Evaluation Of Pozzolanic Activity Of Lithium Slag And Study On Microstructure Characteristics Of Composite Cementitious Materials

In recent years,with the continuous expansion of lithium salt application fields and the rapid growth of market demand,China's lithium salt industry has developed rapidly.At the same time,a large number of industrial by-product lithium slag has been produced,and its stockpiles have increased year by year.Compared with slag and fly ash,the utilization rate of lithium slag is relatively low?the actual utilization rate is only about10%?.It is mainly used for the admixture and supplementary cementitious materials in the building materials industry,and a small amount is used to prepare new materials.The reason for the low utilization rate is that the basic research results of the phase composition,active component content and reactivity evaluation of lithium slag are few and unclear.And there is a lack of systematic regulation research on the hydration process of cement as supplementary cementitious materials and the relationship between microstructural changes and macroscopic performance.At the same time,breaking through the traditional methods of reactivation and exploring new and efficient lithium slag reactivation technology is also a key research content to expand the consumption range of lithium slag in the cement industry and enhance the capacity of consumption.This topic is based on the research of“pretreatment of low-activity lithium slag and high-efficiency activation technology”in the national key research and development project.Lithium slag from three representative areas was selected as the research object,and its morphology and phase composition and content,especially the active component content,were studied in depth.The degree of reaction of lithium slag in the composite cementitious system was quantitatively characterized.The pozzolanic activity and reaction kinetics of lithium slag were systematically studied.The microstructure and structure of the hydrated gel of lithium slag composite cementitious material were investigated.The relationship between the pore structure characteristics and the microstructure and macroscopic properties of the material is discussed.Finally,for the low hydration activity of the high-content lithium slag composite cementitious material,an inorganic polymer activator is used to enhance its hydration activity and preliminary explore its activation mechanism.The lithium slag contained the dark grey polygonal and flat cake-shaped crystalline aluminosilicate phase,off white irregularly polygonal alkali and alkaline-earth compounds phase,black grey amorphous phase without fixed morphology,grey clubbed gypsum phase,and a small amount of bright approximatively hexagonal and round granular iron-bearing phase.According to the gray distribution curve,the main phases of the lithium slag were composed of 39.78⁴2.65%amorphous aluminosilicate phase?amorphous Al₂O₃·4SiO₂?,6.46⁹.83%alkali and alkaline-earth compounds phase?CaCO₃,KAlSi₃O₈ and CaAl₂Si₂O₈,a little NaAlSi₃O₈?,34.41⁴4.74%crystalline aluminosilicate phase(Al₂Si₄O₁₀?OH?₂,a little quartz),3.96⁷.10%gypsum phase?CaSO₄·2H₂O?,etc.If the production process of lithium slag would be improved,the crystalline aluminosilicate phase might be transformed into more amorphous phases,which will increase the reactivity and the resource utilization of lithium slag.Among the methods for measuring the degree of lithium slag reaction,the hydrochloric acid method is suitable for measuring the degree of reaction of lithium slag and has universal applicability.The early pozzolanic activity of lithium slag was low,and the pozzolanic activity gradually appeared after 7 days.After 28 days,the hydration activity increased slowly.The reaction rate was between 6.30¹1.58%for 7 days,between 14.08¹9.45%for 28days,and between 16.22²1.46%for 90 days.The pozzolanic reaction of lithium slag is consistent with the first-order reaction kinetics model,and the reaction rate constant k?0.0014⁰.0028?reflects the activity of pozzolanic.The reaction rate of lithium slag before 7 d is low,the gel content of hydration products is low,the granular structure is loose,and the contribution to the strength of the system is low.The pozzolanic activity gradually shows after 7 days and contribute to the strength of the system increased rapidly,and the structure of the hydrated product became dense in the the later period of hydration,but some lithium slag particles did not react.The higher the content of active components in lithium slag,the greater the hydration activity,and the more gel content is formed,the greater the contribution to the strength of the system.According to comprehensive evaluation,the pozzolanic activity of lithium slag is lower than that of slag and higher than that of fly ash.In the lithium slag system with different C/LS value,when the C/LS is 0.25,the reaction rate of lithium slag is higher?k=0.0028?,and the activity has been fully excited by CaO,resulting in C-?A?-S-H gel content is the highest.As the C/LS value continues to increase,the lithium slag reaction decreases,and excess CaO will hinder the surface contact reaction of the active component in the lithium slag and the Ca²⁺diffusion and permeation reaction,and will also be the form of Ca?OH?₂ crystal is interposed in the middle of the hydrated gel,and Ca?OH?₂ gradually dissolves during long-term curing,forming a large pore channel,and the strength of the hardened slurry is lowered.In lithium slag-cement composite cementitious materials,early lithium slag can promote cement hydration and accelerate the growth of C-S-H gel.The lithium slag content?40%of the sample has a low degree of lithium slag reaction,and the hydration product fails to fill the pores in the slurry well.In the later period of hydration,the structure of each sample becomes dense,and the hydration product is well filled in the pores.However,the sample with lithium slag content?40%is added with respect to the sample with lithium slag content?20%.Increased structural compactness is significantly reduced,and the degree of reaction becomes lower.Lithium slag incorporation changes the type of hydration products of composite cementitious materials,from calcium-rich C–S–H to aluminum-rich C–A–S–H,the composition of which varies from single to complex and the morphology changes from fibrous to spherical clusters.A small amount of lithium slag only changed the chemical composition of the C–S–H phase.Large amounts of lithium slag changed the type of hydration product to C–?A?–S–H phase,and there were two C–A–S–H phases with high Ca and low Ca.Lithium slag incorporation reduces the Ca/?Si+Al?value of C–A–S–H gel,and as the amount of lithium slag increases,the Ca/?Si+Al?value shows its regional distribution and gradually decreases.The Al/Si value of the C–A–S–H gel is about 0.42,indicating that Al in the C–A–S–H gel does not increase with increasing lithium slag content.The critical pore size of the composite cementite material decreases gradually with the increase of curing time.The porosity of the sample with 20%lithium slag content is lower than that of the pure cement sample.The content of lithium slag is not less than40%,which increases the less damaged holes?interstitial holes?in the compound hardening slurry,and the pores first decrease and then increase,which is caused by the expansion of calcium alum formed in the system through the mechanism of"crystal growth"and"pore formation".For the study of pore distribution content,early lithium slag addition increased the number of macropores>100 nm in the slurry,but a small amount of lithium slag can promote cement hydration and generate partial hydration gel to fill the pores.The pore structure of the hardened slurry is coarsened,and large pores are increased.In the middle period,the macroporous content of each sample decreased,and the gel pore content increased,that is,the large pores of>100 nm were significantly reduced,the gel pores of<10 nm were significantly increased,and the clearance pores of4.5 to 50 nm were greatly increased.When the lithium slag content is less than or equal to 20%,the pore size of the hardened slurry can be refined and the gel pores increase.When the amount of lithium slag is?40%,the content of the pores in the hardened slurry is increased from 4.5 to 50 nm,and the total porosity is increased.The critical pore size of the hardened slurry with lithium slag content of 60%is in the range of less harmful pores,and other samples are in the range of gel pores.The contribution of lithium slag to strength is mainly reflected in the physical"filling effect"of the early lithium slag particles and the chemical"pozzolan effect"of the later lithium slag changing the microstructure of the slurry.Using the fractal theory combined with mercury intrusion method,the fractal dimension of the pore surface of the lithium slag composite hardened slurry is between2.82 and 2.94,the correlation coefficient is 0.99,the pore structure has obvious fractal characteristics,the pore surface area is irregular,and the pore structure is complex.The fractal dimension of pore surface is very poorly correlated with compressive strength and porosity,and shows good correlation with average pore diameter and median pore diameter.The fractal dimension has a certain relationship with the pore surface area,that is,the surface fractal dimension increases with the decrease of the average pore diameter and the median pore diameter,and the pore specific surface area increases with the increase of the fractal dimension.The fractal dimension and the pore size distributions<20 nm and>100 nm show good positive and negative correlations.As the fractal dimension increases,the proportion of harmless pores<20 nm increases significantly,while the proportion of harmful pores>100 nm decreases significantly.Both lithium slag and cement particles showed a strong adsorption tendency for the polymerized aluminum.The complexing ability of the polymerized aluminum to the liquid phase ions is SiO?OH?₃^->Al?OH?₄^->Ca²⁺>Fe³⁺>Mg²⁺.In an alkaline environment,as the aluminosilicate mineral dissolves,the polymerized aluminum and the silicon-aluminum ion in the liquid phase form a complex by complexation and adsorption,the metal cation is precipitated by the adhesion sweeping,thereby lowering the liquid phase.The ion concentration,the active silicate mineral continues to dissolve,and the SiO₄^4-and AlO₄^-consumed in the replenishing solution accelerates the hydration reaction,improves the dissolution content of the vitreous in the lithium slag,and thereby improves the hydration reaction degree of the lithium slag.The addition of polymeric aluminum significantly improves the compressive strength and non-evaporable water content in the lithium slag-cement composite cementitious material.In the early hydration,polymeric aluminum accelerates the mineral phase dissolution and crystal growth in the lithium slag-cement composite cementitious material,and increases the total nucleation of hydration products.A large number of ettringite,hydration calcium aluminates,calcium hydroxide and amorphous hydration gels appeared in the hydration products,which promotes hydration of lithium slag-cement composite cementitious system.