Experimental Study Of A New-type Slag Ultrafine Powder In Affecting Properties Of Cement Based Materials And Hydration Process

As one kinds of latent hydraulic activity mineral admixtures,ground granulated blast furnace slag has been widely applicated in cement based materials such as cement concret.Moreover,its effects on cement properties and the way to improve its latent hydraulic activity have been hot research topics in cement science.In this paper,the effects of a new-type slag ultrafine powder(SUP),separated from ground granulated blast furnace slag,on the properties and hydration process of cement based materials were studied.Accordingly,the effect mechanism of SUP on cement hydration and properties was discussed.The results as follows provide theoretical and technology fundamentals for the application of SUP in cement based materials to improve properties.SUP remarkably affects the physical properties of cement based materials.SUP can reduce the setting time,improve the water retention ability and cohesiveness,reduce the slump loss of concrete.With small dosage,SUP can improve the fluidity and plastic shrinkage resistance;but with big dosage,SUP will decrease the fluidity,increase the plastic shrinkage cracking degree and shorten the initial appearing time of plastic shrinkage cracks.However,with the cohesiveness of concrete being too high,it is adverse for the project application.SUP also improves the mechanical properties of cement based materials.SUP can specially improve the compressive strength,flexural strength,tensile splitting strength,axial compressive strength and the static modulus of elasticity,but increase the drying shrinkage.SUP can improve the flexural strength and deflection.But with different fineness,the influence of SUP is definitely different.SUP with relatively bigger particles can improve the flexibility,whereas SUP with relatively smaller particles decreases the flexibility.Relative to the late compressive strength,the early compressive strength of cement based materials can be further improved by SUP.SUP also improves the durability of cement based materials,such as the carbonization resistance,the chloride ion penetration resistance,the resistance to water penetration and abrasion resistance.With the increase of its dosage and fineness,the influence degree of SUP on the properties of cement based materials gradually enhances.Based on the properties evolution of cement based materials with SUP,it can be concluded that the dosage of SUP in cement based materials should not exceed 30 percent(the weight to cement).In other words,in cement based materials the dosage of SUP with its particle size less than 10 ?m should not exceed 30 percent,while the dosage of SUP with its particle size less than 5 ?m should not exceed 20 percent.SUP can participate and remarkably affect the cement hydration process.SUP can change the cement hydration heat release process,reduce the cement hydration heat and the cement hydration heat evolution rate before the hydration deceleratory period,but can accelerate the the cement hydration heat and the cement hydration heat evolution rate after the hydration deceleratory period,and accordingly increases the hydration degree.No new hydration product formed in the cement paste with SUP.But SUP greatly affects the formation rate and content of hydration products such as ettringite,portlandite and CSH gel,causes the content of ettringite and portlandite gradually decreasing while the CSH gel content and polymerization degree increasing after hydrated for 3 days.SUP also can remarkably improve the microstructure,optimize the pore structure and interfacial structure,decrease the pore volume and pore diameter,promote the densification of hydration products.With the hydration time prolonging and SUP dosage increasing,the influence of SUP on cement hydration process gradually enhances.According to the analysis of properties and hydration process evolution,combined with the chemical components&particles distribution of SUP,the mechanism of SUP improving the properties of cement based materials can be attributed to the micro-aggregate effect and the secondary hydration effect of SUP.During the different cement hydration process stages,the mechanism of SUP is obviously different.The key factor for SUP improving the properties of cement based materials during the initial cement hydration period(in 12 hours)is due to its micro-aggregate effect.The key factor for SUP improving the properties of cement based materials during the early cement hydration period(in Id to 7d)is due to its secondary hydration effect.The key factor for SUP improving the properties of cement based materials during the late cement hydration period(28d)is due to the synergism of the micro-aggregate effect and the secondary hydration effect.