Study On The Properties And Mechanism Of Carbon Nanotubes Reinforced Alkali-activated-slag Composite

At present,global warming is a rising threat to sustainable development of human society;as billions of tons of industrial solid waste such as steel slag,slag and fly ash are produced every year.Alkali-activated materials such as alkali-activated slag?AAS?,which ulitilize the solid waste and reduce CO₂ emission,are the hotspot of current research on the sustainable building industry.However,brittleness of AAS results in insufficient resistance to deformation,while shrinkage of AAS leads to micro-crack within the material and deterioration of the flexural strength.Therefore,in view of above problems,this study introduced carbon nanotubes?MWCNTs-COOH?as a nano-reinforcement into the alkali-activated slag to investigate their effects on the workability,mechanical strength and microstructure of the AAS-CNTs composite.The composites prepared in this study were distinguished in terms of CNTs dosage.Then,mechanical properties,pore structure and hydration products of the composites were studied to disclose the working mechanism of CNTs in the composite.Rheological test revealed that CNTs can significantly increase the apparent viscosity and the yield stress of the fresh pastes of the AAS at low shear rate.Ignorable influence of CNTs on the initial and final setting of the AAS was also identified.Besides,the addition of 0.1 wt.-%?by weight of slag?CNTs significantly improved the flexural strength,and elastic modulus of the AAS by 37.69% and 21.28%,respectively,reducing the shrinkage of the AAS by 12.31%simultaneously.Morphology of the AAS-CNTs composites was observed by means of scanning electron microscopy?SEM?and the results showed that individual CNT can be readily found in the AAS matrix,which acts as a bridge between the adjacent hydration products;MIP test confirmed that CNTs optimized the pore size distribution and reduced the capillary pores of the AAS.In addition,XRD and TGA tests revealed that CNTs had negligible influence on the crystalline phases of the AAS paste and the amount of hydration products at the curing age of 28 days.Nevertheless,isothermal calorimetry test on the composites indicated that the presence of CNTs delayed the hydration kinetics of AAS at early age.XPS test suggested that chemical reaction between the carboxyl group from the CNTs and the hydration product of the AAS was not taken place.The results of nano-indentation test emphasized that the introduction of the CNTs increased the elastic modulus of the AAS by the composition of CNTs and the hydration products at microscopic scale.