Mechanical and Biological Characterization of Geopolymers for Potential Application as Biomaterials

Michelina Catauro, Flavia Bollino, Ishu Kansal, Elie Kamseu, Isabella Lancellotti, Cristina Leonelli

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Submitted: 28 March, 2011
Posted: 19 May, 2012

Topics Covered

Abstract

In this study three different geopolymer compositions have been investigated and characterized as potential biomaterials. The first two geopolymer formulations are mainly composed of metakaolin, with some silica additions in order to achieve a Si/Al molar of 2.10 while the third one contains a reduced amount of metakaolin and comprises mainly of silica gel with composition: H₂₄AlK₇Si₃₁O₇₉ with Si/Al = 31. Further, NaOH pellets and sodium silicate (Na₂SiO₃) were added in the first two formulations in different concentrations as activator and ligand, respectively, while KOH additions were made to the third geopolymer formulation (separately or jointly with potassium silicate solution). Room temperature consolidation was followed by thermal activation of composition with Si/Al=31 at 60 °C for 150 min and at 500 °C for 180 min. The work presents exhaustive microstructural characterization (infrared spectroscopy, electron microscopy and X-ray diffraction) of as synthesized geopolymer samples in conjunction with their mechanical and in vitro bioactivity evaluation. The materials were composed of amorphous aluminosilicates with a limited amount of zeolitic phases, found on the top surface. The compressive strength of the first two compositions was higher than 15 MPa and flexural strength around 2 MPa after 2 days of curing at room temperature. Compressive strength tests were carried out on composition with on Si/Al = 31 geopolymer on both activated sample series and demonstrated that when added separately the activator leads to more fragile specimens (0.90 MPa vs. 1.95 MPa). To the best of authors’ knowledge the effect of geopolymer preparation on mechanical properties of thermally activated Si/Al = 31 formulation has never been proved before. The bioactivity was successfully tested with the soaking of the samples in a simulated body fluid (SBF) for 3 weeks. The formation of a layer of hydroxyapatite on the surface of the materials was shown both by SEM micrograph and EDS analysis.

Keywords

Geopolymers, FTIR, Bioactivity, Compressive Strength, Molar Ratio Si/Al = 31

Introduction

In the biomaterials field, some systems based on amorphous silicate network like bioactive glasses¹, calcium phosphates^2,3, aragonite⁴, are successfully applied in bone surgery. As a matter of fact numerous synthetic aluminosilicates^5,6, possess chemical properties which offer employment as bone graft biomaterials. Among these aluminosilicates also geopolymers can be listed. They can be defined as cementitious materials that are formed by mixing aluminosilicate powders (e.g., metakaolin) with alkali or alkali-silicate solution^7-10.

The formation of [M_z(AlO₂)_x(SiO₂)_y.MOH.H₂O] gel, which essentially relies on the dissolution of aluminosilicates materials, is a dominant step in geopolymerisation. When the gel phase hardens, the separate aluminosilicate particles are bound together with the gel which acts as binder¹¹. Authors^12,13 have described the reaction process of the gel formation indicating that Al-Si solid particles in alkaline solution conduct to the formation of monomers -OSi(OH)₃ and Al(OH)^4-. The successive reactions between these monomers, alkali ions and water results in the formation of dimer and longer chains¹⁴. The final geopolymers are then formed with tightly packed 3D semi-crystalline microstructure with good mechanical properties^15,16. Typically, better strength behaviour is obtained for mixtures with Si/Al molar ratio in the range of 1.65-2.10 combined with Na/Al ratio near 1. Increased silica content usually reduces the mechanical strength while multiple alkali sources, KOH and NaOH, can act in a synergistic way to promote samples of optimal chemical and mechanical characteristics^13,14,17. The compressive strength of geopolymer materials depends on a number of factors including gel phase strength, the ratio of gel phase/undissolved Al-Si particles size, the distribution and the hardness of the undissolved Al-Si particles, the amorphous nature of geopolymers or the degree of crystallinity as well as the surface reaction between the gel phase and the undissolved Al-Si particles.

High mechanical strength of geopolymers results on the rapid solidification within hours and rapid early strength development, as it is already known for phosphatic dental cements¹⁸. In addition a good level of porosity can be easily introduced at room temperature by adding specific additives¹⁹. Both these characteristic make these materials attractive for hard tissue prostheses or bone fillers and bindings.

Even though these types of materials are particularly interesting for their fast strength development, their bioactivity or at least biocompatibility has not yet been investigated extensively. The aim of the present paper is to investigate the application of metakaolin based geopolymers as potential biomaterials as well as studying the effect of sample preparation on silica rich metakaolin geopolymer already proposed in literature as bony filling⁶. The preparation and characterization of a total of three different formulations are reported.

Methods and Materials

Geopolymers Preparation

Metakaolin (MK) was used as the principal source of aluminosilicate. The choice was based on the fact that it improves mechanical strength and reduces the transport of water and salts in the final product. Metakaolin, with a molar ratio Si/Al = 2.10, was prepared by calcining kaolinitic clay at 700 °C in muffle oven for 2 hrs.

Two different formulations of geopolymer materials, containing different amount of NaOH and with two different Na/Al ratios, were prepared as shown in Table 1.

Laboratory grade NaOH pellets and distilled water were used to prepare 8M alkaline solution. Sodium silicate solution (grade N, RM3, 0 from Ingessil s.r.l., Verona, Italy) with a mol/mol ratio of SiO₂/Na₂O equal to 2.99 was used.

Designated quantities of sodium hydroxide and silicate solutions were mixed with 100 g batches of metakaolin (solid/liquid ratio of 1.66). No additional water was added apart from that of NaOH and Na-silicate²⁰. Overall porosity, 60vol%, is visible in Figure 1a.

Table 1. Formulation of the two geopolymers, with Si/Al = 2.10, prepared for this study (Na silicate solution used: 30 ml per 100 g MK)

Figure 1. Optical micrograph of (a) GP120 (BGP130 has similar microstructure) and (b) BGP3 P (BGP3 S has similar microstructure) bulk samples.

The resulting slurries were stirred mechanically for about 10 min to reach good homogenization, and then poured in a closed polyethylene mould for 12 hrs at room temperature before demoulding. The curing time was 30 days for both the compositions.

In order to reduce the impact of non-bonded Al atoms in the final product, an increase of silica was operated by SiO₂ additions to MK. Thus the percentage of Al present in the final formulation decreased to Si/Al =31, but, at the same time, the effect of 3D bonded silica chains increased the network connectivity, decreasing Al release^4-6.

Table 2. Silica rich formulation and details of activation: P refers to sample prepared using pellets of KOH directly added to K- silicate (water added =35ml). S refers to sample prepared using 8M KOH solution (water added =25ml).

Following literature formulation of Oudadesse et al.^4-6 a third geopolymer sample is here proposed with theoretical formula: H₂₄AlK₇Si₃₁O₇₉. The raw materials used were KOH, K₂SiO₃ and SiO₂ apart from the previously mentioned metakaolin. Laboratory grade KOH pellets and distilled water were used to prepare 8M alkaline solution (4.45 g of solution in 1.225 g of MK) or were directly added to the potassium silicate solution (1.995 g in 7.164 mol). Potassium silicate solution (grade N, RM3, 0 from Ingessil s.r.l., Verona, Italy) with a mol/mol ratio of SiO₂/K₂O equal to 3.10 was used. In both procedures, 1.225 g MK was added with 14.655 g of SiO₂ and consolidation was performed as reported in Table 2. The SiO₂ added was completely amorphous in nature which was produced hydrothermally from natural silicates²¹.

Thermal cycles for consolidation the 3D silica network were applied, as reported in Table 2, following literature indications^4-6. Microstructure of BGP3 P sample is visible in Figure 1b.

Geopolymer Characterization

The final geopolymers were tested for crystalline phases evolution with X-Ray diffraction (XRD) on both surface and bulk of each sample (Philips Electronic Instruments, model PW 1730).

The compressive strength was determined by using mechanical testing machine Instron, USA. The end surfaces of specimens and surface were polished flat and parallel to avoid the requirement for capping. The cylinders (2 cm diameter x 4 cm thickness) were centred in the compression-testing machine and loaded to complete failure. The compressive strength was calculated by dividing the maximum load (N) at failure by the average cross-sectional area (m²). The final result is the average value of three tested specimens.

Fourier transform infrared (FTIR) transmittance spectra were recorded in the 400-4000 cm^-1 region using a Prestige 21 Shimatzu system, equipped with a DTGS KBr (Deuterated Tryglycine Sulphate with potassium bromide windows) detector, with resolution of 2 cm^-1 (45 scans). KBr pelletised disks containing 2 mg of sample and 200 mg KBr were prepared. FTIR spectra were elaborated by Prestige software (IRsolution).

Bioactivity Tests

In order to study their bioactivity, one disc per each composition was soaked in a simulated body fluid (SBF) solution with ions concentration nearly equal to those in human blood plasma (Table 3), at 37 °C and in a polystyrene bottle, as scheduled by bioactivity test in vitro²². The SBF solution was prepared by dissolving reagent grade chemicals NaCl, NaHCO₃, KCl, MgCl₂•6H₂O, CaCl₂, Na₂HPO₄, Na₂SO₄ (Sigma-Aldrich) in ultra-pure water and buffered at pH 7.4 using HEPES sodium salt (C₈H₁₈N₂O₄SNa) (Sigma-Aldrich) and 1M NaOH.

Table 3. Simulated body fluid (SBF) ionic concentration (mM).

In these tests the ratio between the total surface (ST) of the material in contact with the SBF solution and the volume of such solution (V_SBF) influence the reaction of formation of a hydroxyapatite layer. A constant ratio of ST/V_SBF =10 mm²/ml was used.

After an immersion periods of 21 days, the materials were removed from the SBF, gently washed with ultra-pure water, and dried at 40 °C. The ability to form an apatite layer was studied by Environmental Scanning Electron Microscopy (ESEM QUANTA 200) observations coupled with energy dispersion spectroscopy- EDS (X-EDS Oxford INCA-350) analyses.

Results and Discussion

Sodium Based Geopolymers

Sodium based geopolymer composition proposed here are capable of offering a controlled porous material when appropriately added with a poring agent as reported in literature¹⁹. Final bulk density value was about 1.56-1.55 g/cm³ with a porosity of 38 ± 5.1 vol. %; compressive strength 15.0 ± 4.4 MPa, bi-axial flexural strength 8.0 ± 4.5 MPa. Since the mechanical characterization has been reported elsewhere^19,23, in this contribution we are focused only on their application as biomaterials. With this in mind we started with a complete microstructural characterization of the two formulations GP120 and BGP130.

The two geopolymer formulations were analysed by XRD on both the surface and bulk in order to evaluate material’s structure uniformity and the nature of the geopolymeric gel. The bulk of both samples show traces of the kaolinitic clay which is, not fully transformed into metakaolin, as it always happen in industrial processes. XRD pattern of GP120 (Figure 2) is reported as an example since for BGP130 the same crystalline phases were identified. The geopolymer appears largely amorphous with traces of crystalline phases. In particular the following phases were identified: monoclinic kaolin Al₂Si₂O₅(OH)₄ (JCPDF file 05-0221), monoclinic nacrite 2M Al₂Si₂O₅(OH) ₄ (JCPDF file 16-0606), which is a phyllosilicate clay mineral polymorph of kaolinite, along with traces of another phyllosilicate with monovalent cation and possibly muscovite 1M-like structure, KAl₂Si₃AlO₁₀(OH)₂ (JCPDF file 07-0025). Furthermore, as a consequence of the reaction between metakaolin and the alkaline solutions, Na-aluminosilicates were developed, such as Na₄Al₂Si₂O₉ (JCPDF file 10-0031) and Na₆Al₄Si₄O₁₇ (JCPDF file 10- 0033). During geopolymerization the dissolution and nucleation of gel are generally exothermic reactions so the energy of gel growth to polysialates is not enough to conduct to crystalline phases. Therefore a significant broad band due to the amorphous nature of the geopolimeric gel is also evident in all the XRD patterns for both the compositions.

Kaolinite and nacrite were observed on the surface probably as traces of the unreacted kaolinitic clay used in the synthetic process along with Na- aluminosilicate Na₂Al₂SiO₆ (JCPDF file 30-1148) due to the reaction with the alkaline solution (Figure 3). In addition, a carbonate phase, possibly trona Na₃H(CO₃)₂.2H₂O (JCPDF file 29-1447) was also evident, due to the reaction with the atmosphere.

Figure 2. XRD pattern of GP120 bulk sample (K = kaolinite, N = nacrite, M = muscovite, Na = Na-aluminosilicates)

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Figure 3. XRD pattern of GP120 sample surface (T = trona, K = kaolinite, N = nacrite, M = muscovite, Na = Na₂Al₂SiO₆)

In the geopolymers FTIR spectra, an approximate relationship between the frequency of the absorption bands and the ratio of Si:Al in the aluminosilicate framework was observed by Milkey²⁴: the higher the Al inclusions, the lower the wavelength. In Figures 4 and 5 the spectra of samples GP120 and BGP130, respectively, inner and surface parts are shown. All the spectra present the strongest vibration typical of all aluminosilicates (see Table 3), which are assigned to internal vibrations of Si-O-Si and Si-O-Al. In addition other common feature are present: two absorption bands at about 3450 and 1650 cm^-1 resulting from hydration water, a strong Si-O stretching vibration at about 1080-1100 cm^-1 and Si-O bending vibration at about 450-470 cm^-1. Moreover, the bands between 600-800 cm^-1 are due to Al-O-Si vibrations^25,26 : in particular, the bands at 798 cm^-1 are due to Al-O stretching vibrations. The surface spectra (Figures 4B and 5B) show Al-OH stretching vibrations at 914-916 cm^-1 while Si-OH bending vibrations, at 850 cm^-1, are present only in BGP130 surface spectrum (Figure 5a). The presence of a band at 1460 cm^{- 1} in Figure 4b and 5b indicates that sodium carbonate species are present on the surface of this sample confirming the XRD results which shows the reactivity of the surface with the atmosphere. The absence of this band in Figure 4a and 5a indicates that sodium carbonate species are not present in the samples inner part²⁷.

Figure 4. FTIR of sample GP120 (a) inner and (b) surface part

Figure 5. FTIR of sample BGP130 (a) inner and (b) surface part.

Table 4. IR characteristic bands and their interpretation

The microstructural characterization has been conducted by scanning electron microscopy (ESEM/EDS) in order to compare the two geopolymer formulations and their different behaviour (Figure 6). Both the samples do not show porosity in the micrometric range. The overall porosity¹⁹ has been measured with Archimede’s method. The absence of pores of micrometric size, confirmed by ESEM micrographs, is due to a well designed formulation. In addition the good mechanical properties are justified by the nanometric size of pores. These pores, generally called gel pores (< 15 nm) are too small to affect the mechanical properties²⁸.

GP120 shows a homogeneous microstructure with some incompletely dissolved grains embedded in a compact geopolymeric gel. EDS analysis of grains revealed that they contain alumina and silica in an atomic ratio near to 1 suggesting that they are metakaolin particles. On the other hands, BGP130 composition shows a two-phase microstructure characterized by two geopolymeric gels with different Na content. The grainy one contains higher amount of Na (average Na/Al = 0.64) with respect to the smooth one (average Na/Al = 0.53) (Figure 6b and 6c). In addition, a particle rich in potassium is also evident which can be attributed to muscovite, clayey crystalline phase identify by XRD. Both the geopolymers exhibit good mechanical properties; therefore the sample integrity is not degraded by the presence of granular inclusions.

Figure 6. ESEM micrographs of (a) GP120, (b) and (d) BGP130. (c) reports EDS data collected in area 1 and 2 of image in (b).

Moreover an evaluation of the morphology of the apatite deposition, after bioactivity tests, and a qualitative elemental analysis were also carried out by SEM accompanied by EDS as reported in Figure 7, which shows ESEM micrographs of GP120 soaked in SBF for 21 days. The layer of apatite is not visible on sample surface, proving that this particular formulation is not bioactive. BGP130, on the contrary, is bioactive: in fact the characteristic apatite globular crystals are clearly visible²⁹, as shown in Figures 7,b and c. These studies were considered sufficient to identify the presence of apatite on the geopolymer surface; additional XRD investigations were not possible due to the extremely low amount of the newly formed crystalline phase.

Figure 7. ESEM micrograph of (a) GP120 and (b) BGP130 after soak in SBF for 21 days. (c) enlargement of (b).

A possible explanation for this different bioactive behaviour is the presence of a greater number of –Si–ONa and –Al–ONa groups on the surface of BGP130 (Na/Al = 1.05) when compared to GP120 (Na/Al = 0.77). As reported in the literature^30,31 when exposed to SBF, sodium silicate glasses and ceramics released Na⁺ ions via exchange with H₃O⁺ ions in SBF to form Si–OH and Al–OH groups on their surface; this reaction causes a pH increase of SBF solution and, consequently, Si-OH and Al-OH groups are dissociated into negatively charged units Si–O-. These groups combine with Ca²⁺ ions present in the fluid imposing an increase of positive charge on the surface. In addition Ca²⁺ ions combine with the negative charge of the phosphate ions to form amorphous phosphate, which spontaneously transforms into hydroxyl-apatite [Ca₁₀(PO₄)₆ (OH)₂] where the atomic ratio Ca/P is 1.60³². The EDS analysis confirms that the surface layer observed in the SEM micrographs consists of calcium phosphate showing contents of Ca:P (atomic %) 3.14:1.95 for a value of Ca/P=1.61. Before biomedical applications of this porous metakaolin based geopolymer the issue of the potential problems caused by the alkalinity as well as the possible leaching of Al into the SBF should be addressed. As reported in literature, the highly alkaline nature of the material can induce cell death, and the effect of aluminium leached from the implant in living organisms is controversial; some authors^33-35 report that low aluminium concentrations lead to brain disease, although lower concentrations of aluminium may be beneficial and stimulate the proliferation of osteoblasts and new bone formation31. However, with various approaches it is possible to obtain stable geopolymers both in vitro and in vivo^5,36.

Potassium Based Geopolymers

Potassium based geopolymers have already been tested in vitro and in vivo^4-6 and they show high bioactivity/compatibility. Oudanesse et al.⁵ demostrated that amorphus geopolymers of the potassium-poly(sialate)-nanopolymer type, obtained with molar ratios Si:Al = 31, K₂O:SiO₂= 0.54 and thermally-treated at 500 °C, give excellent results in terms of biological compatibility. This geopolymer matrix was able to reduce alkalinity and the amount of free aluminium and to provide high porosity for biological compatibility. Hereafter the mechanical resistance evaluation only is reported being the two activation processes used responsible of differences in gel consolidation.

Mechanical characterization was performed on B-GP2P and B-GP2S after the complete curing at 30 days from the preparation and thermal treatment at 60 °C in Teflon® container for 150 min. Both formulations present a sort of plastic behaviour showing a long plateau before rupture at 1 MPA for B-GP2P and 2MPa for B-GP2S (Figure 8). With the high amount of silica to be activated this behavior is expected³⁷.

(a)

(b)

Figure 8. Compression resistance of (a) B-GP2P and (b) B-GP2S after thermal treatment at 60 °C.

FTIR spectra (Figure 9) of potassium based geopolymers show the typical alluminosilicate vibrations presented in Table 4. Depending on thermal treatment, the bands at about 790 cm^-1, due to Al(IV)-O stretching vibrations, increase and the shoulders at about 960 cm^-1, due to Al(IV)-OH stretching vibrations, decrease. A possible explanation is that during the geopolymerization, there is a progressive formation of Al(IV) species and a reduction of the number of Al(IV)-OH groups. Moreover, it is evident that there are not many differences between geopolymer matrix obtained with procedure S or P (Figure 9), except for B-GP3S and B-GP3P. In B-GP3P spectrum an intense band at about 1420 cm^-1 is evident due to carbonate high amount. The intensity of this band is smaller in all other spectra. Moreover, the shoulder at about 960 cm^-1 decreases in B-GP3P and disappears in B-GP3S.

(a)

(b)

Figure 9. FTIR spectra of geopolymer obtained according to procedures (a) P and (b) S.

Chemical alkali activation according to procedure S, i.e. KOH pellets added to K₂SiO₃ solution shows some un-reacted metakaolin area in untreated B-GP1S material (Figure 10). Similar areas are present also in B-GP1P but here the platelet typical of metakaolin morphology are not so well defined indicating that alkali attack has already begun.

These areas completely disappear after the 60 °C-treatment when a more uniform microstructure is reached (Figure 11). In particular S procedure lead to more bonded microstructure as visible in Fig. 10 from the reduced number of fracture surfaces in sample B-GP2S with respect to B-GP2P, confirming what observed in the compression resistance tests.

Figure 10. ESEM micrograph of (a) B-GP1P and (b) B-GP1S soon after preparation. Yellow circle evidence non reacted metakaolin area.

After the 500 °C, the microstructure maintains as in Figure 11, accordingly to infrared conclusions of very similar materials.

Figure 11. ESEM micrograph of (a) B-GP2P and (b) B-GP2S after 60 °C thermal treatment.

Conclusions

The study presents two formulations (GP120 and B-GP130) of metakaolin based geopolymers activated with NaOH which have been tested in vitro for bioactivity. The hydroxyl-apatite [Ca₁₀(PO₄)₆(OH)₂] formation was observed only on the surface of geopolymer BGP130 after immersioni n SBF solution for 21 days. Their high intrinsic compressive strength (recorded to be > 50 MPa) and the capability to form also porous structures (~60 vol. %), is indicative for their suitability for applications as hard tissue prostheses.

The third formulation proposed in the study, silica rich MK geopolymer activated with KOH/K₂SiO₃, has been tested mechanically to evaluate the consolidation process as a function of the activation procedure. The activation procedure that requires the addition of KOH pellets to K- silicate solution leads to materials with higher compressive strength. The results indicate that the order of addition of the reactants in the activation step is important and affects mechanical properties even though thermal consolidation is performed.

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