Research On The Activation Of Recycled Powder From Waste Concrete Under Chemical-Thermal Coupling

The demolition and renovation of old buildings have produced a large amount of waste concrete.The traditional way of dumping or landfill not only occupies a lot of land resources,but also brings serious secondary pollution to the urban environment.How to deal with the ecological load of waste concrete and complete the recycling of waste concrete has important practical significance for conservation of ecosystem and sustainable development of resources.Compared with the mature recycled aggregate,the overall utilization efficiency of recycled micropowder as an important component of waste concrete is low.Recycled micropowder contains a large number of potential activity silicon aluminum components.Therefore some methods can be taken to stimulate its potential activity and make it become a cementitious material with high hydration characteristics.It is of great significance not only to reduce the huge energy and environmental load brought by cement production,but also to realize the complete and efficient recycling of waste concrete.In this paper,the activation effect of the recycled micropowder was studied by means of chemical reagents,heat treatment and chemical-heat treatment,and the activation mechanism of the recycled micropowder was analyzed by means of mechanical properties,X-ray diffraction and infrared spectrum.（1）When chemical reagents are used for activation,the activation effect order of different reagents is:（TEA）>Na₂SiO₃>Na OH>Ca（OH）₂.The reagent with the best activation effect is 0.01%dosage reagent（TEA）.The strength of 3d and 28d specimens can reach 4.3MPa and 10.2MPa,which is 79%and46%higher than that of the unexcited samples respectively.The activation principle of various chemical reagents is:Ca（OH）₂ itself has cementitious,which provides active calcium components for the system and improves the production of cementitious components.Na OH can provide a strong alkaline environment and promote the fracture of Si-O and Al-O bonds.Na₂SiO₃ can not only play the role of alkali activation,but also supplement active siliceous components.The content of oligomeric silicon oxide tetrahedron in sodium silicate with suitable modulus is high,which can improve the activity of the system,The external sodium silicate itself has cementation and fills the pores of the sample,which can improve the strength of the sample.TEA can produce gel material by providing excess electrons and complexing with the cation in the recycled micropowder,in addition,because of its strong alkalinity,it can also promote the dissolution of the particles surface and accelerate the hydration reaction process.（2）When heat treatment is used for activation,the order of activation effects of different factors is:calcination temperature>holding time>heating rate.The best activation system is heating up to 600℃at the heating rate of 5℃/min,and then holding in the high-temperature furnace for 2 hours.Under the optimum activation system,the compressive strength of the specimen at 3d and 28d is 18.6MPa and40.8MPa respectively.Compared with the unexcited samples,the strength value is increased by 4.2 times and 4.8 times respectively.Compared with the triethanolamine（TEA）excitation group,which has the best activation effect for chemically excited recycled micropowder,the strength values were increased by 3.3times and 3 times,respectively.The effect of thermal activation is significantly better than that of chemical activation.High temperature thermal activation can promote the disintegration of silicon aluminum structure in the system,and change the stable form to metastable structure,and then achieve the purpose of stimulating the activity of recycled micropowder.Under high temperature thermal activation conditions,the hydration product Ca（OH）₂ of the recycled micropowder has basically decomposed completely at400～500℃,and the structure of the silicate anion polymer structure of the silica gel is broken into oligomer structure at 300～400℃.However,ettringite has been completely decomposed within 200℃and has rehydration activity again.（3）When coupled activation of chemical-heat treatment,the best activation system is 0.01%tea chemical reagent+600℃calcination temperature+5℃/min heating rate+60min holding time.Under this system,the compressive strength values of 3d and 28d are 28.8MPa and 51.0MPa respectively,Compared with the single-doped TEA activation group,the compressive strength values of 3d and 28d were increased by 5.7 times and 4 times,respectively.which are 56%and 40%higher than that of obvious thermal activation.Better activation effect is formed after chemical-heat treatment coupling.For the mechanism of chemical-heat treatment coupling activation of recycled micropowder,in the high-temperature environment,the minerals in the recycled micropowder are dehydrated and decomposed.At the same time,the structure of silicon and aluminum will be recombined or broken at a higher temperature,so that silicon,aluminum,calcium and other elements are easier to dissolve under chemical activation and participate in the reaction,Chemical activation is more sufficient on the basis of the metastable silicon aluminum components after thermal decomposition,thus forming more gel material and enhancing the activity of recycled micropowder.According to different activation principles,chemical reagents and heat treatment can effectively improve the gelling activity of recycled micropowder,and coupling them can produce stronger activation characteristics.The above results have an important reference and guiding role for effectively solving the technical bottleneck of low activity and difficult utilization of recycled micropowder of waste concrete,and realizing the efficient recycling of recycled micropowder of waste concrete.