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On the properties of freshly made Portland cement paste. Part I. Origins of the colloidal behaviour of cement paste
CEMENT and CONCRETERESEARCH. Vol. 16, pp. 967-970, 1986. Printed in the USA. 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Journals, Ltd.
NOTES ON THE PROPERTIES OF FRESHLY MADE PORTLAND CEMENT PASTE. PART I. ORIGINS OF THE C O L L O I D A L B E H A V I O U R OF CEMENT PASTE.
S. Chatterji T e k n o l o g i s k Institut 2630 T~strup. Denmark
(Communicated by D.M. Roy) (Received Aug. 5, 1986) ABSTRACT A freshly made Portland cement paste is a very unusual fluid. It behaves like a colloidal suspension, although cement particles are much coarser than conventional colloid size. The origins of this colloidal behaviour of a cement paste have been traced to high density and high viscosity of the paste itself. Its high d e n s i t y and viscosity hinder the sedimentation of cement grains and allow for the colloid behaviour to develop.
A freshly made paste of Portland cement is, in many respects, a very unusual fluid. It behaves like a colloidal suspension even t h o u g h its solid component, i.e. Portland cement has a weight averaged mean particle radius of about 3 ~m which is much bigger than that usually associated with the colloidal state, i.e. less than about 0.3 ~m. Just like other colloidal suspensions, the flow properties of a Portland cement paste can be altered by adding a small amount of an active agent, e.g. superplasticizer, CaCI 2, etc. These changes occur in spite of large average size of Portland cement grains. It is more d i f f i c u l t to m a i n t a i n the increased fluidity of a sup e r p l a s t i c i z e d paste of a fine-grained ASTM Type III cement than a paste of a coarse-grained ASTM Type I cement. It is not clear why a paste made with a coarser grained cement, when superplasticized, can m a i n t a i n its increased fluidity longer than a corresponding paste made with a fine grained cement. The settling behaviour of cement particles in a freshly made cement paste is also unusual. If a freshly made cement paste is allowed to stand undisturbed, cement grains tend to settle to the
967
968
Vol. 16, ~4o. 6 S. C h a t t e r j i
bottom and clear water appears at the top. During this settlement all Portland cement grains settle at the same speed irrespective of their size and density (I). On the other hand, in a suspension of low cement c o n c e n t r a t i o n the individual cement grains settle at a speed appropriate to their size and density. It appears that the settling behaviour of cement grains changes with their c o n c e n t r a t i o n in a suspension. The above examples are only a selection of the unusual properties of freshly made cement paste. It appears that at present there are no generally accepted explanations for these unusual properties. In this and subsequent short notes attention will be drawn to different p h y s i c o - c h e m i c a l processes which govern the properties of cement paste. In each paper one of these p h y s i c o - c h e m i c a l processes will be studied more closely; although it is realised that in a real paste a number of p r o c e s s e s may operate concurrently. The object of this series is to start a general discussion on various aspects of fresh cement paste so that an unified view may emerge. It is known that the Brownian movement is one of the characteristics of solid particles of a colloidal suspension. Any combination of p h y s i c o - c h e m i c a l processes which may allow for a Brownian-like movement of cement grains in a cement paste will also explain the colloidal nature of cement paste. According to Maxwell's kinetic theory, the mean kinetic energy of any molecule, i.e. 0.5 m • ~2 in an e q u i l i b r i a t e d system is constant at any given temperature. This rule of the constancy of the kinetic energy does not stipulate any limit to the mass of the molecule; so it should apply equally to very large and very small molecules. The above rule suggests that in particulate suspensions, where g r a v i t a t i o n a l force plays no significant part, the suspended particles will have the same mean kinetic energy as the molecules of the surrounding medium, i.e. the suspended particles will behave like large molecules. In these systems the suspended particles will show Brownian movement just like molecules of the surrounding medium. This is similar to the assumption that Perrin made in his work on the Brownian movement. The more prominent the role of the g r a v i t a t i o n a l force, the less will be the B r o w n i a n movement and colloidal nature of the suspension. The significance of the above assumption is best illustrated by calculating the s e d i m e n t a t i o n velocity of cement particles in thin suspensions as well as in thick paste and comparing them with those of conventional colloid particles, e.g. colloidal gold The sedimentation v e l o c i t y of a particle, Vs, may be calculated from the following formula: V
= 2 s
where
g
"(Ps - Pm)" 9
r~
........
1
g is the gravitational constant Ps, Pm are the densities of the solid and the surrounding medium rs is the radius of the solid particle q is the v i s c o s i t y of the medium
Vol. 16, No. 6
969 COLLOID, CEMENT PASTE, SETTLING
In the case of an aqueous s u s p e n s i o n of c o l l o i d a l gold the conc e n t r a t i o n s of gold and e l e c t r o l y t e s are so low that Pm is that of water. From e q u a t i o n (I) it may be c a l c u l a t e d that a 0.3 ~m r a d i u s gold p a r t i c l e will have a s e d i m e n t a t i o n v e l o c i t y of 3.6 ~ m / s e c and a 3 ~m r a d i u s gold p a r t i c l e will have a velocity of 360 ~m/sec. In thin s u s p e n s i o n s of cement in w a t e r Pm is same as that of water and a 0.3 ~m radius cement g r a i n will have a s e d i m e n t a tion v e l o c i t y of 0.4 ~ m / s e c and that of a 3 ~m r a d i u s g r a i n 42 ~m/sec. In the case of a thick cement p a s t e Pm will be that of the paste itself (2). This increase in Pm alone will reduce the s e d i m e n t a t i o n v e l o c i t y of a 3 ~m r a d i u s cement grain. For e x a m p l e in a cement paste having a w a t e r / c e m e n t ratio of 0.5, ~m will be 1.83 and the s e d i m e n t a t i o n v e l o c i t y of a 3 ~m c e m e n t g r a i n will be 26 ~m/sec. In the case of a thick paste the choice of q is not very s t r a i g h t forward; the value of D could be e i t h e r that of water or that of the paste. In the case of the s e d i m e n t a t i o n of a large a g g r e g a t e , i.e. g r a v e l in a paste, the v i s c o s i t y of the paste needs to be c o n s i d e r e d . By analogy one may c o n s i d e r that even in the case of cement g r a i n s q of the paste itself should be the p r o p e r choice. In this second case, the sed i m e n t a t i o n v e l o c i t y of a 3 ~m cement g r a i n will be m u c h lower. It will be of interest to c a l c u l a t e the m e a n v e l o c i t y , due to k i n e t i c energy, of both cement and gold p a r t i c l e s of d i f f e r e n t sizes and c o m p a r e them with the c o r r e s p o n d i n g s e d i m e n t a t i o n velocity. The m e a n v e l o c i t y , ~, due to the k i n e t i c e n e r g y is g i v e n by = where
k T m
48kT/~m
......
2
is the B o l t z m a n n c o n s t a n t is the a b s o l u t e t e m p e r a t u r e of the is the mass of the particle.
suspension
In the following table the m e a n v e l o c i t y and the s e d i m e n t a t i o n v e l o c i t y (at r o o m t e m p e r a t u r e ) of d i f f e r e n t size p a r t i c l e s have been collected. Particle radius, ~m
Material
~ ~m/sec
Vs ~m/sec
0.3
Gold
2172
3.0
Gold
69
360
5.2174
3.0
Cement
170
26
0.1529
3.6
Vs/~
0.0016
From the table it can be seen that the m o v e m e n t of a 0.3 ~m radius gold p a r t i c l e will be d e t e r m i n e d by ~, w h e r e a s the m o v e m e n t of a 3 ~m r a d i u s gold p a r t i c l e will be d e t e r m i n e d m a i n l y by Vs. In the case of 3 ~m P o r t l a n d cement p a r t i c l e s in a paste, their m o v e m e n t will be a f f e c t e d both by ~ and Vs, i.e. they will form a weak c o l l o i d a l s y s t e m which will s e d i m e n t on s t a n d i n g .
970
Vol. 16, ~o. 6 S. C h a t t e r j i
From the above calculations it can be seen that in the case of conventional gold colloids particles of 0.3 ~m radius or less will be able to attain the equilibrium with the medium and show Brownian m o v e m e n t and colloidal properties. The larger the particles, the more the g r a v i t a t i o n a l force will determine their movement and the less prominent will be their colloidal nature. A similar calculation showed that in thin suspensions of Portland cement only grains below about 0.8 ~m size will show colloidal behaviour. In the case of thick cement paste larger particles will show colloidal behaviour. This will partly explain the variation of settling behaviour of cement grains with their concentration in a suspension. In a subsequent paper another effect of particle concentration will be discussed. I should like to thank M. Brandt Pedersen, N. Thaulow, and A. Damgaard Jensen for their friendly support and assistance in the development of these ideas. References (I) Wheeler
(2) Steinour
and Chatterji
- a) J.Amer. Ceram. Soc. 55, 461-64, 1972 b) Indian Conc. J. 47, 147-50, 1973 c) Indian Conc.J. 47, 194-97, 1973
- The settling of Portland Cement. Bull. 98. Portland Cement Assoc. 33 West Grand Ave. Chicago 10. Ill. 1958.
NOTES ON THE PROPERTIES OF FRESHLY MADE PORTLAND CEMENT PASTE. PART I. ORIGINS OF THE C O L L O I D A L B E H A V I O U R OF CEMENT PASTE.
S. Chatterji T e k n o l o g i s k Institut 2630 T~strup. Denmark
(Communicated by D.M. Roy) (Received Aug. 5, 1986) ABSTRACT A freshly made Portland cement paste is a very unusual fluid. It behaves like a colloidal suspension, although cement particles are much coarser than conventional colloid size. The origins of this colloidal behaviour of a cement paste have been traced to high density and high viscosity of the paste itself. Its high d e n s i t y and viscosity hinder the sedimentation of cement grains and allow for the colloid behaviour to develop.
A freshly made paste of Portland cement is, in many respects, a very unusual fluid. It behaves like a colloidal suspension even t h o u g h its solid component, i.e. Portland cement has a weight averaged mean particle radius of about 3 ~m which is much bigger than that usually associated with the colloidal state, i.e. less than about 0.3 ~m. Just like other colloidal suspensions, the flow properties of a Portland cement paste can be altered by adding a small amount of an active agent, e.g. superplasticizer, CaCI 2, etc. These changes occur in spite of large average size of Portland cement grains. It is more d i f f i c u l t to m a i n t a i n the increased fluidity of a sup e r p l a s t i c i z e d paste of a fine-grained ASTM Type III cement than a paste of a coarse-grained ASTM Type I cement. It is not clear why a paste made with a coarser grained cement, when superplasticized, can m a i n t a i n its increased fluidity longer than a corresponding paste made with a fine grained cement. The settling behaviour of cement particles in a freshly made cement paste is also unusual. If a freshly made cement paste is allowed to stand undisturbed, cement grains tend to settle to the
967
968
Vol. 16, ~4o. 6 S. C h a t t e r j i
bottom and clear water appears at the top. During this settlement all Portland cement grains settle at the same speed irrespective of their size and density (I). On the other hand, in a suspension of low cement c o n c e n t r a t i o n the individual cement grains settle at a speed appropriate to their size and density. It appears that the settling behaviour of cement grains changes with their c o n c e n t r a t i o n in a suspension. The above examples are only a selection of the unusual properties of freshly made cement paste. It appears that at present there are no generally accepted explanations for these unusual properties. In this and subsequent short notes attention will be drawn to different p h y s i c o - c h e m i c a l processes which govern the properties of cement paste. In each paper one of these p h y s i c o - c h e m i c a l processes will be studied more closely; although it is realised that in a real paste a number of p r o c e s s e s may operate concurrently. The object of this series is to start a general discussion on various aspects of fresh cement paste so that an unified view may emerge. It is known that the Brownian movement is one of the characteristics of solid particles of a colloidal suspension. Any combination of p h y s i c o - c h e m i c a l processes which may allow for a Brownian-like movement of cement grains in a cement paste will also explain the colloidal nature of cement paste. According to Maxwell's kinetic theory, the mean kinetic energy of any molecule, i.e. 0.5 m • ~2 in an e q u i l i b r i a t e d system is constant at any given temperature. This rule of the constancy of the kinetic energy does not stipulate any limit to the mass of the molecule; so it should apply equally to very large and very small molecules. The above rule suggests that in particulate suspensions, where g r a v i t a t i o n a l force plays no significant part, the suspended particles will have the same mean kinetic energy as the molecules of the surrounding medium, i.e. the suspended particles will behave like large molecules. In these systems the suspended particles will show Brownian movement just like molecules of the surrounding medium. This is similar to the assumption that Perrin made in his work on the Brownian movement. The more prominent the role of the g r a v i t a t i o n a l force, the less will be the B r o w n i a n movement and colloidal nature of the suspension. The significance of the above assumption is best illustrated by calculating the s e d i m e n t a t i o n velocity of cement particles in thin suspensions as well as in thick paste and comparing them with those of conventional colloid particles, e.g. colloidal gold The sedimentation v e l o c i t y of a particle, Vs, may be calculated from the following formula: V
= 2 s
where
g
"(Ps - Pm)" 9
r~
........
1
g is the gravitational constant Ps, Pm are the densities of the solid and the surrounding medium rs is the radius of the solid particle q is the v i s c o s i t y of the medium
Vol. 16, No. 6
969 COLLOID, CEMENT PASTE, SETTLING
In the case of an aqueous s u s p e n s i o n of c o l l o i d a l gold the conc e n t r a t i o n s of gold and e l e c t r o l y t e s are so low that Pm is that of water. From e q u a t i o n (I) it may be c a l c u l a t e d that a 0.3 ~m r a d i u s gold p a r t i c l e will have a s e d i m e n t a t i o n v e l o c i t y of 3.6 ~ m / s e c and a 3 ~m r a d i u s gold p a r t i c l e will have a velocity of 360 ~m/sec. In thin s u s p e n s i o n s of cement in w a t e r Pm is same as that of water and a 0.3 ~m radius cement g r a i n will have a s e d i m e n t a tion v e l o c i t y of 0.4 ~ m / s e c and that of a 3 ~m r a d i u s g r a i n 42 ~m/sec. In the case of a thick cement p a s t e Pm will be that of the paste itself (2). This increase in Pm alone will reduce the s e d i m e n t a t i o n v e l o c i t y of a 3 ~m r a d i u s cement grain. For e x a m p l e in a cement paste having a w a t e r / c e m e n t ratio of 0.5, ~m will be 1.83 and the s e d i m e n t a t i o n v e l o c i t y of a 3 ~m c e m e n t g r a i n will be 26 ~m/sec. In the case of a thick paste the choice of q is not very s t r a i g h t forward; the value of D could be e i t h e r that of water or that of the paste. In the case of the s e d i m e n t a t i o n of a large a g g r e g a t e , i.e. g r a v e l in a paste, the v i s c o s i t y of the paste needs to be c o n s i d e r e d . By analogy one may c o n s i d e r that even in the case of cement g r a i n s q of the paste itself should be the p r o p e r choice. In this second case, the sed i m e n t a t i o n v e l o c i t y of a 3 ~m cement g r a i n will be m u c h lower. It will be of interest to c a l c u l a t e the m e a n v e l o c i t y , due to k i n e t i c energy, of both cement and gold p a r t i c l e s of d i f f e r e n t sizes and c o m p a r e them with the c o r r e s p o n d i n g s e d i m e n t a t i o n velocity. The m e a n v e l o c i t y , ~, due to the k i n e t i c e n e r g y is g i v e n by = where
k T m
48kT/~m
......
2
is the B o l t z m a n n c o n s t a n t is the a b s o l u t e t e m p e r a t u r e of the is the mass of the particle.
suspension
In the following table the m e a n v e l o c i t y and the s e d i m e n t a t i o n v e l o c i t y (at r o o m t e m p e r a t u r e ) of d i f f e r e n t size p a r t i c l e s have been collected. Particle radius, ~m
Material
~ ~m/sec
Vs ~m/sec
0.3
Gold
2172
3.0
Gold
69
360
5.2174
3.0
Cement
170
26
0.1529
3.6
Vs/~
0.0016
From the table it can be seen that the m o v e m e n t of a 0.3 ~m radius gold p a r t i c l e will be d e t e r m i n e d by ~, w h e r e a s the m o v e m e n t of a 3 ~m r a d i u s gold p a r t i c l e will be d e t e r m i n e d m a i n l y by Vs. In the case of 3 ~m P o r t l a n d cement p a r t i c l e s in a paste, their m o v e m e n t will be a f f e c t e d both by ~ and Vs, i.e. they will form a weak c o l l o i d a l s y s t e m which will s e d i m e n t on s t a n d i n g .
970
Vol. 16, ~o. 6 S. C h a t t e r j i
From the above calculations it can be seen that in the case of conventional gold colloids particles of 0.3 ~m radius or less will be able to attain the equilibrium with the medium and show Brownian m o v e m e n t and colloidal properties. The larger the particles, the more the g r a v i t a t i o n a l force will determine their movement and the less prominent will be their colloidal nature. A similar calculation showed that in thin suspensions of Portland cement only grains below about 0.8 ~m size will show colloidal behaviour. In the case of thick cement paste larger particles will show colloidal behaviour. This will partly explain the variation of settling behaviour of cement grains with their concentration in a suspension. In a subsequent paper another effect of particle concentration will be discussed. I should like to thank M. Brandt Pedersen, N. Thaulow, and A. Damgaard Jensen for their friendly support and assistance in the development of these ideas. References (I) Wheeler
(2) Steinour
and Chatterji
- a) J.Amer. Ceram. Soc. 55, 461-64, 1972 b) Indian Conc. J. 47, 147-50, 1973 c) Indian Conc.J. 47, 194-97, 1973
- The settling of Portland Cement. Bull. 98. Portland Cement Assoc. 33 West Grand Ave. Chicago 10. Ill. 1958.