Study On Hydration Properties And Strength Model Of Concrete Mixed With Recombined Fly Ash Particles

Fly ash is a common industrial waste,and yet is considered as Supplementary Cementitious Materials in construction field.It is usually considered as raw materials for cement or replaces partial cement in high performance concrete.How to make full use of fly ash has now become a research hotspot and controlling sizes of fly ash particles could effectively improve utilization rate of fly ash.Classification on particle size range of fly ash is not precise and complete in the fly ash particle size research,moreover,characterization on the properties of fly ash with varied particle sizes is not yet comprehensive enough.In addition,influence of fly ash particle size on the hydration and mechanical properties of concrete is also lack of research from quantitative aspect.Fly ash particles with single range sizes were prepared by sieving method and the sizes of fly ash particles conforming to typical distributional functions were then recombined.The properties of the recombined fly ash were also characterized.Subsequently,hydration properties of the cementitious materials mixed with the recombined fly ash were studied.Relationship between fineness of fly ash and compressive of concrete was modelled.Further,carbonation induced corrosion on steel in the concrete was taken as an evaluation criterion for structure failure of concrete.Relationship between fly ash fineness and service time of steel reinforced concrete structure was studied,which could provide convenience for guiding practical engineering application.The main content and results are as follows:（1）The fly ash particles recombined in accordance with typical sizes distribution functions was characterized.The particle size,micro morphology,composition,bond energy strength and activity index of fly ash with different particle size range and different particles sizes distribution functions were characterized using Malven Laser Particle Size Analyzer,SEM,EDS,XRD,XRF,TG-DSC and FT-IR.Results show that the fly ash prepared by sieving method（narrow and concentrated sizes range）and recombined method（recombining fly ash with narrow and concentrated sizes）can obtain fly ash with expected particle size distribution.Fly ash with different fineness were composed of spherical fly ash and irregularly shaped fly ash.Composition of two type of fly ash was basically same,but content was slightly different.Composition of fly ash with different particle size is consistent,and the crystallinity of Si O₂ and Ca O is better than that of Al₂O₃,Ca CO₃ and Ca SO₄.There was a certain amount of Mg-Al（ferrite）spinel in fly ash,but the crystallinity was poor.The fly ash less than 43μm contained a certain amount of unburned carbon particles.Infrared spectral transmittance of fly ash increased with the decrease of its particle size and strength activity index of fly ash increased with decrease of its particle size.Fly ash can meet requirements of the national standard for fly ash activity index,except for the fly ash within 50-80μm range and that conforms to Normal distribution（μ=22.5）.（2）Effect mechanism of particle size of fly ash on hydration behavior of cementitious materials was studied.Hydration heat of the cementitious materials mixed with different particle size distribution of fly ash was recorded by microcalorimeter.Effect of different particle size fly ash on heat,heart flow and degree of hydration was studied.Hydration products were characterized by XRD and TG-DSC method and pore structure of the hydration products was tested by MIP method.Effect of particle size of fly ash on composition and pore structure of hydration products was studied.Results show that the particle size of fly ash has little effect on the hydration heat release of cementitious materials within 10 hours.After 10 hours,particle size of fly ash has obvious influence on total hydration heat release of cement.Fly ash with particle size in the range of 0-26μm can increase total hydration heat release of cement,and that in the range of 26-61μm could slightly reduce total amount of hydration heat release of cement,while fly ash with particle size larger than 61μm can greatly reduce total hydration heat release of cement.Hydration induction period for cement mixed with narrow and concentrated particle size range of fly ash was jenerally lower than that mixed with untreated fly ash,which was not conducive to nucleation process of C-S-H crystal.As particle size of fly ash decreases,hydration degree increases in 7d.Cementitious materials replaced by fly ash with different fineness did not change the hydration products,where hydration products were composed of C-S-H gel,calcium hydroxide,ettringite,and monosulfide calcium sulphoaluminate hydrate,etc.In addition,a small amount of unhydrated dicalcium silicate and tricalcium silicate remained unreacted.The total porosity,average pore size and most probable pore size of hydration products increased with increase of fly ash particle size.（3）Study on the relationship model between the particle size of fly ash and compressive strength of concrete was established.The relationship model between particle size of fly ash and compressive strength of concrete was studied based on an intermediate parameter hydration degreeαand Neville theory.Results show that strength relationship model has a large error,and fitting strength values of different particle sizes of fly ash were 10-20MPa higher than measured values,while it can reflect the strength more accurately.The calculation error of model can be greatly reduced from 30-70%to 3-17%by introducing void fraction of concrete into the model as the correction coefficient.Taking total porosity p（1-p_w）as correction factor of the model could make calculation accuracy of model higher,with an average error of 4.79%.The model optimized by linear regression method and simplified model based on the principle of equivalent particle size substitution also have higher fitting accuracy.（4）Study on service life prediction model based on concrete carbonation depth was researched.Influence of fly ash particle size on carbonation depth of concrete was studied.The prediction model of concrete carbonation depth was established based on the formula of concrete carbonation rate and sum up of influencing factors of concrete carbonation rate at home and abroad.The model was verified by measured carbonation depth.Taking carbonation induced corrosion of steel in concrete as failure mechanism,service life of concrete structure was predicted.The synergetic development relationships among particle size of fly ash,carbonation depth of concrete and service life of concrete structure were investigated.Results show that carbonation depth of concrete increased with increase of particle size of fly ash.Carbonation depth fitting model based on integration of environmental factors had a good fitting effect for concrete prepared with different particle sizes of fly ash.Error expectation of carbonation depth fitted by strength model and carbonation model was slightly higher than that fitted by carbonation model alone.Method of combining two models had a good application effect.Service life of concrete structure decreased with increase of fly ash particle size.Service life of reinforced concrete（mixed with 26-80μm fly ash particles）with protective layer thickness of 50 mm is less than 50 years,which does not meet the construction design specifications.As particle size of fly ash decreases,the reliability of same service life concrete structure increase.Reliability of fly ash with different particle size functions in same service life is greater than that of the control group.