Research & Development

PCA RP348 RESEARCH & DEVELOPMENT Stabilization of Heavy Metals in Portland Cement, Silica Fume/Portland Cement and Masonry Cement Matrices by Javed I. Bhatty, F. MacGregor Miller, Presbury B. West, and Börje W. Öst RP348 99 ABSTRACT The effects of heavy metals on the physical and chemical properties of portland-cement based pastes were studied using different types of cement, four metal oxides, and four soluble metal salts. Type I (high calcium aluminate content) and Type V (low calcium aluminate content) portland cements were used to study the effects of their chemical differences on paste properties and metal stabilization. Fresh pastes were tested for workability, initial setting times, and heats of hydration. Hardened pastes were tested for strength and leachability by both TCLP (Toxic Characteristic Leaching Procedure) and column leaching with acetic acid. The cement matrix was an excellent stabilization matrix, better than could be projected from pH consid- erations alone. The investigation also involved examining Type I portland cement paste treated with three metals to- gether—chromium, cadmium and lead, added at very high (1% by mass of cement) and intermediate (1000 ppm by mass of cement) levels. The leaching solutions were acetic acid, “synthetic acid rain” (pH 3 sulfu- ric/nitric acid), and deionized water. A Type N masonry cement matrix was also investigated. The leachabilities were very low throughout the pH range of 6-11 for lead and chromium. At very high pH values, the leachabilities of lead and chromium were significantly higher. Cadmium leachability was neg- ligible above pH 10, but became rapid and highly significant below pH 9. At pH values below 6, which generally corresponded to the extraction of nearly all calcium present, all metals were rendered highly leachable. For all the metals, the leachability in masonry cement-stabilized matrix was higher than with Type I during the intermediate leaching steps. A series of tests involved testing of a contaminated soil matrix, both dry and contaminated with aged SAE 10 W oil (without additives). Cement was used at 8% by weight of total mix. The results of leachability vs pH generally were consistent with those obtained in the multiple metals leaching studies; as long as calcium silicate hydrate was present, metals solubility was very low, irrespective of pH. In an attempt to address the early high pH leaching of lead and chromium, a series of experiments was run with silica fume, in an amount calculated to react with all calcium hydroxide generated from hydration of the calcium silicates. The reasoning was based on the fact that if calcium hydroxide was all used up, it could not displace the larger heavy metal cations from either calcium silicate hydrate or sulfoaluminate or sulfoferrite hydrates. The results indicated that leachabilities of lead and chromium were reduced at pH values greater than 11.0, and also at the low pH values obtained after multiple sequential batch leaches. Since ettringite is known to be much less soluble than calcium monosulfoaluminate hydrate, it was also thought that a system in which extra sulfate is present might yield even lower leachabilities for those metals which were substituted in the ettringite lattice. (This reasoning may not apply to anionically substituted metals, because the extra sulfate may tend to displace them from the ettringite lattice). The results indicated that at very high pH values, the leachability of chromium was reduced with the lower excess gypsum level, and at low pH values, less than about 5.0, with both high gypsum levels. At intermediate pH values, chromium leachability was minimum for the unaltered matrix. Excess sulfate reduced lead leachability also at the low and high pH levels, but increased it at intermediate pH values. Arsenic is a metalloid whose stabilization has presented difficulties in solidification/stabilization sys- tems because of solubility. Because the stabilization of arsenic (III) is more difficult than that of arsenic (V), an oxidizing agent is often added to the system to ensure that arsenic is not reduced to the trivalent state. Three separate regimens were evaluated: Ferrous sulfate was used to produce a very insoluble iron arsen- ate precipitate; this system was evaluated for arsenic (III), arsenic (V), and arsenic (III) which had been treated with hydrogen peroxide to oxidize it to As(V). The neat soil cement matrix and one in which ethyl- enediamine tetraacetic acid had been added as a chelating agent were also tested. Whereas arsenic stabilization was not as effective as that for cadmium, chromium and lead, the optimum system functioned admirably to reduce As leachability. The intermediate pH levels around 9.5 proved the most difficult for As stabilization, unlike the other metals, which were generally least soluble in this pH range. KEYWORDS solidification, stabilization, hazardous waste, trace metals, chromium, cadmium, lead, arsenic, portland cement, contaminated soils, leaching, TCLP PCA R&D Serial No. 2067 Stabilization of Heavy Metals in Portland Cement, Silica Fume/Portland Cement and Masonry Cement Matrices by Javed I. Bhatty, F. MacGregor Miller, Presbury B. West, and Börje W. Öst ISBN 0-89312-149-5 * Senior Scientist, Senior Principal Scientist, Senior Scientist, and Principal Scientist, respectively, Construction Technol- ogy Laboratories, Inc., 5420 Old Orchard Road, Skokie, © Portland Cement Association 1999 Illinois 60077 Stabilization of Heavy Metals in Portland Cement, Silica Fume/Portland Cement and Masonry Cement Matrices 102ii PCA RP348 Contents CONTENTS .......................................................................................................................................... iii EXECUTIVE SUMMARY ...................................................................................................................... 1 INTRODUCTION................................................................................................................................... 3 Solidification/Stabilization of Hazardous Wastes ............................................................................ 3 S/S Processes ................................................................................................................................ 4 CHAPTER 1 - Effects of Salts and Oxides on Paste Properties and Acetic Acid Leachability .......................................................................................................................... 7 MATERIALS AND METHODS............................................................................................................... 7 EXPERIMENTAL................................................................................................................................... 7 RESULTS AND OBSERVATIONS......................................................................................................... 9 Fresh Pastes .................................................................................................................................. 9 Hardened Cement Pastes ............................................................................................................ 12 Metals Interactions with Type I Cement ........................................................................................ 15 Discussion and Interpretation of Results ...................................................................................... 17 CONCLUSIONS .................................................................................................................................. 17 CHAPTER 2 - Leachability of Single Metals from Cement Pastes Using Sequential Batch Acetic Acid Leaching ................................................................................... 19 EXPERIMENTAL WORK.................................................................................................................... 19 Preparation of Paste Specimens ..................................................................................................19 Preparation of Deionized Water.................................................................................................... 20 Preparation of “Pseudo-TCLP” Leaching Solution ....................................................................... 20 RESULTS ............................................................................................................................................ 21 Physical Testing ............................................................................................................................ 21 DISCUSSION ...................................................................................................................................... 21 Physical Properties ....................................................................................................................... 21 CHAPTER 3 - Leachability of Multiple Metals from Paste Matrices Using Acetic Acid, Deionized Water, and "Synthetic Acid Rain" Leachants ............................................... 29 EXPERIMENTAL WORK..................................................................................................................... 29 Preparation of Deionized Water.................................................................................................... 29 Preparation of Synthetic Acid Rain ............................................................................................... 29 Preparation of Acetic Acid Leaching Solution ..............................................................................

Load more