Environmental Engineering Reference
In-Depth Information
these investigators do not realize that the properties of liquid silicates vary signifi-
cantly with concentration, temperature, Na
2
O/SiO
2
ratio, and storage time. Very
limited information can be found on how the Na
2
O/SiO
2
ratio and concentration of
sodium silicate affect the properties of stabilized waste forms.
Butler et al.
46
used Type I portland cement and Type N sodium silicate solution
to solidify Cd, Pb, and phenols. They used a combination of conduction calorimetry,
XRD, and solid-state NMR to obtain information about immobilization mechanisms
of heavy metals and organics in cement matrices. The results from conduction calo-
rimetry showed a greater rate of heat evolution for cement/sodium silicate compared
to blank (control) cement samples, although the total heat output was greater for the
blank cement. Following the course of silicate transformation beginning with the
orthosilicates (SiO
4
4-
) through tenninal silicates (SiO
3
), internal silicates (SiO
2
), and
branching silicates (SiO) with the aid of NMR showed an excellent correlation with
the calorimetry results. Fourier transform infrared spectroscopy (FTIR) and thermo-
gravimetry analysis (TGA) methods have also been used to investigate the solidifica-
tion of Cr, Pb, Ba, Hg, Cd, and Zn in portland cement. The analyses have been found
to support the leaching and mechanical property test results.
Caldwell et al.
46
attempted the solidification of a largely inorganic residue con-
taining trace amounts of organic compounds using several solidification processes:
cement, cement-fly ash, cement-activated carbon, cement-bentonite, and cement-
soluble silicates. Twenty organic compounds at three contaminant levels (10, 100,
and 1000 mg/g of solidified waste) were studied. The compounds were chosen to
represent a wide array of organics: aliphatic/aromatic, volatile/nonvolatile, monocy-
clic/polycyclic, halogenated/nonhalogenated compounds, pesticides, and PCBs.
Results based on a distilled water extraction of the solidified wastes indicated that
the performance of the solidified forms was highly dependent on the compound in
question. Nonvolatile and insoluble organic compounds were contained better than
soluble/nonvolatile contaminants. For the processes studied, the cement-soluble sil-
icates system showed better performance than cement, cement-fly ash, cement-
activated carbon, and cement-bentonite systems.
A site in Florida, which belonged to General Electric Co. and was contaminated
with PCBs (up to 950 mg/kg soil), volatile organic compounds (up to 1500 mg total
VQC/kg soil), chromium (up to 400 mg/kg), copper (up to 900 mg/kg), lead (up to
2500 mg/kg), and zinc (up to 1000 mg/kg), was solidified using a proprietary commer-
cial mix with sodium silicate as an additive in April 1988. Microstructural analyses of
the treated waste showed that the process produced a dense homogeneous product with
low porosity. The treated soil had a compressive strength of up to 5 MPa and perme-
ability up to 21 × 10
-7
cm/s. Results from TCLP tests showed that PCBs were success-
fully immobilized. The viral quality control (VQC) concentrations in the leachates
ranged between 320 and 605 mg/L. Metal concentrations ranged between 120 and 210
mg/L. The detected concentrations were below regulatory levels for all the contaminants.
4.2.5
P
OLYMER
-M
ODIFIED
P
ORTLAND
C
EMENT
A replacement of 10 to 15% (by mass) of the cement binder by some synthetic
organic polymer can greatly improve the flexural strength and impermeability of
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