Preparation Of Refractory Lining UHPC And Mechanism Of Its High-Temperature Resistance

Based on the performance requirements of refractory lining materials for thermal equipment,this work focuses on the preparation of ultra-high performance concrete(UHPC)with high high-temperature resistance.By optimizing the composition of UHPC,the spalling resistance and residual property of UHPC were improved.A cementitious material with excellent performance for heat-resistant engineering and its preparation method was achieved.Firstly,the influence and mechanism of fiber and metakaolin(MK)on the spalling behavior of UHPC were studied,and the materials with high spalling resistance were selected.The results show that polypropylene(PP)fiber and steel fiber can significantly improve the spalling resistance of UHPC.PP fiber content can affect the spalling damage and initial temperature of spalling.The lower content caused the more serious damage and reduced initial temperature of spalling more significantly.The addition of MK increased the critical PP fiber content of UHPC,which was unfavorable to its spalling resistance.The content of MK should be controlled.The occurrence of spalling behavior mostly lay between 300 ℃～500 ℃.Secondly,the change and mechanism of residual properties of UHPC after high temperature were explored,including compressive strength,flexural strength,internal and external structure.UHPC available for refractory lining material was selected.It is found that PP fiber played a negative role on the impermeability and mechanical properties of UHPC after exposure to high temperature,and its dosage should be taken as the critical value.The mechanical properties of UHPC first increased and then decreased between 20 ℃ to 600 ℃.From 20 ℃ to 300 ℃,the hydration reaction promoted by high temperature effect and steam pressure enhanced the mechanical properties of UHPC.From 300 ℃ to 600 ℃,the strength of UHPC decreased under the effect of several adverse factors such as product dehydration,quartz transformation and pore coarsening.The addition of MK improved the residual mechanical properties of UHPC exposed to high temperature of 600 ℃,whose compressive strength were even higher than the strength under normal temperature.This was mainly attributed to the generation of hydration products with high-temperature resistant and increase of decomposition temperature of calcium silicate hydrate(C-(A)-S-H)subjected to high temperature as a result of addition of MK.The appropriate MK content was supposed to be 5%of the total amount of binder.In addition,there were no obvious cracks in the external and interior of the UHPC specimens in this work subjected to high temperature.The volume of pores with the diameter of 10 nm～100nm in UHPC increased after exposure to high temperature,and the number of pores in other sizes showed no obvious changes.The optimal choice as refractory lining materials was selected according to the residual properties.Finally,to verify whether the optimized UHPC was suitable for practical application,its service performance was tested,including its mechanical properties,pH value and impermeability after long-time exposure to 600 ℃.The influence of constant temperature time on the performance was analysed,and the specific application scope was determined.It is shown that as the constant temperature time was extended from 3 h to 6 h,the mechanical properties of UHPC decreased slightly.From 6 h to 9 h,the properties remained stable,and the compressive strength of UHPC can still achieve nearly 150 MPa.At this time,the color of UHPC profile was uniform,indicating that the phase reaction at 600 ℃ had been completed.During this process,the total volume of pores in UHPC increased slightly,and the newly-added pores were mainly small pores with the size of 10 nm～100nm with the pore whose size was above 100 nm unchanged.After exposure to 600 ℃,the pH value of UHPC decreased due to the decomposition of partial hydration products.With the extension of constant temperature time,the decrease in pH value continued in small degree.After holding for 9 h,the pH value of UHPC was higher than 12,remaining the high alkalinity.The UHPC at room temperature and exposed to 300 ℃ exhibited excellent impermeability with the impermeability grades of over P12.After exposure to 600 ℃,the impermeability grade of UHPC decreased to P3,whose impermeability degraded rapidly.Therefore,the refractory lining UHPC prepared in this paper can be applied to projects with working temperature of300 ℃ in contact with hot liquids and those with working temperature of 600 ℃ without contact with liquid.