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Influence of sorbed fluids on compressive strength of cement paste
CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 225-232, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.
INFLUENCE OF SORBED FLUIDS ON COMPRESSIVE S T R ~ G T H OF CEMENT PASTE
B. Robertson and R.H. Mills Department of Civil Engineering University of Toronto Toronto, Canada M55 IA4
(Communicated by D.M. Roy) (Received May 3, 1984) Abstract HCP cylinders made with water/cement ratios = 0.3 and 0.5 were vacuum dried at I00°C before saturation in a range of polar liquids and nonpolar Cyclohexane. Compressive s t r e n g t h v a r i e d from I to 2.3 times the water-saturated strengths and was ordered with increase in polarity. Maximum strength and minimum sorption was obtained with cyclohexane having the largest molecule. The weakening effect of water was associated with its capacity to penetrate regions inaccessible to the other fluids.
Introduction S h r i n k a g e and s w e l l i n g of c e m e n t p a s t e s r e s u l t i n g from loss or gain of moisture arises from the movement of water in narrowly confined spaces separating layered solid in C-S-H gel. Intensive study of these phenomena has led to the d e y e l o p m e n t of s e v e r a l m o d e l s of w a t e r - s o l i d i n t e r a c t i o n (1,2,3). Variation of strength accompanying moisture movement has received comparatively l i t t l e a t t e n t i o n yet it m i g h t p r o v i d e i m p o r t a n t e v i d e n c e in s u p p o r t of a l t e r n a t i v e c o n c e p t s (4,5) of the n a t u r e of w a t e r h e l d at low v a p o u r pressures. M u c h of the p u b l i s h e d work on s t r e n g t h - m o i s t u r e c o n t e n t has i n v o l v e d miniature specimens which were failed in flexure (2,6). Recently Beaudoin (7) adopted a fracture mechanics approach using notched beam specimens. The notch dimensions were 250 x 10 -6 m or somewhere between one and ten thousand times the dimension of spaces between C-S-H layers or seven times the average size of a cement grain. The tip of the notch had the dimension of a semi-infinite plane compared with interlayer (I) space separating solids in C-S-H gel. The concept of material failure through crack propagation (8) leads to a paradoxical situation with respect to oven dried concrete and mortar; and oven or d-dried cement paste (9). However carefully the drying is done, cracking c a n n o t be a v o i d e d , yet the s t r e n g t h may be as m u c h as t w i c e that of the 225
226
Vol. 15, No. 2 B. Robertson and R.H. Mills
undisturbed but saturated material. Such cracks, which are visible in nearly every SEM fractograph, (10) are massive compared with I space dimensions which are too small to be r e s o l v e d in the SEM. Mikhail and Selim (11) showed that in the same HCP, different areas were a v a i l a b l e to sorption of water, methanol, isopropanol and cyclohexane. Feldman (12) differentiated between length change isotherms using Methanol and, alternatively, water as sorbents. It was expected that varying strength characteristics should result from saturation of HCP in fluids with different interfacial energies. The present work was designed to investigate the effects of saturation of HCP with fluids of various molecular dimensions, and "polarity" as defined by R e i c h a r d t (13). The r e f e r e n c e s t a t e s were o v e n - d r i e d HCP and o v e n - d r i e d HCP re-saturated with water. Porosity was varied by choosing two water:cement ratios, 0.3 and 0.5, and the pore structure was varied by steam curing half the specimens.
Experimental Materials and Specimen Preparation Type III Portland cement having the following oxide composition was used: Si02 = 20.8%; A 1 2 0 ~ = 4.7%; Fe203 = 1.9%; CaO = 63.3%; M g O = 3.3%; S03 = 3.9%; K20 = 0.7%; and Ignition loss 1.1%. The Blaine fineness was 574 m2/kg and the specific gravity was 3.21. Pastes were prepared with water/cement ratios, wo 0.3 and 0.5. A s o l i d s u p e r p l a s t i c i s e r was used so that b o t h f r e s h p a s t e s w e r e v e r y fluid. The p a s t e s w e r e p o u r e d into p l a s t i c f i l m c o n t a i n e r s w i t h average diameter = 29.5 and length = 48 mm. The flexible caps were pierced to a l l o w closure without trapping air, and finally sealed with adhesive tape. The specimens were rotated end for end until final set took place in order tO maintain even dispersion of the cement grains. Chemical shrinkage was accommodated by flexure of the plastic cap. =
After one day, one g r o u p of s p e c i m e n s were s t r i p p e d and w a t e r - c u r e d at At this age the ends were g r o u n d f l a t on a l a p p i n g t a b l e 23°C for 28 days. and the s p e c i m e n s were d r i e d to c o n s t a n t w e i g h t in a v a c u u m o v e n at 110°C, cooled in a vacuum desslcator and then subjected to various treatments before test. A s e c o n d g r o u p of s p e c i m e n s were s t e a m c u r e d at 100 kPa p r e s s u r e a f t e r r o t a t i o n and f i n a l set, had t a k e n place. S t e a m c u r i n g l a s t e d 32 hours, and was then f o l l o w e d by the same t r e a t m e n t a s the w a t e r c u r e d s p e c i m e n s in the first group.
Treatment
before crushing
(a) S p e c i m e n s were v a c u u m s a t u r a t e d , in b a t c h e s having the properties listed in Table I.
of ten,
in s e v e n
liquids
Liquids B-E had approximately the same dipole moment but a wide range of m o l e c u l a r volumes; F and G had similar volumes but one had the maximum, and the other zero, dipole moment of the group.
Vol. 15, No. 2
227 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
Table I Liquids used to saturate oven-dried pastes Ref
A B C D E F G
Liquid
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane
V x = M/g ml/mol
18 41 58 77 92 106 108
Dipole Moment c.m/10 -3° 6.23 5.79 5.89 5.85 5.75 9.37 0
Polarity E A kJ/mol
264 232 224 203 210 -
V* = approximate molecular volume - Molecular weight M/specific gravity g E A = empirical value given by Reichardt (13).
(b) S p e c i m e n s , in b a t c h e s of ten, were a l l o w e d per cent of the water lost in oven-drying.
to take up 20, 40, 60 and 80
The fraction of pore space taken up on vacuum saturation is shown in Table 2 as k = VL/V w. Table 2 Values of k = VL/Vw; where V L = Volume of liquid and V w = Volume of water in pores Water:cement ratio: Curing Porosity-per cent A
B C D E F G
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane
NC = Normal water curing
0.3 NC 31.2 1.0 0.70 0.67 0.66 0.65 0.72 0.62
0.5 SC 35.40 1.0 0.92 0.93 0.83 0.82 0.94 0.79
NC 44.7 1.0 0.76 0.75 0.74 0.73 0.79 0.71
SC 44.7 1.0 0.95 0.85 0.88 0.85 0.90 0.83
SC = Steam curing
The liquids are listed A to G in increasing order of V*. It is seen tha~ the volume occupied by the liquids is in descending order with increase in V except for F which is out of phase with the other liquids. Crushing Values These are expressed as a factor equal to the strength in the stated condition d i v i d e d by the s t r e n g t h of w a t e r - s a t u r a t e d specimens. Experimental values are shown in Table 3.
228
Vol. 15, No. 2 B. Robertson and R.H. Mills Table 3 Strength of saturated specimens given as multiples of the water-saturated strength Water:cement ratio: Curing Water Saturated Strength MPa (a) A B C D E F G
0.5
NC
SC
NC
SC
43.4
47.8
29.8
31.2
Saturated Strength Factors
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane (b)
0.3
1.0 2.0 1.9 2.3 2.2 2.0 2.2
1.0 1.5 1.8 1.8 1.8 1.4 1.9
1.0 1.6 1.5 1.7 1.6 1.3 1.8
1.0 1.5 1.8 2.1 1.8 1.8 2.3
Partially Saturated Strength Factors
Water replacement 20% 40% 60% 80%
1.51 1.34 1.32 1.24
0.95 0.86 0.94 1.01
1.41 1.08 1.21 1.16
1.53 1.22 1.27 1.29
Oven-dry
2.5
1.9
1.7
2.3
It is seen that the strength of F-saturated HCP is also out of phase with respect to molecular volume. The strengths of specimens saturated with Cyclohexane do not differ significantly from those of oven-dried specimens. Water Surface area Dry s p e c i m e n s were c r u s h e d and s c r e e n e d t h r o u g h a No. 50 s i e v e and then brought to mass equilibrium in vacuum dessicators over sulphuric acid solutions of varying concentration. BET surface areas were calculated from the sorption isotherms. Results are shown in Table 4. Table 4 Water surface areas calculated by the BET method (14) Water/cement ratio Curing ZBE T m2/gm
0.3 NC 110
0.5 SC 106
NC 138
SC 115
It is clear that the expected change in pore size distribution of steamcured specimens (14) did not take place, probably because of the significant period of water curing which followed steam treatment and which resulted in additional gel formation.
Vol. 15, No. 2
229 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
Sedimentation of finely ground HCP i__n_nvariou_____~s liquids Dry specimens of the 4 HCP's were m i l l e d to pass the 150 um sieve, shaken up in the series A to G liquids and a l l o w e d to settle to a stable volume. The results are shown in Table 5. Table 5 Bulk volume of -150 um material in the fluid shown Sedimination Volume ml/ml of initially oven-dry normally cured w o = 0.3 HCP Liquid
A B C D E F G
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane • unknown
Polarity E A kJ/mol 264 232 224 203 210 * *
Dry Solid Volume Dry Bulk Volume
Volume of Sediment 4.9 4.2 4.1 3.9 4.0 3.6 6.6
1.00 1.80
The sedimentation volume decreases with decrease in polarity in the series A to E. By far the g r e a t e s t s e d i m e n t a t i o n v o l u m e was for n o n - p o l a r C y c l o h e x a n e w h i c h a l s o had n e g l i g i b l e i n f l u e n c e on dry strength. The h i g h b u l k v o l u m e in w a t e r was p a r t l y due to h y d r a t i o n of c l i n k e r residue, e x p o s e d by crushing. Discussion T h i r t y e i g h t y e a r s ago B a n g h a m (15) d e s c r i b e d the d i l a t i o n of p o r o u s solids, which results from adsorption of water, as the result of "... a wedgelike action resulting from m o l e c u l a r bombardment at sharp re-entrant angles at the surface". If, for HCP following Powers and others (1,16,17), one assumes that Tobermorite sheets in the gel are buckled, and also that the separation v a r i e s from 0.4 to 3 nm, the c o n c e p t of w e d g i n g a c t i o n has some appeal. The spaces b o u n d e d by c o n v e r g i n g w a l l s are a p p r o x i m a t e l y w e d g e - s h a p e d . These wedges probably have their axes normal to the surface of clinker residue but in any case are randomly oriented within the gross structure of HCP. Adsorbed water molecules are attracted to sorption sites on solid surfaces towards the a p e x but r e t a i n some f r e e d o m of m o v e m e n t n o r m a l to the plane. In r e g i o n s where the wedge-space narrows to molecular dimensions the rate of collision between water molecules and the surface increases and results in a wedging or disjoining pressure on the solid and, consequently, the solid-solid van der Waal's forces are weakened. When the HCP is dried, water m o l e c u l e s migrate away from the apex and van der Waal's bonds between the solid species are strengthened. Upon re-saturation w i t h w a t e r the p r o c e s s is r e v e r s i b l e e x c e p t for new s o l i d - s o l i d bonds
230
Vol. 15, No. 2 B. Robertson and R.H. Mills
w h i c h are s t r o n g e r than the s o l i d - w a t e r molecules cannot enter spaces where water not a t t e n u a t e the s o l i d - s o l i d b o n d s to therefore, do not have the same weakening
attraction. Substances with large can exert disjoining pressure and do the same extent as w a t e r does, and effect.
This s i m p l e c o n c e p t does not take a c c o u n t of other f a c t o r s w h i c h are of importance. These h a v e b e e n e x t e n s i v e l y r e v i e w e d (18) and o n l y a few are mentioned here: (a)
Some of the water held at low pressures by the gel may be taken up as hydrate water and become part of the solid (19).
(b)
The weakening effect of water taken up at low temperatures may be due to degeneration of silaxane groups (2).
(c)
The change in surface energy resulting from water sorption affects the stress at which Griffith cracks will propogate (20).
The r o l e of Ca(OH) 2 has so far been n e g l e c t e d . In a dry HCP, s o l i d Ca(OH) 2 which is precipitated out of solution probably contributes to strength and the r e v e r s e is true w h e n it is d i s s o l v e d by s o r b e d water. Most authors treat pore water as pure water though it must contain dissolved electrolytes which exert a weakening effect (21). P r e v i o u s work on the f l e x u r a l s t r e n g h t of Q u a r t z (22,23) and G l a s s (24) showed that immersion in water and other organic fluids resulted in reduction of s t r e n g t h which, as in HCP, was m a x i m u m for water, and a p p e a r e d to be related to "polarity" E A (13). FIG. I shows these data plotted on the same diagram as mean values for the two HCP's of this investigation.
STRESS 2.S
RATIO .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
FIG. I 2.8
wo
1,5
I,
Strength of saturated specimens related to water saturated strength = I as function of "polarity" E A =
0.3
(O)
w =0.5 Q Ref. 2~ Glass Ref. 22 Quartz ( ~
t 60
22e E&-kJ/mol
280
Vol. 15, No. 2
231 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
The same trend is evident for the three different materials. It appears that sorption-induced reduction of surface energy may be the principle mechanism which is responsible for the different strengths of materials A, B, C, D and E. The sedminentatlon volume, which is a function of interfacial free energy, did not reveal a consistent trend in the case of HCP though it appeared to be significant in the case of glass (24). Interpolation of two sorption isotherms for the normally cured HCP's of this series indicated that the volume of wedge-space not penetrated by liquids B-G would be filled by water at relative vapour pressures ranging from 0.48 for 2-Pentanone to 0.65 for Cyclohexane. This corresponds to space dimensions of 3 to 5 nm ( 4 ) . This was not p u r s u e d in greater depth b e c a u s e of the scarcity of data.
Conclusion The principal weakening effects of pore water in HCP are
(a)
The Bangham effect which results and
(b)
The s u r f a c e e n e r g y effect w h i c h reduces the amount of e x t e r n a l work necessary to propogate cracks.
in attenuation of solid-solid bonds;
References I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
T.C. Powers, M a t e r i a l s and S t r u c t u r e s r e s e a r c h and testing I, 6 pp 487508, Dec. 1968. P.J. Sereda, R.F. F e l d m a n and E.G. Swenson, H i g h w a y R e s e a r c h Board SR90, pp 58-73, 1966. Z.P. Bazant, Materials and Structures research and testing 3, 13 pp 3-36, 1970. R.H. Mills, Materials and Structures research and testing I, 6 pp 553558, 1968. R.F. Feldman, Cem. Concr. Res. 2, 375, 1972. S. Mindess, Mats. Res. Ser. 2, University of British Columbia, p 96. J.J. Beaudoin, Cem. Concr. Res. 12, pp 705-716, 1982. A.A. Griffith, Phil. Trans. Series A 221, pp 163-198, Royal Society 1920. R~H. M i l l s , Int. Symp. on c o n c r e t e and r e i n f o r c e d c o n c r e t e in hot countries. Haifa 1960. J.J. Beaudoin, and R.H. Mills, Mechanical Behaviour of Materials, pp 184192, Soc. Mat. Sci. Japan 1972. R.S. M i k h a i l and S.A. Sellm, H i g h w a y R e s e a r c h Board SR90, pp 123-134, 1966. R.F. Feldman, V International Symposium on the Chemistry of Cement, Vol. 3, Pp 53-66 (1968). C. Reichardt, Angew. Chem. Vol. 4, I pp 29-40 (1965). T.C. Powers, J.A.C.I. Proceedings, Vol. 43, p 301, 1946. D.H~ Baughan, Sym. Soc. Chem. Ind. London, May 1946. V.S. R a m a c h a n d r a n , R.F. F e l d m a n and J.J. Beaudoin, C o n c r e t e Science, pp 55-59, Heyden and Son 1981. O. Isai, Applied Materials Research, July 1966, pp 154-161. Ref. 16, Chapter 2.
232
Vol. 15, No. 2 B. Robertson and R.H. Mills
19. 20. 21. 22. 23. 24.
R.F. Feldman, Ref. 12, V3, pp 84-85. M.J. Setzer and F.H. Wittman, Applied Physics 3, 403 (1974). P.A. Rehbinder, L.A. S c h r e i n e r and K.F. Zhigach, H a r d n e s s r e d u c e r s in drilling, Academy of Science, Moscow 1944, CSIR Melbourne 1948. M.L. Hammond and S.F. Ravitz, J. Amer. Ceram. Soc. 396, 215-217, 1956. H. leRoux, D e l a y e d F r a c t u r e of Fused Quartz in Water Vapour, Ph.D. Thesis, University of the Witwatersrand, 1963. D. M c C a m m o n d , A.W. N e u m a n n and N. Natarajan, J. Amer. Ceram. Soc. Vol. 58, No I, pp 15-17, 1975.
Acknowledgement The support of the Natural Sciences Canada is gratefully acknowledged.
and
Engineering
Research
Council
of
INFLUENCE OF SORBED FLUIDS ON COMPRESSIVE S T R ~ G T H OF CEMENT PASTE
B. Robertson and R.H. Mills Department of Civil Engineering University of Toronto Toronto, Canada M55 IA4
(Communicated by D.M. Roy) (Received May 3, 1984) Abstract HCP cylinders made with water/cement ratios = 0.3 and 0.5 were vacuum dried at I00°C before saturation in a range of polar liquids and nonpolar Cyclohexane. Compressive s t r e n g t h v a r i e d from I to 2.3 times the water-saturated strengths and was ordered with increase in polarity. Maximum strength and minimum sorption was obtained with cyclohexane having the largest molecule. The weakening effect of water was associated with its capacity to penetrate regions inaccessible to the other fluids.
Introduction S h r i n k a g e and s w e l l i n g of c e m e n t p a s t e s r e s u l t i n g from loss or gain of moisture arises from the movement of water in narrowly confined spaces separating layered solid in C-S-H gel. Intensive study of these phenomena has led to the d e y e l o p m e n t of s e v e r a l m o d e l s of w a t e r - s o l i d i n t e r a c t i o n (1,2,3). Variation of strength accompanying moisture movement has received comparatively l i t t l e a t t e n t i o n yet it m i g h t p r o v i d e i m p o r t a n t e v i d e n c e in s u p p o r t of a l t e r n a t i v e c o n c e p t s (4,5) of the n a t u r e of w a t e r h e l d at low v a p o u r pressures. M u c h of the p u b l i s h e d work on s t r e n g t h - m o i s t u r e c o n t e n t has i n v o l v e d miniature specimens which were failed in flexure (2,6). Recently Beaudoin (7) adopted a fracture mechanics approach using notched beam specimens. The notch dimensions were 250 x 10 -6 m or somewhere between one and ten thousand times the dimension of spaces between C-S-H layers or seven times the average size of a cement grain. The tip of the notch had the dimension of a semi-infinite plane compared with interlayer (I) space separating solids in C-S-H gel. The concept of material failure through crack propagation (8) leads to a paradoxical situation with respect to oven dried concrete and mortar; and oven or d-dried cement paste (9). However carefully the drying is done, cracking c a n n o t be a v o i d e d , yet the s t r e n g t h may be as m u c h as t w i c e that of the 225
226
Vol. 15, No. 2 B. Robertson and R.H. Mills
undisturbed but saturated material. Such cracks, which are visible in nearly every SEM fractograph, (10) are massive compared with I space dimensions which are too small to be r e s o l v e d in the SEM. Mikhail and Selim (11) showed that in the same HCP, different areas were a v a i l a b l e to sorption of water, methanol, isopropanol and cyclohexane. Feldman (12) differentiated between length change isotherms using Methanol and, alternatively, water as sorbents. It was expected that varying strength characteristics should result from saturation of HCP in fluids with different interfacial energies. The present work was designed to investigate the effects of saturation of HCP with fluids of various molecular dimensions, and "polarity" as defined by R e i c h a r d t (13). The r e f e r e n c e s t a t e s were o v e n - d r i e d HCP and o v e n - d r i e d HCP re-saturated with water. Porosity was varied by choosing two water:cement ratios, 0.3 and 0.5, and the pore structure was varied by steam curing half the specimens.
Experimental Materials and Specimen Preparation Type III Portland cement having the following oxide composition was used: Si02 = 20.8%; A 1 2 0 ~ = 4.7%; Fe203 = 1.9%; CaO = 63.3%; M g O = 3.3%; S03 = 3.9%; K20 = 0.7%; and Ignition loss 1.1%. The Blaine fineness was 574 m2/kg and the specific gravity was 3.21. Pastes were prepared with water/cement ratios, wo 0.3 and 0.5. A s o l i d s u p e r p l a s t i c i s e r was used so that b o t h f r e s h p a s t e s w e r e v e r y fluid. The p a s t e s w e r e p o u r e d into p l a s t i c f i l m c o n t a i n e r s w i t h average diameter = 29.5 and length = 48 mm. The flexible caps were pierced to a l l o w closure without trapping air, and finally sealed with adhesive tape. The specimens were rotated end for end until final set took place in order tO maintain even dispersion of the cement grains. Chemical shrinkage was accommodated by flexure of the plastic cap. =
After one day, one g r o u p of s p e c i m e n s were s t r i p p e d and w a t e r - c u r e d at At this age the ends were g r o u n d f l a t on a l a p p i n g t a b l e 23°C for 28 days. and the s p e c i m e n s were d r i e d to c o n s t a n t w e i g h t in a v a c u u m o v e n at 110°C, cooled in a vacuum desslcator and then subjected to various treatments before test. A s e c o n d g r o u p of s p e c i m e n s were s t e a m c u r e d at 100 kPa p r e s s u r e a f t e r r o t a t i o n and f i n a l set, had t a k e n place. S t e a m c u r i n g l a s t e d 32 hours, and was then f o l l o w e d by the same t r e a t m e n t a s the w a t e r c u r e d s p e c i m e n s in the first group.
Treatment
before crushing
(a) S p e c i m e n s were v a c u u m s a t u r a t e d , in b a t c h e s having the properties listed in Table I.
of ten,
in s e v e n
liquids
Liquids B-E had approximately the same dipole moment but a wide range of m o l e c u l a r volumes; F and G had similar volumes but one had the maximum, and the other zero, dipole moment of the group.
Vol. 15, No. 2
227 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
Table I Liquids used to saturate oven-dried pastes Ref
A B C D E F G
Liquid
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane
V x = M/g ml/mol
18 41 58 77 92 106 108
Dipole Moment c.m/10 -3° 6.23 5.79 5.89 5.85 5.75 9.37 0
Polarity E A kJ/mol
264 232 224 203 210 -
V* = approximate molecular volume - Molecular weight M/specific gravity g E A = empirical value given by Reichardt (13).
(b) S p e c i m e n s , in b a t c h e s of ten, were a l l o w e d per cent of the water lost in oven-drying.
to take up 20, 40, 60 and 80
The fraction of pore space taken up on vacuum saturation is shown in Table 2 as k = VL/V w. Table 2 Values of k = VL/Vw; where V L = Volume of liquid and V w = Volume of water in pores Water:cement ratio: Curing Porosity-per cent A
B C D E F G
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane
NC = Normal water curing
0.3 NC 31.2 1.0 0.70 0.67 0.66 0.65 0.72 0.62
0.5 SC 35.40 1.0 0.92 0.93 0.83 0.82 0.94 0.79
NC 44.7 1.0 0.76 0.75 0.74 0.73 0.79 0.71
SC 44.7 1.0 0.95 0.85 0.88 0.85 0.90 0.83
SC = Steam curing
The liquids are listed A to G in increasing order of V*. It is seen tha~ the volume occupied by the liquids is in descending order with increase in V except for F which is out of phase with the other liquids. Crushing Values These are expressed as a factor equal to the strength in the stated condition d i v i d e d by the s t r e n g t h of w a t e r - s a t u r a t e d specimens. Experimental values are shown in Table 3.
228
Vol. 15, No. 2 B. Robertson and R.H. Mills Table 3 Strength of saturated specimens given as multiples of the water-saturated strength Water:cement ratio: Curing Water Saturated Strength MPa (a) A B C D E F G
0.5
NC
SC
NC
SC
43.4
47.8
29.8
31.2
Saturated Strength Factors
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane (b)
0.3
1.0 2.0 1.9 2.3 2.2 2.0 2.2
1.0 1.5 1.8 1.8 1.8 1.4 1.9
1.0 1.6 1.5 1.7 1.6 1.3 1.8
1.0 1.5 1.8 2.1 1.8 1.8 2.3
Partially Saturated Strength Factors
Water replacement 20% 40% 60% 80%
1.51 1.34 1.32 1.24
0.95 0.86 0.94 1.01
1.41 1.08 1.21 1.16
1.53 1.22 1.27 1.29
Oven-dry
2.5
1.9
1.7
2.3
It is seen that the strength of F-saturated HCP is also out of phase with respect to molecular volume. The strengths of specimens saturated with Cyclohexane do not differ significantly from those of oven-dried specimens. Water Surface area Dry s p e c i m e n s were c r u s h e d and s c r e e n e d t h r o u g h a No. 50 s i e v e and then brought to mass equilibrium in vacuum dessicators over sulphuric acid solutions of varying concentration. BET surface areas were calculated from the sorption isotherms. Results are shown in Table 4. Table 4 Water surface areas calculated by the BET method (14) Water/cement ratio Curing ZBE T m2/gm
0.3 NC 110
0.5 SC 106
NC 138
SC 115
It is clear that the expected change in pore size distribution of steamcured specimens (14) did not take place, probably because of the significant period of water curing which followed steam treatment and which resulted in additional gel formation.
Vol. 15, No. 2
229 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
Sedimentation of finely ground HCP i__n_nvariou_____~s liquids Dry specimens of the 4 HCP's were m i l l e d to pass the 150 um sieve, shaken up in the series A to G liquids and a l l o w e d to settle to a stable volume. The results are shown in Table 5. Table 5 Bulk volume of -150 um material in the fluid shown Sedimination Volume ml/ml of initially oven-dry normally cured w o = 0.3 HCP Liquid
A B C D E F G
Water Methanol Ethanol 2-Propanol l-Butanol 2-Pentanone Cyclohexane • unknown
Polarity E A kJ/mol 264 232 224 203 210 * *
Dry Solid Volume Dry Bulk Volume
Volume of Sediment 4.9 4.2 4.1 3.9 4.0 3.6 6.6
1.00 1.80
The sedimentation volume decreases with decrease in polarity in the series A to E. By far the g r e a t e s t s e d i m e n t a t i o n v o l u m e was for n o n - p o l a r C y c l o h e x a n e w h i c h a l s o had n e g l i g i b l e i n f l u e n c e on dry strength. The h i g h b u l k v o l u m e in w a t e r was p a r t l y due to h y d r a t i o n of c l i n k e r residue, e x p o s e d by crushing. Discussion T h i r t y e i g h t y e a r s ago B a n g h a m (15) d e s c r i b e d the d i l a t i o n of p o r o u s solids, which results from adsorption of water, as the result of "... a wedgelike action resulting from m o l e c u l a r bombardment at sharp re-entrant angles at the surface". If, for HCP following Powers and others (1,16,17), one assumes that Tobermorite sheets in the gel are buckled, and also that the separation v a r i e s from 0.4 to 3 nm, the c o n c e p t of w e d g i n g a c t i o n has some appeal. The spaces b o u n d e d by c o n v e r g i n g w a l l s are a p p r o x i m a t e l y w e d g e - s h a p e d . These wedges probably have their axes normal to the surface of clinker residue but in any case are randomly oriented within the gross structure of HCP. Adsorbed water molecules are attracted to sorption sites on solid surfaces towards the a p e x but r e t a i n some f r e e d o m of m o v e m e n t n o r m a l to the plane. In r e g i o n s where the wedge-space narrows to molecular dimensions the rate of collision between water molecules and the surface increases and results in a wedging or disjoining pressure on the solid and, consequently, the solid-solid van der Waal's forces are weakened. When the HCP is dried, water m o l e c u l e s migrate away from the apex and van der Waal's bonds between the solid species are strengthened. Upon re-saturation w i t h w a t e r the p r o c e s s is r e v e r s i b l e e x c e p t for new s o l i d - s o l i d bonds
230
Vol. 15, No. 2 B. Robertson and R.H. Mills
w h i c h are s t r o n g e r than the s o l i d - w a t e r molecules cannot enter spaces where water not a t t e n u a t e the s o l i d - s o l i d b o n d s to therefore, do not have the same weakening
attraction. Substances with large can exert disjoining pressure and do the same extent as w a t e r does, and effect.
This s i m p l e c o n c e p t does not take a c c o u n t of other f a c t o r s w h i c h are of importance. These h a v e b e e n e x t e n s i v e l y r e v i e w e d (18) and o n l y a few are mentioned here: (a)
Some of the water held at low pressures by the gel may be taken up as hydrate water and become part of the solid (19).
(b)
The weakening effect of water taken up at low temperatures may be due to degeneration of silaxane groups (2).
(c)
The change in surface energy resulting from water sorption affects the stress at which Griffith cracks will propogate (20).
The r o l e of Ca(OH) 2 has so far been n e g l e c t e d . In a dry HCP, s o l i d Ca(OH) 2 which is precipitated out of solution probably contributes to strength and the r e v e r s e is true w h e n it is d i s s o l v e d by s o r b e d water. Most authors treat pore water as pure water though it must contain dissolved electrolytes which exert a weakening effect (21). P r e v i o u s work on the f l e x u r a l s t r e n g h t of Q u a r t z (22,23) and G l a s s (24) showed that immersion in water and other organic fluids resulted in reduction of s t r e n g t h which, as in HCP, was m a x i m u m for water, and a p p e a r e d to be related to "polarity" E A (13). FIG. I shows these data plotted on the same diagram as mean values for the two HCP's of this investigation.
STRESS 2.S
RATIO .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
FIG. I 2.8
wo
1,5
I,
Strength of saturated specimens related to water saturated strength = I as function of "polarity" E A =
0.3
(O)
w =0.5 Q Ref. 2~ Glass Ref. 22 Quartz ( ~
t 60
22e E&-kJ/mol
280
Vol. 15, No. 2
231 SORPTION, WATER, ORGANIC FLUIDS, STRENGTH, PASTE
The same trend is evident for the three different materials. It appears that sorption-induced reduction of surface energy may be the principle mechanism which is responsible for the different strengths of materials A, B, C, D and E. The sedminentatlon volume, which is a function of interfacial free energy, did not reveal a consistent trend in the case of HCP though it appeared to be significant in the case of glass (24). Interpolation of two sorption isotherms for the normally cured HCP's of this series indicated that the volume of wedge-space not penetrated by liquids B-G would be filled by water at relative vapour pressures ranging from 0.48 for 2-Pentanone to 0.65 for Cyclohexane. This corresponds to space dimensions of 3 to 5 nm ( 4 ) . This was not p u r s u e d in greater depth b e c a u s e of the scarcity of data.
Conclusion The principal weakening effects of pore water in HCP are
(a)
The Bangham effect which results and
(b)
The s u r f a c e e n e r g y effect w h i c h reduces the amount of e x t e r n a l work necessary to propogate cracks.
in attenuation of solid-solid bonds;
References I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
T.C. Powers, M a t e r i a l s and S t r u c t u r e s r e s e a r c h and testing I, 6 pp 487508, Dec. 1968. P.J. Sereda, R.F. F e l d m a n and E.G. Swenson, H i g h w a y R e s e a r c h Board SR90, pp 58-73, 1966. Z.P. Bazant, Materials and Structures research and testing 3, 13 pp 3-36, 1970. R.H. Mills, Materials and Structures research and testing I, 6 pp 553558, 1968. R.F. Feldman, Cem. Concr. Res. 2, 375, 1972. S. Mindess, Mats. Res. Ser. 2, University of British Columbia, p 96. J.J. Beaudoin, Cem. Concr. Res. 12, pp 705-716, 1982. A.A. Griffith, Phil. Trans. Series A 221, pp 163-198, Royal Society 1920. R~H. M i l l s , Int. Symp. on c o n c r e t e and r e i n f o r c e d c o n c r e t e in hot countries. Haifa 1960. J.J. Beaudoin, and R.H. Mills, Mechanical Behaviour of Materials, pp 184192, Soc. Mat. Sci. Japan 1972. R.S. M i k h a i l and S.A. Sellm, H i g h w a y R e s e a r c h Board SR90, pp 123-134, 1966. R.F. Feldman, V International Symposium on the Chemistry of Cement, Vol. 3, Pp 53-66 (1968). C. Reichardt, Angew. Chem. Vol. 4, I pp 29-40 (1965). T.C. Powers, J.A.C.I. Proceedings, Vol. 43, p 301, 1946. D.H~ Baughan, Sym. Soc. Chem. Ind. London, May 1946. V.S. R a m a c h a n d r a n , R.F. F e l d m a n and J.J. Beaudoin, C o n c r e t e Science, pp 55-59, Heyden and Son 1981. O. Isai, Applied Materials Research, July 1966, pp 154-161. Ref. 16, Chapter 2.
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Vol. 15, No. 2 B. Robertson and R.H. Mills
19. 20. 21. 22. 23. 24.
R.F. Feldman, Ref. 12, V3, pp 84-85. M.J. Setzer and F.H. Wittman, Applied Physics 3, 403 (1974). P.A. Rehbinder, L.A. S c h r e i n e r and K.F. Zhigach, H a r d n e s s r e d u c e r s in drilling, Academy of Science, Moscow 1944, CSIR Melbourne 1948. M.L. Hammond and S.F. Ravitz, J. Amer. Ceram. Soc. 396, 215-217, 1956. H. leRoux, D e l a y e d F r a c t u r e of Fused Quartz in Water Vapour, Ph.D. Thesis, University of the Witwatersrand, 1963. D. M c C a m m o n d , A.W. N e u m a n n and N. Natarajan, J. Amer. Ceram. Soc. Vol. 58, No I, pp 15-17, 1975.
Acknowledgement The support of the Natural Sciences Canada is gratefully acknowledged.
and
Engineering
Research
Council
of