Study On Preparation And Properties Of Biochar Concrete By Sludge And Water Hyacinth Co-pyrolysis

With the increase of sewage treatment rate in China,the amount of sludge is increasing.The sludge from industrial wastewater treatment is more harmful because it contains a lot of toxic substances such as heavy metals,and many of them are hazardous wastes.Water hyacinth is also a serious ecological problem.In addition,China is the country with the largest output and use of cement,which releases a lot of carbon dioxide and accelerates the greenhouse effect.In order to solve the above three problems at the same time,sludge water hyacinth biochar was prepared by co-pyrolysis of sludge and water hyacinth,and added to concrete to prepare sludge water hyacinth biochar concrete with good performance.The heavy metals in the sludge are effectively fixed in the concrete,eliminating the environmental hazards.In view of the good carbon sequestration effect of biochar,its partial replacement of cement can help to alleviate the greenhouse effect in concrete production.Firstly,sludge water hyacinth biochar was prepared by co-pyrolysis of industrial sludge and water hyacinth,and characterized by physical and chemical properties,element composition,group type,phase composition and surface morphology.The effects of pyrolysis temperature,water hyacinth addition ratio and pyrolysis time on the properties of sludge water hyacinth biochar were studied.The main results are as follows: pyrolysis temperature,water hyacinth addition ratio and pyrolysis time have significant effects on the physicochemical properties of sludge water hyacinth.Increasing pyrolysis temperature and prolonging pyrolysis time decreased the yield and volatile content of biochar,but increased the aromatic degree,crystallinity,p H value,ash content and fixed carbon content of biochar;Increasing the proportion of water hyacinth decreased the biomass carbon yield and ash content,but increased the carbon content,aromatization,p H value,fixed carbon and volatile content;With the increase of temperature and the proportion of water hyacinth within 60%,the pore morphology of sludge water hyacinth biochar was gradually enriched.Then,sludge water hyacinth biochar was added to concrete to replace cement with different mass ratios to prepare biochar concrete.Through the characterization test of group type,phase composition and surface morphology,as well as the test of compressive strength,splitting tensile strength,water absorption and porosity,the effects of pyrolysis temperature,water hyacinth addition ratio and biochar addition ratio on the structural characterization and basic properties of concrete were discussed.The main results are as follows: the overall performance of sludge water hyacinth biochar concrete is good and meets the needs of practical engineering,and the performance is improved to varying degrees compared with the blank and fly ash control group.Based on the experimental analysis,when the pyrolysis temperature is550 ℃,the water hyacinth addition ratio is 50% and the biochar addition ratio is 5%,the overall performance of biochar concrete is good,and the improvement degree is higher than that of the control group.Its compressive strength is 44.52 MPa,splitting tensile strength is 4.91 MPa,water absorption is 4.31% and porosity is 6.80%.In order to investigate the environmental friendliness of sludge water hyacinth biochar concrete,the migration and transformation of heavy metals in each stage were deeply analyzed.Based on the analysis of the concentration,chemical form and leaching toxicity of heavy metals in biochar concrete,the solidification effect of heavy metals in biochar concrete was explored,and the potential ecological risk of biochar concrete was evaluated by quantitative evaluation method.The main results are as follows: the addition of pyrolysis temperature and water hyacinth with a large proportion of sludge water hyacinth biochar are more conducive to the solidification of heavy metals in concrete.The concentration of heavy metals in biochar concrete was lower than the limit specified in the Agricultural Sludge Pollutant Control Standard(GB 4284-2018).The leaching amount of heavy metals was lower than the limit specified in the Identification Standard for Leaching Toxicity of Hazardous Wastes(GB 5085.3-2007).Heavy metals mostly exist in the residue state,the stability is higher than that of biochar and sludge,and the curing effect is good.The potential risk index RI decreased from 436.14(sludge)to 1.44(biochar concrete).In conclusion,the sludge water hyacinth biochar with good performance can be obtained by co-pyrolysis of sludge and water hyacinth;Compared with plain concrete and fly ash concrete,the basic performance of sludge water hyacinth co-pyrolysis biochar concrete prepared by adding it into concrete has been improved in varying degrees;Moreover,the heavy metals in the sludge are effectively fixed in the biochar concrete,eliminating the potential ecological risk.Therefore,the preparation of sludge water hyacinth co-pyrolysis biochar concrete provides an effective way for the resource utilization of industrial sludge,the solution of water hyacinth problem and the realization of pollution and carbon reduction.