Hydration Characteristics of Accerelating Agent Based on Calcium Aluminate
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Relationships between characteristics and hydraulic reactivity of fused calcium aluminate and crystalline calcium aluminate, hydration characteristics of accelerating agent based on calcium aluminate were investigated to obtain the fundamental data concerning the hydration characteristics in the initial stage of quick setting process.<BR>(1) The structure of the Al<SUB>2</SUB>O<SUB>3</SUB> group in fused calcium aluthinate resembled the γ Al<SUB>2</SUB>O<SUB>3</SUB> type while that of the crystalline calcium aluminate a Al<SUB>2</SUB>O<SUB>3</SUB> type. The heat of diffolution of the fused calcium aluminate is greater than that of the crystalline one by over 50 cal/g.<BR>(2) The heat evolution rate in hydration of the fused calcium aluminate immediately after the addition of water is greater than that of crystalline calcium aluminate by 2 or 3 times. The hydration during the initial period of fused calcium aluminate increases as fineness increases. This ten-dency is greater than that of the crystalline calcium aluminate.<BR>(3) When appropriate amounts of Na<SUB>2</SUB>Al<SUB>2</SUB>O<SUB>4</SUB>, Al (OH) <SUB>3</SUB>, Ca (OH) <SUB>2</SUB>, Na<SUB>2</SUB>CO<SUB>3</SUB> and CaSO<SUB>4</SUB> are added to the calcium aluminate and mixed to the cement paste, quick setting is obtained; quick setting occurs in 30 seconds after the addition of water when only 7% of accelerating agent is added. The acceleration effect is freater when fused calcium aluminate is used compared with the case of crystalline calcium aluminate.<BR>(4) The amount of heat evolution in hydration of the crystalline calcium aluminate immediately after the addition of water is less than that of fused calcium aluminate. However, since the lime saturation ratio in liquid phase is maintained in high level, vivid hydration of alite is promoted. (5) The hydrates produced in cement paste containing of accelerating agent immediately after the addition of water are calcium aluminate hydrate, monsulfate hydrate and ettringite. Especially amount of ettringite is remarkably different between accelerating agents based on fused calcium aluminate and crystalline calcium aluminate.
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