Study On The Performance Of Sludge Biochar Cement And Evaluation Of Its Ability To Fix Heavy Metals

In recent years,with the development of economy,the production of rural and urban sludge is increasing,and the harm to the surrounding environment and water sources is becoming more and more serious.Traditional sludge treatment methods can no longer meet the needs of people,and it is urgent to develop new economic and green methods for urban sludge treatment.Sludge biochar is the pyrolysis of charcoal under anaerobic or less oxygen conditions using sludge as the biomass source to form charcoal with rich pore structure,surface functional groups and strong adsorption capacity.The disposal method of sludge pyrolysis into biochar not only can solve a large number of toxic substances such as bacteria,virus and insect eggs in sludge,but also produces a product sludge biochar with great potential for soil improvement,soil remediation,water restoration,building materials utilization and carbon fixation,so it has received wide attention.The research on biochar modified cement-based materials is a hot spot in the construction and structural direction in water conservancy and civil engineering fields,but there is still relatively little research on combining sludge biochar with cement-based materials to produce sludge biochar eco-cement,thus reducing the waste and damage to resources and environment in cement industry;the effects of cracking temperature,alternative admixture and particle size of sludge biochar on physical properties,compressive strength and thermal conductivity of cement have not yet been studied.The effects of sludge biochar cracking temperature,alternative admixture amount and particle size on the physical properties,compressive strength and thermal conductivity of cement have not been thoroughly studied;whether sludge biochar cement is harmful to the environment and the effect of slag silicate cement on the fixation of toxic substances such as heavy metals in sludge biochar are still unclear.Therefore,in this thesis,local municipal sludge will be selected as the biomass and produced by pyrolysis in a muffle furnace to produce sludge biochar samples at300-700 ℃.The influence of cracking temperature on the structural composition and properties of sludge biochar will be studied using physicochemical analysis and characterization techniques such as Fourier infrared spectroscopy(FTIR),X-ray diffraction technique(XRD),and scanning electron microscopy(SEM);The effects of different pyrolysis temperatures,different alternative admixtures and different particle sizes of sludge biochar on the basic physical properties,compressive strength and thermal conductivity of 325 and 425 slag silicate cements were investigated by controlled variable method and characterized by FTIR and XRD techniques to elaborate the mechanism of sludge biochar to enhance the performance of cements;and then different soaking times and different particle sizes of sludge biochar cement specimens were carried out to study the toxicity leaching of sludge biochar cement samples.The toxicity leaching study of sludge biochar cement specimens with different soaking times and particle sizes was then carried out to investigate the fixation of four heavy metal ions,Cu,Zn,Pb and Cd,in sludge biochar cement by atomic absorption spectrophotometer using flame atomic absorption spectrophotometry.This study on sludge biochar cement provides new ideas for rural and urban sludge treatment methods,also reduces the amount of cement,improves the performance of cement,alleviates the resource and environmental problems caused by the production of cement,and provides a way to fix heavy metals in sludge.This study has important implications for the promotion and application of sludge biochar cement in the field of water conservancy construction.The main findings of the thesis are as follows.(1)The preparation and characterization of sludge biochar revealed such a pattern:the sludge biochar yield gradually decreased and stabilized with the increase of pyrolysis temperature.The p H of the sludge biochar changed from weakly acidic to alkaline with the increase of pyrolysis temperature.The water absorption and retention rate tends to increase and then decrease with the increase of pyrolysis temperature;the surface functional groups gradually disappear with the increase of biochar cracking temperature,the hydroxyl and methylene groups disappear,the fatty structure changes to aromatic structure,the silica substances become more stable,the carbonate and cellulose gradually decompose,and the ash increases;the physical phase analysis of sludge biochar shows that there are more silica and calcium carbonate and iron salts,and the crystallinity of sludge is stronger after burning into biochar,reaching the strongest at 500 degrees Celsius,and various metal compounds are more stable.(2)Detailed experimental tests on the basic physical properties of sludge biochar cement were carried out.It was tested that the mass fraction of sludge biochar replacement cement should not exceed 5%.The standard consistency water consumption of cement showed an increasing trend with the increase of sludge biochar substitution ratio;the flow degree changed more slowly with the increasing of sludge biochar substitution ratio in a slow increasing trend,and the flow degree of cement gradually became larger with the increase of the temperature of burning biochar,and the trend was more gentle;the setting time increased with the increase of sludge biochar substitution cement,and its setting time including The setting time(including initial setting time and final setting time)showed a linear increasing trend as the amount of sludge biochar replaced cement increased.The influence of the cracking temperature of sludge biochar on the setting time of cement is relatively small.(3)The results of compressive strength of sludge biochar cement: the early strength and late strength of cement partially replaced by sludge biochar were generally higher than those of the control group.The compressive strength of cement basically tended to increase first and then decrease with the increase of sludge biochar substitution amount,cracking temperature and particle size,among which the highest compressive strength was 65.18 MPa with 1% substitution amount at 700 degrees Celsius,which increased 50.01% compared with the blank control group.The compressive strength of the sludge biochar at 500 degrees Celsius can replace up to 5%of cement and still meet the requirements.The strength of cement was better improved when the particle size of biochar was 200 mesh.(4)Effect of different temperatures and different substitution amounts of sludge biochar on the thermal conductivity of cement: The effect of sludge biochar firing temperature on the thermal conductivity and thermal diffusion coefficient of cement is small and basically shows a slow rising trend.The substitution amount of sludge biochar has a great influence on the thermal conductivity of cement.With the increase of substitution ratio,the thermal conductivity of cement decreases,and the thermal diffusion coefficient decreases first and then increases with the increase of substitution ratio.The sludge biochar particle size has little effect on the thermal conductivity of cement.(5)The leaching test results of heavy metal ions from sludge biochar cement test blocks: the particle size and soaking time of the samples have an effect on the leaching concentration of the four heavy metals,and the precipitation concentration of copper,zinc,lead and cadmium ions is very small when 5% of sludge or sludge biochar is substituted for part of the cement,and the standard is not exceeded.The use of sludge biochar cement does not cause adverse effects on the environment.