What aspects should be paid attention to when using concrete admixtures?

Usually, substances added to concrete to improve the properties of concrete during the mixing process and before mixing are collectively called admixtures. During the concrete mixing process, appropriate amounts of concrete admixtures need to be added to improve the performance of the concrete. The mass of admixtures mixed into concrete shall generally not exceed 5% of the mass of cement.

The wide application of concrete admixtures is the primary symbol of modern concrete engineering. Some quality problems of the mixture during construction are usually related to the viscosity, thixotropy, dilatancy and other characteristics of modern concrete mixtures. However, there are many factors that affect the quality of concrete projects, most of which are application problems. Therefore, there are several points that should be paid attention to in the selection and use of water-reducing agents:

(1) It is not appropriate to use product “generation” and “grade” to evaluate water-reducing agents, because any product has certain advantages. There are disadvantages, there is no good or bad, only suitable and inappropriate;

(2) The dosage of concrete admixture is very small, especially the polycarboxylic acid-based high-efficiency water-reducing agent, the dosage is only 0.2% to 0.5% of cement dosage. Its products are produced through molecular design and scientific technical processes. The raw materials are of high purity. As long as they are qualified products produced according to the design, their quality is stable. Cement is very different. Currently, commercially available cements are generally too fine, require large amounts of water, have high C3A content in the clinker, have too high or too low alkali content, are optimized based on SO3 condensation time only, and have factory cement temperatures that are too high, etc. etc.;

(3) The influence of mineral admixtures and aggregates cannot be ignored. The dominant aspect of compatibility between cement and admixtures should be between cement and concrete. However, the current basic research in this area is very weak, the mechanism is unclear, and the formation method is unscientific, which brings many contradictions to the project. In particular, the simple concept of strength has caused many doubts about the actual quality of concrete. Some concrete companies do not adhere to their own material selection criteria for large, important and important raw materials such as cement and sand, but try to solve their own big problems with small amounts of concrete admixtures. This is actually putting the cart before the horse. Concrete companies should be advised Use the composite cement or mixed material cement supplied by the cement plant after optimization; the cement company should produce cement according to the needs of the concrete company. This is the basic way to solve the problem.

The adaptability problem of cement and water-reducing admixture

During a certain concrete mixing work, it was found that after a small amount of water-reducing admixture was added to the cement, the hardening state of the cement changed abnormally. , a few minutes later, the cement solidification speed suddenly became faster, and the so-called rapid setting phenomenon occurred. Through analysis, the cause of the abnormal phenomenon was finally determined: the gypsum, mixed materials, mineral raw materials and other substances in the cement were not suitable for the water-reducing agent. In view of the above situation, when practitioners find that the fluidity of concrete becomes poor or rapid setting occurs after mixing with qualified water-reducing admixtures, they cannot use the construction method of forcibly adding water. Instead, they should start from the adaptability of cement and water-reducing agents to find out. Solve countermeasures to ensure the quality of finished concrete products to the greatest extent. When using different types of admixtures in combination, great attention must be paid to their compatibility to prevent the performance of the concrete from being adversely affected, such as excessive retardation, false setting or rapid setting, or collapse that does not meet the requirements. Drop rate and poor cohesion, water retention, fluidity, etc.

Therefore, before formal use, the corresponding concrete admixture compatibility test must be done to ensure that it meets the corresponding requirements before it can be officially put into use.

Several important issues that should be paid attention to during the use of concrete admixtures:

(1) Ready-mixed concrete companies are in continuous production, and the quality of sand and gravel fluctuates, and the engineering requirements for concrete Workability and strength grade requirements are also different, resulting in the production mix ratio and admixture dosage needing to be adjusted in time. Therefore, polycarboxylic acid admixtures with a solid content (water-reducing component) of about 10% to 12% should be used to facilitate management and control. ;

(2) Admixtures newly put into production must be tested for compatibility with cement, concrete workability and strength in advance, and verified to meet concrete production quality requirements;

(3) Since both naphthalene-based and polycarboxylic acid-based water-reducing agents add functional retarding and air-entraining components, there are problems with the uniformity of dissolution between them, and stratification and aggregation occur over time and temperature. , fermentation and other phenomena. This will have a serious impact on the workability and strength of concrete, so admixtures that have not been used for more than 1 month must be subject to relevant experimental verification;

(4) The mixing time during production must be appropriately selected. Especially when constructing at low temperatures or in winter, the mixing time should be extended;

(5) During the transportation of ready-mixed concrete, attention should be paid to maintaining the homogeneity of the concrete to avoid delamination, and at the same time, attention should be paid to delaying the initial setting time. Features such as slump loss.

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