Fly Early Activity To Stimulate Research And Its Mechanism

Recently, with the development of modem concrete technology and being deeply understood for the potential effect of fly ash in concrete, the study of using high volumes of fly ash concrete (HVFC) in civil engineering is paid more and more attention, especially in hydraulic structure of large volume concrete or large dam. However, researches are also indicated that the problem of over-low early strength should be concerned because of high volumes fly ash in HVFC. Therefore, if fly ash can be activated through suitable technological way and the distribute of its potential activity can be adjusted during hydrating period, then the early strength of HVFC can be obviously increased, the technology of utilizing fly ash on a large scale will be undoubtedly promoted to be perfect and practical. Meanwhile, the mechanism of activating fly ash is theoretical foundation on the design and application of HVFC and it is also a popular and difficult point in this'field.This paper focuses attention on the investigations of fly ash activator and the mechanism of activating.Chemical measure was taken to activate fly ash. Firstly, the mortar was selected as test carrier (fly ash 60%+cement 40%, by weight), seeking for activators that showed remarkable result. Secondly, examining the effect of combining activators and getting final prescriptions after optimizing activators dosage by means of orthogonal design. Lastly, the results of these prescriptions in HVFC (fly ash is 60% of total binder) were tested.Test results indicate that seven groups of optimized prescription were remarkable on activating fly ash in the mortar system. Comparing compressive strength with control group without activator, there was more than 150% growth in 3-day age and the highest one up to 210%. Besides, due to the dosage of activator used were in the range of 1.0%~1.5% (disregarding superplasticizer), it was economical and feasible. The early compressive strength of HVFC with some activators was also increased obviously, the highest one was up to 55% in 3-day age and its strength grade was improved from C25 to C35.According to the whole course of investigation, the result of activators was mainly depended on their chemical characteristics. Besides, there were two factors that should not be ignored: One was the effect of activators of reducing water-binder ratio of mortar or concrete, the other was the leveling of mixing.X-ray diffraction (XRD) technique was adopted to investigate the quantity change of Ca(OH)2 and the character of hydrated products of mortar system with or without activators. At the same time scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) technique were used to systematic analyze the morphology of hydrated products and elementary composition of microstructure. In addition, nuclear magnetic resonance (NMR) technique was used to test fly ash-cement system for the first time at home. And the activating influence of activators on structure of silicon-oxygen and aluminum-oxygen was analyzed. Besides, the macroscopic qualitative research was also combined to prove the effect of activating.The results show that fly ash was activated in early hydrate. The structure of silicon-oxygen and aluminum-oxygen was partly depolymerized into little activated molecular. And the net-modified ion (such as K, Na) or net-neutral ion (Al) from activator destroyed low activated glassiness in fly ash. Thus the more hydrated products owning early strength was appeared, such as hydrate calcium silicate, hydrate aluminum silicate and ettringite. What merited attention was, an unanimous result was received by XRD and SEM analysis: A large amount of C-S-H( I ) was produced owing to the function of activator and the result was identical with that of NMR analysis. 29Si-NMR analysis show that micro-structure of fly ash was changed from concentrated high-polymerization to scattered low-polymerization by the activator and the glassiness in fly ash was activated. 27A1-NMR analysis express that it is the function of activator to dest...