Preparation Of Phosphogypsum(PG)based Artificial Aggregate And Its Application In The Asphalt Mixture

As a big country of phosphogypsum(PG)by-products,China is faced with the increasingly serious problem of PG stockpiling.It makes the resource utilization of PG become an urgent demand at the present stage.However,most of the current resource utilization technologies are limited by various factors,such as secondary pollution,high energy consumption,complex process and so on.The complex pretreatment and purification process,low utilization rate and utilization volume are the most severe restrictions on PG resource utilization.This paper prepared artificial aggregate with PG as the primary raw material to solve this problem.And the artificial aggregate was used in asphalt mixture gradation design to open up a new way of PG resource utilization.First,PG,slag powder,and cement are raw materials for building the PG slag cement hydration reaction system.Two kinds of the aggregate preparation process,the crushing method and disk method,were used to prepare the crushing method aggregate and disk method aggregate,respectively.By comparing the strength evaluation methods of aggregates,it is found that the surface failure mode of the disc method aggregates is different from the point failure mode of the natural aggregates,which makes the test results of the crushing value larger.However,the data measured by the injection mould method and cutting method designed in this paper have more significant variability.On this basis,the unconfined compressive strength method was used for aggregate strength evaluation to explore the influence of PG dosage,curing conditions and curing time on aggregate strength to determine the optimal technological conditions of the 80% mass dosage of PG and seven days 60 ℃immersion curing.The multi-factor analysis of strength data shows that increasing curing temperature can effectively promote the growth of intensity.Scanning electron microscopy(SEM),X-ray diffractometer(XRD)and X-ray diffractometer(CT)were used to characterize the microstructure,phase,component content change and pore distribution of aggregates at different hydration stages.The results show that the calcium vanadite crystals produced by the hydration reaction of the PG slag cement system are crosslinked with calcium silicate gel.And a large amount of unreacted PG is wrapped by hydration products as filler,thus producing strength for aggregate.As the hydration reaction goes on,the hydration products increase.Among them,the grain size of calcium vanadinite becomes fine.At the same time,the continuous dissolution of PG resulted in the growth of micron pores,which had a more prominent effect on aggregate strength after seven days of curing.The section microscopic images of the disk method aggregate show that the lamellar structure of PG makes it express orientation during aggregate growth,which leads to the core-shell form of the disk method aggregate.In addition,the surface of artificial aggregate was coated with watersoluble silicone resin to reduce the water absorption of aggregate.The results show that the water absorption of the aggregate can be reduced to the standard value range(≤2.0%)when the spraying amount is 2.8%,while the water absorption of the aggregate without spraying treatment is 18.35%.Secondly,two artificial aggregates substitute natural aggregates for asphalt mixture gradation design.Four gradation types were designed respectively: natural aggregate mixture(without artificial aggregate),30 vol.% crushing mixture(crushing aggregate volume content is 30%),30 vol.% disk method mixture(disk method aggregate volume content is 30%),50 vol.% disk method mixture(disk method aggregate volume content is 50%).The Marshall test results of the mixtures showed that due to the high water absorption of the artificial aggregates,the effective asphalt content of the three mixtures was low,and the effective asphalt volume percentage was lower than 8%.The mixture of 30 vol.% disk method completely meets the design requirements of the specification,but when the disk method aggregate content is increased to 50%,the aggregate absorbs more asphalt,resulting in low asphalt saturation and a high void ratio.At the same time,due to the higher water absorption of the aggregate,the voidage of the mixture with 30 vol.% crushing method is slightly higher than the specification requirements.Finally,the four mixtures carried out rutting,low-temperature bending,and water damage resistance tests.The rutting test shows that the spherical structure of the disk aggregate makes it easier to produce rutting under load,but the rutting stability of the two artificial aggregate mixtures is above 1000 times /mm as required by the specification.The low-temperature bending test results show that the low-temperature performance of the three artificial aggregate mixtures is lower than that of natural aggregate mixtures.Among them,under the same dosage,the mixture of 30 vol.% disk method has better low-temperature performance.The water damage test of the mixture shows that the fracture energy of the disk method mixture increases because the aggregate can be further hydrated to enhance the strength in the water damage test.Among them,the water damage resistance of 30%vol.% disk method is better than that of the natural aggregate mixture.It is found that the fracture surface of the natural aggregate mixture extends along with the asphalt aggregate interface,and the aggregate itself does not damage.The strength of the crushed aggregate is low,so the fracture surface runs directly through the aggregate.However,the rough surface of the disk method aggregate makes the section of aggregate and asphalt bond more closely.The unique core-shell structure of aggregate makes cracks develop along the inner side of the shell.