Fluoride removal capacity of cement paste

The objectives of this study are to assess the feasibility of using the cement paste as an alternative agent for fluoride removal and to investigate fluoride removal capacity of the cement paste. Initially, screening experiments were conducted to evaluate Ca-bearing materials (cement, cement paste, lime). The cement paste was competitive to lime, common fluoride removal agent. Various Ca-bearing hydrates such as portlandite, calcium silicate hydrate (CSH), and ettringite in the cement paste were identified to remove fluoride by precipitating CaF2 and/or adsorbing F– ions. In the batch slurry experiments using cement paste and lime simultaneously, 50–67% of lime can be substituted by cement paste to satisfy fluoride effluent limitation of 15 mg/L. Fluoride removal reactions in cement paste slurries were strongly affected by pH, and an optimal pH for the cement paste slurries exists between 7.0 and 11.5. From the result of the column experiment to observe the successive fluoride removal capacity of cement paste, the real hydrofluoric acid wastewater concentration of 1150 mg/L immediately reduced to the level of less than 15 mg/L. These results indicate that the cement paste generally has advantageous characteristics as an economical and viable substitute for lime to remove fluoride.