Study On Chloride Transport And Modification Of Recycled Powder Mortar

With the accelerated urbanization in China,the production of construction waste has increased year by year and will remain high in the future;the traditional construction waste disposal methods,mainly piling and landfilling,not only occupy a large amount of land,but also cause a series of environmental and social problems.Therefore,construction waste is prepared into recycled aggregates and recycled powder through resourceful disposal,which has been applied in construction projects on a large scale and is inevitably exposed to a chloride attack environment,which is the main cause of corrosion of reinforcement within concrete and its durability degradation.Previous relevant studies have less considered the influence of the complexity of construction waste and the chloride transport of different CWP mortars;studies on the chloride transport of fully recycled mortars with compounded recycled aggregates and recycled powders are rarely involved,and the research on chloride transport and modification of CWP mortar is still in the initial stage.Therefore,this topic focuses on the main components of construction waste in China,preparing waste concrete into recycled aggregate and recycled concrete powder,preparing waste hardened cement paste into recycled hardened cement paste powder,grinding mortar waste into recycled mortar powder,and using them to prepare recycled mortar respectively.This topic not only studied the basic properties and chloride transport of recycled mortar;further explored the basic properties and chloride transport of compounded recycled aggregate and recycled powder mortar;in addition,focusing on the adverse effects of chloride permeability of recycled mortar,this topic proposed an effective regulation and modification method,aiming to achieve the safe application of recycled mortar and fully recycled mortar.The main research results are summarized as follows.(1)With the admixture of CWP,the pore size and porosity of recycled mortar expands and the compressive strength decreases,and the mixed active admixture can refine the pore structure of recycled mortar,the total porosity decreases and the compressive strength increases.The chloride diffusion of mortar was elevated as CWP replacement rate and particle size increased,and the free chloride concentration increased but chloride diffusion coefficient decreased as chloride exposure time was prolonged.Mixing both CWP and silica fume or metakaolin could prepare sustainable mortar with a similar chloride resistance to plain mortar.The total chloride concentration of mortar was elevated as CWP was incorporated,whereas the chloride binding capacity reduced with the growth of CWP substitution rate.When CWP replacement percentage was identical,the mortar with CWP ground from cement paste waste had better chloride binding capacity than the mortar with CWP ground from mortar and concrete waste.Through optimizing CWP replacement and active admixture content,good strength and chloride resistance can be guaranteed in CWP blended mortars.(2)The chloride diffusion of recycled mortar rises with the usage of RA or CWP;the mix of RA raises the chloride binding capacity of mortar,but mixing CWP reduces the chloride binding capacity.The recycled mortar including CWP ground from hardened paste waste has better chloride binding capacity than the recycled mortar including CWP ground from mortar and concrete waste.The chloride diffusion resistance of fully recycled mortar with both RA and CWP is inferior to that of recycled mortar with single RA or CWP.Mixing various CWPs or active admixture can improve the chloride diffusion resistance of fully recycled mortar,and a significant improvement occurs on chloride diffusion resistance when metakaolin or silica fume is blended into fully recycled mortar.New calcium oxide and C2S were generated in heat-modified CWP,and the micro-structure of heat-modified CWP blended paste was finer than that of un-modified CWP blended paste;specifically,the average pore diameter of paste with 50%800℃-modified CWP was 35.5-38.7%smaller than that of paste with 50%un-treated CWP.(3)Incorporating high-content CWP declines the mechanical strength and obviously rises the chloride permeability of mortar.When CWP replacement level is 30%or 50%,the chloride permeability of mortar containing heat-modified CWP is lower compared with mortar containing un-modified CWP,and the reduction in chloride permeability is more distinct when CWP heat-modified temperature is 600-800℃.The 120-d maximum chloride concentration of mortar with 50%800℃-modified CWP is 22.7-27.5%smaller than that of mortar with 50%un-modified CWP.The positive effect of CWP heat-modified treatment on the chloride resistance improvement of CWP blended mortar becomes obvious following the increment of CWP replacing level.Optimizing CWP type and heat-modified temperature can prepare high-content CWP blended cement-based materials with good chloride resistance.