Alkali-Activated Fly Ash-Slag Cement Based Nuclear Waste Forms
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ALKALI-ACTIVATED FLY ASH-SLAG CEMENT BASED NUCLEAR WASTE FORMS
W. JIANG, X. WU AND D.M. ROY Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
ABSTRACT This paper is based on the results of an in-progress research project on Alkali-Activated Cement System at MRL. The objective of this research is to establish the potential for large volume use of fly ash and slag as main components of the cement system. Alkali-activated Fly ash-slag Cement (AFC) was studied as a matrix for immobilization of nuclear waste. AFC is characterized by high early strength, high ultimate strength, low porosity, lower solubilities of the hydrates, and high resistance to chemical corrosion as well as to freezing and thawing. All these advanced properties are particularly favorable to the immobilization the nuclear wastes. INTRODUCTION Nuclear waste pollution of the environment is one of the most important concerns of our time in an international scope. Immobilization of radioactive and hazardous industrial waste in cement-based materials is an economically attractive and widely accepted waste management methods. 1' 2 Since unused industrial by-products as fly ash and slag also may cause land, air, and water pollution, the utilization of those by-products, to save considerable energy and costs and at same time to solve many ecological problems. 3 Therefore large volume use of fly ash and slag in nuclear waste immobilization provides a double benefit. In addition, the limitation of fly ash and slag-based waste forms can be modified by reasonable changes in the relative proportions of the mixure ingredients and/or by additives. BACKGROUND The latent hydraulic properties of fly ash and slag are influenced by the chemical and phase composition, the surface area and by the type of activation (chemical, thermal, and mechanical). 4 They require 'activation' to initiate hydration, and the availability of a mechanism for continuing the hydration process. In practice the activation depends on two methods (1) alkali (hydroxide) and (2) hydrothermal. Alkali Activation During alkaline activation fly ash and slag are affected by a high concentration of OH- in the mix. The siliceous fly ash and slag are partly activated due to a high concentration of OH" ions, leaching and partly dissolving the fly ash and slag particles, which are taken into solution as hydroxocomplexes. These are mostly condensed silicate and aluminate anions.The low concentration of Ca 2+ ions in solution owing to the lower solubility of calcium hydroxide in a highly alkaline medium allows the formation of low-calcium silicates. These hydrate complexes are mostly condensed aluminates or aluminosilicates. The utilization of alkali activated slag in nuclear waste solidification has been investigated. 5. 6. 7.s, 9 Hydrothermal Activation Hydrothermal processing involves the chemistry of hot water under pressure to carry out dissolution, leaching, and precipitation reactions. The hydrothermal processing of fly ash is based on sol-gel reaction a
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