Since concrete admixtures have been widely used, people have found that the same admixture often shows different application effects on different types of cement, especially the rheological properties of fresh concrete and its changes over time. Again, this phenomenon can be explained by the "compatibility of concrete admixtures and cement". The compatibility of cement and admixtures refers to the degree of change in cement slurry fluidity and loss over time due to the quality of the cement or admixture when using the same admixture or cement, as well as the degree of change in the amount of admixture used to obtain the same fluidity. .
The so-called compatibility of admixtures with cement refers to the phenomenon that the same admixture has significantly different or even completely opposite effects due to different cements under the same conditions. Just like a concrete admixture at the same dosage, due to the different mineral composition, setting agent, mixing material, fineness, etc. of the cement, the plasticizing effect and water reduction enhancement effect at the same dosage are obviously different. Another example is Molasses and calcium lignosulfonate, which have a retarding effect, greatly shorten the setting time in some cements, and even final setting occurs in less than an hour.
The compatibility of admixtures with concrete, the most critical of which is the adaptability of admixtures with cement. Adaptability is also called compatibility. In accordance with the technical specifications for the application of concrete admixtures, admixtures that have been tested and meet relevant standards are added to concrete prepared with cement that can use this type of admixture according to regulations. If the expected effect can be produced, we consider that the cement is consistent with This kind of admixture is adaptable; on the contrary, if it cannot produce the desired effect, there is incompatibility between the cement and this kind of admixture.
The incompatibility between concrete admixtures and cement will cause fluctuations in concrete quality, and in severe cases will affect construction. Therefore, construction units and cement and admixture production units must take the following necessary measures to reduce or avoid the occurrence of incompatibility between cement and concrete admixtures. Strengthen the control of the temperature of the materials in the mill. Since controlling the hydration of C3A depends on the balance of sulfate ions in the pore solution, cement manufacturers must pay attention to controlling the content and dissolution rate of calcium sulfate in the cement and strengthen the control of the temperature of the materials in the mill. Control to avoid too high and too low temperatures to maintain the stability of the composition and content of gypsum in cement.
Grinding the cement mixture separately. The ultra-fine mixture in the cement can assist in "water reduction" and improve the fineness of the mixture in the cement. Without reducing the amount of the mixture, It can increase the strength of cement; while maintaining the strength of cement, it can increase the amount of mixed materials, reduce cement production costs, and improve the adaptability between cement and admixtures. Therefore, grinding cement mix materials separately in cement production plants is a feasible technical measure to produce high-quality cement. Change the mixing method of concrete admixtures. When preparing concrete, you can use post-mixing method or batch addition method to add admixtures to improve the workability of concrete. When added later, only a small amount of the concrete admixture is absorbed by the ettringite, so the slump loss is small. The reason why the post-mixing method improves the adaptability of admixtures to cement is: C3A, C. Cement with a higher mineral content such as tAF and gypsum has a greater adsorption force on admixtures, making the plasticizing effect of concrete admixtures poorer.
In the post-mixing method, because these minerals react quickly with water, the adsorption amount of their hydration products to admixtures is reduced, thereby improving the effect.
For this reason, during the use of concrete admixtures, when cement is available, cement that is more adaptable to the admixture should be selected. When admixtures are available for selection, admixtures that are more adaptable to cement should be selected, and measures should be taken to improve the adaptability of admixtures to cement. </p
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