© 2006 by Taylor & Francis Group, LLC

Abrasive Blasting and Heavy-Metal Contamination 95
• The combination of sodium silicate and silica fume provided higher compressive
strength and lower permeability than separate use of these compounds.
• The set time is proportional to the lead/portland cement ratio; decreasing
the ratio decreases the set time. The compressive strength of the mortar,
on the other hand, is inversely proportional to this ratio.
• For the most highly contaminated mixtures, accelerators were required to
achieve setting.
• All the mortars studied had TCLP leaching concentrations below EPA
limits. No correlation between the types and amounts of metals in the
wastes and the TCLP leaching results was found.
• The proper accelerator and amounts of accelerator necessary should be
determined for each batch of blasting debris (where the batch would be
all the debris from a repainting project, which could be thousands of tons
in the case of a large structure) by experimenting with small samples of
debris, accelerators, and portland cement.
Webster and colleagues [30] have investigated the long-term stabilization of
toxic metals in portland cement by using sequential acid extraction. They mixed
portland cement with blasting debris contaminated with lead, cadmium, and
chromium. The solid mortar was then ground up and subjected to the TCLP
leaching test. The solid left after filtering was then mixed with fresh acetic acid
and the TCLP test was repeated. This process was done sequentially until the pH
of the liquid after leaching and filtering was below 4. Their findings are summarized
here:
• The amount of lead leaching was strongly dependent on the pH of the
liquid after being mixed with the solid; lead with a pH below 8 began to
leach, and the amount of lead leaching rose dramatically with each sequential
drop in pH.
• Cadmium also began leaching when the pH of the liquid after being
mixed with the solid dropped below 8; the amount leaching reached
a maximum of 6 and then fell off as the pH continued to drop. This
could be an artificial maximum, however, because the amount of
cadmium was low to begin with; it could be that by the time the pH
had dropped to 5, almost all the cadmium in the sample had leached
out.
• The authors suggest that the ability of the calcium matrix to resist breakdown
(due to acidification) in the concrete is important for the stabilization
of lead and cadmium.
• Chromium began leaching with the first extractions (pH 12); the amount
leached was constant with each of the sequential extractions until the pH
dropped below 6. Because the amount of chromium in the debris was also
low, the authors suggest that chromium has no pH dependency for leaching;
instead it merely leaches until it is gone. This finding was supported
© 2006 by Taylor & Francis Group, LLC