| The sensitivity and mechanism of the Polycarboxylate superplasticizer(PC) to mud is one of the hottest spots in the research of concrete technology in recent years. In this paper, the adsorption of PC on the main mineral kaolin, montmorillonite and illite was studied, and the adsorption equilibrium of PC with three mineral surfaces and its influence factors were quantitatively determined by UV method, and the mechanism of it was discussed. The main contents and results are drawn from this work as follows:1. At 25℃, the adsorption and adsorption equilibrium of PC on these three kinds of clay(kaolin, montmorillonite, illite) were determined, in the solid-liquid ratio of 3:1, 10:1, 6:1 respectively. The results show that the time of illite coming to the adsorption equilibrium is the shortest which is just 5 minutes, while kaolin needs 30 minutes and montmorillonite needs 60 minutes; The saturation adsorption capacity is 1.60 mg/g, 25.8 mg/g, 4.78 mg/g, corresponding, the order for saturation adsorption of are: montmorillonite illite > kaolin. The Zeta potential of the three minerals in water was-10 mV,-35 mV and-21 mV; and when they came to the adsorption equilibrium, the Zeta potential was-25 mV,-51 mV and-29 mV, respectively.2. The effect of temperature on the adsorption and the adsorption heat were determined. The results showed that as the temperature increased from 15 to 45, the adsorption capacity of the three minerals increased; their heat of adsorption were-32.40 J J/mol,-103.70 J/mol,-63.47 J/mol. with the order of montmorillonite > illitie > kaolin. Further studies shows the adsorption curve of kaolin is approximate to Langnuir equation, while montmorillonite and illite are similar to Freundlich equation. With the time of equilibrium adsorption, it can be deduced that the adsorption of PC on the surface of kaolin is mainly monolayer adsorption, and belongs to electrostatic adsorption; but the interlayer of montmorillonite and illite is weak and they has a large interlayer spacing, so the crystal has a large internal surface area, which is dominated by intercalation adsorption.3. The influence of pH, SO42- and Ca2+ ion concentration on the adsorption was determined. When the solution pH changes from 5 to 12, the saturated adsorption amount of PC by these three clay minerals all decreased; The presence of SO42-and Ca2+ ion will inhibit the adsorption of PC to these three kinds of clay minerals, and the inhibition of Ca2+ is stronger than SO42-.4. The PC3:1, PC6:1 and PC2:1 with different density of branch density were selected to determine their adsorption capacity at three mineral surfaces. The results showed that the adsorption capacity of kaolin was 1.05 mg/g, 1.60 mg/g, 0.83 mg/g respectively, and the order was: PC3:1 > PC2:1 > PC6:1. While the adsorption capacity of montmorillonite was 33.8 mg/g, 25.8 mg/g and 21.8 mg/g, the order of its size was: PC2:1 > PC3:1 > PC6:1. The adsorption capacity of illite was 6.47 mg/g, 4.78 mg/g, 0.79 mg/g, and the order of its size was: PC2:1 > PC3:1 > PC6:1. It shows that the selectivity of kaolin, montmorillonite and illite to absorb PC is different. The adsorption of kaolin is the mainly by external surface, and the adsorption of intercalation is assistant, so it prefer to adsorb PC3:1 which has a higher density of branched chain and a higher anion active site density relatively; while montmorillonite and illite are mainly adsorbed by the intercalation, so it is easier to adsorb PC2:1 which has a highest branched density. |
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