What are the effects of water-reducing admixtures on the strength of concrete? They all need to be obtained through practice. After a large number of repeated practical studies by relevant technical personnel, we finally came to a conclusion that the effects of water-reducing admixtures on concrete have the following points :
1. The effect of water reducing agent on the rheological properties of fresh concrete
In order to produce new concrete with high fluidity, it is necessary to reduce the concentration of fine particles in the concrete. The viscous structure of fluid friction is conducive to completely dispersing the fine particles of concrete into the water medium. There are many characteristics that affect the melting of concrete, such as the mineral composition of concrete, the shape and size of concrete particles, the specific degree of mineral crystals, as well as operating conditions and environmental factors. The various factors mentioned above are directly or indirectly controlled.
Reliability of concrete particles in adhesion slurry. Different media conditions can change the charge on the concrete particles in the slurry, that is, change the electrostatic repulsion between the particles.
When appropriate additives are added to the freshly mixed soil, the potential difference required by the concrete particles is expanded, and the electrical repulsion between the concrete particles is greatly increased, resulting in a reduction in the viscosity of the freshly mixed soil, which results in Promote the reliability of the entire decentralized system and improve liquidity.
At the same time, there is an intermediate state between people in the mixed mortar body from the sealing liquid to the cohesion state, that is, the thixotropic state. This is caused by the redispersion of the cohesive structure in the cement paste when the shear rate increases. The main manifestation is that the friction decreases and the viscosity decreases when the shearing speed increases. That is, the slurry does not become agglomerated when it is still. If the slurry that has been agglomerated is stirred or shaken, it regains fluidity. Usually, adding appropriate admixtures to the plastering mortar can promote the strong thixotropy of fresh concrete. It will show good fluidity if it is slightly shaken due to the adsorption and solvation film of additives on the surface of concrete particles and the increase in potential difference. Fresh concrete that does not require a superplasticizer is much less thixotropic.
2. The influence of water reducing agent on the workability of fresh concrete
There are many factors that can endanger the adhesion of fresh concrete. The key are factors such as concrete, asphalt mixture, water requirements, properties and uses of water reducing agents, temperature. When other criteria are the same, adhesion is closely related to the type and dosage of admixtures. The workability of fresh concrete is usually measured using slump value measurements. There must be a first period of waiting time for transportation and parking after the concrete mixing station for irrigation, which makes the concrete's adhesion worse and makes the construction project difficult. Experiments have shown that adding water-reducing admixture can improve the initial workability of concrete, but the slump loss is often greater than the benchmark concrete without water-reducing admixture. The reasons are:
(1 ) The ability of minerals in cement to adsorb water-reducing agents. The main minerals in cement have the ability to adsorb water-reducing agents in the order of C3A > C4AF > C3S > C2S. As soon as water is added and stirred, more dispersants are forced to gather on the surface of the cement particles, and the concentration of water-reducing agents in the entire liquid phase decreases. When pouring, the The amount of water-reducing agent used for cement dispersion is gradually insufficient, so the slump gradually decreases with time.
(1) Bubble overflow and water evaporation. Even non-non-air-entraining admixtures must be carefully introduced when mixed into concrete. During transportation and other processes, bubbles continue to flow out and subside, accompanied by water evaporation. The performance of superplasticizers is particularly obvious. .
(2) Due to the effects of dispersion and wetting after adding water reducing agent, the initial hydration rate of cement is too fast, the hydration products increase, the amount of solids increases, and the viscosity of the entire system increases, resulting in slump. The value decreases rapidly, especially under high temperature conditions.
3. The influence of water-reducing agent on concrete setting time
Concrete setting time is a key parameter at the construction site, especially for the construction of large-volume concrete projects. important. Adding an appropriate amount of retardant can delay the setting time of concrete and facilitate the solution of problems during construction
Adding water-reducing agent will increase the slump of concrete. Adding too much water-reducing agent will cause concrete to bleed. water, even segregation.
Theoretically, adding a small amount of water-reducing agent will only increase the slump, but will not affect the water-cement ratio and the strength of the concrete. But if you are careless, it will definitely affect the strength.
The impact on strength can be considered from the following aspects:
1) Test block production. The water-reducing agent increases the slump of the concrete, and the concrete is prone to segregation and bleeding, so the requirements for the production of test blocks are very high. If the test blocks are not produced in a standardized manner, the strength will be reduced.
2) Concrete slump. If the slump of the prepared concrete is below 200mm, adding less admixtures will have little effect; if the slump is relatively large, above 220mm, then the impact of the secondary addition of admixtures will be greater.
3) Adjust the concrete. If the slump of the concrete delivered to the construction site is very small, it is necessary to add admixtures again for adjustment. Stronger than adding water. However, special attention should be paid to the completion of concrete pouring quickly, otherwise problems such as pump blockage may occur.
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