Study On Sulfate Corrosion Resistance And Micro-structure Characteristic Of Fly Ash Lightweight Aggregate Concrete

Lightweight aggregate concrete not only meets the demand of strength, but also reduces dead weight which can effectively improve the bridge span, lower the thickness of beam and decrease bridge piers due to its better corrosion resistance for matrix and less treatment fees for foundation. And all these characteristics provide the related theory and technical support for concrete preparation and application in bridge and high-rise buildings.In this paper, cement was replaced by equivalent fly ash, to prepare lightweight aggregate concrete applied to long-span bridge and high-rise building. By systematic experiment, early mechanical property and sulfate corrosion resistance of lightweight aggregate concrete were researched, including preparing concrete, mixing performance and early mechanical property etc. Based on the systematic test, effect of sulfate on fly ash lightweight aggregate concrete during drying-wetting cycle were researched; meanwhile, mercury intrusion porosimetry and industrial computed tomography were introduced to access the internal meso-crack propagation law and pore change, and then the related factors were summarized as follows:(1) Based on the test of mix proportion, effects of fly ash, coarse aggregate types and pre-wetting time on lightweight aggregate concrete were studied. As the results show, the higher fineness of fly ash, the better working performance of lightweight aggregate concrete; Compared with crushed lightweight aggregate, spherical lightweight aggregate concrete has better fluidity and less water retention, coupling with segregation; At the same water-binder ratio and when the range of fineness modulus of sand was from 2.4 to 2.8, the workability of lightweight aggregate concrete reached to the optimum value.(2) Cement was replaced by different fineness or quantity of fly ash, the early compression strength decreased, and then the mechanical property of lightweight aggregate concrete were researched. As the results show, compression strength of LWAC at 3 days are lower than the standard samples due to the weaker early activity; the higher fineness of fly ash, the fast increase degree of LWAC, however after 28 days, it has a weaker effect on the compressive strength. (3) 30% ordinary sand was replaced by grade I fly ash of the same volume, meanwhile, cement was replaced by equivalent ultra-fine fly ash or silica fume, to investigate the effect of sulfate corrosion resistance on lightweight aggregate concrete. As the research results show, after a period of drying-wetting cycle for 20 times with the solution ranging from 0% to 10%, the compression strength reached to the maximum value and then decreased fast; Under the condition of drying-wetting cycles, mineral admixtures could improve the corrosion resistance of lightweight aggregate concrete to a certain extent, enhance the adhesive property between lightweight aggregate and mortar, improve lightweight aggregate-mortar interfacial transition zone, and delay further erosion of sulfate on the structure of lightweight aggregate concrete.(4) Based on preparing the lightweight aggregate concrete, mercury injection test was introduced to investigate the effect of mineral admixtures on the internal pore of lightweight aggregate concrete. As the results show, mineral admixtures make the internal aperture of lightweight aggregate concrete decrease, and the volume fraction of aperture between 50nm and 100nm of fly ash lightweight concrete increased obviously. The volume fraction of aperture less than 100nm of lightweight aggregate concrete admixed ultra-fine fly ash or silica fume was 1.39 times and 1.46 times of that of ordinary concrete.(5) Herein two kinds of lightweight aggregate concrete were prepared with crushed shale ceramisite and spherical shale ceramisite respectively. Tests of industrial CT and drying-wetting cycle in sulfate solution were adopted to study the CT numbers in the section, crack propagation law and influencing factors of crack propagation rate of lightweight aggregate concrete. As the results shown, after 50 drying-wetting cycles subjected to 5% Na2SO4 solution, the failure surface is mainly throughout the aggregate. Referring to the relationship curve between CT numbers in the section and stress, the process ranging from the appearance of mesoscale crack to the failure of the whole structure could be divided into four stages:pressure consolidation, crack initiation and development, crack propagation and penetration, damaged.