Study On Hydration Characteristics And Performance Improvement Mechanisms Of High-volume Fly Ash Concrete

Partially replacing cement with fly ash in concrete is an effective approach for reusing fly ash.When using more fly ash than cement in concrete,high-volume fly ash(HVFA)concrete can be obtained.There are many advantages for this type of concrete:gradually-increasing later-age strengths,lower shrinkage,and lower production cost.In this context,HVFA concrete has been successfully used in the construction,including mass concrete,self-compacted concrete,roller-compacted concrete,and engineered cementitious composite.However,its application is limited,resulting from its low early-age strengths and low frost resistance.The low hydration degree of fly ash at the early-age leads to the reduce mechanical properties of HVFA concrete,while the unburnt carbon present in fly ash would impose negative effect on the air-entrainment,compromising the freezing-thawing(F-T)durability.This dissertation proposed a comprehensive approach to improve the strength and freezing-thawing resistance of HVFA concrete besides the air-entraining.The optimal mix design for HVFA system which performed best in the mechanical properties was firstly selected through the design of experiment.Hydration characteristics of fly ash particles in selected HVFA system then investigated.In the HVFA system that already adopted the optimal mix design,appropriate approaches were applied to improve the transport properties,matrix’s microstructure properties,and interfacial transition zone properties,respectively.HVFA concretes with the best performance in the aforementioned three sections were subjected to further testing.A multi-phase sphere model was proposed for HVFA concrete and the frost resistance was used to verify this model.In the range of influencing factors,the optimal mix design for HVFA mortar entailed the weight percentage of fly ash in the total cementitious material at 60%,water-to-cementitious material ratio(w/cm)at 0.3,and without any other mineral admixtures.Different phases in fly ash exhibited different characteristics during the hydration of fly ash.Ca-rich phase participated in the hydration of fly ash faster than Si-rich phase or Al-rich phase.All the class C fly ash showed similar hydration degree in mortars containing different amount of fly ash,with the values exceeding60%,based on the BSE image analysis results.Admixing appropriate non-polymeric admixtures or polymeric admixtures in HVFA mortar can improve its transport properties(reduce its water sorptivity),while not reduce the mechanical properties.Polymeric admixtures worked better in reducing the gas permeability of mortar,while non-polymeric admixtures showed greater potentials in refining pore structure and reducing the critical pore radius.The addition of graphene oxide at the dosages ranging from 0.01%to 0.1%enhanced the hydration degree of fly ash and hindered the propagation of the cracks at the same time.The admixed graphene oxide could increase the microhardness,scratch surface roughness,scratch hardness,and friction coefficient of paste in HVFA concrete.When the dosage of graphene oxide exceeded 0.05%,the agglomeration of graphene oxide occurred,which negatively affected the microstructure of HVFA paste.Coating the coarse aggregate with graphene oxide-or nanosilica-contained cement paste could target the nanomaterials in the ITZ of concrete successfully.With only one-tenth or even less of the admixed amount than that usually used in the direct-mixing,the addition of nanosilica in the ITZ reduced the porosity and increased the hydration products,densifying the ITZ microstructure.The splitting tensile strength,water sorptivity,chloride migration,and the freezing-thawing resistance of HVFA concrete were all improved to some extents.As a hydrophilic nanomaterial,graphene oxide could significantly reduce the F-T resistance of HVFA concrete when admixed.HVFA concretes containing chemical admixtures or containing treated coarse aggregate exhibited increased F-T resistance.The proposed four-phase sphere model considering the presence of the unhydrated fly ash could gave the predicted values of the dynamic modulus of elasticity(E_d),which matched well with the measure values.This confirmed that the damage mainly occurred in the cementitious paste,in which the equivalent w/cm characterized by the water absorption increased rapidly,reducing the E_d.Focusing on HVFA concrete,this work deeply studied its strength and freezing-thawing durability.The reasons for improving the frost resistance of HVFA concrete were investigated from the views of hydration mechanism of fly ash particles and modification mechanism of various measures.These measures can enlarge the application of this environmentally friendly concrete in the field.Modification mechanism of these measured would be used to solve some possible problems when employing HVFA concrete in the field.