United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/056 Jan. 1990
&EPA Project Summary
The Morphology and
Microchemistry of
Solidified/Stabilized
Hazardous Waste Systems
F. K, Cartledge, H. C. Eaton, M. E. Tittlebaum
A study was carried out to gather
data that can be interpreted in terms
of the mechanisms operating during
cementitious solidification/stabiliza-
tion of hazardous waste streams
containing water-soluble organics.
Most of the work was carried out with
phenols and ethylene glycol as the
model wastes and Type I Portland
cement as the fixing agent. Solidified
products containing from 2% to 50%
by weight of organics (measured
relative to cement) were studied at
various cure time up to 1 year. The
products were studied at various time
Intervals using extractions with
solvents of varying polarity, optical
microscopy, scanning electron
microscopy and energy dispersive
elemental analysis, transmission
electron microscopy, x-ray powder
diffraction, and several physical tests
(setting times and unconfined com-
pressive strength).
The report describes and interprets
effects of the organics on the
developing cement matrix and the
nature of the interactions involved.
The organics were not effectively im-
mobilized toward water leaching.
Depending on the chemical nature of
the organic, the distribution of the
model waste in the cement matrix
was either homogeneous at the
micron level or quite heterogeneous.
Effects on the matrix were very con-
centration dependent and were much
more dramatic with ethylene glycol
than with the phenols. These effects
ranged from microscopic changes in
morphologies and compositions of
cement phases to macroscopic prop-
erties such as compressive strength.
Although there was evidence of
chemical reaction between the phe-
nols and the fixing agents (spe-
cifically, salt formation), the reaction
did not result in chemical fixation of
the organic.
At low concentrations, the organics
did not greatly alter the hydration
chemistry of the cement system. The
probable consequence of this would
be little interference in immo-
bilization of inorganics in cases in
which mixed inorganic/organic waste
streams are encountered. Specific
experiments designed to test this
conclusion, however, have not yet
been carried out.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The report summarized here describes
the first of three phases of work aimed at
thoroughly characterizing wastes
solidified in a cement matrix, or whatever.
Phase I, the fundamental research aspect
of the work is an effort to understand
solidification mechanisms with the use of
cement and several water-soluble
organics. Phase II is an extension of
Phase I and involves cooperation with the
Waterways Experiment Station of the
U.S. Army Corps of Engineers (WES) in a
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project to characterize interferences by
various inorganic and organic materials in
solidification of metal sludges with the
use of cement and fly ash fixing agents.
Phase III of the work involves the use of
x-ray diffraction (XRD) and scanning
electron microscopy (SEM) to charac-
terize vendor-produced samples of
wastes solidified by processes, which are
usually proprietary. Phase III is a part of
the U.S. EPA/Environment Canada joint
project to evaluate test methods for
solidified products. Phases II and III are
not completed and will be described in
detail in a future report.
Solidification/stabilization is a common
technology used to immobilize wastes
before landfilling, roadbed construction,
etc. The technology has been most com-
monly studied as a potential technique to
immobilize toxic metal ions or radioactive
waste. Complete detoxification of metals
by chemical, biological, or other means is
impossible short of elemental trans-
formation; hence, methods that dilute
and/or isolate the metals are necessary
as part of waste management schemes.
Although many vendors of solidification/
stabilization technology have presented
leaching data claiming the efficacy of
various immobilization techniques, claims
concerning the nature of chemical
interactions between waste metal ions
and fixing agents have remained un-
substantiated.
The aim of the Phase I research was to
gain information about the mechanisms
by which solidification/stabilization
schemes operate and to define the nature
of the interferences associated with the
presence of various classes of com-
pounds, both inorganic and organic.
Conceptually, there are two broad
classes of mechanisms; namely, physical
entrapment and chemical interaction.
Physical tests, spectroscopy, and micros-
copy all were used to gain information
about the interactions of various
compounds with typical solidification/
stabilization matrices.
The two principal constituents of most
commercial fixing agents are cementi-
tious materials (Portland cement, fly ash,
etc.) and soluble silicates. A limitation
often cited is that the processes are
incompatible with organics, but details
about the types of organics that interfere
or the concentrations at which inter-
ference is detrimental are not available.
The same is true with many inorganic
materials. A potential concern is that
interferences, even in small amounts, can
alter the process sufficiently to sub-
stantially decrease the ability of the fixing
agents to immobilize metal ions. Indeed,
it is well known that admixtures can alter
the setting characteristics of Portland
cement. It is not clear whether, and at
what concentrations, various materials
commonly present in waste streams
interfere with the complex setting
reactions and result in a significantly
altered cement matrix. The question is of
obvious interest with respect to solidifica-
tion/stabilization technology, since com-
plex mixed inorganic/organic waste
streams are commonly encountered.
Research Methods
The methods used in Phase I were
adaptations of procedures used to char-
acterize cement (or other inorganic)
matrices, as well as common techniques
to characterize solidified waste form.
Samples were prepared using Type I
Portland cement in a 0.4 watercement
ratio. The organic compounds were ethyl-
ene glycol (EG), p-bromophenol (pBP),
and p-chlorophenol (pCP). The required
amount of organic compound (0.2 g, 0.4
g, 1.0 g, or 2.0 g) was weighed out into a
screw-cap vial, and 10.0 mL of Portland
cement was added followed by 4.0 ml of
deionized water. The mixture was stirred
with a glass stirring rod, for 1 minute by
hand. The vials were allowed to stand in
the dark at room temperature for periods
of from 12 hours to 1 year before being
tested.
The test methods and the information
they were expected to afford are briefly
summarized here:
Extraction with Solvents of
Varying Polarity
The samples containing various con-
centrations of organics were ground to a
standard size and extracted with three
solvents: dichloromethane (DCM), di-
methyl sulfoxide (DMSO), and deionized
water. The amount of organic compound
recovered in the extractions was deter-
mined as a function of solvent polarity
and time of cure for the sample. The data
were used to reach qualitative con-
clusions concerning the environment of
the organic compound in the cement
structure, and trends in extractability
were compared with qualitative differ-
ences in sample morphological compo-
sition observed by microscopy.
Microscopy (Optical and SEM)
and Energy-Dispersive X-Ray
Analysis (EDXA)
Optical and SEM observations of sur-
faces of solidified samples were made to
characterize changes occurring as a
result of having added wastes to cerr
Because numerous micrographs I
been published for cements alone
with various additives, a large amoui
data are available for comparisons. E
can provide an elemental profile i
solid surface, with certain limitati
These methods in combination with tl
described below can determine whe
the waste has influenced the formatic
the mineral phases characteristic of
matrix.
Transmission Electron
Microscopy (TEM) and Select
Area Diffraction (SAD)
TEM has been used to study cer<
materials and minerals for the pas
years, and several studies of cer
have been reported. TEM, however,
not been applied to studies of solid
hazardous chemical wastes. The r
advantage of TEM is the high sp
resolution, which can exceed 0.2 nn
addition, the SAD pattern can pro
information about crystal structure
the orientation of crystalline phases. I
of these instrumental characteristics
important for waste research.
X-Ray Powder Diffraction (XR
Powder diffraction analysis allows
crystalline mineral constituents of
matrix to be identified by t
characteristic set of diffraction li
Many XRD studies of cements have t
carried out. Although limitations
associated with the complex nature o
formed solid matrix, some min
constituents are easily identifiable.
presence or absence of specific mil
phases can be a function of admixtun
the cement formulation; that
presumably also be the case w
wastes are the admixtures.
Physical Testing
Selected physical characteristics o
solidified samples have been studie
define strength and practical workal
of waste/cement mixes. Specific
compressive strengths and setting ti
(initial and final) of cements contai
organics were studied. The compre:
strength test reflects the amoun
applied loading the solidified wastes
withstand before failure in a landfill.
setting time tests indicate the
needed for the solidified waste
become workable. In addition, these
were conducted because they have
been used by the cement industr
standard indicators of mechar
performance.
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The Use of Polar Organlcs
Most of the Phase 1 studies involved
relatively polar organic molecules,
although some preliminary work with
nonpolar organics is also reported.
Because of their relatively high water
solubility, polar organics are the ones
most likely to be present in substantial
concentrations in mixed aqueous waste
streams that also contain metal ions.
Thus, polar organics are good candidates
as solidification/stabilization interferences.
Glycols and phenols are typical of such
species and are also major articles of
commerce and, hence, constituents of
waste streams. Apart from the fact that
EG, pCP, and pBP are typical of real
wastes, the reason for choosing halo-
genated phenols is that the presence of
the heavy atoms enables elemental
analysis by EDXA to locate the organic
within the complex hydrated cement
matrix.
These polyhydroxy compounds are, in
fact, among the classes of organics that
alter the setting characteristics of
Portland cement. The polyhydroxy
organic EG was chosen for several
reasons. There is evidence in the litera-
ture that EG has profound effects on
already set cement and the mineral
constituents thereof. When hydrated
;ement of tricalcium silicate (C/S) is
.reated with EG, etching occurs, but it is
unclear whether mainly calcium silicate
hydrate gel (CSH) or calcium hydroxide
(CH) is dissolved. A further consideration
is that EG is completely miscible with
water. Hence, cement pastes can be
prepared with any concentration of EDG
in water, and in all cases, a homog-
eneous liquid phase is being mixed with
the unhydrated cement. The mixing prob-
lems are also minimized with the
phenols, which are substantially water
soluble. When the very water-in-soluble
organic 1-decanol was solidified, as the
right ratio of organic to cement varied
from 50:1 to 1:1, the product of solid-
ification varied from a solid, which was
outwardly indistinguishable from hard-
ened cement, to a two-phase system with
hardened material and supernatant liquid
(shown by gas chromatograph analysis to
be pure 1-decanol). With the EG
samples, there was great variation in the
physical appearance of the mixes as
functions of both concentration and time
of cure.
Results and Discussion
The systems studied in Phase I repre-
sent the most thoroughly characterized
combinations of organics and cement
pastes. In particular, the combinations
included those with relatively high con-
centrations of organics—those typical of
situations in which organics would con-
stitute a large fraction of a waste mixture
being solidified. Two questions concern-
ing the Phase I study of organics are
particularly relevant to solidification prac-
tice:
1. Do cementitious matrices have the
ability to immobilize the organics
themselves?
2. How do organics alter the cement
matrix and thus affect immobilization
of inorganic constituents?
The Phase I study has effectively
demonstrated that cement alone is inef-
fective in immobilizing water-soluble
organics such as EG or phenols. The
second question is not completely an-
swered to date. Although many effects of
organics on the cement matrix at various
concentrations have been documented.
The leaching studies needed to deter-
mine whether the organics constitute
interferences in inorganic immobilization
remain to be carried out or interpreted.
Phases II and III will report on some
leaching studies at 3 later date.
The following paragraphs summarize
the information obtained thus far.
EGlCement
1. For ratios of up to 1:5 EG:cement, the
same basic reaction chemistry went
on during cement hydration in the
presence or absence of EG. SEMI
EDX analysis was able to identify
phases apparently identical to those
in hydrated cement alone (although
not necessarily in the same propor-
tions as in cement alone). At con-
centrations of 1:5 and greater, the
samples did not effectively set even
at 28 days or longer.
2. EG slowed the rate of hydration of
cement, even in the 1:50 mixtures. At
1:5 ratios, final set was not reached.
3. There was a small percentage of
strongly bound (although mostly
reversibly bound) EG. Solvents in
which EG is very soluble (DCM and
DMSO) and that penetrate the
cement matrix very effectively under
our extraction conditions did not
always give high recoveries of EG.
4. Despite the statements in the pre-
ceding paragraph, EG was not
effectively immobilized toward water
leaching even at low loadings of EG
on cement. High percentage recover-
ies (up to 100% in a single extraction
after grinding) were observed at all
loadings of EG on cement at all times
of cure.
5. The principal effect of EG on the
microstructure of cement involved the
CSH gel phase. There were notice-
able effects on the morphology of this
phase, particularly the presence of
aggregates of small grains. EG also
appeared to absorb to the surfaces of
Ca(OH)2 crystals. Fine crystals with
d-spacings uncharacteristic of com-
mon cement phases could be ob-
served, but not identified by TEM.
6. Overall crystalinity of the matrix was
diminished with increasing EG con-
centration, particularly above about
10% EG by weight.
7. Below 1:10 EG:cement, 28 day com-
pressive strengths were comparable
with those of cement pastes alone,
but above 1:10 there were significant
decreases in strength.
8. Increasing EG concentration in-
creased porosity of the matrix, as
judged qualitatively by SEM.
pCP/Cement
1. Unconfined compressive strength of
pCP/cement was lower than for the
control at 7 days but was approxi-
mately equal at 28 days.
2. Times for initial and final set were
approximately doubled. pCP slowed
the hydration of cement more than
did pBP but less than did EG.
3. In 1:10 pCP:cement samples percent
recovery in water extraction de-
creased with time of cure from 93%
(12 hours) to 60% (90 days). There
was no effective immobilization at
any concentration or time of cure.
4. SEM showed significant morpholog-
ical differences of pCP/cement when
compared with cement pastes: more
porous at 12 and 24 hours but less
porous at 7 and 28 days.
5. EDX showed homogeneous distribu-
tion of Cl at the 10 p level. pCP was
not found in the CH phase, and it was
not found in a separate phase as was
pBP.
6. XRD showed new peaks, probably
the Ca salt of pCP.
7. XRD showed significantly decreased
crystalinity.
8. Despite the observations above, pCP
allowed the basic reaction chemistry
of setting cement to proceed, as
shown by the presence of the normal
cement mineral phases.
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pBP/Cement
1. Unconfined compressive strength of
pBP/cement was lower than was the
control at 7 days but approximately
equal at 28 days.
2. Time for initial set increased
approximately 20%; time for final set
increased approximately 75%.
3. In 1:10 pBP:cement samples, percent
recovery in water extraction varied
between 45% and 55% over the
period from 12 hours to 90 days, but
dropped to a low of 9% at 1 year.
Again, effective immobilization was
not achieved at any concentration or
any time of cure.
4. Electron Probe Micro Analysis
showed inhomogeneous distribution
of Br at the 10 n level. There were
distinct Br-rich phases, presumably
the Ca salt.
5. XRD showed new peaks, probably
the Ca salt of pBP.
6. XRD showed significantly decreased
crystalinity.
7. The presence of pBP allowed the
basic reaction chemistry of setting
cement to proceed.
Conclusions
Clearly neither the phenols used in this
study nor ethylene glycol was effectively
immobilized by cement alone at any
concentration studied. We assume that
this will also be the case with other
relatively water-soluble organics. The
only evidence of reaction between the
organics and the fixing medium was salt
formation. Indeed, when salt formation
occurred, as in the case of the phenols,
the organic remained relatively water
soluble and easily extractable from the
solidified matrix. There appeared to be
several distinguishable environments for
the organics in the cement matrix:
1. a separate microscopically distin-
guishable phase, as with the Ca salt
of pBP; as such, it was easily ex-
tractable.
2. an environment from which the or-
ganic was rather easily extracted by
water but not by less polar solvents;
probably involved hydrogen bonding
by the organic hydroxy groups in
interlayer spaces of CSH gel.
3. an environment in which the organic
was quite loosely bound and easily
extracted by any solvent in which it
was moderately soluble; this environ-
ment may also have involved the
CSH phase.
The idea that there is a definite number
of binding sites in the cement matrix at
which the organic may be effectively
immobilized was not borne out by our
work. Even at the lowest concentrations
of organics in the cement matrix, water
extracted the organics efficiently.
The cement matrix was affected in dra-
matic ways by the presence of organics.
Presumably the ability of the matrix to
contain other wastes, such as metal ions,
will also be affected by the changes
induced by the presence of organics. The
matrix changes depended on the nature
and the concentration of the organic. The
changes in easily observable or meas-
urable properties of the matrix were
greatest with EG, less with pCP, and
least with pBP. At or below a 1:10 ratio of
organic to cement, the effects of all three
organics on the easily measurable prop-
erties of the matrix were rather minor. All
gave homogeneous matrices that looked
like set cement; set times were affected
to a significant degree by EG but not
much by pCP and pBP; compressive
strengths at 28 days were comparable to
those of cement pastes without additives.
Nevertheless, while SEM, TEM, and XRD
indicated that some cement hydration
reactions were taking place in the same
way that they do in the absence of the
organics, the situation was much more
complex. New phases were formed; the
usual phases appeared to undergo solid
solution formation giving shifted XRD
peaks; new morphologies appeared with
increasing frequency and increasing con-
centration of organics. Although
changes can be documented, the effi
of such changes on some of
measurable properties, particularly c
pressive strength, were minor. The n
important point would be whether
changes affected the immobilizing at
of the matrix toward a variety of was
Some information of the latter kind she
be available from the WES project:
results of our participation in that v
will be reported as Phase II of
project.
Nevertheless, additional work need;
be done to bridge the gap between
work done here and field application!
solidification technology. Cementiti
solidification/stabilization is most c>
monly used to immobilize metal ions.
are now investigating the effectivenes
immobilization, as measured by Toxi
Characteristics Leaching Prpced
(TCLP), as various metal contair
wastes are solidified, increasing
waste-to-cement ratios gradually i
failure occurs. At the same ti
increasing amounts of organic in
ferences are being added. In additioi
monitoring metal and organic c
centration in TCLP leachates, the m;
changes are being monitored using
techniques developed in this study.
can thus have a thorough characteriza
of the matrix in cases where
mobilization if effective and can w;
changes in the matrix that occur as
approach failure of the system. We c<
thereby identify what changes
associated with decreasing (or incn
ing) effectiveness of immobilization.
The full report was submitted in
fillment of Contract No. CR 812318-1
by Louisiana State University under
sponsorship of the U.S. Environme
Protection Agency.
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F. K. Cartledge, H. C. Eaton, and M. E. Tittlebaum are with Louisiana State
University, Baton Rouge, LA 70803.
Charles I. Mashnl is the EPA Protect Officer (see below).
The complete report, entitled "The Morphology and Microchemistry of
Solidified/Stabilized Hazardous Waste Systems," (Order No. PB 90-134 156/AS;
Cost: $23.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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