In order to improve the early strength of concrete, some manufacturers will add triethanolamine, calcium chloride and industrial salt to the superplasticizer, especially in winter or prefabricated component factories and cement products factories that require high early strength.
The chloride salt brought into the admixture will do great harm to the volume stability and durability of concrete, including:
1. The invasion of chloride-based corrosion medium into concrete will reduce the alkalinity of concrete, destroy the passivation film on the surface of steel bar, and there will be a large potential difference in different parts of the surface of steel bar, resulting in electrochemical reaction, forming anode and cathode.
2. The coarse volume expansion of crystal particles precipitated by repeated hydrolysis and crystallization of chloride salt in concrete makes the concrete crack and reduce the durability of concrete.
3. It is generally believed that the amount of chloride in ordinary reinforced concrete will not necessarily rust if the amount of chloride in ordinary reinforced concrete is less than 0.5%. However, when sodium chloride reduces the freezing point of the solution, when the concrete gradually freezes from the outside to the inside, the chloride ion content in the unfrozen solution becomes higher and higher, that is, the chloride ion concentration of the remaining solution will be more than 0.5%, causing local rust. Therefore, in the actual production, although a small amount of chlorine salt may form a higher concentration, corrosion may occur. And sodium chloride and calcium chloride will not enter the molecular structure of hydration products, but exist in a free state in the void, which is easy to absorb moisture and increase the shrinkage of hardened concrete, which is also disadvantageous to the durability.
Effect of alkali content on concrete additives
When the alkali content in cement is high, the effect of superplasticizer is reduced. The setting time is shortened and the early strength is increased. Too high or too low alkali content in cement has an adverse effect on the compatibility of superplasticizer and cement. This may be due to the fact that it changes the solubility of gypsum and thus affects the hydration of C3A. The best soluble alkali content is 0.4%-0.6% (equivalent Na2O). When the adaptability between admixture and cement is serious, some aminosulfonate superplasticizer should be added to replace nano superplasticizer (or polycarboxylic acid system), and the total amount of water reducing components should be appropriately increased to improve the adaptability of the product.
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