Preparation And Properties Of Lignin-Based Water Reducing Agent For Recycled Concrete

The construction waste generated every year in my country accounts for a considerable proportion of solid waste.The traditional way of dealing with construction waste has caused serious environmental pollution and resource waste.And,the natural materials used in construction are increasingly depleted.Therefore,the rational use of existing waste resources and the realization of turning waste into treasure has become a consensus reached by all countries in the world.Compared with natural concrete,recycled concrete has the characteristics of rough surface,large porosity,strong water absorption,low apparent density,etc.,resulting in poor workability,high water demand,low strength,and large slump loss over time.and other shortcomings,making it more difficult to recycle concrete.At present,the research hotspot of researchers is how to make up for the defects of recycled aggregate and realize the reuse of waste concrete.One of the ways to improve the performance of concrete is to add admixtures in concrete,among which water reducer is a commonly used admixture in concrete.In this paper,sodium lignosulfonate（LS）was used as raw material to prepare water reducing agent by chemical modification method of oxidation and grafting.MLS-AM-TPEG water reducing agent was synthesized with oxidized lignin（MLS）,acrylamide（AM）and isoprene polyoxyethylene ether（TPEG）as monomers.MLS-AM-HPEG water reducing agent was synthesized by using oxidized lignin（MLS）,acrylamide（AM）and isobutenol polyoxyethylene ether（HPEG）as monomers.And,research on the properties of recycled concrete containing modified water reducing agent.（1）The optimal process conditions for the final synthesis of MLS-AM-TPEG:the content of AM is 65%of the content of oxide-lignin,the content of TPEG is 35%of the content of oxide-lignin,and the content of potassium persulfate is 8%of the content of oxide-lignin.The reaction temperature was 85°C and the reaction time was 4h.And,The optimal process conditions for the final synthesis of MLS-AM-HPEG:the content of AM is 75%of the content of oxide-lignin,the content of HPEG is 25%of the content of oxide-lignin,and the content of potassium persulfate is 8%of the content of oxide-lignin.The temperature was 75°C and the reaction time was 4h.（2）Infrared spectroscopy and volume exclusion chromatography analysis showed that polyoxyethylene ether groups（-CH₂CH₂O）and amide groups（-NHC=O）were successfully introduced into the LS molecular chain,and the introduction of new functional groups caused an increase in the molecular weight of the water reducing agent after lignin modification.Thermogravimetric analysis showed that the glass transition temperature Tg increased after lignin was modified.Scanning electron microscope analysis showed that the molecular chains introduced by LS turned the originally smooth spherical surface into a rough and uneven flocculent structure.（3）The adsorption study showed that in the mixed system containing recycled aggregate and cement,the adsorption of MLS-AM-TPEG and MLS-AM-HPEG by particles conformed to the Langmuir isotherm adsorption model,belonging to the monolayer adsorption,and the adsorption of the two was saturated.The adsorption amounts were 15.64mg/g and 17.56mg/g;the LS adsorption conformed to the Freundlich isotherm adsorption model,which belonged to the multi-molecular layer adsorption,and the saturated adsorption amount was 21.38mg/g.The kinetic study shows that in the mixed system of recycled aggregate and cement,the adsorption of the three superplasticizers all meet the second-order adsorption kinetics,and the adsorption of particles to the superplasticizer is mainly chemical adsorption,and there are also physical adsorption and molecular weight.The adsorption rate of the larger MLS-AM-TPEG was lower than that of the slightly smaller molecular weight MLS-AM-HPEG.（4）The application performance study shows that when the amount of MLS-AM-TPEG and MLS-AM-HPEG water reducer is 0.8%of the cement amount in the recycled concrete,the slump of recycled concrete is 205mm and 210mm,respectively.The water rates are 27.4%and28.3%,which are 19.2%and 20.1%higher than that of LS.When the replacement rate of recycled aggregate is 20%,the compressive strength can reach 43.4MPa and 43.8MPa respectively after 28 days of curing,meeting the engineering design requirements.