Study On The Preparation And Mechanism Of Alkali-activated Slag Tunnel Fireproof Coatings Containing Metakaolin And Magnesium Oxide

With the development of the economy,the fire prevention of tunnel construction has attracted the attention of the engineers.The tunnel fireproof coating with excellent fireproof performance and lower cost has become the focus of the research.An orthogonal test was used to study the influence of different dosage of metakaolin and magnesium oxide on the bond strength and fire resistance of alkali-activated slag tunnel fireproof coatings.Effect of the concentration of sodium hydroxide on its fire resistance,bond strength,dry density and drying time was studied.The influence of polypropylene fiber,silane powder,and silica fume on its impermeability was studied.Finally,an alkali-activated slag tunnel fireproof coatings of metakaolin and magnesium oxide with excellent fire resistance and lower cost was developed(PY coatings,P stands for metakaolin and Y stands for magnesium oxide).The type and morphology of hydration products and pore structure were characterized by XRD,MIP,TG-DSC and SEM.The results of the macro-micro study are as follows.1.When the dosage of metakaolin is 9.9%,the dosage of magnesium oxide increases from 0%?3.3%?6.6%.As the magnesium oxide is fire-resistant and acts as a filler,the fire resistance and bond strength of the PY coating continuously increases;when the dosage of magnesium oxide is 6.6%and the dosage of metakaolin was increased from 3.3%?6.6%?9.9%,Due to the dissolution of silicon and aluminum in metakaolin,hydration products such as C-S-H and hydrotalcite formed.the dissolved substances fill the pores of the PY coatings and the compactness was enhanced.The fire resistance and bond strength of the PY coating also continuously increase.Compared with the amount of metakaolin,the addition of magnesium oxide to the alkali-activated slag tunnel fireproof coatings have a greater improvement in the fire resistance and bond strength.The enduring fire limit time of the preferred group PY69(The contents of metakaolin and magnesium oxide were 9.9%and 6.6%,respectively)is 182 min,and the bond strength is 0.263 MPa.2.With the increase of sodium hydroxide concentration(1%?14%),the bond strength of PY69 coating is increased,drying time is reduced,dry density and fire resistance are little influenced by the hydration reaction fully,the pore structure is more compact and the distribution of pore size is more reasonable.The PY69-12 group.with sodium hydroxide concentration of 12%has better comprehensive performance.3.The bonding and bridging effects of polypropylene fiber inhibit the development of micro cracks;Silica fume mainly plays a role of physical filling;Due to the presence of hydrophilic functional groups in the silane powder,it will be hydrolyzed in an alkaline environment to form silanol group,and this hydroxyl group react with the hydration product of PY69-12 and combines with each other and bonds together.The internal interface transition region becomes dense and the pore diameter is small.The order of enhancing the impermeability of PY69-12 is:silane powder>polypropylene fiber>silica fume,and its optimum dosage is 0.6%(namely PY69-12-P0.6,among them,P0.6 indicates that the yield of the silane powder is 0.6%).4.For PY69-12-P0.6,the fire-resistant limit time is 196.5 min,the bond strength is 0.423 MPa,and the anti-permeability performance is good.The comprehensive performance of PY69-12-P0.6 is superior to the market-available coatings(namely Wei Cheng coating and Tianan Puning coating).PY69-12-P0.6 has lower cost and good application prospects compared to the market-available coatings.