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Properties of epoxy-cement mortar systems
85
Materials Science and Engineering, 18 (1975) 85--95 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Properties of E p o x y - - C e m e n t Mortar Systems
PORE-FENG SUN*, J.A. S A U E R and E.G. NAWY
Rutgers University, New Brunswick, N.J. (U.S.A.) (Received June 27, 1974)
SUMMARY
The possible benefits to be obtained by addition of an epoxy resin system to cement mortar have been explored. The e p o x y resin system, consisting of a low molecular weight liquid resin, Epon 828, and either an amidoamine or a di-/3-hydroxyalkylamine as a curing agent, is first premixed and then varying percentages of the blended e p o x y resin are added to a standard cement; sand; water mix. With 5% of added epoxy (by weight of the mortar) tensile strengths above 900 p.s.i, and compressive-strengths above 8000 p.s.i, have been realized with either normal Type I or early Type III cement. Both of the hardeners used gave improved strength properties b u t the hydroxyalkylamine gives superior strength and workability. Additional tests were made in which the cement: water phase of the mortar was progressively replaced by the blended epoxy system. For 20% replacement, strength properties were improved b u t they diminished at 40% replacement, as the added e p o x y interfered with hydration of the cement. At still higher replacements, strength values rose again; and, for 100% replacement, the average tensile strength exceeded 1800 p.s.i, and the water absorption was effectively zero.
1. INTRODUCTION
Cement and concrete are widely used in many structural applications. While cement mortar and concrete are strong in compression, they are relatively weak in tension and have limited freeze - thaw resistance [ 1]. * Now at the University of Iowa, Dept. of M e c h a n i c s and Hydraulics
In an effort to improve their strength qualities and extend the range of usefulness of mortar and concrete systems, investigators have begun to experiment with and study the properties of cement and concrete systems that have been modified in some manner. The systems studied include: polymer impregnated concretes or PIC systems [2 - 8] containing impregnated m o n o m e r that is subsequently polymerized; polymer concretes or PC systems [3,9,10] in which a polymer is used as the binder instead of cement; and p o l y m e r - cement concretes or PCC systems, in which the binder includes both a polymer and hydrated cement [3,11 - 14]. The most successful of the systems studied to date in terms of the percentage improvement in strength and durability as compared with the control, or plain concrete, is the PIC system, and this method has been applied to porous ceramics as well as to cement mortars and concrete [7]. Some of the steps involved in preparing PIC systems include precasting of the cement concrete to the desired shape, drying the precast material or slabs, evacuating the water, impregnating m o n o m e r (as styrene, methyl methacrylate, b u t y l acrylate or other easily polymerizable material) under vacuum, and then polymerizing the m o n o m e r by use of radiation and by use of chemical catalysts or thermal treatment or both. In some instances thermosetting resins have also been used in place of thermoplastic t y p e monomers to improve the strength and modulus of PIC type composites [3,8]. It has been reported that PCC systems, based on direct addition of m o n o m e r to the cement or concrete mix followed by polymerization of the monomer, have n o t been successful to date [2]. However, some PCC systems, in
86 which latexes of poly(vinyl acetate), vinylidene chloride copolymers, or styrene - butadiene copolymers have been added to the mix, have resulted in product improvement and such systems have been used in a number of applications [3,14]. PC, or polymer concrete, systems with no cement phase tend to have high strength and good resistance to water absorption and freeze thaw conditions [3,9,10]. However, PC systems are expensive by comparison with ordinary cement mortars or concretes. They tend to be used for rather specialized applications as protective coatings of polymer mortar on concrete or reinforced concrete and as polymer concrete in highly corrosive atmospheres [9]. The present study is an attempt to improve the strength and durability properties of mortar and concrete through the direct addition of suitable components to the mortar or concrete mix in the field. The procedure consists essentially of adding to the mortar or concrete mix a blended epoxy resin system, i.e., a system consisting of appropriate proportions of a low molecular weight epoxy resin and a liquid hardener or curing agent. The materials used and the results obtained from five series of polymer - cement mortar systems are here presented and discussed; in a second paper [15] discussion is given of the results obtained when similar epoxy materials are used to modify concrete. In both cases, significant improvements in strength properties can be realized by relatively minor additions of an epoxy resin system to the mortar or concrete mix. Still greater improvements, though with greater expense, can be achieved by more fully replacing the hydrated cement phase by the cured epoxy system. The epoxy resin systems used in the present investigation have the following attractive characteristics: 1. Both the resin and hardener can be obtained in liquid form. 2. The resin and hardener can be premixed prior to incorporation in the cement - water sand mixture. Also, with proper choice of components, subsequent curing can be delayed until the mixture has been properly worked and formulated. 3. Resin - hardener systems can be obtained which can cure at room temperature. 4. The resulting properties of cured epoxy
systems can be varied by proper selection of resin, hardener and such other ingredients as accelerators and plasticizers. 5. The epoxy - cement mortar systems can be prepared on the site where they are to be used and no special facilities are required. In the present investigation, several different epoxy formulations have been investigated and two types of cement were used. The epoxy cement specimens were then cured for various time periods ranging from 3 to 28 days. Various physical measurements and tests were made on the resulting products.
2. MATERIALS AND TEST SPECIMENS Two types of Portland cement were used: normal Type I and high early strength Type III. The aggregate used was a natural graded sand, dried before use, having a sieve gradation as follows: U.S. Standard sieve No. #4 #8 #16 ~30 #50 #100 Percentage passing 100 99.5 95 72.7 28.5 8.67 For the polymer modifier of the cement, several epoxy resin systems were tried. In each of these the starting epoxy resin was Epon-828. This resin has an epoxide equivalent weight of 180 - 195 and a viscosity, at ambient temperature, of 110 to 150 poises. Two types of hardener were used. The first one was a fatty amidoamine, sold under the trade name of Lancast A. The second curing agent, known as CA-640, is a di-fl-hydroxyalkylamine. This was chosen for study as it is expected to function better in an aqueous medium. It has the following chemical structure {with n = 7 to 9):
CH3-(CH2)~-O--CH2--CH~CH2--NH E I OH CI2H4 NH I
CI2H4 NH I
Ctt3~--O- CH2--CH--CI-I2--NH Both curing agents will cure and set the epoxy resin to a three-dimensional network structure at ambient temperature.
87 TABLE 1 C o m p o s i t i o n o f e p o x y - c e m e n t m o r t a r t e s t series A, B, C and D Specimen type
Cement
Curing agent
Epoxy/ cement mortar (%)
Epoxy/ cement (%)
A1 A2 A3 A4
Type I
Amido
0.0 0.05 0.10 0.15
0.0 0.2 0.4 O.6
B1 B2 B3 B4
Type I
CA-640
0.02 0.05 0.10 0.15
0.1 0.2 0.4 0.6
C1 C2 C3 C4
T y p e III
Amido
0.0 0.05 0.10 0.15
0.0 0.2 O.4 0.6
D1 D2 D3
T y p e III
CA-640
0.05 0.10 0.15
0.2 0.4 0.6
C e m e n t : sand : w a t e r = 1 : 2.75 : 0.3 for all m o r t a r s
To study the influence of an epoxy modifier on the properties of cement mortar specimens, various percentages of the blended resin, consisting of the low molecular weight Epon 828 liquid resin mixed with the liquid curing agents, were ~idded to the mortar, prepared with a fixed sand : cement : water ratio. Four series of tests were run at a water : cement ratio deliberately chosen at a low value, 0.3, in order to prevent segregation of the fine aggregates at high resin contents. In these tests the cement : sand ratio was maintained at 1 : 2.75 by weight for all specimens. The percentage of the epoxy system, resin plus hardener, added to the cement mortar was varied from 0 to 15% by weight of the mortar. This
corresponded to an epoxy : cement ratio varying from 0.0 to 0.6. The specific compositions of the test specimens for each of these four series of tests are given in Table 1. It is noted that specimens designated A and B respectively were both prepared from normal Type 1 cement, but had different hardeners, while test specimens designated C and D were equivalent specimens prepared using Type III early cement. In an effort to determine the best quantitative c o m b i n a t i o n of the cement and epoxy binders, a fifth series of specimens were prepared and tested. In this series, designated E, the water : cement ratio was maintained at 0.4 for all specimens but a percentage of the ce-
TABLE 2 Compositionratiosofepoxy-cementmortartestseriesE Specimen type
Cement
Sand
Water
Epoxy
% o f c e m e n t and w a t e r r e p l a c e d by epoxy
E1 E2 E3 E4 E5 E6
1.0 0.8 0.6 0.4 0.2 0.0
2.75 2.75 2.75 2.75 2.75 2.75
0.40 0.32 0.24 0.16 0.08 0.00
0.0 0.28 0.56 0.84 1.12 1.40
0 20 40 60 80 100
Water : c e m e n t = 0.4 for all m o r t a r s
88 ment and water was progressively replaced by the polymer resin, here based on Epon 828 and the CA-640 curing agent. The a m o u n t of sand was maintained constant. The compositions of the various specimens included in this series, varying from 0 to 100% replacement of the cement - water system by the epoxy, are given in Table 2.
3. MOLDING AND CURING PROCEDURE After some trial and error experimentation, the following procedure was adopted. Dry sand and fresh cement were mechanically mixed by hand at room temperature for about two minutes in a mixing pan. Water was then added to the mixture in the desired proportions and mixing continued until a homogeneous product was obtained. The epoxy system, in which the liquid resin and liquid hardener were premixed in stoichiometric ratios shortly before use, is then added to the sand cement mortar and mixing continued until a uniform dispersion is attained. With the amido hardener, the proportions of resin to hardener were 100 : 45, for both Tests A and C; for the CA-640 hardener, the proportions were 100 : 75 for the three test series Tests B, D and E. Test specimens were prepared from the above mixture by placing the material in successive layers in cylindrical molds 2 in. in diameter and 4 in. in length. Because of the adhesive nature of cure epoxy systems, all the mold surfaces were first treated with a mold release agent, Rust Band 385, after heating the molds to 70°C. The molded specimens were stored in a moisture room for 24 hours before t h e y were stripped from the molds. The specimens were then marked for identification and stored under water for 3, 7, 14 or 28 days respectively until ready for testing.
4. TEST PROCEDURES Both compressive and tensile splitting strength tests were conducted. The specimens for compression tests were carefully capped in accord with ASTM Standard C617, with a mixture of granular material and sulfur, a vitrobond product. Strength measurements were carried out on
specimens corresponding to each of the test conditions described in Table 1. For these four series of tests, designated A, B, C and D repectively, strength values were recorded after curing times of 3 days, 7 days, 14 days and 28 days. Generally, three specimens were tested for each test condition and the average of these three values accepted as the nominal compressive or tensile strength for the specific variables involved. For the series E tests, with specimens having compositions as indicated in Table 2, tensile and compressive strength measurements were made only for specimens given a 7-day cure and a 28-day cure. Again, tests were made for each set of test variables on three identical specimens, and the average values noted. To determine how the addition of a polymer c o m p o n e n t modifies the stiffness of specimens, measurements were made of the elastic modulus for specimens of series E. For these tests, 2 in. X 4 in. cylindrical specimens were dried in air for five days. In order to m o u n t SR-4 strain gages on the surface for strain measurements, the dried specimens were sanded and cleaned with acetone. The gages were then m o u n t e d near the center of the specimen at two diametrically opposite positions and the specimens allowed to dry in air for another two days. The tests were carried out at a constant loading rate of about 25 p.s.i./sec and the strain was determined from a strain indicator. The maximum applied load did not exceed 40% of the ultimate strength. The modulus was determined from the slope of the stress strain curve and two specimens were tested for each set of variables involved in the group E test specimens. Determinations were also made, for series E specimens, of free water content so that data would be available as to the effect on porosity of partial replacement of a cement phase by a polymer phase. Tests were made on two specimens for each of the series E variables. These measurements were made as follows. First, specimens were selected which had been submerged in water for at least 28 days. These specimens were removed from the water environment, wiped dry and weighed. They were then placed in an oven for 2 to 4 days at a temperature of 230°F in order to remove the water present. The oven-dried samples were cooled to ambient temperature
89 and reweighed. The per cent of free water present in the specimens was calculated from the equation: W1--W 2 % free water c o n t e n t - - × 100 W2 where W1 is the weight of the saturated specimen and W2 is the weight of the oven-dried specimen. In addition to the above tests, optical and scanning electron microscope (SEM) pictures of the fracture surfaces of several p o l y m e r cement mortar specimens were taken. The SEM micrographs were obtained with an Itek SEM apparatus. The specimens to be examined were coated with Au - Pd in a vacuum evaporator to provide a conducting surface and to overcome charging. The secondary electron image of the fracture surface was then observed at various magnifications in order to obtain information about the nature of the bond between the resin phase and the sand particles.
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RES,N CONTENT (°/.I Fig. 1. Tensile and c o m p r e s s i v e s t r e n g t h s v s . resin cont e n t o f series A e p o x y - c e m e n t mortars. S p e c i m e n s p r e p a r e d f r o m : T y p e I c e m e n t , E p o n 828 and amidoa m i n e curing agent.
5. T E S T R E S U L T S AND D I S C U S S I O N
A. Effect
of ep o xy c o n t e n t on strength
The effects of adding an increasing a m o u n t of epoxy, at a fixed cement : sand : water ratio of 1 : 2.75 : 0.3, on the tensile and compressive strengths are shown graphically in Figs. 1 - 4 for series A, B, C and D test specimens. For series A specimens, wherein the liquid epoxy Epon 828 and an amidoamine hardener were added to Type 1 cement, it is observed from Fig. 1 that significant improvements in both tensile and compressive strength are realized for polymer additions of some 5 - 10%. For the specimens cured for 28 days, only 5% epoxy by weight of the mortar is needed to provide the maximum strength benefit. For this a m o u n t of added epoxy, the tensile strength is raised on the average from 480 p.s.i, to 730 p.s.i., an improvement of over 50%, and the compressive strength from 3,570 p.s.i, to 5.390 p.s.i., also an improvement of over 50%. For the specimens cured for 3, 7 or 14 days, it appears that m a x i m u m gain in strength can be realized at about 10% resin c o n t e n t but, for the 28-day cured specimens, there appears to be no advantage in raising the epoxy c o n t e n t by more than 5%. From these results it is clear that appreciable
strength improvements in cement mortar are possible by relatively minor additions of epoxy components to the mix. In the instances cited above, the added epoxy c o m p o n e n t a m o u n t e d to only 1/5 by weight of the cement. In series B specimens the CA-640 hardener was combined with the Epon 828 and then both added to the cement mortar. Specimens were prepared with 2%, 5%, 10% and 15% of epoxy by weight of the cement mortar. The observed increases in strength are shown in Fig. 2. The m a x i m u m strength gain appears to be achieved by addition of only about 5 wt. % of epoxy while further increases in epoxy c o n t e n t have no appreciable strengthening influence. For the specimens cured for 28 days, the tensile splitting strength has increased by 90% from the original 480 p.s.i. to 910 p.s.i., and the compressive strength has increased by 125% from the initial value of 3570 p.s.i, to 8130 p.s.i. It can be assumed that 5% of added epoxy improves workability and the bond of the hydrated cement to the filter particles; but if an excess a m o u n t of epoxy is added, it appears to hinder development of a continuous cement gel and hence lower strengths may result. From these test results, it is evident that the
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Fig. 2. Tensile and compressive strengths v s . resin content o f series B e p o x y - c e m e n t mortars. S p e c i m e n s prepared from: T y p e I c e m e n t , E p o n 8 2 8 and CA6 4 0 curing agent.
Fig. 3. Tensile and compressive strengths v s . resin content o f series C e p o x y - c e m e n t mortars. S p e c i m e n s prepared from: T y p e III cement, E p o n 8 2 8 and amidoamine curing agent.
specific hardener used in the epoxy formulation was a significant factor in the added strength. Although the first hardener tried worked satisfactorily and resulted in an increase of 50% in both tensile and compressive strengths at only 5% of added polymer, the second hardener, the hydroxyalkylamine, effectively raised the strength values approximately 50% more. The higher strengths for the latter type hardener probably result from a better bond between the matrix and the sand particles and also because of greater workability and hence greater homogeneity in the
can be drawn from use of emulsified polymers in other types of PCC systems [11]. For the specimens cured for 28 days and having 5% of epoxy resin by weight of mortar 14 28
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When high early Type III cement was substituted for the normal type I and the Lancast A hardener used there was slight beneficial strength increase for 5% added resin as seen in Fig. 3. However, at higher resin contents, both the tensile and compressive strengths decreased. With the hydroxyalkylamine hardener, there was a marked improvement in strength at 5% added resin level as seen in Fig. 4 while little benefit was gained by further addition of the resin component. Thus addition of approximately 5% of epoxy by weight of the mortar or 20% of epoxy by weight of the cement appears to be an optimum value for strength improvement. Similar conclusions
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Fig. 4. Tensile and compressive strengths v s . resin content o f series E e p o x y - c e m e n t mortars. S p e c i m e n s prepared from: T y p e III c e m e n t , E p o n 8 2 8 and CA640 curing agent.
91
the observed increases in strength were not as large for the Type III cement specimens as for the Type I cement specimens. For the Type III specimens, the tensile strength increased from 745 p.s.i, for the zero resin content specimens to 940 p.s.i, for the 5% resin content specimens. The corresponding increase in compressive strength was from 6,370 p.s.i, to 8,370 p.s.i.
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B. Effect of epoxy content on workability The results discussed above clearly demonstrate that polymer - cement mortars can be prepared so as to have tensile strengths above 900 p.s.i, and compressive strengths above 8000 p.s.i. Furthermore, these relatively high values of both tensile and compressive strength can be realized by addition of only 5% of epoxy by weight of the cement mortar or 20% of epoxy by weight of the cement itself. Addition of the polymer c o m p o n e n t permits better workability at lower water : cement ratios. The control samples with water : cement c o n t e n t of 0.3 and no resin have poor workability. This low workability can be improved by the addition of more water; but added water results in strength reduction. However, upon addition of blended epoxy resin of about 20% by weight of the cement, the total liquid cement ratio increased sufficiently to give good workability. Also during the curing period, the epoxy resin sets to a three-dimensional network. This network together with that of the hydrated cement phase acts as a binder for the sand particles and, as the test results have shown, results in a polymer - cement mortar that develops both tensile and compressive strength values 50 - 100% higher than the control samples depending upon the particular hardener used. C. Effect of replacement o f cement - water by epoxy In series E mortar mixes the water : cement ratio was maintained constant at 0.4 but the cement and water were replaced by epoxy at levels of 20%, 40%, 60%, 80% and then 100%. The measured compressive strengths and tensile strengths for series E specimens are plotted in Fig. 5 as a function of the resin c o n t e n t , i.e., as a funCtion of the per cent of the cement and water replaced by the epoxy. The results are given for specimens that were al-
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Fig. 5. Tensile a n d c o m p r e s s i v e s t r e n g t h s v s . resin cont e n t of series E e p o x y - c e m e n t m o r t a r s . S p e c i m e n s p r e p a r e d f r o m : T y p e I c e m e n t , E p o n 828 a n d CA640 curing agent.
lowed to cure for 7 days and also for specimens cured for 28 days. For 20% resin content, Fig. 5 shows that increases in strength values have been obtained. However, when the resin c o n t e n t is further increased both tensile and compressive strength values fall below original control values. Somewhat similar results have been reported for other types of PCC systems [11] upon addition of resin beyond 15 or 20%. One reason for the decrease in strength values may be that the a m o u n t of resin present is large enough to seriously interfere with the hydration of the cement. Evidently at polymer : cement ratios of 0.2 or less, there is little interference, a continuous cement gel is maintained, and strengths remain high or increase. At resin contents above 40%, Fig. 5 shows that the tensile and compressive strengths start to rise sharply from their minimum values with increased resin replacement of the cement. At 80% replacement of the cement - water combination by the epoxy, the strengths are again well above the original control values. For example, for 80% replacement the average 28-day tensile strength is 925 p.s.i. (a 34%
92
increase) and the average compressive strength is 5540 p.s.i. (a 14% increase). For 100% replacement, i.e., for the specimens in which the only binder present is the epoxy, the average tensile strength has risen to 1860 p.s.i. (a 169% increase) and the average compressive strength to 8630 p.s.i. (a 77% increase). It is interesting to note that for these specimens the 28 day strengths are 3 to 5 times the 7 day strengths. This indicates that polymerization and further curing of the resin are continuing even after a 7-day period. D. Effect o f resin content on Young's modulus Measurements were made of Young's modulus for specimens designated in Table 2 by the letters E l , E2, E4 and E6. Specimens E1 contained as a binder only a hydrated cement phase and specimens E6 only the cured epoxy phase. The E2 specimens had a 20% epoxy replacement and the E4 specimens a 60% epoxy replacement of the cement - water paste. A typical stress - strain curve for an E2 specimen is shown in Fig. 6. The loading curve is effectively linear while the unloading curve is concave upward and a small permanent set remains after unloading. The effect o f resin replacement on modulus is shown in Fig. 7. Although there is some scatter in the data, it appears that the modulus falls off essentially linearly from an initial value for the cement sand system of 4.34 X 106 p.s.i, to a final value of 1.47 X 106 p.s.i, for the epoxy - sand system. The modulus or stiffness thus decreases essentially in proportion to the reduction of
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Fig. 7. Elastic m o d u l u s v s . p e r c e n t a g e o f c e m e n t paste replaced by resin for series E e p o x y - c e m e n t m o r t a r specimens.
the hydrated cement phase and its replacement by the less stiff epoxy network. This result differs from that obtained in PIC systems. In these systems, where the polymer tends to fill in the voids and pores, modulus and density increase with a m o u n t of m o n o m e r added [2,4]. E. Effect of resin content on water retention Permeability was investigated for the series E epoxy cement mortars whose composition is given in Table 2. Weight measurements were made on water saturated specimens stored for 28 days in water and on the same specimens after both 2 days and 4 days of oven drying. The calculated free water c o n t e n t values are given as a function of the percentage of cement and water replaced by epoxy in Fig. 8. For the samples subject to 96 hours of drying, the water c o n t e n t was 7.47% for the control samples, 5.59% for 20% epoxy replacement and 0.13% for 100% epoxy replacement. The actual variation is only approximately linear as Fig. 8 shows. Nevertheless, it appears that IO
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Fig. 8. Percentage w a t e r a b s o r p t i o n v s . p e r c e n t a g e o f c e m e n t paste r e p l a c e d by resin for series E e p o x y c e m e n t m o r t a r specimens.
93
Fig. 9. Scanning electron micrograph of the fracture surface of an epoxy - cement mortar specimen fractured in tension. w a t e r c o n t e n t decreases in p r o p o r t i o n t o the decrease in the c e m e n t b i n d e r phase and the p r o b a b l e decrease in n u m b e r o f pores. It is a p p a r e n t f r o m the test results t h a t if e p o x y resin alone is used as the b i n d e r (series E6 specimens) t h e free w a t e r c o n t e n t is alm o s t nil and h e n c e freeze - t h a w c o n d i t i o n s s h o u l d have little e f f e c t o n e p o x y mortars. Such a p o l y m e r - m o r t a r c o m b i n a t i o n , which consists o f 1.4 parts p o l y m e r and 2.75 parts sand, w o u l d have e x c e l l e n t strength p r o p e r t i e s (tensile strength above 1 5 0 0 p.s.i, a n d compressive strength a b o v e 8 0 0 0 p.s.i. ) a n d g o o d d u r a b i l i t y p r o p e r t i e s , as far as t e m p e r a t u r e and h u m i d i t y changes are c o n c e r n e d . In m a n y applications, h o w e v e r , the use o f such p o l y m e r m o r t a r s m a y n o t be justified on e c o n o m i c a l grounds as the cost p e r p o u n d is v e r y m u c h higher for an e p o x y b i n d e r t h a n f o r a c e m e n t binder.
In the e p o x y - m o r t a r specimens, if high strengths are desired in a s h o r t e r aging p e r i o d t h a n 28 days, curing c o u l d be a c c e l e r a t e d by e x p o s u r e to elevated t e m p e r a t u r e s or possibly by selection o f s o m e o t h e r t y p e o f h a r d e n i n g agent.
F. Fracture behavior and bond strength The f r a c t u r e surfaces o f several o f the e p o x y m o d i f i e d m o r t a r test samples, particularly t h o s e b r o k e n in the tensile splitting test, were e x a m i n e d in an a t t e m p t t o d e t e r m i n e , if possible, the influence o f the e p o x y m o d i f i e r on the n a t u r e o f the b o n d existing b e t w e e n sand particles and h y d r a t e d c e m e n t . Figure 9 is a low m a g n i f i c a t i o n scanning e l e c t r o n micrograph o f o n e such specimen. T h e d a r k gray areas are the sand particles and the light gray areas r e p r e s e n t the e p o x y - c e m e n t matrix. In general, the b o n d b e t w e e n the m a t r i x and
94
Fig. 10. Higher m a g n i f i c a t i o n s c a n n i n g e l e c t r o n m i c r o g r a p h o f p o r t i o n o f f r a c t u r e surface o f a n e p o x y - c e m e n t m o r t a r s p e c i m e n f r a c t u r e d in t e n s i o n .
the filler appears to be quite satisfactory and the fracture surface has generally passed direct~ ly through the sand particles. Figure 10 shows a higher magnification SEM picture of a portion of the fracture surface for an epoxy modified sample. Some matrix cracks appear to have been arrested by nearby filler particles. For the large particle in the upper center of the picture, the bond at the matrix - sand interface on one side of the particle has been broken. The effect of the modifying agent in improving the bond between sand particles is apparent from the manner in which Group B specimens, in which a low 0.3 water : cement ratio was present, behaved when tested on the flow table. For the control specimens (A1), with no added epoxy, there was p o o r cohesion between the sand particles and the molded
paste tended to disintegrate when tested on the flow table. However, for Group B1 specimens, with resin : mortar ratio of only 0.02, cohesion was already much improved and for Group B2 specimens, with 5% added epoxy, both cohesion and workability were significantly improved. Thus better packing and fewer voids may contribute to the high observed strength values found for these epoxy modified B2 specimens. Visual inspection of the fracture surfaces of the Series E specimens, with increasing amounts of cement paste replaced b y epoxy, show that the densest structure, and the minim u m size voids, were obtained for the E2 specimens having the minimum e p o x y replacement (20%) as, with larger concentrations of epoxy, larger size voids appeared to be present. These probably arise from the highel viscosity of the ep-
95
o x y resin compared with water and the increasing difficulty of removing entrapped air from the system.
CONCLUSIONS
1. Addition of 5% (by weight of the mortar) of a premixed epoxy system, consisting of Epon 828 resin and a CA-640 curing agent, has increased the tensile strength of a standard c e m e n t - sand - water mix by some 90% and the compressive strength by over 100%. For these epoxy - cement mortar specimens the average 28-day tensile strength is above 900 p.s.i, and the average 28-day compressive strength above 8000 p.s.i. These values can be reached with either T y p e I or Type III cement. 2. If the premixed epoxy system is made from Epon 828 plus an amidoamine hardener (Lancast A), and again 5% of the blended resin added to the standard mix, the resultant tensile and compressive strength improvements are of the order of 50%. 3. There is no strength advantage gained by increasing the resin content b e y o n d 5% by weight of the mortar or the epoxy : cement ratio b e y o n d 0.2. At these figures, workability is good, strength properties are maximum, and the bond strength of the matrix to the filler particles is high. 4. The elastic modulus and the free water content both decrease essentially linearly if an epoxy system is used to partly replace the cement and water phase. For a PC system in which only the cured e p o x y is present as the binder, the water content is essentially nil, the compressive strength is high (above 8500 p.s.i.) and the tensile strength is exceptionally good (above 1800 p.s.i.).
ACKNOWLEDGEMENTS
Appreciation is expressed to R.J. Schutz, Vice-President of Sika Chemical Co., for his
aid in supplying materials and for his advice. The authors wish also to acknowledge the assistance of Chia-Chuan Li in carrying o u t the tests described herein, the assistance of C.F. Cook, Jr. in obtaining scanning electron micrographs, and the helpful suggestions of Prof. D. Kline and his co-workers.
REFERENCES 1 A.M. Neville, Properties of Concrete, Wiley, New York, reprinted, 1970. 2 L.E. Kukacka and G.W. DePuy (eds.), Fourth Topical Rept., Concrete - Polymer Materials, AEC and Dept. of Interior, Jan. 1972; see also prior reports #1, 1968, #2, 1969, ~"3, 1971. 3 V.I. Solomatev, Polymer - Cement Concretes and Polymer Concretes, Moscow, 1967 ; translation available from Isotopes Information Center, Oak Ridge National Laboratory, USAEC. 4 J.A. Manson, W.F. Chen, J.W. Vanderhoff, Y.N. Liu, E. Dahl-Jorgenson and H. Mehta, Polymer Pre-prints, 14 (1973) 1203. 5 J. Gebauer and R.W. Coughlin, Cement Concrete Res., 1 (1971) 187. 6 D.G. Manning and B.B. Hope, Cement Concrete Res., 1 (1971) 631. 7 J. Gebauer, D.P.H. Hasselman and R.E. Long, Am. Ceram. Soc. Bull., 51 (1972) 471. 8 D.A. Whiting, P.R. Blankenhorn and D.E. Kline, Polymer Preprints, 14 (1973) 1155; J. Testing Evaluation, ASTM, 2 (1974) 44. 9 N.A. Moshchanskii and V.V. Paturoev (eds.), Structural Chemically Stable Polymer Concretes, Moscow, 1970; translation available NTIS, U.S. Dept. Commerce. 10 R. Bares, Resin Concretes, Chem. of Ind., April 1970, p. 482. 11 J. Geist, S. Amagna and B. Mellor, Ind. Eng. Chem., 45 (1953) 759. 12 H.B. Wagner, Ind. Eng. Chem., Prod. Res. Develop., 4 (1965) 131; 5 (1966) 149 and 6 (1967) 223. 13 J. Hosek, J. Am. Concrete Inst., 63 (1966) 141. 14 J.E. Isenburg and J.W. Vanderhoff, Polymer Preprints, 14 (1973) 1197. 15 P.F. Sun., E.G. Nawy and J.A. Sauer, to be published.
Materials Science and Engineering, 18 (1975) 85--95 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Properties of E p o x y - - C e m e n t Mortar Systems
PORE-FENG SUN*, J.A. S A U E R and E.G. NAWY
Rutgers University, New Brunswick, N.J. (U.S.A.) (Received June 27, 1974)
SUMMARY
The possible benefits to be obtained by addition of an epoxy resin system to cement mortar have been explored. The e p o x y resin system, consisting of a low molecular weight liquid resin, Epon 828, and either an amidoamine or a di-/3-hydroxyalkylamine as a curing agent, is first premixed and then varying percentages of the blended e p o x y resin are added to a standard cement; sand; water mix. With 5% of added epoxy (by weight of the mortar) tensile strengths above 900 p.s.i, and compressive-strengths above 8000 p.s.i, have been realized with either normal Type I or early Type III cement. Both of the hardeners used gave improved strength properties b u t the hydroxyalkylamine gives superior strength and workability. Additional tests were made in which the cement: water phase of the mortar was progressively replaced by the blended epoxy system. For 20% replacement, strength properties were improved b u t they diminished at 40% replacement, as the added e p o x y interfered with hydration of the cement. At still higher replacements, strength values rose again; and, for 100% replacement, the average tensile strength exceeded 1800 p.s.i, and the water absorption was effectively zero.
1. INTRODUCTION
Cement and concrete are widely used in many structural applications. While cement mortar and concrete are strong in compression, they are relatively weak in tension and have limited freeze - thaw resistance [ 1]. * Now at the University of Iowa, Dept. of M e c h a n i c s and Hydraulics
In an effort to improve their strength qualities and extend the range of usefulness of mortar and concrete systems, investigators have begun to experiment with and study the properties of cement and concrete systems that have been modified in some manner. The systems studied include: polymer impregnated concretes or PIC systems [2 - 8] containing impregnated m o n o m e r that is subsequently polymerized; polymer concretes or PC systems [3,9,10] in which a polymer is used as the binder instead of cement; and p o l y m e r - cement concretes or PCC systems, in which the binder includes both a polymer and hydrated cement [3,11 - 14]. The most successful of the systems studied to date in terms of the percentage improvement in strength and durability as compared with the control, or plain concrete, is the PIC system, and this method has been applied to porous ceramics as well as to cement mortars and concrete [7]. Some of the steps involved in preparing PIC systems include precasting of the cement concrete to the desired shape, drying the precast material or slabs, evacuating the water, impregnating m o n o m e r (as styrene, methyl methacrylate, b u t y l acrylate or other easily polymerizable material) under vacuum, and then polymerizing the m o n o m e r by use of radiation and by use of chemical catalysts or thermal treatment or both. In some instances thermosetting resins have also been used in place of thermoplastic t y p e monomers to improve the strength and modulus of PIC type composites [3,8]. It has been reported that PCC systems, based on direct addition of m o n o m e r to the cement or concrete mix followed by polymerization of the monomer, have n o t been successful to date [2]. However, some PCC systems, in
86 which latexes of poly(vinyl acetate), vinylidene chloride copolymers, or styrene - butadiene copolymers have been added to the mix, have resulted in product improvement and such systems have been used in a number of applications [3,14]. PC, or polymer concrete, systems with no cement phase tend to have high strength and good resistance to water absorption and freeze thaw conditions [3,9,10]. However, PC systems are expensive by comparison with ordinary cement mortars or concretes. They tend to be used for rather specialized applications as protective coatings of polymer mortar on concrete or reinforced concrete and as polymer concrete in highly corrosive atmospheres [9]. The present study is an attempt to improve the strength and durability properties of mortar and concrete through the direct addition of suitable components to the mortar or concrete mix in the field. The procedure consists essentially of adding to the mortar or concrete mix a blended epoxy resin system, i.e., a system consisting of appropriate proportions of a low molecular weight epoxy resin and a liquid hardener or curing agent. The materials used and the results obtained from five series of polymer - cement mortar systems are here presented and discussed; in a second paper [15] discussion is given of the results obtained when similar epoxy materials are used to modify concrete. In both cases, significant improvements in strength properties can be realized by relatively minor additions of an epoxy resin system to the mortar or concrete mix. Still greater improvements, though with greater expense, can be achieved by more fully replacing the hydrated cement phase by the cured epoxy system. The epoxy resin systems used in the present investigation have the following attractive characteristics: 1. Both the resin and hardener can be obtained in liquid form. 2. The resin and hardener can be premixed prior to incorporation in the cement - water sand mixture. Also, with proper choice of components, subsequent curing can be delayed until the mixture has been properly worked and formulated. 3. Resin - hardener systems can be obtained which can cure at room temperature. 4. The resulting properties of cured epoxy
systems can be varied by proper selection of resin, hardener and such other ingredients as accelerators and plasticizers. 5. The epoxy - cement mortar systems can be prepared on the site where they are to be used and no special facilities are required. In the present investigation, several different epoxy formulations have been investigated and two types of cement were used. The epoxy cement specimens were then cured for various time periods ranging from 3 to 28 days. Various physical measurements and tests were made on the resulting products.
2. MATERIALS AND TEST SPECIMENS Two types of Portland cement were used: normal Type I and high early strength Type III. The aggregate used was a natural graded sand, dried before use, having a sieve gradation as follows: U.S. Standard sieve No. #4 #8 #16 ~30 #50 #100 Percentage passing 100 99.5 95 72.7 28.5 8.67 For the polymer modifier of the cement, several epoxy resin systems were tried. In each of these the starting epoxy resin was Epon-828. This resin has an epoxide equivalent weight of 180 - 195 and a viscosity, at ambient temperature, of 110 to 150 poises. Two types of hardener were used. The first one was a fatty amidoamine, sold under the trade name of Lancast A. The second curing agent, known as CA-640, is a di-fl-hydroxyalkylamine. This was chosen for study as it is expected to function better in an aqueous medium. It has the following chemical structure {with n = 7 to 9):
CH3-(CH2)~-O--CH2--CH~CH2--NH E I OH CI2H4 NH I
CI2H4 NH I
Ctt3~--O- CH2--CH--CI-I2--NH Both curing agents will cure and set the epoxy resin to a three-dimensional network structure at ambient temperature.
87 TABLE 1 C o m p o s i t i o n o f e p o x y - c e m e n t m o r t a r t e s t series A, B, C and D Specimen type
Cement
Curing agent
Epoxy/ cement mortar (%)
Epoxy/ cement (%)
A1 A2 A3 A4
Type I
Amido
0.0 0.05 0.10 0.15
0.0 0.2 0.4 O.6
B1 B2 B3 B4
Type I
CA-640
0.02 0.05 0.10 0.15
0.1 0.2 0.4 0.6
C1 C2 C3 C4
T y p e III
Amido
0.0 0.05 0.10 0.15
0.0 0.2 O.4 0.6
D1 D2 D3
T y p e III
CA-640
0.05 0.10 0.15
0.2 0.4 0.6
C e m e n t : sand : w a t e r = 1 : 2.75 : 0.3 for all m o r t a r s
To study the influence of an epoxy modifier on the properties of cement mortar specimens, various percentages of the blended resin, consisting of the low molecular weight Epon 828 liquid resin mixed with the liquid curing agents, were ~idded to the mortar, prepared with a fixed sand : cement : water ratio. Four series of tests were run at a water : cement ratio deliberately chosen at a low value, 0.3, in order to prevent segregation of the fine aggregates at high resin contents. In these tests the cement : sand ratio was maintained at 1 : 2.75 by weight for all specimens. The percentage of the epoxy system, resin plus hardener, added to the cement mortar was varied from 0 to 15% by weight of the mortar. This
corresponded to an epoxy : cement ratio varying from 0.0 to 0.6. The specific compositions of the test specimens for each of these four series of tests are given in Table 1. It is noted that specimens designated A and B respectively were both prepared from normal Type 1 cement, but had different hardeners, while test specimens designated C and D were equivalent specimens prepared using Type III early cement. In an effort to determine the best quantitative c o m b i n a t i o n of the cement and epoxy binders, a fifth series of specimens were prepared and tested. In this series, designated E, the water : cement ratio was maintained at 0.4 for all specimens but a percentage of the ce-
TABLE 2 Compositionratiosofepoxy-cementmortartestseriesE Specimen type
Cement
Sand
Water
Epoxy
% o f c e m e n t and w a t e r r e p l a c e d by epoxy
E1 E2 E3 E4 E5 E6
1.0 0.8 0.6 0.4 0.2 0.0
2.75 2.75 2.75 2.75 2.75 2.75
0.40 0.32 0.24 0.16 0.08 0.00
0.0 0.28 0.56 0.84 1.12 1.40
0 20 40 60 80 100
Water : c e m e n t = 0.4 for all m o r t a r s
88 ment and water was progressively replaced by the polymer resin, here based on Epon 828 and the CA-640 curing agent. The a m o u n t of sand was maintained constant. The compositions of the various specimens included in this series, varying from 0 to 100% replacement of the cement - water system by the epoxy, are given in Table 2.
3. MOLDING AND CURING PROCEDURE After some trial and error experimentation, the following procedure was adopted. Dry sand and fresh cement were mechanically mixed by hand at room temperature for about two minutes in a mixing pan. Water was then added to the mixture in the desired proportions and mixing continued until a homogeneous product was obtained. The epoxy system, in which the liquid resin and liquid hardener were premixed in stoichiometric ratios shortly before use, is then added to the sand cement mortar and mixing continued until a uniform dispersion is attained. With the amido hardener, the proportions of resin to hardener were 100 : 45, for both Tests A and C; for the CA-640 hardener, the proportions were 100 : 75 for the three test series Tests B, D and E. Test specimens were prepared from the above mixture by placing the material in successive layers in cylindrical molds 2 in. in diameter and 4 in. in length. Because of the adhesive nature of cure epoxy systems, all the mold surfaces were first treated with a mold release agent, Rust Band 385, after heating the molds to 70°C. The molded specimens were stored in a moisture room for 24 hours before t h e y were stripped from the molds. The specimens were then marked for identification and stored under water for 3, 7, 14 or 28 days respectively until ready for testing.
4. TEST PROCEDURES Both compressive and tensile splitting strength tests were conducted. The specimens for compression tests were carefully capped in accord with ASTM Standard C617, with a mixture of granular material and sulfur, a vitrobond product. Strength measurements were carried out on
specimens corresponding to each of the test conditions described in Table 1. For these four series of tests, designated A, B, C and D repectively, strength values were recorded after curing times of 3 days, 7 days, 14 days and 28 days. Generally, three specimens were tested for each test condition and the average of these three values accepted as the nominal compressive or tensile strength for the specific variables involved. For the series E tests, with specimens having compositions as indicated in Table 2, tensile and compressive strength measurements were made only for specimens given a 7-day cure and a 28-day cure. Again, tests were made for each set of test variables on three identical specimens, and the average values noted. To determine how the addition of a polymer c o m p o n e n t modifies the stiffness of specimens, measurements were made of the elastic modulus for specimens of series E. For these tests, 2 in. X 4 in. cylindrical specimens were dried in air for five days. In order to m o u n t SR-4 strain gages on the surface for strain measurements, the dried specimens were sanded and cleaned with acetone. The gages were then m o u n t e d near the center of the specimen at two diametrically opposite positions and the specimens allowed to dry in air for another two days. The tests were carried out at a constant loading rate of about 25 p.s.i./sec and the strain was determined from a strain indicator. The maximum applied load did not exceed 40% of the ultimate strength. The modulus was determined from the slope of the stress strain curve and two specimens were tested for each set of variables involved in the group E test specimens. Determinations were also made, for series E specimens, of free water content so that data would be available as to the effect on porosity of partial replacement of a cement phase by a polymer phase. Tests were made on two specimens for each of the series E variables. These measurements were made as follows. First, specimens were selected which had been submerged in water for at least 28 days. These specimens were removed from the water environment, wiped dry and weighed. They were then placed in an oven for 2 to 4 days at a temperature of 230°F in order to remove the water present. The oven-dried samples were cooled to ambient temperature
89 and reweighed. The per cent of free water present in the specimens was calculated from the equation: W1--W 2 % free water c o n t e n t - - × 100 W2 where W1 is the weight of the saturated specimen and W2 is the weight of the oven-dried specimen. In addition to the above tests, optical and scanning electron microscope (SEM) pictures of the fracture surfaces of several p o l y m e r cement mortar specimens were taken. The SEM micrographs were obtained with an Itek SEM apparatus. The specimens to be examined were coated with Au - Pd in a vacuum evaporator to provide a conducting surface and to overcome charging. The secondary electron image of the fracture surface was then observed at various magnifications in order to obtain information about the nature of the bond between the resin phase and the sand particles.
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5. T E S T R E S U L T S AND D I S C U S S I O N
A. Effect
of ep o xy c o n t e n t on strength
The effects of adding an increasing a m o u n t of epoxy, at a fixed cement : sand : water ratio of 1 : 2.75 : 0.3, on the tensile and compressive strengths are shown graphically in Figs. 1 - 4 for series A, B, C and D test specimens. For series A specimens, wherein the liquid epoxy Epon 828 and an amidoamine hardener were added to Type 1 cement, it is observed from Fig. 1 that significant improvements in both tensile and compressive strength are realized for polymer additions of some 5 - 10%. For the specimens cured for 28 days, only 5% epoxy by weight of the mortar is needed to provide the maximum strength benefit. For this a m o u n t of added epoxy, the tensile strength is raised on the average from 480 p.s.i, to 730 p.s.i., an improvement of over 50%, and the compressive strength from 3,570 p.s.i, to 5.390 p.s.i., also an improvement of over 50%. For the specimens cured for 3, 7 or 14 days, it appears that m a x i m u m gain in strength can be realized at about 10% resin c o n t e n t but, for the 28-day cured specimens, there appears to be no advantage in raising the epoxy c o n t e n t by more than 5%. From these results it is clear that appreciable
strength improvements in cement mortar are possible by relatively minor additions of epoxy components to the mix. In the instances cited above, the added epoxy c o m p o n e n t a m o u n t e d to only 1/5 by weight of the cement. In series B specimens the CA-640 hardener was combined with the Epon 828 and then both added to the cement mortar. Specimens were prepared with 2%, 5%, 10% and 15% of epoxy by weight of the cement mortar. The observed increases in strength are shown in Fig. 2. The m a x i m u m strength gain appears to be achieved by addition of only about 5 wt. % of epoxy while further increases in epoxy c o n t e n t have no appreciable strengthening influence. For the specimens cured for 28 days, the tensile splitting strength has increased by 90% from the original 480 p.s.i. to 910 p.s.i., and the compressive strength has increased by 125% from the initial value of 3570 p.s.i, to 8130 p.s.i. It can be assumed that 5% of added epoxy improves workability and the bond of the hydrated cement to the filter particles; but if an excess a m o u n t of epoxy is added, it appears to hinder development of a continuous cement gel and hence lower strengths may result. From these test results, it is evident that the
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Fig. 2. Tensile and compressive strengths v s . resin content o f series B e p o x y - c e m e n t mortars. S p e c i m e n s prepared from: T y p e I c e m e n t , E p o n 8 2 8 and CA6 4 0 curing agent.
Fig. 3. Tensile and compressive strengths v s . resin content o f series C e p o x y - c e m e n t mortars. S p e c i m e n s prepared from: T y p e III cement, E p o n 8 2 8 and amidoamine curing agent.
specific hardener used in the epoxy formulation was a significant factor in the added strength. Although the first hardener tried worked satisfactorily and resulted in an increase of 50% in both tensile and compressive strengths at only 5% of added polymer, the second hardener, the hydroxyalkylamine, effectively raised the strength values approximately 50% more. The higher strengths for the latter type hardener probably result from a better bond between the matrix and the sand particles and also because of greater workability and hence greater homogeneity in the
can be drawn from use of emulsified polymers in other types of PCC systems [11]. For the specimens cured for 28 days and having 5% of epoxy resin by weight of mortar 14 28
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When high early Type III cement was substituted for the normal type I and the Lancast A hardener used there was slight beneficial strength increase for 5% added resin as seen in Fig. 3. However, at higher resin contents, both the tensile and compressive strengths decreased. With the hydroxyalkylamine hardener, there was a marked improvement in strength at 5% added resin level as seen in Fig. 4 while little benefit was gained by further addition of the resin component. Thus addition of approximately 5% of epoxy by weight of the mortar or 20% of epoxy by weight of the cement appears to be an optimum value for strength improvement. Similar conclusions
14
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91
the observed increases in strength were not as large for the Type III cement specimens as for the Type I cement specimens. For the Type III specimens, the tensile strength increased from 745 p.s.i, for the zero resin content specimens to 940 p.s.i, for the 5% resin content specimens. The corresponding increase in compressive strength was from 6,370 p.s.i, to 8,370 p.s.i.
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B. Effect of epoxy content on workability The results discussed above clearly demonstrate that polymer - cement mortars can be prepared so as to have tensile strengths above 900 p.s.i, and compressive strengths above 8000 p.s.i. Furthermore, these relatively high values of both tensile and compressive strength can be realized by addition of only 5% of epoxy by weight of the cement mortar or 20% of epoxy by weight of the cement itself. Addition of the polymer c o m p o n e n t permits better workability at lower water : cement ratios. The control samples with water : cement c o n t e n t of 0.3 and no resin have poor workability. This low workability can be improved by the addition of more water; but added water results in strength reduction. However, upon addition of blended epoxy resin of about 20% by weight of the cement, the total liquid cement ratio increased sufficiently to give good workability. Also during the curing period, the epoxy resin sets to a three-dimensional network. This network together with that of the hydrated cement phase acts as a binder for the sand particles and, as the test results have shown, results in a polymer - cement mortar that develops both tensile and compressive strength values 50 - 100% higher than the control samples depending upon the particular hardener used. C. Effect of replacement o f cement - water by epoxy In series E mortar mixes the water : cement ratio was maintained constant at 0.4 but the cement and water were replaced by epoxy at levels of 20%, 40%, 60%, 80% and then 100%. The measured compressive strengths and tensile strengths for series E specimens are plotted in Fig. 5 as a function of the resin c o n t e n t , i.e., as a funCtion of the per cent of the cement and water replaced by the epoxy. The results are given for specimens that were al-
/J 1
20
40
60
80
I00
20
/ w N j o
i
0
20
=
40
i
i
60
80
RESIN CONTENT
I00
(%)
Fig. 5. Tensile a n d c o m p r e s s i v e s t r e n g t h s v s . resin cont e n t of series E e p o x y - c e m e n t m o r t a r s . S p e c i m e n s p r e p a r e d f r o m : T y p e I c e m e n t , E p o n 828 a n d CA640 curing agent.
lowed to cure for 7 days and also for specimens cured for 28 days. For 20% resin content, Fig. 5 shows that increases in strength values have been obtained. However, when the resin c o n t e n t is further increased both tensile and compressive strength values fall below original control values. Somewhat similar results have been reported for other types of PCC systems [11] upon addition of resin beyond 15 or 20%. One reason for the decrease in strength values may be that the a m o u n t of resin present is large enough to seriously interfere with the hydration of the cement. Evidently at polymer : cement ratios of 0.2 or less, there is little interference, a continuous cement gel is maintained, and strengths remain high or increase. At resin contents above 40%, Fig. 5 shows that the tensile and compressive strengths start to rise sharply from their minimum values with increased resin replacement of the cement. At 80% replacement of the cement - water combination by the epoxy, the strengths are again well above the original control values. For example, for 80% replacement the average 28-day tensile strength is 925 p.s.i. (a 34%
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increase) and the average compressive strength is 5540 p.s.i. (a 14% increase). For 100% replacement, i.e., for the specimens in which the only binder present is the epoxy, the average tensile strength has risen to 1860 p.s.i. (a 169% increase) and the average compressive strength to 8630 p.s.i. (a 77% increase). It is interesting to note that for these specimens the 28 day strengths are 3 to 5 times the 7 day strengths. This indicates that polymerization and further curing of the resin are continuing even after a 7-day period. D. Effect o f resin content on Young's modulus Measurements were made of Young's modulus for specimens designated in Table 2 by the letters E l , E2, E4 and E6. Specimens E1 contained as a binder only a hydrated cement phase and specimens E6 only the cured epoxy phase. The E2 specimens had a 20% epoxy replacement and the E4 specimens a 60% epoxy replacement of the cement - water paste. A typical stress - strain curve for an E2 specimen is shown in Fig. 6. The loading curve is effectively linear while the unloading curve is concave upward and a small permanent set remains after unloading. The effect o f resin replacement on modulus is shown in Fig. 7. Although there is some scatter in the data, it appears that the modulus falls off essentially linearly from an initial value for the cement sand system of 4.34 X 106 p.s.i, to a final value of 1.47 X 106 p.s.i, for the epoxy - sand system. The modulus or stiffness thus decreases essentially in proportion to the reduction of
~L
C3
0
_on
2
>"
I
,
0
20
L
i
i
40
60
80
I00
PERCENT OF CEMENT GROUTREPLACED BY RESIN
Fig. 7. Elastic m o d u l u s v s . p e r c e n t a g e o f c e m e n t paste replaced by resin for series E e p o x y - c e m e n t m o r t a r specimens.
the hydrated cement phase and its replacement by the less stiff epoxy network. This result differs from that obtained in PIC systems. In these systems, where the polymer tends to fill in the voids and pores, modulus and density increase with a m o u n t of m o n o m e r added [2,4]. E. Effect of resin content on water retention Permeability was investigated for the series E epoxy cement mortars whose composition is given in Table 2. Weight measurements were made on water saturated specimens stored for 28 days in water and on the same specimens after both 2 days and 4 days of oven drying. The calculated free water c o n t e n t values are given as a function of the percentage of cement and water replaced by epoxy in Fig. 8. For the samples subject to 96 hours of drying, the water c o n t e n t was 7.47% for the control samples, 5.59% for 20% epoxy replacement and 0.13% for 100% epoxy replacement. The actual variation is only approximately linear as Fig. 8 shows. Nevertheless, it appears that IO
:~0
OVEN DRYING FOR
2 DAYS - 4 DAYS - -
8 w
6
4
2
0
0.2
0.4 STRAIN
0.6 x
0.8
I0 s
Fig. 6. Stress - strain curve in c o m p r e s s i o n o f an E2 e p o x y - c e m e n t m o r t a r s p e c i m e n , w i t h resin replacing 20% o f c e m e n t and water.
0
20
40
60
80
I00
PERCENT OF CEMENT GROUT REPLACED BY RESIN
Fig. 8. Percentage w a t e r a b s o r p t i o n v s . p e r c e n t a g e o f c e m e n t paste r e p l a c e d by resin for series E e p o x y c e m e n t m o r t a r specimens.
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Fig. 9. Scanning electron micrograph of the fracture surface of an epoxy - cement mortar specimen fractured in tension. w a t e r c o n t e n t decreases in p r o p o r t i o n t o the decrease in the c e m e n t b i n d e r phase and the p r o b a b l e decrease in n u m b e r o f pores. It is a p p a r e n t f r o m the test results t h a t if e p o x y resin alone is used as the b i n d e r (series E6 specimens) t h e free w a t e r c o n t e n t is alm o s t nil and h e n c e freeze - t h a w c o n d i t i o n s s h o u l d have little e f f e c t o n e p o x y mortars. Such a p o l y m e r - m o r t a r c o m b i n a t i o n , which consists o f 1.4 parts p o l y m e r and 2.75 parts sand, w o u l d have e x c e l l e n t strength p r o p e r t i e s (tensile strength above 1 5 0 0 p.s.i, a n d compressive strength a b o v e 8 0 0 0 p.s.i. ) a n d g o o d d u r a b i l i t y p r o p e r t i e s , as far as t e m p e r a t u r e and h u m i d i t y changes are c o n c e r n e d . In m a n y applications, h o w e v e r , the use o f such p o l y m e r m o r t a r s m a y n o t be justified on e c o n o m i c a l grounds as the cost p e r p o u n d is v e r y m u c h higher for an e p o x y b i n d e r t h a n f o r a c e m e n t binder.
In the e p o x y - m o r t a r specimens, if high strengths are desired in a s h o r t e r aging p e r i o d t h a n 28 days, curing c o u l d be a c c e l e r a t e d by e x p o s u r e to elevated t e m p e r a t u r e s or possibly by selection o f s o m e o t h e r t y p e o f h a r d e n i n g agent.
F. Fracture behavior and bond strength The f r a c t u r e surfaces o f several o f the e p o x y m o d i f i e d m o r t a r test samples, particularly t h o s e b r o k e n in the tensile splitting test, were e x a m i n e d in an a t t e m p t t o d e t e r m i n e , if possible, the influence o f the e p o x y m o d i f i e r on the n a t u r e o f the b o n d existing b e t w e e n sand particles and h y d r a t e d c e m e n t . Figure 9 is a low m a g n i f i c a t i o n scanning e l e c t r o n micrograph o f o n e such specimen. T h e d a r k gray areas are the sand particles and the light gray areas r e p r e s e n t the e p o x y - c e m e n t matrix. In general, the b o n d b e t w e e n the m a t r i x and
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Fig. 10. Higher m a g n i f i c a t i o n s c a n n i n g e l e c t r o n m i c r o g r a p h o f p o r t i o n o f f r a c t u r e surface o f a n e p o x y - c e m e n t m o r t a r s p e c i m e n f r a c t u r e d in t e n s i o n .
the filler appears to be quite satisfactory and the fracture surface has generally passed direct~ ly through the sand particles. Figure 10 shows a higher magnification SEM picture of a portion of the fracture surface for an epoxy modified sample. Some matrix cracks appear to have been arrested by nearby filler particles. For the large particle in the upper center of the picture, the bond at the matrix - sand interface on one side of the particle has been broken. The effect of the modifying agent in improving the bond between sand particles is apparent from the manner in which Group B specimens, in which a low 0.3 water : cement ratio was present, behaved when tested on the flow table. For the control specimens (A1), with no added epoxy, there was p o o r cohesion between the sand particles and the molded
paste tended to disintegrate when tested on the flow table. However, for Group B1 specimens, with resin : mortar ratio of only 0.02, cohesion was already much improved and for Group B2 specimens, with 5% added epoxy, both cohesion and workability were significantly improved. Thus better packing and fewer voids may contribute to the high observed strength values found for these epoxy modified B2 specimens. Visual inspection of the fracture surfaces of the Series E specimens, with increasing amounts of cement paste replaced b y epoxy, show that the densest structure, and the minim u m size voids, were obtained for the E2 specimens having the minimum e p o x y replacement (20%) as, with larger concentrations of epoxy, larger size voids appeared to be present. These probably arise from the highel viscosity of the ep-
95
o x y resin compared with water and the increasing difficulty of removing entrapped air from the system.
CONCLUSIONS
1. Addition of 5% (by weight of the mortar) of a premixed epoxy system, consisting of Epon 828 resin and a CA-640 curing agent, has increased the tensile strength of a standard c e m e n t - sand - water mix by some 90% and the compressive strength by over 100%. For these epoxy - cement mortar specimens the average 28-day tensile strength is above 900 p.s.i, and the average 28-day compressive strength above 8000 p.s.i. These values can be reached with either T y p e I or Type III cement. 2. If the premixed epoxy system is made from Epon 828 plus an amidoamine hardener (Lancast A), and again 5% of the blended resin added to the standard mix, the resultant tensile and compressive strength improvements are of the order of 50%. 3. There is no strength advantage gained by increasing the resin content b e y o n d 5% by weight of the mortar or the epoxy : cement ratio b e y o n d 0.2. At these figures, workability is good, strength properties are maximum, and the bond strength of the matrix to the filler particles is high. 4. The elastic modulus and the free water content both decrease essentially linearly if an epoxy system is used to partly replace the cement and water phase. For a PC system in which only the cured e p o x y is present as the binder, the water content is essentially nil, the compressive strength is high (above 8500 p.s.i.) and the tensile strength is exceptionally good (above 1800 p.s.i.).
ACKNOWLEDGEMENTS
Appreciation is expressed to R.J. Schutz, Vice-President of Sika Chemical Co., for his
aid in supplying materials and for his advice. The authors wish also to acknowledge the assistance of Chia-Chuan Li in carrying o u t the tests described herein, the assistance of C.F. Cook, Jr. in obtaining scanning electron micrographs, and the helpful suggestions of Prof. D. Kline and his co-workers.
REFERENCES 1 A.M. Neville, Properties of Concrete, Wiley, New York, reprinted, 1970. 2 L.E. Kukacka and G.W. DePuy (eds.), Fourth Topical Rept., Concrete - Polymer Materials, AEC and Dept. of Interior, Jan. 1972; see also prior reports #1, 1968, #2, 1969, ~"3, 1971. 3 V.I. Solomatev, Polymer - Cement Concretes and Polymer Concretes, Moscow, 1967 ; translation available from Isotopes Information Center, Oak Ridge National Laboratory, USAEC. 4 J.A. Manson, W.F. Chen, J.W. Vanderhoff, Y.N. Liu, E. Dahl-Jorgenson and H. Mehta, Polymer Pre-prints, 14 (1973) 1203. 5 J. Gebauer and R.W. Coughlin, Cement Concrete Res., 1 (1971) 187. 6 D.G. Manning and B.B. Hope, Cement Concrete Res., 1 (1971) 631. 7 J. Gebauer, D.P.H. Hasselman and R.E. Long, Am. Ceram. Soc. Bull., 51 (1972) 471. 8 D.A. Whiting, P.R. Blankenhorn and D.E. Kline, Polymer Preprints, 14 (1973) 1155; J. Testing Evaluation, ASTM, 2 (1974) 44. 9 N.A. Moshchanskii and V.V. Paturoev (eds.), Structural Chemically Stable Polymer Concretes, Moscow, 1970; translation available NTIS, U.S. Dept. Commerce. 10 R. Bares, Resin Concretes, Chem. of Ind., April 1970, p. 482. 11 J. Geist, S. Amagna and B. Mellor, Ind. Eng. Chem., 45 (1953) 759. 12 H.B. Wagner, Ind. Eng. Chem., Prod. Res. Develop., 4 (1965) 131; 5 (1966) 149 and 6 (1967) 223. 13 J. Hosek, J. Am. Concrete Inst., 63 (1966) 141. 14 J.E. Isenburg and J.W. Vanderhoff, Polymer Preprints, 14 (1973) 1197. 15 P.F. Sun., E.G. Nawy and J.A. Sauer, to be published.