Characteristics of coal gangue fine aggregates after calcination and its effects on the improvement of mortar properties

Coal gangue (CG) is an industrial solid waste generated from coal mining and washing.Environmental issues caused by its long-term accumulation are becoming increasingly serious,and using it as aggregate has significant environmental and economic benefits.Calcined coal gangue fine aggregates (CGFAs) are prepared by using CG as raw material,and the effect of calcination on physico-chemical properties and microstructure of CGFAs is systematically explored with the testing methods such as XRD,FTIR and MAS NMR.Workability,mechanical properties,water absorption and drying shrinkage properties of CGFAs mortar and river sand (RS) mortar are comparatively investigated.Moreover,the influence mechanism of calcined CGFAs on improving the properties of mortar is analyzed based on the aggregate characteristics.The results show that raw coal gangue fine aggregate (CGFAraw) contains much more coal powder,impurities,edges and corners,and with lower strength.Compressive strength of cement mortar mixed with CGFAraw at 28 days is only 27.1 MPa,and has higher water absorption and drying shrinkage.After calcination,the water absorption and strength of CGFAs increase.When the temperature is above 600 ℃,the vibration peaks of O—H and Al—OH in CGFAs disappear,the structure of Si and Al transformed,and the pozzolanic activity released.Calcined CGFAs absorb the mixing water in mortar manufacturing,ultimately reduce the fluidity of mortar.Thermally activated CGFAs at 600-900 ℃ can significantly enhance the mechanical properties of mortar and present a higher strength at ealy stage.Among them,the compressive strengths of mortar prepared by coal gangue fine aggregate calcined at 750 ℃ (CGFA750) are 64.7% and 92.6% higher than those of CGFAraw mortar at 3 and 28 days respectively,and also exceed those of RS mortar.Interface between CGFAraw and cement matrix is poor,which induce the much larger porosity,water absorption and drying shrinkage to mortar.Calcined and thermally activated CGFAs reduce the effective water-cement ratio in matrix,and have a function of internal curing due to the water released from aggregate interior.With the joint action of activated fine powder and Si and Al components on the surface,the matrix pores are compacted and interface bonding is enhanced,which reduces the porosity and water absorption,as well as the drying shrinkage of cement mortar.Where,the drying shrinkage of CGFA900 mortar is 69.5% lower than that of CGFAraw mortar at 90 days.The quality of CGFAraw with more weak components,lower strength and complex composition is worse than RS,and the treatment of thermal activation (600-900 ℃) is an effective approach to improve the properties of CGFAs and mortar.