Environmental Engineering Reference
In-Depth Information
hydraulic cements. The polymer includes a monomer, prepolymer-monomer mix-
ture, or a dispersed polymer (latex). To effect the polymerization of the monomer
or prepolymer-monomer, a catalyst is added to the mixture. Curing of latex polymer
cement concrete is generally shorter than for conventional concrete because the
polymer forms a film on the surface of the product and retains some of the internal
moisture needed for continuous cement hydration. Therefore, polymer-modified
cement concrete can be cast in place in field applications.
Polymer-modified cement can exhibit good ductility and excellent resistance to
penetration of water, aqueous salt solutions, and freeze/thaw cycles. The use of an
improper polymer may have negative effects on the properties of the cement due to
the incompatibility between the cement and the polymers. To achieve a substantial
improvement over unmodified concrete, fairly large proportions of these polymers
are required. The improvement does not always justify the additional cost.
Polymers are sometimes added into hydraulic cements to improve the effective-
ness of S/S of some contaminants, especially organic wastes. The USEPA even
acknowledges that organics interfere with the cement stabilization process particu-
larly when the organic concentration exceeds 1% total organic carbon by mass.
48
In
one study,
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latex-modified cement systems were used for S/S of inorganic wastes
containing lead and chromium. Results of freeze/thaw durability tests conducted on
the synthetic wastes show little or no weight loss after 50 cycles. In addition,
unconfined compressive strength tests show no loss of strength due to the presence
of lead and chromium. Preliminary TCLP and extraction procedure tests indicated
latex-modified cement systems had considerable improvement over regular portland
cement. In another study, it is noticed that a mercury-containing sludge solidified
by polymer latex-modified cement shows much higher strength and lower leachabil-
ity than that by regular portland cement.
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The author attributed this to the formation
of a cross-linked structure in the solidified monoliths, which enhances the cohesive
or chemical bonding between the contaminants and binder.
A latex polymer was added to portland cement to stabilize several pure polynu-
clear aromatic compounds present in the K051 waste, which include naphthalene,
phenanthrene, and pyrene.
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Leach test results indicate that the retention levels were
very high for all these compounds and was over 99% for pyrene in the polymer-
modified cement-stabilized waste form. FTIR analysis indicated the formation of
chemical bonding between the additives and the organic contaminants. It was
reported that the polymer enhances the encapsulation and penetration of the cement
system into the interstitial spaces of the waste.
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4.2.6
M
ASONRY
C
EMENT
In North America, masonry cement is usually an interground cement of 50% Type
I portland cement and 50% limestone. The replacement of 50% portland cement
with limestone powder decreases the initial pH of the system. On the other hand,
limestone has a significantly high acid-neutralization capacity. A comparison
between Type I portland cement and masonry cement indicated, in the early leaching
stages, masonry cement-solidified waste forms showed lower pH and lower metal
leachability. In a more acidic solution, portland cement outperforms the masonry
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