The role of fly ash in concrete mainly includes three aspects: “morphological effect”, “pozzolan effect” and “micro-aggregate effect”.

The use of fly ash in concrete has both favorable aspects, such as reducing the heat of hydration, improving the later strength of concrete, improving the workability of concrete, etc.; it also has unfavorable aspects, such as reducing the early strength of concrete, prolonging the curing time, and resisting carbonization. The performance declines, and the two aspects can be combined to better understand and use fly ash in concrete.

(1) Morphological effect
The morphological effect of fly ash is the comprehensive effect of physical characteristics such as the appearance, internal and external structure, density and particle gradation of fly ash particles. Generally speaking, the morphological effect of fly ash can also be regarded as a physical effect. The morphological effect of fly coal can change the workability of the concrete mixture. The spherical glass microbead particles in fly ash can make the particles in the slurry evenly dispersed, reduce the friction between the particles, and increase the concrete mixture. liquidity. This is the positive aspect of fly ash, the positive aspect, and has the effect of reducing water and making the mixture homogeneous and dense. However, if the interior contains relatively coarse, loose, porous, irregular microbead particles and more unburned carbon content, it will lead to an increase in the water demand of fly ash and a decrease in the working performance of the concrete mixture, which is called a negative effect. The positive effect of fly ash morphological effect should be brought into full play, and the negative effect should be suppressed and overcome by certain means.

(2) Active effect
The active effect of fly ash is the most important basic effect of fly ash, which can play the role of cementitious material in concrete. The activity of fly ash refers to the chemical effect produced by the active ingredients in fly ash. The level of its activity depends on factors such as the speed of chemical action, its ability and the structure of its reaction product, the nature of its chemical composition, and the number of glass bodies. The active effect of fly ash can be enhanced by improving the concrete ambient temperature and chemical excitation. Silica (SiO2) and alumina (Al2O3) in fly ash can produce secondary hydration reaction under the excitation of cement hydration product Ca(OH)2 to generate calcium silicate hydrate (C-S-H), aluminum hydrate Calcium acid (C-A-H) fills the capillary pores and enhances the strength of the concrete. The hydration of fly ash is very slow. In the early stage, the physical filling effect of fly ash is basically dominant. With the increase of age, secondary hydration can proceed slowly. Concrete using fly ash has a good potential for later strength development. The improvement of fly ash concrete’s later strength growth must depend on the continuous maintenance of concrete curing temperature and humidity.

(3) Micro-aggregate effect
The micro-aggregate effect of fly ash means that the fine particles in fly ash are evenly distributed in the concrete slurry, which enhances the structural hardness of the hardened slurry. The advantages of the micro-aggregate effect of fly ash are:

(1) The fly ash in the concrete slurry densifies the capillary pores and improves the strength of the fly ash concrete;
(2) The solid and thick-walled hollow glass beads in fly ash have high strength, which can enhance the effect of cement paste. The glass beads are dispersed in the hardened cement paste and combined with the cement paste for curing time Longer and denser. At the interface between fly ash and cement slurry, the hardness of fly ash hydrated gel is greater than that of cement gel.

The three basic effects of fly ash coexist and exert influence together, and the three effects cannot be simply isolated. It is generally believed that for fresh concrete, morphological effects and micro-aggregate effects play a major role. With the development of hydration, the main influences on the properties of hardening concrete and hardened concrete are active effect and micro-aggregate effect.

Water demand behavior and water reduction effect of fly ash

In the production practice, it is found that adding better quality fly ash to the concrete will not increase the water consumption but reduce the water consumption, and has the advantage of water-reducing behavior. The water-reducing effect of fly ash in concrete is usually explained by the ball bearing effect of fly ash microbeads, and its water-reducing behavior and water-reducing effect mainly depend on its micro-aggregate effect and morphological effect. The water demand of fly ash is an important physical index of fly ash, which is defined as the amount of water required when the mixture of fly ash and water reaches a certain fluidity. The better, the higher the utilization value of fly ash. Fly ash particles are also considered as a mineral water-reducing agent. Although the water-reducing effect of fly ash is not as high as that of superplasticizers, it can still improve the working performance of fresh concrete.

The water demand of fly ash concrete is closely related to the density, fineness, particle gradation, water demand ratio, volume stability, oxide content, loss on ignition and alkali content of fly ash. Water ratio, loss on ignition and fineness are the main factors affecting the water demand of fly ash concrete. The water demand ratio of fly ash has a great relationship with the particle morphology of fly ash. The more spherical vitreous particles with smooth surface in fly ash, the less water demand; and the more porous particles, the greater the water demand; Within a certain range, the finer the fly ash, the smaller the water demand ratio. Thomas et al. obtained the relationship between fly ash fineness and water demand ratio in different loss on ignition ranges according to a large number of experiments:

Y=A+BX
In the formula: Y——the water demand ratio of fly ash concrete and ordinary concrete (%);
X——fineness of fly ash (45μm sieve allowance, %); A, B: test constant,
When the loss on ignition is 3% to 4%, A=88.76, B=0.25, the correlation coefficient is 0.86; when the loss on ignition is 5% to 11%, A=89.32, B=0.38, the correlation coefficient is 0.85 .

Water demand ratio is an important indicator of fly ash, which can comprehensively reflect the particle morphology and gradation of fly ash. In the mix ratio of fly ash concrete, the water demand of fly ash itself is the basic factor, however. The water demand ratio of fly ash (when the content of fly ash is 30%) cannot be directly regarded as the water reduction rate of fly ash concrete, and the water reduction rate of fly ash in concrete is determined after concrete trial mixing.