1. Basic characteristics of fly ash
The basic characteristics of fly ash mainly refer to the activity of fly ash, which can be divided into two aspects: chemical activity and physical activity.
The physical activity of fly ash includes the micro-form of fly ash and the effect of aggregate, which has nothing to do with the chemical properties of fly ash. It can improve the cementing activity of concrete products and improve the performance of concrete products (such as wear resistance, strength, permeability resistance, etc.).The physical activity of fly ash is the main source of fly ash pre-activity.The effect of fly ash morphology is mainly manifested in the distribution of fly ash particle size, particle morphology and other characteristics can play a role in enhancing the filling and lubrication of cement-based materials.
The chemical activity of fly ash refers to that the soluble components of fly ash react chemically with water and limestone at room temperature to form the gel of calcium silicate aluminate hydrate and calcium silicate hydrate, and then make it harden in air or water and appear strength properties.Fly ash activity mainly comes from the hydration of Al2O3 and Si02 under alkaline conditions.
2.The effect of fly ash in concrete
(1) Reduce temperature rise
Of hydration reaction of cement is exothermic reaction, 1 kg gives off heat the hydration of cement can reach 500 j, due to the poor thermal conductivity of concrete, its internal hydration reaction of hydration heat accumulation condition, lead to rapid temperature rising, but its surface temperature is lower, the internal and external temperature difference, inside the concrete tensile stress, the tensile stress can exceed the concrete tensile strength, along with the time, the concrete internal temperature drop, is likely to appear again tensile stress, this could lead to concrete cracking.This is related to the temperature of the internal hydration heat release, which can effectively prevent the cracking of concrete by limiting the temperature of the peak temperature rise of concrete.
(2) Improve the workability of fresh concrete
Fly ash can improve the workability of fresh concrete mainly by reducing the water consumption of concrete when it has the same fluidity as ordinary concrete.The reduction of water consumption provides basic conditions for improving the workability of fresh concrete and reduces the probability of shrinkage cracks in hardened fly ash concrete.In addition, it can improve the strength of hardened concrete and reduce the change of volume of hardened concrete.Therefore, the water reduction rate of fly ash is the primary performance parameter of fly ash concrete.
(3) Delay the setting time of concrete
The chemical composition and characteristics of fly ash make it not only fill compactness and micro aggregate in concrete, but also have different degree of surface adsorption and effect of volcanic ash.Is due to the effect of fly ash on above, can improve the concrete internal hole structure, impact on the process of concrete hydration gel component, coordinating the development of concrete strength, effectively improve the concrete hydration products structure and composition, performance, optimize the structure of the concrete interface transition zone to promote comprehensive performance of concrete is improved.
With the continuous progress of the society, the application of fly ash is also being studied. As a kind of active admixture, fly ash plays an important role in the further development of high-performance concrete, which has received more and more attention.Because of its own unique advantages, not only can improve the performance of concrete, but also can obtain higher economic and social benefits, fly ash development prospects are extremely bright.