Study On Rehydration Of Reactive Powder Concrete By Different Fineness Of Cement

Reactive powder concrete is one kind of high performance concrete which has high strength and durability. It is designed based on the principle of maximum density and low water-cement ratio, which lead to a lot of unhydrated cement particles in hardened reactive powder concrete. The volume of hydration products is 2.1 times more than unhydrated cement particles, which makes the unhydrated cement particles become a potential hazard.In this paper, through establishing the hydration dynamics model and contrasting the change of cement hydration degree that before and after rehydration, simulation results were compared with the experiment results of chemical combination water and hydration heat. The effect of hydration on the strength properties of reactive powder concrete was observed by macroscopic strength test. The mechanisms of rehydration were accordingly analyzed through the test of fundamental frequency, porosity, SEM microscopic. The main conclusions are as follows.The degree of hydration kinetics model can accurately reflect actual cement hydration. Combined the results of chemically bound water and heat of hydration, it reveals that rehydration promote hydration degree of cement with low water-cement ratio significantly. After high temperature accelerated hydration for 50 days, hydration degree of reactive powder concrete with higher/medium/lower fineness cement increased by 14.8%/26.6%/16.0% compared to the end of curing stage.The compressive strength and flexural strength of reactive powder concrete with different fineness cement of different periods were measured. The compressive strength reached the maximum at high temperature water bath for 21d, which increases by 20.9%/18.2%/10.8% for reactive powder concrete with higher/medium/low fineness cement compared to the end of curing stage. The flexural strength is not obviously increased. After high temperature water bath for 21d, the compressive strength and flexural strength decreased significantly. The compressive strength of high temperature water bath for 80 days decreases by 17.0%/20.3%/15.6% compared to high temperature water bath for 21 days. There is small elevation of flexural strength after high temperature water bath for 30 days. Reactive powder concrete has been subjected to the dual role of self-repair and self-injury. Before high temperature water bath for 21 days, self-repair’s effect is greater than self-injury’s effect, macroscopic properties overall rise. After high warm water curing for 21 days, self-injury’s effect greater than self-healing’s effect, macroscopic properties of overall decline.Test of fundamental frequency found that its development trend is similar to the compressive strength. Before high temperature water bath for 21 days, the fundamental frequency increases, then decreased gradually. Porosity research shows that during the same period, from the end of curing period to high temperature water bath for 21 days, porosity of 0-50nm aperture of reactive powder concrete significantly reduced. After high temperature water bath for 80 days, porosity of 0-50nm aperture of reactive powder concrete gradually increased. Overall porosity of reactive powder concrete is gradually reduced from the beginning of high warm water curing. It was clearly observed in SEM electron microscope image. After high warm water curing, there is new hydration products that fix defects produced in internal pores and cracks of reactive powder concrete. At the last period of high warm water curing, there are a large number of 0.1 μm wide cracks appear inside of reactive powder concrete. Rehydration of reactive powder concrete has a destructive effect.