Regulation And Mechanism Of Adjustable Solidification Agent On Cement Hydration Process

This paper is based on National 973 Key Basic Research Development Program in China, hydration process, development of initial structure and mechanism of cement with chemical admixtures, which is the project of "The formation mechanism of modern concrete cementitious paste microstructure" (NO.2009CB623201)Multiple measurements are unfolded in this paper, such as setting time, drying shrinkage, mechanical properties and chemically combined water etc.. Through these macro-performances, the hydration process, the initial structures and the changes of volume of cement hydration are studied. Further more, the structure formation and development of paste together with adjustable solidification agent (set retarding admixture and flash seting admixture) are investigated systematically and comprehensively. In addition, regulative mechanisms of chemical admixtures to hydration process are discussed thoroughly by microstructure analysis techniques.Results showed that cement hydration process was postponed perfectly by Set Retarding Admixture. Hydration of C3A was delayed or aceelerated under the contral of retarders, but hydration of C3S was postponed. Na3PO4 react with hydration production of Ca(OH)2, reduces the concentration of Ca+, formed insoluble calcium phosphate on the surface of the clinker minerals, but Na5P3O10 together with Ca+ created a kind of stable complex, Na3PO4,Na5P3O10 not only delay the hydration of C3A, but also slow down the hydration of C3S.The hydroxyl (-OH) of sodium gluconate molecule can be formed hydrogen bonding on the surface of cement particles,which can prevent hydration of cement, carboxyl (COO-) can form a stable complex with Ca2+, and delay the hydration of C3S, sodium gluconate can decrease the surface tension of the solution increase the hydration marauder of cement mineral particles, accelerate the early hydration of C3A, The hydration of ettringite deposited on the surface of the cement mineral particles, inhibiting the hydration of cement, Regulative action of retarders on cement hydration heat process was unfolded a saddle dual-peak phenomenon. With increaseing of organic retarders, the first peak was raised,and the second peak became broad and weak. Chemical shrinkage of cement was reduced under the control of set retarding admixture. With increasing amount of organic retarders,initial chemical shrinkage was increase.after that, chemical shrinkage was reduced.The tape of sodium meta-aluminate and aluminum sulfate liquid flash seting admixture can aceelerate the hydration of C3A,also accelerate the hydration of C3S, made the cement paste rapid condensation, and greatly improve the early strength of cement, but, due to the cement paste rapid condensation, hardening, made structure loose, and generate a large number of small ettringite, which wrapped on the surface of hydration products, inhibiting the hydration of C3S and C3A hydration late, result the late strength of paste loss more. Compared to sodium meta-aluminate flash seting admixture, aluminum sulfate liquid flash seting admixture can less reduce late strength, and can less drying shrinkage in relative age, sodium meta-aluminate flash seting admixture compared high alkali content, easy to lead cement occurred alkali-aggregate reaction, therefore,in higher durability required major projects should be used cautiously.Each set retarding admixture exist an optimal dosage,In a suitable dosage range, although the early strength of cement paste has been reduced, did not affect the development of late strength, or even slightly higher, but too much paste mixed with retarder not only reduce the early strength, and will reduce the later strength of cement paste, the variety performance of the latter also have adverse effects. Incorporation of the flash seting admixture can reduce the late strength of cement, so in practical engineering applications, should be considered the performance requirements of setting time, the late strength, volume stability, durability, etc.., to choose the right type of flash seting admixture, and determine the appropriate dosage.