Study On The Frost Resistance Of Geopolymer Concrete And Its Structure In Cold Area

Concrete is currently the most widely used building material in civil engineering.The production of cement is characterized with high energy consumption and large carbon emissions,and the resulting environmental pollution problems are becoming more and more serious;at the same time,the impact of industrial waste generated in the industrial production process on the ecological environment cannot be ignored.Therefore,the cement free construction material,fly ash-slag based geopolymer concrete that can fully consume industrial waste is expected to solve the contradiction between construction development and environmental protection.Existing studies have shown that geopolymer concrete has the advantages of rapid strength development,high temperature resistance,and good corrosion resistance,but the research on its frost resistance is relatively scarce.For concrete structures served in cold areas,poor frost resistance will bring great safety risks.In view of this deficiency,this dissertation studied the frost resistance of geopolymer concrete,and extended to the frost resistance of reinforced geopolymer concrete members considering the load effect,which can provide a reference for the application of geopolymer concrete in cold areas.In chapter 2,to prepare geopolymer concrete with excellent workability and mechanical properties,the following 7 variables were gradually determined through the three stages of tests for paste,mortar and concrete:including liquid/binder ratio（L/B）,water glass/sodium hydroxide solution mass ratio（SS/SH）,modulus of sodium silicate solution（Ms）,percentage of slag,curing manner,cement sand ratio and sand coarse aggregate ratio.The geopolymer concrete GPC-10 requiring high temperature curing was obtained,the 3d compressive strength reached 80MPa,and the workability was good.On this basis,increasing slag content was used to replace high temperature curing.The geopolymer concrete GPC-50（50%slag content）which can reach high strength under standard curing conditions was obtained,and the 28d compressive strength reached 80 MPa.Because the curing humidity has a significant influence on the strength development of geopolymer concrete with different calcium content,the humidity curing method had been further studied.A compressive strength development formula considering the curing humidity was established.In chapter 3,the frost resistance of geopolymer concrete was studied.A set of Class F fly ash-based geopolymer concrete with slag gradient was prepared.Rapid freeze-thaw cycle testing was carried out according to GB/T 50082-2009.The frost resistance was evaluated by mass loss,relative dynamic elasticity modulus and compressive strength loss.In addition,the microstructure and mineralogy were characterized by use of scanning electron microscopy（SEM）,mercury intrusion porosimetry（MIP）,energy dispersive X-ray spectrosocpy（EDS）and X-ray diffraction（XRD）.The results show that the frost resistance of geopolymer concrete increased with the increase of slag content.Also,125 freeze-thaw cycles marked an inflection point for the freeze-thaw resistance of GPC-50.The compressive strength damage rate was changed here,and the damage characteristics of the inflection point were tested by SEM and MIP;an initial crack with a width of 1～3μm was produced in the gel.Finally,through EDS and XRD,the reasons for the improvement of frost resistance were analyzed;low-calcium product N-A-S-H was produced in GPC-10,while high-calcium products C-A-S-H and C-S-H were produced in GPC-50.The different roles and destinations of water in the reaction process affect the compactness of the gel structure,thereby changing the frost resistance of the geopolymer concrete.In Chapter 4,fibers were used to improve the weak frost resistance of low-calcium geopolymer concrete.Different fibers and different volume content were taken into account.The results show that because of the high elastic modulus and good dispersion of PVA fiber,0.3%PVA fiber has the best effect on improving both the mechanical properties and frost resistance of GPC.The addition of fiber can not inhibit the initiation of microcracks but can suppress their propagation,resulting in a faster decrease in the relative dynamic elastic modulus and a slower mass loss of the fiber-reinforced specimens.In chapter 5,the frost resistance of geopolymer concrete under axial load was studied.A compressive stress of 10MPa or 20MPa was applied to the specimens,and then freeze-thaw cycles were performed.The test results show that the compressive stress less than 0.25f_cu can improve the frost resistance of geopolymer concrete.The water penetration depth and MIP test results verified that the applied compressive loading further compacted the concrete,which significantly improved the frost resistance.Besides,due to the high elastic modulus of S fiber,S fiber-reinforced GPC has better frost resistance than the other two fiber concretes under no load.However,when the load is applied,S fiber inhibited the compaction of the matrix,causing its frost resistance degrading to the weakest of the three types of fiber reinforced concrete.Finally,in chapter 6,the coupling damage of sustained flexural loading and freeze-thaw cycling on reinforced beams was analyzed.One specimen per beam type was used to conduct a coupling test of the freeze-thaw cycling and sustained flexural loading and then compared with both the control specimen（without any damage）and the specimen subjected to individual freeze-thaw cycling.The sustained flexural loading applied is about 30%of the ultimate bearing capacity.Analyses were conducted on the damage of the beams in terms of the following four aspects:failure mode,ultimate flexural capacity,bending stiffness,and surface cracking.It is found that,due to the rapid decline in the tensile strength and bond strength of the materials,the diagonal tension failure and debonding failure occurred before the flexural failure in some beams.The sustained flexural loading inhibited the damage of the compression zone and avoided the over-reinforced failure of the beam.In terms of the ultimate bearing capacity,sustained flexural loading was harmful to GPC beam,but beneficial to OPC beam.In addition,the sustained flexural loading can improve the performance of the beams;it can inhibit the stiffness degradation of a beam and relieve the fatigue damage produced by the drastic temperature variation.