| The emissions of industrial solid waste and CO2 are increasing with the rapid development of metallurgy,electric power,building materials and other industries in China,and the resource and environmental problems caused by them have become an important factor restricting the low-carbon,green and sustainable development of the industries.The resource efficient utilization of solid waste and the development of low-carbon green cement have become two urgent problems to be solved.The preparation of cementitious materials with high content of solid waste to replace the use of cement is an effective way for the resource efficient utilization of solid waste and the development of low-carbon green cement with the policy orientation of strengthening the construction of ecological civilization and moving towards carbon peak and carbon neutralization.In this work,Bayer red mud,desulfurized gypsum and fly ash were used as typical alkaline solid waste,sulfate solid waste and silicon-aluminum character solid waste to prepare new low-carbon green cementitious materials of red mud and other solid waste.The composite synergistic effect among the three solid wastes in the hydration process of cementitious materials and the relationship between performance and microstructure evolution of cementitious materials were systematically studied.Firstly,the mechanism of enhancing the pozzolanic activity of red mud under thermal activation was studied,and the effects of temperature on the phase,pozzolanic activity and polymerization degree of red mud were described.With the increase of temperature(100-1000℃),the phase of red mud changes from crystalline materials to amorphous materials to crystalline materials.Its pozzolanic activity first increased and then decreased,and the polymerization degree first decreased and then increased.There was a good linear relationship between polymerization degree and pozzolanic activity.The pozzolanic activity of red mud was the highest and polymerization degree was the lowest at 700℃.The microstructure results showed that the intensity of crystal diffraction peak of RM700 decreased significantly and the area of amorphous dispersion peak increased.The micro morphology presents a disorder and loose structure.The polymerization structure with relatively stable Si-O and Al-O bonds was destroyed,and fracture bonds and activation points were formed in the structure.Secondly,the composite synergistic effect of red mud,fly ash and desulfurization gypsum in the hydration process of cementitious materials was studied,and the effects of composite synergistic effect on the properties and microstructure of cementitious materials was analyzed.The mechanical properties of cementitious materials were significantly improved under the composite synergistic effect.When the content of the three solid wastes was 50%,the compressive strength of cementitious materials at 28 days was 50.60 MPa,and the mechanical properties at each age were higher than standard of#42.5 Portland cement.The microstructure results showed that the hydration products of cementitious materials mainly included ettringite,C-S-H and C(N)-A-S-H gel.Under the synergistic action of alkalinity and sulfate,the active silicon-aluminum substances in red mud and fly ash can quickly participate in the reaction,which makes the hydration degree of cementitious materials moved towards a higher polymerization degree.A large number of gel with ettringite in the cementitious materials interlaced,which reduced its porosity and formed a dense structure,thus improving its mechanical properties.The leaching results showed that the cementitious materials had a good solidification effect on Na+.Thirdly,the content effects of each solid waste on the properties of cementitious materials was studied,and the prediction statistical models of water demand,setting time and compressive strength of cementitious materials were established.Within the set range(red mud=17.5-35.0%,desulfurized gypsum=2.5-12.5%,fly ash=10.0-25.0%),fly ash significantly reduced the water demand of cementitious materials,and desulfurized gypsum significantly increased the water demand of cementitious materials,and red mud significantly reduced the setting time of cementitious materials,and desulfurization gypsum significantly increased the setting time of cementitious materials.The optimum design results showed that the compressive strength of cementitious materials at 28 d was 53.13 MPa when the maximum content of red mud was 32.1%.Through the verification of several test points,the deviation between the predicted values of water demand,setting time and compressive strength equations and the measured values was less than 10%,which showed that the models have good significance.Then,the influence of Ca/(Si+Al)molar ratio on the properties and microstructure of cementitious materials was studied,and the relationship between Ca/(Si+Al)ratio and the evolution of properties and microstructure of cementitious materials was clarified.With the increase of Ca/(Si+Al)ratio(1.52-1.92),the mechanical properties of the cementitious materials,the formation,polymerization degree and the structure of Al(Ⅳ)of hydration products gel increased first and then decreased,and the porosity decreased first and then increased.A large amount of gel and ettringite were formed in the hydration later stage of cementitious materials,and the contribution of gel to the mechanical properties of cementitious materials was greater than ettringite.With the increased of hydration age,the gel content,polymerization degree and Al(Ⅳ)in cementitious materials increased significantly.When Ca/(Si+Al)ratio was 1.68,the mechanical properties of cementitious materials,polymerization degree and[AlO4]-content of gel were the highest.The distribution of Na in the system was consistent with the distribution of Al in the gel,which indicates that Na+was mainly involved in charge balance and is solidified.Finally,the dissolution characteristics of Si and Al elements in red mud and fly ash,and the hydration process of cementitious materials were studied,and the hydration mechanism of cementitious materials was clarified.Compared with alkali solution,the dissolution concentrations of Si and Al elements of red mud and fly ash in alkali and sulfate solution increased significantly.The dissolution process of particles and elements in red mud and fly ash were affected by many factors,and the temperature had the greatest influence.With the increase of temperature,the dissolution concentration of Si and Al elements increases significantly.The results of dissolution kinetics showed that the dissolution process of Si and Al elements in red mud and fly ash under the synergistic action of alkali and sulfate was mainly controlled by internal diffusion.The hydration process of cementitious materials was mainly in the order of crystal nucleation and crystal growth→phase boundary reaction→diffusion.The synergistic action of alkali and sulfate can change the pozzolanic reaction of red mud and fly ash from diffusion process to phase boundary reaction process,which significantly increased the rate and degree of pozzolanic reaction.The hydration processes of cementitious materials mainly includes three basic stages:depolymerization of active silicon-aluminum substances in red mud and fly ash,condensation of C-S-H and C(N)-A-S-H gel,ettringite formation,and gel polymerization to the formation of three-dimensional network structure.The results showed that the properties of road base materials met the requirements of relevant standards,and its benefits of economic,social and environmental were remarkable.The results of this work provide scientific basis and theoretical guidance for the building materials collaborative utilization of alkaline solid waste,sulfate solid waste and silicon-aluminum character solid waste. |
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