Conversion Of Contaminated Dredged River Sediment Into Lightweight Construction Materials And Heavy Metal Solidification

Large quantities of contaminated river sediment are dredged continuously worldwide,which is generally dumped in landfill area resulting in land occupation and soil-groundwater pollution.Conventional treatment of river dredged sediment will bring huge cost impact and secondary pollution risk.The compositions of river sediment are similar to those in clay,and it has a large prospect to produce building materials.In order to consume the large amounts of dredged river sediment,an innovative approach is demonstrated in this study to transform contaminated sediment into eco-friendly foamed concrete by adding cement,foam and all kinds of additives without heating and pressure.In this study,the effects of basic process parameters and various additives on the compressive resistance,fracture resistance,dry density,thermal conductivity,and water resistance,etc.of foamed concrete were studied in detail.Moreover,the solidification effects of heavy metals were studied and the economic and environmental benefits were also analyzed.The main contents and conclusions of this study are as follows:（1）The effects of the addition ratio of sediment and foam volumes on the sediment-based foamed concrete were studied.A systematic evaluation of product properties was conducted,which included dry density,mechanical strength characteristics,thermal conductivity,water stability and pore formation in the microstructure.It was found that compressive strength of the product decreased with the incremental dosages of sediment.A balanced benefit with regard to both the performances of the concrete and the maximum sediment percentage was found at a sediment/cement ratio of 1:1（w/w）,at which a compressive strength of16.8 MPa was achieved.With the foam percent rising from 0%to 80%（v/v）,the compressive strength and dry density of foamed concrete decreased from 18.8 to 2.1 MPa,and from 1442 to 620.6 kg·m^-3,respectively.Accordingly,the thermal conductivity declined from 0.516 to0.173 W·m^-1·K^-1,and water resistance coefficient decreased from 0.73 to0.43.When the foam addition was 54%（v/v）of the slurry,850 kg·m^-3 dry density,3.99 MPa compressive strength and 0.2368 W·m^-1·K^-1 were achieved,which conformed to the standard requirement of foamed concrete in China（JG/T 266-2011）.Studies have found that the effect of foam volume on product performance is greater than the effect of the sediment addition ratio.（2）The effects of active additives which partially replaced the cement on product properties were studied.It was found that addition of 0-10%silica fume can comprehensively improve product performances,while lime and fly ash were not observed the obvious improvement on the product performances.With 10%（w/w）silica fume addition,combined with 40%（w/w）cement,a highly qualified product could be obtained around 4.95 MPa compressive strength,0.186 W·m^-1·K^-1 thermal conductivity,and 0.70 water resistance coefficient.By means of XRD and SEM characterization,it was found that silica fume can induce secondary hydration reaction and exert pore filling effect.According to Image pro plus analysis,it was found that silica fume narrows the pore distribution and made pore distribution more homogenous and denser.（3）The effects of admixtures including water glass,sodium metasilicate,cow manure（CM350,CM500）,and barley grass biochar（BG350 and BG500）on product performances were studied.Water glass and sodium metasilicate did not improve product peoperties.Barley grass biochar improved product performances more than cow manure biochar did,and the effects of high-temperature biochar is higher than that of low-temperature biochar.The addition of 2%BG500 improved the compressive strength by 15%and the thermal insulation performance by4.8%because of its activity in the hydration reaction and pore-filling effect.（4）The environmental safety of foamed concrete prepared by sediment was studied.Three levels of heavy metals contaminated sediments were used to prepare the foamed concrete,and the leaching behavior of heavy metals from the foamed concrete was investigated using three types of leaching procetrures,TCLP,SPLP and H₂O extraction.It was found that heavy metals in the three sediments can be immobilized by 94–100%for Cd,Ni,Pb,Cu and Zn,even in an acidic environment.With the increase of heavy metal concentration in sediment,the immobilization rate of Cr decreased,but with incorporation of silica fume and biochar,the immobilization rate of Cr could be increased from about 30%to higher than 60%.（5）Through the analysis of the economic and environmental benefits,it was found that when the cost of dredged sediment was a part of the income of product,the profit of the foamed concrete was 1057 CNY per cubic meter.After mixing 10%silica fume,2%BG500 and silica fume and biochar,respectively,the profit per cubic meter of foamed concrete was985 CNY,977 CNY and 907 CNY,respectively.Although addition of these materials might reduce the product profit,it could increase the market prospects of the products significantly due to the improved properties.The utilization of sediment to produce foamed concrete could consume more than 30 tons of solid waste a day.According to the life cycle assessment,it was found the overall environmental impact of the production process of river sediment based foamed concrete was far below the traditional foam concrete.This study provides new opportunities for hazardous dredged river sediment recycling,which is prospective because it not only consumes a large amount of sediment but also avoids depletion of resources and creates profits.