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94 Corrosion Control Through Organic Coatings encapsulate the metal ions in silicate- or metal-silicate gel matrices. Either way, the metals are removed from solution before they precipitate on the cement grains. Compared with lead, cadmium and chromium have negligible effects on the hardening properties of portland cement [26, 27]. 5.4.2 ATTEMPTS TO STABILIZE BLASTING DEBRIS WITH CEMENT The University of Texas at Austin has done a large amount of research on treatment of spent abrasive media by portland cement. Garner [28] and Braband [29] have studied the effects of concrete mix ingredients, including spent abrasives and counteracting additives, on the mechanical and leaching properties (TCLP) of the resulting concrete. They concluded that it is possible to obtain concrete using spent abrasive with adequate compressive strength, permeability resistance, and leaching resistance. Some of their findings are summarized here: • The most important factors governing leaching, compressive strength, and permeability were the water/cement ratio and the cement content. In general, as the water/cement ratio decreased and the cement content increased, leaching decreased and compressive strength increased. • As the contamination level of a mix increased, compressive strength decreased. (It should be noted that this is not in agreement with Shively’s [24] results [see section above].) • Mixes with lower permeability also had lower TCLP leaching concentrations. • Mixing sequence and time were important for the success of the concrete. Best performance was obtained by thoroughly mixing the dry components prior to adding the liquid components. It was necessary to mix the mortar for a longer period than required for ordinary concrete to ensure adequate homogenization of the waste throughout the mix. • Set times and strength development became highly unpredictable as the contamination level of the spent abrasives increased. • Contamination level of the spent abrasives was variable. Possible factors include the condition and type of paint to be removed, the type of abrasive, and the type of blasting process. These factors contribute to the particle size of the pulverized paint and its concentration in the spent blasting abrasives. • No relationship was found between the leaching of the individual metals and the concrete mix ingredients. Salt and colleagues [4] have investigated using accelerating additives to counteract the effects of lead and other heavy metals in the spent abrasive on the set, strength, and leaching of mortars made with portland cement and used abrasive debris. Some of their findings are summarized here: • Sodium silicate was most effective in reducing the set time of portland cement mixed with highly contaminated debris, followed by silica fume and calcium chloride. Calcium nitrite was ineffective at reducing the set time for highly contaminated wastes. © 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
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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