Environmental Engineering Reference
In-Depth Information
minimized the detrimental effects of organic materials on the cement hydration
reaction and contaminant leachability.
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Silica fume is a by-product of producing
silicon metal or ferrosilicon alloys. Silica fume consists primarily of amorphous
(non-crystalline) silicon dioxide (SiO
2
). The individual particles are extremely small,
approximately 1/100th the size of an average cement particle. Because of its fine
particles, large surface area, and the high SiO
2
content, silica fume is a very reactive
pozzolan. Cement containing silica fume can have very high strength and can be
very durable.
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8.7
MULTITALENTED OR MULTIPURPOSE REAGENTS
There are a few additives that have wide usage. This is due in part to the multiple
positive characteristics that they impart to the grout or the resulting stabilized waste
form. These select additives include flyash, soluble silicates, kiln dust, and ferrous
sulfate. Chapter 4 describes the use of flyash, soluble silicates, and cement kiln dust
as cementing components.
The characteristics of flyash are described in detail in Chapter 4. The use of
flyash with portland cement has many advantages, including increased viscosity,
preventing phase separation, acting as a pozzolan, binding additional water, decreas-
ing the pore pH, adsorbing metal ions, and maybe also retarding setting of the
cement. The resulting product typically has increased strength, decreased perme-
ability, and increased durability in tests such as freeze-thaw and wet-dry resistance.
Thus, a cement-flyash combination is a popular system where critical physical
properties are required.
The greatest potential disadvantage of the cement-flyash process is the weight
and volume increase associated with large additions of flyash. The flyash:cement
ratio, by weight, is typically 2:4, with total weight increases of 50 to 150% corre-
sponding to volume increases of 25 to 75%.
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This is especially true with low solids
waste streams, where the flyash acts as a bulking agent to increase viscosity and
prevent phase separation until the mass sets. Where the increase is not important,
as in some remedial action projects, portland flyash cement may be the optimum
choice. Waste components, which interfere with cement setting and hardening, will
likely also interfere with those functions in the portland flyash cement system.
Soluble silicates combined with portland cement act as immobilization agents for
metals,
18
as anti-inhibition agents for cement setting,
18
and to reduce permeability.
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Lime kiln dust and cement kiln dust have been used on hundreds of remedial
solidification projects in the United States over the last 25 years. They have also
been used extensively at central hazardous waste management facilities (TSDFs).
Attractive characteristics or features of kiln dust include functioning primarily as an
absorbent or bulking agent and removing free water as additional water is consumed
in their pozzolanic reactions. In addition, lime kiln dust with its high calcium oxide
content is used to neutralize acidic wastes. Kiln dust is used for stabilization of
organics and inorganics. Systems with kiln dust usually set slower that portland
cement systems and have less strength. The chemical properties for kiln dusts vary
widely from source to source because of how these materials are produced.
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