| Recently, demand of concrete water reducer become huge as large-scale of constructionin China is doing. For example, the consumption of water reducer reached 4.84 million tons in2009. The application of superplasticizer can reduce building energy consumption andimprove the durabilityof the project, so it brings significant economic and social benefits. Butthe study of synthesis and dispersion mechanism of superplasticizer is not enough.Superplasticizer is water-soluble polymer with structural characteristics and surfacephysico-chemical properties of surface active agents. In this paper, superplasticizer was cuttedinto 3~4 fractions by ultrafiltration membrane technology, then the structure of fractions arecharacterized and dispersion propertities of each fraction were also studied. Adsorptionkinetics, thermodynamics, zeta potential and adsorbed layer thickness data of differentfractions after being adsorbed on cement particles were to sheld light on dispersionmechanism of superplasticizer fractions. Conclusions here may provide useful direction andreference for optimizing the synthesis craft and in-depth understanding dispersion mechanismof superplasticizer.In this paper, five superplasticizers, including the modified pulp black liquorsuperplasticizer (GCL1-JB), naphthalene sulfonate formaldehyde condensate superplasticizer(FDN), sulfonated acetone formaldehyde condensate superplasticizer (SAF), amino acidphenol formaldehyde condensate superplasticizer (ASP) and polycarboxylate superplasticizer(PC) were studied.The results show that:(1) Different molecular weight fractions of superplasticizer had distinct difference inintrinsic viscosity. GPC test result showed that dividing the superplasticizers into fractions ofdifferent molecular weight was achieved by ultrafiltration membrane technology, andmolecular weight distribution of fractions got narrower. Elemental analysis result showed thatfractions'molecular weight were higher, degree of sulfonation would be lower, and thesingle-molecule sulfonic group content was higher.(2) From the results obtained from the study on the effect of fractions of fivesuperplasticizers on the dispersion of cement paste, it was found that the higher molecularweight fractions, the better the water-reducing dispersion performance. SAF fractions of high molecular weight enjoyed better dispersion performance than other type superplasticizers。When dosage of water reducers was 0.5%, and fluidity of cement paste reached 250 mm,water-cement ratio of SAF was 0.31, but water-cement ratio of high molecular weight SAFfraction (SAF-D) was only 0.21. When fluidity of cement paste was up to 250mm and dosageof polycarboxylate superplasticizer was 0.2%, corresponding water-cement ratio was 0.25.And when water-cement ratio was 0.21 and dosage of polycarboxylate superplasticizer withhigh molecular weight was 0.2%, fluidityof cement paste was up to 268 mm.(3) From the results obtained from the study of adsorption characteristics of SAF, FDNand ASP fractions, it was found that the higher molecular weight fraction, the larger amountof it on cement particles would be adsorbed, and so were zeta potential and adsorbed layerthickness. High molecular weight fraction adsorbed to the surface of cement particles inducedgreater electrostatic repulsion and steric hindrance, so the cement system would show betterdispersion performance. Further study of SAF showed that high molecular weight fractionwas of higher absolute value of Gibbs free energy, zeta potential and adsorbed layer thickness,and might preferentially adsorbed onto the cement particles, which was related to it'smolecular structure and adsorption configuration. High molecular weight molecule withhigher sulfonic group content per chain and better molecular chain flexibility, was proposed tobe coil configuration in solution, which enhanced interaction with the cement particles. Andhigh molecular weight molecule adsorbed on cement particles formed loop on cementparticles to show multi-charge structure which induced higher electrostatic repulsion andstrong short-range steric, and as a result dispersion performance of cement paste was greatlyincreased.
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