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92 Corrosion Control Through Organic Coatings stabilized. The effect, nonsoluble lead, is extremely temporary; after a short time, it leaches precisely as if no treatment had been done [13]. 5.3.3 STABILIZATION OF LEAD WITH CALCIUM SILICATE AND OTHER ADDITIVES 5.3.3.1 Calcium Silicate Bhatty [14] has stabilized solutions containing salts of cadmium, chromium, lead, mercury, and zinc with tricalcium silicate. Bhatty proposes that, in water, tricalcium silicate becomes calcium silicate hydrate, which can incorporate in its structure metallic ions of cadmium and other heavy metals. Komarneni and colleagues [15–17] have suggested that calcium silicates exchange Ca 2+ in the silicate structure for Pb 2+ . Their studies have shown that at least 99% percent of the lead disappears from a solution as a lead-silicate-complex precipitate. Hock and colleagues [13] have suggested a more complex mechanism to explain why cement stabilizes lead: the formation of lead carbonates. When cement is added to water, the carbonates are soluble. Meanwhile, the lead ions become soluble because lead hydroxides and lead oxides dissociate. These lead ions react with the carbonates in the solution and precipitate as lead carbonates, which have limited solubility. Over time, the environment in the concrete changes; the lead carbonates dissolve, and lead ions react with silicate to form an insoluble, complex lead silicate. The authors point out that no concrete evidence supports this mechanism; however, it agrees with lead stabilization data in the literature. 5.3.3.2 Sulfides Another stabilization technique involves adding reactive sulfides to the debris. Sulfides — for example, sodium sulfide — react with the metals in the debris to form metal sulfides, which have a low solubility (much lower, for example, than metal hydroxides). Lead, for example, has a solubility of 20 mg/liter as a hydroxide, but only 6 × 10 −9 mg/liter as a sulfide [18]. If the solubility of the metal is reduced, the leaching potential is then also reduced. Robinson [19] has studied sulfide precipitation and hydroxide precipitation of heavy metals, including lead, chromium, and cadmium; he saw less leaching among the sulfides, which also had lower solubility. Robinson also reported that certain sulfide processes could stabilize hexavalent chromium without reducing it to trivalent chromium (but does not call it sulfide precipitation and does not describe the mechanism). Others in the field have not reported this. Means and colleagues [20] have also studied stabilization of lead and copper in blasting debris with sulfide agents and seen that they could effectively stabilize lead. They make an important point: that mechanical–chemical form of a pulverized paint affects the stabilization. The sulfide agent is required to penetrate the polymer around the metal before it can react with and chemically stabilize the metal. In their research, Means and colleagues used a long mixing time in order to obtain the maximum stabilization effect. © 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
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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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