. 5 R. Zimbelmann
of environmental c o n d i t i o n s . By an increase of the water/cement r a t i o from 0.35 to 0.40 and O.50, an increase of the porous intermediate layer and of the transi t i o n area was obtained, but no change of the thickness and s t r u c t u r e of the contact layer could be determined. Changing the p o s i t i o n or o r i e n t a t i o n of the contact l a y e r , no change of the contact layer was determined. As expected the intermediate layer was t h i c k e r with the aggregate placed above the cement paste than with the cement paste app l i e d on the aggregate. At a v e r t i c a l o r i e n t a t i o n of the aggregate l a y e r , no change could be determined in comparison to placing the cement paste on the aggregate. Using b l a s t furnace cement, the contact zone had the same s t r u c t u r e as for Portland cement, but the thickness of the layers, e s p e c i a l l y of the contact layer, was e s s e n t i a l l y smaller than f o r Portland cement. Summary cement paste was applied in a mould on d i f f e r e n t minerals and stones with one polished surface. By bond tests the bond strength of the cement stone was pursued u n t i l an age of 80 days. I t could be determined that the curves of bond strength for the d i f f e r e n t stones used as aggregates, with the exception of the d u r a t i o n , run p a r a l l e l to each other. From t h i s , i t can be deduced that the adhesion of these minerals and stones depends (besides mechanical indentation in an unpolished status) mainly on the e f f e c t of physical forces; an influence of chemical connections could not be determined. As for limestone, the development of the bond strength d i f f e r s e s s e n t i a l l y from that of other aggregates. Chemical procedures could possible play an important r o l e , but the main reason for t h i s behavior can be assumed to be in the formation of e p i t a x i s growths. The chemical and s t r u c t u r a l composition of cement stone was determined by electron microscope and x-ray i n v e s t i g a t i o n s on f r a c t u r e faces of specimens tested in the bond t e s t , where the p o s i t i o n of the bond f r a c t u r e was l o c a l i z e d . The contact zone shows a m u l t i - l a y e r s t r u c t u r e in which the contact l a y e r , that is to say the layer adhering to the aggregate surface, is composed of calcium hydroxide with e t t r i n g i t e c r y s t a l s s i t u a t e d at the aggregate surface. This follows d i r e c t l y to the cement stone, a very porous layer c o n s i s t i n g mainly of needle-and panel-shaped hydrates of the cement, the so-called intermediate layer which by a t r a n s i t i o n area and, under decrease of the p o r o s i t y , changes over i n to the firm and dense cement stone. The t r a n s f e r of load from the cement stone to the aggregate is mainly done by big panel-shaped calcium hydroxide c r y s t a l s and, to a smaller part, by the CSH pressing i n t o the contact layer. In the early stage, rupture occurs to the formation of the contact zone in the s t i l l weak intermediate layer and, with the increasing h y d r a t i o n , in the proper adhesion surface. At a higher age adhesion seems to become weaker owing to f u r t h e r c r y s t a l l i z a t i o n procedures at the cont a c t layer. References I. 2. 3.
L. Struble and J. Skalny, A Review of the Cement-Aggregate Bond, Cem. Concr. Res. I0, 277-286 (1980). T.T.C-:--Hsu, et a l , Microcracking of Plain Concrete and the Shape of the Stress-Strain~Curve, ACI Proc. 60, 209-224 (1963). W. Richartz, Uber die Gefuge- und Festigkeitsentwicklung des Zementsteins, Betontechnische Berichte 1969, pp. 67-83, Betonverlag GmbH, DUsseldorf.