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thawing cycles
Building and Environment, Vol. 32, No. 3, pp. 199-202, 1997 0 1997 Elsevier Science Ltd. All riehts reserved Printed in Great Britain 036&1323/97 117.00+0.00
Pergamon PII: SO360-1323(96)00054-6
Ice Formation in Pores in Polymer Modified Concrete-II. The Influence of the Admixtures on the Water to Ice Transition in Cementitious Composites Subjected to Freezing/Thawing Cycles AGNIESZKA J. KLEMM* PIOTR KLEMMt
(Received 22 Augusf 1995; revised 8 May 1996; accepted 2 October 1996)
This paper presents results which are part of a larger study on the influence of two admixtures (methyl hydroxy ethyl cellulose, MHEC, and polyvinyl acetate, PVA) on properties of water contained in pores and capillaries of cementitious materials subjected to freezing and thawing. An attempt has been made to explain some aspects of frost deterioration based on the Everett and Hynes model of phase transition. The Differential Scanning Calorimetry technique was used to record the process of water to ice transition in cementitious composites, which were initially subjected to different curing conditions including freezing and thawing. A study of the phase transition of water in polymer modified concrete revealed variations in the ice concentration depending upon the mix composition and number offreezing and thawing cycles. It has been found that admixtures have strong effects on the phase transition particularly after prolonged exposure to alternate freezing and thawing. 0 1991 Elsevier Science Ltd.
INTRODUCTION
resist heterogeneous stresses due to water and ice migration, at least until the time of definite freezing. During the freezing process, ice creates a convex meniscus with water [3]. Hence, Piincapillary > pwincapdlary, and in accordance with the Laplace equation (1):
THE durability of a cementitious material is understood to be the ability of the material to maintain its structural integrity, protective capacity and aesthetic qualities over a prolonged period of time. The most common type of physical durability is associated with freezing and thawing, which may be experienced during the winter months in many countries. The problem is more acute when the concrete is permeable to water, is exposed to water and the freezing and thawing cycles are frequent. This is likely to occur for horizontal slabs and unprotected concrete road surfaces exposed to climates such as those found in the UK or in North America. The number of cycles is crucial, since climates in which the concrete is frozen for much of the year are less severe than those climates in which freezing and thawing cycles occur regularly. The mechanism of frost deterioration has been previously considered by many authors. For the present study the Everett and Hynes model of phase transition has been applied [l, 21. It is assumed that the ice phase remains in a state of internal equilibrium, which means that chemical potential is constant in a total volume and that a porous material has elastic properties, so it may
Pi - Pw = 2~,,/r,,
(1)
where pi and pW are the pressures above ice and water phases; biw is the surface tension at the interface; and r,,, is the radius of meniscus curvature. When the temperature is constant, the chemical potential bi) of ice increases with volume rise. Each increment of the chemical potential pi is associated with an increase of pressure pi and it alters due to the ice growth. The ice cannot be created in capillaries at a temperature of 0°C but it is essential to apply a supercooling. The ice propagation to the thinner capillary will not occur until the pressure above the ice phase rises up to a certain value, which is obviously related to a rise of the chemical potential up to a value of picap.During supercooling, the chemical potential rises as well as the pressure. This, in turn, leads to an increment of interface curvature until the radius of the meniscus and the radius of the capillary become equal. From this moment each additional heat abstraction will determine an ice migration in a capillary. At a temperature, T, ice inside a pore, being in equilibrium with water in a capillary, remains under a pressure p,. The migration process of water from capillary to
*Department of Energy and Environmental Technology, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 OBA, U.K. t Lodz Technical University, Lodz, Poland. 199
200
A. J. Klemm and P. Klemm
pore lasts until plce in the pore becomes equal to plce in the capillary. Unfortunately, the above model is not free from problems, in particular the ice plasticity seems to be too small to maintain thermodynamic equilibrium over a period of time in the total volume of ice [4]. Nevertheless, it offers an approximate picture of the process which is suitable for most comparative purposes. In order to analyse the mechanism of water to ice which may occur in capillary-porous transition, materials, and to explain deterioration processes in cementitious materials subjected to freezing and thawing cycles, the Differential Scanning Calorimetry technique was used. The gain (exothermic process) and losses (endothermic process) in enthalpy of the material, measured during the calorimetric experiment, can be related to physical and chemical changes in the material during the heating or cooling. The crystallization process in cementitious materials is extremely complicated and is determined by many parameters-particularly by microstructure and chemical composition of the material. In fact, the phase transition tests, which were done for various composites, distinctly indicate differences in the phase transition in their capillaries and, as a consequence, in their microstructure [5]. EXPERIMENTAL
PROCEDURE
The results presented in this paper were obtained using the mixes given in Table 1. Samples were made in the form of prisms 50 x 50 x 200 mm and they were cured in their moulds for 24 h before being demoulded. The samples were than stored under normal laboratory conditions with no water access, at an average temperature of 20°C. After a predetermined period of curing (28days) they were placed in containers with water, for freezing and thawing tests. Saturated specimens destinated for freezing/thawing tests were then placed into a climatic cabinet from Ringway Climatic Ltd, London. The testing procedure involved six cycles of freezing and thawing every 24 hours. During each cycle the temperature inside the cabinet was lowered to -20°C held at that temperature for 3 h and then raised to + 20°C and held for 1 h. This is effectively similar to the ASTM test Standard C666-75 for freezing and thawing [6]. When the specimens were taken out of the cabinet at the end of the required number of cycles (100 or 300 cycles), they were left at room temperature until further testing. Calorimetric measurements were carried out on small samples, approximately 50-100 mg weight, extracted from the rep-
resentative prisms. Specimens were firstly saturated with water and than placed in standard aluminium crucibles of about 0.05 cm3 and normal pressure inside 1200 hPa. Saturation time was standardized at 48 hours to make sure that samples reached their maximum content. The small receptacles were then sealed carefully and placed one after other inside the chamber of the apparatus together with an empty reference receptacle. The experiment was carried out using the automatic mode of the UNIPAN 605M system linked with an IBM PC computer, allowing the processing and analysis of the calorimetric measurements.
Test results The results presented here have been obtained from a calorimetric experiment with a cooling rate assumed to be equal to 2Kmin-‘. The high rate of thermal energy abstraction has caused the creation of amorphous ice. Dry masses of all examined composites were about 60 mg. The amounts of heat emission AQ for the three stages of phase transition, i.e. nucleus creation, spontaneous growth of nucleus and growth of ice crystals, have been separately estimated during the experiment. The above data have been used to analyse the process for each composite. The value of the latent heat of solidification, AH,, has been assumed to be equal to 306.6 J cm3 [7]. A V,,, and A VWare the increments of ice and water volume, and AK is the increment of ice concentration. T, is the temperature of the beginning of the phase transition. The calculation procedure was presented in [8]. Tables 2, 3 and 4 present main parameters obtained for different sets
Table 2. The main parameters composites Mix
AQ [J]
A
0.96545 1.99030 2.50113 2.54281 2.54451
B C D E
A V,,,
obtained for different normally cured
[cm’1
314.8 x lo-’ 649.0x10-’ 815.7 x lo-’ 829.3 x lo-’ 829.9 x lo-’
AV, [cm’] 17.3 x 11.2x 15.6x 16.0 x 14.0 x
lO-3 IO-’ 10m3 10m3 lo-’
sets of
A, [%]
Tp [K]
18 58 52 52 46
250.0 249.0 251.3 249.8 250.8
Table 3. The main parameters obtained for different sets of composites subjected to 100 cycles of freezing and thawing Mix
AQ [J]
A
0.78721 1.21251 1.24339 1.65611 2.0006
B C D E
A v,,, 256.0 395.0 405.5 540.0 652.0
bn’l x x x x x
lo-’ 1O-5 1O-5 lO-5 10m5
AV, [cm’] 14.2 12.3 14.9 14.2 9.1
x x x x x
lo-’ 1O-3 lo-” lo-’ 1O-3
A, [%]
Tp [K]
18 32 27 38 60
259.0 256.3 257.3 255.5 252.3
Table 1. Mix compositions (proportions by mass) Table 4. The main parameters obtained for different sets of composites subjected to 300 cycles of freezing and thawing
Mix compositions OPC
Sand
MHEC
PVA
Mix
A
1
0.0000
A
1
C D E
1 1 1
0.0010
B
1 1
0.0035 0.0060 0.0035 0.0025
0.0050 0.0150 0.0300 0.0500
B C D E
1
1 1
AQ [Jl 0.78678 0.71914 0.78685 1.49513 0.92580
AViceWI 256.6 234.0 405.5 540.0 652.0
x x x x x
lo-’ 1O-5 lo-’ 10m5 10m5
AV, [cm’] 11.6 12.3 18.6 14.8 10.3
x x x x x
lo-’ lo-’ lo-’ 10m3 10m3
A, [%]
r,
22 19 14 33 23
255.1 254.8 256.8 253.1 253.4
Ice Formation in Pores in Polymer Mod@ed Concrete-11 Table 5. Ice concentration
Mix
cured composites
A B C D E
3.6 11.2 1.9 9.4 9.1
Normally
(in per cent) during
second stage
Composites after Composites after 300 F/T cycles 100 F/T cycles 4.2 2.5 2.2 6.0 4.2
3.3 8.9 4.2 6.3 11.7
of normally cured composites and those subjected to 100 and 300 cycles of freezing and thawing. The values of ice concentration in composites during the second stage of the phase transition are shown in Table 5. DISCUSSION All data obtained during the experiment should be treated as approximate in value, because of the assumptions which were made. Nevertheless, it gives a picture of the whole process, and changes which may occur as a consequence of freezing and thawing. The second stage of transition, i.e. spontaneous growth of an ice nucleus, is believed to be the most dangerous stage for the material, mainly because a comparatively high increase of ice volume takes place during a very short period of time. During this stage the lowest ice concentrations with increasing number of freezing and thawing cycles were recorded for all tested samples except sample A, where after 300 cycles a small gain in ice concentration appeared (see Table 5). The total ice concentration recorded for all the cases revealed some variations with increasing number of freezing and thawing cycles (Table 6). The ice concentration significantly deceased with time for all the composites, excluding sample A, where the small increase after 300 cycles was recorded. However the most significant differences were observed when the samples were compared to each other after the same number of freezing and thawing cycles. The lowest values for normally cured composites [5] and after 100 cycles were observed in the cases of samples A and E. Both were characterized by the comparatively low percentage of MHEC (respectively, 0.1
Table 6. Total ice concentration
Mix
Normally cured composites
Composites after 100 F/T cycles
Composites after 300 F/T cycles
A B C D E
18 58 52 52 46
18 32 27 38 34
22 19 11 33 23
and 0.25%). After 300 cycles the lowest ice concentrations were recorded for samples B and C-for the composites containing the highest quantity of MHEC. The lowest ice concentration for samples A and E, in the first two cases, could be associated with the rehydration process which appeared here with higher intensity. The curing regime applied to the samples, with a comparatively low content of MHEC which provides water retention, could be the reason for drying. The hydration process halted during curing was then reactivated when the samples were saturated with water during the freezing and thawing cycles. As a consequence, the intensified rehydration process affected the amount of free water able to freeze at this stage of curing. The behaviour of samples B and C would suggest that some changes in the structure of pastes occurred. The pore size distribution after 300 cycles in these two samples did not facilitate the type of ice growth which could be dangerous to the material [9]. The high number of large pores, exceeding 1400nm in diameter, provided some kind of protection from frost damage. Since the said pores are not fully saturated, because of the absence of capillary suction and the higher rate of evaporation, the phase transition can take place without damage due to the ice expansion [lo]. The pores with diameters below 1400 nm were not the dominant feature in these cases [9]. CONCLUSIONS Calorimetric measurements help describe the mechanism of water to ice transition which may occur in capillary-porous materials. These measurements present additional information to the porosity tests, and they are essential for an explanation of deterioration processes in cementitious materials subjected to freezing and thawing cycles. Recording the temperature changes in materials saturated with water and the latent heat abstraction enables the determination of the growth of ice concentration as a function of time. Results, obtained during calorimetric experiments in connection with physical principles, allow the observation of phase transition and interpretation of the destructive effects of the process on the mechanical properties of the composites. Indeed, a study of the phase transition of water contained in the normally cured composites and subjected to freezing and thawing cycles by the differential scanning calorimetry revealed some variations in the ice concentration, depending upon the mix composition and number of freezing and thawing cycles applied. Generally a significant decrease in ice concentration with increasing number of freezing and thawing cycles was found to be associated with the comparatively high quantity of MHEC. This would suggest strong effects of admixtures on the phase transition, particularly after prolonged periods of alternate freezing and thawing.
REFERENCES 1. 2. 3.
of frost damage to porous solids. Transactions of the Faraday 1961, 56, 1541-1551. D. H. and Hynes, J. M., Capillary properties of some model pore systems with special to frost damage. Rilem Bulletin, 1965, 27, 31-38. A., Billups, R. and Rooney, R., Capillary-cone method for determination of surface tension Journal of Chemistry and Physics, 1967, 26, 13561351.
Everett, D. H., Thermodynamics Society, Everett, reference Skapski, of solids.
201
202
A. J. Klemm and P. Klemm 4. 5. 6. I. 8.
9.
10.
Blachere, J. R. and Young, J. E., Failure of capillary theory of frost damage as applied to ceramics. Journal qf the American Ceramics Society, 1974,51,212-216. KIemm, A. J. and Klemm, P., Ice formation in pores in polymer modified concrete-I. The influence of the admixtures on the water to ice transition. Building and Environment, 1997, 32, 195-198. ASTM Standard C666-75, Standard Method qf Test,for Resistance of Concrete to Rapid Freezing and Thawing, part 14. ASTM, Philadelphia, 1975, pp. 371-375. Klemm, P., Jablonski, M. and Klemm, A. J., Phase transition in capillary-porous materials. In Physics of Materials and Structures. Lodz Technical University Press, Lodz, 1993. Jablonski, M., Klemm, P. and Klemm, A. J.. Calorimetric experiment in cementitious composites. In Proceedings of the IV Corzference on Building Physics in Theory and Practice. Lodz, Poland, 1993, pp. lOG108. Klemm, A. J., The influence of admixtures on the microstructural features and mechanical properties of cementitious materials subjected to freezing and thawing cycles. Ph.D. thesis, Strathclyde University, Glasgow, 1994. Ravagnioli, A., Evaluation of the frost resistance of pressed ceramic products. Transactions qf the British Ceramics
Societ),,
1975. 75, 92-95.
Pergamon PII: SO360-1323(96)00054-6
Ice Formation in Pores in Polymer Modified Concrete-II. The Influence of the Admixtures on the Water to Ice Transition in Cementitious Composites Subjected to Freezing/Thawing Cycles AGNIESZKA J. KLEMM* PIOTR KLEMMt
(Received 22 Augusf 1995; revised 8 May 1996; accepted 2 October 1996)
This paper presents results which are part of a larger study on the influence of two admixtures (methyl hydroxy ethyl cellulose, MHEC, and polyvinyl acetate, PVA) on properties of water contained in pores and capillaries of cementitious materials subjected to freezing and thawing. An attempt has been made to explain some aspects of frost deterioration based on the Everett and Hynes model of phase transition. The Differential Scanning Calorimetry technique was used to record the process of water to ice transition in cementitious composites, which were initially subjected to different curing conditions including freezing and thawing. A study of the phase transition of water in polymer modified concrete revealed variations in the ice concentration depending upon the mix composition and number offreezing and thawing cycles. It has been found that admixtures have strong effects on the phase transition particularly after prolonged exposure to alternate freezing and thawing. 0 1991 Elsevier Science Ltd.
INTRODUCTION
resist heterogeneous stresses due to water and ice migration, at least until the time of definite freezing. During the freezing process, ice creates a convex meniscus with water [3]. Hence, Piincapillary > pwincapdlary, and in accordance with the Laplace equation (1):
THE durability of a cementitious material is understood to be the ability of the material to maintain its structural integrity, protective capacity and aesthetic qualities over a prolonged period of time. The most common type of physical durability is associated with freezing and thawing, which may be experienced during the winter months in many countries. The problem is more acute when the concrete is permeable to water, is exposed to water and the freezing and thawing cycles are frequent. This is likely to occur for horizontal slabs and unprotected concrete road surfaces exposed to climates such as those found in the UK or in North America. The number of cycles is crucial, since climates in which the concrete is frozen for much of the year are less severe than those climates in which freezing and thawing cycles occur regularly. The mechanism of frost deterioration has been previously considered by many authors. For the present study the Everett and Hynes model of phase transition has been applied [l, 21. It is assumed that the ice phase remains in a state of internal equilibrium, which means that chemical potential is constant in a total volume and that a porous material has elastic properties, so it may
Pi - Pw = 2~,,/r,,
(1)
where pi and pW are the pressures above ice and water phases; biw is the surface tension at the interface; and r,,, is the radius of meniscus curvature. When the temperature is constant, the chemical potential bi) of ice increases with volume rise. Each increment of the chemical potential pi is associated with an increase of pressure pi and it alters due to the ice growth. The ice cannot be created in capillaries at a temperature of 0°C but it is essential to apply a supercooling. The ice propagation to the thinner capillary will not occur until the pressure above the ice phase rises up to a certain value, which is obviously related to a rise of the chemical potential up to a value of picap.During supercooling, the chemical potential rises as well as the pressure. This, in turn, leads to an increment of interface curvature until the radius of the meniscus and the radius of the capillary become equal. From this moment each additional heat abstraction will determine an ice migration in a capillary. At a temperature, T, ice inside a pore, being in equilibrium with water in a capillary, remains under a pressure p,. The migration process of water from capillary to
*Department of Energy and Environmental Technology, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 OBA, U.K. t Lodz Technical University, Lodz, Poland. 199
200
A. J. Klemm and P. Klemm
pore lasts until plce in the pore becomes equal to plce in the capillary. Unfortunately, the above model is not free from problems, in particular the ice plasticity seems to be too small to maintain thermodynamic equilibrium over a period of time in the total volume of ice [4]. Nevertheless, it offers an approximate picture of the process which is suitable for most comparative purposes. In order to analyse the mechanism of water to ice which may occur in capillary-porous transition, materials, and to explain deterioration processes in cementitious materials subjected to freezing and thawing cycles, the Differential Scanning Calorimetry technique was used. The gain (exothermic process) and losses (endothermic process) in enthalpy of the material, measured during the calorimetric experiment, can be related to physical and chemical changes in the material during the heating or cooling. The crystallization process in cementitious materials is extremely complicated and is determined by many parameters-particularly by microstructure and chemical composition of the material. In fact, the phase transition tests, which were done for various composites, distinctly indicate differences in the phase transition in their capillaries and, as a consequence, in their microstructure [5]. EXPERIMENTAL
PROCEDURE
The results presented in this paper were obtained using the mixes given in Table 1. Samples were made in the form of prisms 50 x 50 x 200 mm and they were cured in their moulds for 24 h before being demoulded. The samples were than stored under normal laboratory conditions with no water access, at an average temperature of 20°C. After a predetermined period of curing (28days) they were placed in containers with water, for freezing and thawing tests. Saturated specimens destinated for freezing/thawing tests were then placed into a climatic cabinet from Ringway Climatic Ltd, London. The testing procedure involved six cycles of freezing and thawing every 24 hours. During each cycle the temperature inside the cabinet was lowered to -20°C held at that temperature for 3 h and then raised to + 20°C and held for 1 h. This is effectively similar to the ASTM test Standard C666-75 for freezing and thawing [6]. When the specimens were taken out of the cabinet at the end of the required number of cycles (100 or 300 cycles), they were left at room temperature until further testing. Calorimetric measurements were carried out on small samples, approximately 50-100 mg weight, extracted from the rep-
resentative prisms. Specimens were firstly saturated with water and than placed in standard aluminium crucibles of about 0.05 cm3 and normal pressure inside 1200 hPa. Saturation time was standardized at 48 hours to make sure that samples reached their maximum content. The small receptacles were then sealed carefully and placed one after other inside the chamber of the apparatus together with an empty reference receptacle. The experiment was carried out using the automatic mode of the UNIPAN 605M system linked with an IBM PC computer, allowing the processing and analysis of the calorimetric measurements.
Test results The results presented here have been obtained from a calorimetric experiment with a cooling rate assumed to be equal to 2Kmin-‘. The high rate of thermal energy abstraction has caused the creation of amorphous ice. Dry masses of all examined composites were about 60 mg. The amounts of heat emission AQ for the three stages of phase transition, i.e. nucleus creation, spontaneous growth of nucleus and growth of ice crystals, have been separately estimated during the experiment. The above data have been used to analyse the process for each composite. The value of the latent heat of solidification, AH,, has been assumed to be equal to 306.6 J cm3 [7]. A V,,, and A VWare the increments of ice and water volume, and AK is the increment of ice concentration. T, is the temperature of the beginning of the phase transition. The calculation procedure was presented in [8]. Tables 2, 3 and 4 present main parameters obtained for different sets
Table 2. The main parameters composites Mix
AQ [J]
A
0.96545 1.99030 2.50113 2.54281 2.54451
B C D E
A V,,,
obtained for different normally cured
[cm’1
314.8 x lo-’ 649.0x10-’ 815.7 x lo-’ 829.3 x lo-’ 829.9 x lo-’
AV, [cm’] 17.3 x 11.2x 15.6x 16.0 x 14.0 x
lO-3 IO-’ 10m3 10m3 lo-’
sets of
A, [%]
Tp [K]
18 58 52 52 46
250.0 249.0 251.3 249.8 250.8
Table 3. The main parameters obtained for different sets of composites subjected to 100 cycles of freezing and thawing Mix
AQ [J]
A
0.78721 1.21251 1.24339 1.65611 2.0006
B C D E
A v,,, 256.0 395.0 405.5 540.0 652.0
bn’l x x x x x
lo-’ 1O-5 1O-5 lO-5 10m5
AV, [cm’] 14.2 12.3 14.9 14.2 9.1
x x x x x
lo-’ 1O-3 lo-” lo-’ 1O-3
A, [%]
Tp [K]
18 32 27 38 60
259.0 256.3 257.3 255.5 252.3
Table 1. Mix compositions (proportions by mass) Table 4. The main parameters obtained for different sets of composites subjected to 300 cycles of freezing and thawing
Mix compositions OPC
Sand
MHEC
PVA
Mix
A
1
0.0000
A
1
C D E
1 1 1
0.0010
B
1 1
0.0035 0.0060 0.0035 0.0025
0.0050 0.0150 0.0300 0.0500
B C D E
1
1 1
AQ [Jl 0.78678 0.71914 0.78685 1.49513 0.92580
AViceWI 256.6 234.0 405.5 540.0 652.0
x x x x x
lo-’ 1O-5 lo-’ 10m5 10m5
AV, [cm’] 11.6 12.3 18.6 14.8 10.3
x x x x x
lo-’ lo-’ lo-’ 10m3 10m3
A, [%]
r,
22 19 14 33 23
255.1 254.8 256.8 253.1 253.4
Ice Formation in Pores in Polymer Mod@ed Concrete-11 Table 5. Ice concentration
Mix
cured composites
A B C D E
3.6 11.2 1.9 9.4 9.1
Normally
(in per cent) during
second stage
Composites after Composites after 300 F/T cycles 100 F/T cycles 4.2 2.5 2.2 6.0 4.2
3.3 8.9 4.2 6.3 11.7
of normally cured composites and those subjected to 100 and 300 cycles of freezing and thawing. The values of ice concentration in composites during the second stage of the phase transition are shown in Table 5. DISCUSSION All data obtained during the experiment should be treated as approximate in value, because of the assumptions which were made. Nevertheless, it gives a picture of the whole process, and changes which may occur as a consequence of freezing and thawing. The second stage of transition, i.e. spontaneous growth of an ice nucleus, is believed to be the most dangerous stage for the material, mainly because a comparatively high increase of ice volume takes place during a very short period of time. During this stage the lowest ice concentrations with increasing number of freezing and thawing cycles were recorded for all tested samples except sample A, where after 300 cycles a small gain in ice concentration appeared (see Table 5). The total ice concentration recorded for all the cases revealed some variations with increasing number of freezing and thawing cycles (Table 6). The ice concentration significantly deceased with time for all the composites, excluding sample A, where the small increase after 300 cycles was recorded. However the most significant differences were observed when the samples were compared to each other after the same number of freezing and thawing cycles. The lowest values for normally cured composites [5] and after 100 cycles were observed in the cases of samples A and E. Both were characterized by the comparatively low percentage of MHEC (respectively, 0.1
Table 6. Total ice concentration
Mix
Normally cured composites
Composites after 100 F/T cycles
Composites after 300 F/T cycles
A B C D E
18 58 52 52 46
18 32 27 38 34
22 19 11 33 23
and 0.25%). After 300 cycles the lowest ice concentrations were recorded for samples B and C-for the composites containing the highest quantity of MHEC. The lowest ice concentration for samples A and E, in the first two cases, could be associated with the rehydration process which appeared here with higher intensity. The curing regime applied to the samples, with a comparatively low content of MHEC which provides water retention, could be the reason for drying. The hydration process halted during curing was then reactivated when the samples were saturated with water during the freezing and thawing cycles. As a consequence, the intensified rehydration process affected the amount of free water able to freeze at this stage of curing. The behaviour of samples B and C would suggest that some changes in the structure of pastes occurred. The pore size distribution after 300 cycles in these two samples did not facilitate the type of ice growth which could be dangerous to the material [9]. The high number of large pores, exceeding 1400nm in diameter, provided some kind of protection from frost damage. Since the said pores are not fully saturated, because of the absence of capillary suction and the higher rate of evaporation, the phase transition can take place without damage due to the ice expansion [lo]. The pores with diameters below 1400 nm were not the dominant feature in these cases [9]. CONCLUSIONS Calorimetric measurements help describe the mechanism of water to ice transition which may occur in capillary-porous materials. These measurements present additional information to the porosity tests, and they are essential for an explanation of deterioration processes in cementitious materials subjected to freezing and thawing cycles. Recording the temperature changes in materials saturated with water and the latent heat abstraction enables the determination of the growth of ice concentration as a function of time. Results, obtained during calorimetric experiments in connection with physical principles, allow the observation of phase transition and interpretation of the destructive effects of the process on the mechanical properties of the composites. Indeed, a study of the phase transition of water contained in the normally cured composites and subjected to freezing and thawing cycles by the differential scanning calorimetry revealed some variations in the ice concentration, depending upon the mix composition and number of freezing and thawing cycles applied. Generally a significant decrease in ice concentration with increasing number of freezing and thawing cycles was found to be associated with the comparatively high quantity of MHEC. This would suggest strong effects of admixtures on the phase transition, particularly after prolonged periods of alternate freezing and thawing.
REFERENCES 1. 2. 3.
of frost damage to porous solids. Transactions of the Faraday 1961, 56, 1541-1551. D. H. and Hynes, J. M., Capillary properties of some model pore systems with special to frost damage. Rilem Bulletin, 1965, 27, 31-38. A., Billups, R. and Rooney, R., Capillary-cone method for determination of surface tension Journal of Chemistry and Physics, 1967, 26, 13561351.
Everett, D. H., Thermodynamics Society, Everett, reference Skapski, of solids.
201
202
A. J. Klemm and P. Klemm 4. 5. 6. I. 8.
9.
10.
Blachere, J. R. and Young, J. E., Failure of capillary theory of frost damage as applied to ceramics. Journal qf the American Ceramics Society, 1974,51,212-216. KIemm, A. J. and Klemm, P., Ice formation in pores in polymer modified concrete-I. The influence of the admixtures on the water to ice transition. Building and Environment, 1997, 32, 195-198. ASTM Standard C666-75, Standard Method qf Test,for Resistance of Concrete to Rapid Freezing and Thawing, part 14. ASTM, Philadelphia, 1975, pp. 371-375. Klemm, P., Jablonski, M. and Klemm, A. J., Phase transition in capillary-porous materials. In Physics of Materials and Structures. Lodz Technical University Press, Lodz, 1993. Jablonski, M., Klemm, P. and Klemm, A. J.. Calorimetric experiment in cementitious composites. In Proceedings of the IV Corzference on Building Physics in Theory and Practice. Lodz, Poland, 1993, pp. lOG108. Klemm, A. J., The influence of admixtures on the microstructural features and mechanical properties of cementitious materials subjected to freezing and thawing cycles. Ph.D. thesis, Strathclyde University, Glasgow, 1994. Ravagnioli, A., Evaluation of the frost resistance of pressed ceramic products. Transactions qf the British Ceramics
Societ),,
1975. 75, 92-95.