Experimental Study On Waste Concrete Recycled Micro-powder As Supplementary Cementitious Material

With the rapid expansion of urbanization,a large number of old buildings have been dismantled,and a large amount of construction waste has been discharged.The total amount of construction waste has has reached more than 1/3 of the total national waste.The recycling of construction waste has become the inevitable direction of sustainable development of cities in the future.Recycled concrete powder is a kind of powder particles produced by crushing construction and demolition wastes,and its rational utilization is an effective way to improve the recycling of construction and demolition wastes.In this paper,the recycled concrete powder produced in the process of waste concrete crushing is taken as the research object,the basic properties of recycled concrete powder produced during the crushing of waste concrete are tested,and it is used as supplementary cementitious materials to replace cement to prepare recycled concrete powder paste,recycled concrete powder mortar and recycled concrete powder concrete.Through microscopic analysis and functional test,the influence of recycled concrete powder on mechanical properties and microstructure of paste,mortar and concrete is investigated.The specific research contents and results are as below:1.The effect of recycled concrete powder on the consistency and compressive strength of cement paste was studied by cement paste test.The results showed that with the increase of recycled concrete powder content,the water consumption of cement paste with standard consistency and cement setting time increased.When fly ash and silica fume are mixed in recycled concrete powder paste,the water consumption of cement paste with standard consistency and the cement setting time are obviously reduced.When the recycled concrete powder content is 15%,and other mineral admixtures content is 15%(fly ash: silica fume = 3: 2),the 28-day compressive strength of paste is the highest and higher than that of the control group.2.The hydration heat and hydration products of recycled concrete powder cement were determined by I-Cal 4000 HPC hydration calorimeter,X-ray diffractometer and thermogravimetric analysis,and the pore structure of recycled concrete powder paste was determined by NMR.The results indicated that recycled concrete powder accelerated the hydration process of cement,but did not change the crystal type of hydration products.With the increase of recycled concrete powder content,the hydration heat release rate and 72 h total heat release of samples are both lower than those of the control group,the cement hydration heat release peak is ahead of time,the number of harmful pores and multi harmful pores is more.The addition of fly ash and silica fume in recycled concrete powder paste can increase the peak value of hydration exothermic peak and enhance pozzolanic reaction.The compactness of the microstructure of the sample improves.When the content of recycled concrete powder is 15%,the content of other mineral admixtures is 15%,and the ratio of fly ash to silica fume is 3: 2,the hydration reaction of the sample is the most complete and the heat release is the highest among all the admixtures.3.A total of 13 groups of cement mortar mix ratios were designed.Through the cement mortar test,the effects of recycled concrete powder single mixing(10%,20%,30%,40%),recycled micro powder combined with fly ash and silica fume(the recycled concrete powder content is 21%,15%,28%,20%,the total amount of fly ash and silica fume is 9%,15% respectively and fly ash and silica fume ratio is 3:2 and 4:1)on mortar fluidity,3d,7d,28 d compressive strength and anti-corrosion resistance were considered.The results display that with the increase of recycled concrete powder content,the fluidity of mortar decreases gradually,and the compressive strength and flexural strength of mortar at each curing ages drop gradually.When recycled concrete powder is mixed with fly ash and silica fume,the fluidity of mortar increases with the increase of fly ash content.When the content of recycled concrete powder is 15% and the content of fly ash and silica fume is 15%(fly ash: silica fume =3: 2),the 28 d compressive strength and flexural strength of the motar are the highest,reaching 50.6MPa and 9MPa respectively,which are higher than those of the control group.4.The pore structure of recycled concrete powder mortar was measured by MIP test,and the correlation between 28 d compressive strength and flexural strength of recycled concrete powder mortar and pore structure was discussed by grey correlation theory.The results show that with the increase of recycled concrete powder content,he most probable pore size and total pore volume of mortar increase,the content of harmless pores in the mortar decreases,the content of harmful pores first increases and then decreases,the content of multi harmful pores first decreases and then increases,and the total porosity and average pore size of recycled concrete powder mortar increase.When fly ash and silica fume are mixed with recycled concrete powder mortar,compared with the mortar only with recycled concrete powder,the content of harmless pores increases,the content of harmful pores decreases,the content of more harmful pores increased slightly and the most probable pore size and total pore volume decrease.The ratio of pore size distribution less than 20 nm is the primary interfering factor affecting the 28 d mechanical strength of recycled concrete powder mortar,while the average pore size is the secondary interfering factor affecting the 28 d compressive strength and the total pore volume is the secondary interfering factor affecting the 28 d flexural strength of recycled micro-powder mortar.5.The influence of recycled concrete powder content on the strength and micromorphology of concrete was studied by concrete test.The results show that with the increase of recycled powder content,the recycled powder concrete compressive strength decreases and the internal structure of concrete becomes more loose.When fly ash and silica fume are mixed in recycled concrete powder concrete,the compressive strength of concrete increases slowly in early age,and increases sharply in mid-late age.The amount of CSH gel in concrete increases greatly,the compactness of samples increases.When the replacement rate of recycled concrete powder is 21%,the replacement rate of fly ash and silica fume is 9%,and the ratio of fly ash to silica fume is 3:2,the 28 d compressive strength of concrete is the highest among all the mixed proportions,which is 47.45 Mpa.