Nanoindentation Study of Na-Geopolymers Exposed to High Temperatures

Transcription

1 Nanoindentation Study of Na-Geopolymers Exposed to High Temperatures I. Beleña and W. Zhu* Abstract. This paper reports the usefulness of nanoindentation as a characterization and monitoring tool for studying thermal behaviour of Geopolymer materials. The influence of the manufacturing process of Na-Geopolymers in their micromechanical properties and thermal behaviour has been studied. Two types of metakaolin-based geopolymer panels with almost identical composition were prepared by injection and pouring methods. Micro-mechanical properties of the two samples exposed to high temperatures up to 1000 ºC were studied by nanoindentation technique, supplemented by X-ray diffraction (XRD), Nuclear magnetic resonance (NMR), Thermogravimetric analysis (TGA) and Microscopy. Remarkable differences in micro-mechanical properties and thermal behaviour between the two samples were found. Statistical nanoindentation has been successfully used to provide information about the micro-mechanical properties of different phases in the material and their volume distributions. 1 Introduction Geopolymers are ceramic-like materials obtained by alkali activation of aluminosilicates raw materials at high ph environment, atmospheric pressure and temperatures below 100 ºC. These materials are inorganic polymers consisting in 3D amorphous networks of SiO 4 and AlO 4 tetrahedron alternately linked by sharing all the oxygen atoms. The Al 3+ forth-coordinated is charge balance achieved by the presence of alkali cations (usually Na + or K + ). Their properties are much influenced by the Si/Al ratio in the final network. The starting aluminosilicate materials are disaggregated in a high alkali media and the SiO 4 and AlO 4 oligomers (dimers, trimers ) condense giving as a result an amorphous aluminosilicate polymer and water [1]. I. Beleña Technological Institute of Construction of Valencia (AIDICO), Technological Park, Valencia, Spain irene.belenya@aidico.es W. Zhu Advanced Concrete and Masonry Centre, University of the West of Scotland, Scotland, UK Wenzhong.Zhu@uws.ac.uk

2 170 I. Beleña and W. Zhu Due to the low energy requirements in the production and good mechanical performance, thermal behaviour and durability, Geopolymer has attracted increasing interest as an ecologically friendly fireproof building material. Many studies about the thermal behaviour of geopolymers with sodium or potassium silicate have been reported [1-4]. The metakaolin-based geopolymers (with a potassiumcontaining activator) have shown to achieve thermal stability up to ºC. The chemical composition, microstructure and macro-mechanical properties have been widely studied. No attempt, however, has been made to determine the micromechanical properties and their changes in different conditions for these materials. The aim of the present work was to study the micro-mechanical properties of Na-geopolymers using nanoindentation technique, which has been used to study properties of nano/micro-scale features (e.g. Elastic modulus and Hardness) in many different materials, including composites or multiphase materials [5-8]. In this study, geopolymers with the same starting materials and composition were made using two different processes, i.e. direct pouring and injection. Micromechanical properties of the two samples and their changes due to high temperature exposure up to 1000 ºC were studied by Nanoindentation and supplemented by Microscopy, XRD, MAS-NMR and TGA. 2 Experimental 2.1 Materials and Specimens Geopolymers tested were synthesised using Metakaolin (Sigma & Aldrich Chemistry, S.A.), Sodium Silica (Massó y Carol, S.A.) and Sodium Hydroxide (Prolabo, S.A.) (Table 1). The specimens were prepared using a previous optimized composition [9] and different manufacturing processes (Table 2). Table 1 Raw material composition and physical properties % oxides (wth) SiO 2 Al 2 Fe 2 Na 2 O K 2 O P.C* d (g/cm 3 ) d 50 (µm) BET (m 2 /g) Metakaolin Na 2 SiO 4.xH 2 O Table 2 Details of specimens used Manufacture process Chemical composition Sample Moulding Curing Drying SiO 2 /Al 2 Na 2 O/Al 2 H 2 O/ Na 2 O %Solid NaMk-I Injection Closed mould Two weeks NaMk-P Pouring Open mould 24h In the air In absorbent material

3 Nanoindentation Study of Na-Geopolymers Exposed to High Temperatures 171 Fig. 1 Optical images of the typical tested specimens (intent spacing being 30 µm): a) NaMk-I unheated, b) NaMk-P unheated 2.2 Testing Details The methodology and operating principle for the nanoindentation technique have been reviewed and presented in detail elsewhere [5-8]. Briefly, the test consists of making contact between a sample surface and a diamond indenter of known geometry, followed by a loading-unloading cycle while continuously recording the load, P, and indentation depth, h. The P-h curve obtained is a finger print of the mechanical properties of the test area. Most commonly, the elastic modulus (E) and hardness (H) of the test area are determined by analysing the initial part of the unloading data according to a model for the elastic contact problem. For studying multiphase composite materials, a refined statistical indentation method has been used, which involves testing and statistically analyzing a large number of indentation points within a representative sample area [7-8] The nanoindentation apparatus used in this study was Nanoindenter XP with a Berkovich indenter. In this study, all testing was programmed in such a way that the loading started when the indenter came into contact with the test surface and the load maintained for 30 seconds at the pre-specified maximum value before unloading. In order to provide statistical analysis of the micro-mechanical properties of different phases in the specimen, a grid of 8x15 indentation points with a indent spacing of 30 µm was selected, as shown in Fig.1. For studying the effect of high temperature exposure on properties of the geopolymers, selected samples were placed in an electrical furnace at various temperatures up to 1000ºC. The temperature was set to increase at 1ºC/min until 100ºC and 2ºC/min until the specified temperature and then maintained for 24 hours before natural cooling. 3 Results and Discussions 3.1 Micromechanical Properties of the Geopolymers before Heating The results obtained from the statistical indentation test of the two geopolymer samples are presented in Table 3. Generally, for both samples, three peaks were

4 172 I. Beleña and W. Zhu observed in the frequency-mechanical property (i.e. E and H) plots, which led to the determination of E and H values and volume fractions for the three different phases in each sample. Microscopic (optical and electron) analysis of the indented area has been used to identify the nature of these phases. In Fig.1 the grey background mass is the geopolymer matrix, the brightest irregular shaped pieces are the quartz (present in metakaolin as an impurity) and the slightly bright/white pieces are those incompletely reacted metakaolinite raw material. Table 3 Micromechanical properties and volume fractions of phases present in test samples Phase NaMk-I NaMk-P E, GPa H, GPa V% E, GPa H, GPa V% Geopolymer (GP) Metakaolinite (Mk) Quartz (Q) Results in Table 3 appear to indicate that the micromechanical properties of the geopolymeric matrix are significantly higher in NaMk-I than in NaMk-P sample, as well as there is a higher amount of unreacted metakaolinite in NaMk-P than in NaMk-I. This is probably due to the difference in curing and drying of the two processes. For NaMk-P the curing time was short (24 h) and in open mould with water likely evaporating from the top surface, while the NaMk-I was cured in closed mould for two weeks, thus likely leading to more complete polymerization. 3.2 Thermal Behaviour Study of Geopolymer Samples Nanoindentation tests were carried out on selected geopolymer samples which had undergone exposure at various high temperatures (Table 4). They show a moderate increase in the mechanical properties of the geopolymer matrix and partially reacted metakaolin phases in both specimens with the rising exposure temperature up to 400ºC. It is believed that such changes are probably due to the dehydration of both phases, together with a progressive increase in the development of new bonds (as suggested by the thermal analysis results). The quartz phase remained unchanged until 400 o C but disappeared at 700 o C. There seemed to be little change in the GP and the MK phases between 400ºC and 700ºC. Dramatic changes in the sample were observed at high temperature exposure between 700 to 1000ºC. The mechanical properties of the GP phase in both specimens were more than tripled, likely due to a partial softening of geopolymer structure from 788ºC and forming a vitreous phase upon cooling, as suggested by the TDA results. In the 1000 o C sample, the MK phase was no longer present, but a new phase, Mullite was observed in both samples. Nepheline was also found in the NaMk-I.. Optical images of the 1000 o C specimens (Fig.2) revealed that no MK and quartz phases were present in both samples, and many pores (up to 150 um) were developed in NaMk-I sample. Different phase identification was support by XRD. It is

5 Nanoindentation Study of Na-Geopolymers Exposed to High Temperatures 173 Table 4 Properties of different phases in geopolymer samples heated to high temperatures Tª NaMk-I NaMk-P ºC Phase E H % Phase E H % GP Mk Q GP GP Mk Mk Q Q GP Mk GP GP Nepheline Mullite Mullite Fig. 2 Optical images of specimens after the 1000 o C exposure: a) NaMk-I, b) NaMk-P believed that in the injection process, migration and accumulation of Na + in pores occurred due to the long and closed curing, leading to the formation of nepheline at about 1000 o C accompanied with a volume reduction and resulting voids. 4 Conclusions Statistical nanoindentation has been successfully used to provide information about the micro-mechanical properties of different phases and their volume distributions in the geopolymer samples. The manufacturing process was found to significantly affect the micro-mechanical properties and thermal stability of geopolymer materials produced. A long curing period in closed mould associated with the injection process appeared to be responsible for a higher degree of reaction and better mechanical properties of the geopolymeric phase. The long and closed curing, however, seemed to result in higher Na + concentration in pore solutions, which led to formation of nepheline and large porosity at ºC. On the

6 174 I. Beleña and W. Zhu other hand, the sample produced by direct pouring process using open mould and short curing period was found to lead to an incomplete reaction of the starting materials and, thus, lower mechanical properties of geopolymers. Generally, the mechanical properties of the geopolymer matrix showed a moderate increase when the sample was exposed to temperature up to 400ºC, no significant change at o C, and then an increase more than three times at 1000 o C. Acknowledgments. We are indebted to the Valencian Institut of Small and Medium Size Industries (IMPIVA) and European Social Fund for the financial support through the Program High Specialization in Industrial Technologies with the project High Specialization in Composed Nanomaterials with application in the Construction Sector. IMAETA /2005/1. References 1. Davidovits, J.: Geopolymer, Chemistry and Applications. Institute Géopolymère (Geopolymer Institute) (2008) 2. Hammell, J.A., Balaguru, P.N., Lyon, K.E.: Strength retention of fire resistant aluminosilicate-carbon composites under wet-dry conditions. Compos. Part B-Eng. 31, (2000) 3. Barbosa, V.F.F., McKencie, K.J.D.: Thermal behaviour of inorganic geopolymers and composites derived from sodium polysialate. Mater. Res. Bull. 38, (2003) 4. Barbosa, V.F.F., McKencie, K.J.D.: Synthesis and thermal behaviour of potassium sialate geopolymers. Matter Lett. 57, (2003) 5. Oliver, W.C., Pharr, G.M.: Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology. J. Mater. Res. 19, 3 20 (2004) 6. Ficher-Cripps, A.C.: Nanoindentation. Springer, Heidelberg (2002) 7. Ulm, F., Vandamme, M., Bobko, C., Ortega, J.A.: Statistical indentation techniques for hydrated nanocomposites: concrete, bones and shale. J. Am. Ceram. Soc. 90, (2007) 8. Zhu, W., Hughes, J., Bicanic, N., Pearce, C.: Micro/nano-scale mapping of mechanical properties of cement paste and natural rocks by nanoindentation. Mater. Charact. 11, (2007) 9. Beleña, I., Tendero, M.J.L., Tamayo, E.M., Vie, D.: Estudio y optimización de los parámetros de reacción para la obtención de material geopolimérico mediante 29 Si y 27 Al RMN y DRX. Boletín de la Sociedad Española de Cerámica y Vidrio 43 (2004)