- Email: [email protected]
Novel hybrid organic-geopolymer materials
Applied Clay Science 73 (2013) 42–50
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Novel hybrid organic-geopolymer materials Claudio Ferone a, Giuseppina Roviello a,⁎, Francesco Colangelo a, Raffaele Cioffi a, Oreste Tarallo b a b
Dipartimento per le Tecnologie, Facoltà di Ingegneria, Università di Napoli ‘Parthenope’, INSTM Research Group Napoli Parthenope, Centro Direzionale Napoli, Isola C4, 80143 Napoli, Italy Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, 80126 Napoli, Italy
a r t i c l e
i n f o
Article history: Received 23 April 2012 Received in revised form 18 October 2012 Accepted 2 November 2012 Available online 04 December 2012 Keywords: Hybrid geopolymer Epoxy resin Metakaolin
a b s t r a c t Novel hybrid organic–inorganic materials were prepared through an innovative synthetic approach based on a co-reticulation in mild conditions of epoxy based organic resins and an MK-based geopolymer inorganic matrix. A high compatibility between the organic and inorganic phases, even at appreciable concentration of resin, was realized up to micrometric level. A good and homogeneous dispersion (without the formation of agglomerates) of the organic particles was obtained just by hand mixing. These new materials present significantly enhanced compressive strengths and toughness in respect to the neat geopolymer allowing a wider utilization of these materials for structural applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Geopolymers represent an innovative class of ceramic materials 1 characterized by advanced technological properties, as well as low manufacturing energy consumption for construction purposes and engineering applications. Geopolymers are usually obtained through inexpensive and ecofriendly synthetic procedures with low waste gas emission (Duxson et al., 2007; Habert et al., 2011; Komnitsas, 2011; Provis et al., 2010). For these reasons they are considered “green materials”. In the field of civil engineering, geopolymer-based materials are also referred to as “alkali-activated cements” or “chemically-bonded ceramics” which can be obtained from raw materials with low (or zero) CaO content, such as metakaolin (Cioffi et al., 2003), clay (Buchwald et al., 2009; Ferone et al., 2012) and other natural silico-aluminates (Xu and van Deventer, 2000) as well as industrial process wastes such as coal fly ash (Andini et al., 2008, 2010; Ferone et al., 2011), lignite bottom ash (Sathonsaowaphak et al., 2009) and metallurgical slag (Shi et al., 2006). The synthesis of geopolymers can be carried out by mixing reactive silico-aluminate materials with strongly alkaline solutions such as alkali metal (Na, K) hydroxide or silicate. In such reaction environment the silico-aluminate reactive materials are rapidly dissolved. A complex reaction mechanism follows, in which solubilized silica and alumina condensate with the ultimate formation of a three-dimensional geopolymeric network (Davidovits, 1991). This phase is crucial in relation to the final product properties ⁎ Corresponding author. Tel.: +39 081 5476781; fax: +39 081 5476777. E-mail address: [email protected] (G. Roviello). 1 “A ceramic is a nonmetallic, inorganic solid” (Kingery et al., Introduction to Ceramics, Wiley & Sons, 1976). Geopolymers can be properly considered ceramics since they are a class of synthetic aluminosilicate materials presenting an amorphous three-dimensional structure similar to that of an aluminosilicate glass. 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2012.11.001
that strongly depend on the degree of cross linking among the different silico-aluminate polymeric chains. Geopolymers synthesized at temperature lower than 90 °C are amorphous, while zeolite-like crystalline products are obtained at 150–200 °C (Davidovits, 1991). These materials show excellent mechanical properties, low shrinkage, thermal stability, freeze-thaw, acid and fire resistance, long term durability and recyclability, so the application of geopolymer-based materials covers many fields. On the other hand geopolymers are ceramic materials, so they present a typical brittle mechanical behavior with the consequent low ductility and low fracture toughness. This behavior may represent a great limit in several structural applications. This drawback can be overcome by producing geopolymer matrix composites. A great deal of studies (Barbosa and MacKenzie, 2003; Dias and Thaumaturgo, 2005; Li and Xu, 2009; Lin et al., 2008; Menna et al., 2013; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007) was produced about this topic and many types of fillers were tested, such as particulate and various kinds of short and continuous fibers. For example, polyvinyl alcohol, polypropylene, basalt and carbon short fibers were employed as additives to improve geopolymeric mechanical performances. In fact fibers provide a control of cracking by a bridging action during both micro and macrocracking of the matrix thus increasing the fracture toughness of the brittle matrix (Dias and Thaumaturgo, 2005; Li and Xu, 2009; Tiesong et al., 2008; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007). It is worth pointing out that these composites are usually obtained by a physical blending of the two components, the filler being added as a finely divided powder or as an emulsion, 2 2 Sometimes, to obtain a closely mix of the two different phases, polymers rather soluble in water, such as sodium polyacrylate (PAANa), polyacrylic acid (PAA), polyacrylamide (PAm), polyvinyl alcohol (PVA), polyethylene glycol (PEG), were used (Xu and van Deventer, 2000).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
sometimes in the presence of compatibilizers (Dias and Thaumaturgo, 2005; Li and Xu, 2009; Tiesong et al., 2008; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007). Recently, a new class of geopolymeric composites with organic matrix has been developed (Hussain et al., 2004, 2005) with the main aim to improve the fire resistance of organic polymers and to reduce the smoke production arising from their burning. These composites are also named “hybrid” inorganic–organic composites (i.e. a material composed of an intimate mixture of inorganic and organic components, where the components usually interpenetrate on scales of less than 1 μm. IUPAC, 1997). The new class of composites described by Hussain has been obtained by incorporating the geopolymer into the cross-linked polymeric structure, tailoring the chemical compositions of the components. In particular, a bi-functional epoxy resin, Diglycidyl Ether of Bisphenol A (DGEBA), was mixed with a small amount of the geopolymer (20% weight fraction) in the presence of a curing agent (Hussain et al., 2004, 2005). In this way a hybrid material with excellent mechanical properties and improved fire resistance was obtained. The development of hybrid materials represents an extremely interesting and relevant research field for the potentially useful physical properties that could derive from the interfacial interactions of two chemically incompatible phases (De Santis et al., 2007). By tuning the chemical composition of the components, it is feasible to realize different kinds of materials, with different properties, whose applications depend on the ratio between the organic and inorganic phases (Kickelbick, 2007). In this paper an innovative strategy for the chemical incorporation of an appreciable amount of organic polymer into an MK-based geopolymeric matrix, that allows the preparation of novel geopolymer based hybrid materials, is described. The new proposed approach is based on the mixing of a partly crosslinked epoxy resin of tailored composition to a geopolymeric suspension, when both polymerization reactions are not yet completed. In this way, an improved compatibility between the organic and the aqueous inorganic phases is easily realized, affording a homogeneous and stable in time dispersion of the organic microdomains into the inorganic phase, without addition of external additives, even at appreciable concentration of resin. By using this new method, novel organic–inorganic hybrid materials have been prepared and characterized through scanning electron microscopy (SEM), thermal analysis (TGA, DSC), FT-IR spectrometry, dynamic mechanical thermal analysis (DMTA) and compressive strength determination. These new materials present interesting mechanical properties and, in particular, a considerably reduced brittleness in respect to the geopolymeric matrix. For these reasons these hybrid materials could be used for all the applications in which the use of the pure geopolymer is limited by its brittleness such as the realization of thermo-insulating or thermo-resistant panels with improved tenacity. Moreover, these materials can show more useful rheological (tixotropicity) properties in the field of restoration and repair of damaged concrete and masonry.
2. Materials and methods
43
2.2. Analytical methods Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed by a TA Instrument SDT2960 simultaneous DSC–TGA under air flow at a heating rate of 10 °C/min using 10–15 mg of the powdered specimen stored at room temperature for one week. DMTA were performed by using a Triton Technology Ltd instrument, Tritec 2000 DMA. The specimens were clamped in the single cantilever mode. DMTA analyses were performed from room temperature to 260 °C at 2 °C/min using a 1 Hz frequency and having a 1% strain. SEM analysis was carried out by means of a FEI Quanta 200 FEG microscope. EDS analyses were performed by using an Energy Dispersion Spectrometer Oxford Inca Energy System 250 equipped with INCAx-act LN2-free detector, working at 20 kV voltage. FT-IR measurements were performed using a Thermo Nicolet spectrometer (Mod. Nexus). As far as the geopolymer and hybrids specimens, the experiments were carried out by using KBr discs in which few milligrams of the already cured specimens were dispersed. Otherwise, the organic resins were smeared on the surface of a pure KBr disc when they were only partly cured and then kept at room temperature (T = 20 °C) in dry atmosphere in order to complete the crosslinking process. The compressive strength of geopolymer and hybrid specimens was determined using a 500 KN testing machine (MTS mod.810) on 25 × 50 mm (diameter × height) cylindrical specimens prepared into polyethylene molds. The testing was carried out in extension control, with an extension rate of 0.5 mm min −1. Three specimens for each mixture were tested. However, only one stress–strain curve for each type of specimens is shown in the following figures for a better view. A preliminary LCA study was carried out on the hybrid materials. 2.3. Synthetic procedure 2.3.1. Synthesis of resins Two different epoxy resins, named resin 1 and 2, were synthesized and employed. Resin 1 was obtained by homogeneous mixing at room temperature of N,N-diglycidyl-4-glycidyl-oxyaniline (82.0% w/w) and bis(2-aminoethyl)amine (18.0% w/w). Resin 2 was obtained by adding at room temperature N,N-diglycidyl-4-glycidyl-oxyaniline (79.6% w/w) to a mixture of bis-(2-aminoethyl)amine (4.4% w/w) and 2,4-diaminotoluene (16.0% w/w). In both cases, the use of solvents was prevented because the reaction occurs between liquid reagents (the aromatic amine, solid at room temperature, was finely dispersed into the liquid aliphatic one).3 2.3.2. Synthesis of geopolymer The alkaline activating solution was prepared by dissolving solid sodium hydroxide into the sodium silicate solution. The solution was then allowed to equilibrate and cool. The composition of the solution can be expressed as Na2O 1.4SiO2 10.5H2O. Then metakaolin was incorporated to the activating solution with a liquid:solid ratio of 1.4:1 by weight and manually mixed for 15 min. The composition of the whole geopolymeric system can be expressed as Al2O3 3.5SiO2 1.0Na2O 10.4H2O, assuming that geopolymerization occurred at 100%.
2.1. Materials N,N-diglycidyl-4-glycidyl-oxyaniline, bis-(2-aminoethyl)amine, 2,4-diamino-toluene and sodium hydroxide were commercially available (Aldrich, Baker). Reagents were of analytical R grade and used without further purification. Metakaolin, provided by Neuchem S.r.l., has the following composition (%): Al2O3 41.90, SiO2 52.90, K2O 0.77, Fe2O3 1.60, TiO2 1.80, MgO 0.19, CaO 0.17. The sodium silicate solution was supplied by Prochin Italia S.r.l. with the composition of SiO2 27.40 wt.% and Na2O 8.15 wt.%.
2.3.3. Preparation of hybrid composites Before being added to the geopolymeric mixture, both resins have been cured for 45 min at room temperature, i.e. as soon as they started increasing their viscosity and long before their complete crosslinking and hardening (that takes place in about 24 h). In these 3 The reactions have been carried out on limited amounts of reagents (about 20 g) in order to avoid possible combustion events due to the strongly exothermic polymerization reaction.
44
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
conditions, the partly crosslinked polymers obtained were still fluid materials. A 20% w/w of resin was added to the freshly-prepared geopolymeric suspension, and quickly incorporated by a prolonged and accurate mixing by hand in a mortar with a pestle. In this way, hybrid I (geopolymer and resin 1) and hybrid II (geopolymer and resin 2) were obtained. Both hybrids had a homogeneous aspect and started solidifying in few minutes. 2.3.4. Curing treatments All the specimens of geopolymer, hybrids I and II were cured in 99% relative humidity conditions, at room temperature for 7 days (the samples used for the mechanical tests were cured further 21 days in air). Moreover, in order to examine the possible effects of different curing conditions on the thermal behavior of the hybrids, specimens of hybrids I and II were cured at 60 °C for 24 h and then kept at room temperature for 6 days. All treatments have been carried out in 99% relative humidity conditions. 3. Results and discussion 3.1. Synthetic method Hybrids I and II have been prepared by mean of the new synthetic approach proposed in the present paper. This method is based on the incorporation of the resin to the geopolymeric matrix suspension when both polymerization reactions are not yet completed. By following this procedure, a good compatibility between the organic and the aqueous inorganic phases is obtained (see Section 3.2.3) thanks to the numerous hydroxyl tails formed during the epoxy ring opening reaction that make the organic phase “temporarily hydrophilic” increasing the compatibility with the aqueous inorganic phase. The pivotal step of this procedure is the mixing of the two phases when both the polymerization reactions (of the organic and inorganic phases) were started but not yet completed: as a matter of fact, an early mixing of the reagents, when the two organic components were not yet reacted, produced the separation of an organic phase made by the triglycidyl compound, while the amine converged in the aqueous one; on the contrary, a tardy mixing of the two components (the cured organic resin and the geopolymer) strongly reduces their compatibility and the homogeneity of the dispersion of the two phases. For this reason a careful realization of the synthetic procedure is essential to produce innovative and interesting materials. In addition, the choice of the resins has been done in order to obtain an improved compatibility between the organic and inorganic phases. As a matter of fact, instead of the most common commercial diglycidyl (DGEBA, etc.), we have used a triglycidyl that promotes the crosslinking presenting more reactive sites. Moreover, since the ultimate properties (thermal, mechanical, etc.) of the final hybrid materials are strongly affected also by the nature of the resin component, in order to improve their thermal stability we have chosen to realize a resin containing also an aromatic amine. Finally, it is worth pointing out that we have experimented different w/w ratios between the organic and inorganic matrices, ranging from 5 to 30% w/w. Preliminary results indicate that for very low organic resin concentration (5% w/w) the mechanical properties of the hybrid are not significantly affected by the organic resin, since it turned out to be brittle like the neat geopolymer; for the highest resin concentration studied (30% w/w) instead, a significant improvement in the mechanical properties of the material but minor phase segregation phenomena have been observed. In this paper we report only the results obtained in the case of the hybrids containing 80% weight geopolymer and 20% weight resin since this composition turned out to be the best compromise between the necessity of keeping a good compatibility of two, at least in principle, dramatically
different phases and the need to produce a significant improvement on the mechanical properties of the inorganic matrix. It is worth pointing out that, in agreement with the expectations of green chemistry, in the proposed procedure the use of solvents is completely avoided. Preliminary data obtained using LCA methodology suggest that these new materials can be considered “eco-friendly building materials”.
3.2. Characterization 3.2.1. Thermal analysis (TGA/DSC) Simultaneous thermo-gravimetric and differential scanning calorimetry analyses were performed on the geopolymer cured at room temperature and at 60 °C for 24 h and then for 6 days at room temperature, on the resins 1 and 2, and on the hybrids I and II. Fig. 1 shows the weight losses and the DSC thermograms for the unmodified geopolymers. In both cases the weight loss starts at about 30 °C, has a maximum rate around 120–130 °C and is completed at 450 °C. This loss can be attributed to the removal of water molecules adsorbed or differently linked to the silicate molecules (Kong et al., 2007). The overall weight loss is 28% for the geopolymer cured at room temperature while is 24% for the one cured at 60 °C and a combustion residual of 72 and 76% respectively remained at 800 °C. These phenomena are associated with the strong endothermic peaks characterizing the DSC curves centered at 123 °C and 133 °C respectively. The geopolymer cured at 60 °C shows also a broad endothermic peak at about 300 °C, probably due to zeolitic-like water molecules (Knowlton et al., 1981). It is worth pointing out that the variation of enthalpy ΔH associated to the removal of water is in qualitative agreement with the weight loss of the specimen. Moreover, by examining the TGA and DSC curves reported in Fig. 1 it is clear that the different curing temperatures have the only effect of removing part of the water contained in the specimen and of increasing of few degrees the temperature at which the loss of water has been observed. Fig. 2 shows the weight loss and the thermograms for the unmodified resins 1 and 2 cured at room temperature. Both pure resins showed a two-step degradation mechanism in which the most part of the decomposition process is completed below 500 °C and implies a weight loss of about 70%. The degradation is completed at about 700 °C and no combustion residual remained. For resin 1, the initial degradation started at around 320 °C with a high degradation rate. The second degradation step starts at about 500 °C and has a lower degradation rate. As shown by the DSC curve, these two steps are accompanied by two exothermic peaks
Fig. 1. TGA (black curves) and DSC (red curves) of the geopolymer cured at room temperature (continuous lines) and at 60 °C (dashed lines).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Fig. 2. TGA (black curves) and DSC (red curves) of the resin 1 (continuous lines) and resin 2 (dashed lines) cured at room temperature for 7 days.
(the first one centered at 352 °C and the second one centered at 583.5 °C) of opposite extent in respect to the corresponding weight losses (ΔH is 295 J/g and 1930 J/g respectively). Resin 2 showed a very similar behavior, with degradation temperatures only slightly higher than resin 1 (for example, the degradation temperatures at 10% weight loss were found 342 °C for resin 1 and 349 °C for resin 2) possibly due to the presence of the aromatic ring. A substantially identical behavior (not shown) has been observed also for the specimen cured at 60 °C. Figs. 3 and 4 show the weight losses of the hybrids I and II after different curing conditions, respectively. Hybrid I shows a complex degradation mechanism involving three main steps: the first step begins at room temperature and finishes at about 220 °C corresponding to a weight loss of 23%; the second step ends at 406 °C and involves a weight loss of about 11% while the third one ends at 750 °C with a further weight loss of 14%. The combustion residual at 800 °C is around 53%. From the comparison of the TGA and DSC curves for the pure geopolymer and resin reported in Figs. 1 and 2, respectively, the first degradation step can be associated with the loss of water of the geopolymeric matrix while the remaining two correspond to the degradation of the dispersed organic phases. It is worth pointing out that in the hybrid the relative extent of the second and third weight loss steps (around 400 °C and 600 °C, respectively) is inverted in respect to the corresponding phenomena that have been recorded for the pure resin (see Fig. 2). Moreover it is worth noting that the peak temperature of water loss for the hybrid is higher than that of the pure geopolymer (see
45
Fig. 4. TGA (black curves) and DSC (red curves) of the hybrid II cured at room temperature (continuous lines) and at 60 °C (dashed lines).
Fig. 1): probably the polar groups of the resin tend to restrain the water molecules delaying their evaporation. A very similar behavior has been recorded also in the case of hybrid II (see Fig. 4), the most significant difference being the degradation of the resin that takes places in a continuous way up to 600 °C. It is worth pointing out that in both cases the thermal treatment up to 800 °C results in a complete removal of the organic phase from the hybrid (see SEM images in Section 3.2.3). Degradation temperatures and weight losses for all the studied systems are summarized in Table 1. 3.2.2. Dynamic mechanical thermal analysis Fig. 5 shows the dynamic-mechanical properties of the pure resin 1 and of hybrid I cured at room temperature. DMTA measurements are carried out at very low load, in the elastic deformation field. In the explored temperature range, the cured resin (curve 5a) shows a progressive drop of the storage modulus of one order of magnitude that is likely due to the thermal softening. As a result of the high stiffness of the crosslinked resin, the corresponding loss factor (tanδ, curve 5a′) is very small, being always lower than 8 × 10 −2 in all the examined temperature ranges. As far as hybrid I is considered, on account of the geopolymeric matrix, its storage modulus (curve 5b) turns out to be always higher than that of the pure resin and, in the considered temperature range, it decreases less than one order of magnitude. The slight increase of the modulus observed in the region between 175 °C and 225 °C could be due to the completion of the curing process of the resin that maybe, at room temperature, was partly inhibited by the presence of the inorganic components. Moreover, thanks to the higher stiffness of inorganic phase, hybrid I shows a lower drop of the storage modulus in respect to the pure resin. This greater stiffness is the cause also of the lower tanδ values in respect to those of the pure resin 1 (curve 5b′). Table 1 Thermal properties of the geopolymer, resins and hybrids. Weight loss Curing temperature starting temperature (°C) (°C)
Fig. 3. TGA (black curves) and DSC (red curves) of the hybrid I cured at room temperature (continuous lines) and at 60 °C (dashed lines).
Geopolymer 25 60 Resin 1 25 Resin 2 25 Hybrid I 25 60 Hybrid II 25 60
30 30 260 290 30 30 30 30
Temperature at 10% weight loss (°C)
Weight loss ending temperature (°C)
Combustion residual at 800 °C (weight %)
101 120 342 349 109 140 112 154
450 450 700 670 750 670 640 655
72 76 0 0 53 57 54 59
46
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Fig. 5. Storage moduli (black curves) and mechanical loss tangents (tanδ, circle graphs) of resin 1 (continuous black curve and red graph) and of hybrid I (dashed black curve and blue graph) both cured for 7 days at room temperature.
Finally, it is worth pointing out that, at variance with the pure geopolymer that is very brittle at room temperature (Zhao et al., 2007), thanks to the presence of the organic resin, hybrid I is significantly less brittle. As a matter of fact, while the neat geopolymer specimen is broken at room temperature during the test (carried out with the same condition of load used for the hybrid systems and resins), the hybrid specimen is safe up to 220 °C. 3.2.3. Microstructural analysis Fig. 6 shows micrographs of freshly obtained fracture surfaces of geopolymer and hybrids I and II cured at room temperature. All the specimens show an amorphous structure (few crystals can be seen only in the geopolymer specimen, see Fig. 6a′, as confirmed by X-ray diffraction analysis of the starting metakaolin) indicating that the geopolymerization process has been successfully carried out in all cases. Moreover, a compact and homogeneous morphology is clearly observed for all the specimens. In particular, from the examination of images 6b, b′ and 6c, c′ referring to hybrids I and II, a very good homogeneity and uniformity of the microdispersion of the organic phase in the inorganic one is evident. The dimension of the resin particles is in the range of 1–20 μm. For all the specimens examined, no agglomerations phenomena were observed. It is worth pointing out that inorganic–organic hybrids (20%/80% w/w) described in literature (Hussain et al., 2004, 2005) are characterized by a dispersed phase of millimetric dimension. This result is even more interesting if we consider the fact that this uniform dispersion at micrometric level was obtained just by simple manual mixing. It is likely to be expected that a fine tailoring of the average diameter of the microdispersed organic phase (that reasonably influences the mechanical properties of the hybrids) can be obtained simply by mechanically controlling the mixing step. In addition, in all cases, the strict adhesion between the phases is evident: in particular in the case of the hybrid II (image 6c and 6c′), the interaction between the geopolymer matrix and the organic resin microsphere is so good that the particles of resin have been scratched when the specimens were broken to prepare the SEM samples. A more detailed investigation of the interphase region between organic and inorganic phases confirming this close interaction has been carried out by energy-dispersive X-ray spectroscopy (EDS) analysis on hybrid II and shown in Fig. 7 and Table 2. In particular, we have recorded EDS spectra on i) the region of a polymeric particle that, up to 5000 magnifications, does not present visible traces or grains of the inorganic phase, but shows traces of Si atoms (region 1 of Fig. 7, spectrum 1 of Table 2); ii) the region around the polymer particle (i.e. the cavity obtained by the removing of the polymer
particle) that, up to 5000 magnifications, does not present visible traces or grains of polymer, but reveals the presence of C atoms (region 2 of Fig. 7, spectrum 2 of Table 2); iii) the area between the polymer particles and the geopolymeric matrix in which the analysis pointed out a significant presence of all the elements revealed (region 3 of Fig. 7, spectrum 3 of Table 3). These observations confirm the close interaction between the different phases. Moreover, comparing Fig. 6a, a′, b, b′ and c, c′ it can be easily seen that the number and the extension of the microcracks that characterize the fracture surface of the geopolymer specimen is strongly reduced by adding the organic resin. Therefore, in agreement with what observed by DMTA analysis (described in Section 3.2.2) and with the compressive strength determinations (described in Section 3.2.5), the resin seems to prevent the cracking growth and propagation improving the mechanical properties and enhancing the fracture toughness of the brittle inorganic matrix. As a matter of fact, a sort of crack deflection mechanism typical of particle reinforced ceramic matrix composites could be expected to occur (Boccaccini et al., 1997; Monette and Anderson, 1993). Finally, Fig. 6d and d′ show the images of a specimen of hybrid I after thermal treatment at 800 °C for 1 h. It is apparent that, in good agreement with the TGA results shown in Section 3.2.1, the organic phase has been completely removed. In this way a macroporous material characterized by uniformly dispersed pores of very similar diameter (see Fig. 6d′) has been obtained. Analogous results (not reported in this paper) have been obtained also for the specimens cured at 60 °C. 3.2.4. FT-IR analysis FT-IR spectra of the starting metakaolin, of the geopolymer, the two resins and of the two hybrids are shown in Fig. 8. The FTIR spectrum of the geopolymer (curve 8b) is a typical one, with broad bands at about 3430 cm −1 and 1635 cm −1 due to O\H stretching and bending modes of adsorbed molecular water, with a strong Si\O stretching vibration at 1050 cm −1, which is lower in wave number than that of the metakaolin (curve 8a), indicating the condensation of Si\O tetrahedra in geopolymer. It is worth pointing out that the characteristic metakaolin Si\O\Al bond at 810 cm −1, after the geopolymerization, is replaced by several weaker bands in the range from 600 cm −1 to 800 cm −1. Finally, the signal at about 460 cm −1 is due to Si\O bending vibration. (Aronne et al., 2002; Catauro et al., 2003, 2004; Clayden et al., 1999; Felahi et al., 2001; Ortego and Barroeta, 1991; Wang et al., 2005; Zhang et al., 2008a, 2008b). The FT-IR spectra of resins 1 and 2 (curves 8c and 8d respectively) are very similar to each other. For both resins, the presence of some unreacted \NH groups is revealed by the broad peak at around 3350 cm −1 (due to the \NH stretching) and at around 1515 cm −1 (due to \NH bending). This last band, together with that at 830 cm −1, can be also attributed to p-disubstituted benzene rings. The signals in the wavenumber range of 2920–2820 cm −1 and at about 1460 cm −1 are due to \CH2\ symmetric and asymmetric stretching and bending, respectively. Finally, signals in the region of 1300–1050 cm −1 can be assigned to C\N, C\C and C\O stretching. In addition, the absence of bands at 971, 917 and 775 cm −1 due to terminal epoxy rings reveals a successful curing process (Felahi et al., 2001; Hummel and Scholl, 1971). As far as the hybrids, their spectra (curve 8e: hybrid I; curve 8f: hybrid II) are characterized by the main bands of the pure organic and inorganic components. In particular a strong absorption band at about 3430 cm −1 and 1640 cm −1 due to water, at about 1050 cm −1 due to Si\O stretching and at about 450 cm −1 due to Si\O bending vibration can be observed. The bands in the region of 800–600 cm −1 are associated to Si\O\Al vibrations (Barbosa and Mackenzie, 2003; Frost et al., 1996; Parker and Frost, 1996; Zaharaki et al., 2010).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
47
Fig. 6. SEM micrographs (amplification is 5 × 102 in a, b, c, d and is 5 × 103 in a′, b′, c′, d′) of the specimens: a, a′) geopolymer cured at room temperature for 24 h; b, b′) hybrid I cured at room temperature; c, c′) hybrid II cured at room temperature for 24 h; d, d′) hybrid I hold at 800 °C for 1 h.
48
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50 Table 3 Average compressive strength, average stress at a 0.015 strain (σ0.015) and average stress at a 0.018 strain (σ0.018) of specimens.
Geopolymer Hybrid I Hybrid II
Compressive strength (MPa)
σ0.015 (MPa)
σ0.018 (MPa)
8.2 28.2 29.0
0.0 21.4 20.6
0.0 15.4 16.9
could absorb an aliquot of the load by plastic deformation; and on the other side it could perform a toughening effect by a typical crack deviation mechanism (Swain, 1989), as evidenced also by SEM observations (see Fig. 6).
Fig. 7. SEM micrographs (amplification 5 × 103) of the hybrid II cured at room temperature for 24 h. The regions selected for the EDS analysis are indicated by white rectangles. The chemical characterization for each region is reported in Table 2.
3.2.5. Compressive strength determination The average compressive strengths and the average stress retained by the specimens at a deformation of 0.015 and 0.018 of the geopolymer, hybrid I and hybrid II specimens are given in Table 3. It can be seen that hybrid specimens have higher strength than the geopolymer one. The addition of 20% resin results in compressive strengths which are 3.4 and 3.5 times higher than that of the geopolymer specimen, respectively for resin 1 and resin 2. This effect may be likely explained by considering that metakaolin based geopolymer is subjected to microcracking caused by the evaporation of the high amount of water added with the activating solution. The incorporation of the resin has an effect of immobilizing water molecules and effectively postpone the water evaporation (Zhang et al., 2010a, 2010b) as evidenced by thermal analysis (see Figs. 3 and 4). Significant are also the differences of the mechanical behaviors of the geopolymer specimen in respect to the hybrid ones, which are illustrated in Fig. 9. The geopolymer specimen shows a typical brittle ceramic behavior and fracture at a strain lower than 0.01. The hybrid specimens show a gradual decrease of the load with increasing displacement, after the maximum is reached, which denotes a sort of pseudo-plasticity behavior. As evidenced by the data reported in Table 3, hybrid specimens I and II still retain 76% and 71% of the compressive strength, respectively, at a deformation of 0.015 and retain 55% and 58%, respectively, at a deformation of 0.018. The hybrid specimens show progressive fracture behavior rather than a catastrophic one and consequent higher fracture energy. This behavior may be likely explained by considering the propagations of cracks proceeding in a progressive and controlled way with increasing strain (Li and Xu, 2009) owing to the presence of the resin phase. The organic phase may play a dual role: on one side it
Fig. 8. FT-IR spectra of a) metakaolin, b) geopolymer, c) resin 1, d) resin 2, e) hybrid I and f) hybrid II.
Table 2 Chemical characterization of hybrid II cured at room temperature for 24 h obtained by energy-dispersive X-ray spectroscopy (EDS) in the regions indicated by white rectangles shown in Fig. 7. All the elements have been analyzed and normalized. All results are in weight%. Spectrum
In stats.
C
O
Na
Al
Si
Total
Spectrum 1 Spectrum 2 Spectrum 3
Yes Yes Yes
65.54 8.14 20.73
29.15 18.65 49.67
1.49 7.89 6.64
1.48 21.16 8.09
2.35 44.16 14.87
100.00 100.00 100.00
Fig. 9. Mechanical behavior of the geopolymer (black curve), hybrid I (dashed red curve) and hybrid II (dotted blue curve) specimens.
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
4. Conclusions Through an innovative synthetic approach based on a co-reticulation in mild conditions of epoxy based organic resins and an MK-based geopolymer inorganic matrix, novel hybrid organic–inorganic materials were prepared. A high compatibility between the organic and inorganic phases, even at appreciable concentration of resin (20% w/w), was realized up to micrometric level. A good and homogeneous dispersion (without the formation of agglomerates) of the organic particles was obtained just by hand mixing. These new materials show good technological properties: in particular, in respect to the neat geopolymer, they present significantly enhanced compressive strengths and toughness. By comparing our novel hybrid organic–inorganic composite materials with traditional geopolymer it is worth pointing out that: i) in agreement with the expectations of green chemistry, in the proposed synthetic procedure the use of solvents is completely avoided; ii) the strength of our material is 3.5 times higher than geopolymer control specimens; iii) hybrid materials show higher deformation before cracking. From an environmental point of view this means that it is possible to save material, to use smaller section for the same load condition, to reduce the number of cracks obtaining more durability and so a longer service life. Starting from this consideration we can conclude that the hybrid composite materials described in the present paper seem to have all the conditions to be an environmental friendly material. Finally, the possible use of wastes coming from different activities (mining, metallurgic, municipal, construction and demolition) instead of a raw material (metakaolin) for the obtainment of hybrid geopolimeric materials with our new synthetic approach could further reduce the environmental impact of the material we have studied. A more complete technological characterization, including the examination of the structural integrity of the material studied under freeze-thaw cycles, together with other technological characterizations (flame resistance, porosimetry, mechanical properties and so on) will be carried out in a subsequent paper. Acknowledgment Thanks are due to Prof. Antonio Roviello and Prof. Finizia Auriemma of the Università degli Studi di Napoli Federico II for useful discussions and suggestions. References Andini, S., Cioffi, R., Colangelo, F., Grieco, T., Montagnaro, F., Santoro, L., 2008. Coal fly ash as raw material for the manufacture of geopolymer-based products. Waste Management 28, 416–423. Andini, S., Cioffi, R., Colangelo, F., Ferone, C., Montagnaro, F., Santoro, L., 2010. Characterization of geopolymer materials containing MSWI fly ash and coal fly ash. Advances in Science and Technology 69, 123–128. Aronne, A., Esposito, S., Ferone, C., Pansini, M., Pernice, P., 2002. FTIR study of the thermal transformation of barium exchanged zeolite A to celsian. Journal of Materials Chemistry 12 (10), 3039–3045. Barbosa, V.F.F., MacKenzie, K.J.D., 2003. Thermal behavior of inorganic geopolymer and composites derived from sodium polysialate. Materials Research Bulletin 38, 319–331. Boccaccini, A.R., Bücker, M., Bossert, J., Marszalek, K., 1997. Glass matrix composites from coal fly ash and waste glass. Waste Management 17, 39–45. Buchwald, A., Hohmann, M., Posern, K., Brendler, E., 2009. The suitability of thermally activated illite/smectite clay as raw material for geopolymer binders. Applied Clay Science 3, 300–304. Catauro, M., Raucci, M.G., De Gaetano, F., Marotta, A., 2003. Sol–gel synthesis, characterization and bioactivity of polycaprolactone/SiO2 hybrid material. Journal of Materials Science 38, 3097–3102. Catauro, M., Raucci, M.G., de Gaetano, F., Buri, A., Marotta, A., Ambrosio, L., 2004. Sol– gel synthesis, structure and bioactivity of polycaprolactone/CaO • SiO2 hybrid material. Journal of Materials Science. Materials in Medicine 15, 991–995. Cioffi, R., Maffucci, L., Santoro, L., 2003. Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue. Resources, Conservation and Recycling 40, 27–38.
49
Clayden, N.J., Esposito, S., Aronne, A., Pernice, P., 1999. Solid state 27Al NMR and FTIR study of lanthanum aluminosilicate glasses. Journal of Non-Crystalline Solids 258, 11–19. Davidovits, J., 1991. Geopolymers: inorganic polymeric new materials. Journal of Thermal Analysis 37, 1633–1656. De Santis, R., Catauro, M., Di Lucy, S., Manto, L., Raucci, M.G., Ambrosio, L., Nicolais, L., 2007. Effects of polymer amount and processing conditions on the in vitro behaviour of hybrid titanium dioxide/polycaprolactone composites. Biomaterials 28, 2801–2809. Dias, D.P., Thaumaturgo, C., 2005. Fracture toughness of geopolymer concretes reinforced with basalt fibers. Cement and Concrete Composites 27, 49–54. Duxson, P., Fernandez-Jimenez, A., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S.J., 2007. Geopolymer technology: the current state of the art. Journal of Material Science 42, 2917–2933. Felahi, S., Chikhi, N., Bakar, M., 2001. Modification of epoxy resin with kaolin as a toughening agent. Journal of Applied Polymer Science 82, 861–878. Ferone, C., Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2011. Mechanical performances of weathered coal fly ash based geopolymer bricks. Procedia Engineering 21, 745–752. Ferone, C., Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2012. Use of reservoir clay sediments as raw materials for geopolymer binders. Advances in Applied Ceramics. http://dx.doi.org/10.1179/1743676112Y.0000000064. Frost, R.L., Fredericks, P.M., Shurvell, H.F., 1996. Raman microscopy of some kaolinite clay minerals. Canadian Journal of Applied Spectroscopy 41, 11–14. Habert, G., d'Espinose de Lacaillerie, J.B., Roussel, N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. Journal of Cleaner Production 19, 1229–1238. Hummel, D.O., Scholl, F., 1971. Infrared Analysis of Polymers, Resins and Additives An Atlas, vol. 1. Wiley Interscience, NewYork. Hussain, M., Varley, R.J., Cheng, Y.B., Simon, G.P., 2004. Investigation of thermal and fire performance of novel hybrid geopolymer composites. Journal of Material Science 39, 4721–4726. Hussain, M., Varley, R.J., Cheng, Y.B., Mathys, Z., Simon, G.P., 2005. Synthesis and thermal behaviour of inorganic–organic hybrid geopolymer composites. Journal of Applied Polymer Science 96, 112–121. IUPAC, 1997. Compendium of chemical terminology. (the “Gold Book”) Compiled by A. D. McNaught and A. Wilkinson2nd ed. Blackwell Scientific Publications, Oxford. (XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. http://dx.doi.org/10.1351/goldbook). Kickelbick, G., 2007. Introduction to Hybrid Materials. In: Kickelbick, G. (Ed.), Hybrid Materials: Synthesis, Characterization, and Applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. http://dx.doi.org/10.1002/9783527610495.ch1. Knowlton, G.D., White, T.R., Laurence McKague, H., 1981. Thermal study of types of water associated with clinoptilolite. Clays and Clay Minerals 29 (5), 403–411. Komnitsas, K., 2011. Potential of geopolymer technology towards green buildings and sustainable cities. Procedia Engineering 21, 1023–1032. Kong, D.L.Y., Sanjayan, J.G., Sagoe-Crentsil, K., 2007. Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures. Cement and Concrete Research 37, 1583–1589. Li, W., Xu, J., 2009. Mechanical properties of basalt fiber reinforced geopolymeric concrete under impact loading. Materials Science and Engineering A 505, 178–186. Lin, T., Jia, D., He, P., Wang, M., Liang, D., 2008. Effects of fiber length on mechanical properties and fracture behavior of short carbon fiber reinforced geopolymer matrix composites. Materials Science and Engineering A 497, 181–185. Menna, C., Asprone, D., Ferone, C., Colangelo, F., Balsamo, A., Prota, A., Cioffi, R., Manfredi, G., 2013. Use of geopolymers for composite external reinforcement of RC members. Composites Part B 45, 1667–1676. http://dx.doi.org/10.1016/ j.compositesb.2012.09.019. Monette, L., Anderson, M.P., 1993. Effect of particle modulus and toughness on strength and toughness in brittle particulate composites. Scripta Metallurgica et Materialia 28, 1095–1100. Ortego, J.D., Barroeta, Y., 1991. Leaching effects on silicate polymerization, a FTIR and 29Si NMR study of lead and zinc in Portland cement. Environmental Science and Technology 25, 1171–1174. Parker, R.W., Frost, R.L., 1996. The application of drift spectroscopy to the multicomponent analysis of organic chemicals adsorbed on montmorillonite. Clays and Clay Minerals 44, 32–39. Provis, J.L., Duxon, P., van Deventer, J.S.J., 2010. The role of particle technology in developing sustainable construction materials. Advanced Powder Technology 21, 2–7. Sathonsaowaphak, A., Chindaprasirt, P., Pimraksab, K., 2009. Workability and strength of lignite bottom ash geopolymer mortar. Journal of Hazardous Materials 168, 44–50. Shi, C., Krivenko, P.V., Roy, D.M., 2006. Alkali-Activated Cements and Concretes. Taylor and Francis, Abington, UK. Swain, M.V., 1989. Toughening mechanism for ceramics. Materials Forum 13, 237–253. Tiesong, L., Dechang, J., Peigang, H., Meirong, W., Defu, L., 2008. Effects of fiber length on mechanical properties and fracture behavior of short carbon fiber reinforced geopolymer matrix composites. Materials Science and Engineering A 497, 181–185. Wang, H., Li, H., Yan, F., 2005. Synthesis and mechanical properties of metakaolinite based geopolymer. Colloids and Surfaces A: Physicochemical and Engineering Aspects 268, 1–6. Xu, H., van Deventer, J.S.J., 2000. The geopolymerisation of alumino-silicate minerals. International Journal of Mineral Processing 59, 247–266.
50
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Zaharaki, D., Komnitsas, K., Perdikatsis, V., 2010. Use of analytical techniques for identification of inorganic polymer gel composition. Journal of Materials Science 45, 2715–2724. Zhang, Y., Sun, W., Li, Z., 2006. Impact behavior and microstructural characteristics of PVA fiber reinforced fly ash-geopolymer boards prepared by extrusion technique. Journal of Materials Science 41, 2787–2794. Zhang, Y., Sun, W., Li, Z., 2008a. Infrared spectroscopy study of structural nature of geopolymeric products. Journal of Wuhan University of Technology-Material Science Edition 23, 522–527. Zhang, Y., Sun, W., Li, Z., Zhou, X., Eddie, C.C., 2008b. Impact properties of geopolymer based extrudates incorporated with fly ash and PVA short fiber. Construction and Building Materials 22, 370–383.
Zhang, Y.J., Li, S., Xu, D.L., Wang, B.Q., Xu, G.M., Yang, D.F., Wang, N., Liu, H.C., Wang, Y.C., 2010a. A novel method for preparation of organic resins reinforced geopolymer composites. Journal of Material Science 45, 1189–1192. Zhang, Y.J., Wang, Y.C., Xu, D.L., Li, S., 2010b. Mechanical performance and hydration mechanism of geopolymer composite reinforced by resin. Materials Science and Engineering A 527, 6574–6580. Zhao, Q., Nair, B., Rahimian, T., Balaguru, P., 2007. Novel geopolymer based composites with enhanced ductility. Journal of Materials Science 42, 3131–3137.
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Novel hybrid organic-geopolymer materials Claudio Ferone a, Giuseppina Roviello a,⁎, Francesco Colangelo a, Raffaele Cioffi a, Oreste Tarallo b a b
Dipartimento per le Tecnologie, Facoltà di Ingegneria, Università di Napoli ‘Parthenope’, INSTM Research Group Napoli Parthenope, Centro Direzionale Napoli, Isola C4, 80143 Napoli, Italy Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, 80126 Napoli, Italy
a r t i c l e
i n f o
Article history: Received 23 April 2012 Received in revised form 18 October 2012 Accepted 2 November 2012 Available online 04 December 2012 Keywords: Hybrid geopolymer Epoxy resin Metakaolin
a b s t r a c t Novel hybrid organic–inorganic materials were prepared through an innovative synthetic approach based on a co-reticulation in mild conditions of epoxy based organic resins and an MK-based geopolymer inorganic matrix. A high compatibility between the organic and inorganic phases, even at appreciable concentration of resin, was realized up to micrometric level. A good and homogeneous dispersion (without the formation of agglomerates) of the organic particles was obtained just by hand mixing. These new materials present significantly enhanced compressive strengths and toughness in respect to the neat geopolymer allowing a wider utilization of these materials for structural applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Geopolymers represent an innovative class of ceramic materials 1 characterized by advanced technological properties, as well as low manufacturing energy consumption for construction purposes and engineering applications. Geopolymers are usually obtained through inexpensive and ecofriendly synthetic procedures with low waste gas emission (Duxson et al., 2007; Habert et al., 2011; Komnitsas, 2011; Provis et al., 2010). For these reasons they are considered “green materials”. In the field of civil engineering, geopolymer-based materials are also referred to as “alkali-activated cements” or “chemically-bonded ceramics” which can be obtained from raw materials with low (or zero) CaO content, such as metakaolin (Cioffi et al., 2003), clay (Buchwald et al., 2009; Ferone et al., 2012) and other natural silico-aluminates (Xu and van Deventer, 2000) as well as industrial process wastes such as coal fly ash (Andini et al., 2008, 2010; Ferone et al., 2011), lignite bottom ash (Sathonsaowaphak et al., 2009) and metallurgical slag (Shi et al., 2006). The synthesis of geopolymers can be carried out by mixing reactive silico-aluminate materials with strongly alkaline solutions such as alkali metal (Na, K) hydroxide or silicate. In such reaction environment the silico-aluminate reactive materials are rapidly dissolved. A complex reaction mechanism follows, in which solubilized silica and alumina condensate with the ultimate formation of a three-dimensional geopolymeric network (Davidovits, 1991). This phase is crucial in relation to the final product properties ⁎ Corresponding author. Tel.: +39 081 5476781; fax: +39 081 5476777. E-mail address: [email protected] (G. Roviello). 1 “A ceramic is a nonmetallic, inorganic solid” (Kingery et al., Introduction to Ceramics, Wiley & Sons, 1976). Geopolymers can be properly considered ceramics since they are a class of synthetic aluminosilicate materials presenting an amorphous three-dimensional structure similar to that of an aluminosilicate glass. 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2012.11.001
that strongly depend on the degree of cross linking among the different silico-aluminate polymeric chains. Geopolymers synthesized at temperature lower than 90 °C are amorphous, while zeolite-like crystalline products are obtained at 150–200 °C (Davidovits, 1991). These materials show excellent mechanical properties, low shrinkage, thermal stability, freeze-thaw, acid and fire resistance, long term durability and recyclability, so the application of geopolymer-based materials covers many fields. On the other hand geopolymers are ceramic materials, so they present a typical brittle mechanical behavior with the consequent low ductility and low fracture toughness. This behavior may represent a great limit in several structural applications. This drawback can be overcome by producing geopolymer matrix composites. A great deal of studies (Barbosa and MacKenzie, 2003; Dias and Thaumaturgo, 2005; Li and Xu, 2009; Lin et al., 2008; Menna et al., 2013; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007) was produced about this topic and many types of fillers were tested, such as particulate and various kinds of short and continuous fibers. For example, polyvinyl alcohol, polypropylene, basalt and carbon short fibers were employed as additives to improve geopolymeric mechanical performances. In fact fibers provide a control of cracking by a bridging action during both micro and macrocracking of the matrix thus increasing the fracture toughness of the brittle matrix (Dias and Thaumaturgo, 2005; Li and Xu, 2009; Tiesong et al., 2008; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007). It is worth pointing out that these composites are usually obtained by a physical blending of the two components, the filler being added as a finely divided powder or as an emulsion, 2 2 Sometimes, to obtain a closely mix of the two different phases, polymers rather soluble in water, such as sodium polyacrylate (PAANa), polyacrylic acid (PAA), polyacrylamide (PAm), polyvinyl alcohol (PVA), polyethylene glycol (PEG), were used (Xu and van Deventer, 2000).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
sometimes in the presence of compatibilizers (Dias and Thaumaturgo, 2005; Li and Xu, 2009; Tiesong et al., 2008; Zhang et al., 2006, 2008a, 2008b, 2010a, 2010b; Zhao et al., 2007). Recently, a new class of geopolymeric composites with organic matrix has been developed (Hussain et al., 2004, 2005) with the main aim to improve the fire resistance of organic polymers and to reduce the smoke production arising from their burning. These composites are also named “hybrid” inorganic–organic composites (i.e. a material composed of an intimate mixture of inorganic and organic components, where the components usually interpenetrate on scales of less than 1 μm. IUPAC, 1997). The new class of composites described by Hussain has been obtained by incorporating the geopolymer into the cross-linked polymeric structure, tailoring the chemical compositions of the components. In particular, a bi-functional epoxy resin, Diglycidyl Ether of Bisphenol A (DGEBA), was mixed with a small amount of the geopolymer (20% weight fraction) in the presence of a curing agent (Hussain et al., 2004, 2005). In this way a hybrid material with excellent mechanical properties and improved fire resistance was obtained. The development of hybrid materials represents an extremely interesting and relevant research field for the potentially useful physical properties that could derive from the interfacial interactions of two chemically incompatible phases (De Santis et al., 2007). By tuning the chemical composition of the components, it is feasible to realize different kinds of materials, with different properties, whose applications depend on the ratio between the organic and inorganic phases (Kickelbick, 2007). In this paper an innovative strategy for the chemical incorporation of an appreciable amount of organic polymer into an MK-based geopolymeric matrix, that allows the preparation of novel geopolymer based hybrid materials, is described. The new proposed approach is based on the mixing of a partly crosslinked epoxy resin of tailored composition to a geopolymeric suspension, when both polymerization reactions are not yet completed. In this way, an improved compatibility between the organic and the aqueous inorganic phases is easily realized, affording a homogeneous and stable in time dispersion of the organic microdomains into the inorganic phase, without addition of external additives, even at appreciable concentration of resin. By using this new method, novel organic–inorganic hybrid materials have been prepared and characterized through scanning electron microscopy (SEM), thermal analysis (TGA, DSC), FT-IR spectrometry, dynamic mechanical thermal analysis (DMTA) and compressive strength determination. These new materials present interesting mechanical properties and, in particular, a considerably reduced brittleness in respect to the geopolymeric matrix. For these reasons these hybrid materials could be used for all the applications in which the use of the pure geopolymer is limited by its brittleness such as the realization of thermo-insulating or thermo-resistant panels with improved tenacity. Moreover, these materials can show more useful rheological (tixotropicity) properties in the field of restoration and repair of damaged concrete and masonry.
2. Materials and methods
43
2.2. Analytical methods Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed by a TA Instrument SDT2960 simultaneous DSC–TGA under air flow at a heating rate of 10 °C/min using 10–15 mg of the powdered specimen stored at room temperature for one week. DMTA were performed by using a Triton Technology Ltd instrument, Tritec 2000 DMA. The specimens were clamped in the single cantilever mode. DMTA analyses were performed from room temperature to 260 °C at 2 °C/min using a 1 Hz frequency and having a 1% strain. SEM analysis was carried out by means of a FEI Quanta 200 FEG microscope. EDS analyses were performed by using an Energy Dispersion Spectrometer Oxford Inca Energy System 250 equipped with INCAx-act LN2-free detector, working at 20 kV voltage. FT-IR measurements were performed using a Thermo Nicolet spectrometer (Mod. Nexus). As far as the geopolymer and hybrids specimens, the experiments were carried out by using KBr discs in which few milligrams of the already cured specimens were dispersed. Otherwise, the organic resins were smeared on the surface of a pure KBr disc when they were only partly cured and then kept at room temperature (T = 20 °C) in dry atmosphere in order to complete the crosslinking process. The compressive strength of geopolymer and hybrid specimens was determined using a 500 KN testing machine (MTS mod.810) on 25 × 50 mm (diameter × height) cylindrical specimens prepared into polyethylene molds. The testing was carried out in extension control, with an extension rate of 0.5 mm min −1. Three specimens for each mixture were tested. However, only one stress–strain curve for each type of specimens is shown in the following figures for a better view. A preliminary LCA study was carried out on the hybrid materials. 2.3. Synthetic procedure 2.3.1. Synthesis of resins Two different epoxy resins, named resin 1 and 2, were synthesized and employed. Resin 1 was obtained by homogeneous mixing at room temperature of N,N-diglycidyl-4-glycidyl-oxyaniline (82.0% w/w) and bis(2-aminoethyl)amine (18.0% w/w). Resin 2 was obtained by adding at room temperature N,N-diglycidyl-4-glycidyl-oxyaniline (79.6% w/w) to a mixture of bis-(2-aminoethyl)amine (4.4% w/w) and 2,4-diaminotoluene (16.0% w/w). In both cases, the use of solvents was prevented because the reaction occurs between liquid reagents (the aromatic amine, solid at room temperature, was finely dispersed into the liquid aliphatic one).3 2.3.2. Synthesis of geopolymer The alkaline activating solution was prepared by dissolving solid sodium hydroxide into the sodium silicate solution. The solution was then allowed to equilibrate and cool. The composition of the solution can be expressed as Na2O 1.4SiO2 10.5H2O. Then metakaolin was incorporated to the activating solution with a liquid:solid ratio of 1.4:1 by weight and manually mixed for 15 min. The composition of the whole geopolymeric system can be expressed as Al2O3 3.5SiO2 1.0Na2O 10.4H2O, assuming that geopolymerization occurred at 100%.
2.1. Materials N,N-diglycidyl-4-glycidyl-oxyaniline, bis-(2-aminoethyl)amine, 2,4-diamino-toluene and sodium hydroxide were commercially available (Aldrich, Baker). Reagents were of analytical R grade and used without further purification. Metakaolin, provided by Neuchem S.r.l., has the following composition (%): Al2O3 41.90, SiO2 52.90, K2O 0.77, Fe2O3 1.60, TiO2 1.80, MgO 0.19, CaO 0.17. The sodium silicate solution was supplied by Prochin Italia S.r.l. with the composition of SiO2 27.40 wt.% and Na2O 8.15 wt.%.
2.3.3. Preparation of hybrid composites Before being added to the geopolymeric mixture, both resins have been cured for 45 min at room temperature, i.e. as soon as they started increasing their viscosity and long before their complete crosslinking and hardening (that takes place in about 24 h). In these 3 The reactions have been carried out on limited amounts of reagents (about 20 g) in order to avoid possible combustion events due to the strongly exothermic polymerization reaction.
44
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
conditions, the partly crosslinked polymers obtained were still fluid materials. A 20% w/w of resin was added to the freshly-prepared geopolymeric suspension, and quickly incorporated by a prolonged and accurate mixing by hand in a mortar with a pestle. In this way, hybrid I (geopolymer and resin 1) and hybrid II (geopolymer and resin 2) were obtained. Both hybrids had a homogeneous aspect and started solidifying in few minutes. 2.3.4. Curing treatments All the specimens of geopolymer, hybrids I and II were cured in 99% relative humidity conditions, at room temperature for 7 days (the samples used for the mechanical tests were cured further 21 days in air). Moreover, in order to examine the possible effects of different curing conditions on the thermal behavior of the hybrids, specimens of hybrids I and II were cured at 60 °C for 24 h and then kept at room temperature for 6 days. All treatments have been carried out in 99% relative humidity conditions. 3. Results and discussion 3.1. Synthetic method Hybrids I and II have been prepared by mean of the new synthetic approach proposed in the present paper. This method is based on the incorporation of the resin to the geopolymeric matrix suspension when both polymerization reactions are not yet completed. By following this procedure, a good compatibility between the organic and the aqueous inorganic phases is obtained (see Section 3.2.3) thanks to the numerous hydroxyl tails formed during the epoxy ring opening reaction that make the organic phase “temporarily hydrophilic” increasing the compatibility with the aqueous inorganic phase. The pivotal step of this procedure is the mixing of the two phases when both the polymerization reactions (of the organic and inorganic phases) were started but not yet completed: as a matter of fact, an early mixing of the reagents, when the two organic components were not yet reacted, produced the separation of an organic phase made by the triglycidyl compound, while the amine converged in the aqueous one; on the contrary, a tardy mixing of the two components (the cured organic resin and the geopolymer) strongly reduces their compatibility and the homogeneity of the dispersion of the two phases. For this reason a careful realization of the synthetic procedure is essential to produce innovative and interesting materials. In addition, the choice of the resins has been done in order to obtain an improved compatibility between the organic and inorganic phases. As a matter of fact, instead of the most common commercial diglycidyl (DGEBA, etc.), we have used a triglycidyl that promotes the crosslinking presenting more reactive sites. Moreover, since the ultimate properties (thermal, mechanical, etc.) of the final hybrid materials are strongly affected also by the nature of the resin component, in order to improve their thermal stability we have chosen to realize a resin containing also an aromatic amine. Finally, it is worth pointing out that we have experimented different w/w ratios between the organic and inorganic matrices, ranging from 5 to 30% w/w. Preliminary results indicate that for very low organic resin concentration (5% w/w) the mechanical properties of the hybrid are not significantly affected by the organic resin, since it turned out to be brittle like the neat geopolymer; for the highest resin concentration studied (30% w/w) instead, a significant improvement in the mechanical properties of the material but minor phase segregation phenomena have been observed. In this paper we report only the results obtained in the case of the hybrids containing 80% weight geopolymer and 20% weight resin since this composition turned out to be the best compromise between the necessity of keeping a good compatibility of two, at least in principle, dramatically
different phases and the need to produce a significant improvement on the mechanical properties of the inorganic matrix. It is worth pointing out that, in agreement with the expectations of green chemistry, in the proposed procedure the use of solvents is completely avoided. Preliminary data obtained using LCA methodology suggest that these new materials can be considered “eco-friendly building materials”.
3.2. Characterization 3.2.1. Thermal analysis (TGA/DSC) Simultaneous thermo-gravimetric and differential scanning calorimetry analyses were performed on the geopolymer cured at room temperature and at 60 °C for 24 h and then for 6 days at room temperature, on the resins 1 and 2, and on the hybrids I and II. Fig. 1 shows the weight losses and the DSC thermograms for the unmodified geopolymers. In both cases the weight loss starts at about 30 °C, has a maximum rate around 120–130 °C and is completed at 450 °C. This loss can be attributed to the removal of water molecules adsorbed or differently linked to the silicate molecules (Kong et al., 2007). The overall weight loss is 28% for the geopolymer cured at room temperature while is 24% for the one cured at 60 °C and a combustion residual of 72 and 76% respectively remained at 800 °C. These phenomena are associated with the strong endothermic peaks characterizing the DSC curves centered at 123 °C and 133 °C respectively. The geopolymer cured at 60 °C shows also a broad endothermic peak at about 300 °C, probably due to zeolitic-like water molecules (Knowlton et al., 1981). It is worth pointing out that the variation of enthalpy ΔH associated to the removal of water is in qualitative agreement with the weight loss of the specimen. Moreover, by examining the TGA and DSC curves reported in Fig. 1 it is clear that the different curing temperatures have the only effect of removing part of the water contained in the specimen and of increasing of few degrees the temperature at which the loss of water has been observed. Fig. 2 shows the weight loss and the thermograms for the unmodified resins 1 and 2 cured at room temperature. Both pure resins showed a two-step degradation mechanism in which the most part of the decomposition process is completed below 500 °C and implies a weight loss of about 70%. The degradation is completed at about 700 °C and no combustion residual remained. For resin 1, the initial degradation started at around 320 °C with a high degradation rate. The second degradation step starts at about 500 °C and has a lower degradation rate. As shown by the DSC curve, these two steps are accompanied by two exothermic peaks
Fig. 1. TGA (black curves) and DSC (red curves) of the geopolymer cured at room temperature (continuous lines) and at 60 °C (dashed lines).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Fig. 2. TGA (black curves) and DSC (red curves) of the resin 1 (continuous lines) and resin 2 (dashed lines) cured at room temperature for 7 days.
(the first one centered at 352 °C and the second one centered at 583.5 °C) of opposite extent in respect to the corresponding weight losses (ΔH is 295 J/g and 1930 J/g respectively). Resin 2 showed a very similar behavior, with degradation temperatures only slightly higher than resin 1 (for example, the degradation temperatures at 10% weight loss were found 342 °C for resin 1 and 349 °C for resin 2) possibly due to the presence of the aromatic ring. A substantially identical behavior (not shown) has been observed also for the specimen cured at 60 °C. Figs. 3 and 4 show the weight losses of the hybrids I and II after different curing conditions, respectively. Hybrid I shows a complex degradation mechanism involving three main steps: the first step begins at room temperature and finishes at about 220 °C corresponding to a weight loss of 23%; the second step ends at 406 °C and involves a weight loss of about 11% while the third one ends at 750 °C with a further weight loss of 14%. The combustion residual at 800 °C is around 53%. From the comparison of the TGA and DSC curves for the pure geopolymer and resin reported in Figs. 1 and 2, respectively, the first degradation step can be associated with the loss of water of the geopolymeric matrix while the remaining two correspond to the degradation of the dispersed organic phases. It is worth pointing out that in the hybrid the relative extent of the second and third weight loss steps (around 400 °C and 600 °C, respectively) is inverted in respect to the corresponding phenomena that have been recorded for the pure resin (see Fig. 2). Moreover it is worth noting that the peak temperature of water loss for the hybrid is higher than that of the pure geopolymer (see
45
Fig. 4. TGA (black curves) and DSC (red curves) of the hybrid II cured at room temperature (continuous lines) and at 60 °C (dashed lines).
Fig. 1): probably the polar groups of the resin tend to restrain the water molecules delaying their evaporation. A very similar behavior has been recorded also in the case of hybrid II (see Fig. 4), the most significant difference being the degradation of the resin that takes places in a continuous way up to 600 °C. It is worth pointing out that in both cases the thermal treatment up to 800 °C results in a complete removal of the organic phase from the hybrid (see SEM images in Section 3.2.3). Degradation temperatures and weight losses for all the studied systems are summarized in Table 1. 3.2.2. Dynamic mechanical thermal analysis Fig. 5 shows the dynamic-mechanical properties of the pure resin 1 and of hybrid I cured at room temperature. DMTA measurements are carried out at very low load, in the elastic deformation field. In the explored temperature range, the cured resin (curve 5a) shows a progressive drop of the storage modulus of one order of magnitude that is likely due to the thermal softening. As a result of the high stiffness of the crosslinked resin, the corresponding loss factor (tanδ, curve 5a′) is very small, being always lower than 8 × 10 −2 in all the examined temperature ranges. As far as hybrid I is considered, on account of the geopolymeric matrix, its storage modulus (curve 5b) turns out to be always higher than that of the pure resin and, in the considered temperature range, it decreases less than one order of magnitude. The slight increase of the modulus observed in the region between 175 °C and 225 °C could be due to the completion of the curing process of the resin that maybe, at room temperature, was partly inhibited by the presence of the inorganic components. Moreover, thanks to the higher stiffness of inorganic phase, hybrid I shows a lower drop of the storage modulus in respect to the pure resin. This greater stiffness is the cause also of the lower tanδ values in respect to those of the pure resin 1 (curve 5b′). Table 1 Thermal properties of the geopolymer, resins and hybrids. Weight loss Curing temperature starting temperature (°C) (°C)
Fig. 3. TGA (black curves) and DSC (red curves) of the hybrid I cured at room temperature (continuous lines) and at 60 °C (dashed lines).
Geopolymer 25 60 Resin 1 25 Resin 2 25 Hybrid I 25 60 Hybrid II 25 60
30 30 260 290 30 30 30 30
Temperature at 10% weight loss (°C)
Weight loss ending temperature (°C)
Combustion residual at 800 °C (weight %)
101 120 342 349 109 140 112 154
450 450 700 670 750 670 640 655
72 76 0 0 53 57 54 59
46
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Fig. 5. Storage moduli (black curves) and mechanical loss tangents (tanδ, circle graphs) of resin 1 (continuous black curve and red graph) and of hybrid I (dashed black curve and blue graph) both cured for 7 days at room temperature.
Finally, it is worth pointing out that, at variance with the pure geopolymer that is very brittle at room temperature (Zhao et al., 2007), thanks to the presence of the organic resin, hybrid I is significantly less brittle. As a matter of fact, while the neat geopolymer specimen is broken at room temperature during the test (carried out with the same condition of load used for the hybrid systems and resins), the hybrid specimen is safe up to 220 °C. 3.2.3. Microstructural analysis Fig. 6 shows micrographs of freshly obtained fracture surfaces of geopolymer and hybrids I and II cured at room temperature. All the specimens show an amorphous structure (few crystals can be seen only in the geopolymer specimen, see Fig. 6a′, as confirmed by X-ray diffraction analysis of the starting metakaolin) indicating that the geopolymerization process has been successfully carried out in all cases. Moreover, a compact and homogeneous morphology is clearly observed for all the specimens. In particular, from the examination of images 6b, b′ and 6c, c′ referring to hybrids I and II, a very good homogeneity and uniformity of the microdispersion of the organic phase in the inorganic one is evident. The dimension of the resin particles is in the range of 1–20 μm. For all the specimens examined, no agglomerations phenomena were observed. It is worth pointing out that inorganic–organic hybrids (20%/80% w/w) described in literature (Hussain et al., 2004, 2005) are characterized by a dispersed phase of millimetric dimension. This result is even more interesting if we consider the fact that this uniform dispersion at micrometric level was obtained just by simple manual mixing. It is likely to be expected that a fine tailoring of the average diameter of the microdispersed organic phase (that reasonably influences the mechanical properties of the hybrids) can be obtained simply by mechanically controlling the mixing step. In addition, in all cases, the strict adhesion between the phases is evident: in particular in the case of the hybrid II (image 6c and 6c′), the interaction between the geopolymer matrix and the organic resin microsphere is so good that the particles of resin have been scratched when the specimens were broken to prepare the SEM samples. A more detailed investigation of the interphase region between organic and inorganic phases confirming this close interaction has been carried out by energy-dispersive X-ray spectroscopy (EDS) analysis on hybrid II and shown in Fig. 7 and Table 2. In particular, we have recorded EDS spectra on i) the region of a polymeric particle that, up to 5000 magnifications, does not present visible traces or grains of the inorganic phase, but shows traces of Si atoms (region 1 of Fig. 7, spectrum 1 of Table 2); ii) the region around the polymer particle (i.e. the cavity obtained by the removing of the polymer
particle) that, up to 5000 magnifications, does not present visible traces or grains of polymer, but reveals the presence of C atoms (region 2 of Fig. 7, spectrum 2 of Table 2); iii) the area between the polymer particles and the geopolymeric matrix in which the analysis pointed out a significant presence of all the elements revealed (region 3 of Fig. 7, spectrum 3 of Table 3). These observations confirm the close interaction between the different phases. Moreover, comparing Fig. 6a, a′, b, b′ and c, c′ it can be easily seen that the number and the extension of the microcracks that characterize the fracture surface of the geopolymer specimen is strongly reduced by adding the organic resin. Therefore, in agreement with what observed by DMTA analysis (described in Section 3.2.2) and with the compressive strength determinations (described in Section 3.2.5), the resin seems to prevent the cracking growth and propagation improving the mechanical properties and enhancing the fracture toughness of the brittle inorganic matrix. As a matter of fact, a sort of crack deflection mechanism typical of particle reinforced ceramic matrix composites could be expected to occur (Boccaccini et al., 1997; Monette and Anderson, 1993). Finally, Fig. 6d and d′ show the images of a specimen of hybrid I after thermal treatment at 800 °C for 1 h. It is apparent that, in good agreement with the TGA results shown in Section 3.2.1, the organic phase has been completely removed. In this way a macroporous material characterized by uniformly dispersed pores of very similar diameter (see Fig. 6d′) has been obtained. Analogous results (not reported in this paper) have been obtained also for the specimens cured at 60 °C. 3.2.4. FT-IR analysis FT-IR spectra of the starting metakaolin, of the geopolymer, the two resins and of the two hybrids are shown in Fig. 8. The FTIR spectrum of the geopolymer (curve 8b) is a typical one, with broad bands at about 3430 cm −1 and 1635 cm −1 due to O\H stretching and bending modes of adsorbed molecular water, with a strong Si\O stretching vibration at 1050 cm −1, which is lower in wave number than that of the metakaolin (curve 8a), indicating the condensation of Si\O tetrahedra in geopolymer. It is worth pointing out that the characteristic metakaolin Si\O\Al bond at 810 cm −1, after the geopolymerization, is replaced by several weaker bands in the range from 600 cm −1 to 800 cm −1. Finally, the signal at about 460 cm −1 is due to Si\O bending vibration. (Aronne et al., 2002; Catauro et al., 2003, 2004; Clayden et al., 1999; Felahi et al., 2001; Ortego and Barroeta, 1991; Wang et al., 2005; Zhang et al., 2008a, 2008b). The FT-IR spectra of resins 1 and 2 (curves 8c and 8d respectively) are very similar to each other. For both resins, the presence of some unreacted \NH groups is revealed by the broad peak at around 3350 cm −1 (due to the \NH stretching) and at around 1515 cm −1 (due to \NH bending). This last band, together with that at 830 cm −1, can be also attributed to p-disubstituted benzene rings. The signals in the wavenumber range of 2920–2820 cm −1 and at about 1460 cm −1 are due to \CH2\ symmetric and asymmetric stretching and bending, respectively. Finally, signals in the region of 1300–1050 cm −1 can be assigned to C\N, C\C and C\O stretching. In addition, the absence of bands at 971, 917 and 775 cm −1 due to terminal epoxy rings reveals a successful curing process (Felahi et al., 2001; Hummel and Scholl, 1971). As far as the hybrids, their spectra (curve 8e: hybrid I; curve 8f: hybrid II) are characterized by the main bands of the pure organic and inorganic components. In particular a strong absorption band at about 3430 cm −1 and 1640 cm −1 due to water, at about 1050 cm −1 due to Si\O stretching and at about 450 cm −1 due to Si\O bending vibration can be observed. The bands in the region of 800–600 cm −1 are associated to Si\O\Al vibrations (Barbosa and Mackenzie, 2003; Frost et al., 1996; Parker and Frost, 1996; Zaharaki et al., 2010).
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
47
Fig. 6. SEM micrographs (amplification is 5 × 102 in a, b, c, d and is 5 × 103 in a′, b′, c′, d′) of the specimens: a, a′) geopolymer cured at room temperature for 24 h; b, b′) hybrid I cured at room temperature; c, c′) hybrid II cured at room temperature for 24 h; d, d′) hybrid I hold at 800 °C for 1 h.
48
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50 Table 3 Average compressive strength, average stress at a 0.015 strain (σ0.015) and average stress at a 0.018 strain (σ0.018) of specimens.
Geopolymer Hybrid I Hybrid II
Compressive strength (MPa)
σ0.015 (MPa)
σ0.018 (MPa)
8.2 28.2 29.0
0.0 21.4 20.6
0.0 15.4 16.9
could absorb an aliquot of the load by plastic deformation; and on the other side it could perform a toughening effect by a typical crack deviation mechanism (Swain, 1989), as evidenced also by SEM observations (see Fig. 6).
Fig. 7. SEM micrographs (amplification 5 × 103) of the hybrid II cured at room temperature for 24 h. The regions selected for the EDS analysis are indicated by white rectangles. The chemical characterization for each region is reported in Table 2.
3.2.5. Compressive strength determination The average compressive strengths and the average stress retained by the specimens at a deformation of 0.015 and 0.018 of the geopolymer, hybrid I and hybrid II specimens are given in Table 3. It can be seen that hybrid specimens have higher strength than the geopolymer one. The addition of 20% resin results in compressive strengths which are 3.4 and 3.5 times higher than that of the geopolymer specimen, respectively for resin 1 and resin 2. This effect may be likely explained by considering that metakaolin based geopolymer is subjected to microcracking caused by the evaporation of the high amount of water added with the activating solution. The incorporation of the resin has an effect of immobilizing water molecules and effectively postpone the water evaporation (Zhang et al., 2010a, 2010b) as evidenced by thermal analysis (see Figs. 3 and 4). Significant are also the differences of the mechanical behaviors of the geopolymer specimen in respect to the hybrid ones, which are illustrated in Fig. 9. The geopolymer specimen shows a typical brittle ceramic behavior and fracture at a strain lower than 0.01. The hybrid specimens show a gradual decrease of the load with increasing displacement, after the maximum is reached, which denotes a sort of pseudo-plasticity behavior. As evidenced by the data reported in Table 3, hybrid specimens I and II still retain 76% and 71% of the compressive strength, respectively, at a deformation of 0.015 and retain 55% and 58%, respectively, at a deformation of 0.018. The hybrid specimens show progressive fracture behavior rather than a catastrophic one and consequent higher fracture energy. This behavior may be likely explained by considering the propagations of cracks proceeding in a progressive and controlled way with increasing strain (Li and Xu, 2009) owing to the presence of the resin phase. The organic phase may play a dual role: on one side it
Fig. 8. FT-IR spectra of a) metakaolin, b) geopolymer, c) resin 1, d) resin 2, e) hybrid I and f) hybrid II.
Table 2 Chemical characterization of hybrid II cured at room temperature for 24 h obtained by energy-dispersive X-ray spectroscopy (EDS) in the regions indicated by white rectangles shown in Fig. 7. All the elements have been analyzed and normalized. All results are in weight%. Spectrum
In stats.
C
O
Na
Al
Si
Total
Spectrum 1 Spectrum 2 Spectrum 3
Yes Yes Yes
65.54 8.14 20.73
29.15 18.65 49.67
1.49 7.89 6.64
1.48 21.16 8.09
2.35 44.16 14.87
100.00 100.00 100.00
Fig. 9. Mechanical behavior of the geopolymer (black curve), hybrid I (dashed red curve) and hybrid II (dotted blue curve) specimens.
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
4. Conclusions Through an innovative synthetic approach based on a co-reticulation in mild conditions of epoxy based organic resins and an MK-based geopolymer inorganic matrix, novel hybrid organic–inorganic materials were prepared. A high compatibility between the organic and inorganic phases, even at appreciable concentration of resin (20% w/w), was realized up to micrometric level. A good and homogeneous dispersion (without the formation of agglomerates) of the organic particles was obtained just by hand mixing. These new materials show good technological properties: in particular, in respect to the neat geopolymer, they present significantly enhanced compressive strengths and toughness. By comparing our novel hybrid organic–inorganic composite materials with traditional geopolymer it is worth pointing out that: i) in agreement with the expectations of green chemistry, in the proposed synthetic procedure the use of solvents is completely avoided; ii) the strength of our material is 3.5 times higher than geopolymer control specimens; iii) hybrid materials show higher deformation before cracking. From an environmental point of view this means that it is possible to save material, to use smaller section for the same load condition, to reduce the number of cracks obtaining more durability and so a longer service life. Starting from this consideration we can conclude that the hybrid composite materials described in the present paper seem to have all the conditions to be an environmental friendly material. Finally, the possible use of wastes coming from different activities (mining, metallurgic, municipal, construction and demolition) instead of a raw material (metakaolin) for the obtainment of hybrid geopolimeric materials with our new synthetic approach could further reduce the environmental impact of the material we have studied. A more complete technological characterization, including the examination of the structural integrity of the material studied under freeze-thaw cycles, together with other technological characterizations (flame resistance, porosimetry, mechanical properties and so on) will be carried out in a subsequent paper. Acknowledgment Thanks are due to Prof. Antonio Roviello and Prof. Finizia Auriemma of the Università degli Studi di Napoli Federico II for useful discussions and suggestions. References Andini, S., Cioffi, R., Colangelo, F., Grieco, T., Montagnaro, F., Santoro, L., 2008. Coal fly ash as raw material for the manufacture of geopolymer-based products. Waste Management 28, 416–423. Andini, S., Cioffi, R., Colangelo, F., Ferone, C., Montagnaro, F., Santoro, L., 2010. Characterization of geopolymer materials containing MSWI fly ash and coal fly ash. Advances in Science and Technology 69, 123–128. Aronne, A., Esposito, S., Ferone, C., Pansini, M., Pernice, P., 2002. FTIR study of the thermal transformation of barium exchanged zeolite A to celsian. Journal of Materials Chemistry 12 (10), 3039–3045. Barbosa, V.F.F., MacKenzie, K.J.D., 2003. Thermal behavior of inorganic geopolymer and composites derived from sodium polysialate. Materials Research Bulletin 38, 319–331. Boccaccini, A.R., Bücker, M., Bossert, J., Marszalek, K., 1997. Glass matrix composites from coal fly ash and waste glass. Waste Management 17, 39–45. Buchwald, A., Hohmann, M., Posern, K., Brendler, E., 2009. The suitability of thermally activated illite/smectite clay as raw material for geopolymer binders. Applied Clay Science 3, 300–304. Catauro, M., Raucci, M.G., De Gaetano, F., Marotta, A., 2003. Sol–gel synthesis, characterization and bioactivity of polycaprolactone/SiO2 hybrid material. Journal of Materials Science 38, 3097–3102. Catauro, M., Raucci, M.G., de Gaetano, F., Buri, A., Marotta, A., Ambrosio, L., 2004. Sol– gel synthesis, structure and bioactivity of polycaprolactone/CaO • SiO2 hybrid material. Journal of Materials Science. Materials in Medicine 15, 991–995. Cioffi, R., Maffucci, L., Santoro, L., 2003. Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue. Resources, Conservation and Recycling 40, 27–38.
49
Clayden, N.J., Esposito, S., Aronne, A., Pernice, P., 1999. Solid state 27Al NMR and FTIR study of lanthanum aluminosilicate glasses. Journal of Non-Crystalline Solids 258, 11–19. Davidovits, J., 1991. Geopolymers: inorganic polymeric new materials. Journal of Thermal Analysis 37, 1633–1656. De Santis, R., Catauro, M., Di Lucy, S., Manto, L., Raucci, M.G., Ambrosio, L., Nicolais, L., 2007. Effects of polymer amount and processing conditions on the in vitro behaviour of hybrid titanium dioxide/polycaprolactone composites. Biomaterials 28, 2801–2809. Dias, D.P., Thaumaturgo, C., 2005. Fracture toughness of geopolymer concretes reinforced with basalt fibers. Cement and Concrete Composites 27, 49–54. Duxson, P., Fernandez-Jimenez, A., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S.J., 2007. Geopolymer technology: the current state of the art. Journal of Material Science 42, 2917–2933. Felahi, S., Chikhi, N., Bakar, M., 2001. Modification of epoxy resin with kaolin as a toughening agent. Journal of Applied Polymer Science 82, 861–878. Ferone, C., Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2011. Mechanical performances of weathered coal fly ash based geopolymer bricks. Procedia Engineering 21, 745–752. Ferone, C., Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2012. Use of reservoir clay sediments as raw materials for geopolymer binders. Advances in Applied Ceramics. http://dx.doi.org/10.1179/1743676112Y.0000000064. Frost, R.L., Fredericks, P.M., Shurvell, H.F., 1996. Raman microscopy of some kaolinite clay minerals. Canadian Journal of Applied Spectroscopy 41, 11–14. Habert, G., d'Espinose de Lacaillerie, J.B., Roussel, N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. Journal of Cleaner Production 19, 1229–1238. Hummel, D.O., Scholl, F., 1971. Infrared Analysis of Polymers, Resins and Additives An Atlas, vol. 1. Wiley Interscience, NewYork. Hussain, M., Varley, R.J., Cheng, Y.B., Simon, G.P., 2004. Investigation of thermal and fire performance of novel hybrid geopolymer composites. Journal of Material Science 39, 4721–4726. Hussain, M., Varley, R.J., Cheng, Y.B., Mathys, Z., Simon, G.P., 2005. Synthesis and thermal behaviour of inorganic–organic hybrid geopolymer composites. Journal of Applied Polymer Science 96, 112–121. IUPAC, 1997. Compendium of chemical terminology. (the “Gold Book”) Compiled by A. D. McNaught and A. Wilkinson2nd ed. Blackwell Scientific Publications, Oxford. (XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. http://dx.doi.org/10.1351/goldbook). Kickelbick, G., 2007. Introduction to Hybrid Materials. In: Kickelbick, G. (Ed.), Hybrid Materials: Synthesis, Characterization, and Applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. http://dx.doi.org/10.1002/9783527610495.ch1. Knowlton, G.D., White, T.R., Laurence McKague, H., 1981. Thermal study of types of water associated with clinoptilolite. Clays and Clay Minerals 29 (5), 403–411. Komnitsas, K., 2011. Potential of geopolymer technology towards green buildings and sustainable cities. Procedia Engineering 21, 1023–1032. Kong, D.L.Y., Sanjayan, J.G., Sagoe-Crentsil, K., 2007. Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures. Cement and Concrete Research 37, 1583–1589. Li, W., Xu, J., 2009. Mechanical properties of basalt fiber reinforced geopolymeric concrete under impact loading. Materials Science and Engineering A 505, 178–186. Lin, T., Jia, D., He, P., Wang, M., Liang, D., 2008. Effects of fiber length on mechanical properties and fracture behavior of short carbon fiber reinforced geopolymer matrix composites. Materials Science and Engineering A 497, 181–185. Menna, C., Asprone, D., Ferone, C., Colangelo, F., Balsamo, A., Prota, A., Cioffi, R., Manfredi, G., 2013. Use of geopolymers for composite external reinforcement of RC members. Composites Part B 45, 1667–1676. http://dx.doi.org/10.1016/ j.compositesb.2012.09.019. Monette, L., Anderson, M.P., 1993. Effect of particle modulus and toughness on strength and toughness in brittle particulate composites. Scripta Metallurgica et Materialia 28, 1095–1100. Ortego, J.D., Barroeta, Y., 1991. Leaching effects on silicate polymerization, a FTIR and 29Si NMR study of lead and zinc in Portland cement. Environmental Science and Technology 25, 1171–1174. Parker, R.W., Frost, R.L., 1996. The application of drift spectroscopy to the multicomponent analysis of organic chemicals adsorbed on montmorillonite. Clays and Clay Minerals 44, 32–39. Provis, J.L., Duxon, P., van Deventer, J.S.J., 2010. The role of particle technology in developing sustainable construction materials. Advanced Powder Technology 21, 2–7. Sathonsaowaphak, A., Chindaprasirt, P., Pimraksab, K., 2009. Workability and strength of lignite bottom ash geopolymer mortar. Journal of Hazardous Materials 168, 44–50. Shi, C., Krivenko, P.V., Roy, D.M., 2006. Alkali-Activated Cements and Concretes. Taylor and Francis, Abington, UK. Swain, M.V., 1989. Toughening mechanism for ceramics. Materials Forum 13, 237–253. Tiesong, L., Dechang, J., Peigang, H., Meirong, W., Defu, L., 2008. Effects of fiber length on mechanical properties and fracture behavior of short carbon fiber reinforced geopolymer matrix composites. Materials Science and Engineering A 497, 181–185. Wang, H., Li, H., Yan, F., 2005. Synthesis and mechanical properties of metakaolinite based geopolymer. Colloids and Surfaces A: Physicochemical and Engineering Aspects 268, 1–6. Xu, H., van Deventer, J.S.J., 2000. The geopolymerisation of alumino-silicate minerals. International Journal of Mineral Processing 59, 247–266.
50
C. Ferone et al. / Applied Clay Science 73 (2013) 42–50
Zaharaki, D., Komnitsas, K., Perdikatsis, V., 2010. Use of analytical techniques for identification of inorganic polymer gel composition. Journal of Materials Science 45, 2715–2724. Zhang, Y., Sun, W., Li, Z., 2006. Impact behavior and microstructural characteristics of PVA fiber reinforced fly ash-geopolymer boards prepared by extrusion technique. Journal of Materials Science 41, 2787–2794. Zhang, Y., Sun, W., Li, Z., 2008a. Infrared spectroscopy study of structural nature of geopolymeric products. Journal of Wuhan University of Technology-Material Science Edition 23, 522–527. Zhang, Y., Sun, W., Li, Z., Zhou, X., Eddie, C.C., 2008b. Impact properties of geopolymer based extrudates incorporated with fly ash and PVA short fiber. Construction and Building Materials 22, 370–383.
Zhang, Y.J., Li, S., Xu, D.L., Wang, B.Q., Xu, G.M., Yang, D.F., Wang, N., Liu, H.C., Wang, Y.C., 2010a. A novel method for preparation of organic resins reinforced geopolymer composites. Journal of Material Science 45, 1189–1192. Zhang, Y.J., Wang, Y.C., Xu, D.L., Li, S., 2010b. Mechanical performance and hydration mechanism of geopolymer composite reinforced by resin. Materials Science and Engineering A 527, 6574–6580. Zhao, Q., Nair, B., Rahimian, T., Balaguru, P., 2007. Novel geopolymer based composites with enhanced ductility. Journal of Materials Science 42, 3131–3137.