Study On Hydration Mechanism And Properties Of Alkali-Activated Lithium Slag Composite Cementitious Metarial

The associated lepidolite ore of Yichun Tantalum Niobium mine in Jiangxi Province is the largest lepidolite ore resource in the world.Due to the low grade of lepidolite,each ton of lithium carbonate produced will produce 20～30 tons of solid waste lithium slag.The comprehensive utilization of lithium slag is about 23%,but the actual utilization rate is only about 10%.Because the active aluminum silicate phase in lithium slag is small,the activity of lithium slag is low,but the content of SiO₂ and Al₂O₃ in lithium slag is high,so it has potential activity.Based on this,the feasibility study of using lepidolite lithium slag as cement mineral admixture in Yichun area of Jiangxi Province was carried out.By means of high temperature calcination,mechanical milling and alkali excitation,the activity of lithium slag was improved.Combined with the XRD analysis,FTIR analysis,SEM analysis and the mechanical properties of the lepidolite lithium slag in Yichun area of Jiangxi Province,the best activation method was evaluated.Because of the low activity of lithium slag,the experiment adopts the method of double mixing mineral powder and fly ash to prepare cement mortar instead of part of cement.Then,the effects of water-binder ratio,NaOH concentration and lithium slag content on slurry flow properties and mechanical properties of cement mortar were studied,and the working and mechanical properties of alkali excited lithium slag cementitious materials were determined.The hydration products and microstructure of alkali-excited lithium slag cementing materials were studied by XRD,SEM/EDS and TG/DTG analysis,and the hydration mechanism of alkali-excited lithium slag cementing materials was revealed.The following conclusions can be drawn from the above research:（1）High temperature calcination can significantly improve the activity of lithium slag,and the optimal calcination temperature is 700℃.Because the internal structure of kaolin in lithium slag is dehydroxylated at 500～700℃,the crystal surface structure is destroyed,thus forming amorphous SiO₂ and Al₂O₃,and improving the activity of lithium slag.（2）Lithium slag is easy to grind,so the specific surface area of lithium slag can be increased with low energy consumption,and the activity of finely ground lithium slag can be significantly improved.Lithium slag is a loose porous structure with strong water absorption,so the mixing of lithium slag will reduce the fluidity of cement mortar.When the water-binder ratio is 0.325,the particle size of lithium slag is 20μm and the dosage is 25%,the fluidity of slurry prepared is 200mm,which meets the specification requirements.Considering the mechanical properties and working properties,the optimum water-binder ratio of alkali excited lithium slag composite cementitious material is 0.325.（3）Lithium slag has pozzolanic activity,and with the increase of lithium slag content,the mechanical properties of the mortar specimens decrease first,then increase and then decrease.Because active SiO₂ and Al₂O₃ in lithium slag can undergo secondary hydration reaction with Ca（OH）₂ produced by cement hydration,the generated calcium silicate hydrate and calcium aluminate hydrate gel can fill the void of matrix,reduce porosity,and improve the strength of cement sand specimen.Considering the mechanical properties and maximizing the reuse of lithium slag,the optimal dosage of lithium slag is 25%.（4）The Si-O and Al-O covalent bonds were broken by the dissolution reaction of SiO₂and Al₂O₃ in the lithium slag with OH^-in the alkaline solution,and the[AlO₄]^5-tetrahedron and[Si O4]^4-tetrahedron were dissolved out to form Si-O-Si and al-o-Al oligomers.[Al O4]^5-tetrahedron and[Si O4]^4-.With the occurrence of deploy condensation reaction,large amounts of C-A-S-H gel substances will be generated.Therefore,with the increase of NaOH concentration,the strength of specimens increases,but when the concentration of NaOH is1.5mol/L and 1mol/L,the mechanical properties of specimens have little difference.Considering the economic effect,1mol/L NaOH concentration is appropriate.