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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