Characteristics Of Cement With Ultrafine Circulating Fluidized Bed Fly Ash

With the development of clean coal-fired technology,the circulating fluidized bed combustion technology has developed rapidly in the field of thermal power generation due to its advantages of high efficiency and low pollution.Due to the lower combustion temperature,the formation mechanism of circulating fluidized bed fly ash?CFA?and pulverized coal fly ash is fundamentally different.There are huge differences in the physical and chemical characteristics of the two,but CFA also has pozzolanic activity,even self-hardening,and can be used as a secondary cementitious material in cement.The physical and chemical properties of CFA are usually very poor?such as porous surface,high sulfur and high calcium,etc.?,which can easily cause problems such as workability and late expansion of cement concrete.The mechanical powder grinding of industrial waste such as fly ash can significantly improve the surface properties,hydration activity of the particles,and improve its application performance in cement.At present,ultra-fine fly ash and ultra-fine slag powder have been widely used in cement concrete.In order to improve the performance of CFA in cement concrete,this paper takes the CFA as the research object,and ultrafine grinding is carried out by the air flow mill.Based on the analysis of its physical and chemical characteristics,the rheological properties,hydration and hardening properties and volume stability of the Ultrafine circulating fluidized bed fly ash(UCFA,D₅₀<5?m)cement were systematically studied,and the optimization of cement performance was also discussed.The main findings are as follows:?1?Ultrafine grinding can significantly improve the morphology and pore structure of particles,increase bulk density and powder flowability,and reduce the water requirement ratio.Ultrafine grinding destroys part of the crystalline structure of fly ash and increases the lattice defects,thereby increasing the dissolution rate of active silica-alumina in CFB fly ash.This enhances the pozzolanic activity of CFB fly ash,promotes the formation of hydration products in the system and optimizes the microstructure,thereby improving the macroscopic mechanical properties of the cementitious material.?2?Ultrafine grinding makes CFA have better compatibility with polycarboxylate superplasticizer,and has better effect on fluidity of CFA-cement,but does not change the fluid model of the paste.Ultrafine grinding improves the surface roughness and porous structure of the particles,which contributes to the increase of free water content in the system and effectively improves the thixotropy of the cement paste.When the content of CFB fly ash is 10wt%,the De of the system particles decreases,and the value of n increases,the yield stress?₀ and the plastic viscosity?of the paste decrease.The rheological properties of the fresh cement paste are improved,and the cement paste has the lowest thixotropy.?3?The CFA prolonged the hydration induction period of the cement,the second exothermic peak was postponed,the peak value was reduced,and the peak width was broadened.Ultra fine grinding improved this phenomenon and increased the total heat release.Ultrafine grinding results in a higher degree of hydration of the CFA,a faster hydration rate,more hydration products and a denser paste structure.The UCFA has a higher contribution to strength,especially to the later period.?4?The volumetric expansion rate of cement increases with the increase of CFA content,and the expansion rate curve conforms to the logarithmic function model:F?t?=a-bln?t+c?Ultrafine grinding promotes the dissolution of II-CaSO₄ and accelerates the transformation of II-Ca SO₄ into CaSO₄?2H₂O.This accelerates the release of early expansion and reduces the overall rate of expansion.The sources of expansion of CFA-mixed cements at different ages are different:the main source of 3d-expansion is the volume expansion of Ca-Al and SO₄^2-generated AFt,and the expansion of II-CaSO₄ to CaSO₄?2H₂O.The main source of the 28d-expansion is the dissolution of aluminum from CFB fly ash to produce delayed AFt and the expansion that occurs during the conversion of II-CaSO₄ to CaSO₄?2H₂O.Longterm-expansion is mainly due to the expansion of CaO cores in CFA particles exposed to CH.The particle structure of CFA is similar to the ideal particle model in which the center is the CaO core,the middle is the amorphous and the surface is enriched with the II-anhydrite.?5?The UCFA has a good improvement effect on the strength of the composite cement,especially the late strength.The amount of fly ash in the ultra-fine circulating fluidized bed should not be more than 20%,and 5%is the optimal amount.When UCFA:RCFA:limestone powder=60:25:15,excellent composite mineral admixtures can be prepared.The water demand ratio was 97.5%,the 7d and 28d activity indexes were 74.1%and 91.1%,respectively.When the content is less than 30%,the initial slump of the concrete does not change,the slump loss of 1h decreases,and the workability improves.