| Hot mix asphalt mixture not only generates a large amount of emissions during the construction process,but also requires a large amount of energy consumption,which poses a threat to the environment and also affects the health of workers.Warm mix asphalt mixture may replace traditional hot mix asphalt mixture in the future due to its excellent energy-saving and emission reduction characteristics.However,the current energy consumption reduction brought about by warm mixing technology is not enough to cover the cost increase it brings.Phosphogypsum,as a typical industrial solid waste,has not been effectively utilized in large quantities and is still treated by stacking.Because phosphogypsum has crystalline water molecules,it will be dehydrated during heating,forming water vapor in asphalt,which will produce foam and reduce the viscosity of asphalt materials.Phosphogypsum,as a warm mixing agent material for asphalt,can not only reduce the cost of warm mixing technology,but also facilitate the green,sustainable,and high-value recycling of phosphogypsum.In this study,the preparation process of phosphogypsum warm mix asphalt additives was designed based on the basic principle of recrystallization to obtain high-purity phosphogypsum warm mix additives,optimize the preparation process parameters,and solve the variability problem of solid waste phosphogypsum.Through the dynamic shear rheological test,bending beam rheological test,infrared spectroscopy analysis,thermogravimetric analysis,and gas chromatography-mass spectrometry analysis,the high,medium,and low-temperature rheological properties and ageing behaviour of phosphogypsum warm mix asphalt were clarified.The reaction and viscosity reduction mechanism of phosphogypsum warm mix asphalt was revealed,and the emission reduction effect of phosphogypsum warm mix asphalt was explored.Finally,the influence of phosphogypsum warm mix additives on the performance of asphalt concrete pavement was verified,providing a theoretical basis and technical support for the promotion and application of phosphogypsum warm mix asphalt mixtures.Firstly,the phosphogypsum warm mix agent for asphalt mixture was prepared using the atmospheric pressure acidification method,and the process flow of temperature rise dissolution,cooling crystallization,and filtration separation was designed.Fluorescence spectroscopy was used to analyze the purity of the phosphogypsum warm mixing agent,which was over 99%.The influence of material source and aging time on the preparation purity was evaluated.Fourier transform infrared spectroscopy was used to analyze that phosphogypsum warm mix agent and asphalt do not produce new chemical functional groups,and the two are mainly physical reactions.The presence of crystalline water functional groups in phosphogypsum warm mix asphalt was discovered.The release law of crystalline water in phosphogypsum was clarified through thermogravimetric analysis,revealing the mechanism of viscosity reduction.The crystalline water in phosphogypsum warm mix asphalt starts to release at 100 ℃,with the fastest release rate at 140 ℃,and is completely released at 150 ℃.The cooling effect of phosphogypsum warm mix asphalt was revealed based on the Saal fitting formula.Phosphogypsum warm mixed 70 # base asphalt and SBS-modified asphalt can effectively reduce construction temperatures by 7-26 ℃ and 6-29 ℃.The optimal factor combination and aspect ratio of the phosphogypsum warm mixing agent were determined through the response surface analysis method,with a stirring temperature of 80.63 ℃,a stirring time of 22.59 minutes,and a hydrochloric acid concentration of 18.18%.The maximum aspect ratio was obtained as 69.95.The experiment verified that the relative error between the measured value of the length-to-diameter ratio and the predicted value of the model is less than 5%.Secondly,based on the principles of rheology,analyze the evolution of rheological properties of phosphogypsum warm mix asphalt under the conditions of original,shortterm ageing,and long-term ageing.Establish a rheological main curve based on the principle of time temperature equivalence to evaluate the viscoelastic properties of phosphogypsum warm mix asphalt over a wide temperature(frequency)range.Multiple stress creep recovery tests were used to investigate the creep and deformation recovery characteristics of phosphogypsum warm mix asphalt under multiple stress states.Evaluate the fatigue performance of phosphogypsum warm mix asphalt through the principles of viscoelastic damage and fatigue failure criteria.The viscoelastic and lowtemperature creep characteristics of phosphogypsum warm mix asphalt were analyzed based on the Burgers model.Phosphogypsum increases the complex modulus and creep recovery rate of asphalt,reduces cumulative strain and irreparable flexibility,and maintains the proportion of viscous elastic components,thereby improving the hightemperature performance of asphalt materials.Phosphogypsum increases the yield stress of asphalt,reduces the maximum cumulative damage sustained,and improves the fatigue life of asphalt.The Burgers model was used to fit the two evaluation indicators of phosphogypsum warm mix asphalt relaxation time and low-temperature comprehensive compliance,which are consistent.The elastic components in the viscoelastic composition of aged phosphogypsum warm mix asphalt increase,and the macroscopic manifestation is that phosphogypsum warm mix asphalt becomes hard,improving high-temperature stability.Ageing increases the peak stress tolerance,reduces the tolerable strain,increases creep stiffness,reduces creep rate,and has adverse effects on fatigue and low-temperature performance.The elastic components in the viscoelastic composition of aged phosphogypsum warm mix asphalt increase,and the macroscopic manifestation is that phosphogypsum warm mix asphalt becomes hard,improving high-temperature stability.Aging increases the peak stress tolerance,reduces the tolerable strain,increases creep stiffness,reduces creep rate,and adversely affects fatigue and low-temperature performance.The influence of aging on the chemical functional groups of phosphogypsum warm mix asphalt was investigated using Fourier transform infrared spectroscopy.Aging leads to an increase in the content of carbonyl and sulfoxide functional groups in phosphogypsum warm mix asphalt.Thirdly,the emission reduction effect of volatile organic compounds and characteristic ions in phosphogypsum warm mix asphalt was confirmed by gas chromatography-mass spectrometry analysis.Due to the cooling and adsorption effects of phosphogypsum warm mix agent in asphalt,the total volatile organic compounds of phosphogypsum warm mixed asphalt are significantly reduced at mixing and compaction temperatures.Among them,phosphogypsum warm mix 70# base asphalt and SBSmodified asphalt reduce the characteristic ion toluene by more than 73% and 42%,respectively,and xylene by more than 69% and 28% during the construction stage.Finally,the road performance of phosphogypsum warm mix asphalt mixture was verified through immersion marshall test,rutting test,low-temperature beam bending test,and four-point bending fatigue test.A fatigue life prediction equation for phosphogypsum warm mix asphalt mixture was proposed based on fatigue data.The above research designed and prepared phosphogypsum warm mix additives from the aspects of experimental design and performance characterization,revealed the mechanism of viscosity reduction,discussed the rheological properties and aging behavior of phosphogypsum warm mix asphalt,explored emission reduction effects,and verified the road performance of asphalt mixtures,providing theoretical and technical support for the design and development of phosphogypsum warm mix additives. |
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