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The use of solvent exchange to monitor diffusion characteristics of cement pastes containing silica fume
CEMENT and CONCRETE RESEARCH. Vol. 18, pp. 321-324, 1988. Printed in the USA 0008-8846/88 $3.00+00. Copyright (c) 1988 Pergamon Press plc.
NOTE THE USE OF SOLVENT EXCHANGE TO MONITOR DIFFUSION CHARACTERISTICS PASTES CONTAINING SILICA FUME.
OF CEMENT
D.C. HUGHES SCHOOL OF CIVIL ENGINEERING UNIVERSITY OF BRADFORD WEST YORKSHIRE, U.K. BD7 IDP
(Communicated by C.D. Pomeroy) (Received Aug. 20, 1987)
The durability of concrete is strongly related to its permeability or diffusion characteristics and, hence, a simple and credible measurement of these parameters is desirable. Solvent exchange, using either methanol, ethanol or propanol, has the advantage of being quickly and simply determined using inexpensive apparatus. The technique was initially developed, for this purpose, by Parrott (1,2) and following work on alite pastes he concluded that any of the above solvents were suitable. Early doubts, expressed by Day (3), concerning the suitability of methanol have been recently extended by Taylor and Turner (4) to include propan-2-ol. They concluded that both solvents are strongly sorbed within the structure of mature C3S pastes and unsuitable for use as a drying agent prior to thermal analysis. Feldman (5), measuring the expansion/contraction of Type I cements undergoing solvent exchange, suggested that, whilst methanol is capable of entering interlayer space, the same does not occur with propan-2-ol. The diffusion coefficients obtained using propan-2-ol were of the same order as those obtained by others for the diffusion of CIions in OPC pastes. Feldman proposed that propan-2-ol may be a suitable medium for establishing the diffusion characteristics of pastes and concrete. This Note, without considering possible sorption effects, draws attention to the limitations of propan-2-ol in certain systems. Work is in progress at the University of Bradford to examine the efficiency of several silicate based liquid and mortar surface repair treatments in modifying the pore structure of existing concrete. Solvent exchange was proposed as a technique for monitoring such changes. In order to assess the suitability of several solvents a model material of strongly modified pore structure was chosen, namely an OPC - silica fume paste, and solvent exchange monitored using methanol, ethanol, propan-l-ol and propan-2-ol. EXPERIMENTAL DETAILS Two pastes were prepared, as shown in Table i, placed in 33 mm diameter cylinders, rotated overnight and cured for a further 27 days in water 321
Vol. 18, No. 2
322 D.C. Hughes
(200C) which had been conditioned by the prior immersion of spare samples. Four discs, 3mm thick, were sawn for exchange measurement in each solvent. They were placed in a small quantity of curing solution and stored overnight, in a vacuum, to maximise saturation. Since the technique was to be a constituent of a large experimental programme the generous use of solvent adopted by Feldman was not feasible. The discs were immersed in 200ml of solvent which was replaced after 80% of the expected exchange had been measured. Previous work had shown this to yield similar results to those obtained using much larger solvent volumes.
TABLE I Mix Details, by Weight Ratio
MIX
OPC SILICA FUME* WATER SUPER PLASTICISER**
OPC
OPC-SILICA FUME
i 0.55 -
0.88 0.12 0.55 0.0055
* ELKEM, EIO0 ** CORMIX SPI
Results and Discussion Results in units of weight loss per unit volume of sample versus the square root of exchange time are sho~rn in Figs I (OPC) and 2 (OPC - silica fume). The data for methanol and propan-2-ol only are displayed, since they exhibit the trends to be discussed. Where appropriate, the data for ethanol and propan-l-ol are described. The form of the exchange for both methanol and propan-2-ol exhibited by the OPC paste is as expected, with methanol exchanging at the faster rate. However, a marked difference occurs with the OPC-silica fume paste. In this case the propan-2-ol apparently exchanges at the faster rate, exhibiting a maximum in the weight loss before gaining weight until equilibrium is achieved. This form of behaviour was also exhibited by the OPC-silica fume paste immersed in both ethanol and propan-l-ol. Methanol yielded the standard form. Not only is the propan-2-ol exchange apparently quicker than that of methanol, but also quicker for OPC-sillca fume than OPC. It might be expected that the permeability of a paste containing such a fine reactive pozzolana would be at least 1-2 orders of magnitude less than that for the OPC paste. A possible explanation for the anomalous behaviour of OPC-silica fume pastes may lle in the refined pore structure; the pozzolanic reaction would be expected to reduce pore sizes. It may be that some pores are of an entry size whereby propan-2-ol is able to withdraw the pore solution, with the subsequent introduction of propan-2-ol being time dependent. Feldman (6) ha~
Vol.
18, No. 2
323 SOLVENT EXCHANGE,
DIFFUSION,
SILICA Fb~ME, PASTES
1L
TIME
0
0-00
(mins) =
100 I
I
200
I
A
E
•
O
600
930
0"04 ~0 (/3
0
.J
T
0"08
ua
0"12 FIG i Diffusion of methanol (0) and propan-2-ol into OPC pastes
(O)
I
(mins) 2
TIME 0
0'00
200
100 I
I
I
O
1190 fminsJ
E
o.os 0 _J
"r
_o o.1o
W
0"15
FIG 2 Diffusion of methanol (0) and propan-2-ol into OPC-silica fume pastes
(O)
295
324
Vol. 18, No. 2 D.C. Hughes
suggested that the pore structure of fly ash cement blends comprises significant porosity to which access is gained only at high pressure (small entry size) during mercury intrusion porosimetry. The true entry size is not determinable since the high pressure access is likely to be gained only at the expense of physical damage to the very fine pore structure of the blended paste. The entry of propan-2-ol into pores of such a restricted entrance may be a function of the character of both the pore structure and propan-2-ol molecule and the interaction between propan-2-ol, pore solution and the hydrated structure as modified by sorbed propan-2-ol. Such a consideration is outside the scope of the current work. Whatever the reason it would appear that propan-2-ol is inappropriate to monitor diffusion characteristics of OPC-silica fume pastes. In contrast the use of methanol yields the expected form and order of exchange for both OPC and OPC-silica fume pastes: the times for half exchange (T~) were 600 and 1190 minutes respectively. Whilst the existence of time dependent methanol diffusion, albeit on a reduced scale, cannot be eliminated by the current data, and acknowledging methanol-paste interaction, the use of methanol appears to provide a comparltive basis for the evaluation of the inclusion of silica fume in OPC pastes. Taylor and Turner (4) have suggested that both methanol and propan-2-ol are unsuitable as drying agents prior to thermal analysis of C3S pastes. This is due to the strong sorption of both solvents within the paste structure. Before the use of either solvent is considered as a technique to measure diffusion or pore structure characteristics, a critical appraisal should be made of the significance of the known solvent-structure interactions. References I. 2. 3. 4. 5. 6.
Parrott, L.J. Cem. Contr. Res., ii, 651, (1981) Parrott, L.J. Mater. Struct., 17, 131, (1984) Day, R.L. Cem. Concr.Res., ii, 341, (1981) Taylor, H.F.W. and Turner, A.B. Cem. Concr. Res. Feldman, R.F. Cem. Concr. Res., 17, 602, (1987) Feldman, R.F. Cem. Concr. Res., 16, 452, (1985)
17, 613,
(1987)
NOTE THE USE OF SOLVENT EXCHANGE TO MONITOR DIFFUSION CHARACTERISTICS PASTES CONTAINING SILICA FUME.
OF CEMENT
D.C. HUGHES SCHOOL OF CIVIL ENGINEERING UNIVERSITY OF BRADFORD WEST YORKSHIRE, U.K. BD7 IDP
(Communicated by C.D. Pomeroy) (Received Aug. 20, 1987)
The durability of concrete is strongly related to its permeability or diffusion characteristics and, hence, a simple and credible measurement of these parameters is desirable. Solvent exchange, using either methanol, ethanol or propanol, has the advantage of being quickly and simply determined using inexpensive apparatus. The technique was initially developed, for this purpose, by Parrott (1,2) and following work on alite pastes he concluded that any of the above solvents were suitable. Early doubts, expressed by Day (3), concerning the suitability of methanol have been recently extended by Taylor and Turner (4) to include propan-2-ol. They concluded that both solvents are strongly sorbed within the structure of mature C3S pastes and unsuitable for use as a drying agent prior to thermal analysis. Feldman (5), measuring the expansion/contraction of Type I cements undergoing solvent exchange, suggested that, whilst methanol is capable of entering interlayer space, the same does not occur with propan-2-ol. The diffusion coefficients obtained using propan-2-ol were of the same order as those obtained by others for the diffusion of CIions in OPC pastes. Feldman proposed that propan-2-ol may be a suitable medium for establishing the diffusion characteristics of pastes and concrete. This Note, without considering possible sorption effects, draws attention to the limitations of propan-2-ol in certain systems. Work is in progress at the University of Bradford to examine the efficiency of several silicate based liquid and mortar surface repair treatments in modifying the pore structure of existing concrete. Solvent exchange was proposed as a technique for monitoring such changes. In order to assess the suitability of several solvents a model material of strongly modified pore structure was chosen, namely an OPC - silica fume paste, and solvent exchange monitored using methanol, ethanol, propan-l-ol and propan-2-ol. EXPERIMENTAL DETAILS Two pastes were prepared, as shown in Table i, placed in 33 mm diameter cylinders, rotated overnight and cured for a further 27 days in water 321
Vol. 18, No. 2
322 D.C. Hughes
(200C) which had been conditioned by the prior immersion of spare samples. Four discs, 3mm thick, were sawn for exchange measurement in each solvent. They were placed in a small quantity of curing solution and stored overnight, in a vacuum, to maximise saturation. Since the technique was to be a constituent of a large experimental programme the generous use of solvent adopted by Feldman was not feasible. The discs were immersed in 200ml of solvent which was replaced after 80% of the expected exchange had been measured. Previous work had shown this to yield similar results to those obtained using much larger solvent volumes.
TABLE I Mix Details, by Weight Ratio
MIX
OPC SILICA FUME* WATER SUPER PLASTICISER**
OPC
OPC-SILICA FUME
i 0.55 -
0.88 0.12 0.55 0.0055
* ELKEM, EIO0 ** CORMIX SPI
Results and Discussion Results in units of weight loss per unit volume of sample versus the square root of exchange time are sho~rn in Figs I (OPC) and 2 (OPC - silica fume). The data for methanol and propan-2-ol only are displayed, since they exhibit the trends to be discussed. Where appropriate, the data for ethanol and propan-l-ol are described. The form of the exchange for both methanol and propan-2-ol exhibited by the OPC paste is as expected, with methanol exchanging at the faster rate. However, a marked difference occurs with the OPC-silica fume paste. In this case the propan-2-ol apparently exchanges at the faster rate, exhibiting a maximum in the weight loss before gaining weight until equilibrium is achieved. This form of behaviour was also exhibited by the OPC-silica fume paste immersed in both ethanol and propan-l-ol. Methanol yielded the standard form. Not only is the propan-2-ol exchange apparently quicker than that of methanol, but also quicker for OPC-sillca fume than OPC. It might be expected that the permeability of a paste containing such a fine reactive pozzolana would be at least 1-2 orders of magnitude less than that for the OPC paste. A possible explanation for the anomalous behaviour of OPC-silica fume pastes may lle in the refined pore structure; the pozzolanic reaction would be expected to reduce pore sizes. It may be that some pores are of an entry size whereby propan-2-ol is able to withdraw the pore solution, with the subsequent introduction of propan-2-ol being time dependent. Feldman (6) ha~
Vol.
18, No. 2
323 SOLVENT EXCHANGE,
DIFFUSION,
SILICA Fb~ME, PASTES
1L
TIME
0
0-00
(mins) =
100 I
I
200
I
A
E
•
O
600
930
0"04 ~0 (/3
0
.J
T
0"08
ua
0"12 FIG i Diffusion of methanol (0) and propan-2-ol into OPC pastes
(O)
I
(mins) 2
TIME 0
0'00
200
100 I
I
I
O
1190 fminsJ
E
o.os 0 _J
"r
_o o.1o
W
0"15
FIG 2 Diffusion of methanol (0) and propan-2-ol into OPC-silica fume pastes
(O)
295
324
Vol. 18, No. 2 D.C. Hughes
suggested that the pore structure of fly ash cement blends comprises significant porosity to which access is gained only at high pressure (small entry size) during mercury intrusion porosimetry. The true entry size is not determinable since the high pressure access is likely to be gained only at the expense of physical damage to the very fine pore structure of the blended paste. The entry of propan-2-ol into pores of such a restricted entrance may be a function of the character of both the pore structure and propan-2-ol molecule and the interaction between propan-2-ol, pore solution and the hydrated structure as modified by sorbed propan-2-ol. Such a consideration is outside the scope of the current work. Whatever the reason it would appear that propan-2-ol is inappropriate to monitor diffusion characteristics of OPC-silica fume pastes. In contrast the use of methanol yields the expected form and order of exchange for both OPC and OPC-silica fume pastes: the times for half exchange (T~) were 600 and 1190 minutes respectively. Whilst the existence of time dependent methanol diffusion, albeit on a reduced scale, cannot be eliminated by the current data, and acknowledging methanol-paste interaction, the use of methanol appears to provide a comparltive basis for the evaluation of the inclusion of silica fume in OPC pastes. Taylor and Turner (4) have suggested that both methanol and propan-2-ol are unsuitable as drying agents prior to thermal analysis of C3S pastes. This is due to the strong sorption of both solvents within the paste structure. Before the use of either solvent is considered as a technique to measure diffusion or pore structure characteristics, a critical appraisal should be made of the significance of the known solvent-structure interactions. References I. 2. 3. 4. 5. 6.
Parrott, L.J. Cem. Contr. Res., ii, 651, (1981) Parrott, L.J. Mater. Struct., 17, 131, (1984) Day, R.L. Cem. Concr.Res., ii, 341, (1981) Taylor, H.F.W. and Turner, A.B. Cem. Concr. Res. Feldman, R.F. Cem. Concr. Res., 17, 602, (1987) Feldman, R.F. Cem. Concr. Res., 16, 452, (1985)
17, 613,
(1987)