Study On Mechanical Properties And Durability Of Fiber-polymer Mortar And Concrete

With the continuous increase of the construction scale of highways,bridges,airports,buildings,etc,cement concrete has become the most used construction material in China,and the consumption of cement ranks first in the world,which seriously damaged the ecological environment.As a new type of environmentally friendly cementitious material,geopolymer has great advantages and development potential in alkali-activated cementitious materials.Its biggest advantage is the simple preparation process and low production energy consumption,which can reduce the ecological environment damage caused by infrastructure construction.However,in terms of the mechanical properties of geopolymer mortar and geopolymer concrete,the emphasis on compressive strength has limitations,the mechanism of freeze-thaw damage is not clear,and the fatigue life of geopolymer concrete under the alternating effects of freeze-thaw and fatigue is lacking.Therefore,geopolymer mortar and geopolymer concrete are rarely used in road repairs and new roads in seasonal freezing areas.In order to enable geopolymer concrete to be used in seasonal freezing area projects,systematically study the mechanical properties and frost resistance of geopolymer mortar and geopolymer concrete,analyze the mechanism of freeze-thaw damage,and propose effective improvement methods.It is of great practical significance to predict the freeze-thaw fatigue life of geopolymer concrete and evaluate the damage degree.This article combines the "Fundamental Research Funds for the Central Universities(2572017BB05)",combined with the requirements of pavement materials in seasonal freezing areas,and conducts research on the mechanical properties and strength formation mechanism of geopolymers,and conducts research on fiber geopolymer mortar and fiber geopolymer.Experimental research and mechanism analysis on the mechanical properties,frost resistance and freeze-thaw-fatigue properties of concrete was carried out.The aim is to develop fiber geopolymer mortar repair materials with good mechanical properties and durability and fiber geopolymer concrete that meets the requirements of roads in seasonal freezing areas.The main research contents and results of this paper are as follows:(1)Metakaolin is used as Silicon aluminum raw material,and water glass mixed with NaOH is used as alkali activator to prepare geopolymers,and the mechanical properties of geopolymers are improved by mixing fly ash and slag.Through single factor test,the influence of sodium silicate modulus,Na2O equivalent,fly ash and slag content on the setting time and strength of geopolymer pure slurry were studied.The XRD,FTIR and SEM on the phase,molecular structure and micro morphology of geopolymer were performed to characterize and analyze microstructure and reaction mechanism of geopolymers.The research results show that the setting time of geopolymers increases with the increase of modulus,and decreases with the increase of Na2O equivalent.Fly ash and slag function as retarders and early strength agents,the n(SiO2)/n(A12O3)is an important factor affecting its strength.Metakaolin(P100F0S0M1.4N16),fly ash-metakaolin(P60F40S0M1.2N12),slag-metakaolin and fly ash(P60F0S40M1.2N14),fly ash,slag-metakaolin(P60F20S20M1.2N12)has the best mechanical properties when its molar ratio is 2.97,3.34,3.31 and 3.27,respectively.By XRD,FTIR,and SEM analysis of the geopolymer reaction mechanism,P60F20S20M1.2N12 has the most complete polymerization reaction.The formation of amorphous aluminosilicate is the most condensed,the cementation between the gels is the best,and the mechanical properties are the best.(2)On the basis of P100F0S0M1.4N16,P60F40S0M1.2N12,P60F0S40M1.2N14 and P60F20S20M1.2N12 geopolymer slurries,the effect of polypropylene fiber length and blending method on the flexural strength,compressive strength and toughness of geopolymer mortar(compressioncompression ratio)was researched.the best blending method of fiber is finally determined.The frost resistance research is carried out,and fiber reinforcement is analyzed.The mechanical properties and frost resistance mechanism of geopolymer mortar.The research results show that the flexural strength and toughness of geopolymer mortar is the best when 15 mm fiber is added,and the compressive strength of geopolymer mortar mixed with 12 mm fiber is the best.The mechanical properties of geopolymer mixed with 9 mm and 18 mm fiber are the best,and it is better than the geopolymer mortar with single fiber blend.P60F20S20M1.2N12 geopolymer mortar has a coMPact structure,and the blending of 9 mm and 18 mm fibers has the best frost resistance,and the number of freeze-thaw cycles can be up to 90 times.(3)Based on the design of fiber geopolymer mortar mix ratio,the mechanical properties,frost resistance and salt freezing resistance of fiber geopolymer concrete were studied.The frost resistance mechanism of geopolymer concrete was analyzed through the pore structure.The research results show that P60F20S20M1.2N12 geopolymer concrete mixed with 9 mm and 18 mm polypropylene fiber has the best mechanical properties,and the flexural and compressive strengths reach 6.5 MPa and 45.3 MPa,respectively.The frost resistance of geopolymer concrete varies with the internal porosity of the structure.And the ratio of harmful pores increases and decreases,P60F20S20M1.2N12>P60F0S40M1.2N14>P100F0S0M1.4N16>P60F40S0M1.2N12,and the number of freeze-thaw cycles with polypropylene fiber is increased by 25,25,25 and 50 times respectively.Incorporating polypropylene fibers can also effectively improve the saltfreezing resistance of geopolymer concrete.Based on the Lemaitre effect variation principle,the freeze-thaw damage equation of P60F20S20M1.2N12 geopolymer concrete was established by the least squares linear regression prediction method.(4)Taking P60F20S20M1.2N12 polymer concrete as the research object,two freeze-thaw medium environments of water and 3.5%NaCl solution fatigue characteristics after freezing and thawing were studied.Based on the theoretical basis of fatigue mechanics and the Weibull distribution model,the freeze-thaw fatigue equation under different reliability probabilities of geopolymer concrete and the freeze-thaw fatigue damage evolution equation under different freeze-thaw cycles have been established.The research results show thatP60F20S20M1.2N12 geopolymer concrete has better resistance to salt freezing,which can be applied to repairing and constructing road surface in seasonal freezing area.The fatigue equations under different reliability probabilities established for can provide references and basis for the fatigue design of geopolymer concrete pavements.