In this work, we present the possibility to reduce the amount of fluoride ions in silica gelwaste by using different techniques or to immobilize these ions by creating products ofcommercial value. The leaching of fluoride ions from silica gel waste to the liquid medium wasdone under static and dynamic conditions. It was determined that the removal of fluoride ionsfrom this compound depends on various factors, such as dissociation, solubility, the w/s ratio,reaction temperature, leaching conditions, the adsorption properties of silica gel waste, and others.The obtained results showed that, by applying different techniques, the quantity of fluoride ionscan be reduced by 60%, while obtained water was neutralized by calcium hydroxide.Additionally, it was determined that silica gel waste is a promising raw material for thehydrothermal synthesis of a stable compound containing fluoride ions – cuspidine.

In this work, the influence of various hydroxide and salt additives on the removal of F− ions from silica gel waste, which is obtained during the production of AlF3, was examined. The leaching of the mentioned ions from silica gel waste to the liquid medium was achieved by the application of different techniques: (1) leaching under static conditions; (2) leaching under dynamic conditions by the use of continuous liquid medium flow; and (3) leaching in cycles under dynamic conditions. It was determined that the efficiency of the fluoride removal from this waste depends on the w/s ratio, the leaching conditions, and the additives used. It was proven that it is possible to reduce the concentration of fluorine ions from 10% to <5% by changing the treatment conditions and by adding alkaline compounds. The silica gel obtained after the leaching is a promising silicon dioxide source.

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By producing 1 ton of Portland cement clinker in environment releasing about 0.85 tons of CO 2 : 70% of limestone decarbonation and 30% of electricity and thermal consumption. High specific CO 2 emissions results take the responsibility of Portland cement industry for about 5% of global CO 2 emissions. One of the ways to reduce CO 2 emissions is the use of Portland cement substituting materials. Properly treated ashes could become not a waste of biofuel but a valuable raw material for new construction materials. This paper presents results about the characterization of the biomass bottom ash sourced from the combustion of plant biomass located in Lithuania, and the study of new cement formulations incorporated with the biomass bottom ash. The study includes a comparative analysis of the phase formation and the setting of cement with bottom ash composite. Techniques such as X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), measurement of hydration temperature were used to determine the structure and composition of the formulations.

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