Application And Theoretical Research Of Activated Coal-series Kaolin As Cement Admixture

The utilization of coal-series kaolin after roasting activation as cement admixture will recycle this discharged resources,enhance hydration properties,mechanical strength and corrosion resistance and reduce CO2 emission and energy consumption,which performs high efficient utilization of resource and ecological environmental protection.However the related researches focused on the cement factors and lacked of systematic in-depth study about the properties of coal-series kaolin,meanwhile there were also lack of some researches about the activation and hydration mechansisms of cement containing activated coal-series kaolin,which had negative effects on the comprehensive application of coal-series kaolin in cement and concrete fields.Based on the mineralogical properties of coal-series kaolin from Yichang,Hubei,the pozzolanic activity of coal-series kaolin was activated by roasting and controlling granularity.The influence of mineralogical properties of coal-series kaolin on pozzolanic activity and strength was revealed.Besides the hydration properties and mechanisms,hydration dynamics and corrosion resistance was systematic studied.In the research process the mentioned problems and deficiencies were aslo discussed.The main conclusions are as follows:The main valuable mineral was kaolinite and its Hinckley Index(HI)was 0.84.After roasting activation,kaolinite would transform into metakaolinite with high pozzolanic activity.The pozzolanic activity was mainly depended on activation temperature and fineness.The optimal activation temperature(800°C)and finer particle size had higher pozzolanic activity.Modified Chapelle test and active aluminum dissolution method would overestimate the pozzolanic activity of metakaolinite with remained hydroxyls in structure after incomplete dehydroxylation due to the strong agitation,temperature and pressure.The Frattini test could accurately assess pozzolanic activity but the assessment time was long.There are three effects of metakaolinite in cement mortars such as dilution,physical and chemical effects in the strength contrast method.The dilution effect had a positive correlation with replacement level and physical effect was mainly affected by fineness in the early ages,while chemical effect was primarily influenced by activation temperature and fineness in the later ages.The pozzolanic activity had a positive correlation with chemical effect.Based on the relationship between pozzolanic activity and strength,strength contrast method was not suitable for the pozzolanic activity measurement at 3 days.The association of the four assessment methods will be convenience,accuracy and celerity.The hydration dynamics was modelled by Knudsen extrapolation equation and Krstulovic-Dabic model based on the heat data of cement pastes admixed with metakaolinite.The results showed that the hydration was controlled successively by NG-I-D stage.In the nucleation and crystal growth(NG)stage,the strong physical effect of metakaolinite would accelerate the hydration reaction and reduce hydration resistance,leading to the reduction of reaction order n but increase of reaction rate constant K1'.In the phase boundary reaction(I)and diffusion(D)stages,physical effect gradually diminished and hydration rate radically reduced,demonstrated the slowing of hydration process.The chemical effect had some acceleration effect in D stage,leading to a slight increase of reaction rate constant K3'.As the cement admixture,metakaolinite had the strong influence on hydration progress that was reflected in variations of combained water,rection degree of metakaolinite and hydration degree ?OPC.Results showed that the major factor was chemical effect.Due to the dilution effect,the utilization of metakaolinite as cement admixture would reduce the total combained water,while the excellent physical and chemical effects would increase the equivalent combained water when considering the dilution and chemical effects.Based on the rection degree of metakaolinite and CH content,the CH decoupling method showed that metakaolinite would consume hydration products CH at 3-28 days,leading to the reduction of Ca(OH)2 but the increase of hydration degree ?OPC,which performed the the strong acceleration of metakaolinite in cement hydraton.Comparing with these two methods,CH decoupling method was more accurate than equivalent combained water method when studying the influence of metakaolinite on hydration process.The related hydration mechanisms was analyzed by the phase transformation,morphology,Ca/Si value and thermal stability.The results showed that the pozzolanic reaction of metakaolinite would increase polymerization degree of C-S-H,decrease Ca/Si value,densify the paste texture,refine the pore size and enhance the thermal stability,showing the excellent hydration properties.When suffering Mg SO4 and seawater,the generated densified C-S-H gels would hinder the corrosion of inimical ions,leading to the reduction of expansion,strength loss and corrosion products,which provided an excellent corrosion resistance to Mg SO4 and seawater.