Study On Preparation And The Correlation Of Structure And Properties Of Polycarboxylate Superplasticizer

High performance polycarboxylate superplasticizer (PC) belongs to a kindof anionic surfactant. Because of the higher water-reducing ratio, lower slumploss, and bigger effect to the strength of concrete, it has been paid more attentionby concrete engineering world. For the performance and requirements ofsuperplasticizers in concrete admixtures industry, development of newpolycarboxylate superplasticizer is still one of the hot spot in the research. In thispaper, carboxylic acid groups, sulfonic acid groups, hydroxyl groups andpoly(ethylene oxide)(PEO) side chain were introduced on the molecularstructure of PC, and then a new polycarboxylate superplasticizer (RPC) wassynthetized by free radical polymerization. Meanwhile, synthesis method, thecorrelation of molecular structure and properties of RPC was discussed and theeffect of RPC on the hydration process of cement paste was studied. The mainresearch contents and the results are as follows:1. The research of polymerization process and conditions of RPC:Polycarboxylate superplasticizer was synthetized by polymerization ofacrylic acid (AA), methyl propylene sulfonic acid sodium (MAS), maleicanhydride modified β-cyclodextrins (MAH-β-CD) and allyl poly(ethylene oxide)ether (APEG) using ammonium persulfate ((NH4)2S2O8, APS) as the inititor. Themolecular structure and relative molecular weight of PC was characterized byFT-IR, NMR and GPC. Using orthogonal design and single factor variablemethod, the optimal monomer ratio and synthesis process conditions weredetermined: the monomer ratio of n(AA): n(MAS): n(MAH-β-CD): n(APEG)is5:0.5:0.1:1, the dosage of inititor is5%and the reaction temperature andtime is90°C and5h, respectively. The relative molecular weight of RPC is1.2×10~5on the condition of the optimum synthesis process.2. The research of the correlation of structure and properties:The molecular structure of PC emerged some certain regularity, that is to say, a certain percentage of active anionic groups, such as carboxylic acid andsulfonic acid groups, was introduced on the main chain of PC, which was used toprovide the electrostatic repulsion. Meanwhile, different length and quantity ofPEO side chains were grafted on the main chain of PC, which used to providethe steric hindrance repulsion. In order to banlance the molecular structure, therelative ratio of anionic groups and the length and quantity of side chains wereadjusted. According to this theory, the fluidity of cement paste, adsorptionamount and zeta potential were determined, and then the correlation of structureand properties was discussed. The experimental results show that the property ofRPC increases with increasing the content of sulfonic acid groups andMAH-β-CD side chains, and first increases and then decreases with increasingthe content of carboxylic acid groups and the length of PEO side chain. Theadsorption amount and ζ potential are1.5mg/g and-4.5mV when the dosage ofRPC is0.2%, respectively, and increase with increasing the dosage of RPC. Theapplication experiment results show that the water-reducing ratio,1h slump andsetting time are33.5%,218mm,365min and435min, respectively.3. The research of the hydration process of cement paste:The effect of PC on the hydration process and the hydration productsstructure was studied and the hydration mechanism was discussed using FT-IR,DSC-TG, XRD and SEM. The results show that the early hydration process wasobviously delayed and the hydration products microstructure was improved bythe new polycarboxylate superplasticizer. The formation and growth of hydrationproducts, such as ettringite (AFt) was decreased, and the growth of portlandite(CH) and hydration calcium silicate (C-S-H) gel after7days was promoted whendoped with new PC. All these results are benefit for the development of the latestrength of hardened cement or concrete. The whole development process of themicrostructure of hardened cement can be observed by SEM. SEM results showthat the microstructure of hardened cement doped with new PC are morehomogeneity and the porosities are more little, which are benefit for thedevelopment of later strength and durability of concrete.