Effect Of Low-calcium Circulating Fluidized Bed Fly Ash On Properties Of Cement-based Materials And Its Mechanism

With the increasing awareness of environmental protection,more and more efforts have been made to control pollution in China.In this context,clean and efficient circulating fluidized bed（CFB）combustion boilers have been vigorously promoted and widely used in coal-fired enterprises.CFB combustion boiler by-product-circulating fluidized bed fly ash also increases,in the context of energy conservation and emission reduction,its use is imminent.In this paper,low-calcium circulating fluidized bed fly ash（LCFA）was mixed into ordinary Portland cement（OPC）to prepare cement-based materials.The effects of LCFA dosage on mechanical properties,physical properties and durability of cement-based materials was studied,and the hydrated mechanism of OPC-LCFA system was revealed;In addition,water glass was used to modify OPC-LCFA system,and its modification effect and mechanism were discussed.The main research results of this paper are as follows:（1）The LCFA used in the experiment has a loose multi-pore microstructure,and the main mineral components are quartz,hematite,anhydrite and calcite.It has certain self-hardening characteristics,and its pozzolanic activity is better than F-type fly ash（FA）.And the addition of LCFA is more conducive to the strength development of cement-based materials than FA.（2）Under constant temperature curing condition,the compressive strength of samples decrease gradually with the increase of LCFA dosage;Compared with the samples of constant temperature curing,the compressive strength of cement-based materials can be significantly improved by water curing.When the optimal LCFA dosage is 2 0%,the compressive strength of the water curing samples on 2 8 d and 9 1 d reaches 9 4 MPa and 1 0 2 MPa,respectively;Under water curing condition,when LCFA and FA are mixed to replace OPC,the mixture dosage is 2 0%,LCFA:FA=3 is the optimal ratio for preparing OPC-LCFA-FA system,and the compressive strength of the water curing 91d sample reaches 93MPa.（3）In OPC-LCFA system,the addition of LCFA can reduce the fluidity of slurry,increase water demand,delay setting time,and accelerate drying shrinkage of samples;Through the study on the durability of cement-based materials by LCFA,it can be concluded that the addition of LCFA reduces the chloride resistance and carbonization resistance of the samples,while the surface morphology and strength of samples after adding LCFA are better,and the sulfate erosion resistance of samples are improved.（4）The modifying study of water glass on OPC-LCFA system shows that water glass can shorten setting time,reduce bleeding,slow down drying shrinkage and mass loss;The addition of water glass can improve the physical properties of OPC-LCFA system,promote its early hydration reaction and improve its early strength,but water glass has no modified effect on its long-term strength.（5）The durability of OPC-LCFA system was modified by water glass.The results show that adding water glass can effectively improve the chloride resistance,sulfate erosion resistance and carbonation resistance of OPC-LCFA samples.This is mainly due to the increase of hydroxyl concentration in the system by appropriate incorporation of LCFA and sodium silicate,which accelerates the rupture of Si-O and Al-O bonds on the surface of LCFA particles,reduces the degree of polymerization of Si-O-Al network polymers,and forms free and unsaturated active bonds on the surface of LCFA.Thus,it is easy to react with Ca（OH）₂to form more hydration products such as C-S-H,C-A-H and C-A-S-H.At the same time,gelatinous hydration products migrate and diffuse with the change of ion concentration in the system,combine with the undissolved LCFA,and gradually fill and compact the internal structure.（6）By studying the properties of LCFA in cement-based materials,the maximum possibility of preparing green and low-carbon building materials by LCFA and OPC is verified,which provides technical reference for the resource utilization of LCFA.