Abstract:

Soluble and insoluble phosphate and fluoride were incorporated into cement respectively,and setting and hydration of pastes were investigated by using XRD and micro calorimeter.The results indicate that,when equivalent content of P₂O₅ or F varies from 0.5% to 1.5%,setting time of cement is significantly increased by blended soluble phosphates such as Na₂HPO₄ and Na₃PO₄ and insoluble CaF₂,whereas insoluble phosphates such as CaHPO₄ and Ca₃(PO₄)₂ and soluble NaF have not obvious retarding effect on cement setting, even flash setting is seldom observed.Microscopic analysis on soluble phosphate and insoluble fluoride discloses that they retard cement setting due to lengthened induction period.As for soluble Na₂HPO₄ and Na₃PO₄,they can decrease PH value,produce hydration products of aluminum phase and yield silicate solid solution,whereas insoluble CaF₂ retards hydration of cement by absorption effect of hydrogen bond,which comes from the combination of CaF₂ with H⁺ of hydration products located on surface of cement particles.

Key words: cement, phosphate, fluoride, retarding, hydration