| With the reform of energy saving,emission reduction and the low-carbon in domestic cement industry,the active utilization of industrial wastes and the development of low carbon economy will be a vital direction for the future development of cement industry.The preparation of alkali-activated fly ash(AAFA)cementitious materials does not need to be calcined,the energy-consumption of production is low and the process of production is simple.Its development will be a useful way to efficiently utilize fly ash and complete the low-carbon reform of cement industry.The slow development of early strength is regarded as one of the main problem that restricts the application of AAFA cementitious materials.In view of the good conductive properties of AAFA based cementitious materials and the electrophoresis characteristics of hydration products "N-A-S-H" gels,direct current(DC)assisted curing AAFA based cementitious materials was used in this paper,the best curing method and the optimum proportion were obtained through the analysis of the factors affecting its mechanical properties.The evolution laws of the mineral composition and microstructure of hydration products under auxiliary DC electric curing were analyzed by means of XRD,SEM,NMR and MIP,and the AAFA concretes was prepared on the basis of the above research.Moreover,high temperature resistance of AAFA at 100-800 ℃was to be analyzed.The results showed that the inner temperature of AAFA cementitious material specimen first rose and then decreased under auxiliary DC electric curing.Its setting time could be effectively shortened and its early strength could be increased under short time(1 h)auxiliary DC electric curing.When the Na OH content is 15 wt.% and the water cement ratio is 0.38,the compressive strength of 3 d can reach 13.9 MPa at first 1 h with11 V DC voltage electric curing and then cured at room temperature.The migration rate of OH-in alkali solution into cementitious material system was to be accelerated and the internal temperature of slurry was to be increased under auxiliary DC electric curing.Under the dual effects of heat and electrochemistry,the dissolution of active Si O2 and active Al2O3 on the surface of fly ash beads was accelerated in the early stage of hydration action,and more [Si O4]4-monomers and more [Al O4]4-monomers were formed.At the same time,the migration rate and collision probability of [Si O4]4-and [Al O4]4-were increased,and the rate at which Q4(0Al)converts to Q4(1Al),Q4(2Al),Q4(3Al),Q4(4Al)was to be accelerated,which enhanced the reaction that monomers reconstitute polymers under the effect of direct current field,more aluminosilicate gels and hydroxysodalites were to be formed,which resulted in the increase of the proportion of harmless pores,the reduction of the ratio of multiple harmful pores,the decrease of porosity and the compactness of the structure.Early age compressive strength was to be increased.At high temperature,the apparent color of AAFA concrete specimen changed from dark color to khaki color with the increase of temperature,and a few cracks appeared in appearance.The drying compressive strength and the compressive strength after 200 ℃of 28 d were to be increased slightly.Because the AAFA cementitious materials was to be further hydrated at high temperature,which made its structure more compact and then made the compressive strength increased.However,when the temperature is above400 ℃,the compressive strength decreased continuously,the loss of the compressive strength reached 40% at 800 ℃.The internal water of the specimen was to be took off at high temperature,which made the appearance of the specimen change,the structure became more loose,and the bonding force between the paste and aggregate was to be decreased,and then the compressive strength of the specimen was to be decreased. |
