Coupling effect of NaOH and NaNO3 on the solidified fly ash-cement matrices containing Cs+: Reaction products, microstructure and leachability

Abstract The low- and intermediate-level radioactive waste liquid is a solution with high concentration of sodium nitrate (100–500 g/L). In order to immobilize the radioactive waste effectively, a certain content of sodium hydroxide (0.82–10%) would be added into the solidified fly ash-cement matrices as alkali activator. This study was aimed to investigate the coupling effect of sodium nitrate and sodium hydroxide on the reaction products, microstructure and leachability of solidified fly ash-cement matrices. The results indicated that a rather high content of sodium hydroxide (above 5%) was necessary for the crystallization of zeolite Na–P1 and sodium nitrate had participated in the formation of zeolite of chabazite and acted as a source of Na. Sodium hydroxide promoted the enhancement of pore structure and increased the compressive strength while sodium nitrate generated an opposite effect. The absorption performance for Cs+ was that: chabazite > zeolite Na–P1 > N-A-S-H gel. The cumulative leaching rate of Cs+ decreased with the increasing content of sodium hydroxide and the increasing concentration of sodium nitrate. However, on the contrary, an excessive amount of sodium nitrate (500 g/L) would accelerate the cumulative leaching rate of Cs+ because of the extreme degradation of the pore structure.