Effects Of Adsorption Groups Of Polymers On The Rheological Behavior Of Cementitious Materials

Polycarboxylate?PCE?superplasticizers are the most widely used superplasticizers in modern concrete,which could greatly reduce the water to binder ratio and improve the flowability.However,a series of compatibility problems still exsist between polycarboxylate superplasticizer and concrete raw materials.Some scholars have adjusted the adsorption process of comb-like polymers by modifying the adsorption groups.Yet,the PCE structure?e.g.grafting density,charge density?and the results might not be comparable between each other and the fundamental effect of different adsorption groups on the rheological properties of cement pastes was still not so clear.In this thesis,a series of PCEs modified with different types?carboxyl group,sulfonate group and phosphate group?and proportions?1/3 and 2/3?of adsorption groups,different substitution methods?equimolar number substitution/equivalent charge amount substitution?,comparable molecular weight and the same side chain length have been synthesized and characterized by NMR and FT-IR.Based on the characterization of the solution and adsorption behavior,dispersion performance,and rheological properties of cement pastes,the effects and mechanism of PCE with different adsorption groups on the flow behavior of cement paste is explored.The main findings are as following:?1?The solution conformation of PCEs with different adsorption groups is influence by the intramolecular electrostatic repulsive force between anionic adsorption groups,the complexation of Ca²⁺and anionic adsorption group,and the“shielding effect”caused by the ions in solution.The results of hydrodynamic radius indicate that the complexing ability of different adsorption groups follows the order?from large to small?:-OPO₃^2-,-COO^-,-SO₃^-.The sulfonate-group-modified polymer and phosphate-group-modified polymer have a short backbone length,a small single-molecule occupying surface area.The phosphate-group-modified polymer synthesized by equivalent subsitituion has a higher side chain density and a larger saturated adsorption layer thickness.The phosphate-group-modified polymer synthesized by equimolar number substitution method has a larger amount of negative charge and strong Ca²⁺complexing ability.Under the experimental conditions,the saturated adsorption amount of PCEs substituted by 1/3 ratio and 2/3ratio of phosphate group with equimolar number substitution method is higher than that of traditional polycarboxylate superplasticizer by 76.9%and 82.7%,respectively.The saturated adsorption amount of sulfonate-group-modified PCE is much lower due to really low complexing ability with Ca²⁺and therefore really low adsorption affinity toward the surface of cement particles.?2?The fluidity of cement pastes is basically consistent with the adsorption amount of PCEs at higher PCE dosage.Compared with PCE with only carboxylic group,the dispersing performance of phosphate-group-modified polymer?substitution ratio 1/3 equimolar number?is improved by 27.5%under the experimental conditions.While at lower PCE dosage,the dispersing performance of phosphate-group-modified polymer with 2/3 ratio of phosphate group is slightly lower than PCE with only carboxyl group due to the“bridge effect”.The sulfonate-group-modified polymer has the worst dispersing performance due to its lower adsorption amount.?3?The phosphate-group-modified polymer has pronounced sulfate resistance.Under the experimental conditions,the sulfate sensitivity of polymer modified with 1/3 ratio of phosphate group characterized by the variety of yield stress is roughly 65.4%lower than that of carboxyl group.The effects of polymers with different adsorption groups on the rheological properties of cementitious material and the“adsorption-dispersion”mechanism is systematically explored from microscopic to macroscopic view.The improving effect of phosphate-group-modified polymer on the sulfate compatibility is further explored.The results provide a constructive reference for the development of new high-performance superplasticizers.