Research On Synthesis And Reaction Mechanism Reaction Mechanism Of Shrinkage-reducing Polycarboxylate Superplasticizer

Although concrete is of great significance to engineering construction, it is a much vexedproblem how to reduce its shrinkage and improve durability of concrete structure.Shrinkage-reducing agent (SRA) is one of effective method to prevent shrinkage of concrete.However, SRA cannot be widespread used, because it is expensive and costly. Whilepolycarboxylate superplasticizer (PCs) is becoming to be one of indispensable admixtures toconcrete, because of its excellent properties. Recently, many scholars are making an effort tograft reductible groups onto lateral chain of PCs, or aggregate reduced groups on the mainchain of PCs, so that, PCs with excellent performance of SRA can lower the shrinkage ofconcrete without high costs. Our research, according to principles of molecular structuredesign and the molecular structure of PCs as well as its action mechanism, was conducted tosynthesize a kind of shrinkage-reducing polycarboxylate superplasticizer (SRPC) withreduced groups grafted to lateral chain of PCs， which can possess high water reducing ratio,high slump loss resistant as well as bring down shrinkage with low-content. Moreover, themolecular structure characterization and properties tests were made.The influences of raw material ratio, reaction temperature and time on esterification rate wereinvestigated firstly. The results showed that the raw material ratio, reaction temperature andtime were3.5:1,130℃and4.5h respectively for the optimum esterification rate. Contrastedvacuum distillation with vacuum infiltration, it was found that the method of vacuuminfiltration was more effective and scientific. And it was showed that the molecular structureof compound ester was the expected one and the ester was of high conversion rate and purity,after the molecular structure of compound ester was characterized by infrared spectrum (IR)and nuclear magnetic resonance (NMR). Moreover, the effects of monomer ratio and initiatordosage on rheological property were studied, and the results showed that the optimal mixingproportion for flowability of SRPC was n(APEG):n(DGMA):n(MAA):n(MAS)=1:2:2:0.125and the dosage of initiator was3%of grass mass of monomer, while best mixture ratio forrheology was n(APEG):n(DGMA):n(MAA):n(MAS)=1:2:4:0.33with the same dosage of initiator. The rheological model of cement with ideal SRPC whose yield stress was very lowwas proved to be Newton-type. The molecular structure of SRPC was represented by IR,NMR and GPC. And the results found that SRPC was grafted successfully shrinkage-reducinggroups on having expected group. Meanwhile, the molecular weight distribution of SRPC wastemperate. In contrast, SRPC, PCs and SRA sold in market possess the same mainfunctional groups.Besides, the effects of PCs, SRA and SRPC on shrinkage reducing property as well asstrength performance of cement mortar were also researched. It is found that shrinkagereducing ability of0.3%SRPC was much better than2%SRA until the day cement mortarscured for14days, and SRA doped with PCs demonstrated more excellent shrinkage reducingperformance than that without PCs.Furthermore, the influence of the surface tension of capillary pore solution, pore structuresand hydration of cement pastes on the shrinkage reducing properties was analyzed, accordingto the shrinkage mechanism. There were some results as follows: the surface tension ofwater was decreased to45.8mN/m by SRPC; the quantity of capillary below50nm of cementpaste with SRPC cured for28days decreased more than that with SRA, and the former hadmore optimal pore size distribution; the accumulative hydration heat of cement doped withSRPC was cut down, which might be related to the better shrinkage reducing performanceafter14days; with integration of TG-DTA it came out that both SRPC and SRA could delaythe early hydration of cement, which could be beneficial to enhance the anti-shrink efficiency.