Uptake of Metal Ions by Autoclaved Cement Pastes
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UPTAKE OF METAL IONS BY AUTOCLAVED CEMENT PASTES Caijun Shi', Robert L. Day', Xuequan Wu2 and Mingshu Tang2 'Department of Civil Engineering, The University of Calgary, Calgary, Alberta T2N 1N4, Canada 2 Department of Silicate Engineering, Nanjing Institute of Chemical Technology, Nanjing, Jiangsu 210009, P. R. China ABSTRACT This paper deals with the hydration products and pore structure of Portland cement (PC) and alkali-phosphorus slag cement (APSC) pastes, and the uptake of metal ions (Sr"÷, Co2 ÷, Cd2", and Cu 2") in pastes which have been hydrated at 150°C for 25 days. Results indicate that the hydration products of PC are crystalline Ca(OH)2 and C2SH(A); for APSC the products are poorly crystallized low-basic CSH(B) and crystalline tobermorite. The PC pastes have a lower total porosity than APSC pastes, but the PC pastes consist mainly of larger pores with r> IOOOA, while the APSC pastes consist mainly of smaller pores with r< 100A. The two types of paste were immersed for 19 days at room temperature in Sr 2", Co 2", Cd2` and Cu2 + solutions with concentrations of 100, 200, 500 and 1000 ppm. It was found that all Ca(OH), in the PC pastes was dissolved. Except for Sr"2, the uptake of metal ions by PC paste is mainly due to the formation of insoluble hydroxides. The uptake of metal ions by APSC pastes is due to physical adsorption and the formation of some insoluble compounds. The APSC pastes show a stable structure after immersion in these metal ion solutions. INTRODUCTION The solidification of radioactive nuclear wastes with cements is a simple technology that uses readily available raw materials [1]. The mechanism by which wastes are contained include: (1) the wastes are mechanically encapsulated by the cementitious material; (2) some components of the wastes are physically adsorbed onto the surfaces of the hydration products of the cement; (3) some waste components are integrated into the hydration products. Komarneni et al [2] found that all the poorly crystallized hydrated calcium silicates produced during the hydration of Portland cement below 100*C have ion-adsorption and ion-exchange properties. Crystalline tobermorite and xonotlite formed at high temperature have good ion-exchange capabilities, somewhere between zeolites and clays [3-7]; such ionexchange properties may be beneficial for the solidification of radioactive wastes. The main hydration products of alkali-slag cements, hydrated at room temperature, are poorly crystallized low basic CSH(B) [8-11]. Compared to PC pastes, these pastes have a denser and more stable structure [12-151. Alkali-slag cement pastes should be more effective than PC pastes when they are used to solidify radioactive nuclear wastes. The disposal of radioactive nuclear wastes is a three-step process: (1) the wastes are first solidified using some form of cement; (2) the solidified material is encapsulated by steel containers; (3) the containers are embedded underground to depths of several hundreds to thousands of meters [16]. The cement must produce a solidified material
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