| Cement and admixture chemistry is theoretical basis for designing and improving the performance of concrete. Gel permeation chromatography, zeta potential analysis, X-ray diffraction, differential scanning calorimetry and scanning electron microscopy were used to evaluate the effect of polycarboxylate superplasticizer(PCs) on cement hydration in this paper and the effect of PCs on the crystallization behavior and microstructure of hydration products were discussed. The adsorption property of PCs is the an important clue to study the hydration of cement and minerals. The main research contents and innovation points are as follows:1. The adsorption behavior of PCs with different structure on C3 S, Ca(OH)2 as well as their impact on C3 S hydration were studied. Results showed that adsorption amounts of PCs on C3 S is 5.5~6.5 mg/g during 60 min hydration, the corresponding zeta potential of C3 S is-3.2 mV. Adsorption capacity of Ca(OH)2 is 1.9 mg/g, the corresponding zeta potential is-4.32 mV. The results indicate that the adsorption site of PCs is occurred both on C3 S and its hydration layer and play an important role on C3 S hydration. Before induction period, the hydration heat is increased by adsorption of PCs, and promoted the dissolution of C3 S. During the induction and reduction period, C3 S hydration was delayed and the distribution and morphology of hydration products were obviously changed. The size of Ca(OH)2 is much smaller compared with the reference sample without PCs. The higher dosage and carboxylic density of PCs, the greater retardation effect on C3 S. The simulation experiment on synthesis of Ca(OH)2 crystal show that the size of Ca(OH)2 crystals in the absence of PCs is about 0.5~1 μm, while the size of Ca(OH)2 crystals reduced to 0.2~0.5 μm in the presence PCs.The results showed that the function of PCs in Portland cement paste are not only reducing water amount and improving pore structure of concrete, but also affect the hydration behavior of C3 S as well as the size of hydration product. This results provide a new basis for PCs to improve mechanical properties of concrete.2. The adsorption behavior of PCs with different structure on C3 A, Ca2SO4·2H2O, AFt as well as their impact on hydration of C3 A were studied. The results showed that the zeta potential of C3 A, AFt and Ca2SO4·2H2O were +12、+4.1和-0.6 mV. The maximum adsorption amounts of PCs on C3 A is 9~12 mg/g during 60 min hydration, while the adsorption amount of PCs on AFt and Ca2SO4·2H2O were 13~15 mg/g and about 0.21 mg/g, respectively. The PCs adsorbed mainly on the surface of C3 A and AFt, PCs with higher density of carboxylic group showed greater of adsorption capacity. The effects of PCs on C3 A hydration can be described that before induction, the solubility of C3 A and Ca2SO4·2H2O were inhibited, and the induction period was extended 1~2 h, and the transformation process of AFt to AFm was suppressed during the acceleration period. PCs with lower density of carboxyl group showed stronger inhibition effect. The strong adsorption of PCs on AFt resulted in delaying the growth and morphology change of AFt. The size of AFt synthesized in the absence of PCs is about 2 μm with long needle-like morphology, while in the presence of PCs, the corresponding size was changed to about 1 μm, which showed short rodlike morphology.3. The adsorption behavior of PCs with different structure on cement as well as their impact on cement hydration were studied. The results show that the adsorption amount of the three kinds of PCs is 1.3~1.6 mg/g during 60 min hydration, the equilibrium zeta potential is between-9 to-12 mV. Although the adsorption amount is lower than pure C3 S and C3 A, it is consistent with the conventional dosage of PCs in concrete engineering. The reason could be related to content of C2 S and grinding aid. The effect of PCs on cement hydration can be described that the induction period was extended, the hydration rate was slowed down. Increasing the dosage of PCs lead to prolong the induction period, this effect was consistent with the hydration of C3 S and C3 A in the presence of PCs. In addition, due to the covering of C-S-H gel on hydration phase of Ca(OH)2 and AFt, their characteristics of morphology are not obvious. But from the results of PCs on C3 S and C3 A hydration, it indicated that PCs can refine the crystal size of Ca(OH)2 and AFt, This role would contribute the strength of cement stone.4. In order to further verify that the adsorption of admixture is the main reason lead to retardation of cement and mineral, the adsorption behavior of the organic phosphonic acid HEDP on cement and its impact on the cement hydration were studied in this paper. The results showed that the adsorption ability of HEDP is stronger than PCs. During the dosage range of 0.02~0.2%, the adsorption rate of HEDP is more than 98% of the dosage and did not reach saturated. The corresponding exothermic peak shifted from normal cement hydration 11 h to 50 h. The retarding effect of HEDP depend on its dosage, increasing the dosage of HEDP lead to the extent of the induction period. The further study of lithofacies analysis showed that the formation and growth of AFt and Ca(OH)2 were inhibited significantly by the adsorption of HEDP. The morphology of C-S-H was changed from a short fiber-like structure to a unique microvilli-like structure. The retarding mechanism of HEDP on cement hydration can be considered as “adsorption” and “complexation precipitation” inhibition. |
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