Hydraulic Cement Systems for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 63

encapsulate the heavy metals present in the chromium filter cake into a calcium-
alumino-silicate matrix, thereby rendering them essentially immobile. 59 The product
of this reaction is identified as a chemically stabilized filter cake (CSFC). Several
leaching tests on the treated waste indicated the concentrations of Cr in leachates
are well below the corresponding levels of regulatory concern. Based on the test
results, it can be concluded that the chromium CSFC did not meet the criteria for
which EPA hazardous waste code F006 was listed in 40 CFR Part 261.32, nor did
it meet any other hazardous criteria/characteristics that might cause the waste to be
considered as hazardous. In addition, the materials used in the treatment process are
not expected to introduce any additional constituents of concern. The manufacturer
submitted a delisting petition for excluding the CSFC from the list of hazardous
wastes.
Debroy and Dara 60 evaluated the immobilization of zinc and lead present in
waste sludges by chemical fixation and encapsulation methods using lime-pozzolan
and fiber-reinforced lime-pozzolan admixtures. Fixation and coating with sodium
silicate solution produced good results for immobilizing lead and zinc ions, but
encapsulation of heavy metal hydroxide sludge in simple and fiber-reinforced
lime-fly ash mixtures was more efficient. Fiber additives to the admixture may
increase the structural strength of the monolith. Coating of an encapsulated admix-
ture may provide an additional immobilization barrier.
Mining tailings are waste materials left over after mineral ores have been milled
and the valuable minerals extracted. Lime is often mixed with mining tailings for
backfill, or with acid-generating tailings to increase the pH to a level where the
solubility of most metals is at its lowest. 61,62 Pozzolanic reactions happen between
lime and the minerals in the tailings, whose rate and products are dependent on the
size and the nature of the minerals in the tailings.
Lime is often used to treat contaminated soils alone. Of course, lime provides
an alkaline environment for precipitation of heavy metals. The addition of lime to
a fine-grained soil initiates several reactions, which can be cataloged as physical
adsorption, ion exchange, and pozzolanic reaction. Physical adsorption and ion
exchange take place rapidly and produce immediate changes in soil plasticity, work-
ability, and the immediate uncured strength and load-deformation properties. 20
In most cases, the silica and aluminum in soils are reactive, and a soil-lime
pozzolanic reaction may happen. The pozzolanic reaction results in the formation
of various cementing compounds, which improve the strength and durability of the
mixture. Since the pozzolanic reactions are very slow and time dependent, the
strength development is gradual but continuous for long periods of time amounting
to several years in some instances. An increase in temperature accelerates the poz-
zolanic reaction between lime and soil, and the strength development, significantly.
Laboratory results have confirmed that heavy metals, such as lead and chromium,
were included and immobilized by the soil-lime pozzolanic reaction products. 63
Thus, lime treatment of contaminated soil, to some extent, belongs to the use of
lime-pozzolan cements.