| With the increasing energy shortage and serious environmental pollution,developing“green and low-carbon”new environmentally friendly materials to replace petrochemical resources has become urgent.Chemical admixtures are indispensable functional components in the modern construction field,and their synthetic raw materials are mainly derived from petrochemical products,which have led to the consumption of petrochemical resources and aggravation of environmental problems.Using biomass as raw materials for preparing concrete chemical admixtures is a green,energy-saving and environmentally friendly method,which brings broad prospects for overcoming environmental problems and obtaining excellent application performance.In addition,the severe loss of workability of fresh concrete directly affects the mechanical properties,durability,and volume stability of concrete.Therefore,it is urgent to design and develop bio-based chemical admixtures,which have significant“green and low-carbon”potential while maintaining the excellent performance of concrete.Poly（γ-glutamic acid）（γ-PGA）is a biomass raw material containing various active sites such as carboxyl and amino groups,which is easy to be chemically modified and has excellent water retention characteristics.However,there are no systematic studies on the application ofγ-PGA biomass in concrete.Based on the above research background,in this study,two solid wastes,coffee grounds and soybean meal,were used as raw materials to ferment the biomass raw materialγ-PGA,and the comb-like andπ-likeγ-PGA-based concrete chemical admixtures were synthesized by the molecular design principles.By analyzing the molecular structural characteristics,adsorption behavior,interface properties,water absorption-release behavior,and hydration behavior,the mechanism of action betweenγ-PGA-based chemical admixtures and cement was revealed,and the relationship betweenγ-PGA-based chemical admixtures and their application performance in cement pastes was established.In addition,the environmental impact and carbon footprint emissions of the product resource extraction,design,production,and consumption stages were systematically evaluated using life cycle assessment（LCA）method.The synthesis of novel bio-based admixtures not only provides a new perspective and research direction for the development of novel chemical admixtures with high-energy-efficient and low-cost,but also has significant implications for optimizing resource and energy utilization,reducing petrochemical resource consumption,and reducing waste emissions and environmental pollution.This is expected to provide theoretical guidance for the sustainable design and development of concrete science and technology.The research content and main results of this paper are as follows:（1）Low-molecular-weight biomass raw materialγ-PGA was obtained by solid fermentation using coffee grounds and soybean meal as the main raw materials.Comb-likeγ-PGA-based chemical admixtures（C-γ-PGA-g-β-CD）andπ-likeγ-PGA-based chemical admixtures（S-β-CD-g-γ-PGA）were prepared by substitution-condensation and Schiff base reaction-substitution modification techniques usingγ-PGA andβ-cyclodextrin（β-CD）as raw materials,respectively.The molecular structures of the synthesized products were characterized using infrared spectroscopy（FT-IR）,nuclear magnetic resonance（NMR）,and other characterization methods,confirming that they conform to the expected design.Furthermore,the molecular structural stability of theγ-PGA-based chemical admixtures was quantitatively described by antibacterial activity,storage stability,and cement environmental stability.（2）Comb-like andπ-likeγ-PGA-based admixtures were added to the cement slurry,and the changes in the fluidity and flow retention of the slurry,particle size distribution of the cement particles,rheological properties,and workability of the concrete with the dosage of chemical admixtures or time were studied.The results showed that,compared withγ-PGA andβ-CD,C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA could improve the fluidity and fluidity retention of the slurry,reduce the yield stress and plastic viscosity,improve the slump and slump flow of the concrete,and obtain a small and uniform particle size distribution.The dispersibility of C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA was lower than that of traditional polycarboxylate superplasticizers（PCE）,but the dispersion retention was better than that of PCE.（3）By analyzing the molecular structure characteristics and combining with the Flory-Huggins lattice model,C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA had stronger molecular chain rigidity,and the molecular chain was more easily stretched in the pore solution and adsorbed on the surface of cement particles.This was reflected in the larger hydrodynamic radius,lower Zeta potential values,and higher adsorption amounts.The adsorption behavior complied with the Langmuir model and the pseudo-second-order kinetic model.The thickness of adsorption layer of C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA on the surface of cement particles was 3.49 nm and 3.81 nm,respectively,which was higher than that ofγ-PGA（1.76 nm）andβ-CD（2.05 nm）,and slightly lower than that of PCE（4.02 nm）.This established a stronger electrostatic repulsion and space steric hindrance interface force on the surface of cement particles,which was conducive to improving the dispersibility of the slurry.（4）The results of the water absorption-release behavior of C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA in cement paste showed that these chemical admixtures exhibited excellent water absorption-release and water retention capacity.Compared with the blank sample,the water retention rate of mortar was increased by 46.60%and 45.93%for C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA,respectively,which was higher than that ofγ-PGA（41.38%）,β-CD（44.36%）,and PCE（37.83%）.Low-field NMR（LF-NMR）results showed that C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA exhibited significant molecular carrying of free water signal peaks at P-T23,indicating strong competition for water absorption and storage capacity,which compensated for the internal moisture of mortar,reduced capillary pressure,formed uniform small pores and dense internal structure of mortar,reduced the water absorption,inhibit the self-shrinkage and drying shrinkage of mortar.Based on this,the water migration kinetics model was established.It was concluded thatγ-PGA-based chemical admixtures mainly carry free water by hydrogen bonding and cavity water storage capacity,and release free water through humidity gradients and ion exchange,thereby exhibiting good dispersion and retention properties.（5）The mechanism ofγ-PGA-based chemical admixtures on cement hydration behavior was explored from the aspects of complexation,hydration kinetics,and hydration product morphology.The results showed thatγ-PGA,β-CD,C-γ-PGA-g-β-CD,S-β-CD-g-γ-PGA,and PCE significantly delayed cement hydration,and C-γ-PGA-g-β-CD exhibited the most persistent delay effect,followed byβ-CD and S-β-CD-g-γ-PGA.In addition to affecting cement hydration through adsorption and complexation,the water absorption-release behavior ofγ-PGA-based chemical admixtures also significantly affected cement hydration,and the stronger the water absorption-release ability,the more significant the retarding effect.The hydration reaction of cement pastes containing C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA was mild and gradual with a long duration,which slowed down the diffusion reaction and avoided the generation of microcracks.As a result,the hardened mortar exhibited excellent mechanical properties and resistance to sulfate attack.（6）The environmental impact and carbon footprint emissions ofγ-PGA-based chemical admixtures and PCE were evaluated using LCA technology.The results showed that the impact of producing 1 kg（solids）of C-γ-PGA-g-β-CD,S-β-CD-g-γ-PGA,and PCE on ecosystem diversity（ED）was 27%,14%,and 59%,respectively,while the impact on human health（HH）was 33%,17%,and 50%,respectively,and the impact on resource availability（RA）was 9%,10%,and 81%,respectively.When the fluidity of cement pastes was the same at 300 min,the endpoint environmental impact of C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA were lower than that of PCE.Through carbon footprint accounting,it was found that C-γ-PGA-g-β-CD and S-β-CD-g-γ-PGA reduced carbon emissions by about 70%compared to PCE,indicating excellent environmental benefits. |
