Study On Heavy Metal Immobilization Property Of Binary Blended Cementitious Material

With city expansion and industrial technology development,the production of solid wastes containing heavy metals in China has grown rapidly.Due to the low degradation,migration concealment,bioaccumulation and biological toxicity of heavy metals,it is necessary to properly treat and dispose these solid wastes to reduce the migration,release,circulation and enrichment of heavy metals and ensure human health and ecological environment safety.Cement solidification technology can effectively immobilize heavy metals through physical encapsulation and chemical stabilization.At the same time,it has technical advantages such as wide sources of materials,strong availability and large treatment capacity,and is widely used in the harmless treatment and resource utilization of solid waste s containing heavy metals.However,the cement solidification technology has the disadvantage of high carbon emissions caused by the high use of Portland cement.The use of supplementary cementitious materials in the solidification treatment can effectively reduce the carbon emissions in the treatment process,which has broad application prospects under the background of the Dual Carbon target era.This paper focused on the immobilization property,modification mechanism and immobilization application of supplementary cementitious material blended cement,aiming to aiming to provide a theoretical basis for high-performance and low-carbon design of cement solidification technology.The specific contents are as follows:Through the study on the influence of four heavy metals,Cu²⁺,Zn²⁺,Ni²⁺and Cd²⁺,on the hydration of Portland cement and their chemical binding mechanism s,it was found that Cu²⁺and Zn²⁺had a"poison effect"on the nucleation and growth of hydration products at early stage,leading to a significant extension of the hydration induction period and a retardation of the early strength development of cement paste,while Ni²⁺and Cd²⁺did not inhibit the early hydration.There were three chemical binding forms of heavy metals in cement paste:double hydroxides formed by coprecipitation with Ca²⁺,high aluminum binding phase through replacing Ca in AFt crystal structure and C-S-H binding phase.In the C-S-H binding phase,the-Si O^-and（-Si OOH）₂Al^-sites of the C-S-H aluminosilicate chain can achieve chemical binding to heavy metals through ion exchang e,surface complexation or formation of a thriple layer structure with Ca²⁺.The binding mechanism of different heavy metals in C-S-H depended on the specific correlation between the binding of heavy metals and the chemical composition of C-S-H.The influence of three types of supplementary cementitious material,silica fume,metakaolin and slag on the cement immobilization property was studied.It was found that silica fume reduced the capillary porosity and increased the tortuosity of pores of immobilization systems,which enhanced the physical encapsulation capacity to heavy metals and improved the immobilization property.Metakaolin and slag increased the Si/Ca and Al/Ca of C-S-H,which improved the availability of-Si O^-and（-Si OOH）₂Al^-sites in C-S-H and contributed to enhance the chemical binding capacity of C-S-H to heavy metals.However,metakaolin and slag at high replacement level caused the decrease in the capillary porosity and the increase in the pore tortuosity,which was not conducive to the physical encapsulation to heavy metals and resulted in the deterioration of the immobilization property.In view of the adverse effects of metakaolin and slag at high replacement level on the immobilization property,sodium sulfate was introduced as a modifier and its modification effect and mechanism were studied.It was found that sodium sulfate promoted the hydration of C₃S and C₃A as well as the pozzolanic reaction of metakaolin and slag,and inhibited the transformation of AFt to AFm phase.This improved the reaction degree of immobilization systems,thus increasing the amount of C-S-H and AFt.On the one hand,it enhanced the filling capacity of hydrated products to the matrix space,which improved the physical encapsulation capacity to heavy metals.On the other hand,it increased the capture probability of heavy metals by hydration products,thus improving the chemical binding capacity of the immobilization system to heavy metals.By synergistically enhancing the physical encapsulation and chemical binding capacity to heavy metals,the immobilization property of blended cement modified by sodium sulfate was significantly improved.Silica fume blended cement,sodium sulfate modified metakaolin blended cement and sodium sulfate modified slag blended cement were applied to the solidification treatment of municipal solid waste incineration fly ash,and the ir mechanical properties and heavy metal leachabilities were studied.Results showed that the compressive strength of immobilization systems reach 20-30 MPa at the mass ratio of municipal solid waste incineration fly ash to binder of 2:3,which met the mechanical property requirements of building material utilization.At the same time,the TCLP leaching concentrations of Cu²⁺and Zn²⁺were below 40μg/L and Ni²⁺and Cd²⁺respectively below 4μg/L and 0.6μg/L.The potential heavy metal leaching toxicity of immobilization systems was at a safe level,which was reliable to ensure human health and ecological environment safety in the building material utilization.