Regarding hazards of using flyash in concrete:
"Findings to date indicate that little, if any,
fly ash exhibits characteristics defined as hazardous in the Federal
regulations. Therefore, Subtitle C regulations will have no
significant impact of the use of fly ash in cement and concrete.
A few commenters suggested that EPA limit the use of
fly ash in concrete, restricting its use in potable water sources or
in storage areas for food. The rationale given for these suggestions
was the potential for leaching of trace metal elements out of the fly
ash. The commenters provided no documentation as to the likelihood or
extent of leaching when fly ash is used in concrete.
While it is true that fly ash contains trace amounts
of certain elements, which can be toxic in larger concentrations, it
is unlikely that fly ash as used in concrete would exhibit leaching
characteristics. First, the permeability of concrete containing fly
ash is negligible compared to the permeability of fly ash as typically
disposed. This reduced permeability prevents water or other liquids
from penetrating concrete and providing a leaching medium through
which contaminants could travel.
Second, when used in concrete, fly ash becomes an
integral part of the final product. The surface area of individual fly
ash particles, from which leaching of trace constituents takes place,
is so greatly reduced in this application as to be almost nonexistent.
It is not possible through conducting leaching tests or raw fly ash to
estimate the leaching, if any, which would take place in a concrete
containing fly ash. Thus, the commenters suggestion that dams and
pipes not be constructed using fly ash appears to have no technical
basis.
(c) Radioactivity Issues. At the same time as
original proposal of the hazardous waste regulations (December 18,
1978), EPA issued an advance notice of proposed rulemaking that it was
considering establishing 5 Pico curies per gram [pCi/g] of radium-226
as a criterion for listing wastes as hazardous. The notice also
requested comment on other criteria which might tend to affect the
radiation hazard. Among these is the emanation rate of radon from the
waste. The agency has, at this date, taken no further action on this
proposed rulemaking to establish general criteria for hazardous
radioactivity levels in wastes.
Where resource recovery is practiced, an important
consideration in assessing the hazard is the proposed use of the waste
material. While some proportion of fly ash generated in the U.S. has
more than 5 pCi/g of radium-226, the physical structure of fly ash is
such that its contribution to radiation exposure is probably less than
that of most normal constituents of concrete which generally fall
below this level. This is explained below.
A few commenters expressed concern to EPA that fly
ash used in the construction of habitable structures could pose a
threat to public health due to radioactivity. The source of the
radiation threat is due to radium-226, a radioactive isotope which
occurs naturally in soil, sand, and mineral deposits as well as in fly
ash. The radium-226 content of soil generally ranges from .2 to 3 pCi/g.
Limited measurements of radioactivity in cement show that the
redium-226 content of cement can be as high as 5 pCi/g, but typically
averages close to 1 pCi/g. Limited measurements of fly ash presently
generated in the U.S. show a radium-226 content ranging from 1 to 8
pCi/g with an average of roughly 4 /Ci/g.
There are two pathways of radiation exposure from
radium-226 in building materials. The pathway of primary concern is
from inhalation of radon-222 and its short-lived decay products.
Radon-222, an inert gas with a radioactive half-life of 3.8 days, is
the first generation decay product of radium-226. Because it is an
inert gas, it can readily migrate from the building material into the
indoor air of a home. Although the rate at which radon is created
within a building material is proportional to its radium content, the
intrinsic structure of the material may, in some cases, prevent most
of the radon from escaping. When air containing radon and its
radioactive decay products is breathed for long periods of time, a
person's risk of lung cancer is increased.
Gamma radiation from radium-226 and its decay
products is the other exposure pathway. The amount of gamma radiation
emission from a building material is proportional to its radium
content, but the total exposure a person receives will also depend on
other factors such as shielding, distance from the material, and
exposure time. Exposure to gamma radiation results in an increased
risk of many types of cancer.
When fly ash is used as a partial cement replacement
in concrete, the fly ash content of the final concrete product is
between 2 and 3 percent (assuming a 15-25 percent cement replacement
rate and an 8 to 1 ratio of aggregate and water to cementitious
material). Since the average radium-226 content of fly ash exceeds
that of cement by a few pCi/g, the use of fly ash as a cement
replacement in habitable structures will, on the average, result in a
slight increase in the gamma radiation exposure to people (less than a
mill roentgen per year). However, in some instances, where fly ash
with a lower than average radium content replaces a cement with a
higher than average radium content, the result would be less gamma
radiation exposure.
The use of fly ash as a cement replacement will also
affect the quantity of radon emitted by the building material.
Although the rate at which radon is created is directly proportional
to the radium content, other factors may inhibit radon emanation from
a material. Because fly ash is produced at high temperatures, it has a
glassy structure which keeps most of the radon from escaping. The
fraction of radon which escapes from fly ash (emanation fraction) has
been measured at no more than a few percent. In contrast, typical soil
and soil like materials tend to have an emanation fraction in the
neighborhood of 20 percent. Thus, although fly ash on the average, has
a greater radium content than the cement it replaces, the use of fly
ash as a partial cement replacement is likely to reduce the radon gas
contribution of the final concrete product.
During the proposal period for this guideline EPA
has been investigating this issue more thoroughly. Tests recently
conducted for EPA substantiate the conclusions above, i.e., that the
radon emanation rate of fly ash in its raw state and as used in
concrete is only a few percent compared to the absolute radium
concentration. Thus, while fly ash use in cement would, on the
average, result in a small increase in gamma radiation exposure, this
small increase in gamma exposure is likely to be offset by a decreased
radon exposure. In light of this, EPA believes that the use of
typically-occurring fly ash in concrete does not constitute a
significantly different radiation risk, than the risk from the cement
it replaces, and neither of these is significantly different from the
radiation risk posed by common soil."
