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Morphology and microstructure of autoclaved clinker and slag-lime pastes in presence and in absence of silica sand
CEMENT and CONCRETE RESEARCH. Printed in the United States.
Vol. 7, pp. 231-238, 1977.
Pergamon Press,
Inc
MORPHOLOGY AND MICROSTRUCTURE OF AUTOCLAVED CLINKER AND SLAGLIME PASTES IN PRESENCE AND IN ABSENCE OF SILICA SAND
S. A. Abo-El-Enein, N. A. Gabr and R. Sh. Mikhail Faculty of Science Ain Shams University, Cairo, Egypt
(Communicated by R. Kondo) (Received Nov. 29, 1976; in final form Jan. 24, 1977)
ABSTRACT Development of microstructure in four hydrothermal reactions has been undertaken using scanning electron microscopy. These are clinker, clinker-sand, slag-lime and slag-lime-sand hydrothermal reactions. The microstructure of clinker hydration products displayed crumpled foils and tabular masses of calcium silicate hydrates; few cubic crystals of hydrogarnet appeared only during the initial stage of the reaction. In clinker-sand mixture the C-S-H phase was the only product identified. In slag-lime hydration the microstructure displayed both of the hydrogarnet crystals and the C-S-H phase. The hydration of slaglime-sand mixture (an optimum composition) was associated with th formation of ill-crystallized tobermorite and crystalline 11i tobermorite as the main products. Der Ablauf von der Mikrostruktur in vier hydrothermischen Reaktionen wurde mit Hilfe des Rasterelektronenmikroskops untersucht; diese sind Klinker, mit Klinker-Sand, mit Schlacke-Kalk und mit Schlacke-Kalk-Sand. Die Mikrostruktur von Klinkerhydrataionsprodukten zeigt verdriickteBelBge und massive Teilchen von CalciumKrjstalle von Hydrogarnet silikathydration; wenige kubiche sind nur zu Beginn des Prozesses zu Finden. Im Klinker-Sand Gemisch ist nur die C-S-H phase zu erkennen. Im Schlacke-Kalk Hydratationsprozess erscheinen Kristalle von Hydrogarnet und die C-S-H Phase. Die Hydratation von Schlacke-Kalk-Sand Gemisch (Optimale Zusammensetzung) wir der Formation von Kristallinarmen Tobermorit und Kristallinen 11il! Tobermorit als Haupthydratationssprdukte zugeordnet.
231
232
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail Introduction
The strength development in the hydrothermally hardened cement paste is mainly attributed to the binding centres present in the pastes as represented by the hydration product, as well as the total porosity. Therefore, the study of morphological and microstructural characteristics of the autoclave hydration products themselves is required to understand the physical and engineering properties of the hydrothermally treated hardened pastes. Early investigators generally used dispersion techniques to enable the hydration products to be observed d i r e c t l y by transmission electron microscopy, TEM, ( I - 3 ) . Others used TEM replication techniques of the fracture surfaces (4-8). In recent years scanning electron microscopy, SEM, has been extensively used for the study of the hardened pastes (9-20). The simple sample preparation techniques and the large depth of focus of the SEM makes i t a very useful tool to investigate the microstructural features of cement hydration products. This paper is one of a series of papers on the hydrothermal r e a c t i v i t i e s of blast-furnace slag, clinker and s i l i c a sand under a steam pressure of I0 atm. The f i r s t paper dealt with the experimental techniques and kinetics of four hydrothermal reactions (21). The second dealt with the corrosion of embedded reinforcing steel in neat cement pastes and mortars (22). The present paper deals with the morphology and microstructure of clinker and slag-lime pastes alone and in the presence of s i l i c a sand. The observed microstructural features offer better understanding of the more complex microstructure of the hydrothermally treated specimens and its relation to the strength development. Materials and Experimental The granulated blast-furnace slag, portland cement clinker and s i l i c a sand used in this investigation were of Blaine area of 3500 cm2/g. The chemical compositions of slag and clinker used in this study were given in an e a r l i e r paper (21). Fine powder of highly purified calcium hydroxide was used as the lime source. Four solid mixtures were studied hydrothermally: (I) clinker (I00 wt. %); ( i i ) clinker (50 wt.%) and sand (50 wt.%); ( i i i ) slag (80 wt.%) and calcium hydroxide (20 wt.%); and (iv) slag (25 wt.%), Calcium hydroxide (25 wt.%) and sand (50 wt.%). The methods of mixing, moulding and autoclaving were discussed in an e a r l i e r investigation (21). The hydrothermally hardened specimens have been studied in the range 0.5 up to 24 hours. For scanning electron microscopic (SEM) examination, the entire specimens were dried in nitrogen atmosphere (free from C02) at I05°C for 24 hours in order to remove the free water. Freshly fractured specimens (water/solid ratio=O.20) were placed in a vacuum evaporator with cathode rays and coated with a thin layer of gold of about 300-400A thickness. The specimens were stored in a dessicator prior to SEM examination. Leitz-AMR I000 scanning electron microscope was used in this investigation. Results and Discussion I.
Autoclaved Clinker Hydration Products (a) Clinker without Admixture. The i n i t i a l hydration products obtained after 0.5 hour hydration of clinker were mainly the hexagonal calcium aluminate hydrate phase which made t h e i r appearance among the hexagonal calcium hydroxide phase; the semi-crystalline tobermorite was also formed as radiating and interlocking fibres between the clinker grains (Fig. I ) . After 6 hours autoclaving of clinker, there appeared a single hexagonal crystal of calcium hydroxide linked with an interlocking fibrous structure of C-S-H; the hexagonal particles of calcium hydroxide also appeared in the structure (Fig. 2).
Vol. 7, No. 3
233 SLAG, CLINKER, AUTOCLAVED PASTES
FIG. 1 Clinker (0.5 h): Semicrystalline crumpled f o i l s of calcium s i l i c a t e hydrates, C-S-H, among various types of hydration products.
FIG. 2 Clinker (6 h): A big hexagonal crystal linked with interlocking fibres of C-S-H. The small hexagonal particles of calcium hydroxide also appear in the structure.
In the final stage of clinker hydrothermal curing, after 24 hours autoclaving, the microstructure displayed crumpled fibrous particles and massive tabular structure of tobermorite-like calcium s i l i c a t e hydrate as the dominant hydration products (Fig. 3). The results of X-ray examination and the solvent extraction methods showed that the calcium hydroxide phase was formed in the i n i t i a l stage and consumed in the final stage of the hydrothermal s o l i d i f i c a t i o n process; and the tobermorite-like calcium s i l i c a t e hydrate were the only phases identified in the final stage of the hydrothermal reaction. (b) Clinker-Sand Mixture. The microstructure of clinker-sand mixture after 0.5 hour of the hydrothermal process displayed a mixture of amorphous films of
FIG. 3 Clinker (24 h): Crumpled particles of C-S-H and massive tabular structure showing parallel cleavage of CH.
FIG. 4 Clinker-Sand (2 h): Radiating fibres and interlocking needles of C-S-H as representing the main hydration products.
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Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail
FIG. 5a Clinker-Sand (6 h): Mesh structure consisting of intertwined bundles of o r i ented needles and massive plates of C-S-H as the main hydration product.
FIG. 5b Clinker-Sand (6 h): Long radiating fibres and plates of C-S-H.
hydration products and unhydrated grains of cement minerals. After 2 hours the radiating fibres and the interlocking needles of C-S-H were the main hydration products (Fig. 4). In the intermediate stage of the hydration reaction, namely after 6 hours autoclaving, the microstructure displayed either bundles of oriented needles and massive plates or fine fibres and t i n y needle-like crystals (Figs. 5a and 5b). After 24 hours of steam curing the specimen had a dense microstructure in which plates had a s t r i a t e d and oriented appearance; the interlocking rods, long fibres and plates of w e l l - c r y s t a l l i z e d C-S-H were more predominant (Fig. 6). The amorphous hydrates could also be distinguished.
FIG. 6 Clinker-Sand (24 h): Close-textured, tabular and massive structure composed of plates which show striated and oriented appearance.
FIG. 7 Slag-Lime (0.5 h): Interlocking fibres and semi-crystalline f o i l s of tobermorit~ like hydrates (C-S-H). The cubic crystals of hydrogarnet phases were also dis tinguished in the structure.
Vol. 7, No. 3
235 SLAG, CLINKER, AUTOCLAVEDPASTES
The results of X-ray examination showed that the calcium hydroxide phase was completely consumed in the final stage of the hydrothermal reaction between clinker and sand. 2.
Autoclaved Slag Hydration Products
(a) Slag-Lime Mixture. In the early stage of the hydrothermal reaction, after 0.5 hour autoclaving at I0 atm, the hydration products appeared as interlocking fibres and semi-crystalline f o i l s of tobermorite-like hydrates (C-S-H); the cubic crystals of hydrogarnet phase were also distinguished in the structure (Fig. 7). In the intermediate stage of the hydrothermal s o l i d i f i c a t i o n process, after 6 hours autoclaving~ various forms of hydration products were formed. Among these hydrates i t was possible to distinguish fibrous and rod-like crystals, rosette-like hydrates arranged on plate-like hydration products and cubic hydrogarnet crystals having sizes of about I-2 micron in Fig. 8. After 24 hours of steam curing at I0 atm, there appeared cyrstallized fibres and rods of C-S-H phase and hydration products stacked as parallel layers with the distinction of the cubic hydrogarnet crystals (Fig. 9). From X-ray examination i t was possible to distinguish tobermorite-like C-S-H, hydrogarnet of composition C3ASH4 and calcium hydroxide among the hydration products produced during the various stages of the hydrothermal reaction. (b) Slag-Lime-Sand Mixture (An Optimum Composition). In slag-lime-sand hydrothermal reaction at I0 atm. of saturated steam, the i n i t i a l products obtained after 0.5 hour of hydration appeared as nearly amorphous C-S-H phase covering the grains of unhydrated cement components (Fig. I0). After 2 hours of hydration, the fibres formed grow into tabular and plate masses which bridge the unhydrated grains creating either an intersecting mesh arrangement or interlocking fibres and crumpled f o i l s of C-S-H (Fig. I I ) . In the intermediate stage of the hydrothermal reaction between slag, lime and sand, the space between the partly hydrated grains becomes f i l l e d in, so
FIG. 8 Slag-Lime (6 h): Rod-like and interlocking fibres of C-S-H; the cubic habit of hydrogarnet becomes more cominant.
FIG. 9 Slag-Lime (24 h): Hydration products stacked as parallel layers. The cubic habit of hydrogarnet crystals also appear.
236
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail
FIG. I0 Slag-Lime-Sand (0.5 h): Nearly amorphous C-S-H covering the grains of unhydrated cement components.
FIG. II Slag-Lime-Sand (2 h): Interlocking fibres and crumpled f o i l s of calcium s i l i c a t e hydrates f i l l i n g the pores between partly hydrated grains of cement components.
that the products obtained after 6 hours of hydration appeared as fibrous and rod-like crystals which display an oriented structure (Fig. 12). After 24 hours of steam curing at I0 atm., the main hydration products obtained were massive plates and fibrous particles with closely packed structure (Fig. 13). The results of X-ray analysis showed that the only phase i d e n t i f i e d , in slag-lime-sand hydrothermal reaction, was tobermorite-like calcium s i l i c a t e hydrate with different degrees of c r y s t a l l i n i t y . The high compressive strength obtained, using this optimum composition of slag-lime-sand mixture, was primarily associated with the formation of i l l -
FIG. 12 Slag-Lime-Sand (6 h): Fibrous and rod-like crystals of hydration products display an oriented structure.
FIG. 13 Slag-Lime-Sand (24 h): Massive plates and fibrous particles among the various types of hydration products with closely packed structure.
Vol. 7, No. 3
237 SLAG, CLINKER, AUTOCLAVED PASTES I
I
I
I
t
.--o--- C l i n k e r c~ 1200 E 0
~. 1000 ,,.C: ,,,i-, C: (b .L ,,E,.-,
:
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Autoclaving time (hours) FIG. 14 Compressive strength as a function of autoclaving time c r y s t a l l i z e d tobermorite (C-S-H(1) and C-S-H(II)) and of c r y s t a l l i n e II A tobermorite as the main hydration products. 3.
Strength Development
The dependence of the compressive strength on the autoclaving time is shown in Fig. 14 for the various pastes investigated. Obviously, the strength shows a f i r s t increase with the time of hydration up to 6 hours, then i t decreases a f t e r 12 hours and f i n a l l y increases a f t e r 24 hours of the hydrothermal s o l i d i f i c a t i o n process. I t was found that not only the change in the chemical composition (21) but also the change in the microstructure and the physical state of the formed hydration products are mainly responsible for the strength changes in the hydrothermally treated specimens. The f i r s t increase in the compressive strength up to 6 hours autoclaving is mainly a t t r i b u t e d to the formation and l a t e r s t a b i l i zation of the i n i t i a l hydration product. After 12 hours hydration, however, the well c r y s t a l l i z a t i o n of the i n i t i a l hydrates and/or t h e i r partial transformation into other hydrates might cause a reduction in the compressive strength. Later, the new formation of inner hydration products causes a marked increase in the compressive strength as a r e s u l t of the increase in the total contents of the binding centres in the specimen. Acknowledgement One of the authors (S.A. Abo-EI-Enein) would l i k e to express his grateful appreciation to the I n s t i t u t fur mechanische Verfahrenstechnik-Universit~t Karlsruhe-West Deutschland and to Prof. Dr. Ing. H. Rumpf for all f a c i l i t i e s offered during the present research.
238
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail References
I.
A. Grudemo, Proc. 4th Inter. Symp. Cement, Washington, D.C. National Bureau of Standards, Monograph 43, Vol. I I , 615 (1960).
2.
A. Grudemo, The Chemistry of Cements, Ed. H.F.W. Taylor, Academic Press, New York, Vol. 2, 371 (1964).
3.
H. G. Midgley, Proc. Symp. Structure of Concrete, Southhampton, England, paper 24 (1968).
4.
L. E. Copeland, E. Bodor, T. N. Chang and C. H. Weisse, J. Portland Cement Assoc. Res. Develop. Lab. 9, 61 (1967).
5.
L. E. Cope!and and D. L. Kantro, Proc. 5th Inter. Symp. Chem. of Cement, Tokyo, Vol. II (1968).
6.
W. Richartz, Beton, 19, 203 and 245 (1969).
7.
H. G. Midgley, Proc. B r i t .
Ceram. Soc., 13, 89 (1969).
8.
T. D. Ciach, J. E. G i l l o t t , Res. I , 13 (1971).
E. G. Swenson and P. J. Sereda, Cement Concr.
.
T. D. Ciach and E. G. Swenson, Cement Concr. Res., ~, 159 (1971).
I0.
S. Chatterji and J. W. Jeffery, Nature, 209, 1233 (1966).
II.
R. Sierra, J. Mecroscupie, 7, 491 (1968).
12.
R. B. Williamson, Science, 164, 549 (1969).
13.
R. L. Berger, D. S. Cahn and (1970). S. Diamond, Proc. 3rd Annual (1970). R. B. Williamson, Report No. fornia, Berkeley, California
14. 15.
J. D. McGregor, J. Amer. Ceram. Soc., 53, 57 S.E.M. Symp., Chicago, I I I .
Inst. Tech., 385
UCSESM 70-23, Struct. Wngr. Lab., Univ. Cali(1970).
16.
M. H. Marcinowski and M. E. Taylor, J r . , J. Appl. Phys. 4__]_I,4753 (1970).
17.
M. Daimon, S. Ueda and R. Kondo, Cement Concr. Res. ~, 391 (1971).
18.
F. V. Lawrence, Jr. and J. F. Young, Cement Concr. Res. 3, 149 (1973).
19.
R. I. Berger, F. V. Lawrence, J r . , and J. F. Young, Cement Concr. Res. 3, 497 (1973). S. Goto, S. A. Abo-EI-Enein and R. Kondo, to be published.
20. 21. 22.
R. Sh. Mikhail, S. A. Abo-EI-Enein and N. A. Gabr, Egyptian J. of Chemist r y , submitted for publication. S. A. Abo-EI-Enein, M. A. Shater, S. M. Abd EI-Wahab and R. Sh. Mikhail, Egyptian J. of Chemistry, submitted for publication.
Vol. 7, pp. 231-238, 1977.
Pergamon Press,
Inc
MORPHOLOGY AND MICROSTRUCTURE OF AUTOCLAVED CLINKER AND SLAGLIME PASTES IN PRESENCE AND IN ABSENCE OF SILICA SAND
S. A. Abo-El-Enein, N. A. Gabr and R. Sh. Mikhail Faculty of Science Ain Shams University, Cairo, Egypt
(Communicated by R. Kondo) (Received Nov. 29, 1976; in final form Jan. 24, 1977)
ABSTRACT Development of microstructure in four hydrothermal reactions has been undertaken using scanning electron microscopy. These are clinker, clinker-sand, slag-lime and slag-lime-sand hydrothermal reactions. The microstructure of clinker hydration products displayed crumpled foils and tabular masses of calcium silicate hydrates; few cubic crystals of hydrogarnet appeared only during the initial stage of the reaction. In clinker-sand mixture the C-S-H phase was the only product identified. In slag-lime hydration the microstructure displayed both of the hydrogarnet crystals and the C-S-H phase. The hydration of slaglime-sand mixture (an optimum composition) was associated with th formation of ill-crystallized tobermorite and crystalline 11i tobermorite as the main products. Der Ablauf von der Mikrostruktur in vier hydrothermischen Reaktionen wurde mit Hilfe des Rasterelektronenmikroskops untersucht; diese sind Klinker, mit Klinker-Sand, mit Schlacke-Kalk und mit Schlacke-Kalk-Sand. Die Mikrostruktur von Klinkerhydrataionsprodukten zeigt verdriickteBelBge und massive Teilchen von CalciumKrjstalle von Hydrogarnet silikathydration; wenige kubiche sind nur zu Beginn des Prozesses zu Finden. Im Klinker-Sand Gemisch ist nur die C-S-H phase zu erkennen. Im Schlacke-Kalk Hydratationsprozess erscheinen Kristalle von Hydrogarnet und die C-S-H Phase. Die Hydratation von Schlacke-Kalk-Sand Gemisch (Optimale Zusammensetzung) wir der Formation von Kristallinarmen Tobermorit und Kristallinen 11il! Tobermorit als Haupthydratationssprdukte zugeordnet.
231
232
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail Introduction
The strength development in the hydrothermally hardened cement paste is mainly attributed to the binding centres present in the pastes as represented by the hydration product, as well as the total porosity. Therefore, the study of morphological and microstructural characteristics of the autoclave hydration products themselves is required to understand the physical and engineering properties of the hydrothermally treated hardened pastes. Early investigators generally used dispersion techniques to enable the hydration products to be observed d i r e c t l y by transmission electron microscopy, TEM, ( I - 3 ) . Others used TEM replication techniques of the fracture surfaces (4-8). In recent years scanning electron microscopy, SEM, has been extensively used for the study of the hardened pastes (9-20). The simple sample preparation techniques and the large depth of focus of the SEM makes i t a very useful tool to investigate the microstructural features of cement hydration products. This paper is one of a series of papers on the hydrothermal r e a c t i v i t i e s of blast-furnace slag, clinker and s i l i c a sand under a steam pressure of I0 atm. The f i r s t paper dealt with the experimental techniques and kinetics of four hydrothermal reactions (21). The second dealt with the corrosion of embedded reinforcing steel in neat cement pastes and mortars (22). The present paper deals with the morphology and microstructure of clinker and slag-lime pastes alone and in the presence of s i l i c a sand. The observed microstructural features offer better understanding of the more complex microstructure of the hydrothermally treated specimens and its relation to the strength development. Materials and Experimental The granulated blast-furnace slag, portland cement clinker and s i l i c a sand used in this investigation were of Blaine area of 3500 cm2/g. The chemical compositions of slag and clinker used in this study were given in an e a r l i e r paper (21). Fine powder of highly purified calcium hydroxide was used as the lime source. Four solid mixtures were studied hydrothermally: (I) clinker (I00 wt. %); ( i i ) clinker (50 wt.%) and sand (50 wt.%); ( i i i ) slag (80 wt.%) and calcium hydroxide (20 wt.%); and (iv) slag (25 wt.%), Calcium hydroxide (25 wt.%) and sand (50 wt.%). The methods of mixing, moulding and autoclaving were discussed in an e a r l i e r investigation (21). The hydrothermally hardened specimens have been studied in the range 0.5 up to 24 hours. For scanning electron microscopic (SEM) examination, the entire specimens were dried in nitrogen atmosphere (free from C02) at I05°C for 24 hours in order to remove the free water. Freshly fractured specimens (water/solid ratio=O.20) were placed in a vacuum evaporator with cathode rays and coated with a thin layer of gold of about 300-400A thickness. The specimens were stored in a dessicator prior to SEM examination. Leitz-AMR I000 scanning electron microscope was used in this investigation. Results and Discussion I.
Autoclaved Clinker Hydration Products (a) Clinker without Admixture. The i n i t i a l hydration products obtained after 0.5 hour hydration of clinker were mainly the hexagonal calcium aluminate hydrate phase which made t h e i r appearance among the hexagonal calcium hydroxide phase; the semi-crystalline tobermorite was also formed as radiating and interlocking fibres between the clinker grains (Fig. I ) . After 6 hours autoclaving of clinker, there appeared a single hexagonal crystal of calcium hydroxide linked with an interlocking fibrous structure of C-S-H; the hexagonal particles of calcium hydroxide also appeared in the structure (Fig. 2).
Vol. 7, No. 3
233 SLAG, CLINKER, AUTOCLAVED PASTES
FIG. 1 Clinker (0.5 h): Semicrystalline crumpled f o i l s of calcium s i l i c a t e hydrates, C-S-H, among various types of hydration products.
FIG. 2 Clinker (6 h): A big hexagonal crystal linked with interlocking fibres of C-S-H. The small hexagonal particles of calcium hydroxide also appear in the structure.
In the final stage of clinker hydrothermal curing, after 24 hours autoclaving, the microstructure displayed crumpled fibrous particles and massive tabular structure of tobermorite-like calcium s i l i c a t e hydrate as the dominant hydration products (Fig. 3). The results of X-ray examination and the solvent extraction methods showed that the calcium hydroxide phase was formed in the i n i t i a l stage and consumed in the final stage of the hydrothermal s o l i d i f i c a t i o n process; and the tobermorite-like calcium s i l i c a t e hydrate were the only phases identified in the final stage of the hydrothermal reaction. (b) Clinker-Sand Mixture. The microstructure of clinker-sand mixture after 0.5 hour of the hydrothermal process displayed a mixture of amorphous films of
FIG. 3 Clinker (24 h): Crumpled particles of C-S-H and massive tabular structure showing parallel cleavage of CH.
FIG. 4 Clinker-Sand (2 h): Radiating fibres and interlocking needles of C-S-H as representing the main hydration products.
234
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail
FIG. 5a Clinker-Sand (6 h): Mesh structure consisting of intertwined bundles of o r i ented needles and massive plates of C-S-H as the main hydration product.
FIG. 5b Clinker-Sand (6 h): Long radiating fibres and plates of C-S-H.
hydration products and unhydrated grains of cement minerals. After 2 hours the radiating fibres and the interlocking needles of C-S-H were the main hydration products (Fig. 4). In the intermediate stage of the hydration reaction, namely after 6 hours autoclaving, the microstructure displayed either bundles of oriented needles and massive plates or fine fibres and t i n y needle-like crystals (Figs. 5a and 5b). After 24 hours of steam curing the specimen had a dense microstructure in which plates had a s t r i a t e d and oriented appearance; the interlocking rods, long fibres and plates of w e l l - c r y s t a l l i z e d C-S-H were more predominant (Fig. 6). The amorphous hydrates could also be distinguished.
FIG. 6 Clinker-Sand (24 h): Close-textured, tabular and massive structure composed of plates which show striated and oriented appearance.
FIG. 7 Slag-Lime (0.5 h): Interlocking fibres and semi-crystalline f o i l s of tobermorit~ like hydrates (C-S-H). The cubic crystals of hydrogarnet phases were also dis tinguished in the structure.
Vol. 7, No. 3
235 SLAG, CLINKER, AUTOCLAVEDPASTES
The results of X-ray examination showed that the calcium hydroxide phase was completely consumed in the final stage of the hydrothermal reaction between clinker and sand. 2.
Autoclaved Slag Hydration Products
(a) Slag-Lime Mixture. In the early stage of the hydrothermal reaction, after 0.5 hour autoclaving at I0 atm, the hydration products appeared as interlocking fibres and semi-crystalline f o i l s of tobermorite-like hydrates (C-S-H); the cubic crystals of hydrogarnet phase were also distinguished in the structure (Fig. 7). In the intermediate stage of the hydrothermal s o l i d i f i c a t i o n process, after 6 hours autoclaving~ various forms of hydration products were formed. Among these hydrates i t was possible to distinguish fibrous and rod-like crystals, rosette-like hydrates arranged on plate-like hydration products and cubic hydrogarnet crystals having sizes of about I-2 micron in Fig. 8. After 24 hours of steam curing at I0 atm, there appeared cyrstallized fibres and rods of C-S-H phase and hydration products stacked as parallel layers with the distinction of the cubic hydrogarnet crystals (Fig. 9). From X-ray examination i t was possible to distinguish tobermorite-like C-S-H, hydrogarnet of composition C3ASH4 and calcium hydroxide among the hydration products produced during the various stages of the hydrothermal reaction. (b) Slag-Lime-Sand Mixture (An Optimum Composition). In slag-lime-sand hydrothermal reaction at I0 atm. of saturated steam, the i n i t i a l products obtained after 0.5 hour of hydration appeared as nearly amorphous C-S-H phase covering the grains of unhydrated cement components (Fig. I0). After 2 hours of hydration, the fibres formed grow into tabular and plate masses which bridge the unhydrated grains creating either an intersecting mesh arrangement or interlocking fibres and crumpled f o i l s of C-S-H (Fig. I I ) . In the intermediate stage of the hydrothermal reaction between slag, lime and sand, the space between the partly hydrated grains becomes f i l l e d in, so
FIG. 8 Slag-Lime (6 h): Rod-like and interlocking fibres of C-S-H; the cubic habit of hydrogarnet becomes more cominant.
FIG. 9 Slag-Lime (24 h): Hydration products stacked as parallel layers. The cubic habit of hydrogarnet crystals also appear.
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Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail
FIG. I0 Slag-Lime-Sand (0.5 h): Nearly amorphous C-S-H covering the grains of unhydrated cement components.
FIG. II Slag-Lime-Sand (2 h): Interlocking fibres and crumpled f o i l s of calcium s i l i c a t e hydrates f i l l i n g the pores between partly hydrated grains of cement components.
that the products obtained after 6 hours of hydration appeared as fibrous and rod-like crystals which display an oriented structure (Fig. 12). After 24 hours of steam curing at I0 atm., the main hydration products obtained were massive plates and fibrous particles with closely packed structure (Fig. 13). The results of X-ray analysis showed that the only phase i d e n t i f i e d , in slag-lime-sand hydrothermal reaction, was tobermorite-like calcium s i l i c a t e hydrate with different degrees of c r y s t a l l i n i t y . The high compressive strength obtained, using this optimum composition of slag-lime-sand mixture, was primarily associated with the formation of i l l -
FIG. 12 Slag-Lime-Sand (6 h): Fibrous and rod-like crystals of hydration products display an oriented structure.
FIG. 13 Slag-Lime-Sand (24 h): Massive plates and fibrous particles among the various types of hydration products with closely packed structure.
Vol. 7, No. 3
237 SLAG, CLINKER, AUTOCLAVED PASTES I
I
I
I
t
.--o--- C l i n k e r c~ 1200 E 0
~. 1000 ,,.C: ,,,i-, C: (b .L ,,E,.-,
:
Clinker-sand
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///~\ -'-/
~/
//
800 600
>
m
400
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0 (._)
0
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1 2
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t 12
I 2&
Autoclaving time (hours) FIG. 14 Compressive strength as a function of autoclaving time c r y s t a l l i z e d tobermorite (C-S-H(1) and C-S-H(II)) and of c r y s t a l l i n e II A tobermorite as the main hydration products. 3.
Strength Development
The dependence of the compressive strength on the autoclaving time is shown in Fig. 14 for the various pastes investigated. Obviously, the strength shows a f i r s t increase with the time of hydration up to 6 hours, then i t decreases a f t e r 12 hours and f i n a l l y increases a f t e r 24 hours of the hydrothermal s o l i d i f i c a t i o n process. I t was found that not only the change in the chemical composition (21) but also the change in the microstructure and the physical state of the formed hydration products are mainly responsible for the strength changes in the hydrothermally treated specimens. The f i r s t increase in the compressive strength up to 6 hours autoclaving is mainly a t t r i b u t e d to the formation and l a t e r s t a b i l i zation of the i n i t i a l hydration product. After 12 hours hydration, however, the well c r y s t a l l i z a t i o n of the i n i t i a l hydrates and/or t h e i r partial transformation into other hydrates might cause a reduction in the compressive strength. Later, the new formation of inner hydration products causes a marked increase in the compressive strength as a r e s u l t of the increase in the total contents of the binding centres in the specimen. Acknowledgement One of the authors (S.A. Abo-EI-Enein) would l i k e to express his grateful appreciation to the I n s t i t u t fur mechanische Verfahrenstechnik-Universit~t Karlsruhe-West Deutschland and to Prof. Dr. Ing. H. Rumpf for all f a c i l i t i e s offered during the present research.
238
Vol. 7, No. 3 S. A. Abo-EI-Enein, N. A. Gabr, R. Sh. Mikhail References
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