Post Date:7,Nov,2022
The role of concrete admixtures is to improve the flow properties of concrete and reduce the amount of cementitious materials in concrete. Therefore, concrete admixtures are widely used in various construction fields.
The mechanism of action of concrete admixtures：
The commonly used naphthalene-based admixtures and polycarboxylate-based admixtures are organic compounds with relatively high molecular weight (generally 1500-10000) and belong to the category of surfactants.
The molecule of the surfactant has a bipolar structure, one end is a non-polar lipophilic group (or a non-polar hydrophobic group), and the other end is a polar hydrophilic group. After the surfactant is dissolved in water, it can play various functions such as dispersing, wetting, emulsifying, foaming and washing while reducing the surface tension.
A. Adsorption-dispersion
The fluidity of a concrete mix depends on the amount of free water in the concrete. After the admixture is added to the concrete, the cement particles disperse each other due to the directional adsorption of the admixture molecules on the surface of the cement particles, resulting in electrostatic repulsion between them. As a result, the flocculation structure of the cement is destroyed, and a large amount of free water is released, which greatly increases the fluidity of the concrete mixture.
B. Wetting
Due to the directional arrangement of admixture molecules on the surface of cement particles, a monomolecular solvated water film is formed. This water film increases the contact area between cement particles and water on the one hand, and has a certain wetting effect on the other hand. Therefore, the cement is fully hydrated and the strength of the cement increases rapidly.
The basic functions of concrete admixtures:
1. Without reducing the unit water consumption, the water-binder ratio remains unchanged, which improves the workability of fresh concrete and improves the fluidity; due to the greatly increased contact area between cement particles and water, the cement is fully hydrated, although the water-binder ratio Invariant, the strength of concrete often has a certain improvement.
2. Under the condition of maintaining a certain degree of work, reduce the water consumption, reduce the water-binder ratio, and improve the strength of concrete.
3. Under the condition of maintaining a certain strength, reduce the amount of cementitious materials, reduce water consumption, keep the water-binder ratio unchanged, and save cement and other cementitious materials.
How to properly source and use concrete admixtures:
Properly procuring and using admixtures can generate enormous economic and technical value. It can not only improve the strength of concrete, but also reduce the cost of concrete mix ratio.
The specific method is as follows:
a. Test link
The testing and testing of various technical indicators of admixtures is an important link before purchasing negotiation. Through the test, the qualification standards of various technical indicators of the admixture should be determined. Including the solid content of admixtures, water reduction rate, density, slurry fluidity, concrete water reduction rate and other technical indicators. It is suggested that concrete water reduction rate be used as a key indicator to measure the quality level of admixtures.
b. Procurement
After the eligibility criteria for admixtures are clarified, procurement negotiations can begin. It is suggested that the admixture manufacturers should invite bids according to the qualified standards determined by the test. On the premise that the supply quality of the admixture is not lower than the bidding requirements, the supplier shall be determined according to the principle of winning the bid at a low price.
At the same time, the selection of concrete admixture manufacturers should comprehensively consider the manufacturer's production scale, transportation distance, transportation capacity, supply experience and supply quality level of large-scale mixing plants or large-scale engineering projects, and after-sales service capabilities and levels. As a single indicator for manufacturer screening.
c. Acceptance link
The mixing station should test the admixtures before the admixtures are put into storage, and the test results can be put into storage only after the test results are qualified according to the standards signed in the contract. It is recommended to distinguish between key indicators and reference indicators. Through long-term practice, the author believes that the key indicators of admixtures are water-reducing rate (mortar) and concrete water-reducing rate; reference indicators are density (specific gravity), solid content and fluidity of cement paste. Due to the testing time, the technical indicators that are generally tested in the acceptance link are the density, the fluidity of the cement paste and the water reduction rate (mortar).
Post time: Nov-07-2022