| Alkali-activated slag(AAS)is a promising product,proven to have cementitious properties capable of possessing compressive strengths equal to or higher than those of OPC.AAS as an alternative to Portland cement has been found to have low hydration heat,high rate development of mechanical properties and perfect durability,etc.However,the application of AAS so far is very limited because of its high shrinkage and rate of carbonation.These drawbacks are closely related to the shrinkage microcracking and microstructure of AAS.Therefore,in this research,a study has been conducted to investigate the potential mitigation strategies of macro-micro-nano natural cellulose fibers on the AAS shrinkage and microcracking of alkali-activated slag pastes.Firstly,this research investigated the effect of the macro natural cellulose fibers(NCFs)on mechanical properties,autogenous shrinkage,drying shrinkage and microstructure of AAS based on the optimum mix proportion.Shrinkage,internal relative humidity(IRH),strength and heat flow were tested;microstructural analysis was conducted as well,in order to illustrate the influence of macro NCFs on the AAS microstructure.The results show that macro NCFs efficiently implement outstanding mitigation of AAS autogenous shrinkage and drying shrinkage by about 35–66.9% and 33.9–51.5%,respectively.Furthermore,the inclusion of macro NCFs can enhance the flexural and compressive strength by around 4.6–8.0% and 4.7–9.0%,respectively,when compared to the control AAS reference paste.It was evident from the experiments that the evolution of internal RH,which is supposed to be responsible for the high AAS shrinkage,corresponded well to the development of AAS deformation.Secondly,this research studied the potential of shrinkage microcracking mitigation strategies in AAS paste by combining a low amount of cellulose micro-nano fibers which act as a filler of voids,individual pores,as well as a crack control.Strength,shrinkage,porosity and pore size distribution were tested.Quantitative and qualitative analysis was conducted as well to illustrate the influence of CFs on the AAS shrinkage microcracking and internal pore structure.The results showed that compared to AAS reference,the inclusion of CFs substantially enhanced the mechanical properties and mitigated the shrinkage by around 4.8-21.4% and 12.5-21%,respectively.MIP and BET analysis results demonstrated that the addition of CFs into AAS changed the pore size distribution and the internal pore structure,which results in low porosity by range of 12-42% and a minimum mesopores in the range of 1-100 nm.Surface morphology microstructural observations proved that the CFs-AAS has more homogeneous matrix and denser microstructure in microscale 1-50 ?m by SEM.Acoustic emission(AE)measurements systematically linked microcracks initiation with the AAS deformations and internal pore microstructure in the early age.Even though the limitation of X-CT resolution,the evolution of AAS pore size distribution in the early age is interesting and confirmed the influence of CFs as a voids-pores filler.Therefore;it is worth analyzing the AAS shrinkage mitigation to provide an explanation for AAS shrinkage microcracking phenomenon by studying the effect of pore size distribution on the shrinkage.The 3D reconstructed structure by X-CT investigation showed that AAS reference have a much higher proportion of pore sizes within the micro-mesopore region than CFs-AAS pastes.Furthermore,the volumetric ratios change of pores seems to be an important parameter in determining the magnitude of shrinkage,rather than the number of pores.Finally,the addition of macro-micro-nano cellulose fibers could modify the AAS internal relative humidity and internal pore structure,which finally mitigated the high shrinkage and microcracking behaviour of AAS.Consequently;it is feasible to enhance the short-long term performance of AAS by means of the internal curing,filling pores and controlling the microcracks. |
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