Hydraulic Cement Systems for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 69

a very porous and unstable structure. However, the main hydration products of alkali-
activated slag cement are C-S-H (B) and tobermorite or xonotlite, which give the
hardened pastes a much less porous and stable structure. 100 They also have an obvious
cation ion exchange capacity and enhance the chemical fixation of contaminants. 101-104
Alkali-activated calcined metakaoline/fly ash cements have also been proven as
effective binders for S/S of wastes. 105-111 They are sometimes called geopolymer or
hydroceramics. Some details can also be found in Chapter 6.

4.6 CEMENT KILN DUSTS

Cement plants generate cement kiln dust (CKD) as a means of removing alkalis,
chlorides, and sulfates from the kiln system. There are two types of cement kiln
processes: wet-process kilns, which accept feed materials in a slurry form; and dry-
process kilns, which accept feed materials in a dry, ground form. In each type of
process the dust can be collected in two ways: (1) a portion of the dust can be separated
and returned to the kiln from the dust collection system (e.g., cyclone) closest to the
kiln, or (2) the total quantity of dust produced can be recycled or discarded.
CKD is composed primarily of fine ground particles of limestone, clay, or shale,
lime, sodium, potassium chlorides, sulfates, metal oxides, calcium silicates, and
other salts. It is enriched in sodium and potassium chlorides and sulfates, as well
as volatile metal compounds. The lime content can be up to 15%. Trace constituents
in CKD (including certain trace metals such as cadmium, lead, and selenium, and
radionuclides) are generally found in concentrations less than 0.05% by mass.
Because some of these constituents are potentially toxic at low concentrations, it is
important to assess their levels (and mobility or leachability) in CKD before con-
sidering their use. CKD is a fine powdery material similar in appearance to portland
cement.
Approximately 12.9 million metric tons of CKD are produced annually. About
64% of the total CKD generated (or about 8.3 million metric tons) is reused within
the cement plant. Approximately 6% of the total CKD generated is utilized off-site.
The chemical and physical characteristics of CKD that is collected for use outside
of the cement production facility will depend in great part on the method of dust
collection employed at the facility. The most common beneficial use of CKD is its
use as a stabilizing agent for wastes, where its absorptive capacity and alkaline
properties can reduce the moisture content, increase the bearing capacity, and provide
an alkaline environment for waste materials. Chemical Waste Management con-
ducted a great deal of work on CKD-based S/S process and made a specification as
summarized in Table 4.4. 1 There is also a patented process using CKD and acid to
produce a neutral, environmentally stable and hardened solid product. 112
Calcium silicates in CKD can act as binders. Reactive silica from original or
heated clay in a CKD can react with lime to form a weak binder directly. Thus, a
CKD with high lime and calcium silicate contents is the most desirable for S/S
purposes.
In August 1999, the U.S. Environmental Protection Agency proposed new regu-
lations for management of CKD, which was designated as “high-volume, low-toxicity”
special wastes requiring individualized treatment under the Resource Conservation