Concrete admixtures, referred to as admixtures, refer to substances added to improve and adjust the properties of concrete. Since admixtures can improve and adjust the properties of concrete, admixtures are used more and more widely in engineering. However, improper use of admixtures not only fails to improve and adjust the properties of concrete, but may even greatly affect the performance of concrete and cause engineering accidents. , this article puts forward some problems and corresponding measures in the use of concrete admixtures.
1. When using concrete admixtures, do not select the "optimal dosage" through trial mixing
1.1 Consequences
The dosage of admixtures in concrete is related to Compared with sand, stone, cement and water, although it is small, it has a significant impact. To the performance of concrete (such as workability, strength, setting time, etc.) and economic indicators. Especially when adding retarder, retarding water-reducing agent, and air-entraining agent, the dosage will not be effective if the dosage is too small. Once the dosage is exceeded, the poured concrete will not solidify and harden or will seriously degrade.
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Low strength, causing engineering accidents.
1.2 Corresponding measures
(1) The amount of admixture should be expressed as a mass percentage of the total mass of the cementitious material (cement cow admixture) in the concrete or in mL /kg cementitious material expressed.
(2) The appropriate dosage of admixture (water-reducing agent) is reflected in the fact that when the dosage is small, the fluidity of the concrete mixture will increase less; when the dosage is too much, the fluidity will not improve. The increase is not proportional; only when the amount of water reducing agent is slightly increased within a narrow optimal dosage range, the fluidity of the mixture will be significantly improved. The suitable dosage range is the interval between the rising section and the gentle upper section, similar to a "saddle shape". The manufacturer's product manual provides the dosage range (appropriate dosage) of a certain admixture, and the user must determine the reasonable dosage of the admixture through concrete trial mixing, that is, the most technically and economically suitable admixture. Best dosage. The dosage of different types of admixtures has certain rules. Usually, the dosage of inorganic salt early strength agent is 1﹪~2% of the mass of the cementitious material; the dosage of organic retarder is 0.02﹪~0.1%; air entrainment The dosage of the agent is 0.02﹪~0.1%; the ordinary water-reducing agent is 0.2%~0.3%; the high-efficiency water-reducing agent is 0.5%~1.0%. The dosage of the same kind of admixture when used in different concretes is also different. For example, when the superplasticizer is used in steam-cured concrete, the dosage is 0.3﹪~0.5﹪, when it is used in ordinary concrete, the dosage is 0.5﹪, and when it is used in flowing concrete, the dosage is 0.5%. The dosage for normal concrete is 0.75%, and the dosage for high-strength concrete is 1.0%. When the sodium sulfate early strength agent is used in steam-cured concrete, the dosage is 1.0%. If the dosage exceeds 2%, the specimen after steam-curing will swell and the strength will decrease. In addition, the dosage of composite superplasticizer is half that of single superplasticizer at the same water reduction rate. The post-mixing method not only reduces the amount of superplasticizer, but also has the same effect on other admixtures.
(3) There are many factors that affect the dosage of admixtures, such as cement type, fineness, mineral composition, and mixed materials. For example, the dosage of high-efficiency water-reducing agent for slag cement is less than that of ordinary Portland cement. The dosage of cement superplasticizer with large surface area and high C3A content should be more; the initial fluidity of the mixture, temperature, etc.
2. Not paying attention to the compatibility issue between admixtures and cement (adaptability issue)
2.1 Consequences
Every concrete admixture has It has its own unique function. Adding this kind of admixture can improve the performance of concrete in one or more aspects. For example, adding water reducing agent can increase the fluidity of concrete while maintaining the same water consumption, or reduce the unit water consumption while maintaining the same fluidity, thereby increasing the strength of concrete and improving the durability of concrete. From this, the concept of adaptability and unsuitability of concrete admixtures and cement can be understood in this way: according to the technical specifications for the application of concrete admixtures, a certain admixture that has been tested to meet the relevant standards is added to the application. This variety can be used according to regulations. In concrete prepared with cement with admixtures, if it can produce the desired effect, the cement is compatible with the admixture; on the contrary, if it cannot produce the desired effect, there is a gap between the cement and the admixture. Not adaptable. For example, several ordinary Portland cements are used and mixed with a certain high-efficiency water-reducing agent (which has been tested to meet the quality standards of high-efficiency water-reducing agents) to prepare concrete. When the preparation conditions are the same, the concrete prepared by one cement is There is a serious deficiency in water reduction rate, which means that this cement is not suitable for the high-efficiency water-reducing agent, while other cements are suitable for the high-efficiency water-reducing agent. For another example, when an accelerating setting agent that has been tested and meets the requirements of relevant quality standards is added to the concrete prepared by a certain cement but no quick setting effect is obtained, or a retarder is added but a false setting effect is obtained, it can be considered as Due to the incompatibility between admixtures and cement. There are adaptability problems between almost all kinds of admixtures and cement. However, water-reducing admixtures are currently the most commonly used, and when they are not compatible with cement, they can be responded to intuitively and quickly (such as poor fluidity, The water reduction rate is low, or the mixture is hardened and heated, and the fluidity is lost too quickly, etc.).
2.2 Corresponding measures
The compatibility of cement and admixtures is a very meaningful scientific issue, and it is also a practical engineering issue. This issue arises from engineering accidents. Therefore, it is necessary to cause production units and engineering application departments toof great importance.
3. Excessive use of lignocalcium water-reducing agent
3.1 Consequences
Intrinsic sulfonate surfactant is composed of natural polymer substances Modified, its molecular weight is generally 1,000 to 25,000. In colloidal chemistry, it is classified as a macromolecular surfactant. Theoretically, it is impossible for particles adsorbed with macromolecular protective agents to be very close. This protective film will have a strong hindering effect on hydration diffusion. Therefore, adding wood calcium superplasticizer will delay the setting time. As the dosage increases, this effect is strengthened. In addition, lignocalcium water-reducing agent contains a certain amount of sugar and other reducing substances. Since sugars are polyhydroxy carbohydrates with strong hydrophilicity, after adsorption, they thicken the solvated water film on the surface of cement minerals, increase the diffusion layer and electrokinetic potential, and reduce the diffusion ion concentration, thus producing a retarding effect. , increasing the dosage will further strengthen this retarding effect, so the use of excessive dosage should be avoided.
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