Performance Research Of Weakly Alkalinity Alkali-Activated Slag Cement Immobilsing Nuclear Wastes

The energy sources is substantial basic on which human depend to survive and develop. With reduced oil, gas and coal production and the obligation for implementing the Kyoto Protocol, nuclear power is regarded as a kind of most new important potential source. Production of nuclear power must bring some nuclear wastes, especially a mass of high, intermediate and low level radioactive wastes come from nuclear power plants and other nuclear power use. More than 95% of the bulk of the wastes is intermediate and low level radioactive wastes (ILW and LLW). So, how to deal with nuclear wastes safely is one of the importent barriers for promoting nuclear power.Research and application indicate that it is economical and effective to immobilise nuclear wastes by using cementitious materials, which can be Porland cement or composite Porland cement. Using composite cement system to immobilise nuclear wastes is a primary manner now, which has several advantages of higher efficiency, lower heat of hydration and inexpensive price comparatively. Nonetheless, the high internal pH (typically above 13) will bring security hidden trouble, because it can cause the corrosion of metals such as aluminium and magnesium in ILW and LLW, excess generation of hydrogen and leading to expansion. Although replacement of OPC with up to 90% blast furnace slag (BFS) is used, the pH still remains about 12.5. So, it's very important to choose an immobility system have lower internal alkalinity.The hydration of cinker minerals is primary alkalinity source of Porland cement system and composite cement system, so it's an effectual technique measure to reduce alkalinity by avoiding cinker use. The alkali-activated slag cement (AASC) system being immobility concretion have been used in this paper, and reduce system's alkalinity by using neutral salt Na2SO4 and CaSO4 to activate ground granulated blastfurnace slag (GGBS). In this paper, the setting time and Water for standard consistency,compressive strength, alkalinity, free water content, Al corrosion and Cs+ leaching rate of neutral salt-slag-fly ash cementitious materials were studied, and microcosmic structure of this system was analysed. The result of testing shows:①The setting time of BFS/OPC is short (3～5.5h). Although this system could get to a higher early strength, evening strength increased slowly and the strength at 60 days is lower than 25.0MPa. The system's ability to immobilising Cs+ was weak, which at 28 days curing age, the Cs+ leaching rate was more than 60% all the same. On the other hand, because the alkalinity were high of this system at every age, which alkalinity of fresh paste was more than 13.0 especially, marked metal corrosion happened in system. So, this system didn't adapt to immobilise nuclear wastes directly.②The initial setting time of neutral salt-slag-fly ash binding materials could be controlled more than 4h and the final setting time could be controlled within 48h. Early strength developed slowly, but compressive strength could get to 20~35.0 MPa at 28 days ages and evening strength increased continuously. The alkalinity could be controlled within 12.0 at every age, and with an increase of the PFA content, the alkalinity decreased much more, less than 11.0 indeed under certain condition. The free water content of this system was low and could controled between 10.0 and 15.0 percent. The lower alkalinity and free water content became to an advantage to avoid metal corrosion.③In the neutral salt-slag-fly ash binding materials, there didn't happen metal corrosion evidently under proper activator content and Cs+ leaching rate was low, which is less than 32.0% at 28 days under this researching condition. So this system can satisfy the basic demand of immobilising nuclear wastes.④The main hydration products of the neutral salt-slag-fly ash binding materials are low-alkali hydrous calcium silicate, ettringite and calcium sodium sillo-aluminate hydrate similar to thomsonite, and due to these products combining one another to form compact structure, the strength and immovable ability for nuclear ion of concretions increases. In addition, some quantity of PFA are remained in concretions at hydration evening however.⑤During hydration, there didn't have ettringite and thomaonite products in BFS/OPC binding material and BFS are remained in concretions at hydration evening however. The immobilising ability for nuclear ion was weak due to looser structure.