Durability of cement-based materials in simulated radioactive liquid waste: Effect of phosphate, sulphate, and chloride
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Durability of cement-based materials in simulated radioactive liquid waste: Effect of phosphate, sulphate, and chloride ions A. Guerrero, S. H´ernandez, and S. Go˜ni Institute of Construction Science Eduardo Torroja (CSIC), C/Serrano Galvache, s/n 28033 Madrid, Spain (Received 17 October 1996; accepted 24 September 1997)
The durability of a specific backfilling pozzolanic cement mortar, which is employed in Spain, in concrete containers for the storage of low (LLW) and medium level wastes (MLW), has been studied by means of the K¨och–Steinegger test at the temperature of 40 ±C during a period of 365 days. Mortar samples were immersed in a simulated radioactive liquid waste very rich in sulphate (0.68 M), phosphate (0.89 M), and chloride (0.51 M) ions. The changes of the microstructure were followed by x-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Pore solution was extracted at different periods in order to see the changes of the chemical composition caused by the diffusion of those ions inside the microstructure.
I. INTRODUCTION
Studies on the durability of matrices made with cement and liquid radioactive wastes are numerous, especially those concerning the leaching attack and diffusion of radioactive ions toward the biosphere.1–17 However, little attention has been paid to the implications of the high levels of dissolved salts that accompany the radioactive liquid wastes for the durability of cement materials and concrete barriers employed in its storage.18 Such concentrated solutions contain a great amount of sulphate, phosphate, borate, chloride, etc. The aggressiveness of sulphate to cement-based materials is well established; nevertheless, the degree of the attack depends to a great extent on the other accompanying ions. For example, the aggressiveness of sulphate is lower in sea water, although its concentration is still sufficient to cause severe damage to concrete. The mechanism of sulphate attack is also different if the ions come from the outside the concrete or if are introduced during mixing. In the first case, topochemical formation of ettringite and gypsum with directional crystal growth and swelling of ettringite by water absorption are two hypotheses that have been proposed, among others, to explain expansion and cracking.19 In the second case, and depending on the alkalinity of the pore solution and temperature, problems related to delayed ettringite formation can occur.20 A very recent paper of Li et al.18 reported the formation of a new mineral called “U” phase (a sodium substituted AFm phase, very aggressive), which occurred only in the simulated cemented low level wastes containing high amounts of Na2 SO4 (10–15%). The authors pointed out that this phase can form only at very high alkaline concentration, not compatible with traditional cement paste. This phase transforms into ettringite when J. Mater. Res., Vol. 13, No. 8, Aug 1998
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