Investigation Of Effect Of Sodium Phosphate On Properties Of Hydration Of Alkali-activated Slag Cement

Alkali-activated slag cement is a new type of cementitious materials,which is formed by solution of alkali metal and slags. However,the setting time of alkali-activated slag cement is too short so that it can be hard used in practical constructions. Na3PO4 can obviously effect the setting time of AAS, however, no common agreement has been reached on the law of retarding effect of sodium phosphate in AAS until now. Setting time, slump,slump loss with time and compressive strength of AAS with Na3PO4 、water-glass(M=2.5) and Na OH were studied. The ratio of water to slag was 0.29.TAM air calorimeter was sued to study the effect of Na3PO4 on the hydration procsee of AAS. The kinetics parameters of AAS was analysed by Knusdon formular. The hydration products of AAS with Na3PO4 were been analysed by XRD and SEM. The following conclusions could be drawn:The setting time of AAS was effected evidently by the relative amount of Na3PO4 in mixed alkaline.In Na3PO4-Na OH, first, when the relative amount was less than 20%, continuity of particles of slags were enhanced,the setting time was retarded. While the relative amount was more than 80%, Na3PO4 and Na OH were separated out from liquid phase and pasete was quick setting, however, paste was not hardened after a long time. In Na3PO4-water glass, when the relative amount of Na3PO4 was less than 20%, degree of hydration of AAS was reduced,the setting time was retarded. While the relative amount was more than 80%, Na3PO4 and water glass were separated out from liquid phase and pasete was quick setting,The process of hydration of AAS was effected significantly by the relative amount of Na3PO4 in mixed alkaline. In Na3PO4-Na OH, when the relative amount was less than 20%, the rate of hydration was low. According to the kinetics parameters, reaction rate in acceleration was reduced, chemical reaction was controlled by dense diffusion,in deceleration and decay, chemical reaction were controlled by dense diffusion and porous diffusion. While the relative amount was more than 80%, the hydration heat of AAS was low, slag can not be activated in normal way. In Na3PO4-water glass, when the relative amount of Na3PO4 was less than 20%, the peak of hydration heat was reduced, the time of acceleration was retarded. According to the kinetics parameters, reaction rate in acceleration was reduced, chemical reaction was controlled by dense diffusion,in deceleration and decay, chemical reaction were controlled by dense diffusion and porous diffusion, rate of chemical reaction was increased with time. While the relative amount was more than 80%, AAS can not hydrate in normal way.The production of AAS activated with Na3PO4 has no Ca3(PO4)2 crystal.It was not the mechanism that retarding the setting time of AAS.