Study On Composition Design And Mechanical Performance Of Ultra-high Performance Geopolymer Concrete

Ultra-high performance concrete(UHPC)with excellent mechanical properties and durability is prepared based on the design of the theory of close packing of raw materials,and through employing ultra-fine powders,and superplasticizer,so as to reduce the water-to-binder ratio,optimize porosity and improve the matrix compactness.However,high amount of Portland cement(800-1200 kg/m~3)is required in the fabrication of UHPC,which significantly increases the preparation cost and environmental burden.Geopolymer with good mechanical performance and durability,which is produced by chemically activating industrial solid wastes containing aluminosilicates,is regarded as a green binder with lower production cost and carbon emission.Therefore,utilizing geopolymer to prepare UHPC would be helpful to realize the effective combination of performance indexes and green goals,which meets the new requirements of high performance construction and building materials under the national policy“Carbon Peaking&Carbon Neutrality”and has outstanding practical significance.However,there have been few researches on the preparation of ultra-high geopolymer concrete(UHPGC),and the theory on the composition design and fabrication technology of UHPGC still remained rudimentary.Therefore,given the inherent characteristics of geopolymer and UHPC,this study was conducted based on the three essential scientific issues:(1)Development of composition design with multiple performance requirements of UHPGC binder system,(2)Mechanism of hardening and reaction product formation of UHPGC and(3)Steel fiber reinforcing and toughening mechanism in UHPC.First,based on the simplex-centroid design method with axial design,the influence of each component amount variation of slag-fly ash and silica fume-based geopolymer mortars with activators having different modulus on the flowability,initial and final setting times and compressive strength was quantitively studied.Then,a binder composition range of slag,fly ash and silica fume-based geopolymer mortar that meets the requirement of high workability,acceptable setting times and high compressive strength was acquired,as the basis of the binder composition for UHPGC.The results showed that incorporating silica fume could significantly improve the flowability and setting time when the activator modulus was greater than 1.0.Furthermore,the combination of slag,fly ash and silica fume had synergic effects on the improvement of the compressive strength and the decrease of the porosity.Particularly,when the activator modulus was 1.5 to 2.0,the content of slag was more than 50%,and the contents of fly ash and silica fume were less than 30%,the geopolymer mortar met the performance requirements of high flow performance(>210 mm),reasonable setting time(>120 min)and high compressive strength(>90 MPa).In addition,the addition of silica fume significantly affected the rheological properties,reaction kinetics and product formation of slag-fly ash-silica fume based geopolymer.The plastic viscosity and yield stress of geopolymer paste decreased with the increase of silica fume dosage from 0%to 20%.The addition of silica fume increased the initial heat release rate,but delayed the acceleration period and reduced the cumulative heat release.Second,after establishing the binder composition of UHPC,FTIR,SEM,TEM and the combination of laser diffraction-based particle size distribution(LD-PSD)analysis and selective dissolution treatment were conducted to investigate the influence of silica fume dissolution on the reaction product formation and microstructure of UHPGC matrix,so as to clarify the physicochemical effect of silica fume in UHPGC.The results showed that the incorporation of silica fume induced the deceleration of reaction,the increases of the polymerization degree of C-A-S-H and N-A-S-H,and promoted more N-A-S-H gel formed.The reactivity of fly ash was inhibited by adding silica fume.LD-PSD analysis indicated that the particle size distribution of the residue powders after the selective dissolution treatment presented the residue from the sample at the later age was finer,implying that more fly ash mainly participated in the reaction after.The physicochemical effect of silica fume in UHPGC can be summarized as follows:at the early stage of reaction,silica fume dissolves rapidly,thereby consuming OH~-,changing the concentration of silicate ions in liquid phase and its polymerization degree.As a result,the parameters of the activator were changed and the dissolution of other precursors was impacted;As the reaction continues,silica fume and other raw materials participate in the reaction.Combined with SEM and TEM,it was found that there were still unreacted silica fume particles in the later reaction stage,which refined the pore structure and provided a possibility to improve the mechanical properties.Third,the influence of steel fiber content,length,diameter and shape on the mechanical performance of UHPGC was systematically investigated,and the key parameters affecting the reinforcing and toughing efficiency of steel fiber on UHPGC were discussed in order to provide scientific basis for rational selection of steel fiber suitable for UHPGC.The results showed that UHPGC had a better flowability.The compressive strength of steel fiber-reinforced UHPGC under different curing conditions could reach 150 MPa,which is comparable to that of UHPC,but the flexural performance is inferior to that of UHPC.The effect of steel fiber aspect ratio on the flowability and mechanical property of UHPGC was controlled by the actual number of fibers in the specimen.Different from UHPC,UHPGC matrix had high brittleness and was sensitive to fiber deformation ratio.Steel fiber with high deformation ratio possessed an inferior reinforcing and toughening efficiency on UHPGC.Long and thin straight steel fibers had the best reinforcing and toughening efficiency on UHPGC.The mechanical property of UHPGC specimens contain2%straight steel fibers with a length of 13 mm and a diameter of 0.12 mm are higher than those of other fibers with a volume fraction of 3%,so as to reduce the amount of steel fibers without sacrificing mechanical property.Fourth,the coupled effects of the amounts of steel fiber and silica fume on the mechanical and fracture properties of UHPGC were systematically investigated.It was found that the elastic modulus of UHPGC was lower and the splitting tensile strength was higher,so the existing empirical equations were difficult to be suitable for UHPGC.The fracture resistance curve of UHPGC was different from that of UHPC,and the stress intensity factor K_Iincreased gently when the effective fracture length was less than 60 mm,indicating that the UHPGC matrix was highly brittle.The mechanical and fracture properties of UHPGC decreased sharply when the silica fume content was more than 10%,which was attributed to the rapid dissolution of silica fume that changed the activator characteristics.When the silica fume content exceeded 10%,the homogeneity of the matrix was improved,which improved the bonding property of UHPGC matrix between steel fiber.Thus,on the premise of ensuring the mechanical properties of UHPGC,the dosage of silica fume could be increased and the dosage of steel fiber could be reduced.Fifth,the hybrid steel fiber reinforcement method was proposed and the contribution of each type of steel fiber in the steel fiber hybrid reinforcement to the compressive strength,four-point bending and direct tensile performance of UHPGC was systematically investigated.The results showed that the different steel fiber hybrid combinations had different reinforcing and toughening efficiency on different mechanical performance of UHPGC.The straight steel fiber with lengths of 8 mm and 13 mm had significant effect on improving the compressive strength,bending and direct tensile properties of UHPGC.The hybrid reinforcement of three straight steel fibers with different lengths(6 mm,8 mm and 13 mm)had a negative synergistic effect on the flowability,compressive strength and four-point bending property of UHPGC,but exhibited a significant positive synergistic effect on the improvement of direct tensile property.On the basis of these results,comparing the mechanical property of UHPC,an optimized composition range of UHPGC steel fiber hybrid reinforcement with compressive strength>150 MPa,bending strength>18 MPa,ultimate axial tensile strength>5 MPa and strain>1500μεwas proposed.