| Frequent fires in tunnels pose a serious threat to the safety of structures.As a traditional building material,concrete loses almost its strength when the temperature exceeds 1000°C.The fire will cause the concrete structure to collapse and da mage.Therefore,it is very necessary to reinforce a layer of heat-resistant thermal insulation material for concrete to improve its fire resistance.Geopolymers and lightweight thermal insulation fillers have superior performance in high temperature resistance and thermal insulation respectively.A large number of studies have shown that geopolymers prepared with metakaolin and fly ash as raw materials have good high temperature resistance.The light-weight heat-insulating fillers,mainly microbeads,vermiculite and perlite,have good heat-insulating properties due to their large porosity.Therefore,the use of geopolymer as a cementitious material,compounded with lightweight insulation fillers,to prepare a composite material with both high temperature resistance and thermal insulation performance will have very broad application prospects.In this paper,potassium sodium silicate(compounded with potassium hydroxide to adjust modulus)is used as alkali activator,fly ash and metakaolin are mixed as raw materials,and microspheres,vermiculite and perlite are used as heat insulation fillers to prepare geopolymer mortar.First study the influence of the concentration(40%,44%and 48%)and modulus(0.75,1,1.25,and 1.5)of the alkali activator on the mechanical properties,microstructure and phase components of the geopolymer mortar.Optimum alkali concentration and modulus;secondly,study the influence of the ratio of insulation filler on the mechanical properties and insulation performance of the mortar,and select the best ratio of insulation filler;then test the optimum ratio at room temperature and high temperature(200 ℃,400 ℃,600 ℃,800 ℃ and 1000 ℃)compressive strength and related thermal parameters(thermal conductivity and specific heat capacity)of the mortar.X-ray diffraction(XRD),scanning electron microscopy(SEM)and other test methods are used to characterize the phase composition and microstructure of the geopolymer,and the law of the various properties of the geopolymer changing with the above factors is proved.Finally,Abaqus finite element software is used to simulate and calculate the temperature field of the concrete slab under the protective layer of geopolymer mortar under different thicknesses.The research results show that:(1)The geopolymer mortar specimen has the best alkali concentration(44%)and modulus(1).The combination ratio has the highest compressive strength at room temperature,22.24 MPa,and 55 after 1000℃.% Relative residual compressive strength,the microstructure of the specimen is relatively dense at room temperature,and there are a large number of flocculent hydration products.After a high temperature of 1000 ℃,a large number of crystals are embedded and stacked.(2)With the increase in the content of various types of insulation fillers,the compressive strength of geopolymer mortar generally decreases,and its fire resistance time is generally increased.Considering the high temperature residual comp ressive strength and fire resistance time,the final determination of the insulation filler The mixing ratio is 16% of microbeads,10% of vermiculite,and 14% of perlite.(3)When the temperature is lower than 400℃,the compressive strength of the geopolymer mortar specimens is almost always stable or slightly increased.When the temperature is higher than 400℃,the compressive strength of the mortar specimens decreases significantly.The specimen under the best ratio still has a residual compressive strength of 10.53 MPa and a strength retention rate of 52% after being subjected to a high temperature of 1000℃,and the thermal conductivity at a high temperature of 1000℃ under this ratio is 0.1907(W/m k),The fire resistance time can reach 72 min.(4)When the temperature is lower than 400℃,the main hydration product of the pure geopolymer slurry of the best mix ratio group is amorphous gel,in addition to a small amount of potassium feldspar and muscovite;it starts to occur at 800℃ The crystal phase transforms to generate leucite and kacinite.After the high temperature of 1000 ℃,the diffraction peaks of leucite and saccharite are stronger.(5)When the geopolymer mortar under the best mix ratio is at room temperature to400°C,the number of hydration products keeps increasing and the microstructure is dense.As the temperature continued to rise,cracks began to appear in the matrix,and the number of hydration products began to decrease significantly,accompanied by the generation of a large number of pores.After the high temperature of 800°C and 1000°C,the structure of the hardened body appears porous,and a large number of crystalline substances with regular shapes are connected to each other and stacked on each other.(6)The simulation results show that the geopolymer mortar coating can effectively prevent heat transfer to the concrete and delay the rise of concrete temperature.As the thickness of the coating increases,its thermal insulation performance is more excellent,and the temperature gradient inside the concrete gradually decreases;along the concrete slab The node temperature in the thickness direction decreases gradually,and the closer the distance to the fire surface is,the greater the temperature gradient;on the concrete surface,perpendicular to the direction of the concrete slab,as the distance from the center fire point increases,the temperature and temperature gradient gradually decrease.(7)Considering the temperature distribution and temperature gradient of the concrete structure under different thickness coatings,it is finally confirmed that the optimal thickness of the protective layer of the geopolymer mortar is 10 mm. |
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