Preparation Of Functional Aggregate And Its Effect On Volume Deformation Of Cement-based Materials

Aggregate is one of important component in concrete, which occupies about75%～80%by volume. Generally, it is regarded as inert filler in concrete, which plays arole of supporting skeleton. However, only a few researchers focused on that effect ofaggregate on volume deformation performance of cement-based materials. In this paper,functional design on aggregate based on the basic principle of material design, thepreparation technology of composite material and artificial lightweight aggregate,roasting theory and crushing technology. The functional aggregate with low thermalexpansion coefficient, high elasticity modulus, internal porous and rough surface wasprepared based on based on the rich resources of coal gangue and fly ash in Huainanand Huaibei. The preparation of sintered body and its optimization design wereresearched. Functional aggregates were prepared by adjusting mix proportion of rawmaterials, dosage of foaming components and sintering temperature. Furthermore, theexpansion mechanism of functional aggregate was also analyzed. Effect of functionalaggregate on volume deformation performance of cement-based materials wasinvestigated, as well as revealing the mechanism of functional aggregate improvevolume stability of cement-based material. The main works and conclusions arepresented as follows:Preparation of sintered body and optimization design based on responsesurface method.(1) The mineralogical compositions of sintered body were mullite, quartz andβ-cristobalite. With coal gangue and fly ash ratio decreased, the compressive strengthand linear shrinkage of sintered body increased, the diffraction peaks of quartz mineralphase reduced, the diffraction peaks of β-cristobalite increased, and the diffractionpeaks of mullite had no significant change. Sintering temperature had markedlyimpacted on the compressive strength and linear shrinkage of sintered body For coalgangue and fly ash ratio=1:1, sintered body with compressive strength of63.88MPa,linear shrinkage of8.84%, porosity of15.28%and average pore diameter of203.9nmwas prepared at1250℃.(2) Fitting the second order response surface models of volume shrinkage ratio, apparent density, vacuum water absorption is accurate and reasonable by responsesurface method. It could be applied to analyze and predict the properties of sinteredbody. Moreover, the interaction effects between temperature-specific surface area andtemperature-Si/Al on performance of sintered brick are strongly significant.Considering energy consumption and the properties of sintered body, The technicalparameters for preparation of excellent sintered brick were determined by responsesurface methodology, which is sintering temperature of1225-1250℃, specific surfacearea of600-640m2·kg-1and Si/Al=2.5-2.6.Preparation of functional aggregate and its expansion mechanism(1) Dolomite powder was able to achieve expansion to form porous structure in thepreparation process of coal gangue system and coal gangue-fly ash functional aggregate.Under the appropriate dolomite powder content and sintering temperature, themineralogical compositions of functional aggregate were cordierite, mullite, anorthiteand quartz, Cordierite has low thermal expansion coefficient and an excellent thermalshock resistance.(2) With the increase of dolomite powder content, the porosity and pore diameterof coal gangue functional aggregate radually increase, the apparent porosity andcompressive strength of coal gangue functional aggregate gradually decrease, thecordierite and anorthite content increase, accompanied with a reduction in mullite. Thecoal gangue and fly ash ratio had no significant effect on porosity, apparent density andcompressive strength. However, sintering temperature had markedly significant effecton composition, structure and performance of two kinds of functional aggregates. Thecoal gangue functional aggregate with porosity of40.71%, apparent density of1528kg/m3, pore diameter of0.5-1.5mm, compressive strength of5.76MPa wereprepared at dolomite powder content of10%and1250℃for30min, as well as coalgangue-fly ash functional aggregate with porosity of54.86%, apparent density of1169kg/m3, pore size of0.1-1.5mm, compressive strength of4.78MPa was prepared with coalgangue and fly ash ratio=5:5at1250℃for30min.(3) Expansion mechanism of functional aggregate was analyzed. It includes twoaspects. Namely, CO2gas was generated from the decomposition of dolomite powder,which strongly escaped. The liquid phase with suitable viscosity was generated so that it trapped the CO2gas released at high temperature. The expansion process offunctional aggregate was achieved by the two aspects.Effect of functional aggregate on volume deformation performance ofcement-based materials(1) The deformation of concrete was restrained and its rate was reduced effectivelywhen replacing nature stone with function aggregate under the condition of lowwater-cement ratio. Porous structure of function aggregate enabled it to absorb water inthe early time of concrete blending and release it gradually latterly, which controlled therelative humidity of concrete and reduced its capillary tention. At the same time,interlocking structure was formed between cement and function aggregate due to itsrough surface and then restricted the shrinkage of set cement.(2) Plastic cracking in early stage was delayed and drying shrinkage was reducedwhen replacing river sand part or all with function aggregate. Crack extension of mortarin plastic stage was reduced and drying shrinkage of cement paste was restraint due tothe rough form of function aggregate.(3) The coefficient of thermal expansion of cement-based materials was reducedwith using the aggregate with high content of cordierite or improving the proportion ofaggregate, and thermal stability of cement-based materials was improved.