Environ. Sci. Technol. 1993, 27, 1312-1318
Mechanisms of Metal Stabilization in Cementitious Matrix: Interaction of Tricalcium Aluminate and Copper Oxide/Hydroxide Tzong-Tzeng Lln,t Cheng-Fang Lln,’*t Wen-Cheng J. Wel,* and Shang-Lien Lot
Graduate Institute of Environmental Engineering and Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, 106 Republic of China This present study investigated two fixation systems. The first incorporated different copper-bearing species [such as Cu powder, Cu(N03)~solution, and Cu(OH)a/CuO sludge] into cement, while the second was CuO solidified in tricalcium aluminate (3Ca0-&03, abbreviated as CA). The interactions between the fixation systems and the copper species were characterized using X-ray diffractometer (XRD) and both scanning and transmission electron microscopy (SEM and TEM, respectively) equipped with energy-dispersive X-ray analysis (EDAX). The XRD examinations indicated that both two-fixation systems provided the appropriate alkaline and humidified environment for fixative hydration which enhanced the solubility of the copper species. The characterizations of the interfacial microstructures by SEM between C A paste and CuO revealed that the interaction zone is present about 2 pm thick and that the CuO is entrapped by the hydration products of C3A. Microchemical analyses by SEMIEDAX and TEMIEDAX of hydrated C3A and CuO indicated that the concentration gradient of copper species exists in the hydrated C3A and that no diffraction patterns of metal copper, CuO, Cu(OH)2, and CuCO3 are observed in the same region. Thus, it was apparent that the CuO was stabilized in C3A by two major mechanisms. The first is the heterogeneous solid solution of copper species in the hydrated C3A, and the second is that CuO can be physically entrapped within the hydration products of C3A.
Introduction
Solidification/stabilization (S/S) technology has been used for over 20 years to treat industrial wastes. More recently, the technology has been applied to contaminated soils and safe disposal of combustion residuals. Briefly, solidification involves the addition of inorganic materials (e.g., cement) to the waste to produce a solid material. Thus, the solidified wastes can be disposed of with minimum environmental impact. The benefit of the stabilization techniques is to limit the solubility or mobility of contaminants within the solid matrix. The S/S technology has received considerable attention because of regulations in hazardous wastes management (1). The selection of type and quantity of fixing agents and additives (such as fly ash, slag, soluble silicate, etc.) has usually relied on trial-and-error method and empirical formulas (2). Most researchers evaluated the effectiveness of different S/S processes for hazardous waste control by measuring the physical properties (such as strength, density, permeability, porosity, and durability) and the various leaching procedures. Very few studies have been performed to disclose the mechanisms involved in the S/S processes. There is still a controversy over whether the + Graduate Institute of Environmental Engineering. t
Institute of Materials Science and Engineering.
1312
Envlron. Sci. Technol., Vol.
27, No. 7, 1993
waste is physically entrapped in the inorganic matrix or chemical stabilization reactions occur between the waste and the inorganic materials (2). It is very important to note that a better understanding of the S/S reaction mechanisms would improve the problems encountered in the S/S systems, including mass enlargement, volume expansion, and hydration interferences, and, hence, would reduce or modify the leachability of stabilized wastes placed in landfills or other environments. Recent studies have shown that zinc has been retained considerably more effectively than mercury. Their findings were based on leaching tests and porosity measurementa. Because the observed leachability was not consistent with the measured porosity of the solidified matrix, they reported that permeability is not an important factor in controlling the movement of metal speciesin the matrix. Thus, it was stated that chemical stabilization rather than physical encapsulation is the major factor in confining the metal mobility (2, 3). Some studies reported that the hydration and mechanical strength of cement are retarded and that porosity is generally increased with the addition of lead nitrate. By SEM observations, it was hypothesized that lead precipitates form a colloidal membrane around the cement grains, thus retarding the cement hydration (4,5). Although other studies have been devoted to define the microchemistry of SIS processes, the reaction mechanisms have not yet been interpreted withmicrostructural and microchemical evidences (6-10). One of the limitations of the above studies is that, because the interface between cement phases and metal waste is fragile and submicron in scale, it is fairly difficult to observe their interactions with ordinary optical or SEM microscopy alone. Hence, there is really a research need where the interactions between cement constituent and metal waste should be fully investigated on a microscale basis. Through the gathering of these pieces of information, the insight of cement-based solidification processes will not be a mystery anymore. The present study employed an energy-dispersive X-ray analysis (EDAX) and X-ray diffractometer (XRD) techniques to observe microchanges in the cement phase and the incorporated copper species. We extended our studies by evaluating the interactions between tricalcium aluminate (a major cement constituent, 3CaO-Alz03, abbreviated as C3A) and CuO using the SEMIEDAX and TEM (transmission electron microscopy)/EDAX techniques.
Materials and Methods Sample Preparation. Copper powder (99.7 % ,Merck, Darmstadt, Germany) used in this work was passed through a no. 325 mesh sieve (0.053 mm). Cu-bearing sludge was prepared by mixing Cu(N03)2 with NaOH at pH = 10 for 10-15 min, was washed, and was vacuum dried. A wastelcement ratio of 0.4 was used in all 0013-936X/93/0927-1312$04.00/0
0 1993 American Chemical Society
5
15
25
35
C
A
45
25
15
35
45
55
65
coco
>
5z
>
t ffl
W !-
2 W I-
z
z
W
L I-
5W a
W
cu
2 c
2 9W
A
P
02
A A
I
A
ai
CH
2-Theta Flgure 1. XRD patterns of solldlfled cement samples: (a) control hydrated cement; (b) Cu powder solidifled wlth cement; (c) Cu(NO& solution solldlfled wlth cement; (d) Cu(OH)&uO sludge solidified with cement. Symbols are as follows: A, tricalclum sillcate (3Ca0-S102); B, dlcalcium silicate (2Ca0-S102); CC, calclum carbonate (CaCO3); CH, calcium hydroxlde [Ca(OH)2];CO, copper oxlde (CuO); CU, copper (Cu); CAH, calcium aluminate hydrate (4Ca0-AI2O3-13H20); ET, ettringlte (3Ca0-AI2O3-3CaSO4-31 H20).
Flgure 2. XRD patterns of solidified tricalclum alumlnate (C3A) samples: (a) controlhydrated C3A sample; (b) CsA/CoO solldlfled matrlx. Symbols are as follows: A, calcium alumlnate hydrate (3Ca0-AI2036H20); B, calcium alumlnum oxMe carbonate hydrate (3Ca0-AI2O3CaC03-1 1H20);C, calcium carbonate (CaC03); D, glbbsite (yAln033H20); E, copper oxlde (CuO).
solidification samples, Le., 4 g of copper powder or dried copper hydroxideloxide, or 4 mL of 1000 ppm copper nitrate solution was mixed with 10 g of cement, and the mixture was controlled in a waterlcement ratio which equals 0.4. The solidified matrices were cured at 25 "C in a well-humidified environment for 28 days. Tricalcium aluminate (3Ca@&03, abbreviated as C d ) was prepared by mixing a 3:l molar ratio of reagent-grade calcium carbonate (Cerac, 99.95 % purity, Milwaukee, WI) and alumina (Alcoa AlGSG, Pittsburgh, PA), homogenized with isopropyl alcohol in a ball mill, followed by drying at 85 "C for 2 h, calcining at 950 OC for 2 h, sintering a t 1350 OC for 5 h, and air quenching. Thereafter, the solidified C3AICuO sample for XRD was prepared first by completely mixing equal weights of C3A and CuO, then water was added based on a waterlsolid (solid = C3A powder + CuO powder, C3A=CuO) ratio of 0.4 to carry out the hydration and/or other relevant reactions. The curing process proceeded the same as the blank cement sample. The copper foil of 0.1-mm thickness (JM Science, 99.8 % purity, Ward Hill, MA) was heat-treated to produce thin layers (thickness, about 2-3 pm) of CuO. The sandwich sample used for SEM/EDAX analysis was composed of C3A paste (outside layer), CuO (middle layer), and copper foil (inside layer). It was prepared by immediately coating the heat-treated copper foil with C d paste. This sandwich sample was cured following the above curing procedure. The microtomed sample for TEM/EDAX was prepared using the solidified C3AICuO sample. The matrix was
polished and the 100-pm slice was embedded in G-1epoxy (Gatan Instrument Co., Pleasanton, CAI. After being cured on a hot plate for 2 h at 60 "C, the epoxy-mount sample was trimmed to a trapezium. The thin section was cut with a diamond knife to approximately 50-100 nm thick using a microtomer (LKBNoba ultramicrotomer) operated a t a speed of 1 mmls. Analysis. Pulverized and sieved samples were used for XRD analysis with a Philips Powder X-ray diffractometer (Model PW1710). Operation parameters are as follows: Cu K a radiations, 30 kV, and 20 mA. The measured intensities and 28 values were recorded and analyzed by the computer system. The samples were scanned from 5 to 75" 28, and the scan rate was fixed at 0.050 2e/s. All fine-polished sandwich samples were coated with thin gold film for morphology observations using SEM and microprobe analysis. The element distribution was obtained by the technique using line scanning mode in EDAX (SEM Philips 515 with a EDAX 9100 detector). The SEM was operated at 20 keV accelerating voltage under a vacuum chamber pressure of less than 5 X 1od Torr. The thin sections were put on a copper grid and coated with thin carbon film for TEM observations using TEM (JEOL 2000 FX analytical electron microscopy) equipped with Link System 10000 EDAX operated a t 160 kV. The TEM/EDAX analyses from the microtomed specimens are performed by using bright field (BF)images, selected
2-Theta
Envlron. Sci. Technol., Vol. 27, No. 7, 1993
1313
A b
9. E M mbcqaphs ol hyaatsd CBRYMt samples: (a) pUe hy&ated cement showlng the presence of nwdle-llkeenrlngiie (AH): (b) Cu gah sciidlfled by csmanl showlng the abwnce of ettrlcgke: (c) Cu(No& sokmon SolIdlRed by camtm wlm caklurn alurnlnate wlfata h-te ( A h ) lndcated WVJI arrow.
area diffraction (SAD), and microbeam EDAX examinations (electron beam diameter, 0.05 Mm). Results and Discussion
X-ray Diffraction. XRD is an excellent tool to study the chaugea in crystallinityand appearance or disappearance of phases (11). Any physical and chemical changea in cement cauaed by the addition of different coppercontaining compounds can be characterized by comparing the XRD composite patterns. The Joint Committee on Powder Diffraction Standardn (JCPDS)is used to identify various crystalline phases. The XRD of the heat-treated tricalcium aluminate (3Ca0-&0a abbreviated aa Cd) sample fits well that from the JCPDS (pattern number: 8-5). thus confirming acceptable methodology in terms of sample preparation. The qualitative XRD pattern for the solidifiedcement are shown in F w e 1. All samples contain unhydrated tricalcium silicate and dicalcium silicate, a IS14 E m . 86. Tsbrm.. Val. 21. No. 7. lW3
1
Ica
kb)
V
V
L
J
cu Fe
\ * A
AAu All1
CURSOR (KEV) =OS. 200
EDAX
M4.
(a) SEM nlcmoaph ol cumnt wlkMbd wlth C u m :@) rqbn B poh(sd oul h npue 3a by mlcmpobe amly& ol EDAXID be rlch h Ah (c) rqbn C pMed oul h F W e 3a by rmcmpob. a n a W of EDAX to be lean In AI.
strong intensity of calcium hydroxide, and calcium carbonate, as well as the hydration prcducta of aluminate components such aa ettringite (3CaO-AbO~3CaSOc aa Aft) or calcium aluminum hydrate 3 1 H ~ 0abbreviated , (4CaO-~203-13HzO,abbreviatedasC~,s). The results clearly show the presence of complex hydration producta of cement and also Cu-bearing additives without any unidentifiable peaka that could be attributed to new chemical interactions. Although no reaction product is directly verified in the traces, it is still possible that the constituents of cement might be joined in the reaction, but it is difficult tn distinguish with this technique. Therefore. due to the complex nature of cement itself, the
CURSOR (KEV) -05.320
EOAX
constituent element of cement was prepared for studying the poesible interactions with copper oxide. The qualitative XRD pattern for hydrated CsA sample and C&CuO solidified matrix are presented in F i v e 2. T h e major hydration producta of CsA paste are calcium aluminum hydroxide (3CaO-AbO&-IzO, abbreviated as C&), calcium aluminum oxide carbonate hydrate (3CaO-Alz03-CaCO~11Hz0,abbreviated as C~ACHII), CaC03, and gibbsite (y&0p3HzO, abbreviated as 7-AH3) (Figure 2a). The crystallinestructures of hydrated CsA are unaffected by the addition of CuO (Figure 2b). Unhydcated C d is not found in the pattern (Figure 2). These hydration reactions of both C A paste and C& CuO solidifed matrix are fairly rapid hecause the formation is completed during a 2Sday curing period. Additionally, Figure 2 shows that Ca(OH)z can react with COztoformCaCOs,whichthenrenctawithCd&toform C ~ C H I I Thus, . the hydration reaction of C A not only supplies CuO with alkaline and humidified environment but also continuously forms to new crystalline materials. Since metal oxides and hydroxides have amphoteric characteristics, the solubility of metal oxide/hydroxide becomesgreaterinamorealkalineenvironment(12).Thus, the solubility of CuO under such alkaline environment is greatly enhanced. The fate of dissolved copper species is of great concern hecam it might further react with other hydration producta of C d paste. Since detailed information on C A paste and CuO interactions cannot he obtained by qualitative XRD, the possibility still erieta that C&CuO reactions would generate producta in a undetectable small quantity. Therefore, the resulta are not able to conclusively d e f i e the complex interactions between hydrated C d and CuO. It needs further analysis of the specimen by using other instrumentations.
CURSOR (KEY)-05.800
EOAX
(KEV) -05.040
EOAX
CURSOR
Micmatructoral Observations Uaing 8EM. It is I.easonabletoassumethatittheadditionofthemetalions alters the physical structure of the matrix, the ability of the matrix to immobilize metal ions w i l l also he affectad (11). Therefore, useful information may be obtained by the observation of morphological characteristics of the hydration products of cement by SEM. The micrographs may reveal that either ettringite (3CaO-AlzOp3CeSO431Hz0, abbreviated as Aft) or calcium aluminate sulfate hydrate (3CaO-AlzO&&O~-l3HzO, abbreviated as Afm) is the main constituent in the solidified samples. Figure 3a shows that the hydrated cement is in a needlelike Aft phase and that the same morphology is not present in the cement solidified with copper grain (Figure 3b). The copper nitrate-containing solidified matrix does contain some platelet-like Afm phase (Figure 3c). In general, the C A presented in the hydrated cement attacks the Aft, depriving it of some of the sulfate content and gradually converting it to Afm (13.14). The Afm phase generally forms clusters or rosettes of irregular plates. The formation and quantity of needle or platelet structure from the composite material will eventually determine the flexural strength and toughness (15,IQ. Elemental Analysis by SEWEDAX. Microchemical analysis using the SEM/EDAX technique provided useful information on the interaction of phase boundary or interfacial zone. Using the SEM/EDAX technique, it was observed thatthevieinityofacoppergraininthehydrated cement (Bin Figure 4a) contains high concentrations of aluminum, calcium, and silicone and low concentrations ofironandcopper (Figure4b). Incontrast,themicroprohe analysis of hydrated cement (C in Figure 4a) measured low concentrations of aluminum and iron but high concentrations of Ca and Si (Figure k). To ensure the
-.
w. T.-
w. 27. m.7. isas
is16
I
npUn 6. SEM micrOqaphs of tha c4AICuo sandwich sample. riots tha Cuo layar In 2-,un rmcklu~~yl lorn& next to Cu. Ihe acanninga ot (a) AI ecmwmt. (b) Ca alamm. and (c) Cu alamm.
F l p n 7. la) TEM brlght-Reld Image sho% tha intmiaca of and Cuo and tha-r D and E analylsd by EDAX; (b) and (c) SAD pattans of tha Cuo and C&!+ reglons. respeeavety.
validity of the changes in the concentration of elements, approximately 10 spot analysea were performed on the vicinity of Cu grain and the matrix from the polished section. By semiquantitative analysis, atomic ratios of AVCa, Si/Ca, and (Si+Fe)/Ca were calculated. Al/Ca elemental ratio (0.26) adjacent to the Cu grain is significantly higher than in the matrix (0.12). while the Si/Ca ratio (0.42) and (Si+Fe)/Ca ratio (0.45) are unchanged. Since the At species are obviously a composition of CaA, C A should play a role on the interaction of hydrated cement and Cu grain. We were, therefore, able to justify
and to continue our study using the less complex C& CuO system. The micronMcture from the polished sandwich sample isshown in Figure 5s. It clearly reveals that an interaction zone in the middle is about 2 pm thick and that CuO in entrapped by the hydrated CJA. The microchemical analysea hy EDAX show that there is a decreasing concentration gradient of calcium and aluminum from point B to point D (Figure 6LA). Conversely, the concentration of copper element increases. The interaction zone may be, therefore, formed by the reaction of
1118 Envton. 86. Techlol.. Vd. 27.
No. 7. 1883
) cu
.0
10.260
KOU
Ca AI I
cu
-8.
rauEDAX~otUlOCycuOmlcmtomsdSampls: (a)EDAX lpeclMlofarss D ldenwisdhFlpue 7s: (b)EDAX wockml 01areaE ldenwisd h FipUe 7a.
hydration prcducta of C A and CuO. Thin hypothesis is verified using the EDAX line-scanning technique for AI, Ca, and Cu (Figure 6a-c). The results of XRD and SEWEDAX analyses of the CSlyCuO system are- s ' as follows: (1)there is an alkaline environment; (2) the hydration reactions continue in the humidified environment; (3) the CuO can be entrapped by the hydrated CA; and (4) there is a concentrationgradient ofcopperspecies intothe hydrated C A region. This gradient resultsfrom very slow diffusion within the hydrated C A phase about 0.5 wm (Figure 6c) duringa28daycuringperiod. Basedontheaboveresults, it is intended to p r o m that a solid solution of copper species would occur in the interaction zone during a 2 8 day curing period at 25 "C under a humidified environment. TEMIEDAX Andyds. The TEWEDAX W ~ SUIIdertaken to further the understanding of the interfacial microstructure and microchemistry of the hydrated C A paste and the CuO. Figure 7a shows a typical interface of CAI& and CuO where CuO has been entrapped by CAI& with no intermediate layer. The SAD diffraction pattern taken on both sides of the interface indicate only the presence of CuO and C3AHs (Figure 7b and c, respectively). Thisdemonstratenthatno reaction product betweenbothtworegionsarisea. TwoEDAXspeetrafrom regions D and E (Figure 78) are shown in Figure 8a.b.
AI
I
They clearly show the Ca, Al, and Cu in the C3AH6 region and that the copper concentration decreases from the CuO to the region. This confirms the SEM/EDAX analysis where diffused copper species is in the hydrated C3A region. A second interface, that of gibbsite (yAl2033H20, abbreviated as 7-AH3) and CuO, is shown in Figure 9a. It appears that CuO grain is entrapped by yAH3. The results of EDAX spectra analyzed at three different places identified in Figure 9a shown in Figure 9b-d clearly reveal that the concentration gradient of copper element appears in the ?-AH3 region. It is, therefore, suggested that the solid solution of copper species also exists in the ./-AH3 region. While the SEM/EDAX technique reveals the interfacial morphology, it does not distinguish among the various hydration products of C3A. However, this information can be obtained by TEM/EDAX analysis. The results presented in Figures 7-9 can be summarized as follows. Significant amounts of copper element are present in the hydrated C3A region. No distinguishable crystalline copper compound is found in mixture within different hydration products of C3A. No intermediate phases or new chemical products of copper-related compounds exist in the typical interfaces. The TEM/EDAX results strongly support those of SEM/EDAX analyses where the interaction zone between hydrated C3A and CuO is very likely to be presented as a solid solution of copper species. In other words, macroscopically CuO is only entrapped by the hydrated C3A. Heterogeneous arrangement of the copper species within the lattice is very likely, and this will produce no diffraction patterns by TEM. However, the truth is that the concentration gradient of the copper species does appear in the hydrated Cdregion. Thus, we suggestthat the copper speciesacting as a solute which dissolves and diffuses in solid hydration products of C3A (as solvent) to form a heterogeneous solid solution and CuO being physically encapsulated within the hydration products of C3A are the likely processes of the interactions of C3A paste and CuO. Conclusions The use of XRD, SEM/EDAX, and TEM/EDAX analyses has provided valuable information regarding the mechanistic information of the cement-based solidification process. Both hydrated cement and tricalcium aluminate (C3A)provided the alkaline and humidified environment, which increases the solubility of copper species. The microchemical changes of the interface in the cement fixation system showed that C3A should play an important role on the interaction of hydrated cement and Cu grain. The microstructural analysis of the interaction zone which is about 2 pm in thickness by the interaction between C3A paste and CuO shows that CuO is entrapped by the hydration products of C3A. The microchemical data observed by SEM/EDAX and TEM/EDAX indicate that a concentration gradient of copper species exists in the hydrated C3A and that no diffraction patterns of metal
1318 Environ. Sci. Technol., Vol. 27, No. 7, 1993
copper, CuO, Cu(OH12, and CuCO3 are observed in the hydrated C3A. These dissolved copper species diffuse and bind in solid hydration products of C3A to form a heterogeneous solid solution. It is suggested that the interactions of C3A paste and CuO consist of the heterogeneous solid solution of copper species in the hydrated C3A and CuO physically entrapped within the hydration products of C3A. This work using cement constituent, C3A, provides a microscale observation on the interactions of a model S/S processes. The results would be very significant for understanding and predicting the long-term stability of solidified waste matrix and will be helpful for future work on the interactions between other solidifying agents and metal wastes. Acknowledgments This study was funded by the National Science Council of Republic of China under Contract NSC 81 0421-E-00210-z. Literature Cited (1) Barth, E. F. J . Hazard. Mater. 1990,24, 103. (2) Poon, C. S.; Peters, C. J.; Perry, R. Sei. Total Enuiron. 1985, 41, 55. (3) Poon, C. S.; Clark, A. I.; Peters, C. J.; Perry, R. Waste Manage. Res. 1985, 3, 127. (4) Thomas, N. L.; Jameson, D. A.; Double, D. D. Cem. Concr. Res. 1981, 11, 143. (5) Alford, N. McN.; Rahman, A. A.; Salih, N. Cem. Concr. Res. 1981, 11, 235. (6) Walsh, M. B.; Eaton, H. C.; Tittlebaum, M. E.; Cartledge, F. K.; Chalasani, D. Hazard. Waste Hazard. Mater. 1986, 3, 111. (7) Chalasani, D.; Cartledge, F. K.; Eaton, H. C.; Tittlebaum, M. E.; Walsh, M. B. Hazard. Waste Hazard. Mater. 1986, 3, 167. (8) Skipper, D. G.; Eaton, H. C.; Cartledge, F. K.; Tittlebaum, M. E. Cem. Concr. Res. 1987,17, 851. (9) Cartledge, F. K.; Bulter, L. G.; Chalasani, D.; Eaton, H. C.; Frey, F. P.; Herrera, E.; Tittlebaum, M. E.; Yang, S. L. Environ. Sei. Technol. 1990, 24, 867. (10) Chou, A. C.; Eaton, H. C.; Cartledge, F. W.; Tittlebaum, M. E.Hazard. Waste Hazard. Mater. 1988,5, 145. (11) Cartledge,F. K.;Eaton, H. C.; Tittlebaum, M. E. Morphology and Microchemistry of SolidifiedIStabilized Hazard. Waste Systems; Risk Reduction Engineering Laboratory: Cincinnati, OH, 1989; EPAl600/2-89/056;p 61. (12) Stumm, W.; Morgan, J. Aquatic Chemistry, 2nd ed.; Wiley-Interscience: New York, 1981; Chapter 5. (13) Eaton, H. C.; Walsh, M. B.; Tittlebaum, M. E.; Cartledge, F. K.; Chalasani, D. Microscopic Characterization of the SolidificationlStabilizationof Organic Hazardous Wastes; Energy Sources and Technology Conference and Exhibition, 1985. (14) Lea, F. M. The Chemistry of Cement and Concrete, 3rd ed.; Edward Arnold London, 1970; pp 223-229. (15) Faber, K. T. Ceram. Eng. Sei. Proc. 1984, 6, 478. (16) Faber, K. T.; Evans, A. G. Acta Metall. 1983,31, 565.
Received for review July 2, 1992. Revised manuscript received December 10, 1992. Accepted March 22, 1993.
