Experimental Study On Strength Mechanism And Durability Of Stabilized Iron Tailings With Composite Cementitious Material

Driven by policies related to the comprehensive utilization of solid waste and the development of circular economy,the utilization of tailings and industrial wastes such as slag,calcium carbide slag,and fly ash have become a research hotspot.Lianyungang intends to introduce advanced concentrators to deal with the imported iron ore,while the large amount of fine iron tailings formed by imported iron ore will be piled up.After the fine iron tailings with poor engineering properties was solidified/stabilized,it may be used as subgrade or pavement base materials.The traditional binder is Ordinary Portland Cement,which is high energy consumption,less environmentally friendly,and also consuming large amounts of non-renewable resources.Therefore,it is urgent to development alternative binder with high-performance and environmental friendly,and then use the stabilized Lianyungang fine iron tailings as base course material,which has important social and economic benefits.This study is supported by the National Natural Science Foundation of China（Grant No.41702349）.A new binder（namely,ASF）using the industrial wastes is developed to stabilize the typical iron tailings in Lianyungang,and the strength mechanism and durability of the stabilzed iron tailings are studied.The conclusions provide the theoretical basis for the application of iron tailings based pavement engineering.The main contents and conclusions of this study are summarized as follows:（1）The hydration characteristics of the ASF binder,and the interaction mechanism of the hydration products with iron tailings particles are investigated,through X-ray diffraction（XRD）,scanning electron microscopy（SEM）and energy dispersive spectrum（EDS）analysis.The results show that the typical iron tailings in Lianyungang has samll sized particles,high plasticity index,and water content.Polymerization and hydration occurs during the reaction process of ASF binder,and the typical hydration products are amorphous N-A-S-H and C-（A）-S-H gels.Compared with OPC hydration products such as C-S-H,ettringite and Ca（OH）₂,the ASF binder has more hydration products,better encapsulates of iron tailings particles,and has better filling and bonding effects.（2）The effects of the dosage of binder and the curing age on the strength performance of the stabilized iron tailings are investigated by the unconfined compressive strength tests（UCTs）,and the relationship between strength and p H,conductivity（EC）and dry density value was established.In addition,SEM and mercury intrusion porosimetry（MIP）tests are performaned to further reveal its strength growth mechanism.The results show that the strength of the stabilized sample has a notable positive correlation with p H,EC,dry density,and pores volume content（d<0.1μm）,and negative correlation with the Gaussian fitting parameters.Benefit from the encapsulation,filling and bonding effect of the hydration products,the strength of stabilized sample increases significantly.Compared with the OPC stabilized sample,the p H value of the ASF stabilized sample is smaller,while the pores structure and strength is better.（3）Based on the unconfined compressive strength test,the feasibility of using stabilized iron tailings as pavement base course material is discussed.The results show that,with 3%ASF dosage,the stabilized sample meets the criterion of the sub-base course under all traffic conditions,or the low grade pavement base course.When the ASF dosage increases to 8%,it meets the criterion of high grade highway base course,whereas the 8%OPC stabilized sample was only allowed in low grade pavement base course.（4）The effect of dry-wet and freeze-thaw cycle to the variation of strength,p H,EC and density of the stabilized samples,and also the evolution of micro-pore structure are investigated,by a series of UCTs,SEM and MIP tests.The results show that the dry-wet and the freeze-thaw cycle increase the pores amouts and volume.The pore volume（d<0.1μm）decreases significantly,while the Gaussian fitting parameters increase,which leading to the decrease in strength.Compared with the OPC stabilized samples,the resistance of ASF stabilized samples is weaken to dry-wet cycle,but better to freeze-thaw cycle.（5）Similar to the semi-dynamic leaching test,the experiment is carried out to investigate the ion dissolution characteristics of the stabilized samples under the effect of SO₄^2-and Cl^-solution.The ion concentration and its relationship with p H and EC value are investigated,and the diffusion coefficient are calculated.In addtion,it provides a basis whether it is safe when using the ASF（mainly industrial waste）stabilized iron tailings as pavement materials.The results show that the cumulative mass release of ions and the change of ion concentration with time are both related to the ASF dosage and the type of erosive ion solution.The Ca diffusion coefficient(D_avg value)of the ASF stabilized sample is about 1.3 to 1.5 times that of the OPC stabilized sample;the Si and Al ions are 45%to 66%and 3.9 to 4.8 times,respectively.In addition,the mass release of heavy metals such as As and Zn is extremely low,and thus the environmental stability is confirmed.（6）The mechanism of the erosive environment on the strength behavior of the stabilized sample is investigated,based on the pores evolution and mass release of Ca.The results show that the stabilized samples are typically heterogeneous under the erosive environment.The internal of the stabilized sample is enhanced,that the pore volume content（d<0.1μm）is significantly higher,and the Gaussian fitting parameters are smaller.However,the shallow of the stabilized sample is degraded,that the increasing release of Ca and decreasing of p H in pore structure inhibited the hydration process and resulted in the significantly reduction of hydration products.It shows a negative correlation between the strength and cumulative mass release of Ca and its D_avg.In addition,the resistance of ASF stabilized sample to SO₄^2-and Cl^-attack betters than that of the OPC stabilized sample.