Effect of Fly Ash on Concrete Properties
Release Time:
2020-08-03
Source:
Effect of Fly Ash on Concrete Properties
(1) Operational Performance
Fly ash can optimize the particle-size distribution of cementitious systems, reduce porosity, release “filling water” between cement particles, and enhance concrete workability.
Fly ash contains a large amount of spherical glassy particles, which act as “ball bearings” to provide lubrication, reduce interparticle friction, and improve the workability of concrete.
Fly ash exhibits significantly lower activity than cement, which can reduce slump loss in concrete. High-quality fly ash has a lower adsorption capacity for admixtures compared with cement; therefore, using high-quality fly ash is equivalent to increasing the dosage of admixtures, resulting in improved initial slump and enhanced slump retention.
Fly ash has a lower density than cement; when used to replace an equivalent amount of cement, it increases the paste content in concrete, thereby enhancing its cohesiveness, improving its resistance to segregation, reducing water exudation and bleeding, and optimizing its workability. This results in concrete with better fluidity, compactness, and homogeneity, making it easier to place and finish.
(2) Mechanical Properties
Fly ash itself does not undergo hydration; it can only undergo secondary hydration in the presence of cement hydration products. Consequently, when fly ash is substituted for cement on an equal-weight basis, early-age strength will decrease. However, as secondary hydration progresses, the concrete’s mid- and later-age strength will eventually match or even exceed that of concrete without fly ash.
As the replacement ratio of fly ash for cement increases, early-age strength gradually decreases; however, concrete incorporating fly ash exhibits faster late-age strength development, and within a certain range (<50%), its late-age strength increases with increasing fly ash content.
(3) Durability
Replacing a portion of the cement with fly ash, reducing the water-to-cement ratio, or increasing the fly ash content while maintaining the same water-to-cement ratio can enhance the impermeability of concrete.
The early-stage carbonation depth of fly-ash concrete increases rapidly, while the later-stage increase is relatively slower. As the fly-ash content rises, the carbonation rate accelerates and the internal alkalinity of the concrete decreases, which can trigger reinforcement corrosion. Therefore, the fly-ash dosage should be controlled, and a rational concrete mix design should be adopted.
