Solidification Mechanism And Experimental Study Of Pollution Factors Of Red Mud-based Cementitious Materials

With the sustained and rapid development of China’s national economy and the overall promotion of Transportation Power Strategy,the infrastructure construction has maintained high-speed development.As the most widely used construction material,Portland cement has the problem of high energy consumption and non-renewable raw materials.In recent years,the preparation of cementitious materials from industrial solid waste has become a research focus in the field of infrastructure construction.In this paper,the problems of high content of alkaline components and heavy metals in the preparation of red mud cementitious materials are systematically studied.The occurrence forms and leaching characteristics of pollution factors(alkaline component and heavy metals)in red mud were clarified.The solidification mechanism of pollution factors by red mud-based geopolymer cementitious materials hydration products was investigated,and the action mechanism of solidification agent to improve the solidification rate of pollution factors is analyzed.The compound solidification agent suitable for alkaline component and heavy metals of red mud-based cementitious materials was selected.The improvement law of compound solidification agent on the solidification rate of pollution factors of red mud-based cementitious materials was revealed.The leaching characteristics of pollution factors of red mud-based cementitious materials under acid or alkaline curing conditions were analyzed,and the safe application of red mud-based cementitious materials was realized.The main research contents and results are as follows:(1)The occurrence forms of alkaline component and heavy metals in red mud were systematically analyzed,and the leaching characteristics of pollution factors in acid or alkaline conditions were studied.The results show that the alkaline component of red mud is mainly composed of Na component,and soluble Na component mainly exists in the form of NaHCO3 and Na2CO3,accounting for 8.87 wt.%and 6.35%wt.%,respectively.With the increasing acidity or alkalinity of leaching conditions,the leaching concentrations of Na,Al,Si and Fe all showed an increasing trend,and the leaching concentration of Na+ was particularly significant,which was the most sensitive to acid-alkaline environment.The contents of heavy metal elements from high to low are Cr,Pb,Ni,Cu,As and Cd,which are 754 mg/kg,78.2 mg/kg,41.8 mg/kg,33.2 mg/kg,15.0 mg/kg and 0.77 mg/kg,respectively.Based on Nemerow comprehensive index method and potential ecological risk assessment method,the heavy metal pollution level of red mud is medium pollution,and the potential ecological hazard level is medium pollution.(2)The leaching characteristics of Na+ of red mud-based cementing material were studied.The solidification mechanism of Na+by solidification agents including hydroxyapatite,732 H cation exchange resin,sodium humate and chitosan was analyzed.The time-varying law of Na+ leaching characteristics under the action of hydration product self-cementation and solidification agent synergistic were revealed.With the increase of curing age,the Na+leaching amount of red mud-based cementitious material gradually decreases,and the solidification rate increases.When the curing age is 120 d,the solidification rate of Na+tends to be stable,reaching the value of 63.64%,and the hydration products solidify Na+ through self-cementation.The improvement effect of solidification agents on Na+ solidification rate is as follows:hydroxyapatite<732 H type cation exchange resin<sodium humate<chitosan,the optimal Na+.solidification agent was chitosan with the content of 3%,which can make the Na+ solidification rate reaches 73.76%when curing time is 28 d.The improvement of Na+solidification agent is that it contains active groups,has the ability to adsorb metal ions,and can improve the solidification rate of Na+ through ion exchange reaction.(3)The occurrence forms of heavy metal elements in red mud-based cementitious materials at different curing ages were analyzed,and the improvement law of fly ash,attapulgite and 5 A zeolite on the solidification rate was studied.The pollution factor disposal mechanism of hydration product self-cementing and solidification agent synergistic solidification was established.The occurrence form of heavy metals in red mud-based cementitious materials gradually changes from effective state(exchangeable state,carbonate bound state)to stable state(iron and manganese oxidation state,organic state,residue state)with the increase of curing age.When the curing age is 120 d,the solidification rate tends to be stable,and the solidification rates of As,Cu,Ni,Cd,Cr and Pb reach 99.91%,97.62%,97.02%,96.83%,95.91%and 91.11%,respectively.The enhancing effect of the three heavy metal solidification agents is:5A zeolite<fly ash<attapulgite.The optimal heavy metal solidification agent was attapulgite with the content of 6%,and the solidification rate of Pb and Cr reached 99.74%and 99.40%at the curing age of 28 d,which increased by 8.86%and 4.28%respectively.(4)Based on the optimal principle of mechanical strength and solidification rate of pollution factors,3%chitosan and 6%attapulgite were selected as composite solidification agents of red mud-based cementitious materials.The time-varying law of pollution factors solidification rate of red mud-based cementitious materials modified by composite solidification agents was obtained,and the leaching characteristics of pollution factors in acid or alkaline environment were analyzed.Under the action of composite solidification agent,the pollution factors solidification rate of red mud-based cementitious materials gradually increased,and the solidification rate of Na+,Pb and Cr reached 75.59%,99.95%and 99.74%at the curing age of 90 d.The improvement of solidification rate by composite solidification agent is due to the synergistic solidification of solidification agent and the promotion of geopolymerization reaction.The acid curing environment lead to the reduction of pollution factors solidification rate of red mud based-cementitious material;Alkaline curing environment improved the solidification rate of red mud-based cementitious materials.