Effect of fly ash on concrete
We are the large steel castings manufacturer, today we would like to share the Effect of fly ash on concrete for you.
Many important properties of concrete are significantly improved after adding proper amount of high quality fly ash.
There are, of course, individual performance degradation. That is to say, fly ash has more positive effects on concrete, but also has adverse or negative effects, especially when the fly ash content is too large or the quality of fly ash is poor.
1.1 positive effect of fly ash on concrete
(1) the workability of concrete mixture is improved
Adding appropriate amount of fly ash can improve the fluidity, cohesion and water retention of concrete mixing materials, make concrete mixing materials easy to pump, pouring and forming, and reduce the slump of the time loss.
(2) the temperature rise of concrete decreases
Mixing fly ash can reduce the amount of cement, and fly ash hydration heat is very little, thus reducing hydration heat, so the construction of concrete temperature rise decreases, can significantly reduce the temperature crack, which is particularly beneficial to mass concrete engineering.
(3) the durability of concrete is improved
Due to the secondary hydration, the compactness of concrete, interface structure is improved, at the same time because the secondary reaction to reduce the number of susceptible to corrosion of calcium hydroxide, so after adding fly ash can improve the permeability resistance of concrete and the resistance to sulfate corrosion resistance and corrosion resistance of magnesium salt, etc. At the same time, because of the great specific surface area of fly ash adsorption ability is strong, can adsorption alkali in cement and fly ash particles, and react with alkali consumption and its amount. The reduction of free alkali can inhibit or reduce the alkali aggregate reaction. Generally, the fly ash content of 3 can avoid the alkali aggregate reaction.
(4) deformation decreases
The creep of fly ash concrete is lower than that of ordinary concrete. The water-reducing effect of fly ash results in dry shrinkage of fly ash concrete and early plastic fracture
Ordinary concrete is basically the same or slightly lower, but poor quality fly ash will increase the shrinkage of concrete.
(5) improved wear resistance
The strength and hardness of fly ash are higher, so the abrasion resistance of fly ash concrete is better than that of ordinary concrete. However, poor curing of concrete will result in reduced abrasion resistance.
(6) cost reduction
Adding fly ash at the same strength level can reduce the amount of cement by about 10% ~ 15%, thus reducing the cost of concrete.
1.2 negative effect of fly ash on concrete
(1) the intensity develops slowly and the early intensity is low
Because the hydration rate of fly ash is lower than that of cement clinker, the early strength of concrete mixed with fly ash is lower than that of ordinary concrete, and the higher the fly ash content is, the lower the early strength will be. However, for high-strength concrete, the early strength reduction of concrete mixed with fly ash is relatively small. The strength of fly ash concrete develops relatively slowly, so in order to ensure the normal development of strength, the curing time should be extended to more than 14d.
(2) carbonization resistance and frost resistance have been reduced
The secondary hydration of fly ash reduces the amount of calcium hydroxide in concrete, which is not conducive to the carbonization resistance of concrete and rust prevention of steel reinforcement. The secondary hydration of fly ash makes the concrete structure more compact and is beneficial to protect steel reinforcement. Therefore, the steel corrosion performance of fly ash concrete is not much worse than that of ordinary concrete. The results of many researches are not completely consistent, some think that the corrosion of steel bar is increasing, others think that the corrosion of steel bar is slowing down. No matter what the result is, when adding fly ash, if the water reducer is used at the same time, it can effectively slow down the reduction of carbonization resistance caused by adding fly ash, so as to improve the protection ability of steel reinforcement.
The frost resistance of fly ash concrete is lower than that of ordinary concrete, especially when inferior fly ash is used. There is a requirement for frost resistance of concrete should be made of high quality fly ash, when antifreezing demanding should be adding air-entraining air content is made to reach the requirements of the numerical value, can ensure the good frost resistance of concrete. Such as the three gorges dam with concrete mixed with 20% ~ 30% of Ⅰ grade fly ash and air-entraining agent, achieve F300 frost resistance.
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