Nanomechanical Characterization of the Carbonated Wollastonite System Warda Ashraf, Jan Olek, and Nannan Tian
Abstract This paper focuses on the nano-mechanical characterization of carbonated calcium silicate mineral (wollastonite (CaSiO3)) using nanoindentation technique. While exposed to carbon dioxide (CO2), the calcium component of wollastonite undergoes carbonation reaction which results in formation of two main products: calcium carbonate (CaCO3) and silica (SiO2). The mechanical properties of these partially reacted wollastonite systems were evaluated using the nanoindentation technique from which the reduced elastic modulus (Er) of silicate phase found to be around 38 GPa. For calcium carbonate phase this value was around 60 GPa. Keywords Wollastonite • Calcium silicate • Carbonation • Nanoindentation • Characterization
1 Introduction Carbonation of calcium silicate minerals is a well-known phenomenon as observed in cases involving CO2 sequestration and studies of ordinary portland cement (OPC). Calcium silicates produce silica gel and calcium carbonate phases during carbonation processes and hence capture the CO2 within the microstructure. This concept is widely utilized to capture and store the CO2 in calcium/magnesium silicate minerals and it is known as ‘mineral carbonation’ process [1–3].
W. Ashraf (*) • J. Olek Lyles School of Civil Engineering, Purdue University, West Lafayette, IN, USA e-mail:
[email protected];
[email protected] N. Tian School of Materials Engineering, Purdue University, West Lafayette, IN, USA e-mail:
[email protected] © Springer International Publishing Switzerland 2015 K. Sobolev, S.P. Shah (eds.), Nanotechnology in Construction, DOI 10.1007/978-3-319-17088-6_8
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The investigation of carbonation process of wollastonite mineral is thoroughly described in [4–6]. The set of equations presented below can be used to explain the carbonation process of wollastonite (CaSiO3):
CO 2 + H 2 O H + + HCO3−
(1)
CaSiO3 + H + + HCO3− SiO 2 ↓ + CaCO3 ↓ + H 2 O
(2)
Similar reaction mechanism can also be observed during the carbonation process of OPC based construction materials [7, 8]. The microstructural phases of hydrated OPC system, such as CSH, C2S and C3S were also found to produce silica gel and calcium carbonate as the reaction products while subjected to carbonation process. In this paper, the silicate and calcium carbonate phases produced during the carbonation process of wollastonite were nanomechanically characterized with the aid of nanoindentation technique.
2 Experimental Study 2.1 Sample Preparation Carbonated wollastonite sample was prepared according to the patent US20130122267 A1 [9]. Powdered natural wollastonite sample was first mixed with a small amount of water (water to solid ratio ~ 0.25) and compacted in 50 mm cube molds. These compacted cubes were then subjected to carbonation process by being exposed for 65 h to pure CO2 (100 % concentration) at the temperature of 60 °C. After the carbonation process was completed approximately 10 mm cube s amples were removed from the 50 mm cubes for experimental investigation. These 10 mm cube samples were polished on successively smaller mesh sizes (upto 0.25 μm) to obtain a very flat surface with minimal roughness.
2.2 Test techniques 2.2.1 SEM/EDS Analysis Scanning electron microscope (SEM) along with energy-dispersive spectroscopy (EDS) analyses were performed using a FEI NOVA nanoSEM field emission scanning electron microscope (FESEM) which was operated in high vacuum mode. The accelerating voltage was 20 kV and the working distance was 10.00 mm.
Nanomechanical Characterization of the Carbonated Wollastonite System Fig. 1 Loading cycles of nanoindentation tests
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2.2.2 Nanoindentations For nanomechanical characterization of the carbonated wollastonite, nanoindentations were performed using three different methods (later referred to as: method – 1, method – 2 and method – 3). In the first method, indentations were performed over a small area, mainly focusing on an unreacted grain and the surrounding area. In this case, a trapezoidal loading cycle with 8,000 μN maximum load was used (shown in Fig. 1). In the next method (method-2), nanoindentations were carried out in a grid pattern for using so called statistical nanoindentation technique [10, 11]. In order to do so, four areas of 80 μm × 80 μm were selected on the polished samples surface and a total of 256 indentations were performed (i.e., 64 indents on each area). For the last part of this experimental study (method – 3), a repetitive and consecutively increasing loading cycle was applied on 98 indentation points to identify the short term creep behavior of this system. In this case, for each indent point a total 5 loading cycles were applied with loads increasing from 2,000 to 6,000 μN (Fig. 1). Nanoindentation tests were performed using a Hysitron Triboindenter 950 system fitted with a Berkovich diamond indenter probe.
3 Results and Discussions 3.1 Microstructural Investigation Using SEM/EDS Figure 2 shows a SEM image of the carbonated wollastonite system along with the EDS patterns for individual microstructural phases. The brightest parts of the SEM image represents the unreacted grains of wollastonite as confirmed by 1:1 atomic ratio of Ca: Si. Silica gel primarily forms as a rim around the wollastonite grains and the calcium carbonate phase is mostly found to fill up the inter-particle spaces.
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Fig. 2 SEM image and EDS patterns identifying different microstructural phases of carbonated natural wollastonite system
Fig. 3 Contour map of reduced modulus, Er (GPa) around carbonated wollastonite grain showing as (a) transparent overlay on BSED image of actual indented area and (b) color filled contour with corresponding color scale
3.2 Nano Mechanical Characterization 3.2.1 Method – 1: Identification of the Phases In this method, the main objective was to use the SEM analysis to identify the nanoindentation marks so that a relative comparison of the reduced elastic modulus for the microstructural phases can be obtained. Figure 3 shows the variation of reduced elastic modulus on and around a wollastonite grain. The center region of the unreacted grain is associated with high values (~160 GPa) of the reduced elastic
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Fig. 4 (a) SPM image showing locations of 64 indentation points on an 80 μm × 80 μm area and (b) corresponding contour map of the moduli (GPa)
modulus. That value was found to be smaller around the edges of the grain. Few indentation points were also identified in regions occupied by the silica gel and calcium carbonate phases. Based on the findings of this test, the approximate values of reduced elastic modulus for silica and calcium carbonate phases were determined to be in the range, respectively, of about 40 and 60 GPa. 3.2.2 Statistical Nanoindentation To obtain the mean reduced modulus of individual microstructural phases, statistical nanoindentation technique [10, 11] was applied. Figure 4 shows a scanning probe microscope (SPM) image identifying the locations of 64 indentation points and the corresponding contour map of reduced elastic modulus values. This map was used to illustrate the locations of the unreacted grains and the pores. The Gaussian frequency distributions of the reduced modulus values for individual microstructural phases are given in Fig. 5a. These distributions were determined from the statistical deconvolution of cumulative frequency distributions as given in Fig. 5b. 3.2.3 Short Term Creep Behavior The load-depth responses of the silica gel, calcium carbonate and unreacted wollastonite phases while subjected to repetitive and increasing loading cycles are given in Fig. 6a. The blunting of the tip of load-depth curve with increased and repetitive load reflects the creep behavior of silica gel and calcium carbonate phases [12]. However, the minimal variation of the modulus values with increasing load (Fig. 6b) also indicates that the effect of the indentation size was not significant at least within this load range.
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b Cumulative Frequency, %
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Calcite
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Fig. 5 (a) Frequency distribution and (b) cumulative frequency distribution plots of reduced modulus, Er (GPa) for carbonated wollastonite sample
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Fig. 6 (a) Load depth response of microstructural phases for different loading functions, (b) variation of moduli (GPa) with increasing loads
4 Conclusions In this paper, the silica gel and calcium carbonate phases that formed during the carbonation of calcium silicate minerals were characterized using nanoindentation technique. The reduced elastic modulus for silica gel and calcium carbonate found to be 38 ± 10 GPa and 61 ± 16 GPa, respectively. The creep effect of silica gel phases was found to be higher than that of calcium carbonate phase. Acknowledgement Solidia technologies are gratefully acknowledged for providing testing materials required for this study. Authors would also like to acknowledge the Rutger University, NJ, USA for the development of the technology.
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References 1. Huijgen, W. J. J., & Comans, R. N. J. (2005). Carbon dioxide sequestration by mineral carbonation: literature review update 2003–2004 (Report). Energy Research Centre of the Netherlands. 2. Goff, F., & Lackner, K. (1998). Carbon dioxide sequestering using ultramafic rocks. Environ mental Geosciences, 5(3), 89–101. 3. Lackner, K. (2003, June). A guide to CO2 sequestration. Science, 300, 1677–1678. 4. Daval, D., Martinez, I., Corvisier, J., Findling, N., Goffé, B., & Guyot, F. (2009, July). Carbo nation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modeling. Chemical Geology, 265(1–2), 63–78. 5. Daval, D., Martinez, I., Guigner, J. M., Hellmann, R., Corvisier, J., Findling, N., et al. (2009, November). Mechanism of wollastonite carbonation deduced from micro- to nanometer length scale observations. American Mineralogist, 94(11–12), 1707–1726. 6. Tai, C. Y., Chen, W. R., & Shih, S. M. (2006, January). Factors affecting wollastonite carbonation under CO2 supercritical conditions. AIChE Journal, 52(1), 292–299. 7. Chun, Y., Naik, T., & Kraus, R. (2007). Carbon dioxide sequestration in concrete in different curing environments. In Conference: Sustainable Construction Materials and Technologies, Coventry, Special Papers Proceedings pp. 18–24. 8. Papadakis, V., Vayenas, C., & Fardis, M. (1991). Fundamental modeling and experimental investigation of concrete carbonation. ACI Materials Journal, 88, 363–373. 9. Riman, R. E., Gupta, S., Atakan, V., & Li, Q. (2013). U.S. 20130122267 A1: Bonding element, bonding matrix and composite material having the bonding element, and method of manufac turing thereof. 10. Constantinides, G., Ravi Chandran, K. S., Ulm, F. J., & Van Vliet, K. J. (2006, August). Grid indentation analysis of composite microstructure and mechanics: Principles and validation. Materials Science and Engineering: A, 430(1–2), 189–202. 11. Ulm, F. J., Vandamme, M., Bobko, C., Alberto Ortega, J., Tai, K., & Ortiz, C. (2007, September). Statistical indentation techniques for hydrated nanocomposites: Concrete, bone, and shale. Journal of the American Ceramic Society, 90(9), 2677–2692. 12. Němeček, J. (2009, September). Creep effects in nanoindentation of hydrated phases of cement pastes. Materials Characterization, 60(9), 1028–1034.
