Preparation And Performance Study Of Phosphogypsum Based Self-leveling Mortar For Backfilling Of Floor Heating

The production and massive storage of phosphogypsum have caused serious social and environmental problems.Using phosphogypsum to prepare self-leveling mortar and applying it to the floor heating backfill system can not only effectively alleviate the storage pressure of phosphogypsum,but also solve the problems of shrinkage,cracking,and bulging caused by cement-based self-leveling mortar.However,the common phosphogypsum based self-leveling mortar matrix has many pores and good thermal insulation performance,which has the disadvantages of long temperature adjustment time,large temperature fluctuation,and low heat utilization rate when applied to ground heating backfill.Therefore,research is urgently needed to regulate and optimize the composition,structure,and heat transfer performance of phosphogypsum based self-leveling mortar,providing strong support for the resource utilization of phosphogypsum and energy conservation and consumption reduction of building structures.The project first studied the calcination process of preparing phosphogypsum from phosphogypsum,and clarified the influence of calcination process on the performance of phosphogypsum using gray correlation analysis.The results show that when the calcination temperature is 170 ℃,the calcination time is 2h,and the aging time is 3h,the properties of phosphorus building gypsum prepared are better.Its standard consistency water requirement is 0.61,initial setting time is 16 min,final setting time is 20 min,and 2h compressive strength is 4.31 MPa.The grey relational analysis results show that the calcination temperature has the greatest impact on the 2h compressive strength and standard consistency water requirement of phosphorus building gypsum,the calcination time has the greatest impact on the setting time of phosphorus building gypsum,and the aging time has a small impact on all properties.Secondly,the effects of cement and high-strength gypsum on the performance of phosphogypsum based self-leveling mortar were studied,and the mix ratio of phosphogypsum based self-leveling mortar was optimized.The results show that with the increase of cement content,the fluidity loss of phosphogypsum based self-leveling mortar increases in 30 minutes,its strength decreases in 1 day,and its strength decreases first and then increases in 28 days;With the increase of the content of high-strength gypsum,the initial fluidity and 30 min fluidity of phosphogypsum based self-leveling mortar increase,while the strength at 1 d and 28 d gradually decreases;When phosphorus building gypsum: cement: high-strength gypsum is mixed in a ratio of 7:1:2,the mortar has excellent working performance and the highest mechanical strength.Its fluidity in 30 minutes is 152 mm,initial setting time is 125 min,final setting time is 146 min,1-day compressive strength and flexural strength are 12.1 MPa and 3.57 MPa,28-day compressive strength and flexural strength are 28.70 MPa and 10.96 MPa,respectively,meeting the performance requirements of G25 grade in the standard JC/T 1023-2021.Finally,the effects of graphite powder,industrial graphene oxide,and expanded graphite/paraffin composite phase change materials on the physical,mechanical,and heat transfer properties of phosphogypsum based self-leveling mortar were studied.The results show that the incorporation of graphite powder into the phosphogypsum based self-leveling mortar reduces fluidity,shortens setting time,and decreases mechanical strength.The adverse effect of finer graphite powder on the performance of the mortar is smaller.The performance of the mortar mixed with 6% 1200 mesh graphite powder meets the standard requirements,and the thermal conductivity is increased by 36.8%;The addition of industrial graphene oxide can lead to a decrease in mortar fluidity and a reduction in setting time,but the mechanical strength will first increase and then decrease.The 28 d compressive strength of the mortar mixed with 0.6% graphene oxide increases to 32.08 MPa,while the thermal conductivity increases by 9.3%.The addition of expanded graphite/paraffin will not affect the fluidity and setting time of the mortar,but will reduce the mechanical strength and significantly increase the thermal conductivity.The thermal conductivity of the mortar mixed with 12% expanded graphite/paraffin will increase by 36.4%.The results of heat transfer simulation experiments for floor heating indicate that the indoor temperature of gypsum board filled with 6% graphite powder during heating is 1.0 ℃ higher than that of ordinary gypsum board,and the indoor temperature of gypsum board filled with 9% expanded graphite/paraffin wax during cooling is 1.2 ℃ higher than that of ordinary gypsum board.High thermal conductivity materials and phase change materials effectively improve the heat transfer efficiency of the phosphogypsum based self-leveling floor heating backfill system,reducing temperature fluctuations.