| Today,advocating for green and sustainable development,the "dual carbon" goal is gradually being achieved.As a new type of environmentally friendly material,geopolymers play an important role in construction,civil engineering,and transportation engineering due to their simple preparation process,low production energy consumption,and the realization of solid waste resource utilization.However,geopolymers also have issues of high brittleness and susceptibility to cracking,which hinders further promotion and utilization.Therefore,this article first studies the effects of different chemical activators(water glass,NaOH solution,KOH solution)and slag replacement rates(10%,20%,30%,40%)on the performance of fly ash geopolymer,in order to determine the benchmark mix ratio of fly ash slag geopolymer.On this basis,carbon fibers with different lengths(6mm,12mm)and dosages(0.2%,0.4%,0.6%,0.8%,1.0%)and nano SiO2 with different dosages(1.0%,2.0%,3.0%)were used to toughen the fly ash slag geopolymer.The working performance,compressive strength,flexural strength,uniaxial tensile performance,fracture performance,shrinkage performance,and high-temperature resistance were studied to determine the optimal length,dosage,and nano SiO2 dosage of carbon fibers,The main research findings are as follows:(1)Firstly,determine the benchmark mix ratio of fly ash slag geopolymer.The sensitive factors for the compressive strength of fly ash geopolymer from large to small are water glass content,modulus,and aging time.Taking into account the compressive and flexural strength,this article determines the optimal activator as A2B3C2,which is water glass modulus 1.4,dosage 20%,and aging time 24 hours.The 28 day compressive and flexural strength of the geopolymer prepared from it are 17.6 MPa and 3.9 MPa,respectively.On this basis,the strength is significantly improved by adding slag.When the substitution rate is 40%,the working performance of the specimen is poor,and the improvement in flexural strength is not significant.Therefore,a slag content of 30%is selected as the basic mix ratio for subsequent modification research.(2)Secondly,the effects of different dosages of carbon fibers on the working performance,compressive strength,flexural strength,uniaxial tensile performance,fracture performance,and shrinkage performance of fly ash slag geopolymer were studied.The results show that the addition of carbon fibers can reduce the working performance of geopolymers,but at the same time,it can effectively improve their mechanical properties,uniaxial tensile properties,fracture properties,and shrinkage properties.At the same dosage,the effect of 12mm carbon fiber on fluidity and setting time is more significant than 6mm carbon fiber,and the polymer exhibits lower drying shrinkage value after 12mm carbon fiber is added;The improvement of 6mm carbon fiber on the pore structure,compressive and flexural strength,uniaxial tensile properties,and fracture properties of geopolymer is better than that of 12mm carbon fiber,and the above properties are achieved at the maximum value in the 6C-3 ratio.(3)Then,at the same fiber length,with the increase of carbon fiber content,the fluidity and setting time of the geopolymer gradually decrease,and the decrease gradually slows down;The porosity and water absorption first decrease and then increase,and both reach their minimum values when the dosage is 0.6%;The addition of carbon fiber can significantly improve the compressive bending strength,uniaxial tensile performance,and fracture performance of geopolymer,with an optimal dosage of 0.6%.At this time,the compressive bending strength of 6C-3 specimens is 35.7MPa and 5.9MPa,respectively.When the dosage exceeds this,the strength decreases;In terms of uniaxial tensile performance,a dosage of 0.6%has the best time-delay performance.The ultimate tensile strength of the 6C-3 specimen is 4.881 MPa,and the ultimate elongation is 0.407%;Under the same fiber length,the initiation toughness and instability toughness of geopolymer show a trend of first increasing and then decreasing with the increase of carbon fiber content.The initiation toughness of 6C-3 is 0.782MPa ·m1/2,which is 71.8%higher than FSG-0,and the instability toughness is 1.174MPa·m1/2,which is 73.5%higher;The drying shrinkage rate of geopolymer gradually decreases with the increase of carbon fiber content.Based on the above performance,the optimal length of carbon fiber is 6mm and the optimal dosage is 0.6%.(4)Then,the effects of different amounts of nano SiO2 on the working performance,compressive strength,flexural strength,uniaxial tensile performance,fracture performance,and shrinkage performance of carbon fiber reinforced fly ash slag geopolymer were studied.The results show that the addition of nano SiO2 reduces the flowability and setting time of freshly mixed polymer mortar;When the dosage is 2.0%,the porosity is the smallest and the compressive strength is the highest.The lower the porosity,the higher the compressive strength;In terms of uniaxial tensile properties,the addition of nano SiO2 did not significantly improve the tensile properties of geopolymers.The most significant improvement was observed when the content was 2.0%,with an ultimate tensile strength of 5.112 MPa and an ultimate elongation of 0.446%;Adding an appropriate amount of nano SiO2 can improve the fracture toughness to a certain extent,which can delay the cracking time of the geopolymer and maintain a more stable degradation and instability of the geopolymer.However,the improvement effect is limited,especially for the improvement of the initiation toughness.With the increase of nano SiO2 content,the drying shrinkage of the geopolymer gradually increases,but the growth of drying shrinkage is not significant in the first three days.When the age increases to 3-14 days,the drying shrinkage of the geopolymer increases faster.Based on the above,the optimal dosage of nano SiO2 was selected as 2.0%.(5)Finally,high-temperature tests were conducted on the geopolymer modified with carbon fiber and nano SiO2 to test its mass loss,strength and strength loss,uniaxial tensile performance,and linear shrinkage performance.The results showed that the mass loss rate was small and the increase was not significant within 400℃,but significantly decreased at 600℃,and significantly decreased above 800℃;At the same temperature,with the increase of carbon fiber content and nano SiO2 content,the compressive and tensile strength of the G-6C and G-NS groups first increased and then decreased.The G-NS-2 specimen with a 2.0%nano SiO2 content showed the best performance at 400℃.At this time,the compressive strength reached 54.4 MPa,an increase of 52.4%compared to room temperature,the initial crack strength was 5.221 MPa,the ultimate tensile strength was 5.765 MPa,and the ultimate elongation was 0.561%,which increased by 8.5%,12.8%,and 0.115%,respectively.As the calcination temperature increases,the linear shrinkage rate of the specimens gradually increases.When the temperature reaches 900℃,the linear shrinkage rate of each group of specimens reaches about 7%.Based on the above experiments,the optimal ratio for comprehensive performance is determined to be NS-2,with a 6mm carbon fiber content of 0.6%and a nano SiO2 content of 2.0%. |
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