Chloride Ions Binding Properties Of Alkali-activated Slag And Its Hydration Products

Reinforced concrete structure is now the most common form of building structure,and its durability is an important research hotspot in the field of construction engineering.Compared with Portland cement,the traditional cementitious material for concrete preparation,alkali-activated slag（AAS）,as a green and environmental friendly cementitious material,has the advantages of low energy consumption,low emission,simple production process,and remarkable social and economic benefits.The main reason for the degradation of long-term performance of concrete structure is the penetration of chloride ion.Therefore,a series of experimental research on the performance of AAS binding chloride ion were carries out in this paper,which will provide some theoretical basis for the further application of alkali activated slag cementitious materials.In this study,C-（A）-S-H and AAS paste were prepared by artificial synthesis method,and the chloride ion binding properties and mechanism were studied.Sodium hydroxide and sodium silicate were selected as alkali activators,and ordinary Portland cement was set as control group.The effects of temperature,p H value,time,activator type and alkali content on chloride ion binding behavior of alkali activated slag were studied by equilibrium method.The microstructure and hydration products of AAS and C-（A）-S-H were analyzed by nuclear magnetic resonance（NMR）,X-ray diffraction analysis（XRD）,Fourier transform infrared spectroscopy（FT-IR）,comprehensive thermal analysis（TG-DSC）and scanning electron microscopy energy dispersive spectroscopy（SEM-EDS）.Comprehensive thermal analysis（TG-DSC）was used to quantitatively analyze the chemical binding and physical adsorption of chloride ions in alkali activated slag included sodium hydroxide activated slag（NAS）and sodium silicate activated slag（WAS）.The main results are as follows:（1）C-（A）-S-H,as the main hydration product of AAS,has physical adsorption on chloride binding.The amount chloride ion adsorbed increases with temperature increasing,decreases with the increase of p H value,and increases with the extension of time,and finally reaches a stable equilibrium state.For morphology,C-S-H and C-A-S-H all show spherical and round particles before adsorption of chloride ions;After adsorption,the morphology of C-S-H changes greatly and became layered,while C-A-S-H has no obvious change;But on the whole,C-S-H and c-a-s-h became more loose.（2）Under the same environment,the binding chloride ion capacity of AAS is as follows:WAS>NAS>OPC.In the range of 2℃to 60℃,the amount of chloride ions bound increased with the increase of temperature,decreased with the increase of p H value,and increased with the extension of time,and reached a dynamic equilibrium state in 7 to 14 days.（3）When the alkali equivalent is the same,the ability of WAS to bind chloride ion is stronger than that of NAS;with the increase of alkali equivalent,the chloride binging ability of both activated slag is also improved,which is mainly because the increase of alkali equivalent promotes the further hydration of slag.（4）The binding effect of AAS on chloride ion includes physical adsorption and chemical binding,in which physical adsorption is the most important.However,with the increase of p H value,the amount of physically adsorbed（mainly in the form of C-（A）-S-H）chloride decreased and the amount of chemically bound（mainly in the form of Friedel’s salt）chloride increased.（5）The main hydration product of AAS is C-（A）-S-H,and there is no Ca（OH）₂which is common in Portland cement hydration products.The results of XRD and SEM showed that the slag was eroded by chloride ions to form Friedel salt.After chloride exposure,the morphology of AAS changed from numerous clusters of small spherical particles to folds,and the hexagonal flake crystal product Friedel’s salt was observed.