The Study On The Cement-based Material Water-reducing Agent Prepared By Modified Starch

Water reducing agent is a kind of admixture that is frequently used in concrete preparation and playing an important role in improving the performance of the concrete, as its water-reducing and plasticizing ability can help the particles of the cement disperse in water more even, reduce the separating trend of the concrete constituents, contribute to the microstructural homogeneity of hardened concrete and effectively improve the strength, durability and impermeability of the concrete. At present, the common water reducing agents used in vast amount in the construction projects are naphthalene, aminosulfonic acid series and poly carboxylic acid series superplasticizers, of which the universal applications in concrete have greatly expanded the field of application in concrete industry. However, these kinds of water reducing agents are toxic themselves or would discharge a large number of poisonous spent liquid during the production process, which is non-biodegradable and so not environment-friendly. The starch is a kind of natural carbohydrate polymer, which can be obtained easily and nontoxic in itself. Therefore, it's a kind of material that is of great value for modification and has been used in a lot of areas. Based on the principle of the molecular function structure design, the starch was modified so as to prepare a kind of water reducing agent with good water reducing capability and the water reducing mechanism of the starch water reducing agent was also investigated in the article.Three different kinds of starches were used as the original materials in this study. In order to determine the feasibility of the molecular functional group design, the experimental parameters and optimal medification process the soluble starch was used as the raw material. The modification technique consists of three stages. Firstly the oxidized starch was prepared through solution method (used hypochlorite as the oxidant under base-catalyzed condition), and then the oxidation-ether starch(hereinafter shortened as OES water reducing agent) prepared through solvent method (used oxirane as the etherification agent to etherify the oxidized starch in the ethanol as remaining the alkaline). The fluidity of the cement paste with the various addition of OES was investigated. The result showed that the water reducing ability of the OES was the best when the carboxyl content was 0.48% and the etherification degree (MS) was 0.5 in the molecular of the modified starch. Secondly, the writer took the corn starch under a certain alkaline as the raw material of the modification to prepare the OES water reducing agent with great dispersing ability through the same modification process. Lastly, the writer prepared also successfully the OES water reducing agent with great dispersing ability by etherifying modification on the oxidized starch of the same carboxyl content oxidized by hydrogen peroxide that was bought from the factory, which laid a foundation for the industrialization development of OES.The test result of the OES water reducing agent showed that the water reducing ability of the OES water reducing agent was better than that of naphthalene superplasticizer when the addition was 0.6%, especially the retention capability of fluidity of cement paste with OES. After 30min the fluidity of the paste with OES water reducing agent didn't lose, instead, it increased 22.4%, and the fluidity of the paste in 2 hours was almost no loss. But OES water reducing agent had an apparent retarding effect on the cement paste. It can be demonstrated that in the early hydration of cement paste with OES the hydration exothermic peak appeared in the time of 75 hours, but it didn't influence the late stage hydration and the 7d compressive strength reached the national standard and 28d compressive strength ratio could be 127%, therefore, it can be used as a brilliant super-retarding superplasticizer of the cement.The Zeta potential, absorption quantity and absorption thickness test results showed that the water reducing dispersing function of the OES water reducing agent would be realized through the stereospecific blockade of the water reducing agent molecular absorbed on the surface of the cement particles. The water reducing agent molecular was absorbed on the surface of the cement particles by the anion carboxylic acid group and the larger molecules of OES water reducing agent and the solvation water film formed by the hydrogen bond jointly provided the stereospecific blockade function.The result of Ca²⁺complexation capacity of the OES test showed that the retarding effect of the OES can mainly contribute to the complexation capability of the OH group in the molecular chains of the OES with Ca²⁺ in the liquids, which can prevent the hydration of C3A and resulting to delay the hydration process of cement. However, it can effectively reduce the retarding effect of OES by formulating OES and quick-setting agent, such as TEA and saturated limewater.