Study On Water Action Mechanism And Concrete Performance Of Low-Grade CFB Ash Concrete

Circulating fluidized bed ash(CFB ash)is the by-product of emerging clean coal-fired technology,including CFB fly ash(CFBFA)and CFB bottom slag(CFBBS).Its large displacement and difficult utilization cause the problems of land occupation and environmental pollution.In theory,CFBBS can replace natural sand and stone to prepare concrete and save resources,but the porous structure of CFBBS makes the water in concrete migrate and may affect workability,mechanical properties and microstructure of concrete.And the conventional calculation method is not suitable for preparing CFB ash concrete,which limits its research and application.Based on this,this paper studied the water storage state in CFBBS mortar and the migration law of water in fresh cement paste and hardened paste,studied the influence of water storage migration on the workability,mechanical properties and microstructure of CFBBS mortar.It is found that in the fresh mixing stage of mortar,the water gradually migrates from cement paste to CFBBS until equilibrium,and the greater the total w/c ratio,the greater the migration amount.This process reduces the effective w/c ratio of cement paste,and leads to the reduction of workability and the improvement of strength;In the hardened stage,the water migrates from CFBBS to hardened cement until the internal water is exhausted,which can maintain the internal humidity of the material.The microscopic test shows that the water migration in the hardened stage of mortar increases the hydration degree of cement,improves the polymerization degree and chain length of C-S-H gel,improves the morphology of the interfacial transition zone,and thus enhances the mechanical properties of the mortar interfacial transition zone.Based on the analysis of the relationship between the effective w/c ratio and compressive strength of CFB ash concrete,the relationship between the effective w/c ratio and the total w/c ratio and time,this paper puts forward the calculation method of the mix proportion of low-grade CFB ash concrete,designs and optimizes the CFB ash concrete with strength grade C30,and reveals the relationship between concrete performance and material composition.It is found that with the increase of total water or with the decrease of cementitious material dosage,the strength and bulk density of concrete decrease;When the total amount of cementitious materials remains unchanged and the replacement rate of CFB fly ash is increased,the strength and bulk density of concrete are slightly reduced;When the P·O 42.5 cement is replaced by the P·S 32.5 cement,the workability of CFB ash concrete changes little and the strength decreases significantly.When the dosage of cement,CFBFA,CFBBS and total water is 280kg/m~3,220kg/m~3,1025kg/m~3 and 320kg/m~3 respectively,the performance of CFB ash concrete is the best.At this time,the initial slump of concrete is 240 mm,the workability is good,the 7d strength is 19.30 MPa and the 28 d strength is 42.38 MPa.In addition,the effects of CFBFA and CFBBS on the long-term mechanical properties and durability of concrete are studied.It is found that a large amount of CFBFA is easy to cause slight strength reduction of concrete in the later stage,and the concrete prepared with CFBBS has obvious expansion,but the specimen morphology is intact after 180 days of curing,and the self shrinkage and dry shrinkage of concrete are significantly improved.The large amount of CFBFA leads to the increase of water absorption,the decrease of chloride ion penetration resistance,freeze-thaw cycle resistance and carbonation erosion resistance of CFB ash concrete;The CFBBS increases the water absorption of concrete,enhances the resistance to chloride ion permeability and carbonation erosion,and the concrete made by CFBBS has excellent sulfate erosion resistance.