- Email: [email protected]
Mechanisms of metal stabilization by cement based fixation processes
The Science of the Total Environment, 41 (1985) 55--71 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
55
MECHANISMS OF METAL STABILIZATION BY CEMENT BASED FIXATION PROCESSES
C.S. POON, C.J. PETERS and R. PERRY
Public Health Engineering Laboratory, Civil Engineering Department, Imperial College, London SW7 2BU (United Kingdom) P. BARNES and A.P. BARKER
Department of Crystallography, Birkbeck College, London WCIE 7HX (United Kingdom) (Received February 27th, 1984; accepted March 24th, 1984).
ABSTRACT The mechanism of zinc and mercury fixation by cement/silicate stabilization processes has been assessed from leaching, scanning electron microscopy, X-ray diffraction and porosimetry studies. The results of these tests correlate closely and suggest two separate mechanisms operating in the interaction between these metals and the cement/silicate system. The presence of zinc has a significant effect upon the hydration and final physical properties of cement. The chemical interaction between mercury and cement/silicate is minimal and the hydration process of cement proceeds normally. Zinc appears to be chemically stabilized by cement based fixation processes through the formation of insoluble compounds at high pH. Mercury and related metals which do not form precipitates at elevated pH levels are held in pore solution. Mobility depends largely upon physical encapsulation by the cement matrix and leachability of these materials is expected to be closely related to porosity of the final product.
INTRODUCTION
Chemical fixation of toxic and hazardous waste using cement, lime, pozzolanas and other inorganic material has been practised for many years. The House of Lords Select Committee on hazardous waste disposal considers these methods to be safe and satisfactory for wastes which can be shown to be suitable [ 1 ]. Inorganically based stabilization processes are generally more favourably applied to inorganic wastes, especially those containing cations. Other methods of stabilization involving organic encapsulation techniques are more suitable for organic and inorganic anionic wastes [2]. Figure 1 illustrates the process options c o m m o n l y available for the stabilization of hazardous wastes.
0048-9697/85/$03.30
© 1985 Elsevier Science Publishers B.V.
56 SOLI D [FICAT ION/ZTABI LIZATION
I
Inorgaz~ic Fixation
1
C.... t
1 1 1 1
[
I
Time
Pozz~lanss (I~FA, Sla~,,)
[
Or
b~i[icate
I
1
?n .... pl~ti . . . . . . .
I
l
c Fn ap u ation
ting
l
Ma .... Fncapsulation
Clays
Fig. 1. General classification of solidification processes.
Waste forms produced and fixation technology are based on very empirical formulations of stabilizing material and waste. Except for tests on certain physical properties and leaching, little research has been done on the mechanisms involved in fixation processes. There is still controversy over whether the waste is physically entrapped in the inorganic matrix or whether a chemical stabilization reaction is taking place between the waste and the inorganic materials. Companies marketing these processes have made numerous and sometimes exaggerated claims of interaction mechanisms, with little scientific support. One reason why so little work has been done in this area is that while the chemistry and hydration of cement and related material alone constitutes a complex science, the addition of waste material makes the chemistry even more difficult to study. However, it is thought that a better understanding of the fixation mechanism would facilitate the design of improved processes to reduce or modify leachability of stabilized wastes placed in landfill environments. Studies into the effect of heavy metal oxides (Cr, Cu, Zn, As, Cd, Hg, Pb) on the physical properties of cement [3--5] have shown that metals interact with the hydration and microstructure of the hydrated cement in the early stages of hardening and seriously affect strength development. Certain metals have been found to promote the growth of ettringite crystals and induce significant changes in the microstructure of the hydrated product of the tricalcium aluminate (C3 A) phase. Stepanovo [6] also found that metal chloride additives of Mn, Co, Ni, Cu and Zn interact with silicate and aluminate components of cement to form complexes whose stability makes a substantial contribution to the final compressive strength of cement. It is claimed that this influence on strength development is related to the stability constant and enthalpy of formation of the complexes. It is also considered that differences in strength are due principally to changes in the hydration to the silicate c o m p o n e n t of cement. Further evidence of metal interaction with the hydration process has been established from detailed studies into the effect of lead nitrate on the hydration and physical properties of cement [7, 8]. These studies have shown that the hydration and mechanical strength of cement are retarded
57 and the porosity is generally increased by the addition of lead nitrate. The mechanism of retardation is though to be through the formation of a colloidal membrane coating around the hydrating cement grains by precipitation of the mixed basic lead salt. It has also been suggested that a similar retarding mechanism is likely to occur with other heavy metals. I n d e e d calcium hydroxide (C--H) crystals, normally found in hydrated cement, are absent in samples containing Cu and Zn, which have been shown to inhibit hydration presumably by forming an impermeable layer around the cement grain [6 ]. The mode of interaction of the metal and the stabilization/solidification system will clearly influence the leachability of the metal [9]. Leaching studies of certain metals stabilized in a cement matrix have shown that the amount present in the leachate is often considerably lower than the calculated value based upon the theoretical solubility product. A variety of fixation mechanisms have been postulated to account for this, involving absorption by cement hydrates, substitution and solid solution in the hydrate structure, and formation of insoluble compounds. However, many of these inferred claims relate to semi-quantitative observations and interpretations, leaving many of the fundamentals to be resolved. More recently, processes for immobilizing radioactive waste by cement and related materials have aroused considerable interest [10, 11]. Studies involving Cs and Sr, for example, have shown that pure cement matrices do not fix these metals very effectively although the leach rate can be significantly reduced by the use of admixtures such as pulverized fly ash (PFA), fumed silica, blast furnace slag and zeolites. The immobilizing mechanism for pure cement systems is thought to involve physical encapsulation by the cement matrix with little or no chemical stabilization taking place since Cs does not form an insoluble precipitate under the alkaline conditions of cement solutions. Sorption of Cs and Sr is thought to be the main fixation mechanism for other pozzolanic materials. In the studies reported here, preliminary work has been initiated to •investigate the mechanism of fixation by the Chemfix process [ 12]. Chemfix is an inorganic solidification process based on a cement and sodium silicate formulation. The solidification process involves the mixing of a predetermined amount of cement to produce a slurry, followed by the addition of sodium silicate. The resulting slurry is transferred or p u m p e d to a nearby lagoon to set and cure. The reagent levels added are related to the strength of the solid to be produced, either soft and clay-like or hard and rock-like. The process is claimed to be based on three phases of reaction [ 1 3 ] ; an initial rapid reaction between the soluble silicate and all polyvalent metal ions to form insoluble metal silicates, followed by a slower reaction between the silicate and cement to form a gel, and lastly the hydration of cement. The ability of Chemfix to retain heavy metal ions and the nature of the processes involved have been assessed using four main diagnostic techniques, leaching tests, scanning electron microscopy (SEM}, p o w d e r X-ray diffraction {XRD) and mercury intrusion porosimetry (porosimetry). Zinc
58
TABLE 1 COMPOSITION OF SAMPLE USED IN SEM, XRD AND POROSIMETRY STUDIES Sample
Composition
A
OPC + water 10 g OPC with 10 ml distilled water (initial w / c = 1.0, however approx 40% of unavailable water
bleed reduces the final w/c to 0.6) B
OPC + water + sodium silicate = Chemfix 10 g OPC with 10 ml distilled water and I ml 20% Na2SiO3
C
Chemfix + Zn
D
Chemfix + Hg
10 g OPC with 10 ml 2% Zn: + solution and 1 ml 20% Na2SiO3 10 g OPC with 10 ml 2% Hg 2 + solution and I ml 20% Na2SiO3
a n d m e r c u r y are p r o t o t y p e c o m p o u n d s s e l e c t e d in t h e s e p r e l i m i n a r y investigations, b a s e d o n a l r e a d y k n o w n industrial e x p e r i e n c e w h i c h has i n d i c a t e d t h a t t h e i r leaching p e r f o r m a n c e s are very d i f f e r e n t .
METHODS AND MATERIALS A s o l u t i o n ( 2 0 0 ml o f 2 0 0 0 p p m ) o f Zn and Hg was solidified b y 50 g o f o r d i n a r y P o r t l a n d c e m e n t (OPC) a n d 12 m l 40% Na2 SIO3. T h e f i x e d p r o d u c t was c u r e d at r o o m t e m p e r a t u r e f o r 28 days. F o r t y g r a m s o f t h e c u r e d p r o d u c t was c r u s h e d i n t o small l u m p s a n d t r a n s f e r r e d t o a c o n t a i n e r . B u f f e r e d acetic acid (100 m l o f 0 . 1 5 M ) was a d d e d a n d t h e m i x t u r e agitated using a r o t a t i o n a l shaker. A f t e r 24 h, t h e slurry was filtered t h r o u g h a 0.45 p m m e m b r a n e . A fresh p o r t i o n o f acetic acid was a d d e d and t h e p r o c e s s r e p e a t e d o v e r several days. Filtrates w e r e p r e s e r v e d in a nitric acid s o l u t i o n a n d a n a l y s e d f o r Hg a n d Zn b y A t o m i c A b s o r p t i o n S p e c t r o s c o p y .
SEM, XRD and porosimetry T h e c o m p o s i t i o n o f t h e f o u r s a m p l e s a n a l y s e d b y these t e c h n i q u e s are p r e s e n t e d in T a b l e 1. S a m p l e s w e r e p r e p a r e d b y shaking either w a t e r or m e t a l s o l u t i o n s w i t h c e m e n t f o r 3 m i n in a plastic c o n t a i n e r ; Na2SiO3 s o l u t i o n was t h e n a d d e d as n e e d e d a n d t h e m i x t u r e s s h a k e n f o r a f u r t h e r 30 s. All s a m p l e s w e r e allowed t o c u r e at r o o m t e m p e r a t u r e . F o r SEM a n d X R D studies, 1-day samples w e r e o v e n dried at 1 0 0 - - 1 0 5 ° C f o r 15 rain. F r a c t u r e s p e c i m e n s w e r e p r e p a r e d and c o a t e d w i t h gold or c a r b o n film p r i o r t o SEM e x a m i n a t i o n using a J e o l 3 5 C F + E D A X s y s t e m while p o w d e r e d s a m p l e s w e r e u s e d f o r X R D analysis b y a Philips P o w d e r X - r a y d i f f r a c t o m e t e r . T h e
59 TABLE 2 L E A C H I N G TEST R E S U L T S Elution
pH Zn 2 ÷ ( p p m ) Hg 2 ÷ ( p p m )
Test no. 1
2
3
4
5
11.9 0.25 42
11.5 0.09 32
11.7 0.05 27
11.4 0.1 16
11.3 0.15 14
porosity studies were performed on 7-day samples using a Carlo Erba Mercury Intrusion Porosimeter.
RESULTS
Leaching tests The results of the leaching tests are presented in Table 2. The values are an average of six samples with an overall standard deviation of 0.1 and 3 for Zn and Hg, respectively. Acetic acid buffered to pH 5 is normally used as leachate in these tests in order to simulate pH conditions which are usually encountered in landfill sites [14]. The shaking action produces an accelerated leaching environment. Under these conditions, the results showed that leachates had a very high initial alkalinity, probably due to the dissolution of hydrated cement. Comparing the leachate concentrations of Zn 2 ÷ and Hg 2 +, Zn 2 ÷ remains at a constant low concentration ( ~ 0 . 5 p p m ) throughout, while for Hg 2÷ the concentration is high initially and decreases through successive elutions. It is evident from these results that Zn 2 + has been retained considerably more effectively than Hg 2+, which is easily leached from the matrix. This difference in behaviour suggests that contrasting mechanisms are involved in the fixation of these metals in Chemfix; a theme which reoccurs in the other diagnostic techniques. In order to gain further insight into the different mechanisms operating in the Chemfix process compared to those in ordinary cement hydration, and to reveal possible causes for differences within the Chemfix process by these metal ions (Zn 2+ and Hg 2 +), cement and Chemfix material were studied using three techniques, SEM (scanning electron microscopy), powder X R D (X-ray diffraction) and mercury intrusion porosimetry (porosimetry). The compositions of the four samples analysed by these techniques are presented in Table 1. SEM For sample A, Figs. 2a and 2b show that ordinary hydration products of
60
Fig. 2a . Micrograph of sample A showing large crystal of C-H, cluster of fibrous C~$-H with some penetrating the C-H crystal. pure cement, appropriate to the high water/cement (w/c) ratios used, are indeed present with fibrils of calcium silicate hydrate (C-S-H), rods of ettringite and massive deposits of calcium hydroxide (C-H) crystals. Common features of C-S-H fibrils penetrating C-H crystals and ettringite crystals embedded in a cluster of C-S-H are observable. The structure is porous, this being a consequence of the high w/c ratio (i.e. 1 initially and ~--0.6 finally). Figures 3a and 3b (sample B) show the effect of adding sodium silicate to the cement matrix. Similar hydration products of C-S-H (typified by Hadley grain), ettringite and C-H are observable, but there are also areas of amorphous structure which do not show any resemblance to ordinary C-S-H of hydrated cement. These areas may be formed from the reaction between Ca 2+ from cement and SiO32- from Na2SiO3 producing a calcium-silicate gel. Formation of such a gel accelerates the setting of cement and also increases the water demand of the system. Therefore no bleeding was observed when the paste was prepared, despite the high water/cement ratio. Crystals of C-H are noticeably much smaller compared with those of pure cement (see Figs. 2a and 2b). The micrograph of sample C (Fig. 4a) shows the effect of adding 2% Zn 2 ÷ to Chemfix. The hydration pattern of cement is significantly altered by the addition of Zn 2 +. No ordinary fibrous C-S-H and massive C-H crystals are identifiable but the morphology is dominated by the large ettringite crystals
61
Fig. 2b. Micrograph of sample A showing long and large crystals of C-H with clevage plane and small rod of ettringite crystal embedded in cluster of C-S-H.
occurring as hexagonal prisms and plates of monosulphate. This effect is shown more clearly by the sample with 6% Zn 2 ÷ (Fig. 4b). The effect of adding Hg 2÷ to Chemfix is illustrated by Figs. 5a and 5b. Comparing Figs. 3a and 5a, it is evident that Hg 2 +, unlike Zn 2 +, does not seriously alter the hydration pattern of Chemfix. Figure 5b further shows the massive deposits of calcium--silicate gel resulting from the interaction between the calcium in cement and the added sodium silicate. XRD
The qualitative XRD studies correlate well with the findings from the SEM analysis. Comparing diffraction traces for samples A and B (Fig. 6) the crystalline structures of hydrated cement were not affected by the addition of sodium silicate; commonly identifiable products are calcium hydroxide (C-H), calcite (CaCO3), ettringite, together with the unhydrated alite (C3S) and belite (C2S) and aluminates (C3A and C4AF) phases. The trace for the sample with zinc addition (sample C) confirms the absence of calcium hydroxide, depressed C3 A, and enlarged ettringite peaks with some other unidentifiable peaks also present. The trace for the mercury sample resembles that o f raw Chemfix.
62
Fig. 3a. Micrograph of sample B showing small crystals of C-H and fibrous C-S-H.
Fig. 3b. Micrograph of sample B showing Hadley grain and gel of Ca-silicate.
63
Fig. 4a. Micrograph of sample C. Sample with 2% Zn showing long and large crystals of ettringite and hexagonal AFm phases.
Porosime try Results from the porosimetry studies are presented in Figs. 7 and 8 (pore size distribution diagrams) and Table 3 (porosity data). Table 3 shows that pure cement (sample A) has the lowest porosity as expected due to its low water/cement ratio. Pore size distribution (Fig. 8) also indicates that a large proportion of the porosity is made up of pores with relatively small radii. Comparing the effect of Zn and Hg on Chemfix, Fig. 7 shows that whilst the porosity distribution of the mercury sample relates closely with that of raw Chemfix, the addition of zinc significantly increases the porosity and shifts the pore size distribution towards a larger pore radius. The porosity data for the four sample types appear to suggest three basic distributions: one centered at 370 •, one centered at 7500 A, and the sum of these two. (A) OPC + Water -- single distribution at 370 A (B) Raw Chemfix (D) Chemfix + Hg -- combination of A + C (C) Chemfix + Zn -- single distribution at 7500 A Such an interpretation is very suggestive of at least two separate mechanisms operating in the interaction between these metals and the cement/silicate system. The idea merits further consideration, but it suffices to 'emphasize here that t h e Chemfix--zinc process is quite distinct from raw
64
Fig. 4b. Micrograph of 6% Zn showing much longer and larger ettringite crystals surrounded by large voids.
Chemfix with or without Hg, and that these trends are clearly reflected also in the leaching, SEM, and XRD analyses. DISCUSSION
The SEM, XRD, porosity and leachability studies provide useful information for the understanding of the mechanism of fixation. In spite of the preliminary status of some parts of this work, it is encouraging to find consistent trends from all four diagnostic tests carried out on the metalbearing solids of the Chemfix stabilization process. The correlation between these tests are indicated in Table 4. The increased pore volume and pore size of the zinc-containing Chemfix samples is caused by the extensive growth of ettringite crystals in the hydrated paste due to the accelerated hydration of C3A as observed in the SEM and XRD analyses. Despite the higher porosity the leachability of Zn is low, which indicates that permeability is not an important factor in determining movement of this metal through the matrix and that chemical stabilization rather than physical encapsulation is the controlling factor in reducing metal mobility. It is often claimed that stabilization of metal involves the formation of insoluble metal silicates but the SEM and XRD examinations did n o t reveal
65
Fig. 5a. Micrograph of sample D showing Hadley grain with hydrated core surrounding an unhydrated grain.
Fig. 5b. Micrograph of sample D showing gel of Ca-silicate.
66
~ ~ U L )
~
U
U
q
I U
4J ~J
*J ~J
I
t
i
Fig. 6. XRD traces. TABLE 3 POROSIMETRY RESULTS Sample
Total pore volume (cm 3/g)
A (water + OPC a)
0.136
B (Chemfix)
0.416
C (Chemfix + Zn)
0.684
D (Chemfix + Hg)
0.376
a OPC = Ordinary Portland Cement a n y identifiable crystalline zinc silicate, t h o u g h a m o r p h o u s gel o f c a l c i u m silicate was o b s e r v e d in b o t h p u r e C h e m f i x a n d m e t a l - b e a r i n g C h e m f i x e d solids. T h e a b s e n c e o f zinc silicate is t o b e e x p e c t e d since t h e s o l u t i o n c o n t a i n i n g Z n was a d d e d t o t h e c e m e n t m i x p r i o r t o s o d i u m silicate
370 A 7500 A single
7500 A
double 370 A 7500 A
Ca (OH)2 : medium ettringite : weak unhydrated cement: strong Ca (OH)2 : absent ettringite: medium
unhydrated cement: medium strong Ca (OH)2 : medium ettringite : weak
C-S-H: a little, reticulated
Ca (OHH)2 : absent ettringite: large hexagonal prism with AFro (monosulphate) Ca-silicate gel: massive
C-S-H: fibrous, hydrated shell (Hadley grain) Ca (OH)2 : small crystals ettringite: small rods Ca-silicate gel: massive
C (Chemfix + Zn)
D (Chemfix % Hg)
double
unhydrated cement: medium strong
C-S-H: fibrous, hydrated shell (Hadley grain) Ca (OI-I)2 : small crystals ettringite: small rods Ca-silicate gel: massive
B (Chemfk)
370
single
unhydrated cement: medium strong Ca (OH)2 : strong ettringite: weak
C-S-H: fibrous Ca (OH)2 : large crystals ettringite: small rods
Porosimetry (7-days sample)
A (OPC + Water)
Powder XRD (1-day sample)
SEM (1-day sample)
Sample
SUMMARY OF CORRELATION OF FOUR STUDIES
TABLE 4
high
low
--
--
Leachability values
O~
68 -
IA)
0.1/., Cement+ water wit :0-6
'~E 01; u
~, o.~
o.o6! u
0.0l
>o o-o2
750o
3;oo
,~oo 7~o
~o
,~o
;s
3~
7's
I~
3'7
Pore r a d i u s nm 0.a m ~ ..... u
Chemfix ÷Zn Chemfix Chem fix ÷ H g
IC)
0"6
o o.=
~
/
~
/.:
~: 0.2
.
.
7500
.
.
1500
750
.
•
~
ID I
.,,i.~.~.~.....~ .~--" " -
3700
~.,,,,~,~---~.,,,, •
,
.
370
150
.
.
75
.
37
.
15 Pore r a d i u s hm
7"5
Fig. 7. Cumulative pore volume.
addition; this order of addition is normal practice in the Chemfix operating procedure. Under these conditions it is thought likely that most of the Zn would be precipitated as the hydroxide or would react with the calcium hydroxide to produce possibly calcium zincate [15]. Although no evidence of either of these Zn compounds has been found by the SEM or XRD tests the absence of C-H crystals in the sample containing Zn indicates that calcium hydroxide plays an important role in the fixation of zinc ions. By contrast, Hg did not seriously affect the normal hydration process of Chemfix as is evident from the SEM, XRD and porosity studies. The formation of C-S-H, C-H and calcium silicate all proceed in the same manner as in pure Chemfix, which indicates that there is little or no interaction of this metal with cement or sodium silicate. This inability of Hg to form an insoluble hydroxide or silicate with the solidifying material means that the metal remains in pore solution or at most is only loosely bound to the hydrated products through sorption. The metal is therefore physically encapsulated within the cement structure and not chemically stabilized to the extent observed for Zn. Mobilization of Hg is rapid once in contact with
69 =o
~7
a
o6 o
~5
N4 o3 E 1
!
I I
7500 3700
I
I
1
i
I I
w
1500
750
I
I |
370
I 150
75
37
15
7-5
37
Pore rodius nm
b
6
~3 ,,2 E
3~oo
7501
~o
7so
3~o
i;0
;s
3~
1~ pore
radius
y's
3'?
nm
C
12 11
lO m
9
.? s ?
i-
~4 w3 £ 22 1
!
7500 3100
750
370
150
750
370
150
75
37
15 7- 5 Pore radius nm
!
37
o o.
!
!
7500
3700
i
1500
!
i
75
37
15 7"5 3"7 Pore radius in nm
Fig. 8. Pore size distribution in sample: (A) c e m e n t + w a t e r ; (B) Chemfix; (C) Chemfix + Zn; (D) Chemfix + Hg.
70
water and the leachability of the fixed product is very high and probably dependent on the permeability of the solidified product. The addition of soluble silicate to cement in the Chemfix process increases the water demand and accelerates the setting. To the waste disposer, this increase in water content saves material costs as a larger volume of waste can be treated for the same amount of cement. However, the penalty of increasing the water/cement ratio is to increase the porosity of the solidified material, resulting in reduced physical strength and higher permeability. The differences in leachability and fixation mechanisms of Zn and Hg in Chemfix and the effect of water/cement ratio on porosity suggest that the solidification process should be operated in accordance with the type of material being stabilized. For Zn, and other similar heavy metals ions which react readily with calcium hydroxide to produce insoluble compounds, a high water/cement ratio can be used which will lower material costs without any adverse effect on leachability. The amount of stabilizing material used will be dictated largely by the desired physical quality of the final product in terms of strength and permeability. For Hg and other metals which do not form insoluble compounds with Chemfix b u t rely on physical encapsulation to retain the metal, a low water/cement ratio should be adopted in order to reduce porosity and thereby lower the permeability and minimize metal mobilization. Reduced leachability can be achieved by the appropriate use of additives such as PFA and fumed silica. These react with C-H to produce amorphous C-S-H which fills the pores and reduces porosity. Further research is being planned using EXAFS (extended X-ray absorption fine structure) and STEM (scanning transmission electron microscopy) to investigate the exact behaviour of these and other heavy metals in stabilization processes and establish the form in which these metals are present in the solidified matrix.
REFERENCES
1 2 3
4
5 6 7
House of Lords Select Committee on Science and Technology, Hazardous Waste Disposal, Her Majesty's Stationary Office, London, 1981. C.S, Poon, C.J. Peters and R. Perry, Use of stabilization processes in the control of toxic waste, Effluent Water Treat. J., 23, 11 (1983) 145--459. C. Tashiro, H. Takahashi, M. Kanaya, I. Hirakida and R. Yoshida, Hardening properties of cement mortar adding heavy metal c o m p o u n d and solubility of heavy metal from hardened mortar, Cem. Concr. Res., 1 (1977) 283--290. C. Tashiro, J. O b a and K. Akawa, The effect of several heavy metal oxides on the formation of ettringite and the microstructure of hardened ettringite, Cem. Concr. Res., 9 (1979) 303--309. C. Tashiro and J. Oba, The effect of Cu (OH)2 on the hydration of C3A, Seventh Int. Congr. Chemistry of Cement, Paris, 2, II (1980) 58--63. I.N. Stepanovo, Hardening of cement pastes in presence of chloride of 3d elements, J. Appl. Chem. c/c Zh. Prikl. Khim, 54 (1981) 885--888. N.L. Thomas, D.A. Jameson and D.D. Double, The effect of lead nitrate on the early hydration of Portland cement, Cem. Concr. Res., 11 (1981) 143--153.
71 8 N. McN. Alford, A.A. Rahman and N. Salih, The effect of lead nitrate on the physical properties of cement paste, Cem. Concr. Res., 11 (1981) 235--245. 9 C.S. Poon, C.J. Peters, R. Perry and C.P.V. Knight, Assessing the leaching characteristics of stabilized toxic waste by use of thin layer chromatography, Environ. Tech. Lett., 5, I (1984) 1--6. 10 F.P. Glasser, A.A. Rahman, R.W. Craford, C.E. McCulloch and M.J. Angus, Immobilization and leaching mechanisms of radwaste in cementbased matrices, DOE Report No. DOE/RW/83.093, 1, Department of the Environment, London, 1983. 11 D.J. Lee, Factors affecting the leachability of caesium and strontium from cemented simulant evaporator waste, AEEW-R1461, Atomic Energy Research Establishment, Winfirth, 1978. 12 R.K. Salas, Disposal of liquid waste by chemical fixation/solidification - - the Chemfix process, In R. Pojasek (Ed.), Toxic and Hazardous Waste Disposal, 1, Ann Arbor Science Publishers, Ann Arbor, MI, 1979, 321--348. 13 J.R. Conner, Ultimate disposal of liquid residues by chemical fixation, Proc. National Conf. on Management and Disposal of Residues from the Treatment of Industrial Waste Waters, Washington DC, Information Transfer Inc., 1975, 197--208. 14 K. Jackson, J. Benedik and L. Jackson, Comparison of three solid waste batch leach testing methods and a column leach test method, Hazardous solid waste testing, First Conf., ASTM 760 (1981) 83--98. 15 C. Tashiro and S. Tatibana, Bond strength between C3S paste and iron, copper or zinc wire and microstructure of interface, Cem. Concr. Res., 13 (1983) 377--382.
55
MECHANISMS OF METAL STABILIZATION BY CEMENT BASED FIXATION PROCESSES
C.S. POON, C.J. PETERS and R. PERRY
Public Health Engineering Laboratory, Civil Engineering Department, Imperial College, London SW7 2BU (United Kingdom) P. BARNES and A.P. BARKER
Department of Crystallography, Birkbeck College, London WCIE 7HX (United Kingdom) (Received February 27th, 1984; accepted March 24th, 1984).
ABSTRACT The mechanism of zinc and mercury fixation by cement/silicate stabilization processes has been assessed from leaching, scanning electron microscopy, X-ray diffraction and porosimetry studies. The results of these tests correlate closely and suggest two separate mechanisms operating in the interaction between these metals and the cement/silicate system. The presence of zinc has a significant effect upon the hydration and final physical properties of cement. The chemical interaction between mercury and cement/silicate is minimal and the hydration process of cement proceeds normally. Zinc appears to be chemically stabilized by cement based fixation processes through the formation of insoluble compounds at high pH. Mercury and related metals which do not form precipitates at elevated pH levels are held in pore solution. Mobility depends largely upon physical encapsulation by the cement matrix and leachability of these materials is expected to be closely related to porosity of the final product.
INTRODUCTION
Chemical fixation of toxic and hazardous waste using cement, lime, pozzolanas and other inorganic material has been practised for many years. The House of Lords Select Committee on hazardous waste disposal considers these methods to be safe and satisfactory for wastes which can be shown to be suitable [ 1 ]. Inorganically based stabilization processes are generally more favourably applied to inorganic wastes, especially those containing cations. Other methods of stabilization involving organic encapsulation techniques are more suitable for organic and inorganic anionic wastes [2]. Figure 1 illustrates the process options c o m m o n l y available for the stabilization of hazardous wastes.
0048-9697/85/$03.30
© 1985 Elsevier Science Publishers B.V.
56 SOLI D [FICAT ION/ZTABI LIZATION
I
Inorgaz~ic Fixation
1
C.... t
1 1 1 1
[
I
Time
Pozz~lanss (I~FA, Sla~,,)
[
Or
b~i[icate
I
1
?n .... pl~ti . . . . . . .
I
l
c Fn ap u ation
ting
l
Ma .... Fncapsulation
Clays
Fig. 1. General classification of solidification processes.
Waste forms produced and fixation technology are based on very empirical formulations of stabilizing material and waste. Except for tests on certain physical properties and leaching, little research has been done on the mechanisms involved in fixation processes. There is still controversy over whether the waste is physically entrapped in the inorganic matrix or whether a chemical stabilization reaction is taking place between the waste and the inorganic materials. Companies marketing these processes have made numerous and sometimes exaggerated claims of interaction mechanisms, with little scientific support. One reason why so little work has been done in this area is that while the chemistry and hydration of cement and related material alone constitutes a complex science, the addition of waste material makes the chemistry even more difficult to study. However, it is thought that a better understanding of the fixation mechanism would facilitate the design of improved processes to reduce or modify leachability of stabilized wastes placed in landfill environments. Studies into the effect of heavy metal oxides (Cr, Cu, Zn, As, Cd, Hg, Pb) on the physical properties of cement [3--5] have shown that metals interact with the hydration and microstructure of the hydrated cement in the early stages of hardening and seriously affect strength development. Certain metals have been found to promote the growth of ettringite crystals and induce significant changes in the microstructure of the hydrated product of the tricalcium aluminate (C3 A) phase. Stepanovo [6] also found that metal chloride additives of Mn, Co, Ni, Cu and Zn interact with silicate and aluminate components of cement to form complexes whose stability makes a substantial contribution to the final compressive strength of cement. It is claimed that this influence on strength development is related to the stability constant and enthalpy of formation of the complexes. It is also considered that differences in strength are due principally to changes in the hydration to the silicate c o m p o n e n t of cement. Further evidence of metal interaction with the hydration process has been established from detailed studies into the effect of lead nitrate on the hydration and physical properties of cement [7, 8]. These studies have shown that the hydration and mechanical strength of cement are retarded
57 and the porosity is generally increased by the addition of lead nitrate. The mechanism of retardation is though to be through the formation of a colloidal membrane coating around the hydrating cement grains by precipitation of the mixed basic lead salt. It has also been suggested that a similar retarding mechanism is likely to occur with other heavy metals. I n d e e d calcium hydroxide (C--H) crystals, normally found in hydrated cement, are absent in samples containing Cu and Zn, which have been shown to inhibit hydration presumably by forming an impermeable layer around the cement grain [6 ]. The mode of interaction of the metal and the stabilization/solidification system will clearly influence the leachability of the metal [9]. Leaching studies of certain metals stabilized in a cement matrix have shown that the amount present in the leachate is often considerably lower than the calculated value based upon the theoretical solubility product. A variety of fixation mechanisms have been postulated to account for this, involving absorption by cement hydrates, substitution and solid solution in the hydrate structure, and formation of insoluble compounds. However, many of these inferred claims relate to semi-quantitative observations and interpretations, leaving many of the fundamentals to be resolved. More recently, processes for immobilizing radioactive waste by cement and related materials have aroused considerable interest [10, 11]. Studies involving Cs and Sr, for example, have shown that pure cement matrices do not fix these metals very effectively although the leach rate can be significantly reduced by the use of admixtures such as pulverized fly ash (PFA), fumed silica, blast furnace slag and zeolites. The immobilizing mechanism for pure cement systems is thought to involve physical encapsulation by the cement matrix with little or no chemical stabilization taking place since Cs does not form an insoluble precipitate under the alkaline conditions of cement solutions. Sorption of Cs and Sr is thought to be the main fixation mechanism for other pozzolanic materials. In the studies reported here, preliminary work has been initiated to •investigate the mechanism of fixation by the Chemfix process [ 12]. Chemfix is an inorganic solidification process based on a cement and sodium silicate formulation. The solidification process involves the mixing of a predetermined amount of cement to produce a slurry, followed by the addition of sodium silicate. The resulting slurry is transferred or p u m p e d to a nearby lagoon to set and cure. The reagent levels added are related to the strength of the solid to be produced, either soft and clay-like or hard and rock-like. The process is claimed to be based on three phases of reaction [ 1 3 ] ; an initial rapid reaction between the soluble silicate and all polyvalent metal ions to form insoluble metal silicates, followed by a slower reaction between the silicate and cement to form a gel, and lastly the hydration of cement. The ability of Chemfix to retain heavy metal ions and the nature of the processes involved have been assessed using four main diagnostic techniques, leaching tests, scanning electron microscopy (SEM}, p o w d e r X-ray diffraction {XRD) and mercury intrusion porosimetry (porosimetry). Zinc
58
TABLE 1 COMPOSITION OF SAMPLE USED IN SEM, XRD AND POROSIMETRY STUDIES Sample
Composition
A
OPC + water 10 g OPC with 10 ml distilled water (initial w / c = 1.0, however approx 40% of unavailable water
bleed reduces the final w/c to 0.6) B
OPC + water + sodium silicate = Chemfix 10 g OPC with 10 ml distilled water and I ml 20% Na2SiO3
C
Chemfix + Zn
D
Chemfix + Hg
10 g OPC with 10 ml 2% Zn: + solution and 1 ml 20% Na2SiO3 10 g OPC with 10 ml 2% Hg 2 + solution and I ml 20% Na2SiO3
a n d m e r c u r y are p r o t o t y p e c o m p o u n d s s e l e c t e d in t h e s e p r e l i m i n a r y investigations, b a s e d o n a l r e a d y k n o w n industrial e x p e r i e n c e w h i c h has i n d i c a t e d t h a t t h e i r leaching p e r f o r m a n c e s are very d i f f e r e n t .
METHODS AND MATERIALS A s o l u t i o n ( 2 0 0 ml o f 2 0 0 0 p p m ) o f Zn and Hg was solidified b y 50 g o f o r d i n a r y P o r t l a n d c e m e n t (OPC) a n d 12 m l 40% Na2 SIO3. T h e f i x e d p r o d u c t was c u r e d at r o o m t e m p e r a t u r e f o r 28 days. F o r t y g r a m s o f t h e c u r e d p r o d u c t was c r u s h e d i n t o small l u m p s a n d t r a n s f e r r e d t o a c o n t a i n e r . B u f f e r e d acetic acid (100 m l o f 0 . 1 5 M ) was a d d e d a n d t h e m i x t u r e agitated using a r o t a t i o n a l shaker. A f t e r 24 h, t h e slurry was filtered t h r o u g h a 0.45 p m m e m b r a n e . A fresh p o r t i o n o f acetic acid was a d d e d and t h e p r o c e s s r e p e a t e d o v e r several days. Filtrates w e r e p r e s e r v e d in a nitric acid s o l u t i o n a n d a n a l y s e d f o r Hg a n d Zn b y A t o m i c A b s o r p t i o n S p e c t r o s c o p y .
SEM, XRD and porosimetry T h e c o m p o s i t i o n o f t h e f o u r s a m p l e s a n a l y s e d b y these t e c h n i q u e s are p r e s e n t e d in T a b l e 1. S a m p l e s w e r e p r e p a r e d b y shaking either w a t e r or m e t a l s o l u t i o n s w i t h c e m e n t f o r 3 m i n in a plastic c o n t a i n e r ; Na2SiO3 s o l u t i o n was t h e n a d d e d as n e e d e d a n d t h e m i x t u r e s s h a k e n f o r a f u r t h e r 30 s. All s a m p l e s w e r e allowed t o c u r e at r o o m t e m p e r a t u r e . F o r SEM a n d X R D studies, 1-day samples w e r e o v e n dried at 1 0 0 - - 1 0 5 ° C f o r 15 rain. F r a c t u r e s p e c i m e n s w e r e p r e p a r e d and c o a t e d w i t h gold or c a r b o n film p r i o r t o SEM e x a m i n a t i o n using a J e o l 3 5 C F + E D A X s y s t e m while p o w d e r e d s a m p l e s w e r e u s e d f o r X R D analysis b y a Philips P o w d e r X - r a y d i f f r a c t o m e t e r . T h e
59 TABLE 2 L E A C H I N G TEST R E S U L T S Elution
pH Zn 2 ÷ ( p p m ) Hg 2 ÷ ( p p m )
Test no. 1
2
3
4
5
11.9 0.25 42
11.5 0.09 32
11.7 0.05 27
11.4 0.1 16
11.3 0.15 14
porosity studies were performed on 7-day samples using a Carlo Erba Mercury Intrusion Porosimeter.
RESULTS
Leaching tests The results of the leaching tests are presented in Table 2. The values are an average of six samples with an overall standard deviation of 0.1 and 3 for Zn and Hg, respectively. Acetic acid buffered to pH 5 is normally used as leachate in these tests in order to simulate pH conditions which are usually encountered in landfill sites [14]. The shaking action produces an accelerated leaching environment. Under these conditions, the results showed that leachates had a very high initial alkalinity, probably due to the dissolution of hydrated cement. Comparing the leachate concentrations of Zn 2 ÷ and Hg 2 +, Zn 2 ÷ remains at a constant low concentration ( ~ 0 . 5 p p m ) throughout, while for Hg 2÷ the concentration is high initially and decreases through successive elutions. It is evident from these results that Zn 2 + has been retained considerably more effectively than Hg 2+, which is easily leached from the matrix. This difference in behaviour suggests that contrasting mechanisms are involved in the fixation of these metals in Chemfix; a theme which reoccurs in the other diagnostic techniques. In order to gain further insight into the different mechanisms operating in the Chemfix process compared to those in ordinary cement hydration, and to reveal possible causes for differences within the Chemfix process by these metal ions (Zn 2+ and Hg 2 +), cement and Chemfix material were studied using three techniques, SEM (scanning electron microscopy), powder X R D (X-ray diffraction) and mercury intrusion porosimetry (porosimetry). The compositions of the four samples analysed by these techniques are presented in Table 1. SEM For sample A, Figs. 2a and 2b show that ordinary hydration products of
60
Fig. 2a . Micrograph of sample A showing large crystal of C-H, cluster of fibrous C~$-H with some penetrating the C-H crystal. pure cement, appropriate to the high water/cement (w/c) ratios used, are indeed present with fibrils of calcium silicate hydrate (C-S-H), rods of ettringite and massive deposits of calcium hydroxide (C-H) crystals. Common features of C-S-H fibrils penetrating C-H crystals and ettringite crystals embedded in a cluster of C-S-H are observable. The structure is porous, this being a consequence of the high w/c ratio (i.e. 1 initially and ~--0.6 finally). Figures 3a and 3b (sample B) show the effect of adding sodium silicate to the cement matrix. Similar hydration products of C-S-H (typified by Hadley grain), ettringite and C-H are observable, but there are also areas of amorphous structure which do not show any resemblance to ordinary C-S-H of hydrated cement. These areas may be formed from the reaction between Ca 2+ from cement and SiO32- from Na2SiO3 producing a calcium-silicate gel. Formation of such a gel accelerates the setting of cement and also increases the water demand of the system. Therefore no bleeding was observed when the paste was prepared, despite the high water/cement ratio. Crystals of C-H are noticeably much smaller compared with those of pure cement (see Figs. 2a and 2b). The micrograph of sample C (Fig. 4a) shows the effect of adding 2% Zn 2 ÷ to Chemfix. The hydration pattern of cement is significantly altered by the addition of Zn 2 +. No ordinary fibrous C-S-H and massive C-H crystals are identifiable but the morphology is dominated by the large ettringite crystals
61
Fig. 2b. Micrograph of sample A showing long and large crystals of C-H with clevage plane and small rod of ettringite crystal embedded in cluster of C-S-H.
occurring as hexagonal prisms and plates of monosulphate. This effect is shown more clearly by the sample with 6% Zn 2 ÷ (Fig. 4b). The effect of adding Hg 2÷ to Chemfix is illustrated by Figs. 5a and 5b. Comparing Figs. 3a and 5a, it is evident that Hg 2 +, unlike Zn 2 +, does not seriously alter the hydration pattern of Chemfix. Figure 5b further shows the massive deposits of calcium--silicate gel resulting from the interaction between the calcium in cement and the added sodium silicate. XRD
The qualitative XRD studies correlate well with the findings from the SEM analysis. Comparing diffraction traces for samples A and B (Fig. 6) the crystalline structures of hydrated cement were not affected by the addition of sodium silicate; commonly identifiable products are calcium hydroxide (C-H), calcite (CaCO3), ettringite, together with the unhydrated alite (C3S) and belite (C2S) and aluminates (C3A and C4AF) phases. The trace for the sample with zinc addition (sample C) confirms the absence of calcium hydroxide, depressed C3 A, and enlarged ettringite peaks with some other unidentifiable peaks also present. The trace for the mercury sample resembles that o f raw Chemfix.
62
Fig. 3a. Micrograph of sample B showing small crystals of C-H and fibrous C-S-H.
Fig. 3b. Micrograph of sample B showing Hadley grain and gel of Ca-silicate.
63
Fig. 4a. Micrograph of sample C. Sample with 2% Zn showing long and large crystals of ettringite and hexagonal AFm phases.
Porosime try Results from the porosimetry studies are presented in Figs. 7 and 8 (pore size distribution diagrams) and Table 3 (porosity data). Table 3 shows that pure cement (sample A) has the lowest porosity as expected due to its low water/cement ratio. Pore size distribution (Fig. 8) also indicates that a large proportion of the porosity is made up of pores with relatively small radii. Comparing the effect of Zn and Hg on Chemfix, Fig. 7 shows that whilst the porosity distribution of the mercury sample relates closely with that of raw Chemfix, the addition of zinc significantly increases the porosity and shifts the pore size distribution towards a larger pore radius. The porosity data for the four sample types appear to suggest three basic distributions: one centered at 370 •, one centered at 7500 A, and the sum of these two. (A) OPC + Water -- single distribution at 370 A (B) Raw Chemfix (D) Chemfix + Hg -- combination of A + C (C) Chemfix + Zn -- single distribution at 7500 A Such an interpretation is very suggestive of at least two separate mechanisms operating in the interaction between these metals and the cement/silicate system. The idea merits further consideration, but it suffices to 'emphasize here that t h e Chemfix--zinc process is quite distinct from raw
64
Fig. 4b. Micrograph of 6% Zn showing much longer and larger ettringite crystals surrounded by large voids.
Chemfix with or without Hg, and that these trends are clearly reflected also in the leaching, SEM, and XRD analyses. DISCUSSION
The SEM, XRD, porosity and leachability studies provide useful information for the understanding of the mechanism of fixation. In spite of the preliminary status of some parts of this work, it is encouraging to find consistent trends from all four diagnostic tests carried out on the metalbearing solids of the Chemfix stabilization process. The correlation between these tests are indicated in Table 4. The increased pore volume and pore size of the zinc-containing Chemfix samples is caused by the extensive growth of ettringite crystals in the hydrated paste due to the accelerated hydration of C3A as observed in the SEM and XRD analyses. Despite the higher porosity the leachability of Zn is low, which indicates that permeability is not an important factor in determining movement of this metal through the matrix and that chemical stabilization rather than physical encapsulation is the controlling factor in reducing metal mobility. It is often claimed that stabilization of metal involves the formation of insoluble metal silicates but the SEM and XRD examinations did n o t reveal
65
Fig. 5a. Micrograph of sample D showing Hadley grain with hydrated core surrounding an unhydrated grain.
Fig. 5b. Micrograph of sample D showing gel of Ca-silicate.
66
~ ~ U L )
~
U
U
q
I U
4J ~J
*J ~J
I
t
i
Fig. 6. XRD traces. TABLE 3 POROSIMETRY RESULTS Sample
Total pore volume (cm 3/g)
A (water + OPC a)
0.136
B (Chemfix)
0.416
C (Chemfix + Zn)
0.684
D (Chemfix + Hg)
0.376
a OPC = Ordinary Portland Cement a n y identifiable crystalline zinc silicate, t h o u g h a m o r p h o u s gel o f c a l c i u m silicate was o b s e r v e d in b o t h p u r e C h e m f i x a n d m e t a l - b e a r i n g C h e m f i x e d solids. T h e a b s e n c e o f zinc silicate is t o b e e x p e c t e d since t h e s o l u t i o n c o n t a i n i n g Z n was a d d e d t o t h e c e m e n t m i x p r i o r t o s o d i u m silicate
370 A 7500 A single
7500 A
double 370 A 7500 A
Ca (OH)2 : medium ettringite : weak unhydrated cement: strong Ca (OH)2 : absent ettringite: medium
unhydrated cement: medium strong Ca (OH)2 : medium ettringite : weak
C-S-H: a little, reticulated
Ca (OHH)2 : absent ettringite: large hexagonal prism with AFro (monosulphate) Ca-silicate gel: massive
C-S-H: fibrous, hydrated shell (Hadley grain) Ca (OH)2 : small crystals ettringite: small rods Ca-silicate gel: massive
C (Chemfix + Zn)
D (Chemfix % Hg)
double
unhydrated cement: medium strong
C-S-H: fibrous, hydrated shell (Hadley grain) Ca (OI-I)2 : small crystals ettringite: small rods Ca-silicate gel: massive
B (Chemfk)
370
single
unhydrated cement: medium strong Ca (OH)2 : strong ettringite: weak
C-S-H: fibrous Ca (OH)2 : large crystals ettringite: small rods
Porosimetry (7-days sample)
A (OPC + Water)
Powder XRD (1-day sample)
SEM (1-day sample)
Sample
SUMMARY OF CORRELATION OF FOUR STUDIES
TABLE 4
high
low
--
--
Leachability values
O~
68 -
IA)
0.1/., Cement+ water wit :0-6
'~E 01; u
~, o.~
o.o6! u
0.0l
>o o-o2
750o
3;oo
,~oo 7~o
~o
,~o
;s
3~
7's
I~
3'7
Pore r a d i u s nm 0.a m ~ ..... u
Chemfix ÷Zn Chemfix Chem fix ÷ H g
IC)
0"6
o o.=
~
/
~
/.:
~: 0.2
.
.
7500
.
.
1500
750
.
•
~
ID I
.,,i.~.~.~.....~ .~--" " -
3700
~.,,,,~,~---~.,,,, •
,
.
370
150
.
.
75
.
37
.
15 Pore r a d i u s hm
7"5
Fig. 7. Cumulative pore volume.
addition; this order of addition is normal practice in the Chemfix operating procedure. Under these conditions it is thought likely that most of the Zn would be precipitated as the hydroxide or would react with the calcium hydroxide to produce possibly calcium zincate [15]. Although no evidence of either of these Zn compounds has been found by the SEM or XRD tests the absence of C-H crystals in the sample containing Zn indicates that calcium hydroxide plays an important role in the fixation of zinc ions. By contrast, Hg did not seriously affect the normal hydration process of Chemfix as is evident from the SEM, XRD and porosity studies. The formation of C-S-H, C-H and calcium silicate all proceed in the same manner as in pure Chemfix, which indicates that there is little or no interaction of this metal with cement or sodium silicate. This inability of Hg to form an insoluble hydroxide or silicate with the solidifying material means that the metal remains in pore solution or at most is only loosely bound to the hydrated products through sorption. The metal is therefore physically encapsulated within the cement structure and not chemically stabilized to the extent observed for Zn. Mobilization of Hg is rapid once in contact with
69 =o
~7
a
o6 o
~5
N4 o3 E 1
!
I I
7500 3700
I
I
1
i
I I
w
1500
750
I
I |
370
I 150
75
37
15
7-5
37
Pore rodius nm
b
6
~3 ,,2 E
3~oo
7501
~o
7so
3~o
i;0
;s
3~
1~ pore
radius
y's
3'?
nm
C
12 11
lO m
9
.? s ?
i-
~4 w3 £ 22 1
!
7500 3100
750
370
150
750
370
150
75
37
15 7- 5 Pore radius nm
!
37
o o.
!
!
7500
3700
i
1500
!
i
75
37
15 7"5 3"7 Pore radius in nm
Fig. 8. Pore size distribution in sample: (A) c e m e n t + w a t e r ; (B) Chemfix; (C) Chemfix + Zn; (D) Chemfix + Hg.
70
water and the leachability of the fixed product is very high and probably dependent on the permeability of the solidified product. The addition of soluble silicate to cement in the Chemfix process increases the water demand and accelerates the setting. To the waste disposer, this increase in water content saves material costs as a larger volume of waste can be treated for the same amount of cement. However, the penalty of increasing the water/cement ratio is to increase the porosity of the solidified material, resulting in reduced physical strength and higher permeability. The differences in leachability and fixation mechanisms of Zn and Hg in Chemfix and the effect of water/cement ratio on porosity suggest that the solidification process should be operated in accordance with the type of material being stabilized. For Zn, and other similar heavy metals ions which react readily with calcium hydroxide to produce insoluble compounds, a high water/cement ratio can be used which will lower material costs without any adverse effect on leachability. The amount of stabilizing material used will be dictated largely by the desired physical quality of the final product in terms of strength and permeability. For Hg and other metals which do not form insoluble compounds with Chemfix b u t rely on physical encapsulation to retain the metal, a low water/cement ratio should be adopted in order to reduce porosity and thereby lower the permeability and minimize metal mobilization. Reduced leachability can be achieved by the appropriate use of additives such as PFA and fumed silica. These react with C-H to produce amorphous C-S-H which fills the pores and reduces porosity. Further research is being planned using EXAFS (extended X-ray absorption fine structure) and STEM (scanning transmission electron microscopy) to investigate the exact behaviour of these and other heavy metals in stabilization processes and establish the form in which these metals are present in the solidified matrix.
REFERENCES
1 2 3
4
5 6 7
House of Lords Select Committee on Science and Technology, Hazardous Waste Disposal, Her Majesty's Stationary Office, London, 1981. C.S, Poon, C.J. Peters and R. Perry, Use of stabilization processes in the control of toxic waste, Effluent Water Treat. J., 23, 11 (1983) 145--459. C. Tashiro, H. Takahashi, M. Kanaya, I. Hirakida and R. Yoshida, Hardening properties of cement mortar adding heavy metal c o m p o u n d and solubility of heavy metal from hardened mortar, Cem. Concr. Res., 1 (1977) 283--290. C. Tashiro, J. O b a and K. Akawa, The effect of several heavy metal oxides on the formation of ettringite and the microstructure of hardened ettringite, Cem. Concr. Res., 9 (1979) 303--309. C. Tashiro and J. Oba, The effect of Cu (OH)2 on the hydration of C3A, Seventh Int. Congr. Chemistry of Cement, Paris, 2, II (1980) 58--63. I.N. Stepanovo, Hardening of cement pastes in presence of chloride of 3d elements, J. Appl. Chem. c/c Zh. Prikl. Khim, 54 (1981) 885--888. N.L. Thomas, D.A. Jameson and D.D. Double, The effect of lead nitrate on the early hydration of Portland cement, Cem. Concr. Res., 11 (1981) 143--153.
71 8 N. McN. Alford, A.A. Rahman and N. Salih, The effect of lead nitrate on the physical properties of cement paste, Cem. Concr. Res., 11 (1981) 235--245. 9 C.S. Poon, C.J. Peters, R. Perry and C.P.V. Knight, Assessing the leaching characteristics of stabilized toxic waste by use of thin layer chromatography, Environ. Tech. Lett., 5, I (1984) 1--6. 10 F.P. Glasser, A.A. Rahman, R.W. Craford, C.E. McCulloch and M.J. Angus, Immobilization and leaching mechanisms of radwaste in cementbased matrices, DOE Report No. DOE/RW/83.093, 1, Department of the Environment, London, 1983. 11 D.J. Lee, Factors affecting the leachability of caesium and strontium from cemented simulant evaporator waste, AEEW-R1461, Atomic Energy Research Establishment, Winfirth, 1978. 12 R.K. Salas, Disposal of liquid waste by chemical fixation/solidification - - the Chemfix process, In R. Pojasek (Ed.), Toxic and Hazardous Waste Disposal, 1, Ann Arbor Science Publishers, Ann Arbor, MI, 1979, 321--348. 13 J.R. Conner, Ultimate disposal of liquid residues by chemical fixation, Proc. National Conf. on Management and Disposal of Residues from the Treatment of Industrial Waste Waters, Washington DC, Information Transfer Inc., 1975, 197--208. 14 K. Jackson, J. Benedik and L. Jackson, Comparison of three solid waste batch leach testing methods and a column leach test method, Hazardous solid waste testing, First Conf., ASTM 760 (1981) 83--98. 15 C. Tashiro and S. Tatibana, Bond strength between C3S paste and iron, copper or zinc wire and microstructure of interface, Cem. Concr. Res., 13 (1983) 377--382.