Influence And Mechanism Of Chemical Admixtures On Cement Hydration Process

Based on State "973" Project in China, hydration process, development of initial structure and mechanism of high C3S content cement with chemical admixtures were unfolded through multiple measurements.Using automatic high efficient hydration equipment together with non-contacting electrical resistivity device, traditional hydration heat model (H) and structure formation model (S) were connected. Using temperature-sensitive of H and structure-sensitive of S, instantaneous formation condition of paste structure was described exactly. Together with chemical shrinkage and mechanics properties measurements, regulative actions of chemical admixtures to hydration process were studied systematically. In addition, regulative mechanisms of chemical admixtures to hydration process were discussed thoroughly.Results showed that water reducers improved cement hydration process. Calcium lignosulfonate mainly retarded hydration of C3S. But solubility of gypsum was descended and aquolysis of C3A was promoted. When high amount of Calcium lignosulfonate was added, formation of AFt and AFm was speed up. Formation of hexagonal aluminate calcium was also promoted. In addition, because of its air-entering action, final properties of cement were descended., Initial hydration of cement was accelerated by superplasticizers because of their high range water reducing and dispersive actions. After that, hydration rate was descended as a result of more initial hydrates and absorption of superplasticizers. Dense of structure and development of final properties were benefited from retardation of hydration. In the same amount, cement hydration process was. controlled better by X404 polycarboxylate superplasticizer than UNF-5 naphthalene series superplasticizer owing to its stereoscopic action. And its adaptability to cement was better.Cement hydration process was postponed perfectly by retarders. Results showed that hydration of C3A was delayed or accelerated under the control of retarders, but hydration of C3S was postponed. Zinc salts mainly created infusible Ca (Zn (OH) 3)2- 2H2O, which covered on cement grains and thus cement hydration was delayed. Retarding effect of Zn2+ was weaken by the addition ofSO42- concentration, therefore retarding action of ZnSO4 was inferior to ZnCl2. Na3PO4 together with Ca2+created Ca3(PO4)2- But Na5P3Oi0 together with Ca2+created a kind of stable complex--CaNa3P3O10, and cement hydration was mademore delayed. There existed dual-critical effect of such organic retarders as sucrose and citrin on cement hydration. First, when the amount was relatively low, cement hydration was delayed. While the amount was comparatively high, initial hydration of cement was accelerated, but paste was not hardened after a long time. Second, there existed a critical amount. When lower than it, retarding effect was increased with increasing amount of retarders. While higher than it, retarding effect was dropped with increasing amount of retarders. Chemical shrinkage of cement was reduced under the control of organic retarders. With increasing amount of organic retarders, initial chemical shrinkage was increased. After that, chemical shrinkage was reduced. Regulative action of retarders on cement hydration heat process was unfolded a saddle dual-peak phenomenon. With increasing amount of organic retarders, the first heat peak was raised, and the second peak became broad and weak. It was because that hydration of C3S was postponed and hydration of C3A was promoted under the control of organic retarders. Formation of AFt was promoted by sucrose. But under the control of citrin solubility of gypsum was descended, while formation of AFt, AFm and hexagonal aluminate calcium were accelerated. Results showed that hydration of C3A was delayed by zinc salts, Na5P3O10, yet hydration of C3A was accelerated under the control of sucrose and citrin.