Composition Design And Corrosion Resistance Research Of Geopolymer-based Ultra High Performance Concrete

In recent years,geopolymer concrete（GPC）has attracted much attention because of its many advantages such as environmental protection,high temperature resistance and low cost.However,co MPared with ordinary concrete,GPC has the shortcomings of short hardening time,high brittleness,unstable strength,and large shrinkage of hardened paste,which does not meet the increasingly high requirements of construction technology.In order to develop geopolymer ultra-high performance concrete（UHPGC）that can meet engineering needs,this project uses mineral powder,fly ash,silica fume and micro silica fume as the cementing material of UHPGC,and chooses three different grades of sand as the aggregate,use water glass and sodium hydroxide to prepare a composite alkali stimulant.By studying the modulus of alkali activator,polycarboxylic acid water-reducing agent,Al₂O₃ and Ca O content of the precursor,Si O₂/Al₂O₃ molar ratio and Ca/（Si+Al）composition,the UHPGC work performance,mechanical properties and corrosion resistance are studied.To further analyze the reaction mechanism by using microscopic detection methods.In addition,different types of steel fibers are used to overcome the brittleness of the geopolymer concrete matrix.The results show:（1）When the modulus of the alkali-excited solution increases,the fluidity of UHPGC decreases and the setting time is prolonged.When the modulus is 1.32,its strength reaches the maximum value;（2）The increase of the polycarboxylic acid water-reducing agent can increase the fluidity of UHPGC,but its setting time and mechanical strength are not obvious;（3）The increase of Al₂O₃ content in the precursor can improve the fluidity and prolong the setting time,and the strength reaches the peak when the Al₂O₃ content is 0.14;（4）Ca O increase in content can reduce fluidity and shorten the setting time,and the strength reaches the peak when the content of Ca O is 0.3;（5）When the content of Ca O is the same,the increase in the ratio of Si/Al reduces the fluidity,and the setting time is shortened first and then prolonged.When Si/Al ratio>3,the strength gradually decreases;（6）When Ca/（Si+Al）is lower than 0.2,higher Si/Al can obtain high strength,and Ca/（Si+Al）is higher than 0.2,the smaller Si/Al is more beneficial to strength.Through microscopic examination,it is found that the main solidified products of UHPGC are zeolites,hydrated calcium silicate,hydrated calcium aluminosilicate and dolomite.When the content of Al₂O₃ accounts for 0.14 of the precursor,the polymerization reaction is relatively sufficient,the overall morphology and structure are smooth and co MPact.In addition,albite appears in a low-calcium system,and the aluminosilicate is gradually converted into calcium silicate hydrate with the increase of the content of Ca O.When the content of Ca O is 0.3,the ratio of Si/Al increases and the number of oligomerized Si（OH）₄ monomers increases,indicating that the polymerization reaction is insufficient.In the case of sodium sulfate solution corrosion,the UHPGC strength loss of the high-calcium system and the high-aluminum system is relatively large,and the high-calcium system mainly undergoes a serious decalcification reaction,resulting in the precipitation of gypsum from the matrix.The high-alumina system also undergoes a decalcification reaction,forming an expanding substance similar to ettringite,which causes the matrix to appear discrete geometric blocks under the action of the expansion stress.The matrix with a Ca O content of 0.3 and a Si/Al ratio of 3 is more capable of decalcification reaction,causing structural chemical shrinkage.By adding five different types of steel fibers,it is found that the greater the length of the steel fibers of the same diameter,the more favorable the flexural strength of UHPGC,while the fluidity and compressive strength are reduced.If the volume is increased,the mechanical properties can be improved,but it is not conducive to fluidity.For steel fibers of the same length,the larger the diameter,the better the fluidity,but the mechanical properties are reduced.It is observed that the straight steel fiber and UHPGC have good bonding performance and coordination performance at the interface of steel fiber-UHPGC,which can improve its brittleness and crack resistance.