United States
Environmental Protection
Agency
Risk Reduction Engineering
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/067 Apr. 1990
svEPA Project Summary
Interference Mechanisms in
Waste
Stabilization/Solidification
Processes
Larry W. Jones
The stabilization/solidification (S/S)
of hazardous wastes involves a
series of chemical treatment proced-
ures. The wastes are normally treated
to complex or to bind the toxic
elements into a stable, insoluble form
(stabilization) or to entrap the waste
material in a solid and/or crystalline
matrix (solidi-fication). Hazardous
wastes contain many constituents
that can interfere with the
binding/trapping process. Possible
Interference mechanisms between
particular waste com-ponents and
commercially available, waste
binding systems should be identified.
The complete report presents
background Information and a
literature review covering portland
cement and pozzolan chemistry, the
effects of admixtures on concrete
setting characteristics, and the
effects of common organic waste
components on the physical and
containment properties of the final
treated waste product. These topics
are presented so that conclusions
may be drawn as to possible types of
interferences that may be
encountered in typical waste binder
systems. A glossary of common
cement terminology and three
bibliographic appendices covering a
compilation of references and
annotated citations for both portland
cement and asphaltic waste
treatment systems are also
presented.
This Project Summary was
developed by U.S. EPA's Risk
Reduction Engineering 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
Waste treatment processes that are
designed to reduce the toxicity or volume
of hazardous wastes (e.g., precipitation or
incineration) usually generate persistent
residues that must be prepared for safe
final or ultimate disposal. In most cases,
this final disposal is by means of secure,
shallow land burial. The disposal problem
is most critical in the case of wastes that
cannot be destroyed or detoxified, such
as heavy metal sludges and brines. Thus,
even with new and improved waste
treatment methods, there will continue to
be a need for technology related to the
safe land disposal of toxic wastes. S/S of
hazardous wastes before final disposal
has been proposed as a method to
prevent release of the hazardous waste
constituents to the environment.
A thorough understanding of the
potential behavior of stabilized/solidified
waste is necessary to make judgments
about the long-term effectiveness of their
containment. The extent to which various
contaminants are held securely in
stabilized/solidified wastes must be
determined for all S/S processes so that
individual processes and delistmg
petitions can be evaluated. There are
several available methods for the S/S of
hazardous wastes. Some of the chemical
components of the complex wastes may
interfere with the proposed S/S process
and cause undesired results (e.g., flash
set, set retardation, spading, etc.).
Literature
The literature search was conducted
with the use of facilities available at the
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U.S. Army Engineer Waterways
Experiment Station (WES). The
Compendex Data Base, National
Technology Information Service Data
Base, and Technical Research
Information Service Data Base were used
for two computer searches. Additional
computer searches used the Data Base
Index (DBI), Chemical Abstracts Data
Base, Engineering Information and
Technical Meetings Data Base (EIMET),
Environment Abstracts Data Based
(ENVIROLINE), Electric Power
Information Abstracts Data Base (EPIA),
and Geological References Data Base
(GEOREF) of the American Geological
Institute. Three bibliographies developed
through these procedures are included in
the report as appendices.
Cement and Pozzolan
PropertiesThat May Affect
Waste Containment
The ability of a stabilized/solidified
waste product to retain/contain a given
hazardous constituent depends primarily
on its resistance to leaching (or
volatilization) of waste constituents and
on its long-term durability. Some
background is necessary to understand
factors controlling leaching and durability
characteristics of cement and pozzolans.
Portland Cement
Comparison of the finished
stabilized/solidified waste product with
standard construction materials is not
entirely accurate since most hazardous
wastes to be stabilized/solidified are
liquid slurries with relatively low solids
content (10% to 40% solids by weight).
The mixture of a solidification agent such
as portland cement with a slurried waste
more closely resembles a hydrated
cement paste rather than a typical
concrete with a large proportion of solid
aggregate (60% to 80% of the final
cement volume).
Voids In Hydrated Cement and
Concrete
The several kinds of voids in hydrated
cement paste greatly influence its final
strength, durability, and permeability
properties. The smallest voids, which
occur within the calcium-silicate-hydrate
gel structure, are 0.5 to 2.5 nm. They
account for around 28% of the porosity of
the solid product. These small voids have
little effect on the strength and
permeability of the final product but
appear to be important in drying
shrinkage and creep.
Capillary voids account for the larger
spaces not filled by solid components. In
well-hydrated, low water/cement ratio
mixes, capillary voids range from 10 to
50 nm, but in high-ratio mixes they may
be as large as 3000 to 5000 nm. Pore
size distribution and not simply total
capillary porosity is generally held to be
a better criterion for evaluating the
characteristic of a cementitious product;
capillary voids larger than about 50 nm
are thought to be detrimental to strength
and impermeability, whereas, voids
smaller than 50 nm are more important to
drying shrinkage and creep. Capillary
voids limit the strength of concrete by
acting as "stress concentrators."
The third type of void, usually called
"air void," is generally spherical and
usually ranges from 0.05 to 0.2 mm but
may range up to 3 mm. Air voids in this
size range are usually introduced
intentionally into the cement mixture to
increase the resistance of the final
product to freeze-thaw (frost) damage,
even though they typically have an
adverse affect on strength and
impermeability.
Pozzolanic Materials
By definition, pozzolans are siliceous
materials that display no cementing
actions by themselves but that contain
constituents that combine with lime at
ordinary temperatures and in the
presence of water to form cementitious
compounds. The main pozzolans used
commercially, at present, are fly ash from
burning of powdered coal, ground blast
furnace slag, and kiln dusts from lime or
cement kilns. Although pozzolanic
reactions are not identical to portland
cement reactions, they are thought to
resemble them. Pozzolanic reactions are
generally much slower than portland
cement reactions, and set-times are
usually measured in days or weeks
instead of hours. Caution must also be
exercised concerning additional
contaminants introduced through these
waste byproducts.
The Strength/Porosity
Relationship
Many factors may influence the
strength of a waste-cement mass. By far
the most important factor in strength of
cement is the relationship between the
water/cement ratio and porosity. The
relationship is usually explained as a
natural consequence: the weakening of
the cement matrix caused by porosity
increases as the water/cement r
increases, and thus, the cement
weakened.
Water/Cement Ratio and Degi
of Hydration
This relationship between concr
porosity and strength is of gr
importance when predicting the ability
a given waste-cement mixture to con
the waste under leaching conditio
Compressive strength then may be
acceptable indicator of the stal
ized/solidified waste's resistance
leaching.
Mineral Admixtures
The use of pozzolanic a
cementitious byproducts such as fly a
blast furnace slag, or kiln dusts
admixtures is an important issue in wa
S/S. When used in addition to, or a!
partial replacement for portland cerm
the presence of the mineral generj
retards the rate of strength gain althoi
incorporating them considerat
improves the ultimate strength a
impermeability (water tightness).
Air Entrainment
Additives causing stable ;
incorporation into the cement paste <
universally deleterious to the ultinru
strength and impermeability of t
concrete, most likely because of t
added large-pore space. The entrain
air, however, greatly increases t
resistance of the products to freezir
thereby increasing their durability unc
freeze-thaw conditions.
The Durability/Permeability
Relationship
Long-term durability of tl
stabilized/solidified waste product is
prime consideration in designing ai
specifying waste S/S systems. Predictii
the long-term integrity of the final was
form requires considering all possib
modes of failure. For cementitioi
stabilized/solidified products, water
generally involved in every form
deterioration; and in porous solid
permeability of the material to wat
usually determines the rate
deterioration. Internal movement ar
changes in the structure of water a
known to cause disruptive volurr
changes of many types of product
Examples are water freezing into ic
formation of ordered structure of wat
inside fine pores, development of osmol
pressures because of different ion
concentrations, and hydrostatic pressu
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buildup by differential vapor pressures.
All of these can lead to large internal
stresses within a moist solid and to its
ultimate failure
Cracking by Crystallization in
Pores
Stabilized/solidified waste products
often contain substantial amounts of salts,
or organic molecules, or both, with
appreciable water solubilities.
Concentration of these materials at or
below the surface of the solid where
evaporation of pore water is occurring
can cause super-saturated solutions to
develop and salt crystals to form in the
pores of the stabilized/solidified product,
which may disrupt its structure.
Wet-Dry Cyc//ng
Wet-dry cycling of normal concrete
products usually does not significantly
damage its structure. As the total
proportion of cement is reduced or as the
water/cement ratio is increased, as is
common for economy in waste S/S
practices, wet-dry cycling may, however,
cause rapid deterioration of the
stabilized/solidified waste product.
Freeze-Thaw Damage
Although there is generally a direct
relationship between strength and
durability, this does not hold in the case
of frost damage. In a manner analogous
to salt crystals, ice crystals forming at
subfreezing temperatures can rapidly
deteriorate water saturated concrete
products.
The hydraulic pressure generated by
freezing pore water depends on the
permeability of the material, the distance
from the surface (escape boundary), and
the rate at which ice is formed. Durability
of cement products to freeze-thaw cycles
can be provided by entraining small air
bubbles into the cement paste. These
provide water escape boundaries. In
medium and high-strength concretes,
however, every 1% increase in the air
content reduces the ultimate strength of
the final product by about 5% and
appreciably increases its permeability.
Deterioration by Chemical
Reactions
The effects of internal waste
constituents as well as aggressive
environmental agents on stabil-
ized/solidified waste products must be
adequately known before the long-term
liability of the stabilized/solidified
product can be assumed. The solid
phase of a well-hydrated Portland cement
paste exists in a stable equilibrium with
the high-pH pore fluid. Large
concentrations of Na +, K +, and OH-ions
bring about a pH of 12.5 to 13.5 in the
pore fluid. Natural C02 sulfates, and
chlorides common in ground- and rain-
waters may bring about aggressive
solutions below pH 6, which can be
detrimental to the stabilized/solidified
waste product.
Cation-exchange reactions can occur
between the external solution and the
cement binder. Anions in acidic solutions
that form soluble calcium salts (such as
calcium chloride, acetate, and
bicarbonate) will leach the calcium from
the stabilized/solidified product. This is
particularly damaging because it
increases the permeability of the
concrete, which increases the rate of
further exchange reactions.
Sulfates attack of waste-concrete
products can be a serious problem and is
an important consideration in the S/S of
sulfate-containing wastes in portland
cement. Concentrations of soluble
sulfates greater than 0.1% in soil or 150
mg/L in water will endanger cement
products, and soils of over 0.5 soluble
sulfates or over 2000 mg/L in water can
have serious effect. Pozzolanic S/S
systems are useful for S/S of high sulfate
wastes since they do not contain free
calcium hydroxide and, thus, are
nonreactive to sulfate.
Radiation and
Stabilized/Solidified Waste
Products
Large doses of gamma radiation do not
appear to affect setting properties or
cause appreciable loss of strength or
increased teachability in portland cement
solidified waste products.
Standard Admixtures for
Portland Cement Products
The properties of cement in both the
fresh and hardened state are typically
modified by adding specific materials to
the cement mixtures. The additives vary
widely in chemical composition and may
modify more than one property of the
cement mixture. Cement admixtures are
germane to the discussion of
stabilized/solidified waste since an
understanding of the effects of common,
well-studies' admixtures gives a basis for
assessing the possible range of effects of
waste constituents on the cement or
pozzolanic matrix.
Admixtures fall broadly into three
categories: (1) surface active molecules
that work on the cement-water system
immediately on addition by influencing
the surface tension of water and by
absorbing onto the surface of cement
particles; (2) set controlling materials that
ionize and affect the chemical reaction
between the cement and the water only
after several minutes or hours; and (3)
finely-ground, insoluble minerals, either
natural materials or byproducts, that
immediately affect the rheological
behavior of the fresh concrete, but whose
chemical effects take several days or
months to manifest themselves. These
types of effects can also be seen in
stabilized/solidified waste products.
Possible Mechanisms That
Affect Organic Compounds in
Portland Cement
Several conceptual models of possible
interference mechanisms of organics on
cementitious and pozzolanic are
addressed.
Adsorption
One possible interfering mechanism is
adsorption of added waste molecules on
the surface of the cement particles; this
adsorption blocks the normal hydration
reactions.
Complexation
Conditions in a cement paste are
favorable for aluminate, ferrite, and
silicate ion complexation. Such
complexation possibly delays the
formation of hydration products. When
cement crystal-forming ions are kept in
solution by complexation, hydration
barriers are established that may retard
the set and weaken the final product.
Precipitation
The formation of insoluble hydration
products by waste constituents reacting
with cement compounds is not thought to
be a realistic mechanism of admixture
interference. If the precipitation reaction
involves the hydration product itself,
however, the precipitated product would
be produced preferentially at the surface
of the hydrating material. This is thought
to be the mechanism of action of
phosphates, borates, and oxalates.
Nucleation
The inhibition of nucleation of
crystalline calcium hydroxide by soluble
silica, which is present in small quantity,
is believed to be the self-retarding set
feature of tricalcium silicate hydration. It
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is postulated that organic retarders act
much the same as silica ions by being
adsorbed onto the calcium hydroxide
nuclei. However, organic interfering
materials may be adsorbed more
effectively and may cover crystal growth
surfaces more completely.
Effects of Wastes on
Cement/Pozzolan Processes
Acids, salts, bases, and organic
materials may be present in hazardous
wastes singly or in variable combinations.
As such, they present a difficult problem
for process designers and regulatory
agencies to predict their single and
collective effect on the durability and
containment of typical cement and
pozzolan S/S processes. Long-term
effects are especially difficult to estimate
because subtle differences in
environmental parameters can have
significant long-term consequences to the
integrity of stabilized/solidified waste. In
the full report, the few studies of the
effects of organic and inorganic waste
constituents on stabilized/solidified waste
properties are discussed in some detail.
Asphaltic and Polymeric Binders
Asphalt and bitumens are often
suggested as S/S agents for low-level
radioactive and soluble hazardous
wastes. Bituminous materials, since they
are immiscible in water and are quite
stable, will effectively isolate soluble
components from contacting water for
long periods of time. Major drawbacks of
the asphaltic S/S process are its
flammability, the need to mix at high
temperature, and the cost of the raw
materials and mixing equipment. The
information most pertinent to intermixing
waste and asphalt is its viscosity
properties and flash point. Studies of the
incorporation of hazardous and
radioactive waste in asphaltic materials
are summarized.
Conclusions and
Recommendations
Although chemical S/S of hazardous
wastes before disposal is increasing in
importance, very little work has b<
done concerning the effects of spec
waste components on the physical <
contaminant properties of the trea
waste product.
There is a clear need for furtl
experimental work to obtain physical <
chemical data about cementitious i
asphaltic treatment systems. Sufficii
basic information should be developed
that waste/binder interactions can
modeled. This would overcome I
current need to test each speci
waste/binder combination for possil
interferences. The variability a
complexity of most waste streams m
however, still preclude su
generalizations.
The full report was submitted
fulfillment of Interagency Agreement t
DW 219306080 by the United Stal
Army Engineering Waterwa
Experiment Station under tl
sponsorship of the United Stat
Environmental Protection Agency.
Larry W. Jones is with the Environmental Laboratory, U.S. Army Engineer
Waterways Experiment Station, Vicksburg, MS 39180-0631.
Car/ton C. Wiles is the EPA Project Officer (see below).
The complete report, entitled "Interference Mechanisms in Waste
Stabilization/Solidification Processes," (Order No. PB90-156 2091 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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