A NOVEL TECHNOLOGY IN WET SCANNING ELECTRON MICROSCOPY

Transcription

1 A NOVEL TECHNOLOGY IN WET SCANNING ELECTRON MICROSCOPY Amnon Katz, Arnon Bentur, Konstantin Kovler National Building Research Institute, Faculty of Civil and Environmental Engineering, Technion Israel Institute of Technology, Israel Abstract Scanning electron microscope (SEM) does not allow observation of wet specimens, mainly due to the high vacuum required for its operation. Therefore, in situ observations at the reaction development of hydraulic binders are impossible. Environmental SEM technology can resolve some of the problems involved with regular SEM operation, but still with some limitations. A novel technology (WETSEM TM ) was developed lately to allow observation of wet processes in a conventional SEM while utilizing all its observation capabilities (i.e. back scattered electrons, secondary electrons and energy dispersive spectroscopy). The paper presents results from an initial study on the early age hydration (0-24 hours) of gypsum, ordinary portland cement and blended cement by using this technology. Pastes of gypsum, cement and cement + metakaolin were prepared at different water/binder ratios and the early age hydration was investigated by discrete and sequential observations. 1. INTRODUCTION The development of early age properties of hydraulic binding materials (gypsum, ordinary Portland cement, etc.) has a strong impact on the properties of the hardened product. At the early age, the basic structure of particles is set and further microstructure is developed in between these particles. The reaction rates are intense at early age (~1 st day), and slow down later on. Thus understanding the reactions at this age, and the development of microstructure are crucial for controlling the properties of the hardened material. Scanning Electron Microscope (SEM) is used to make observations at these systems; however, the high vacuum required for proper operation of the microscope requires initial drying of the sample for observation. The drying treatments may alter the microstructure and prevents accurate observations [1]. Environmental SEM allows, under certain conditions, to make observations at wet specimens. Such microscopes are not readily available and maintaining fully sealed conditions, as close as possible to 100%RH, is not always straightforward [2-5]. This paper presents a technology that allows observation of wet specimens maintained in their natural conditions while making the SEM observations. Early age hydration of cement, cement + metakaolin and gypsum was monitored using this technology. 1

2 2. WETSEM TECHNOLOGY WETSEM technology was developed to allow SEM observation of wet specimens in their close to natural conditions, i.e. there is no need to dry the specimens or to coat them with a conductive film before taking them into the SEM chamber for observations. A small capsule is used to hold the tested material under sealed conditions (Figure 1). A thin membrane allows the electron beam to penetrate into the capsule cavity, and the emitted electrons can escape out of the capsule to the appropriate detector. This mode of operation allows the use of any standard SEM equipped with a back scattered electron (BSE) detector. A special capsule, QX- 202C, was developed (Figure 1a) for viewing hydraulic materials such as Portland cement or gypsum. QX-102 QX-200C Figure 1: Description of WETSEM capsule. 3. EXPERIMENTAL STUDY Pastes of Portland cement and gypsum were prepared. The cement paste was prepared at water/cement ratio of ~0.42 and the gypsum at The effect of additive was tested by replacing 20% of the cement by metakaolin, at two water to binder (w/b) ratios 0.45 and In this study, gypsum was used for "in situ" observations of its hydration. The clear crystalline nature of its hydration products together with its rapid rate of reaction enables one to follow the hydration within a relatively short time and save valuable microscope time. The pastes were hand mixed, and inserted into the capsule cavity by a small spatula or by using a calibrated pipette. Ultrasonic vibration was used to ease mixing of the metakaolin mix at low w/b ratio. 4. RESULTS 4.1. Cement paste Micrographs of cement pastes at age ~4 hours and 1 day are shown in Figure 2. The micrograph at the age of 4 hours shows a mixture of ground particles of various sizes ranging 2

3 from a few microns up to ~20 microns, which are typical to cement grains. In addition, few long crystals are shown in the micrograph. EDS (energy dispersive spectrometry) analysis showed that these crystals contain significant amount of Ca and no aluminum or sulfur, and therefore they are presumably Ca(OH) 2 crystals that start to develop at this very early age. Observation of another sample (Figure 2b), prepared by using a spatula rather than a pipette, showed that the cement grains are located somewhat away from the membrane, probably due to the formation of a thin layer of water over the membrane. In both cases, the grains are dispersed unevenly, which might be associated with the preparation of a small sample. (a) (b) (c) (d) Figure 2: Cement paste, low w/c ratio, (a) 4 ours after mixing (prepared with a pipette), (b) cement grains located away from the membrane with a separating water layer (prepared with a spatula), (c) development of massive Ca(OH) 2 over the membrane at 1 day and (d) observation through the Ca(OH) 2 layer at 1 day. 3

4 It should be noted that due to the limitations of observation, only the particles that are close to the membrane are seen clearly whereas large particles that are located away from the membrane due to wall effect are not seen, or only their tip is seen in the micrograph. Massive development of Ca(OH) 2 over the membrane surface (wall effect) was seen at age 1 day (Figure 2c). This layer made the observation difficult as the hydration products were developed behind this layer. However, some hydration products could be seen through the uncovered areas. Acicular hydration products could be seen around cement grains at a high magnification (Figure 2d), which resemble fibrous C-S-H type I that form at early age [6] Cement + metakaolin Very dense system was seen with the paste containing metakaolin (Figure 3a) prepared at low water/binder ratio. The very fine particles of the metakaolin, typically µm, fill the (a) (b) (c) (d) Figure 3: Cement + metakaolin paste (80:20), w/b=0.45, (a) 4 hours and (b, c, d) 1 day after mixing. 4

5 space between the relatively large cement particles to form a dense particle packing as seen in the micrograph. Large cement grains are engulfed by fine metakaolin particles. Although only 20% of the cement was replaced by the metakaolin, the total number of the fine particles is much higher, which leads to a significant filler effect. This, in turn, thickened the fresh mix during preparation, and led to a better contact between the paste and the membrane (less of a wall effect). Similar paste that was prepared at a higher water/binder ratio (0.65) exhibited the same problem of formation of a thin water layer over the membrane that prevented a direct contact of the cement and metakaolin particles with the membrane at early age. Dense mix with initial formation of hydration products could be seen at age 1 day (Figure 3b, w/b=0.45). Ca(OH) 2 crystals were seen at this age, though to a lesser extent compared with the paste of neat cement. The bulk mass of material that was seen through most of the membrane area was very dense, as seen in the upper right corner of Figure 3b. At the bottom left corner, some less dense material was seen, which allowed better resolution at large magnification, as seen in Figure 3c. A large cement grain is seen in the center of this micrograph, connected by fibrous hydration products to the surrounding small particles. Paste that was prepared at a higher w/b ratio of 0.65 exhibited a more open structure that allowed observation at very high magnification (x16000) (Figure 3d). A dense network of fibrous material connects the large (cement) grains with the small ones (metakaolin). This demonstrates the significant effect of fine particles in forming a denser and more uniform C- S-H structure compared with neat cement paste discussed before 4.3. Gypsum Gypsum paste at age ~4 hours and 1 days is seen in Figure 4a and b, respectively. The micrographs present a network of fibrous crystals which is characteristics of a gypsum system. This network of fiber-like crystals contains voids between the fibers, which is also typical characteristic of gypsum systems. The structure is formed quite rapidly as can be seen from the small differences between the two micrographs. It is possible, though, that the membrane forms a preferred 2-D network of fibers that do not represent the real 3-D structure of the bulk paste that might be more open and develop more slowly. Figure 4: Gypsum paste, w/g=0.6, ~4 hours (left) and 24 hours (right) after mixing. 5

6 Observations during the first hour yielded a "live" view of the development of the microstructure of gypsum system. Observations at 17 minutes after mixing show some grains of hemi-hydrate that dispersed over the field of observation. Few needle-like crystals begin to form in the right hand side of the micrograph. The amount and size of these crystals increase rapidly within a very short time, as seen in the micrographs taken 33 and 57 minutes after mixing. (17 min) (33 min) (57 min) Figure 5: Gypsum paste, w/g=0.6 at early age. 5. DISCUSSION Some possible interactions between the cast paste and the membrane, as well as the effect of the energy of the electron's beam must be considered. The development of the hardened cement paste close to the membrane is influenced by the wall effect, which resembles some characteristics of the interfacial transition zone. The large particles of the cement grains 6

7 cannot compact well close to the membrane surface due to their size and due to the formation of a thin layer of water that wets the membrane. This effect seems to be more pronounced at high water/cement ratio and is sensitive to the method of specimen's preparation. Systems that have less free water, such as mixes that contain fine particles (metakaolin, for example), are less prone to this problem, thus enable better observations. The metakaolin system exhibits better particle compaction close to the membrane, which additionally improves the observations. Effect of local energy interference was seen in some cases. Some small crystals began to develop over the membrane while observing the same spot for a long time at high magnifications (Figure 6). A "long time", for example, was the time required to obtain proper focus of cement paste at age 4 hours (Figure 6a). It is possible that some systems are more prone to this effect, like cementitious system at very early age or gypsum that decomposes at low temperature. (a) (b) Figure 6: Possible effect of energy from the electron's beam in cement (a) and gypsum (b) pastes. 6. CONCLUSIONS Wet specimens of cement and gypsum could be readily prepared and observed in SEM by using the WETSEM technology without interfering the microscope operation. Fibrous C-S-H products could be readily seen in cement paste at 1 day. Fine powder additive, such as metakaolin improves the microstructure of the C-S-H and makes it more uniform and denser. Development of gypsum structure was observed under real-time conditions from the appearance of the first crystals, their growth and widening. The development of hydration products might be affected by the presence of the membrane due to a wall effect. This effect hinders proper compaction of the grains near the membrane or alters the orientation of the hydration products. Observation for an extended time at high magnification at a local spot may lead to local changes in the material in this area. 7

8 7. ACKNOWLEDGMENT The authors express their thanks to Quantomix Ltd. for the technical support and discussions. The guidance of Mr. Ofer Zrihan in carrying in the specimen preparation and testing is highly appreciated. 8. REFERENCES [1] Bisschop, J; van Mier J. G.M., "How to study drying shrinkage microcracking in cementbased materials using optical and scanning electron microscopy", Cement and Concrete Research, Vol. 32(2), 2002, pp [2] Diamond S., Hydraulic cement pastes: Their structure and properties, Cement and Concrete Association, p.2, Slough, UK, 1976 [3] Ye G., Hu J., van Breugel K. and Stroeven P., Characterization of the development of microstructure and porosity of cement-based materials by numerical simulation and ESEM image analysis, Materials and Structures, 35 (254), 2002, pp [4] Sun W., Zhang Y-S., Lin W. and Liu Z-Y., In situ monitoring of the hydration process of K-PS geopolymer cement with ESEM, Cement and Concrete Research, 34(6), 2004, pp [5] Kjellsen K.O. and Jennings H.M., Observations of microcracking in cement paste upon drying and rewetting by environmental scanning microscope, Advanced Cement Based Materials, 3(1), 1996, pp [6] McDonald A.M., Environmental scanning electron microscopy ESEM, Materials World, 6(7), 1998, pp