Study On Adsorption Characteristics And Mechanism Of Waste Concrete Recycled Sand Powder On Superplasticizers

With the vigorous development of my country’s construction industry,the construction of numerous high-rise buildings has also brought a huge amount of construction waste.Recycling construction waste can save energy,alleviate environmental pollution,improve product economy,and help my country’s strategic goal of sustainable development.A large amount of recycled sand powder is produced in the process of resource utilization.Due to its own physical and chemical properties,the amount of water reducing agent is large and the water demand is large,so it is difficult to popularize and apply in cement concrete.In this paper,by studying the adsorption characteristics and mechanism of waste concrete regenerated sand powder and polycarboxylate superplasticizer,it is of great significance for the development and application of polycarboxylate superplasticizer suitable for recycled sand powder.The pore structure,appearance and mineral composition of the reclaimed sand powder were characterized by mercury intrusion test,scanning electron microscope,XRD and other test methods.The results show that the pore structure of the reclaimed sand powder is concentrated in 6nm～13nm and 21nm～30nm,a lot of cement hydration products are attached to the surface,and the crystal phases are Si O₂,calcite and unreacted C₂S.The molecular weight,particle size and molecular structure of the polycarboxylate superplasticizer were characterized by GPC,nanoparticle size,infrared spectroscopy and other methods.The results show that:PCE-1 is a common comb type with a particle size ranging from 4 nm to 10 nm;PCE-2 is a short main chain and long side chain comb type with a particle size ranging from 5 nm to 12 nm;PCE-3 is a long main chain and short side chain comb type,the particle size is 4nm to 9nm.There are vibration absorption peaks of-OH,-CH₂,ether bond（-CH₂-C-CH₂）and out-of-plane（-OH）in the molecules of the three superplasticizers.PCE-1and PCE-3 have vibration absorption peaks of-COOH carboxyl group Peak,PCE-2 has a-COO-ester group attached to a new group with Br^–.The effect of polycarboxylate superplasticizer on the fluidity of recycled sand-powder cement slurry was studied by the slurry and mortar fluidity.The results showed that:in the slurry of recycled micropowder,the saturation point of PCE-1 was high and the slurry fluidity was average;The saturated content of PCE-2 is the lowest and the fluidity of the slurry is well maintained.When 30%of micropowder is added,the fluidity can still reach 135mm;the saturated content of PCE-3 is low,but the slurry loses fluidity when the content of micropowder exceeds 15%;PCE-2,PCE-3 recycled mortar has similar fluidity.The research results of slurry rheological properties show that PCE-2 has more obvious effect on improving the rheological properties of regenerated fine powder slurry and regenerated sand mortar.The pore structure,apparent morphology,adsorption isotherm model,adsorption kinetic model,and chemical interaction before and after adsorption were studied by nitrogen adsorption,scanning electron microscopy,TOC,and infrared spectroscopy.The results show that:（1）The10nm-20nm pores in the regenerated sand powder have pore structure adsorption to the polycarboxylate superplasticizer.After adsorbing the superplasticizer,the 10nm-20nm pores of the regenerated sand powder decrease,and the 4nm-6nm pores increase.The relationship between the water reducing dose is:PCE-3>PCE-1>PCE-2.（2）PCE-1 superplasticizer has a medium adsorption degree,belonging to the Langmuir model,pseudo-second-order kinetic equation,and the adsorption mode is dominated by single-layer surface adsorption;PCE-2superplasticizer has a higher adsorption degree,belonging to the Freundlich model,quasi-second The first-order kinetic equation and adsorption method are dominated by multi-layer surface adsorption;PCE-3 superplasticizer has a low degree of adsorption,which belongs to the DR model,pseudo-first-order kinetic equation,and the adsorption method is dominated by pore structure adsorption.（3）The degree of chemical surface adsorption is:PCE-2>PCE-1>PCE-3,the chemical effects are basically the same,the-OH bond hydroxyl group in Ca（OH）₂is combined with the carboxyl group in the polycarboxylate superplasticizer,generate PC-COO-Ca,PC-COO-Al new compounds,the C=C double bond of-CH₂ in the main chain is combined with the Si-O bond and Al-O bond in the regenerated sand powder;the CH bond outside the benzene ring is combined Also binds to bonds in hydration products.Based on the research on the fluidity,rheology and mechanism of recycled sand powder and superplasticizer for waste concrete,PCE-2 superplasticizer with short main chain and long side chain is more effective in adsorption in recycled sand powder slurry,and has better effect on the fluidity of slurry.It is more favorable and has better application prospects in recycled sand-powder concrete.