Precautions For Mixing Ratio Of Concrete Admixtures
Post Date:4,Mar,2025
1. Strictly control the amount of chloride ions added:
The chloride salt introduced into the admixture will do great harm to the volume stability and durability of concrete. It is mainly manifested in the following three aspects.
A. The corrosive medium dominated by chloride ions invading concrete will reduce the basicity of concrete, destroy the passivation film on the surface of the steel bar, produce a large potential difference in different parts of the surface of the steel bar, generate electrochemical reactions, and form the anode and cathode.
B. The crystal particles precipitated by repeated delixing crystallization of chlorine salts in concrete are coarse, and the volume expands, causing concrete cracking and reducing durability.
C. Melting aggregation It is generally believed that in ordinary reinforced concrete, the chloride content does not exceed 0.5%, which does not necessarily cause rust. However, sodium chloride reduces the freezing point of the solution, and when the concrete is gradually frozen from outside to inside, the chloride ion content in the unfrozen solution will become higher and higher, that is, the chloride ion concentration in the remaining solution is greater than 0.5%, causing local rust. Therefore, in actual production, although the chloride content is small, a high concentration may be formed locally, causing rust. Moreover, unlike calcium chloride, sodium chloride does not enter the molecular structure of the hydration product, but exists in the void in a free state, which is easy to absorb water, and increases the shrinkage rate of hardened concrete, which is also unfavorable to durability.
2. The effect of alkali content in cement on admixtures:
The high content of alkali in cement reduces the effect of water reducing agent. The solidification time was shortened and the early strength was increased. Too high or too low alkali content in cement will adversely affect the compatibility of superplasticizer and cement. This may be because it alters the solubility of gypsum, which affects the hydration of C3A. The optimal soluble base content was 0.4% ~ 0.6% (equivalent Na2O). When the compatibility of admixtures with cement is seriously poor, it should be considered to add part of sulfamate water reducer to replace naphthalene water reducer (or use polycarboxylic acid system), and appropriately increase the total amount of water reducer components to improve the adaptability of products.
3. Cement fineness and freshness:
The finer the cement fineness, the larger the specific surface area, and the larger the surface area of the cement particles that the admixture products need to act on. Fresh cement has high surface activity and strong adsorption to admixtures.
4. Type and amount of grinding aid:
In order to improve the output of the mill and the fineness of the grinding, cement plants usually add a certain amount of grinding aid to the cement grinding process. The type and amount of grinding aid have great influence on the adaptability of the admixture. For example, triethanolamine commonly used in grinding AIDS has a certain early strength effect on concrete at low doses, and when the dose exceeds 40000, the retarding effect will be more obvious. When the admixture products used in winter generally contain sodium chloride or sodium nitrite, this effect will be more obvious. Therefore, the influence of grinding AIDS should be fully considered in the selection or preparation of admixtures.
5. Influence of admixture and dosage:
For the adsorption capacity of materials, generally speaking, coal gangue b> fly ash > slag, and the adaptability of cement and slag is better than cement and coal gangue. In general, volcanic ash mixtures have a large specific surface area and therefore a large adsorption capacity. Different grades of fly ash vary greatly. For example, the first grade ash is fine, the microscopic shape is spherical, the carbon content is low, and the adsorption capacity of the admixture is low. It can also play a very good role in improving the working performance of concrete. However, the fly ash with high fire loss has high carbon content, and the silicon-oxygen tetrahedral structure of carbon determines its large adsorption capacity, so its adaptability to admixtures is poor. Similarly, mineral powder as an admixture will also have a great impact on the adaptability of the admixture according to its quality.
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