Civil Engineering Reference
In-Depth Information
1993; Miller and Klemm, 1997). Experiments performed on pure calcium silicate
hydrates indicate that such phases formed at ambient temperature in the presence of
heavy metal ions are poorly crystallized, and their degree of condensation is increased.
Zn, Pd, and Cd ions are immobilized by C-S-H very effectively, but Cr
6+
is not (Nocun-
Wczelik and Malolepszy, 1997). Some ions may occupy sites in the structure of the C-S-
H phase: for example, it has been reported that Cr
3+
may replace Si
4+
in this phase
(Miller and Klemm, 1997).
A particularly difficult problem is posed by the confinement of cesium, which in the
form of a long-half-life isotope is a common constituent of radioactive waste. In the
system C-S-A a matrix composition low in Al
2
O
3
and nearly equimolar in its SiO
2
and
CaO contents gives the best Cs retention (Bagosi and Cseteny, 1998).
The AFt phase present in hydrated Portland cement pastes possesses a range of
structural sites that can be occupied by both cations and anions. Ions that can be
incorporated into the structure of this phase include borate, chromate, selenate, and lead
(Kumaratsan
et al.,
1990; Glasser, 1992; McCarthy
et al.,
1992; Miller and Klemm,
1997).
The leaching of heavy metals from a hardened Portland cement paste with water or
acids also depends greatly on the existing porosity, and thus low leaching rates can be
obtained only at very low water/cement ratios (van Eijk and Bronwers, 1998). The rate of
leaching is also associated with calcium hydroxide removal from the hardened paste. A
significant leaching of the heavy metals gets under way only after a substantial fraction of
this paste constituent has been dissolved and the resultant increased porosity facilitates
the transport of species by diffusion through the leached shell (van Eijk and Bronwers,
1998).
The way in which organic compounds are bound within the hardened Portland cement
paste, and their effect on hydration and the structure of the hydrates formed, may vary
greatly. Compounds that are insoluble in water usually have no effect, whereas those that
are at least partially soluble may or may not slow down the rate of hydration, alter the
structure of the hydrated paste, and decrease its strength. Compounds that exhibit an
adverse effect on Portland cement hydration include various phenols, chlorophenols, and
ethylene glycol, whereas methanol or ethanol have almost no effect (Grutzek, 1992).
It has been reported that the immobilization of waste in hardened Portland cement may
be improved by adding sodium silicate to the system, as this additive accelerates the
hydration reaction and lowers the amount of portlandite in the hydrated material (Scheetz
and Hoffer, 1995).
In addition to plain Portland cement a variety of
blended cements
have also been used
for waste disposal applications. In addition to Portland clinker or cement they may also
contain granulated blast furnace slag, fly ash, natural pozzolanas, microsilica, or clays. It
has been reported that the addition of fly ash or silica fume reduces the leachability of
heavy metals, mainly by reducing the free calcium hydroxide content of the hardened
paste (van Eijk and Bronwers, 1998). The presence of slag in the cementitious system
reduces the leachability of technetium and chromium (Roy, 1992). The leachability of
both cesium and strontium may be reduced by producing zeolitic phases in systems
containing slag and fly ash, through reactions taking place at elevated temperatures (Roy,
1992). It is also possible to reduce the leachability of Cs by combining Portland cement
