Incorporation Methods and Considerations for Functional Concrete Admixtures
The incorporation methods of functional concrete admixtures are mainly as follows:
I. Pre-mixing Method
1. Operation Mode
Firstly, mix the functional concrete admixture uniformly with cement, aggregates, and other dry materials, then add water for mixing. This method allows the admixtures to be well dispersed on the surface of cement particles, increasing the contact area between the admixture and cement, thereby better leveraging the effect of the admixture.
2. Applicable Situations
For some admixtures that require thorough reaction with cement, such as early strength agents and expansive agents, the pre-mixing method can enhance their effectiveness. When the mixing equipment performance is good and can ensure thorough mixing of dry materials, the pre-mixing method can be adopted.
II. Co-mixing Method
1. Operation Mode
Add the functional concrete admixtures, cement, aggregates, and water into the mixing equipment simultaneously for mixing. This method is simple to operate, convenient, and suitable for the incorporation of most admixtures.
2. Applicable Situations
For admixtures with no strict requirements on the order of incorporation, the co-mixing method can meet their usage requirements. In situations where production efficiency is highly demanded, the co-mixing method can save time and improve production efficiency.
III. Multiple-stage Incorporation Method
1. Operation Mode
Divide the functional concrete admixtures into several portions and add them to the concrete at different mixing stages. For example, add a portion of the admixture at the beginning of mixing, and another portion during the mid or later stages of mixing.
2. Applicable Situations
For admixtures that need to function at different stages, such as high-efficiency water reducers, the multiple-stage incorporation method can better control the properties of concrete. During the concrete production process, if it is necessary to adjust the properties of concrete, the multiple-stage incorporation method can be used for fine-tuning.
IV. Post-mixing Method
1. Operation Mode
First mix cement, aggregates, and water for a period, then add the functional concrete admixtures and continue mixing. This method can prevent adverse reactions between the admixture and cement during early contact and also reduce the loss of the admixture.
2. Applicable Situations
For admixtures that are prone to adverse reactions with cement, such as retarders and air-entraining agents, the post-mixing method can enhance their effectiveness. In cases where the slump loss of concrete is significant, the post-mixing method can effectively restore the fluidity of concrete.
3. Advantages and disadvantages
Compared with other incorporation methods, the post-mixing method has the following：
(1) Advantages
-Reduces Slump Loss
By first mixing cement, aggregates, and water for a period, cement hydration begins but is not extensive. Adding the admixtures later can avoid the strong reaction between the admixture and cement in the early stages, thereby reducing the consumption of admixture due to cement hydration and effectively controlling the slump loss of concrete. This is particularly significant for admixtures like high-efficiency water reducers that require high slump retention. For example, in pumped concrete construction, the post-mixing method can maintain good fluidity of concrete during transportation and pumping, reducing the risk of blockages.
-Improves Admixture Efficiency
As the cement hydration has proceeded to some extent when the admixtures is added later, the admixture can play a more targeted role, avoiding the waste associated with early reactions with cement. This allows for better concrete properties to be achieved with the same amount of admixture, improving the efficiency of admixture use. For example, when using retarders, the post-mixing method can more accurately control the setting time of concrete and prevent excessive retardation.
-Broader Adaptability
For admixtures that may have adverse effects when reacting with cement in the early stages, the post-mixing method can reduce such risks. For instance, if air-entraining agents are incorporated too early, they may be adsorbed excessively by cement particles, affecting their air-entraining effect. The post-mixing method can better ensure the effectiveness of air-entraining agents. Therefore, the post-mixing method has broader adaptability for different types of admixtures and cement.
(2) Disadvantages
-Relatively Complex Operation
Compared to the pre-mixing and co-mixing methods, the post-mixing method requires a second addition of admixtures during the mixing process, which increases the number of steps and time costs in the production process. It is also necessary to ensure the uniformity of admixture addition, otherwise, it may lead to uneven concrete properties. For example, in large-scale concrete production, the post-mixing method may affect production efficiency and requires stricter operational control.
-Greater Difficulty in Quality Control
Since the post-mixing method involves two mixing processes, it demands higher quality control for concrete. If the parameters such as time and speed of the two mixing stages are not properly controlled, it may affect the stability of concrete properties. Moreover, the timing and quantity of the post-mixed admixtures also require certain experience and technical level. For example, if the post-mixed admixture is added too late, it may not fully exert its effect; if the amount is not accurate, it may lead to concrete properties that do not meet requirements.
