A technique for preparing TEM specimens from cementitious materials

CEMENT and CONCRETERESEARCH. Vol. 19, pp. 642-648, 1989. Printed in the USA. 0008-8846/89. $3.00+00. Copyright (c) 1989 Maxwell Pergamon Macmillan

A TECHNIQUE FOR PREPARING TEM SPECIMENS FROM CEMENTITIOUS MATERIALS

Douglas G. Ivey° and Mafia Neuwirth# *Dept. of Mining, MctsUurglcal and Pcu-oleum Engineering University of Alber~ Edmonton, Alberta, Canada, T6G 206 # Alberto Environmental Centre Vegreville, Alberta, Canada, T0B 4L0

(Con~unlcated by G.G. Litvan) (Received Nov. 8, 1988)

Trawm/~ion/seaaning transmission electron microscopy (TEM/STEM) is a valuble analytical technique for studying the many phases presem in cememitio~s ma~ials. Specimenprepar~on is, however, a problem ~m~ing ~ full exploitation of this technique. A relatively simple specimen preparation technique, combining dimpling and ion milling, has been presented here. Successful specimens have been prepaw.d from ordinm7 Portland cement (OPC) and cement s t a ~ heavy metal waste samples. Introducdoq Transmission and scanning transmission electron microscopy (TEM and STEM) have been used increasingly to the study the complex microstructures of hydrated Portland cements (1-6). Recently interest has also focussed on the utilization of cements in solidification/stabilization processes for disposal of ba-~rdons liquid wastes (7-9). TEM/STEM techniques could prove particularly useful in stud~yingthe mechanisms involved in these processes. One of the major fimitations of any TEM work lies in specimen preparation. Electron wanspm-entregions are required, which means thinning a specimen down to a fldckness of about 100rim. This is especially a problem with cement materials because of their porous and bri~e nature. Several techniques have been attempted, including grinding, exu'action and ion milling (1,2,6). There are many drawbacks associated with these techniques and the resultant specimens

id ..For em p.me,.boh

ex tlon. c ques only

the

ervauon oz se~ecum phases aria not me over~ m~crostructure, while grinding also inu'oduces the additional problem of mechanical damage. Ion mill-:ng is the best technique in terms of the finished product, but specimen preparation by this method is very tedious and time consuming. Since specimens can only be mechanically polished to a thickness of 30-50gt, 15-30 hours of ion milling time is required. Often, even after this procedure has been fcllowed, a high voltage electron microscope (HVEM) is required to provide adequate electron pencumion for observation.

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643 TEM, SPECIMEN PREPARATION, WASTE

In this report, we present a relatively si.mple and far less ~ consuming teclmique for the preparation of thin foil specimens from cementiuous materials. This method involves the use of a combined dimpling/ion milling process which permRs thinning of the centre of the specimen while mainlining a r~latively ~hick outer rim, thereby facilitating specimen handling. $.l:~'imens, prepared by this technique, are thin enough to be examined in a 100kV electron rmcroscope, eliminating the need for the somewhat more expensive 1000kV instrument. Exr~erirnentalMethods The starting material utilized in this study was Type H ordinary Portland cement (OPC), which was mixed with distilled water (water/cement ratio-- 0.3) and cured for 28 days under 100% relative humidity. Another batch of samples was mixed with a sludge containing 0.4M concentrations of hydroxides of Cr, V and Cd, which are rtpical heavy metal induswial wastes. The concentrations were somewhat higher than those found in actual industrial wastes. However, this initial study was intended to ascertain whether adequate TEM specimens of these materials could be prepared and whether these elements could be located in particular phases by x-ray spectroscopy. A 2-4ram thick slice was cut from the cured material using a low speed diamond saw. Discs, 3ram in diameter, were machined from this slice,using an ultrasonic disc cutter with a hollow aluminum bit. The cutting action was provided by a SiC slurry positioned between the specimen surface and the drillbit. One of the discs was mechanically polished, from both sides, to a thickness of 200-250g. Initially,30g lapping discs were used to remove most of the material, while finalpolishing was done with 5~t discs. The specimen was then dimpled, one side only, using a Gatan Model 656/3 Dimple Grinder. Prior to dimpling, the specimen was mounted on a stainlesssteel plug with Crystal Bond, which is a low melting point wax that dissolves in acetone. The mounted specimen was subsequently placed on a magnetic turntable,which allowed the specimen to rotatewithin itsown plane. A 15rnm diameter phosphor bronze polishing wheel with sphericaledges was coated with a small amount of Ig diamond paste and lowered onto the surface of the specimen. Using a low rotation speed (2-3 on a scale of 10) and a small load (20g), the polishing wheel gently machined a spherical depression in the centre of the disc. The specimen was carefullydimpled until a small hole (<0.5rnm in diameter) appeared. Because of the large radius of curvature of the polishing wheel, the region immediately surrounding the hole was fairlyflatand opticallytransparent. The specimen was removed from the plug, any Crystal Bond dissolved in acetone, and rinsed in ethanol. Specimens at thispoint possessed some electron transparentregions, however, remnants of the polishing paste remained on the surface of the specimen. Final thinning was undertaken, by means of ion milling, to remove any polishing paste on the surface and to increase the amount of thin area. Specimens were ion milled with Ar as the sputtering species at a potential of 5-6 kV, a current of =0.5 mA/gun and an incidence angle of 1520° to the horizontal. The ion milling time was about 3-6 hours, at a rate o f , ~ l ~ r / g u n , giving a total preparation time of 5-8 hours per specimen. Specimens were examined in an Hitachi H-600, 100kV TEM eqt~ipped with a Kevex beryllium window, x-ray detector. Results and Discussion Successful specimens were obtained, although the amount of thin area in each specimen was quite variable. A number of phases were identified in the TEM with the aid of selected area diffraction (SAD) and x-ray spectroscopy. These preliminary results will be discussed briefly in turn in the following paragraphs.

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Vol. 19, No. 4 D.G. Ivey and M. Neuwirth

Alite was found in all specimens examined, including those mixed with the liquid metal waste (Fig. 1). The crystal structure was identified as being monoclinic, with a : 3.308 nm, b= 0.707 nm, c= 1.856 nm and ~ : 94.17 ° (2). Evidence for the Jeffrey rhombohedral pseudostructure (1,2), with hexagonal axes of a= 0.7 nm and c : 2.50 nm, was also found. The relationship between the monoclinic and hexagonal structures can be seen in Figs. lb and lc and in reference (10). The strong reflections are from the hexagonal subcell, while the weaker, more closely spaced reflections are from the monoclinic ceil. X-ray spectroscopic results revealed the presence of Ca and Si in the appropriate proportions along with small amounts of AI and Fe impurities (Table 1).

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FIG. 1 ~M

bfght field image (a) and SAD patterns (b) ~ d (c) of the alite phase in Type ~ OPC.

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No.

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TEM, SPECIMEN~R~ARATION,WASTE

645

TABLE 1 Background Subtracted Intensities from X-ray Spectrum of Alitc Phase

~

~unts 1.74 3.69 6.40

12112 38842 429

~ Si; Kal, Ka2 Ca; Kctl, Kct2 Fc; Kct 1, Ka2

Calcium silicate hydrate (C-S-H), the major product of hydration, was the most prevalent phase in the OPC specimens not mixed with the heavy metal waste. A bright field image and a selected area diffraction pattern are shown in Fig. 2. The phase was amorphous, as indicated by

150

nm

~

(a)

(b) FIG. 2

Bright field image and SAD pattern from CSH phase in Type II OPC.

the diffuse rings in the SAD pattern. The Ca/Si ratio was quite variable (1.2-2.2) and A1, S, K and Fe were present in small amounts as impurities. Argon, implanted during ion milling, was also detected. Specimens mixed with the heavy metal waste showed two forms of calcium silicate hydrate, both with variable C,a/Si ratios (Fig. 3). One ~ was identified as the amorphous C-S-H phase with similar low levels of A1, S, K and F¢ as well as Mg. A crystalline form was also identified, with a crystal structure matching that of the 1.0rim form of tob~rmorite, ie an orthorhombic structure with a - 0.73rim, b= 1.12rim and c= 2.0$nm. This crystalline phase contained the same impurities as above, as well as small but significant quantities of both V and Cr (Table 2). No Cd was detected. Further analysis is r~luired, but our preliminary results indicate that V and Cr may play some role in the crystallization of C-S-H.

646

Vol. 19, No. 4 D.G.

Ivey ~nd M. Neuwirth

O

(a)

(b) FIG. 3

Bright field image and SAD pattern of calcium silicate hydrate phases from the cement stabilized waste material. SAD pattern in (b) is from the crystalline region of (a) and corresponds to the 1.0nm form of tobermorite. TABLE 2 X-ray Counts of Major Peaks Taken from Representative X-ray Spectra from Phases in Fig. 3 Energy (kV)

Counts

Kc~ Lin,es

Crystalline

1.46 1.74 3.69 4.95 5.42 6.41

159 26853 38080 1678 449 604

A1 Si Ca V Cr Fe

Amorphous

1.74 3.69

17638 26227

Si Ca

Calcium hydroxide (Ca(OH)2), which is another major product of hydration, was not detected in any of the specimens. However, our specimens were not protected from the atmosphere and consequently may have reacted with CO2 to form calcite (CaCO3) (6). Calcite was identified in both waste containing and non-waste containing specimens and a representative diffraction pattern is shown in Fig. 4. Other phases were detected in small amounts. Calcia (CaO) was found in both types of specimens (SAD pattern in Fig. 5), while hydrates of tetracalcium aluminoferrite (4CaO2.A1203.Fe203 or C4AF) (Table 3), containing significant quantifies of V and Cr, were

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TEM, SPECIMEN PREPARATION, WASTE

647

HG. 4 SAD Pattern taken from calcite (CaCO3) region in Type 1I OPC specimen.

observed in the some of the heavy metal specimens. Argon, implanted from the sputtering process, was also detected in the C~AF hydrate. Conclusions A relatively simple TEM specimen preparation technique for cementitious materials has been presented here. The technique.combines dimple grinding with ion milling and significantly reduces preparation time over more conventional methods. Successful specimens were produced from OPC samples as well as cement stabilized heavy metal waste materials. Phases identified

FIG. 5 SAD pattern taken from calcia phase (CaO) of the Type II OPC specimen.

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Vol. 19, No. 4 D.G. Ivey and M. Neuwirth

TABLE 3 X-ray Counts from X-ray Slx~CtrUm of a Hydrate of Tctracalcium Aluminofcrritc (C4AF) from Cement Stabfliz~ Heavy Metal Specimen

~

Counts 1.49 1.75 2.96 3.69 4.49 4.95 5.41 6.39

6520 2122 1216 30996 258 1009 665 6596

~ A1 Si Ar Ca Ti V Cr Fe

include aiitc, amorphous C-S-H, calcite and small amounts of calcia and tctracalcium aluminofcrritc hydrates. Acknowledgements We would like to thank Peter Hannak and Diana Chambers for supplying the cement samples and Rosemary Harris for technical assistance in electron microscopy. References 1. P. Barnes and A. Ghose, "The Microscopy of Unhydrated Portland Cements", from Structure and Performance of Cements, P. Barnes, ed., p. 134, Applied Science Publishers (1983). 2. W. Sinclair and G.W. Groves, J. Am. Cer. Soc., 67, 325 (1984). 3. F.V. Lawrence, Jr., D.A. Reid and'A.A, de Carvalho, J. Am. Ccr. Soe., ~

144 (1974).

4. H.M. Jennings, B.J. Dalgleish and P.L. Pratt, I. Am. Cer. Soc., 6~, 567 (1981). 5. D.D. Double, Mater. Sci. and Eng., 12, 29 (1973.). 6. G.W. Groves, P.J. Lc Sueur and W. Sinclair, J. Am. Cer. Sot., 69, 353 (1986). 7. M. Neuwirth, R. Mikula and P. Hanak, "Comparative Studies of Metal Containment in Solidified Matrices by Scanning and Transmission Electron Microscopy, presented at 4th International Hazardous Waste Symposium on Environmental Aspects of Stabilization/Solidification of Hazardous and Radioactive Wastes, (Atlanta, Georgia, 3-6 May, 1987). 8. H.C. Eaton, M.B. Walsh, M.E. Tittlebaum, F.K. Cartledge and D. Chalasani, "Microscopic Characterization of the Solidification/Stabilization of Organic Hazardous Wastes", presented at Energy Sources and Technology Conference and Exhibition, (Dallas, Texas, 17-21 February, 1985). 9. B.J. Dagleish, P.L. Pratt and R.I. Moss, Cement and Concrete Research, 10, 665 (1980). 10. M. Regourd, "Crystal Chemistry of Portland Cement Phases", from Structure and Performance of Cements, P. Barnes, cd., p. 109, Applied Science Publishers (1983).