Leaching Pattern And Environmental Safety Assessment For Heavy Metals In Fly Ash Mortar

Fly ash is widely used in building materials industry because of its certain cementitious activity.In this study,the representative fly ash of Sichuan（FA1）,Chongqing（FA2）and Guizhou（FA3）in three provinces and cities in southwest China was taken as the research object.On the basis of mastering its basic physical and chemical properties,the risk of harmful heavy metals under the background of high dosage application in plastering mortar was explored.Based on the constructed heavy metal pollution risk assessment system,the characteristic harmful heavy metals were screened out.With the help of the relevant leaching kinetic model,the leaching law of the characteristic heavy metals of the plastering mortar products under the real external environment was deeply analyzed,and the heavy metal safety control system was constructed.The main conclusions are as follows.（1）After XRF,XRD,FTIR,SEM/EDS test and pozzolanic activity index analysis,FA1 showed the highest pozzolanic activity due to its 88.5%aluminosilicate material.The SEM/EDS surface spectrum scanning results showed that the fly ash of the three provinces and cities contained As,Cr,Cd,Hg,Ni and other harmful heavy metal elements to human body in different degrees.After content detection,standard limit value comparison,multi-method pollution risk assessment,valence state analysis,heavy metals Cr,Cd,Hg were used as characteristic heavy metals for risk control research.（2）The optimum mix ratio（mass ratio）of M5 grade fly ash mortar is fly ash:cement:sand:water=116:174:1450:245.The 28 days flexural and compressive strength of M5 grade fly ash mortar of FA1,FA2 and FA3 are 3.2 MPa,8.1 MPa;2.4MPa,9.4 MPa;2.3 MPa,7.6 MPa;the optimum mix ratio of M15 grade fly ash mortar is fly ash:cement:sand:water=160:240:1400:265.The 28 days flexural and compressive strength of M15 grade fly ash mortar of FA1,FA2 and FA3 are 5.4 MPa,19.8 MPa;4.6 MPa,15.7 MPa;4.4 MPa,17.4 MPa respectively.The consistency and fluidity of the two strength grade mortars under the optimal mix ratio basically meet the standard JGJ/T220.The results of SEM/EDS scanning and surface scanning distribution showed that these six groups of samples still contained a certain amount of Cd,Cr and Hg heavy metals,which had the risk of leaching under environmental conditions.（3）The mortar samples were placed outdoors for 60 days to simulate natural leaching.The contents of heavy metals Cr,Cd and Hg in fly ash mortar decreased gradually with time.Due to the uneven distribution of heavy metals in mortar products,there was a trend of increasing heavy metal content between the two tests,but with the continuous extension of the experimental period,the content change trend was still gradually reduced.Under the two strength grades of M5 and M15,the leaching law of Cr is suitable for Parabolic equation fitting,and Cd and Hg are mostly predicted by Elovich equation.The non-uniform distribution of heavy metals in mortar products causes the mismatch of fitting equations at some particle size levels,which is solved by data fitting and correlation coefficient comparison.（4）In fly ash mortar products,Cr,Cd and Hg elements still exist in the form of Cr³⁺,Cr⁶⁺,Cd²⁺and Hg²⁺.Based on the single factor index method,potential ecological hazard index method,geological accumulation index method and human health risk assessment method,the reference model of heavy metal characteristic limit value in high volume fly ash mortar was constructed,and the corresponding limit values were given according to the application scenarios.Among them,indoor and outdoor plastering mortar:Cr<0.14 mg/kg,Hg<0.25 mg/kg,Cd<0.3 mg/kg;outdoor masonry mortar:Cr<75 mg/kg,Hg<0.5 mg/kg,Cd<0.3 mg/kg;indoor masonry mortar:Cr<75mg/kg,Hg<4 mg/kg,Cd<10 mg/kg.