Abstract: The backfill of mixed slurry consisting of alkali wastes with saturated brine into abandoned salt cavern can simultaneously resolve the problems of alkali treatment and the potential geology disaster of abandoned salt caverns. The strength of the backfilled alkali wastes is a key parameter which affects the filling effect. In order to improve the strength of the backfilled alkali wastes, fly ash is used to make compound alkali. A series of mineral analysis, strength experiments and mesoscale testing have been conducted to determine the properties of the compound alkali wastes, showing that: (1) The mixing of fly ash can effectively reinforce the alkali wastes, resulting in the increases in the cohesion, the internal frictional angle and the shear strength; (2) The larger the mixing ratio of the fly ash, the stronger the reinforcing effect is; whereas, the strength increases nonlinearly with the increment of mixing ratio. For cohesion, its maximum increasing speed locates in the mixing ratio range of 0-20%; for internal frictional angle, it is in the mixing ratio range of 20-30%; and for shear strength, the maximum increasing speed locates in the mixing ratio range of 0-20%; (3) The mixing of fly ash can significantly improve the compression consolidation properties, inducing an increase in consolidation coefficient, thus the project time of backfill will greatly be reduced; the mixing ratio of 0-10% is most suited to the improvement of consolidation; (4) By mineralogical analysis, it is shown that the mixing of fly ash can change the components of the alkali wastes, significantly reducing the content of the hydrophilia minerals and changing the sedimentary properties. The meso-scale analysis shows that, owing to fly ash mixed into alkali wastes, the flocculation structure of initial alkali wastes changes into the filling structure where the fly ash acts as main skeleton, in this structure the supporting and connecting effects between particles are also more obvious. Comprehensive analysis shows that the mixing ratio of 20% is the optimal ratio in terms of the improving strength, enhancing consolidation effect, shortening project period as well as saving cost. A value less than 20% is recommended for the practical mixing ratio in practice. The research results provide a good reference for revealing reinforcing mechanism of alkali wastes, as well as optimizing technologies for salt cavern backfill.

Key words: alkali wastes, fly ash, backfill, shear strength, consolidation, optimal blend ratio