Yan-rong Zhang;Xiang-ming Kong;
<正>Comparative study of polycarboxylate(PCE) and polynaphthalene(PNS) type superplasticizers was carried out with the aim of correlating the adsorption,the dispersing capability and the impacts on cement hydration of the two superplasticizers in fresh cement pastes(fcps).Total Organic Carbon(TOC) was used to quantify the adsorption amount of superplasticizers on cement surface.The fluidity of the fcp with fixed water to cement ratio of 0.4 was measured by mini-cone test.Influences of superplasticizers on cement hydration were studied via isothermal calorimetry.To identify the changes in microstructure of fcp,zeta potential and particle size of fcps were also determined by acoustic attenuation spectrometer.Results show that the adsorption behaviors of both PCE and PNS superplasticizers on cement particles follow a typical Langmuir monolayer adsorption model.With the addition of superplasticizers,the zeta potential significantly drops and then stays constant when the adsorption of superplasticizers on cement surface gets saturated.The fluidity of the fcps rises by adding the two types of superplasticizers and reaches constant maximum values at own critical dosage.The maximum spread diameter of the fcp with PCE superplasticizers is notably larger than that with PNS superplasticizers.With the dissembling of flocculates, particles she distribution in the fcp alters by adding superplastickers,and the particle size distribution in the cement paste keep unaffected by further addtion of superplasticizer when dosage is beyond critical dosage,It is also noted that at critical dosage,the surface coverage ratio(called critical coverage ratio) of PNS is about 100%, while the critical coverage ratio of PCE is less than 40%.On the other hand,the addition of superplasticizers leads to a prolonged induction period and a delayed main exothermic peak in the acceleration period of cement hydration.When superplasticizer dosage is beyond its saturated dosage,the cement hydration is hardly influenced by more addition of PNS superplasticizer,but is still affected by the further addition of PCE superplasticizer.It is believed that the impact of PNS superplasticizer on the fluidity and cement hydration mainly depends on the coverage ratio of PNS superplasticizers.By contrast,the PCE superplasticizer affects the fluidity mainly through a steric repulsion. The impact of PCE superplasticizer on the cement hydration is not only related to the coverage ratio but also significantly dependent on the residual concentration of PCE in aqueous phase due to the complexation effect of COO~- with Ca~2. Originality The impact of PNS superplasticizers on the fluidity and cement hydration mainly depends on the coverage ratio of PNS superplasticizers on the cement surface. Specifically speaking,larger coverage ratio leads to higher fluidity and stronger retardation in cement hydration.As the coverage ratio is 100%,the fluidity of fcp and cement hydration are not affected by the addition of PNS superplastickers.By contrast, the highest fluidity of the fcp is obtained as the coverage ratio of PCE superplastickers is less than 40%,which can be ascribed to the coverage ratio and sterk repulsion of PCK As for cement hydration,besides the coverage ratio,the residual concentration of PCE in aqueous phase also contributes a lot to the retardation of cement hydration due to the complexation effect of COO- with Ca~(2+) Therefore,the addition of superplastickers results in a delay in the cement hydration even though the adsorption of PCE superplastickers on the cement surface has already been saturated.
Adsorption;;Dispersing capability;;Hydration;;Superplasticizers;;Critical coverage ratio
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