- Email: [email protected]
Durability tests on polymer-cement mortar
CEMENT and CONCRETE RESEARCH. 0008-8846/83/020287-04503.00/0
Vol. 13, pp. 287-290, 1983. Printed in the USA. Copyright (c) 1983 Pergamon Press, Ltd.
NOTE DURABILITY TESTS ON POLYMER-CEMENT MORTAR V.K. Bhattacharya, K.R. Kirtania, M.M. Maiti and S. Maiti Polymer Division, Materials Science Centre Indian Institute of Technology Kharagpur 721302 India
(Communicated by F.H. Wittmann) (Received Aug. 18; in final form Dec. i, 1982) ABSTRACT Accelerated durability tests on several polymer-impregnated mortars have been studied in aqueous sulphuric acid (3:1 v/v) and hydrochloric acid (3:1 v/v) media. In addition to the reactivity of individual polymers and cement body in these corrosive environments, the cement-polymer interaction has been identified as an important factor which controls the durability properties of the composites. Introduction The mechanical strengthening of polymer-impregnated cement composites has been suggested to be primarily due to pore filling by the polymer which acts as continuous, randomly oriented reinforcing network and repairs the microcracking and reinforces the micro~ores of cement body by penetration into the matrix (i). Apart from this, the impregnated polymer is also believed to enter into some type of interaction with the cement matrix and forms stable bonds, which largely determines the strength and durability properties of the composites. In this communication we present some of our results on the durability characteristics of mortars impregnated with different polymers which have been explained in terms of cement-polymer interaction. Experimental Details of materials used, specimen sizes, method of impregnation of polymer, procedures for measurement of strength properties, ultrasonic velocity and the durability of composites have been reported elsewhere (2). Results and Discussion Results of the accelerated durability tests on mortars impregnated with different polymers are presented in Table i. The data clearly show that polymethyl methacrylate (PMMA) improves the durability of mortar-polymer composites remarkably. The mortar-polystyrene (PS) composites are less durable. Durability of mortar-polyacrylonitrile (PAN) composites, however, is even worse than that of control. The observed durability behavior of mortar-PAN composites is not very unusual. On slow hydrolysis in acid medium PAN g~ves polyacrylic acid (PAA) and 287
0
0
46.0
27.0
32.1
13.8
7.24
7.28
4.91
7.77
4.93
10.17
MortarPMMA
MortarPAN
MortarPS
41.0
0
Mortar
7
42.2
52.5
28.0
72.0
0
0
47.1
14
% weight loss day_s_-
Polymer (% w/w)
75.0
80.0
32.2
88.9
1.2
1.0
94.7
28
in
of M o r t a r - P o l y m e r
H2SO 4 m e d i u m
Behavior
Composite
Durability
1
Drastic reduction in structural dimensions
Reduction in structural dimensions
Considerable expansion in 7 days. Do not crumble down even after 28 days
Considerable r e d u c t i o n in size
Observation
Composites;
Table
3.0
4.8 4.9
7.1 7.9
30.1 21.0
3.66 3.89
4.22 5.41
9.01 11.12
7
28.1
40.0
i0.8
ii.0
6.2
6.2
21.8
14
% weight 21
29.0
41.7
i0.9
11.5
7.2
7.2
27.1
loss
Drastic reduction in structural dimensions
48.3 34.2
12.2
R e d u c t i o n in structural dimensions
Considerable expansion in 7 days. Do not crumble down even after 28 days
Considerable r e d u c t i o n in size
13.0
7.2
7.3
27.3
28
Observation
Environment
in days
Stagnant
HCI m e d i u m
- 30°C,
0
(% w/w)
Polymer
Temperature
m"
Z ©
LJ
O
>o oo oo
Vol. 13, No. 2
289 POLYMER-CEMENT MORTARS, DURABILITY, ACIDS
ammonium salt (NH4CI or (NH4)2SO 4 in HCI and H2SO 4 respectively). The ammonium salt so produced, induces further corrosive action in addition to that of H2SO 4 or HCI which are originally present. A similar observation with ammonium salt has been reported by Ohama (3). Exceptionally higher durability of mortar-PMMA composites compared to those containing PS can well be explained in terms of cement-polymer interaction. Since the pores in cement matrix contribute most to the total surface area exposed to the corrosive medium, a poorer durability obviously would correspond to a situation in which the polymer-filled pores allow relatively easier entry of the medium. Thus the composites with weak cement-polymer interfacial bond strength would exhibit poorer durability properties. On these premises, it may be inferred that cement-PS interfacial bond is weaker than cement-PMMA bond. As regards the dimensional stability, the PS- and PAN-mortar systems undergo considerable reduction in sizes while the PMMA-mortar systems undergo some expansion both in HCI and H2SO 4 media. It appears that the acidity of environment is the most important factor which is responsible for the observed dimensional expansion. Effect of sulphate ion exchange (4), if any, is of minor significance, since the expnnsion has been observed in HCI solution also. A possible role played by the acidity of the media is that the ester groups of P M ~ undergo slow hydrolysis to give polymethacrylic acid (PMA) and methanol which are soluble in water. In the presence of strong acids, the dissociation of PMA is suppressed and it forms a swollen structure by absorbing water. As a result, a considerable stress is produced leading to an expansion of the matrix. Polystyrene, however, does not enter into this sort of chemical reaction with either H2SO 4 or HCI in aqueous solution. Thus structural expansion as observed with PMMA is not possible with PS. There could be some expansion in H2SO 4 medium due to sulphate ion exchange. But its contribution would be much less compared to the reduction in size due to corrosive action of the environment. As regards to the strength and nature of interaction, a polar-polar interaction, in general, is always stronger than a polar-nonpolar interaction. Both cement and sand, the two constituents of mortar, are polar inorganic materials of silicate structure. On the other hand, PS and PMMA are non-polar and polar respectively. Thus the interaction between cement and polymer is stronger in PMMA-based composites than PS-based ones. PAN is also a polar polymer, hence higher durability should have also been observed with PAN-based composites. But PAN itself being very much susceptible to degradation in strongly acidic environment and the degradation products (polyacrylic acid (PAA) and ammonium salt) being corrosive itself, a poorer durability is observed with PAN-based composite system. Some experimental evidences are also available in support of this interaction. Cook and co-workers (5) from their studies on the differential thermal analysis (DTA) of premixed cement-polymer composites have shown that there occurs some definite interaction between cement and polymer, detectable by disappearance or diminution of intensity of some well defined DTA peaks. With hydrolyzable polymers like PMMA, PAN and PVAc etc., the interactions are most pronounced. Specifically, these polymers lower the peak intensity for the dehydration of free Ca(OH)2 in water-cured cement paste which occurs at 500°C. Chandra et al. (6) have shown that the Ca +2 ions of free Ca(OH) 2 of the cement matrix interact with the carboxylate groups of methyl methacrylate-styrene polymer dispersion by crosslinking. These crosslinked products render a greater compatibility between the cement-matrix and the impregnated polymer leading to an enhanced cement-polymer bond strength.
290
Vol. 13, No. 2 V.K. Bhattacharya,
et al.
In a recent publication Chang and co-workers (7) have mentioned that significant improvement in strength properties, which is much more than that due to either PS or PMMA alone, can be achieved by impregnating mortars or concretes with poly (styrene-co-methyl methacrylate) at 50:50 w/w ratio. This copolymer is believed to offer the enhanced cement-polymer interaction through the polar easter-functionality, coupled with the rigidity and toughness imparted by the styryl segments of the copolymer. Summary i. So far as chemical durability of polymer-impregnated mortar in aqueous HCI and H2SO 4 is concerned, PMMA-based composites are much superior to PS- and PANbased ones. 2. Durability of PS-based composites is lower to that of PMMA-based composites because the former being a nonhydrolyzable, non-polar type of polymer gives rise to a weaker cement-polymer interfacial bonding. 3. Durability of PAN-based composites in HCI and H2SO 4 solutions is very much poorer due to decomposition of the impregnated polymer to soluble products, which exert further corrosive action. Acknowledgement The authors wish to thank the Department of Atomic Energy, Bombay, for financial support for the project. Thanks are also due to Shri N. Mallick, U. Bakshi and S. Ghosh for technical assistance. References i. 2. 3. 4. 5. 6. 7.
D.G. Manning and B.B. Hope, Cem. Conc. Res. i, 631 (1971). V.K. Bhattacharya, K.R. Kirtania, M.M. Maiti and S. Maiti, National Seminar on Building Materials, New Delhi, April 1982, IIi(6). Y. Ohama and T. Fukuchi, Proc. Twenty-first Japan Congress on Materials Research 71, 190 (1978). A. Auskern and W. Horn, J. Amer. Ceram. Soc. 54, 282 (1971). D.J. Cook, D.R. Morgan, V. Sirivivatnanon and R.P. Chaplin, Cem. Conc. Res. 6, 757 (1976). S. Chandra and L. Berntsson, Cem. Conc. Res. ii, 125 (1981). T.Y. Chang, H.L. Stephens and R.C. Yen, Transp. Res. Rec., Transp. Res. Board (U.S.), No. 542, 1975.
Vol. 13, pp. 287-290, 1983. Printed in the USA. Copyright (c) 1983 Pergamon Press, Ltd.
NOTE DURABILITY TESTS ON POLYMER-CEMENT MORTAR V.K. Bhattacharya, K.R. Kirtania, M.M. Maiti and S. Maiti Polymer Division, Materials Science Centre Indian Institute of Technology Kharagpur 721302 India
(Communicated by F.H. Wittmann) (Received Aug. 18; in final form Dec. i, 1982) ABSTRACT Accelerated durability tests on several polymer-impregnated mortars have been studied in aqueous sulphuric acid (3:1 v/v) and hydrochloric acid (3:1 v/v) media. In addition to the reactivity of individual polymers and cement body in these corrosive environments, the cement-polymer interaction has been identified as an important factor which controls the durability properties of the composites. Introduction The mechanical strengthening of polymer-impregnated cement composites has been suggested to be primarily due to pore filling by the polymer which acts as continuous, randomly oriented reinforcing network and repairs the microcracking and reinforces the micro~ores of cement body by penetration into the matrix (i). Apart from this, the impregnated polymer is also believed to enter into some type of interaction with the cement matrix and forms stable bonds, which largely determines the strength and durability properties of the composites. In this communication we present some of our results on the durability characteristics of mortars impregnated with different polymers which have been explained in terms of cement-polymer interaction. Experimental Details of materials used, specimen sizes, method of impregnation of polymer, procedures for measurement of strength properties, ultrasonic velocity and the durability of composites have been reported elsewhere (2). Results and Discussion Results of the accelerated durability tests on mortars impregnated with different polymers are presented in Table i. The data clearly show that polymethyl methacrylate (PMMA) improves the durability of mortar-polymer composites remarkably. The mortar-polystyrene (PS) composites are less durable. Durability of mortar-polyacrylonitrile (PAN) composites, however, is even worse than that of control. The observed durability behavior of mortar-PAN composites is not very unusual. On slow hydrolysis in acid medium PAN g~ves polyacrylic acid (PAA) and 287
0
0
46.0
27.0
32.1
13.8
7.24
7.28
4.91
7.77
4.93
10.17
MortarPMMA
MortarPAN
MortarPS
41.0
0
Mortar
7
42.2
52.5
28.0
72.0
0
0
47.1
14
% weight loss day_s_-
Polymer (% w/w)
75.0
80.0
32.2
88.9
1.2
1.0
94.7
28
in
of M o r t a r - P o l y m e r
H2SO 4 m e d i u m
Behavior
Composite
Durability
1
Drastic reduction in structural dimensions
Reduction in structural dimensions
Considerable expansion in 7 days. Do not crumble down even after 28 days
Considerable r e d u c t i o n in size
Observation
Composites;
Table
3.0
4.8 4.9
7.1 7.9
30.1 21.0
3.66 3.89
4.22 5.41
9.01 11.12
7
28.1
40.0
i0.8
ii.0
6.2
6.2
21.8
14
% weight 21
29.0
41.7
i0.9
11.5
7.2
7.2
27.1
loss
Drastic reduction in structural dimensions
48.3 34.2
12.2
R e d u c t i o n in structural dimensions
Considerable expansion in 7 days. Do not crumble down even after 28 days
Considerable r e d u c t i o n in size
13.0
7.2
7.3
27.3
28
Observation
Environment
in days
Stagnant
HCI m e d i u m
- 30°C,
0
(% w/w)
Polymer
Temperature
m"
Z ©
LJ
O
>o oo oo
Vol. 13, No. 2
289 POLYMER-CEMENT MORTARS, DURABILITY, ACIDS
ammonium salt (NH4CI or (NH4)2SO 4 in HCI and H2SO 4 respectively). The ammonium salt so produced, induces further corrosive action in addition to that of H2SO 4 or HCI which are originally present. A similar observation with ammonium salt has been reported by Ohama (3). Exceptionally higher durability of mortar-PMMA composites compared to those containing PS can well be explained in terms of cement-polymer interaction. Since the pores in cement matrix contribute most to the total surface area exposed to the corrosive medium, a poorer durability obviously would correspond to a situation in which the polymer-filled pores allow relatively easier entry of the medium. Thus the composites with weak cement-polymer interfacial bond strength would exhibit poorer durability properties. On these premises, it may be inferred that cement-PS interfacial bond is weaker than cement-PMMA bond. As regards the dimensional stability, the PS- and PAN-mortar systems undergo considerable reduction in sizes while the PMMA-mortar systems undergo some expansion both in HCI and H2SO 4 media. It appears that the acidity of environment is the most important factor which is responsible for the observed dimensional expansion. Effect of sulphate ion exchange (4), if any, is of minor significance, since the expnnsion has been observed in HCI solution also. A possible role played by the acidity of the media is that the ester groups of P M ~ undergo slow hydrolysis to give polymethacrylic acid (PMA) and methanol which are soluble in water. In the presence of strong acids, the dissociation of PMA is suppressed and it forms a swollen structure by absorbing water. As a result, a considerable stress is produced leading to an expansion of the matrix. Polystyrene, however, does not enter into this sort of chemical reaction with either H2SO 4 or HCI in aqueous solution. Thus structural expansion as observed with PMMA is not possible with PS. There could be some expansion in H2SO 4 medium due to sulphate ion exchange. But its contribution would be much less compared to the reduction in size due to corrosive action of the environment. As regards to the strength and nature of interaction, a polar-polar interaction, in general, is always stronger than a polar-nonpolar interaction. Both cement and sand, the two constituents of mortar, are polar inorganic materials of silicate structure. On the other hand, PS and PMMA are non-polar and polar respectively. Thus the interaction between cement and polymer is stronger in PMMA-based composites than PS-based ones. PAN is also a polar polymer, hence higher durability should have also been observed with PAN-based composites. But PAN itself being very much susceptible to degradation in strongly acidic environment and the degradation products (polyacrylic acid (PAA) and ammonium salt) being corrosive itself, a poorer durability is observed with PAN-based composite system. Some experimental evidences are also available in support of this interaction. Cook and co-workers (5) from their studies on the differential thermal analysis (DTA) of premixed cement-polymer composites have shown that there occurs some definite interaction between cement and polymer, detectable by disappearance or diminution of intensity of some well defined DTA peaks. With hydrolyzable polymers like PMMA, PAN and PVAc etc., the interactions are most pronounced. Specifically, these polymers lower the peak intensity for the dehydration of free Ca(OH)2 in water-cured cement paste which occurs at 500°C. Chandra et al. (6) have shown that the Ca +2 ions of free Ca(OH) 2 of the cement matrix interact with the carboxylate groups of methyl methacrylate-styrene polymer dispersion by crosslinking. These crosslinked products render a greater compatibility between the cement-matrix and the impregnated polymer leading to an enhanced cement-polymer bond strength.
290
Vol. 13, No. 2 V.K. Bhattacharya,
et al.
In a recent publication Chang and co-workers (7) have mentioned that significant improvement in strength properties, which is much more than that due to either PS or PMMA alone, can be achieved by impregnating mortars or concretes with poly (styrene-co-methyl methacrylate) at 50:50 w/w ratio. This copolymer is believed to offer the enhanced cement-polymer interaction through the polar easter-functionality, coupled with the rigidity and toughness imparted by the styryl segments of the copolymer. Summary i. So far as chemical durability of polymer-impregnated mortar in aqueous HCI and H2SO 4 is concerned, PMMA-based composites are much superior to PS- and PANbased ones. 2. Durability of PS-based composites is lower to that of PMMA-based composites because the former being a nonhydrolyzable, non-polar type of polymer gives rise to a weaker cement-polymer interfacial bonding. 3. Durability of PAN-based composites in HCI and H2SO 4 solutions is very much poorer due to decomposition of the impregnated polymer to soluble products, which exert further corrosive action. Acknowledgement The authors wish to thank the Department of Atomic Energy, Bombay, for financial support for the project. Thanks are also due to Shri N. Mallick, U. Bakshi and S. Ghosh for technical assistance. References i. 2. 3. 4. 5. 6. 7.
D.G. Manning and B.B. Hope, Cem. Conc. Res. i, 631 (1971). V.K. Bhattacharya, K.R. Kirtania, M.M. Maiti and S. Maiti, National Seminar on Building Materials, New Delhi, April 1982, IIi(6). Y. Ohama and T. Fukuchi, Proc. Twenty-first Japan Congress on Materials Research 71, 190 (1978). A. Auskern and W. Horn, J. Amer. Ceram. Soc. 54, 282 (1971). D.J. Cook, D.R. Morgan, V. Sirivivatnanon and R.P. Chaplin, Cem. Conc. Res. 6, 757 (1976). S. Chandra and L. Berntsson, Cem. Conc. Res. ii, 125 (1981). T.Y. Chang, H.L. Stephens and R.C. Yen, Transp. Res. Rec., Transp. Res. Board (U.S.), No. 542, 1975.