Effect Of Active Admixtures On Hydration And Properties Of Reactive Powder Concrete

Reactive Powder Concrete（RPC）is a new type of cement-based composite material with ultra-high strength,high toughness and excellent durability,which overcomes the shortcomings of traditional Concrete with high brittleness and insufficient durability.In terms of the composition of raw materials,since RPC adopts a variety of active mineral admixtures to replace part of cement,it makes full use of industrial waste to reduce the cement consumption,which reduces the environmental pollution caused by cement production and reduce the consumption of non-renewable resources.It accords with the concept of green development and sustainable development of contemporary society.RPC has a good application prospect in bridge engineering,construction engineering,protection engineering,rail transit engineering and other aspects.The gelation system of RPC is complex,for its active admixture has an important influence on the hydration reaction of slurry,the formation of hydration products,and the evolution of microstructure.This paper adopts a variety of active admixtures to replace part of cement to study the influence of different active admixtures on the macroscopic properties of RPC.Based on this,this research selected pure cement system,cement-silica fume system,cement-silica fume-slag powder system and cement-silica fume-fly ash system four groups of typical gelation systems’ RPC.To explore the influence of different active admixtures on the hydration process of its slurry,explore the evolution process of microstructure,and clarify the formation mechanism of its strength.The main achievements of this research as follows:（1）According to the fluidity test,compression and flexural performance test,and water absorption test’s results,it shows that replacing part of cement with silica fume will reduce the working performance of RPC slurry and water absorption,but the compressive strength and flexural strength will be improved,compared with the gelation system of pure cement.When the silica fume replacement arrives 25%,RPC’s performance will be optimal.Keep the silica fume content unchanged at 25%,replacing part of cement with slag powder or fly ash can improve the working performance of RPC slurry.If the replacement is excessive,the strength of early curing will be reduced and the water absorption will be increased,but with the increase of curing age,the strength will gradually increase and the water absorption will gradually decrease.When the replacement amount of silica fume is 25%and the replacement amount of slag powder or fly ash is 30%,RPC can be guaranteed to have higher strength,lower water absorption,and improve economy and environmental protection.（2）Through setting time,electrical conductivity and chemical binding water tests,the results show that replacing part of cement with active admixture will reduce the hydration reaction rate of RPC slurry in the early hydration stage,and affect the generation and development of hydration products.The effects of different active admixtures on hydration products of RPC slurry were investigated by TG,X-ray diffraction（XRD）and INFRARED spectroscopy.The results show that the substitution of active admixtures for cement will reduce the CH content in RPC slurry,but increase the degree of polymerization of C-S-H gel.The active admixture with the pozzolanic effect which can consume CH and generate crystal nucleus effect to promote the generation of CH by cement.When the cement-silica fume system is hydrated for 28 days,the consumption of CH will be greater than the generation,and the total CH content is only 6.96%.The total CH content of cement-silica fume-slag powder and cement-silica fume-fly ash system is lower than that of cement-silica fume system.Different active admixtures all can increase the degree of polymerization of silica oxide tetrahedron in C-S-H gel,but because of the highest activity of silica fume in the early stage,C-S-H gel is the most generated when the cement-silica fume system is hydrated for 3 days.The addition of silica fume and slag powder at the same time is more conducive to the formation of C-S-H gel at the late hydration stage.（3）This paper applies Mercury injection（MIP）and nitrogen adsorption（BET）to study the influence of different active admixture systems on the micro-structure of slurry.The results show that replacing part of cement with active admixture would reduce porosity,optimize pore structure and make the micro-structure of RPC more compact.When the slurry was hydrated for 28 days,the porosity of cement-silica fume system is only 5.22%,which are mainly harmless pores.If replacing part of cement with slag powder or fly ash,both can reduce the number of harmful pores farther,but adding slag powder is benefit to increase the number of gel pores,while adding fly ash will reduce the number of gel pores.It can be observed that gelation material particles are tightly wrapped by C-S-H gel in the cement-silica fume and cement-silica fume-slag powder system,the micro-structure is very dense by Scanning electron microscopy（SEM）test.（4）Cement-silica fume system has the best macroscopic properties of RPC.Its main reasons as follow:Silica fume can promote cement hydration and increase the polymerization degree of hydration product C-S-H gel;it can reduce the particle gap and generate enough C-S-H gel to fill the pores tightly;and its micro-structure becomes denser and has fewer interface region,which reduces the possibility of defects.