Effect Aging And Its Mechanism Of Action During The Hydration Process Of Complex Binder

Fly ash is a by-product of coal-fired power plants,which is widely applied in high volume civil engineering due to its good micro-aggregate,pozzolanic and morphological effect.However,due to the complex role of fly ash in concrete,the effect aging,effect and mechanism of action are not clear.Different people have different research theories of fly ash,which leads to the fact that the time-effect and mechanism of action of fly ash in cement-based composites are not clear.As a result,there is a big bias in people's understanding and use of fly ash.There is also considerable controversy in the analysis,which limits high volume fly ash to a certain extent.Therefore,to study the effect aging and mechanism of fly ash in concrete,that is,when the role of physical and when the chemical effect are great significance to the rational application for fly ash;In addition,due to the low early activity of fly ash,the early strength of high-volume fly ash concrete is too low,which induces early cracking of the concrete and the later strength and durability are reduced,which also limits its high-volume applications to a large extent.Therefore,how to effectively active the fly ash and explore its activation mechanism have become technical key and difficulties in the research of fly ash utilization.It is also the theoretical basis for the design and application of high-volume fly ash concrete.There were some contrastive study with quartz and fly ash of the similar fineness,including the strength of high,low-calcium fly ash-cement pastes,mortar and concrete chloride impermeability test to explore when it takes place physical or chemical effect ? On these experiments,the relative variation of relative Ca(OH)2 of fly ash-cement pastes and quartz-cement pastes at different ages,the hydration history,the evolution of micromorphology of hydration products,chemically combined water,fly ash hydration degree and pore structure,etc were tested by X-ray diffraction(XRD),thermo-gravimetric analysis(TG-DTG),scanning electron microscopy,micro-area energy spectrum analysis(SEM-EDS),nitrogen adsorption(BET)and infrared spectrum analysis(FIR).It further revealed effect aging,action effect andhydration mechanism in cementious materials.On the basis of the above experiments,commercially available chemical reagents such as calcium hydroxide,sodium silicate,sodium sulfate,and triethanolamine were selected for single-doped,multi-doped,and three-doped experiments.The optimum dosage was selected by orthogonal test.The optimum dosage,the best mix ratio and the best activator type were selected by orthogonal test.The microscopic test methods such as XRD,SEM-EDS and BET were used to reveal the activation mechanism of the activator.Through the above research,the main conclusions were as follows:(1)At 28 days,the flexural and compressive strengths of samples with low-calcium fly ash were 89.9% and 72.2% of the contrast group samples,respectively.Well,the flexural and compressive strengths of samples with quartz powder were 76.9% of the contrast group samples.At 56 days,the strength of the sample with low-calcium fly ash began to exceed the strength of the quartz powder-doped sample.The hydration degree of low-calcium fly ash at 56 days was only 1.36,indicating that fly ash played a physical filling role before 56 days;Volcanic activity began at 56 days.According to SEM-EDS,the surface of a small amount of low-calcium fly ash particles was slightly rough at 56 days.At this time,the hydration products were closely bonded and the structure was denser than earlier.At 180 days,the pore size of the samples with low-calcium fly ash was shifted from macropores(60 nm)to gel pores(10 nm)by BET.The samples with high-calcium fly ash are basically similar to the samples with low-calcium fly ash.The only difference was that the high-calcium fly ash started the pozzolanic effect at28 days.(2)The following conclusions were obtained by testing the low-calcium fly ash with single-doped,mixed-doped and triple-doped activators.The best single-doped activator were CH and TEA.The best dosage were 3% and 0.02% of cementitious material,respectively.The best compound-doped activator were CH+TEA and CH+Na2Si O3.The best dosage were 3.02% and 3% of cementitious material,and the best ratio were 1:150(TEA:CH)and 3:1(CH:Na2Si O3).The optimum dosage of three-doped activator was 1.02% of the cementitious material.The best ratio was2:75:25(TEA:Ca(OH)2:Na2Si O3).Compared with the undoped sample,the growthrate of the strength with single-doped activator did not exceed 25.9% within 7 days,and the compound activator was not more than 54.14%.The strength growth rate of the strength with three-doped activator did not exceed 86.5% within 7 days.(3)The relative content of Ca(OH)2 in the sample with activator decreased more significantly than that of the sample without activator within 7 days.Samples with the activator produced significant C-S-H gels,apparent irregular rose petal-like AFm,and apparent pits on the fly ash surface in the early stages of the later formation without activator.At 7 days,the most probable pore size of the sample with the activator was significantly reduced.