| The feasibility of using ferronickel slag(FNS)to produce alkali-activated cements(AACs)has been proved both in theory and technology.However,the application of FNS in AACs so far has been very limited due to its low activity and complicated composition,as well as the uncertain properties.In this study,the dissolution behaviors and phase structure evolution of FNS in alkali solutions were used to determine the activity of FNS,based on the analysis of the physical and chemical properties of FNS.Accordingly,the relationship between the physical and chemical properties and activity of FNS was established.The influence of the factors,including FNS content,alkali activator types and curing conditions,on the early-age reaction process,mechanical properties development,macro-shrinkage behaviors and microstructure characteristics of alkali-activated FNS/GGBS blend cements have been studied systematically.The achievements of this study will provide a theoretical basis for the efficient utilization of FNS in AACs.The main research contents and conclusions are as follows:(1)The leaching kinetics of FNS in Na OH solution was studied.The effects of molar concentration,leaching time,leaching temperature and particle size on the selective leaching behaviors of various elements in FNS were analyzed.The results suggest that the leaching kinetics of FNS can be described by shrinking core model(SCM)controlled by diffusion through the surface product layer.The activation energy for Si and Al during the leaching process in Na OH solution is 80.47 and 49.45kJ/mol,respectively.The dissolved Ca,Al and Si from FNS in the alkali solutions reacted with Na to form N(C)-A-S-H gel and the dissolved Mg was mainly involved in the formation of hydrotalcite and N-A(M)-S-H gel.The increase of molar concentration and leaching temperature was beneficial to the leaching of Fe.The heavy metals,including Cr,Mn and Ni,could not be leached in the alkali solutions but did in the acidic solutions.A rapid evaluation method for the activity of FNS was proposed,based on the evaluation index of the individual leaching ratio of Si and Al from the leaching process in 5M NaOH solution for 300min at 60°C.(2)The effects of FNS content and alkali activator types on the setting time,flexural strength,compressive strength,autogenous and drying shrinkage of alkali-activated FNS/GGBS blend cements were studied.The results show that blending 20% FNS is beneficial to improve the flexural strength of alkali-activated FNS/GGBS blend cements and has no obvious bad effect on the compressive strength.Blending 60% FNS prolongs the setting time and leads to a significant reduction of mechanical properties.The autogenous and drying shrinkage of FNS/GGBS blend pastes activated by sodium silicate is much higher than those by sodium hydroxide.The addition of FNS in the sodium silicate-activated FNS/GGBS blend cements results in a significant increase of the long-term autogenous shrinkage and drying shrinkage but decreases the early-age autogenous shrinkage.The long-term autogenous shrinkage,long-term drying shrinkage and early-age autogenous shrinkage of sodium hydroxide-activated FNS/GGBS blend cements are all reduced obviously with FNS addition.(3)The embedded ultrasonic monitoring method and low field nuclear magnetic resonance technique were adopted to investigate the early-age reaction process and mechanism of different alkali-activated slag(AAS)pastes.The isothermal conduction calorimetry was used to study the influence of FNS content,alkalinity and modulus of activator solutions and curing temperature on the reaction process of alkali-activated FNS/GGBS blend cements.The results suggest that the early-age reaction process of AAS pastes made with sodium silicate can be divided into five stages:(?)dormant period,(?)acceleration period,(?)deceleration period,(?)second acceleration period and(?)second deceleration period.However,only three featured stages could be identified in those activated with sodium hydroxide.Besides,in the sodium silicate-activated system,stages ? and?belong to the diffusion-controlled stages and stage ? belongs to the dissolution-controlled stage.The increase of initial SiO44-content in sodium silicate solutions improves the reaction degree in the dormant period and acceleration period,meanwhile,advances the second acceleration period and eventually accelerates the whole reaction process.Overall,the reaction rate at the beginning of sodium hydroxide-activated system is faster than that of sodium silicate-activated system.However,the reaction degree of sodium hydroxide-activated system is lower than that of sodium silicate-activated system in the later age.When the curing temperature of 25?and the alkalinity of 4%,the cumulative reaction heat of alkali-activated FNS/GGBS blend cements is significantly reduced with FNS addition.The addition of FNS has no obvious influence on the reaction process of sodium hydroxide-activated hybrid system,but delays the reaction process of the FNS/GGBS blend pastes made with sodium silicate.When the curing temperature of 60? or the alkalinity of 6%,the activity of FNS is effectively activated.Blending 20%FNS is beneficial to increase the cumulative reaction heat of alkali-activated FNS/GGBS blend cements.Compared with the room temperature environment,the delayed effect of the addition of FNS on the reaction process of sodium silicate-activated system is weakened at the high curing temperature of 60?.(4)The backscattered electron images analysis method and nanoindentation test were selected to investigate the microstructure,phase composition and micromechanical properties of FNS particles and alkali-activated FNS/GGBS blend cements.The results show that there are three typical phases identified in FNS,namely FNS? phase(Mg-Si phase),FNS? phase(Si-Ca-Al phase)and FNS? phase(Cr-Fe phase),with the volume fractions of 79.92±4.24%,19.02±4.05% and 1.06±0.38%,respectively.Combined with the results of leaching process,the reactivity sequence of the three phases in FNS is FNS?>FNS?>FNS?.The elastic modulus of FNS?,FNS? and GGBS particles is 127.39±2.68GPa,145.77±4.79GPa and113.87±7.36GPa,respectively.The elastic modulus of N-A-S-H gel in alkali-activated FNS pastes,the C-A-S-H gel and internal reaction product(Ip)in alkali-activated GGBS pastes is 16.99±1.89GPa,25.91±2.55GPa and 35.09±6.40GPa,respectively.The Ca/Si and Ca/(Al+Si)in the alkali-activated FNS/GGBS blend cements are significantly reduced with FNS addition.The Na/Si and Na/(Si+Al)in the gel products of sodium hydroxide-activated hybrid system have no obvious relationship with the additional content of FNS.The addition of FNS is effective in increasing the Na/Si and Na/(Si+Al)in the gel products of the sodium silicate-activated hybrid system in the later age. |
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