© 2006 by Taylor & Francis Group, LLC

Abrasive Blasting and Heavy-Metal Contamination 93
5.4 DEBRIS AS FILLER IN CONCRETE
Solidification of hazardous wastes in portland cement is an established practice [18];
it was first done in a nuclear waste field in the 1950s [4]. Portland cement has several
advantages:
• It is widely available, inexpensive, and of fairly consistent composition
everywhere.
• Its setting and hardening properties have been extensively studied.
• It is naturally alkaline, which is important because the toxic metals are
less soluble at higher pH levels.
• Leaching of waste in cement has been extensively studied.
Portland cement has one major disadvantage: some of the chemicals found in paint
debris have a negative effect on the set and strength development of the cement. Lead,
for example, retards the hydration of portland cement. Aluminum reacts with the
cement to produce hydrogen gas, which lowers the strength and increases permeability
of the cement [4]. Some interesting work has been done, however, in adding chemicals
to the cement to counteract the effects of lead and other toxic metals.
The composition of portland cement implies that, in addition to solidification,
stabilization of at least some toxic metals is taking place.
5.4.1 PROBLEMS THAT CONTAMINATED DEBRIS POSE FOR CONCRETE
Hydration is the reaction of portland cement with water. The most important hydration
reactions are those of the calcium silicates, which react with water to form
calcium silicate hydrate and calcium hydroxide. Calcium silicate hydrate forms a
layer on each cement grain. The amount of water present controls the porosity of
the concrete: less water results in a denser, stronger matrix, which in turn leads to
lower permeability and higher durability and strength [21].
Lead compounds slow the rate of hydration of portland cement; as little as 0.1%
(w/w) lead oxide can delay the setting of cement [22]. Thomas and colleagues [23] have
proposed that lead hydroxide precipitates very rapidly onto the cement grains, forming
a gelatinous coating. This acts as a diffusion barrier to water, slowing — but not stopping
— the rate at which it contacts the cement grains. This model is in agreement with
Lieber’s observations that the lead does not affect the final compressive strength of the
concrete, merely the setting time [22]. Shively and colleagues [24] observed that the
addition of wastes containing arsenic, cadmium, chromium, and lead had a delay before
setting when mixed with portland cement, but the wastes’ presence had no effect on final
compressive strength of the mortar. Leaching of the toxic metals from the cement was
greatly reduced compared with leaching from the original (untreated) waste. The same
results using cadmium, chromium, and lead were seen by Bishop [25], who proposed
that cadmium is adsorbed onto the pore walls of the cement matrix, whereas lead and
chromium become insoluble silicates bound into the matrix itself. Many researchers have
found that additives, such as sodium silicate, avoid the delayed-set problem; sodium
silicate is believed to either form low-solubility metal oxide/silicates or possibly
© 2006 by Taylor & Francis Group, LLC