Preparation Of Titanium-containing Blast Furnace Slag-based Oxalate Cement And Study On The Solidification Of Heavy Metals Nickel And Lea

Acid-base cement,as a new type of inorganic cementing material,is usually prepared through the acid-base reaction between alkali metal oxides and acid components in a certain proportion at room temperature.Due to its high early strength,fast setting and good cohesive property,it is often used in the fields of urgent repair of airport runways and municipal engineering and solidification/stabilization of hazardous materials such as heavy metals and radioactive wastes.The application prospects of the commonly used phosphate cement are restricted due to its high preparation cost（pure metal oxides as the raw material）and the secondary phosphorus pollution caused by the unreactive phosphates.Therefore,in our research,the calcium oxalate cement（COC）,a new acid-base cement,was prepared by using titanium-bearing blast furnace slag（TBBFS）and oxalic acid（OA）as the base and acid components,respectively and used as the matrix material to solidify/stabilize the heavy metals such as nickel and lead.This research provides the new approaches for the utilization of blast furnace slag and the development of acid-base cement materials.The main results are summarized as follows:（1）With the increase of raw material mass ratio（TBBFS/OA）,the strength of oxalate cement at each age increases first and then decreases.When TBBFS/OA=6,the mechanical properties are the highest,and the compressive strength at 3 d,7 d and 28 d reaches 19.95 MPa,23.20 MPa and27.25 MPa,respectively.Its structure and composition were characterized.Augite in titanium-bearing furnace slag,as the main phase involved in the reaction,dissolved in the oxalic acid environment to produce Ca²⁺,and reacted with oxalic acid to produce calcium oxalate hydrate products.The overall structure showed a relatively compact microstructure.（2）With the increase of retarding agent content,the compressive strength of oxalate cement increased first and then decreased,and the setting time was gradually extended from 7.17 min to20.52 min.Considering the performance indexes such as compressive strength,fluidity and setting time,1%of retarder was selected as the best proportion.When the dosage is 1%,its mechanical properties are better,the initial setting time is 11.32 min,and it has good construction operability.（3）Water resistance of COC cement was studied,and the results showed that:The compressive strength of COC cement test blocks prepared based on different raw material ratios decreased in both water and constant temperature and humidity chamber,but the strength retention rate of test blocks cured in constant humidity chamber was slightly higher than that of test blocks cured in water.When TBBFS/OA=6,The strength retention rates in water and constant temperature and humidity box were 84.58%and 88.4%,respectively.（4）Different kinds of heavy metals（Ni,Pb and mixed Ni and Pb）were treated by COC cement solidification/stabilization,and the effects of different heavy metal ion contents on the compressive strength and toxic leaching characteristics of heavy metals were investigated.The results showed that compared with the blank sample,the 28 d compressive strength of the heavy metal cured body decreased to different degrees.When the content of Ni,Pb and Ni/P is 5%,4%and 2%,the compressive strength retention rates are 63.1%,79.8%and 74.8%,respectively.The leaching concentrations of cured weight metal ions were all less than 5 mg/L,lower than the national standard limit of toxic heavy metal leaching.（5）The phase composition and structure of the heavy metal solidified body were characterized and analyzed.The curing mechanism of COC cement material for Ni and Pb mainly includes chemical precipitation and physical encapsulation.First,heavy metal ions combine with Ti O₃^2-,Si O₃^2-,C₂O₄^2-and other anions in the slurry to form heavy metal insoluble salts.Secondly,the gelled salt generated in the hydration process of COC cement constitutes a physical coating and forms a dense microstructure,which hinders the leaching of heavy metals.