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Adhesion at interface of geopolymer and cement mortar under compression: An experimental study
Construction and Building Materials 35 (2012) 204–210
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Adhesion at interface of geopolymer and cement mortar under compression: An experimental study Tzuu-Hsing Ueng a, Syuan-Jhih Lyu b, Hsiu-Wen Chu c, Hung-Hui Lee d, Tai-Tien Wang c,⇑ a
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan Graduate Institution of Engineering Technology-Doctoral, National Taipei University of Technology, Taipei, Taiwan c Institute of Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan d Environmental Information and Engineering Department, National Defense University, Taoyuan, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 November 2011 Received in revised form 21 February 2012 Accepted 1 March 2012
Keywords: Geopolymer Interface strength Interface stiffness Failure mode
a b s t r a c t Based on a simple mechanical model that elucidates the effects of distinct components of a cement mortar specimen that contains a geopolymer interlayer, this study undertakes a series of laboratory tests to determine corresponding representative parameters for each component in the model. Failure modes, deformational moduli and strength parameters of cement mortar, geopolymer, their interface, and the composite specimen, are thus obtained. The apparent angle of friction of the interface is close to that of the geopolymer and markedly exceeds that of the cement mortar. The interface adhesion is 34–43% as strong as the cohesions of the two compositional components. The comprehensive failure envelope for the composite specimen can be used to predict the possible failure mode and strength under various stresses when geopolymer is used as an adhesive material to repair defects in concrete. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymer, one of the three-dimensional aluminosilicate minerals with semicrystalline to noncrystalline structures introduced in early 1990s [1,2], exhibits similar mechanical behavior to that of conventional Portland cement, including thermo-stability. Geopolymer also exhibits excellent anti-acid and anti-alkali properties [3–5]. The raw materials used to make geopolymer are abundant and inexpensive, and geopolymerization reactions don not require a high temperature or energy. Therefore, geopolymer has attracted attention as a practical alternative to Portland cement [6–8]. The parameters that represent engineering characteristics of geopolymer, and factors that affect them, have been extensively discussed [3,9–14]. However, before geopolymer can be used to repair defects in concrete made of Portland cement, the mechanical characteristics of a geopolymer–concrete composite or layered member must be well understood. The most important property is the adhesion of the interface between geopolymer and concrete. Experimental investigations have been performed to evaluate the interface adhesion between geopolymer and various materials, such as ceramics, aluminum, concrete or cement mortar, and even natural siliceous aggregates [15–17]. Interface adhesions have been studied in specific stress states corresponding to testing standards [18–20]. However, modern numerical simulations, which are ⇑ Corresponding author. Tel.: +886 2 2771 2171x2773; fax: +886 2 2778 7579. E-mail address: [email protected] (T.-T. Wang). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.03.008
typically performed for a design task to repair structural defects in concrete and which generally consider potential stress states acting on the interface of concrete and repairing materials, require comprehensive knowledge of the mechanical characteristics of an adhering interface [21–23]. Therefore, the deformational moduli and strength parameters of an adhering interface between geopolymer and concrete should be determined. To characterize the adherence at an interface between geopolymer and cement mortar, this study performs a series of laboratory tests. A simple but comprehensive mechanical model of factors affecting various components at the interface is used to determine representative mechanical parameters. The failure modes, deformational moduli, and strength parameters for each component and for the integrated specimen are thus obtained. The comprehensive failure envelope obtained for the cement mortar specimen with a geopolymer interlayer in this study can be used to describe the possible failure mode, deformation, and strength of the specimen in various stress states. 2. Methodology To determine the adhesion behavior of an interface between two distinct materials, the mechanical characteristics of the materials must be investigated. The fundamental concept of this study is to characterize such mechanical behavior using a simple model and corresponding representative parameters. Fig. 1 shows the mechanical model adopted to present the behavior of the adhering interface between cement mortar and geopolymer. The springs and sliding elements are used to describe the deformational and failure characteristics, respectively, in the cement mortar component and in the geopolymer component. A spring in a normal
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Fig. 2. Dimensions of prism cement mortar specimen that contains a geopolymer interlayer.
Fig. 1. Simplified model utilized to simulate mechanical characteristics of a cement mortar specimen that contains a geopolymer interlayer.
orientation to the plane of the interface and a spring tangential to the plane of the interface are adopted to model deformations in their respective directions, and another sliding element is used to model the failure at the interface. Notably, this study focuses on the interface under compressive stress, and only considers the associated shear sliding failure. As such, an element that is used to describe tensile failure of the interface is ignored here.
clearly erroneous test results. Therefore, only the uniaxial compressive test is conducted. Both triaxial and uniaxial compressive tests are performed in displacement control mode with a loading rate of 1 mm/min after a contact loading of 0.2 kN. The stress–strain curves obtained from uniaxial compressive tests are used to determine the deformation characteristics of specimens. The tangent Young’s moduli at one-half the peak strength are measured from stress–strain curves and repreGP sented as deformational parameters ECM 50 and E50 for cement mortar and geopolymer, respectively. The peak stresses under various confining pressures in triaxial compressive tests are used to determine the strength parameters. For simplicity, the Mohr–Coulomb failure criterion with two parameters is adopted. The two parameters are the apparent cohesions, cCM and cGP, and the apparent friction angles, /CM and /GP, of cement mortar and geopolymer, respectively. The mechanical parameters of the adhering interface are discussed below.
3. Experimental results 2.1. Preparation of specimen
3.1. Cement mortar and geopolymer The geopolymer specimen used in this study is composed of kaolin and alkaline solution. Before the specimen is prepared, the kaolin is treated at 700 °C for 3 h to generate metakaolin. Sodium hydroxide is mixed with deionized water at a concentration of OH(M) = 5.4 and kept for at least 3 h to induce exothermic reactions. Sodium silicate solution with a SiO2/Na2O mole ratio of 3.19 is also prepared. The two solutions are then mixed by 15 min of stirring to yield a SiO2/Na2O mole ratio of 1.7 and an H2O/Na2O mole ratio of 8.3. After the exothermic reaction is complete, the solution is mixed with the metakaolin, stirred for an additional 15 min, and cast into a specimen mold. A shake table is employed to remove the air that is mixed into the specimen. The cast specimens are then allowed to stand at room temperature for approximately 24 h. The geopolymer specimens are then removed from the molds and cured at a relative humidity of 80 ± 5% for subsequent handling. Cement mortar is prepared by mixing type I Portland cement and graded river sand at a weight ratio of 1:3. Water is then added to the cement to give a water to cement ratio of 0.85. The cement mortar is made according to the CNS 1230 code [24]. The cast specimens of cement mortar are settled at room temperature for approximately 24 h. The cement mortar specimens are then removed from the molds and cured in saturated limewater. In this study, cylindrical specimens are individually tested to determine the mechanical parameters of cement mortar and geopolymer, and prism specimens are used to determine parameters of the adhering interface. After testing specimens of varying size and shape, those that show the least variation in test results are adopted, i.e., a cylindrical specimen with a diameter of 50 mm and a height of 130 mm and a prism specimen with both length and width of 100 mm and a height of 250 mm. A prism specimen that contains a 20-mm-thick interlay with 12 distinct dip angles is designed to determine the mechanical parameters of the adhering interface between the cement mortar and the geopolymer. Fig. 2 shows the dimensions of the prism specimen. The upper and lower parts of the specimen are firstly prepared by cement mortar. After 28 days of curing, the interlayer is cast in geopolymer. After 14 days of curing to develop the geopolymer, the prism specimens are tested.
Fig. 3 presents typical results obtained from uniaxial and triaxial compressive tests for cement mortar. Under uniaxial compressive stress, the stress–strain curve exhibits brittleness (Fig. 3a), and the failure mode is tensile splitting with local shear failure (Fig. 3b).
2.2. Experimental design Uniaxial and triaxial compressive tests are performed to determine the representative parameters of the cement mortar component and the geopolymer component shown in Fig. 1. Cylindrical specimens subjected to confining pressures of 0, 2, 4, and 8 MPa are tested. The prism specimen cannot be tested using a conventional triaxial compressive test because the concentration of stress at corners leads to
Fig. 3. Typical results from uniaxial and triaxial compressive tests for cement mortar. (a) Stress–strain curves. (b) Specimen after uniaxial compressive test, and failure mode. (c) Specimen after triaxial compressive test under 8 MPa confining pressure, and failure mode.
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Fig. 4. Stress states of cement mortar at failure in p–q stress space and associated failure envelope obtained by linear regression. Abscissa, p, and ordinate, q, respectively represent the average and half of difference between principal stresses.
Fig. 6. Stress states at failure in p–q stress space for geopolymer and associated failure envelope obtained by linear regression.
As the confining pressure increases, the length of specimens at the yielding point significantly decreases (Fig. 3c). The stress–strain curves demonstrate slowly increasing ductility, and the failure mode becomes one of shear failure. Notably, the slopes of the prepeak stress–strain curves under various confining pressures are similar, indicating limited variation of tangential moduli at half peak strength under confining pressures. Accordingly, ECM 50 is calculated from the stress–strain curves that are obtained in the uniaxial compressive tests, and found to have an average of 7.80 GPa. Fig. 4 presents the stress states of cement mortar at failure in p–q stress space. The abscissa represents the average of the principal stresses (p = (r1 + r3)/2) and the ordinate represents half of the difference between the principal stresses (q = (r1 r3)/2). Linear regression analysis shows that these stress states are highly consistent with a linear Mohr–Coulomb failure envelope. The coefficient of determination, r2, is 0.95 for this regression. Simple transformation
from p–q stress space to conventional normal to shear stress space obtains cCM and /CM values of 6.35 MPa and 30.0°, respectively. Fig. 5 presents typical experimental results obtained for a geopolymer in this study. Within the range of confining pressures used in the tests (0–8 MPa), the stress–strain curves consistently exhibit brittleness (Fig. 5a). The failure mode in the uniaxial compressive state is always tensile splitting with failure planes parallel or almost parallel to the loading direction (Fig. 5b). As the confining pressure increases, monoclinic shear failure or conjugated shear failure is observed (Fig. 5c). Fig. 6 presents the stress states at failure for geopolymer. Linear regression indicates that, given a coefficient of determination of 0.97, cGP and /GP are calculated as 8.12 MPa and 45.2°, respectively. 3.2. Prism specimen that contains an interlayer 3.2.1. Failure mode Two major failure modes in a prism cement mortar specimen that contains a geopolymer interlayer are observed under uniaxial compressive loading. Fig. 7 shows these failure modes. The type I failure mode is characterized by fractured plane(s) in the cement mortar and geopolymer, and can be subdivided into types IA and IB. The former involves a major fractured plane in the front or back of the specimen (Fig. 2), such that the fractured plane is approximately parallel to the strike direction of the interlayer, and the latter involves a fractured plane approximately perpendicular to the strike direction of the interlayer, which results in fractured plane(s) on both lateral sides of the specimen. Type II failure mode, which is characterized by shear sliding along the interface between the cement mortar and the geopolymer, can be divided into two subclasses. In type IIA failure, continuous shear sliding is observed, either along the upper or lower interface of the interlayer, or even both. In type IIB failure, shear sliding along the upper interface of the interlayer breaks off and connects with the shear sliding on the lower interface through a short fractured plane within the geopolymer interlayer. Table 1 summarizes the failure modes of the 48 prism specimens tested in this study. Specimens in which the interlayer has a dip angle (b) of between 0° and 45° or 90° exhibit a type I failure. Specimens in which the dip angles is between 50° and 60° exhibit type II failure.
Fig. 5. Typical results from uniaxial and triaxial compressive tests for geopolymer. (a) Stress–strain curves. (b) Specimen after uniaxial compressive test, and failure mode. (c) Specimen after triaxial compressive test under 8 MPa confining pressure, and failure mode.
3.2.2. Stress–strain curve Fig. 8 plots stress–strain curves of the prism cement mortar specimens containing a geopolymer interlayer and failed with
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Fig. 7. Failure modes of cement mortar specimen that contains a geopolymer interlayer with distinct dip angles. (a) Type IA. (b) Type IB. (c) Type IIA. (d) Type IIB.
Table 1 Failure modes of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
IA IA IA IB
IA IA IA IB
IA IB IB IA
IA IA IA IB
IB IA IB IB
IB IA IB IB
IA IA IB IA
IA IB IB IB
IIB IIB IIB IIB
IIB IIA IIA IIA
IIB IIA IIA IIB
IA IB IA IA
various types under uniaxial compressive loading. The peak strengths of the specimens that yielded with type II failure mode
are obviously lower than those with type I failure mode, indicating that the strength of the adhering interface, the adhesion, is lower than the strength of cement mortar and geopolymer. Additionally, since the areas of the upper and the lower interface of the interlayer are equal, the fractured part of the geopolymer interlayer and interface adhesion both affect the peak strength of the specimen with type IIB failure mode; therefore, the peak strength exceeds that associated with the type IIA mode. Moreover, the specimens with type IB failure mode usually have slightly higher peak strength compared to that with type IA mode because of the large area of the fractured plane in the stronger geopolymer interlayer.
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Fig. 8. Stress–strain curves of the prism cement mortar specimens containing a geopolymer interlayer and failed with various types under uniaxial compressive loading. The specimen number that the stress–strain curve represents is listed following the failure type in the legend. The number following the character S indicates test series (1–4), and the second number indicates the dip angel of the interlayer in the specimen.
Tables 2 and 3 respectively list the peak strengths and the tangential moduli at half peak strength for the 48 tested specimens. Fig. 9 plots the variations between the peak strengths and the dip angles of the interlayer. The dip angle of the interlayer substantially affects the peak strength and the failure mode of the specimen. The failure mode associated with the dip angle of the interlayer can be regarded a statistical factor. Tables 4 and 5, respectively, summarize the results of the analysis of variance (ANOVA) for the peak strength and for the modulus. The values for the F statistics, which exceed the critical value for associated 95% confidence intervals, indicate that the failure mode of the specimen and the dip angle of the interlayer adequately explain the variations in the test results.
Fig. 9. Variation of peak strengths and dip angles of prism cement mortar specimens that contain a geopolymer interlayer.
where dSP can be determined from the stress–strain curve of the prism specimen, and dCM and dGP can be derived from those of cylindrical specimens made of cement mortar and geopolymer, respectively. Subtracting deformations caused by cement mortar and geopolymer from the deformation of the prism specimen yields dIF. However, significantly concave sections appearing in initial loading stage result in highly nonlinear stress–strain curves. The incremental form of the deformation for a composite specimen is used instead of Eq. (1):
DdIF ¼ DrðLSP =ESP LCM =ECM LGP =EGP Þ
ð2Þ
IF
4. Discussion The mechanical parameters of the adhering interface are further discussed below. 4.1. Interface stiffness Under a load, the total deformation in a prism specimen dSP is the sum of its components, which are the deformations resulting from cement mortar, dCM, geopolymer, dGP, and the adhering interface, dIF:
dSP ¼ dCM þ dGP þ dIF
ð1Þ
where Dd is the deformation of the prism specimen under an incremental stress, Dr. The LSP and ESP are the length and the deformational modulus of the specimen, respectively. The LCM and LGP are the length of the cement mortar and the geopolymer part of the specimen, respectively. The incremental deformation of the adhering interface includes IF components for normal closure DdIF n and for shear deformation Dds and can be expressed as
DdIF ¼ DdIFn cos b þ DdIFs sin b
ð3Þ
where the cosine and sine of the dip angle b yield the projection of IF IF IF IF DdIF n and Dds in the direction of Dd . The Ddn and Dds result from the incremental stress acting on the adhering interface and can be
Table 2 Peak strengths of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles (Unit: MPa). Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
20.88 21.18 20.13 22.13
19.53 20.46 17.04 24.17
20.03 24.11 23.37 19.63
20.91 21.25 22.30 22.57
22.36 19.44 24.20 22.96
21.27 22.47 24.19 22.16
20.05 19.20 24.67 20.77
18.20 21.22 22.57 21.31
17.81 17.82 21.28 21.49
14.20 7.89 19.65 17.22
14.08 10.80 15.83 21.71
11.61 19.49 17.84 16.28
Table 3 Tangential moduli at half peak strength for rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles (Unit: GPa). Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
7.76 7.91 7.26 7.86
7.83 7.39 6.55 7.63
7.60 7.95 8.21 8.26
7.82 6.74 8.26 8.06
7.74 7.70 8.10 8.64
7.81 7.41 7.99 7.77
7.97 7.85 7.84 8.11
7.49 7.39 7.57 7.95
8.04 7.83 8.21 8.22
7.47 6.10 8.15 8.44
7.09 7.13 8.33 8.22
11.40 16.31 11.48 14.18
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T.-H. Ueng et al. / Construction and Building Materials 35 (2012) 204–210 Table 4 Statistical results of analysis of variance for the peak strength of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Treatment failure mode/dip angle
No. of sample
Sum
Average
Variance
Source of variation
Sum of square error for variation source
Degree of freedom
Mean square
F statistic
Critical value
I/0–45° II/50° II/55° II/60° I/90°
32 4 4 4 4
686.72 78.40 58.97 62.42 65.21
21.46 19.60 14.74 15.60 16.30
3.42 4.25 25.83 20.93 11.51
Treatment Error Total
309.95 293.59 603.55
4 43 47
77.49 6.83
11.35
2.59
Table 5 Statistical results of analysis of variance for the tangential moduli at half peak strengths of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Treatment failure mode/dip angle
No. of sample
Sum
Average
Variance
Source of variation
Sum of square error for variation source
Degree of freedom
Mean square
F statistic
Critical value
I/0–45° II/50° II/55° II/60° I/90°
32 4 4 4 4
248.39 32.31 30.16 30.77 53.37
7.76 8.08 7.54 7.69 13.34
0.17 0.03 1.09 0.45 5.58
Treatment Error Total
114.71 26.83 141.54
4 43 47
28.68 0.62
45.96
2.59
IF
estimated according to the corresponding normal stiffness kn and IF shear stiffness ks as IF
DdIFn ¼ 2ðDr cos2 bÞ=kn DdIFs
¼ 2ðDr
IF cos b sin bÞ=ks
ð4Þ ð5Þ
Notably, both the upper and the lower interfaces to the geopolymer interlayer contribute to the deformation of the specimen and a corresponding constant, 2, is considered. Deformation caused by an incremental increase in stress from IF IF 1/2 to 2/3 peak strength is used to determine the kn and ks of the adhering interface between the cement mortar and the geopolymer. The deformation results for the 44 prism specimens without vertical dip are used to determine these two parameters. The solver function IF in Microsoft Excel is used to obtain kn ¼ 1714 GPa=m and IF ks ¼ 3261 GPa=m. 4.2. Strength of adhesion at interface This study adopts the Mohr–Coulomb failure criterion to describe the failure of the adhering interface, which is the peak shear stress acting on the interface. Failure sf, which is dependent on the adhesion between cement mortar and geopolymer cIF and the associated apparent friction angle /IF, can be written as
sf ¼ cIF þ rfn tan /IF
ð6Þ
where rfn is the normal stress acting on the interface at failure. The sf and rfn can be calculated by performing stress transformation based on the peak strength of the prism specimens with type II failure and the corresponding dip angle of the interlayer. Fig. 10 presents the linear regression results for the 12 prism specimens with type II failure mode: cIF = 2.75 MPa and /IF = 42.5°. The coefficient of determination for this linear regression is 0.82, which implies that the Mohr–Coulomb failure criterion can effectively predict the failure of the adhering interface. The adhesion between the cement mortar and geopolymer that are used herein is only around 34–43% of their apparent cohesions. Clearly, the proportion of the geopolymer in the composition can be improved. Nevertheless, the apparent friction angle of the interface is close to that of the geopolymer and greatly exceeds that of cement mortar. The tested geopolymer may serve as an effective adhesive under highly normal compressive stress states.
Fig. 10. Results of linear regression for prism cement mortar specimens that contain a geopolymer interlayer with type II failure mode.
4.3. Failure criterion for specimen with interlayer Fig. 11 shows the failure envelopes for cement mortar, geopolymer and the associated interface in the normal- and shear (r–s) stress space. When a low normal stress acts on the interface (to the left of point A in Fig. 11), the interface adhesion dominates the failure if the specimen has an interface sliding failure mode. When high normal stress acts on the interface (to the right of point A in Fig. 11), the friction between the cement mortar and the geopolymer fully mobilizes and accordingly contributes to the adhering strength of the interface. Failure of the specimen results from its composition rather than from the interface sliding. In this study the cement mortar, with a shear failure mode, governs this situation. Knowledge of distinct failure modes and associated failure criteria for a specimen containing an interlayer with various dip angles is needed to obtain a comprehensive failure envelope for the specimen (line through crosses in Fig. 11). This comprehensive failure envelope can be used to predict the failure mode and strength of the specimen in various stress states, and it provides a valuable reference for the adoption of geopolymer as an adhesive material for repairing defects in concrete.
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References
Fig. 11. Failure envelopes for cement mortar, geopolymer and associated interface in r–s stress space. The lines plotted through crosses represent comprehensive failure envelope, which dominates the failure of the prism cement mortar specimen that contains a geopolymer interlayer in various stress states.
5. Conclusions Based on a simple mechanical model that takes into account the effects of particular components of a cement mortar specimen that contains a geopolymer interlayer, this study designs and conducts a series of laboratory tests to determine representative parameters and thus elucidate the mechanical characteristics of the interface between cement mortar and geopolymer. Experimental results indicate that, for the tested compositional proportion of geopolymer, the apparent friction angle of the interface is close to that of the geopolymer and is much higher than that of cement mortar. The interface adhesion is about 34–43% comparing with the cohesions of the two components. The normal- and the shear stiffness of the interface between cement mortar and geopolymer are 1714 and 3261 GPa/m, respectively. Accordingly, the comprehensive failure envelope obtained for the specimen can be used to predict the failure mode and strength under various stresses when geopolymer is used as an adhesive material for repairing defects in concrete. Acknowledgment The authors would like to thank the National Science Council, Taiwan, for financially supporting this research under Contract No. NSC 97-2621-M-027-003.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Adhesion at interface of geopolymer and cement mortar under compression: An experimental study Tzuu-Hsing Ueng a, Syuan-Jhih Lyu b, Hsiu-Wen Chu c, Hung-Hui Lee d, Tai-Tien Wang c,⇑ a
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan Graduate Institution of Engineering Technology-Doctoral, National Taipei University of Technology, Taipei, Taiwan c Institute of Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan d Environmental Information and Engineering Department, National Defense University, Taoyuan, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 November 2011 Received in revised form 21 February 2012 Accepted 1 March 2012
Keywords: Geopolymer Interface strength Interface stiffness Failure mode
a b s t r a c t Based on a simple mechanical model that elucidates the effects of distinct components of a cement mortar specimen that contains a geopolymer interlayer, this study undertakes a series of laboratory tests to determine corresponding representative parameters for each component in the model. Failure modes, deformational moduli and strength parameters of cement mortar, geopolymer, their interface, and the composite specimen, are thus obtained. The apparent angle of friction of the interface is close to that of the geopolymer and markedly exceeds that of the cement mortar. The interface adhesion is 34–43% as strong as the cohesions of the two compositional components. The comprehensive failure envelope for the composite specimen can be used to predict the possible failure mode and strength under various stresses when geopolymer is used as an adhesive material to repair defects in concrete. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymer, one of the three-dimensional aluminosilicate minerals with semicrystalline to noncrystalline structures introduced in early 1990s [1,2], exhibits similar mechanical behavior to that of conventional Portland cement, including thermo-stability. Geopolymer also exhibits excellent anti-acid and anti-alkali properties [3–5]. The raw materials used to make geopolymer are abundant and inexpensive, and geopolymerization reactions don not require a high temperature or energy. Therefore, geopolymer has attracted attention as a practical alternative to Portland cement [6–8]. The parameters that represent engineering characteristics of geopolymer, and factors that affect them, have been extensively discussed [3,9–14]. However, before geopolymer can be used to repair defects in concrete made of Portland cement, the mechanical characteristics of a geopolymer–concrete composite or layered member must be well understood. The most important property is the adhesion of the interface between geopolymer and concrete. Experimental investigations have been performed to evaluate the interface adhesion between geopolymer and various materials, such as ceramics, aluminum, concrete or cement mortar, and even natural siliceous aggregates [15–17]. Interface adhesions have been studied in specific stress states corresponding to testing standards [18–20]. However, modern numerical simulations, which are ⇑ Corresponding author. Tel.: +886 2 2771 2171x2773; fax: +886 2 2778 7579. E-mail address: [email protected] (T.-T. Wang). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.03.008
typically performed for a design task to repair structural defects in concrete and which generally consider potential stress states acting on the interface of concrete and repairing materials, require comprehensive knowledge of the mechanical characteristics of an adhering interface [21–23]. Therefore, the deformational moduli and strength parameters of an adhering interface between geopolymer and concrete should be determined. To characterize the adherence at an interface between geopolymer and cement mortar, this study performs a series of laboratory tests. A simple but comprehensive mechanical model of factors affecting various components at the interface is used to determine representative mechanical parameters. The failure modes, deformational moduli, and strength parameters for each component and for the integrated specimen are thus obtained. The comprehensive failure envelope obtained for the cement mortar specimen with a geopolymer interlayer in this study can be used to describe the possible failure mode, deformation, and strength of the specimen in various stress states. 2. Methodology To determine the adhesion behavior of an interface between two distinct materials, the mechanical characteristics of the materials must be investigated. The fundamental concept of this study is to characterize such mechanical behavior using a simple model and corresponding representative parameters. Fig. 1 shows the mechanical model adopted to present the behavior of the adhering interface between cement mortar and geopolymer. The springs and sliding elements are used to describe the deformational and failure characteristics, respectively, in the cement mortar component and in the geopolymer component. A spring in a normal
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Fig. 2. Dimensions of prism cement mortar specimen that contains a geopolymer interlayer.
Fig. 1. Simplified model utilized to simulate mechanical characteristics of a cement mortar specimen that contains a geopolymer interlayer.
orientation to the plane of the interface and a spring tangential to the plane of the interface are adopted to model deformations in their respective directions, and another sliding element is used to model the failure at the interface. Notably, this study focuses on the interface under compressive stress, and only considers the associated shear sliding failure. As such, an element that is used to describe tensile failure of the interface is ignored here.
clearly erroneous test results. Therefore, only the uniaxial compressive test is conducted. Both triaxial and uniaxial compressive tests are performed in displacement control mode with a loading rate of 1 mm/min after a contact loading of 0.2 kN. The stress–strain curves obtained from uniaxial compressive tests are used to determine the deformation characteristics of specimens. The tangent Young’s moduli at one-half the peak strength are measured from stress–strain curves and repreGP sented as deformational parameters ECM 50 and E50 for cement mortar and geopolymer, respectively. The peak stresses under various confining pressures in triaxial compressive tests are used to determine the strength parameters. For simplicity, the Mohr–Coulomb failure criterion with two parameters is adopted. The two parameters are the apparent cohesions, cCM and cGP, and the apparent friction angles, /CM and /GP, of cement mortar and geopolymer, respectively. The mechanical parameters of the adhering interface are discussed below.
3. Experimental results 2.1. Preparation of specimen
3.1. Cement mortar and geopolymer The geopolymer specimen used in this study is composed of kaolin and alkaline solution. Before the specimen is prepared, the kaolin is treated at 700 °C for 3 h to generate metakaolin. Sodium hydroxide is mixed with deionized water at a concentration of OH(M) = 5.4 and kept for at least 3 h to induce exothermic reactions. Sodium silicate solution with a SiO2/Na2O mole ratio of 3.19 is also prepared. The two solutions are then mixed by 15 min of stirring to yield a SiO2/Na2O mole ratio of 1.7 and an H2O/Na2O mole ratio of 8.3. After the exothermic reaction is complete, the solution is mixed with the metakaolin, stirred for an additional 15 min, and cast into a specimen mold. A shake table is employed to remove the air that is mixed into the specimen. The cast specimens are then allowed to stand at room temperature for approximately 24 h. The geopolymer specimens are then removed from the molds and cured at a relative humidity of 80 ± 5% for subsequent handling. Cement mortar is prepared by mixing type I Portland cement and graded river sand at a weight ratio of 1:3. Water is then added to the cement to give a water to cement ratio of 0.85. The cement mortar is made according to the CNS 1230 code [24]. The cast specimens of cement mortar are settled at room temperature for approximately 24 h. The cement mortar specimens are then removed from the molds and cured in saturated limewater. In this study, cylindrical specimens are individually tested to determine the mechanical parameters of cement mortar and geopolymer, and prism specimens are used to determine parameters of the adhering interface. After testing specimens of varying size and shape, those that show the least variation in test results are adopted, i.e., a cylindrical specimen with a diameter of 50 mm and a height of 130 mm and a prism specimen with both length and width of 100 mm and a height of 250 mm. A prism specimen that contains a 20-mm-thick interlay with 12 distinct dip angles is designed to determine the mechanical parameters of the adhering interface between the cement mortar and the geopolymer. Fig. 2 shows the dimensions of the prism specimen. The upper and lower parts of the specimen are firstly prepared by cement mortar. After 28 days of curing, the interlayer is cast in geopolymer. After 14 days of curing to develop the geopolymer, the prism specimens are tested.
Fig. 3 presents typical results obtained from uniaxial and triaxial compressive tests for cement mortar. Under uniaxial compressive stress, the stress–strain curve exhibits brittleness (Fig. 3a), and the failure mode is tensile splitting with local shear failure (Fig. 3b).
2.2. Experimental design Uniaxial and triaxial compressive tests are performed to determine the representative parameters of the cement mortar component and the geopolymer component shown in Fig. 1. Cylindrical specimens subjected to confining pressures of 0, 2, 4, and 8 MPa are tested. The prism specimen cannot be tested using a conventional triaxial compressive test because the concentration of stress at corners leads to
Fig. 3. Typical results from uniaxial and triaxial compressive tests for cement mortar. (a) Stress–strain curves. (b) Specimen after uniaxial compressive test, and failure mode. (c) Specimen after triaxial compressive test under 8 MPa confining pressure, and failure mode.
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Fig. 4. Stress states of cement mortar at failure in p–q stress space and associated failure envelope obtained by linear regression. Abscissa, p, and ordinate, q, respectively represent the average and half of difference between principal stresses.
Fig. 6. Stress states at failure in p–q stress space for geopolymer and associated failure envelope obtained by linear regression.
As the confining pressure increases, the length of specimens at the yielding point significantly decreases (Fig. 3c). The stress–strain curves demonstrate slowly increasing ductility, and the failure mode becomes one of shear failure. Notably, the slopes of the prepeak stress–strain curves under various confining pressures are similar, indicating limited variation of tangential moduli at half peak strength under confining pressures. Accordingly, ECM 50 is calculated from the stress–strain curves that are obtained in the uniaxial compressive tests, and found to have an average of 7.80 GPa. Fig. 4 presents the stress states of cement mortar at failure in p–q stress space. The abscissa represents the average of the principal stresses (p = (r1 + r3)/2) and the ordinate represents half of the difference between the principal stresses (q = (r1 r3)/2). Linear regression analysis shows that these stress states are highly consistent with a linear Mohr–Coulomb failure envelope. The coefficient of determination, r2, is 0.95 for this regression. Simple transformation
from p–q stress space to conventional normal to shear stress space obtains cCM and /CM values of 6.35 MPa and 30.0°, respectively. Fig. 5 presents typical experimental results obtained for a geopolymer in this study. Within the range of confining pressures used in the tests (0–8 MPa), the stress–strain curves consistently exhibit brittleness (Fig. 5a). The failure mode in the uniaxial compressive state is always tensile splitting with failure planes parallel or almost parallel to the loading direction (Fig. 5b). As the confining pressure increases, monoclinic shear failure or conjugated shear failure is observed (Fig. 5c). Fig. 6 presents the stress states at failure for geopolymer. Linear regression indicates that, given a coefficient of determination of 0.97, cGP and /GP are calculated as 8.12 MPa and 45.2°, respectively. 3.2. Prism specimen that contains an interlayer 3.2.1. Failure mode Two major failure modes in a prism cement mortar specimen that contains a geopolymer interlayer are observed under uniaxial compressive loading. Fig. 7 shows these failure modes. The type I failure mode is characterized by fractured plane(s) in the cement mortar and geopolymer, and can be subdivided into types IA and IB. The former involves a major fractured plane in the front or back of the specimen (Fig. 2), such that the fractured plane is approximately parallel to the strike direction of the interlayer, and the latter involves a fractured plane approximately perpendicular to the strike direction of the interlayer, which results in fractured plane(s) on both lateral sides of the specimen. Type II failure mode, which is characterized by shear sliding along the interface between the cement mortar and the geopolymer, can be divided into two subclasses. In type IIA failure, continuous shear sliding is observed, either along the upper or lower interface of the interlayer, or even both. In type IIB failure, shear sliding along the upper interface of the interlayer breaks off and connects with the shear sliding on the lower interface through a short fractured plane within the geopolymer interlayer. Table 1 summarizes the failure modes of the 48 prism specimens tested in this study. Specimens in which the interlayer has a dip angle (b) of between 0° and 45° or 90° exhibit a type I failure. Specimens in which the dip angles is between 50° and 60° exhibit type II failure.
Fig. 5. Typical results from uniaxial and triaxial compressive tests for geopolymer. (a) Stress–strain curves. (b) Specimen after uniaxial compressive test, and failure mode. (c) Specimen after triaxial compressive test under 8 MPa confining pressure, and failure mode.
3.2.2. Stress–strain curve Fig. 8 plots stress–strain curves of the prism cement mortar specimens containing a geopolymer interlayer and failed with
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Fig. 7. Failure modes of cement mortar specimen that contains a geopolymer interlayer with distinct dip angles. (a) Type IA. (b) Type IB. (c) Type IIA. (d) Type IIB.
Table 1 Failure modes of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
IA IA IA IB
IA IA IA IB
IA IB IB IA
IA IA IA IB
IB IA IB IB
IB IA IB IB
IA IA IB IA
IA IB IB IB
IIB IIB IIB IIB
IIB IIA IIA IIA
IIB IIA IIA IIB
IA IB IA IA
various types under uniaxial compressive loading. The peak strengths of the specimens that yielded with type II failure mode
are obviously lower than those with type I failure mode, indicating that the strength of the adhering interface, the adhesion, is lower than the strength of cement mortar and geopolymer. Additionally, since the areas of the upper and the lower interface of the interlayer are equal, the fractured part of the geopolymer interlayer and interface adhesion both affect the peak strength of the specimen with type IIB failure mode; therefore, the peak strength exceeds that associated with the type IIA mode. Moreover, the specimens with type IB failure mode usually have slightly higher peak strength compared to that with type IA mode because of the large area of the fractured plane in the stronger geopolymer interlayer.
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Fig. 8. Stress–strain curves of the prism cement mortar specimens containing a geopolymer interlayer and failed with various types under uniaxial compressive loading. The specimen number that the stress–strain curve represents is listed following the failure type in the legend. The number following the character S indicates test series (1–4), and the second number indicates the dip angel of the interlayer in the specimen.
Tables 2 and 3 respectively list the peak strengths and the tangential moduli at half peak strength for the 48 tested specimens. Fig. 9 plots the variations between the peak strengths and the dip angles of the interlayer. The dip angle of the interlayer substantially affects the peak strength and the failure mode of the specimen. The failure mode associated with the dip angle of the interlayer can be regarded a statistical factor. Tables 4 and 5, respectively, summarize the results of the analysis of variance (ANOVA) for the peak strength and for the modulus. The values for the F statistics, which exceed the critical value for associated 95% confidence intervals, indicate that the failure mode of the specimen and the dip angle of the interlayer adequately explain the variations in the test results.
Fig. 9. Variation of peak strengths and dip angles of prism cement mortar specimens that contain a geopolymer interlayer.
where dSP can be determined from the stress–strain curve of the prism specimen, and dCM and dGP can be derived from those of cylindrical specimens made of cement mortar and geopolymer, respectively. Subtracting deformations caused by cement mortar and geopolymer from the deformation of the prism specimen yields dIF. However, significantly concave sections appearing in initial loading stage result in highly nonlinear stress–strain curves. The incremental form of the deformation for a composite specimen is used instead of Eq. (1):
DdIF ¼ DrðLSP =ESP LCM =ECM LGP =EGP Þ
ð2Þ
IF
4. Discussion The mechanical parameters of the adhering interface are further discussed below. 4.1. Interface stiffness Under a load, the total deformation in a prism specimen dSP is the sum of its components, which are the deformations resulting from cement mortar, dCM, geopolymer, dGP, and the adhering interface, dIF:
dSP ¼ dCM þ dGP þ dIF
ð1Þ
where Dd is the deformation of the prism specimen under an incremental stress, Dr. The LSP and ESP are the length and the deformational modulus of the specimen, respectively. The LCM and LGP are the length of the cement mortar and the geopolymer part of the specimen, respectively. The incremental deformation of the adhering interface includes IF components for normal closure DdIF n and for shear deformation Dds and can be expressed as
DdIF ¼ DdIFn cos b þ DdIFs sin b
ð3Þ
where the cosine and sine of the dip angle b yield the projection of IF IF IF IF DdIF n and Dds in the direction of Dd . The Ddn and Dds result from the incremental stress acting on the adhering interface and can be
Table 2 Peak strengths of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles (Unit: MPa). Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
20.88 21.18 20.13 22.13
19.53 20.46 17.04 24.17
20.03 24.11 23.37 19.63
20.91 21.25 22.30 22.57
22.36 19.44 24.20 22.96
21.27 22.47 24.19 22.16
20.05 19.20 24.67 20.77
18.20 21.22 22.57 21.31
17.81 17.82 21.28 21.49
14.20 7.89 19.65 17.22
14.08 10.80 15.83 21.71
11.61 19.49 17.84 16.28
Table 3 Tangential moduli at half peak strength for rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles (Unit: GPa). Specimen no.
1 2 3 4
Dip angle of interlayer (°) 0
15
20
25
30
35
40
45
50
55
60
90
7.76 7.91 7.26 7.86
7.83 7.39 6.55 7.63
7.60 7.95 8.21 8.26
7.82 6.74 8.26 8.06
7.74 7.70 8.10 8.64
7.81 7.41 7.99 7.77
7.97 7.85 7.84 8.11
7.49 7.39 7.57 7.95
8.04 7.83 8.21 8.22
7.47 6.10 8.15 8.44
7.09 7.13 8.33 8.22
11.40 16.31 11.48 14.18
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T.-H. Ueng et al. / Construction and Building Materials 35 (2012) 204–210 Table 4 Statistical results of analysis of variance for the peak strength of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Treatment failure mode/dip angle
No. of sample
Sum
Average
Variance
Source of variation
Sum of square error for variation source
Degree of freedom
Mean square
F statistic
Critical value
I/0–45° II/50° II/55° II/60° I/90°
32 4 4 4 4
686.72 78.40 58.97 62.42 65.21
21.46 19.60 14.74 15.60 16.30
3.42 4.25 25.83 20.93 11.51
Treatment Error Total
309.95 293.59 603.55
4 43 47
77.49 6.83
11.35
2.59
Table 5 Statistical results of analysis of variance for the tangential moduli at half peak strengths of rectangular cement mortar specimens that contain a geopolymer interlayer with various dip angles. Treatment failure mode/dip angle
No. of sample
Sum
Average
Variance
Source of variation
Sum of square error for variation source
Degree of freedom
Mean square
F statistic
Critical value
I/0–45° II/50° II/55° II/60° I/90°
32 4 4 4 4
248.39 32.31 30.16 30.77 53.37
7.76 8.08 7.54 7.69 13.34
0.17 0.03 1.09 0.45 5.58
Treatment Error Total
114.71 26.83 141.54
4 43 47
28.68 0.62
45.96
2.59
IF
estimated according to the corresponding normal stiffness kn and IF shear stiffness ks as IF
DdIFn ¼ 2ðDr cos2 bÞ=kn DdIFs
¼ 2ðDr
IF cos b sin bÞ=ks
ð4Þ ð5Þ
Notably, both the upper and the lower interfaces to the geopolymer interlayer contribute to the deformation of the specimen and a corresponding constant, 2, is considered. Deformation caused by an incremental increase in stress from IF IF 1/2 to 2/3 peak strength is used to determine the kn and ks of the adhering interface between the cement mortar and the geopolymer. The deformation results for the 44 prism specimens without vertical dip are used to determine these two parameters. The solver function IF in Microsoft Excel is used to obtain kn ¼ 1714 GPa=m and IF ks ¼ 3261 GPa=m. 4.2. Strength of adhesion at interface This study adopts the Mohr–Coulomb failure criterion to describe the failure of the adhering interface, which is the peak shear stress acting on the interface. Failure sf, which is dependent on the adhesion between cement mortar and geopolymer cIF and the associated apparent friction angle /IF, can be written as
sf ¼ cIF þ rfn tan /IF
ð6Þ
where rfn is the normal stress acting on the interface at failure. The sf and rfn can be calculated by performing stress transformation based on the peak strength of the prism specimens with type II failure and the corresponding dip angle of the interlayer. Fig. 10 presents the linear regression results for the 12 prism specimens with type II failure mode: cIF = 2.75 MPa and /IF = 42.5°. The coefficient of determination for this linear regression is 0.82, which implies that the Mohr–Coulomb failure criterion can effectively predict the failure of the adhering interface. The adhesion between the cement mortar and geopolymer that are used herein is only around 34–43% of their apparent cohesions. Clearly, the proportion of the geopolymer in the composition can be improved. Nevertheless, the apparent friction angle of the interface is close to that of the geopolymer and greatly exceeds that of cement mortar. The tested geopolymer may serve as an effective adhesive under highly normal compressive stress states.
Fig. 10. Results of linear regression for prism cement mortar specimens that contain a geopolymer interlayer with type II failure mode.
4.3. Failure criterion for specimen with interlayer Fig. 11 shows the failure envelopes for cement mortar, geopolymer and the associated interface in the normal- and shear (r–s) stress space. When a low normal stress acts on the interface (to the left of point A in Fig. 11), the interface adhesion dominates the failure if the specimen has an interface sliding failure mode. When high normal stress acts on the interface (to the right of point A in Fig. 11), the friction between the cement mortar and the geopolymer fully mobilizes and accordingly contributes to the adhering strength of the interface. Failure of the specimen results from its composition rather than from the interface sliding. In this study the cement mortar, with a shear failure mode, governs this situation. Knowledge of distinct failure modes and associated failure criteria for a specimen containing an interlayer with various dip angles is needed to obtain a comprehensive failure envelope for the specimen (line through crosses in Fig. 11). This comprehensive failure envelope can be used to predict the failure mode and strength of the specimen in various stress states, and it provides a valuable reference for the adoption of geopolymer as an adhesive material for repairing defects in concrete.
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References
Fig. 11. Failure envelopes for cement mortar, geopolymer and associated interface in r–s stress space. The lines plotted through crosses represent comprehensive failure envelope, which dominates the failure of the prism cement mortar specimen that contains a geopolymer interlayer in various stress states.
5. Conclusions Based on a simple mechanical model that takes into account the effects of particular components of a cement mortar specimen that contains a geopolymer interlayer, this study designs and conducts a series of laboratory tests to determine representative parameters and thus elucidate the mechanical characteristics of the interface between cement mortar and geopolymer. Experimental results indicate that, for the tested compositional proportion of geopolymer, the apparent friction angle of the interface is close to that of the geopolymer and is much higher than that of cement mortar. The interface adhesion is about 34–43% comparing with the cohesions of the two components. The normal- and the shear stiffness of the interface between cement mortar and geopolymer are 1714 and 3261 GPa/m, respectively. Accordingly, the comprehensive failure envelope obtained for the specimen can be used to predict the failure mode and strength under various stresses when geopolymer is used as an adhesive material for repairing defects in concrete. Acknowledgment The authors would like to thank the National Science Council, Taiwan, for financially supporting this research under Contract No. NSC 97-2621-M-027-003.
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