The Mechanism And Effect Of Sulfates On Compatibility Of Polycarboxylate-type Superplasticizer And Concrete Constituents

The polycarboxylate-type superplasticizer (simply as PC) as a concrete admixture with high dispersibility and water reducing performance has been widely applied. However, the compatibility between PC and concrete constitutes often exist in practical engineering application. Apart from the basic properties of superplasticizer, sulfates was considered to the most significant factor to compatibility. Research on the mechanism of how sulfates affects compatibility is good for enhancing PC application level, and consummating the theory of compatibility between PC and concrete constitutes.The mechanisms of compatibility between PC and different sulfates were revealed through the influences of four types sulfates, such as K₂SO₄,Na₂SO₄,CaSO₄·2H₂O and CaSO₄·1/2H₂O on the surface tension dispersion, viscosity and setting time of cement paste with PC, and the adsorption capacity on cement particle, dynamics and electric potential.The surface tension meter, ultraviolet spectrometer and infrared spectrometer were used to study the effects of sulfates on the surface tension of cement paste solution which added PC, and on molecular structure. It is found that the sodium sulphate and potassium sulfate could reduce the surface tension dispersion of cement paste with PC and increase its surfactivity and micro air-entraining role. The dihydrate gypsum had poor solubility and dissolution rate in cement paste, so its influence on surface tension was weak. The sulfates had no effect on the molecular structure of PC, but ester PC in the extreme alkalinity could produce hydrolysis reaction which made the polyoxyethylene separated from backbone, to the disadvantage of dispersibility of cement paste.The influence of sulfates on compatibility of PC had been researched by the cement paste fluidity, mortar dispersion degree and the slump of concrete and its loss. The results showed that the sodium sulphate and potassium sulfate had more significant negative effects on dispersibility of PC than insoluble CaSO₄·2H₂O and CaSO₄·1/2H₂O. When the dosage of soluble sulfate was 1%, the fluidity of cement paste had been lost completely. However, when the dosage was 0.2%, it helped to improve the fluidity and resist slump.The ultraviolet spectrometer and Zeta potentiometric analyzer were taken as the analytic tools to research the adsorption capacity and adsorption dynamics of PC, and ζ-potential of cement paste. The compatibility between PC and concrete constitutes which effected by sulfates was centred on. The experimental results indicated that the adsorption of PC on the cement particle was marked reduced by the soluble sulfate, and theζ-potential was also dropped. Therefore the electrostatic repulsion that between particles reduced, too. The average adsorption reduced by 38% and theζ-potential reduced by 18.3% when dosage was 1%, which had adverse impact on compatibility. But the moderate soluble sulfate, such as 0.2% could enhance the adsorption by 10%. The combined action of sulfate and PC leaded viscosity of cement paste low and was beneficial to keep its fluidity. The Langmuir isothermal adsorption equation was appropriate for describing the influences of cement particle on the adsorption capacity of PC and its response to sulfates. Meanwhile the Langmuir second-adsorption dynamics model was appropriate for describing their adsorption dynamics behavior. The sulfates could lessen the adsorption rate constant of this second-adsorption dynamics equation and delay the adsorption equilibrium time of PC to reduce its dispersion effect and affect the dispersion properties of cement paste and the compatibility of PC.