| The rapid on-site curing of cement-based materials is a research difficulty in the field of civil engineering.Creating high temperature curing condition is an important method to break through this difficulty.Under different working conditions,the demand for high temperature curing of cement-based materials is different.Under negative temperature conditions,creating stable positive temperature curing condition for cement-based materials can ensure the smooth formation of its strength;However,under normal temperature conditions,the quality of cement-based materials is closely related to the curing temperature.Especially for the high performance cement-based materials such as active powder concrete(RPC),curing temperature greatly affects its curing efficiency and performance.However,conventional high temperature curing methods cannot be applied to the on-site curing of cement-based materials.Therefore,based on the new curing method of ohmic heating curing,this paper optimizes the cementbased material system,and improves the ohmic heating curing system combined with different material systems to solve the problems that the strength of cementbased materials is difficult to form in negative temperature environment and RPC materials are difficult to be rapidly manufactured in normal temperature conditions,aiming to realize the rapid on-site fabrication of conductive cementbased materials in different working conditions.The main research contents and achievements are as follows:The conductive carbon fibers(CFs)were utilized as conductive filler to improve the electrical properties of the samples,the effect of CFs on the performance of cement-based materials were investigated,and ohmic heating curing was utilized to prepare on-site conductive cement-based materials at-20℃.The results showed that the addition of CFs could effectively improve the electrical conductivity of the samples,and the change of CFs content in a certain range would not cause a significant impact on the performance of the samples.Moreover,the ohmic heating curing method with constant power could significantly improve the early-age mechanical strength of the samples,and the samples possessed excellent long-term hydration and pore structure evolution regularity.This work proves the feasibility of ohmic heating curing on manufacturing cement-based materials at negative-temperature environment according to the perspective of long-term performance development for the samples.After confirming the feasibility of ohmic heating curing on preparing conductive cement-based materials in negative temperature environment,this curing method was utilized to prepare fly-ash modified cement mortar(FA-CM)at-20℃,and the performance development regularity of the samples was thoroughly studied.The results showed that the curing temperature of FA-CM sample could be maintained above 50℃ with the constant power of 30 W,which was conducive to the development of the performance for the sample.The implementation of ohmic heating curing could obviously compensate for the loss of mechanical properties caused by the introduction of fly ash.Under the same amount of fly ash substitution,the samples subjected to ohmic heating curing possess higher mechanical strength.Meanwhile,microstructural analysis methods were used to analyze the microstructure evolution of the samples cured by ohmic heating curing.The hydration degree and product morphology of FA-CM samples were clarified,as well as the strength enhanced mechanism of microscopic pore structure.In order to further broaden the application scenario of ohmic heating curing,conductive carbon nanofibers(CNFs)and non-conductive nano-silica(NS)have been utilized to enhance the heating efficiency of cement-based materials in micro-scale level.The experimental results showed that the combination of CNFs and CFs could build cross-scale conductive networks inside the sample,and this was beneficial for improving the peak temperature and temperature rise rate of OH cured samples with same electrical power.The mechanism of the enhanced heating efficiency for OH cured samples was also clarified.On this basis,NS was further introduced into the sample to further improve the heating efficiency of the ohmic heating cured samples.Based on the hydration characteristics and size characteristics of NS,the specific “squeeze” effect of NS on the microscopic conductive network established by CNFs was proposed,and its key role in densifying the conductive network inside the sample was also clarified.Based on BET analysis results,the enhancement mechanism of NS on the heating efficiency of specimens was further verified.Based on the improvement effect of nano-materials on the heating efficiency of cement-based materials,a new enhanced ohmic thermal curing method was proposed to rapidly prepare RPC at room temperature and standard pressure.The experimental results show that the curing temperature of the sample can reach up to 180℃ in 1 h under the electric power of 50 W.The compressive strength of RPC subjected to 3 h enhanced ohmic heating curing could be higher than 100 MPa,which was comparable to the samples subjected to traditional 3 d high temperature steam curing method,which significantly shortens the curing time of RPC sample without the utilization of complex and high energy consumption equipment.The performance evolution of RPC samples under enhanced ohmic heating curing and traditional curing methods was thoroughly compared.Microstructural analyses were used to disclose the microstructural properties of the samples.It can be concluded that the samples cured by enhanced ohmic heating curing showed high hydration degree and dense pore structure,the mechanism of the improved mechanical properti es for the sample was clarified from the microscopic level.This study provides a new solution for the rapid and high-quality manufacturing of on-site RPC construction.Dolomite powder was used to fully replace silica sand as fine aggregate,in order to realize the preparation of sustainable RPC(SRPC).The performance development regularity and economic benefits of enhanced ohmic heating curing prepared SRPC have been investigated.The experimental results show that the replacement of quartz sand by dolomite has no negative effect on the mechanical properties of RPC samples,and the dolomite powder exhibits great compatibility with the matrix inside the RPC sample.The specific economic benefit of enhanced ohmic heating curing was clarified by thorough economic analysis,and it could be seen that the total preparation cost of 3 h enhanced ohmic heating curing prepared SRPC with dolomite powder as fine aggregate can be reduced by 58.9%compared with the traditional curing method and material system.In this paper,ohmic heating curing as the novel curing method for cementbased materials was utilized to realize the rapid fabrication of cement-based materials in negative-temperature environment,and the on-site high-quality manufacturing of RPC within ultra-short duration was achieved by modifying this curing regime.This work provides effective help for the rapid curing of cementbased materials in different working conditions. |
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