| As a new generation of cement concrete admixtures,polycarboxylate superplasticizers?PCEs?exhibit superior performance to previous generations of superplasticizers?melamine-based,aliphatic,and naphthalene-based?,such us high water-reducing rate,low Slump loss,multi-function and strong molecular structure controllability,so it is widely used in major projects.However,with the continuous deepening of research and application fields of PCE,it was found that the clays in aggregates of concrete have a significant effect on the dispersion properties of polycarboxylate water-reducing agents,among them,the polyoxyethylene?PEO?side chains of PCE can intercalate into the interlaminar of montmorillonite,which has an extremely negative influence on PCE.At present,by replacing the PEO side chains of polycarboxylate molecule and finding suitable sacrificial agents is the main method to improve the mud resistant performance of PCE,but few scholars think about the ways to weaken or prevent the interaction between PEO side chain and clay to improve the mud resistance of PCE.Therefore,in this paper,we successfully developed a modified lignin that can inhibit the interaction between clay and polycarboxylate water-reducing agent,by further studying the interaction mechanism between the modified lignin and various clay minerals,we hope to provide researchers with a new thinking direction.The optimum synthesis conditions of modified lignin were studied by varying the amount of metal catalyst,the pH of the reaction system,the ratio of raw materials and other factors.Infrared spectrum analysis showed that the characteristic absorption peak of sulfonic acid group appeared at 1037 cm-1 and the gel chromatogram showed that the relative molecular mass of the product increased from 19393 to 33441,and the polydispersity coefficient increased from 2.79 to 4.77,which indicated that the sulfonic acid group had successfully inserted into the lignin molecule.The clay minerals studied in this paper are mainly montmorillonite,kaolinite and illite.They were incorporated into the cement system containing PCE only to observe the change in the flow properties of cement slurry.As comparison,after adding a certain amount of modified lignin,the changes in the flow properties of slurry was also observed.Experiments show that the negative impact of montmorillonite is more serious than that of the other two clay minerals.The initial fluidity of the slurry at 1%MMT content drops to 170 mm,the modified lignin has an inhibition effect on montmorillonite,and the fluidity can be restored from 170mm to 260mm.The interaction between modified lignin and clay minerals was analyzed by XRD,TOC and Zeta Potential test methods.It was found that the modified lignin can reduce the interlayer distance of montmorillonite from 1.42 nm to 1.23 nm.The mechanism is that the lignin molecules can be adsorbed on the surface and the side of montmorillonite particles through charge interaction and cationic?bonding.As a result,a negatively charged enveloping network is formed on the periphery of the particles,so that the repulsive forces generated between the montmorillonite particles which compress their own inter-layer spaces,the polycarboxylate thus avoids the large consumption of montmorillonite,and still plays excellent dispersion properties in cement systems.In contrast,the interlayer spacing of kaolinite and illite remains unchanged at 0.72nm and 0.99nm,indicating that the interaction between the two and the polycarboxylate only occurs on the surface of particles,so the modified lignin has less effect on them. |
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