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Microstructural characterization of the transition zone in cement systems by means of A.C. impedance spectroscopy
CEMENTand CONCRETERESEARCH. Vol.23, pp. 581-591,1993. Printedin the USA. 0008-8846/93. $6.00+00. 1993 PergamonPressLtd.
MICROSTRUCTURAL CHARACTERIZATION OF THE TRANSITION ZONE IN C E M E N T SYSTEMS BY MEANS OF A.C. I M P E D A N C E S P E C T R O S C O P Y
PING GU*, PING XIE*, J. J. BEAUDOIN** * Dept. of Civil Eng., University of Ottawa, Ottawa, Ont.,Canada, KIN 6N5 ** Materials Section, Institute for Research in Construction, National Research Council,Ottawa,Ont., Canada, K 1A 0R6 (Communicated by H. Uchikawa) (ReceivedApril27, 1992)
ABSTRACT The microstructure of the transition zone between aggregate and cement paste, and the effect of silica fume, slag, fly ash and latex (EVA) aggregate coatings were investigated by means of A.C. impedance measurements. It has been demonstrated experimentally that the A.C. impedance technique has the potential to describe microstructural changes of the cement-aggregate transition zone induced by surface modification of aggregate. Coatings of pozzolanic materials such as silica fume, slag, fly ash can increase the density of the transition zone. Latex coatings (eg. EVA) actually make the deposition of hydration products on the aggregate surfaces more difficult and give rise to a more porous cement-aggregate interracial zone. INTRODUCTION The transition zone serves as a bridge between matrix and aggregate or reinforcing material. It is potentially a weak link with respect to strength, and durability of a mortar or concrete. This is primarily due to the fact that the volume and size of voids in the transition zone are larger than in the bulk cement paste and the presence of a high concentration of large crystals of calcium hydroxide and ettringite. This has encouraged research on modifying the properties of the transition zone. For example, coating the aggregate surface or reinforcing fibre with chemicals and polymers prior to the mixing[I-4], and use of silica fume, fly ash, blast furnace slag etc.[57] have been investigated. In a recent paper, Monteiro et al[5] demonstrated the effect of silica fume addition on the steel-cement paste transition zone. Results indicated that at a replacement level as low as 5% of silica fume, the amount of preferentially oriented calcium hydroxide in the transition zone is considerably reduced and a denser hydration product is formed. Larbi et al[6] also obtained similar results in their studies of the cement paste-aggregate interface by means of SEM. Electrical conductivity measurements and SEM microscopy were used by Xie Ping and Beaudoin[7] to investigate the microstructure of the transition zone with and without silica fume coated aggregate. Evidence from these studies indicated that the silica fume coating reacts with calcium hydroxide to generate more dense products and remove the water film on the aggregate surface that exists in non-coated aggregate-cement systems. 581
Figure 1. SEM micrographs of various coatings: (A) silica fume xl000; (B) slag x540; (C) fly ash x200 and (D) latex (EVA) xl000.
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Recent studies[8-13] have shown that A.C. impedance spectroscopy (IS) is able to detect microstructural change in hydrating cement. The transition zone in a mortar provides a major contribution to its porosity, conductivity and impedance behaviour. In this work, the impedance behavior of various coatings including silica fume, fly ash, blast furnace slag, and latex (EVA) is compared with the reference sample containing uncoated aggregate. Differences in their A.C. impedance spectra are explained by microstructural differences of the transition zone in mortar. EXPERIMENTAL Materials: Type 10 portland cement, silica fume, slag and fly ash were used. Their chemical composition (wt.%) is given in Table I below. Table I: Oxide Analysis of Cement, Silica Fume, Sla and Fly Ash O.P.Ce Silica Slag Fly ment Fume Ash SiO 2 19.43 95.17 35.30 45.20 CaO 61.21 0.23 36.94 1.63 A120 3 4.18 0.21 10.62 20.70 Fe20 3 MgO Na20
3.20 4.09 0.45
0.13 0.15 0.10
K20 SO 3
0.89 3.93
0.27 0.12
0.58 13.32
24.83 0.97 0.59 2.40
1.41
Specimen Preparation: Quartz sand particles with average radii 0.506 mm were coated with four different materials: silica fume, slag, fly ash and EVA, respectively. The coating procedure[7] is briefly described. The aggregate is wet with a small amount of water to form a water film at the aggregate surface. This is followed by mixing with the selected coating material and drying in hot air. For the slag and fly ash coated samples, 3% of EVA by weight of aggregate was added to the coating system to increase adhesion. The thickness of the coating can be adjusted by controlling the amount of coating material that is used in treating aggregate surfaces. Figure I(A-D) shows the SEM examination of the coatings of silica fume, slag, fly ash and EVA, respectively. The coating layer of silica fume is denser than others because of its small particle size (ca. 0.1 I.tm). The slag and fly ash coatings are about 40-80 ~tm thick and the EVA layer is approximately 5 I,tm. It is noted that the EVA coating is not even and some bare spots are observed (Figld). However all the coating layers, are physically stable and remain intact subsequent to fracture of the mortar specimens. The fresh cement paste and coated aggregate were mixed in a conventional Hobart mixer at a water/cement ratio of 0.35 and an aggregate/cement ratio 0.50, then placed into a 1.2 cm x 1.5 cm stainless steel cylindrical cell described previously[13,14]. The sample was left in a 100% relative humidity environment until tested. Instruments: A 1260 Impedance Gain-Phase Analyzer from Schlumberger Technologies was used for impedance measurements. Data was collected using a frequency scan from 20 Mhz to 1 Hz with 10 readings per decade. SEM micrographs were obtained using a Cambridge Stereoscan $250. Pore size distributions were determined by mercury intrusion at pressures up to 408 Mpa, using an American Instrument Co. porosimeter.
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Electrical Equivalent Circuit Model for a Mortar: The impedance behaviour of a mortar can be modelled by an electrical equivalent circuit (shown in Figure 2), where Rt(agg) is the aggregate resistance. Since the aggregate resistance is usually much larger than that of the solidliquid in the paste, its contribution to the high frequency end resistance can be neglected. Other parameters: Rt(s+l) , Ct(int) and Rt(int) are solid-liquid sum resistance, interfacial capacitance and interfacial resistance respectively as described elsewhere[ 14].
Rt(agg)
Rt(s+l)
C t(int)
Rt(int)
Figure. 2 A simplified electrical equivalent circuit for hydrating portland cement mortar. RESULTS AND DISCUSSION Saturated cement paste or mortar (in equilibrium with 100% relative humidity) contains hydrated ions such as Ca++, Na+, OH- and SO4 = in the pore solution that are adsorbed on surfaces of micropores. In an alternating electric field, these ions and water molecules oscillate in a relatively slow motion compared with their oscillation under the ordinary conditions. Their relaxation times are reduced to MHz range because of geometric limitations, i.e. microstructure profiles, and the chemical and physical interactions among the adsorbed ions. Therefore, impedance spectroscopy and especially the observation of the high frequency arc, can be used to obtain information about the microstructure of a cement paste or a mortar. For instance, a large arc diameter and small capacitance value are indications of a dense matrix and high discontinuity in the pore system. In a mortar, the porosity of the paste-aggregate interface is considered to be more significant than in the cement paste matrix. If the matrix paste porosity remains unchanged, any modification of the transition zone could give rise to different impedance behaviour. In the following discussion, the impedance spectra of mortars containing four modified transition zones produced by coating aggregate with silica fume, slag, fly ash and EVA are analyzed. Results are correlated with porosity and SEM microstructural studies.
(i) Impedance behaviour. A reference mortar with uncoated aggregate was prepared along with the four mortars containing coated aggregate. The impedance data was acquired at three different hydration times: 2, 15 and 27 days and plotted in Figure 3(A-C) respectively. As expected, modifying the paste-aggregate interface leads to a difference in their impedance spectra even at two days hydration time. The difference is also reflected in the growth of the arc diameters at longer hydration times. The analysis of the data shown in Figure 3 involved an equivalent circuit fitting process (see figure 2) and a set of RC parameters for the spectra at 27 days for a typical sample is given in Table 2.
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Table 2: Electrical Parameters of Impedance Spectra for Hydrated Mortars at 27 Days Rt(s+l) Rt(int) Ct(int) Silica Fume Fly Ash Slag Latex(EVA) Reference
Ohms 150 145 140 147 165
Ohms 321 261 389 186 213
nF 1.39 1.71 1.15 2.40 2.09
Analysis of the A.C. impedance spectra in Fig.3 for various coatings indicates that the silica fume and slag coated aggregate possess the largest bulk arc. The latex(EVA) coated aggregate mortar has the smallest one. The size of the arc diameters is in the following order: silica fume, slag > fly ash > reference > latex(EVA) coating at all three different hydration times. Silica fume and slag also have smaller capacitance values than others especially the one coated with latex(Table 2). The reason for the difference of their impedance behaviour is attributed to microstructural variation of the cement-aggregate interfaces since the mixing and curing conditions of these specimens are the same. Large values of the bulk arc diameter and small interfacial capacitance are indications of a dense transition zone structure. The density of the cement-aggregate interfaces is in the order, silica fume or slag > fly ash > reference > latex. This observation is not surprising as silica fume, slag and fly ash contain a high percentage of SiO2 which reacts with CH to form C-S-H. The mechanism of transition zone development in a mortar or concrete could vary significantly with the presence of pozzolanic materials, especially when these materials form coatings on the aggregate surfaces. In a normal freshly compacted mortar or concrete, the formation of the transition zone described by Maso[15] is well accepted. The development starts with formation of a water film around the aggregate surfaces, and subsequent deposition of ettringite and calcium hydroxide on the surface of aggregate. The size of the deposited crystals is dependent on the water/cement ratio. As hydration progresses, a small amount of C-S-H and later generated small crystals of ettringite and calcium hydroxide fill the large empty space that exists between the framework created by the earlier deposited large ettringite and calcium hydroxide crystals. Finally, a zone about 30-50 Ixm wide forms. It basically consists of a layer of ettringite and CH crystals in contact with aggregate surfaces and a layer of poorly crystalline C-S-H gel. However, coating aggregate with pozzolanic materials such as silica fume etc. could reduce or prevent the formation of large crystals on the aggregate surfaces and encourage formation of a layer of C-S-H instead resulting in a denser transition zone. The density of the interfacial zone depends on the activity of the pozzolanic materials. From the impedance results, it appears that silica fume and slag are better coating materials than fly ash, which may be explained by their reactivity. The reference is expected to be worse than pozzolanic coated materials. Experimental data shows that the latex coating is ineffective because the organic polymer coating makes poorest contact between cement and aggregate.
(ii) SEM Examination In order to provide complementary information to support the view that differences in A.C. impedance behaviour of coated aggregate specimens are due to microstructural change of the cement-aggregate interfacial zone, an SEM investigation was conducted. Micrographs are shown in Figure 4A-D for the silica fume, slag, fly ash and latex coated aggregate specimens at 27 days hydration. For the silica fume, slag and fly ash coated specimens, the SEM photos (Figure 4A-C)
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160 -e-,- silica fume
A
%-
fly ash
120 E ¢-
slag --
Latex(EVA)
-~-
reference
80
E
40
i
0 120
160
200
240
280
Real (ohms) 400 silica fume - @ - fly ash
300-
-X-
slag
--
latex(EVA) reference
2001000. 120
220
320
420
520
Real (ohms) 500
C
--e-- silica fume -~-
%` 400-
E i-
--
300" t'-
fly ash
'-X-- slag latex(EVA)
- ~ - reference
200
E 100 i 0 120 220 320 420 520 620
Real (ohms) Figure 3. Impedance spectra of hydrating mortars containing aggregates treated with various coatings at different hydration times. (A) 2 days; (B) 15 days and (C) 27 days.
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show a dense transition area with no significant CH crystals detected. The latex coated aggregate specimen is quite different (Figure 4D). It shows a porous layer with width of 10 lxm contains large crystals CH. Figure 5B is a high magnification of the latex coated cement-aggregate interface. It is noted that the ettringite and CH crystals tend to deposit on the bare spots of aggregate where no latex covering is present, Latex coatings actually increase the porosity of the cement-aggregate interface. This observation explains its impedance behavior. The detachment of the latex layer from the aggregate surfaces is probably due to shrinkage of latex during drying. The reference specimen shown in Figure 5A depicts a typical behavior reported in literature; the transition zone contains ettringite and CH crystals. The transition zone of mortar containing silica fume, slag, fly ash coated aggregate is denser than the reference, and the latex coated aggregate is the least dense.
(iii) Porosity Studies: Specimens hydrated 27 days were also selected for the porosity studies using the mercury intrusion technique. The total pore volume (%) and increment of pore volume (%) vs. pore size distribution curves are plotted in Figure 6 and 7, respectively. The silica fume coated specimen has the lowest pore volume and the latex coated one has largest. The sequence of total porosity of all these studied specimens is silica fume < fly ash < reference < slag < latex coating. This sequence matches the impedance observation except for the slag coated one. It is interesting to examine the increment of pore volume in the pore size range larger than 20 n m ( the pore in transition zone mainly consists of the pore 20nm or more[17]). In Figure 7, it is clear that pore size distributions for those samples with the densification of transition zone by the surface modification of aggregate with pozzolanic materials are mainly in the range of 50-80 nm whereas those for the latex coated one are in the range of 50-200 nm. According to the impedance behavior, slag coated sample should exhibit a lower porosity than the reference. This exception might possibly be explained by consideration of the discontinuity of micropores. To confirm this, helium pycnometry studies were also conducted to determine solid density and porosity values of all five specimens. The comparison of porosity measured by mercury intrusion and helium pycnometry is given in Table.3. Comparing results obtained from the two methods, values from helium pycnometry are generally higher than those from mercury intrusion. It is noted that helium pycnometry involves determination of the absolute volume of solid at a pressure of 2 atm. Helium gas can easily penetrate into small continuous micropores. Moreover, due to the small molecular size, helium molecules can enter pores having least diameter less than 2.5 rim. In the mercury intrusion method, mercury is forced into the pores of the materials at difference pressures up to 414 MPa. It has been reported[16] that mercury can break through the micropore structure and enter discontinuous pores. It does not intrude pores less than 2.5 nm. Data in Table 3 suggests that a significant volume of small continuous pores is present in these specimens with possible Table 3 Porosity Values of Mortars Determined by Mercury Intrusion and Helium Pycnometery Mercury Intrusion Helium Pycnometry Density (g/cm 3) Silica Fume 15.67 19.3 2.60 Slag 17.40 17.7 2.58 Fly Ash 15.87 18.6 2.59 19.45 Latex(EVA) 23.2 2.73 Reference 16.81 20.1 2.62
=igure 4. SEM micrographs of different coated aggregate-cement paste interfaces: (A) silica 'ume xl000; (B) slag xl000; (C) fly ash xl000 and (D) latex (EVA) xl000.
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l
io
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Figure 5 (A). Aggregate-cement paste interface (x4000) of the reference sample. (B). High magnification of latex coated aggregate-cement paste interface (x4000). exception of the slag coated one. The latter has very close porosity values with both methods. The lower density (2.58) of the slag coated sample may be a partial consequence of discontinuous pores. Density is however influenced primarily be the degree of hydration and nature of the hydration products. It does not appear that pore discontinuity can explain the variance of data with respect to the slag coated specimen. The reason is unclear. CONCLUSIONS 1. The A.C. impedance technique has the potential to detect microstructural changes in the cement paste-aggregate transition zone induced by surface modification of aggregate. A large
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Pore Diameter, (microns) Figure 6 Total porosity versus pore size distribution curves for mortar containing aggregate coated with various materials and reference specimens at 27 days.
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i :
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Pore Diameter, (microns) Figure 7 Increment of pore volume (%) versus pore size distribution curves for mortar containing aggregate coatedwith various materials and reference specimens at 27 days. high frequency arc, at a given hydration time usually indicates a dense transition area and a highly discontinuous pore structure in portland cement mortar. 2. Aggregate coatings consisting of pozzolanic materials such as silica fume, slag, fly ash can increase the density of the transition zone. Latex (eg. EVA) coating makes the deposition of hydration products more difficult on the aggregate surfaces and give rise to a more porous cement-aggregate interfacial zone. 3. The density of the transition zone determined by A.C. impedance spectroscopy is in the following order: silica fume, slag > fly ash > reference > latex coated aggregate in mortar specimens. Similar ranking was also obtained in SEM and porosity investigations.
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4. Porosity determined by helium pycnometry is generally higher than by mercury intrusion. There appears to be a certain amount of pore space undetected by mercury in the samples. ACKNOWLEDGMENTS The authors wish to acknowledge Mssrs. B. Myers, Ed Quinn and Gordon Chan for their help with the experimental apparatus. This work is financially supported by NSERC and the Network of Centers of Excellence on High Performance Concrete REFERENCES
[1]. X.Q. Wu, S.F. Han, Q.H. Bian, M.S. Tang, Proc. 8th Inter. Congress on the Chem. of Cem., Brazil, Vol. III, 454 (1986). [2]. X.Q. Wu, D.X. Li, Q.H. Bian, L.Q. Guo, M.S. Tang, Cem. Concr. Res. 17,709 (1987). [3]. X.Q. Wu, D.X. Li, X. Wu, M.S. Tang, Symposia Proc. Mater. Res. Soc., "Bonding in Cementitious Composites", Eds. S.Mindess & S.P. Shah, Pittsburgh, Vol. 114, 35 (1988). [4]. S. Popovics, Materials and Structures, 20, 32 (1987) [5] P.J.M. Monteiro, O.E. Gjorv and P.K. Mehta, Cem. Concr. Res. 19, 114 (1989). [6] J.A. Larbi and J.M.J.M. Bijen, Cem. Concr. Res. 20, 461 (1990). [7] Xie Ping and J.J Beaudoin, "Modification of Transition Zone Microstructure: Silica Fume Coating of Aggregate Surfaces" Cem. Concr. Res. In Press, (1992) [8] McCarter, W. J., Garvin, S., and Bouzid, N., J. Mater. Sci. Lett., 7(10), 1056 (1988). [9] McCarter, W. J., and Brousseau, R., Cem. Conc. Res., 20 891 (1990). [10] Brantervik, K., and Niklasson, G. A., Cem. Conc. Res., 21 469 (1991). [11] Scuderi, C. A., Mason, T. O., and Jennings, H. M., J. Mater. Sci. Lett., 26 349 (1991) [12] Bonanos, N., Steele, B. C. H., Butler, E. P., Johnson, W. B., Worrell, W. L., MacDonald, D. D., and McKubre, M. C. H., " Application of Impedance Spectroscopy", Chpt. 4, Ed., McDonald, J. R., Wiley & Sons, NY, (1987). [13] Christensen, B. J., Mason, T. O., and Jennings, H. M., "Influence of Silica Fume on the Early Hydration Characteristics of Portland Cements using Impedance Spectroscopy (IS)" J. Mater. Sci. Lett., In Press, (1992). [14] Ping Gu, Xie Ping, J.J Beaudoin and R. Brousseau, " A.C. Impedance Spectroscopy (I): A New Equivalent Circuit Model for Hydrated Portland Cement Paste" Cem. Concr. Res. In Press, (1992). [15] F Maso, Proceedings of the Seventh International Congress on the Chemistry of Cements, Vol. 1, Editions Septima, Paris, (1980). [16] Cheng-yi Huang and R.F. Feldman, J. Amer. Ceram. Soc. 82, 740 (1985). [17] H. Uchikawa, Journal of Research of the Onoda Cement Company, 40 (119), (1988). or in "Advances in Cement Manufacture and use", edited by Ellis Gartner Published by Engineering Foundation Conference, _1, p271-294(1989).
MICROSTRUCTURAL CHARACTERIZATION OF THE TRANSITION ZONE IN C E M E N T SYSTEMS BY MEANS OF A.C. I M P E D A N C E S P E C T R O S C O P Y
PING GU*, PING XIE*, J. J. BEAUDOIN** * Dept. of Civil Eng., University of Ottawa, Ottawa, Ont.,Canada, KIN 6N5 ** Materials Section, Institute for Research in Construction, National Research Council,Ottawa,Ont., Canada, K 1A 0R6 (Communicated by H. Uchikawa) (ReceivedApril27, 1992)
ABSTRACT The microstructure of the transition zone between aggregate and cement paste, and the effect of silica fume, slag, fly ash and latex (EVA) aggregate coatings were investigated by means of A.C. impedance measurements. It has been demonstrated experimentally that the A.C. impedance technique has the potential to describe microstructural changes of the cement-aggregate transition zone induced by surface modification of aggregate. Coatings of pozzolanic materials such as silica fume, slag, fly ash can increase the density of the transition zone. Latex coatings (eg. EVA) actually make the deposition of hydration products on the aggregate surfaces more difficult and give rise to a more porous cement-aggregate interracial zone. INTRODUCTION The transition zone serves as a bridge between matrix and aggregate or reinforcing material. It is potentially a weak link with respect to strength, and durability of a mortar or concrete. This is primarily due to the fact that the volume and size of voids in the transition zone are larger than in the bulk cement paste and the presence of a high concentration of large crystals of calcium hydroxide and ettringite. This has encouraged research on modifying the properties of the transition zone. For example, coating the aggregate surface or reinforcing fibre with chemicals and polymers prior to the mixing[I-4], and use of silica fume, fly ash, blast furnace slag etc.[57] have been investigated. In a recent paper, Monteiro et al[5] demonstrated the effect of silica fume addition on the steel-cement paste transition zone. Results indicated that at a replacement level as low as 5% of silica fume, the amount of preferentially oriented calcium hydroxide in the transition zone is considerably reduced and a denser hydration product is formed. Larbi et al[6] also obtained similar results in their studies of the cement paste-aggregate interface by means of SEM. Electrical conductivity measurements and SEM microscopy were used by Xie Ping and Beaudoin[7] to investigate the microstructure of the transition zone with and without silica fume coated aggregate. Evidence from these studies indicated that the silica fume coating reacts with calcium hydroxide to generate more dense products and remove the water film on the aggregate surface that exists in non-coated aggregate-cement systems. 581
Figure 1. SEM micrographs of various coatings: (A) silica fume xl000; (B) slag x540; (C) fly ash x200 and (D) latex (EVA) xl000.
t~
o
.m
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ACIMPEDANCE,MICROSTRUC~RE,TRANSITIONZONE
583
Recent studies[8-13] have shown that A.C. impedance spectroscopy (IS) is able to detect microstructural change in hydrating cement. The transition zone in a mortar provides a major contribution to its porosity, conductivity and impedance behaviour. In this work, the impedance behavior of various coatings including silica fume, fly ash, blast furnace slag, and latex (EVA) is compared with the reference sample containing uncoated aggregate. Differences in their A.C. impedance spectra are explained by microstructural differences of the transition zone in mortar. EXPERIMENTAL Materials: Type 10 portland cement, silica fume, slag and fly ash were used. Their chemical composition (wt.%) is given in Table I below. Table I: Oxide Analysis of Cement, Silica Fume, Sla and Fly Ash O.P.Ce Silica Slag Fly ment Fume Ash SiO 2 19.43 95.17 35.30 45.20 CaO 61.21 0.23 36.94 1.63 A120 3 4.18 0.21 10.62 20.70 Fe20 3 MgO Na20
3.20 4.09 0.45
0.13 0.15 0.10
K20 SO 3
0.89 3.93
0.27 0.12
0.58 13.32
24.83 0.97 0.59 2.40
1.41
Specimen Preparation: Quartz sand particles with average radii 0.506 mm were coated with four different materials: silica fume, slag, fly ash and EVA, respectively. The coating procedure[7] is briefly described. The aggregate is wet with a small amount of water to form a water film at the aggregate surface. This is followed by mixing with the selected coating material and drying in hot air. For the slag and fly ash coated samples, 3% of EVA by weight of aggregate was added to the coating system to increase adhesion. The thickness of the coating can be adjusted by controlling the amount of coating material that is used in treating aggregate surfaces. Figure I(A-D) shows the SEM examination of the coatings of silica fume, slag, fly ash and EVA, respectively. The coating layer of silica fume is denser than others because of its small particle size (ca. 0.1 I.tm). The slag and fly ash coatings are about 40-80 ~tm thick and the EVA layer is approximately 5 I,tm. It is noted that the EVA coating is not even and some bare spots are observed (Figld). However all the coating layers, are physically stable and remain intact subsequent to fracture of the mortar specimens. The fresh cement paste and coated aggregate were mixed in a conventional Hobart mixer at a water/cement ratio of 0.35 and an aggregate/cement ratio 0.50, then placed into a 1.2 cm x 1.5 cm stainless steel cylindrical cell described previously[13,14]. The sample was left in a 100% relative humidity environment until tested. Instruments: A 1260 Impedance Gain-Phase Analyzer from Schlumberger Technologies was used for impedance measurements. Data was collected using a frequency scan from 20 Mhz to 1 Hz with 10 readings per decade. SEM micrographs were obtained using a Cambridge Stereoscan $250. Pore size distributions were determined by mercury intrusion at pressures up to 408 Mpa, using an American Instrument Co. porosimeter.
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Electrical Equivalent Circuit Model for a Mortar: The impedance behaviour of a mortar can be modelled by an electrical equivalent circuit (shown in Figure 2), where Rt(agg) is the aggregate resistance. Since the aggregate resistance is usually much larger than that of the solidliquid in the paste, its contribution to the high frequency end resistance can be neglected. Other parameters: Rt(s+l) , Ct(int) and Rt(int) are solid-liquid sum resistance, interfacial capacitance and interfacial resistance respectively as described elsewhere[ 14].
Rt(agg)
Rt(s+l)
C t(int)
Rt(int)
Figure. 2 A simplified electrical equivalent circuit for hydrating portland cement mortar. RESULTS AND DISCUSSION Saturated cement paste or mortar (in equilibrium with 100% relative humidity) contains hydrated ions such as Ca++, Na+, OH- and SO4 = in the pore solution that are adsorbed on surfaces of micropores. In an alternating electric field, these ions and water molecules oscillate in a relatively slow motion compared with their oscillation under the ordinary conditions. Their relaxation times are reduced to MHz range because of geometric limitations, i.e. microstructure profiles, and the chemical and physical interactions among the adsorbed ions. Therefore, impedance spectroscopy and especially the observation of the high frequency arc, can be used to obtain information about the microstructure of a cement paste or a mortar. For instance, a large arc diameter and small capacitance value are indications of a dense matrix and high discontinuity in the pore system. In a mortar, the porosity of the paste-aggregate interface is considered to be more significant than in the cement paste matrix. If the matrix paste porosity remains unchanged, any modification of the transition zone could give rise to different impedance behaviour. In the following discussion, the impedance spectra of mortars containing four modified transition zones produced by coating aggregate with silica fume, slag, fly ash and EVA are analyzed. Results are correlated with porosity and SEM microstructural studies.
(i) Impedance behaviour. A reference mortar with uncoated aggregate was prepared along with the four mortars containing coated aggregate. The impedance data was acquired at three different hydration times: 2, 15 and 27 days and plotted in Figure 3(A-C) respectively. As expected, modifying the paste-aggregate interface leads to a difference in their impedance spectra even at two days hydration time. The difference is also reflected in the growth of the arc diameters at longer hydration times. The analysis of the data shown in Figure 3 involved an equivalent circuit fitting process (see figure 2) and a set of RC parameters for the spectra at 27 days for a typical sample is given in Table 2.
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Table 2: Electrical Parameters of Impedance Spectra for Hydrated Mortars at 27 Days Rt(s+l) Rt(int) Ct(int) Silica Fume Fly Ash Slag Latex(EVA) Reference
Ohms 150 145 140 147 165
Ohms 321 261 389 186 213
nF 1.39 1.71 1.15 2.40 2.09
Analysis of the A.C. impedance spectra in Fig.3 for various coatings indicates that the silica fume and slag coated aggregate possess the largest bulk arc. The latex(EVA) coated aggregate mortar has the smallest one. The size of the arc diameters is in the following order: silica fume, slag > fly ash > reference > latex(EVA) coating at all three different hydration times. Silica fume and slag also have smaller capacitance values than others especially the one coated with latex(Table 2). The reason for the difference of their impedance behaviour is attributed to microstructural variation of the cement-aggregate interfaces since the mixing and curing conditions of these specimens are the same. Large values of the bulk arc diameter and small interfacial capacitance are indications of a dense transition zone structure. The density of the cement-aggregate interfaces is in the order, silica fume or slag > fly ash > reference > latex. This observation is not surprising as silica fume, slag and fly ash contain a high percentage of SiO2 which reacts with CH to form C-S-H. The mechanism of transition zone development in a mortar or concrete could vary significantly with the presence of pozzolanic materials, especially when these materials form coatings on the aggregate surfaces. In a normal freshly compacted mortar or concrete, the formation of the transition zone described by Maso[15] is well accepted. The development starts with formation of a water film around the aggregate surfaces, and subsequent deposition of ettringite and calcium hydroxide on the surface of aggregate. The size of the deposited crystals is dependent on the water/cement ratio. As hydration progresses, a small amount of C-S-H and later generated small crystals of ettringite and calcium hydroxide fill the large empty space that exists between the framework created by the earlier deposited large ettringite and calcium hydroxide crystals. Finally, a zone about 30-50 Ixm wide forms. It basically consists of a layer of ettringite and CH crystals in contact with aggregate surfaces and a layer of poorly crystalline C-S-H gel. However, coating aggregate with pozzolanic materials such as silica fume etc. could reduce or prevent the formation of large crystals on the aggregate surfaces and encourage formation of a layer of C-S-H instead resulting in a denser transition zone. The density of the interfacial zone depends on the activity of the pozzolanic materials. From the impedance results, it appears that silica fume and slag are better coating materials than fly ash, which may be explained by their reactivity. The reference is expected to be worse than pozzolanic coated materials. Experimental data shows that the latex coating is ineffective because the organic polymer coating makes poorest contact between cement and aggregate.
(ii) SEM Examination In order to provide complementary information to support the view that differences in A.C. impedance behaviour of coated aggregate specimens are due to microstructural change of the cement-aggregate interfacial zone, an SEM investigation was conducted. Micrographs are shown in Figure 4A-D for the silica fume, slag, fly ash and latex coated aggregate specimens at 27 days hydration. For the silica fume, slag and fly ash coated specimens, the SEM photos (Figure 4A-C)
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Real (ohms) Figure 3. Impedance spectra of hydrating mortars containing aggregates treated with various coatings at different hydration times. (A) 2 days; (B) 15 days and (C) 27 days.
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show a dense transition area with no significant CH crystals detected. The latex coated aggregate specimen is quite different (Figure 4D). It shows a porous layer with width of 10 lxm contains large crystals CH. Figure 5B is a high magnification of the latex coated cement-aggregate interface. It is noted that the ettringite and CH crystals tend to deposit on the bare spots of aggregate where no latex covering is present, Latex coatings actually increase the porosity of the cement-aggregate interface. This observation explains its impedance behavior. The detachment of the latex layer from the aggregate surfaces is probably due to shrinkage of latex during drying. The reference specimen shown in Figure 5A depicts a typical behavior reported in literature; the transition zone contains ettringite and CH crystals. The transition zone of mortar containing silica fume, slag, fly ash coated aggregate is denser than the reference, and the latex coated aggregate is the least dense.
(iii) Porosity Studies: Specimens hydrated 27 days were also selected for the porosity studies using the mercury intrusion technique. The total pore volume (%) and increment of pore volume (%) vs. pore size distribution curves are plotted in Figure 6 and 7, respectively. The silica fume coated specimen has the lowest pore volume and the latex coated one has largest. The sequence of total porosity of all these studied specimens is silica fume < fly ash < reference < slag < latex coating. This sequence matches the impedance observation except for the slag coated one. It is interesting to examine the increment of pore volume in the pore size range larger than 20 n m ( the pore in transition zone mainly consists of the pore 20nm or more[17]). In Figure 7, it is clear that pore size distributions for those samples with the densification of transition zone by the surface modification of aggregate with pozzolanic materials are mainly in the range of 50-80 nm whereas those for the latex coated one are in the range of 50-200 nm. According to the impedance behavior, slag coated sample should exhibit a lower porosity than the reference. This exception might possibly be explained by consideration of the discontinuity of micropores. To confirm this, helium pycnometry studies were also conducted to determine solid density and porosity values of all five specimens. The comparison of porosity measured by mercury intrusion and helium pycnometry is given in Table.3. Comparing results obtained from the two methods, values from helium pycnometry are generally higher than those from mercury intrusion. It is noted that helium pycnometry involves determination of the absolute volume of solid at a pressure of 2 atm. Helium gas can easily penetrate into small continuous micropores. Moreover, due to the small molecular size, helium molecules can enter pores having least diameter less than 2.5 rim. In the mercury intrusion method, mercury is forced into the pores of the materials at difference pressures up to 414 MPa. It has been reported[16] that mercury can break through the micropore structure and enter discontinuous pores. It does not intrude pores less than 2.5 nm. Data in Table 3 suggests that a significant volume of small continuous pores is present in these specimens with possible Table 3 Porosity Values of Mortars Determined by Mercury Intrusion and Helium Pycnometery Mercury Intrusion Helium Pycnometry Density (g/cm 3) Silica Fume 15.67 19.3 2.60 Slag 17.40 17.7 2.58 Fly Ash 15.87 18.6 2.59 19.45 Latex(EVA) 23.2 2.73 Reference 16.81 20.1 2.62
=igure 4. SEM micrographs of different coated aggregate-cement paste interfaces: (A) silica 'ume xl000; (B) slag xl000; (C) fly ash xl000 and (D) latex (EVA) xl000.
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Figure 5 (A). Aggregate-cement paste interface (x4000) of the reference sample. (B). High magnification of latex coated aggregate-cement paste interface (x4000). exception of the slag coated one. The latter has very close porosity values with both methods. The lower density (2.58) of the slag coated sample may be a partial consequence of discontinuous pores. Density is however influenced primarily be the degree of hydration and nature of the hydration products. It does not appear that pore discontinuity can explain the variance of data with respect to the slag coated specimen. The reason is unclear. CONCLUSIONS 1. The A.C. impedance technique has the potential to detect microstructural changes in the cement paste-aggregate transition zone induced by surface modification of aggregate. A large
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Pore Diameter, (microns) Figure 7 Increment of pore volume (%) versus pore size distribution curves for mortar containing aggregate coatedwith various materials and reference specimens at 27 days. high frequency arc, at a given hydration time usually indicates a dense transition area and a highly discontinuous pore structure in portland cement mortar. 2. Aggregate coatings consisting of pozzolanic materials such as silica fume, slag, fly ash can increase the density of the transition zone. Latex (eg. EVA) coating makes the deposition of hydration products more difficult on the aggregate surfaces and give rise to a more porous cement-aggregate interfacial zone. 3. The density of the transition zone determined by A.C. impedance spectroscopy is in the following order: silica fume, slag > fly ash > reference > latex coated aggregate in mortar specimens. Similar ranking was also obtained in SEM and porosity investigations.
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4. Porosity determined by helium pycnometry is generally higher than by mercury intrusion. There appears to be a certain amount of pore space undetected by mercury in the samples. ACKNOWLEDGMENTS The authors wish to acknowledge Mssrs. B. Myers, Ed Quinn and Gordon Chan for their help with the experimental apparatus. This work is financially supported by NSERC and the Network of Centers of Excellence on High Performance Concrete REFERENCES
[1]. X.Q. Wu, S.F. Han, Q.H. Bian, M.S. Tang, Proc. 8th Inter. Congress on the Chem. of Cem., Brazil, Vol. III, 454 (1986). [2]. X.Q. Wu, D.X. Li, Q.H. Bian, L.Q. Guo, M.S. Tang, Cem. Concr. Res. 17,709 (1987). [3]. X.Q. Wu, D.X. Li, X. Wu, M.S. Tang, Symposia Proc. Mater. Res. Soc., "Bonding in Cementitious Composites", Eds. S.Mindess & S.P. Shah, Pittsburgh, Vol. 114, 35 (1988). [4]. S. Popovics, Materials and Structures, 20, 32 (1987) [5] P.J.M. Monteiro, O.E. Gjorv and P.K. Mehta, Cem. Concr. Res. 19, 114 (1989). [6] J.A. Larbi and J.M.J.M. Bijen, Cem. Concr. Res. 20, 461 (1990). [7] Xie Ping and J.J Beaudoin, "Modification of Transition Zone Microstructure: Silica Fume Coating of Aggregate Surfaces" Cem. Concr. Res. In Press, (1992) [8] McCarter, W. J., Garvin, S., and Bouzid, N., J. Mater. Sci. Lett., 7(10), 1056 (1988). [9] McCarter, W. J., and Brousseau, R., Cem. Conc. Res., 20 891 (1990). [10] Brantervik, K., and Niklasson, G. A., Cem. Conc. Res., 21 469 (1991). [11] Scuderi, C. A., Mason, T. O., and Jennings, H. M., J. Mater. Sci. Lett., 26 349 (1991) [12] Bonanos, N., Steele, B. C. H., Butler, E. P., Johnson, W. B., Worrell, W. L., MacDonald, D. D., and McKubre, M. C. H., " Application of Impedance Spectroscopy", Chpt. 4, Ed., McDonald, J. R., Wiley & Sons, NY, (1987). [13] Christensen, B. J., Mason, T. O., and Jennings, H. M., "Influence of Silica Fume on the Early Hydration Characteristics of Portland Cements using Impedance Spectroscopy (IS)" J. Mater. Sci. Lett., In Press, (1992). [14] Ping Gu, Xie Ping, J.J Beaudoin and R. Brousseau, " A.C. Impedance Spectroscopy (I): A New Equivalent Circuit Model for Hydrated Portland Cement Paste" Cem. Concr. Res. In Press, (1992). [15] F Maso, Proceedings of the Seventh International Congress on the Chemistry of Cements, Vol. 1, Editions Septima, Paris, (1980). [16] Cheng-yi Huang and R.F. Feldman, J. Amer. Ceram. Soc. 82, 740 (1985). [17] H. Uchikawa, Journal of Research of the Onoda Cement Company, 40 (119), (1988). or in "Advances in Cement Manufacture and use", edited by Ellis Gartner Published by Engineering Foundation Conference, _1, p271-294(1989).