What are the concrete retarders? These are the common ones!

The most feared problem in engineering construction is concrete bleeding. Concrete bleeding is related to almost all aspects of concrete production, such as cementitious materials, aggregate gradation, mix ratio, air content, admixtures, and vibration processes. wait. The emergence of coagulants has alleviated this problem to a certain extent. What are the retarders? In practical applications, polycarboxylic acid water-reducing agents are often combined with a small amount of defoaming components, retarding components, and primers. Gas components, viscosity modification components, etc. are compounded into so-called “end products” to meet different concrete technical performance requirements. Sodium gluconate or sucrose as a retardant component combined with polycarboxylic acid-based water-reducing agent can increase the water-reducing rate to a certain extent, slow down slump loss, and improve the adaptability of the water-reducing agent and cement. What are the common types of retarder? The following types are the most widely used in industry and construction.

Types of retarder

Inorganic retarder

Inorganic retarder includes phosphate, zinc salt, iron sulfate, copper sulfate, and fluorosilicic acid Salt, borax, etc. In recent years, phosphate and metaphosphate retarder have been widely used. The retarding effect of orthophosphoric acid (HgP04) is not great, but the retarding effect of various phosphates is strong. The strongest retarding effect at the same dosage is sodium pyrophosphate (n%p:o7). The retarding effects of various phosphates are in the following order from weak to strong: orthophosphoric acid (H3PO4) < sodium dihydrogen phosphate (NaH2PO4) < disodium hydrogen phosphate (Na2HPO4 • 2H2O) < sodium phosphate (N%P04 • H2O) <Sodium polyphosphate (Serve 6.4.13) <Sodium tripolyphosphate (Serve 5.01.) <Sodium pyrophosphate (Na2P2O7).

Sodium tripolyphosphate is a commonly used phosphate retarder. It belongs to the condensed phosphates. It is a white powder and is easily soluble in water. It has strong ability to complex alkali metals and has a retarding effect. Generally it does not change with the change of operating temperature, and the dosage range is generally (0. 01% ~0. 2%).

Organic retarder

Organic retarder mainly includes three types: hydroxycarboxylic acid, aminocarboxylic acid and its salts and sugars, and polyols and their derivatives. Commonly used hydroxycarboxylic acid retarders include gluconic acid and its salts, citric acid and its salts, tartaric acid and its salts, cylic acid and its salts, etc. These retarders have an obvious retarding effect on concrete, but with the change of temperature Changes in its retarding effect also have corresponding changes. When the temperature rises, the dosage needs to be increased to achieve the corresponding retarding effect. The dosage range is generally 0.01% ~ 0.1%; the retarding effect of polyols and their derivatives The effect is relatively stable, especially when the temperature changes, it still has good stability. The retarding effect of monohydric alcohols is small, but as the alkyl group increases, the retarding effect increases. Ethylene glycol among diols basically has no retarding effect. The retarding effect of diols after propylene glycol gradually increases. Glycerol has a strong retarding effect and can even completely stop the hydration of cement. The dosage of this type of retarder is generally 0. 05% ~ 0. 2%. Increasing the dosage will cause serious retardation and the strength of the concrete will be significantly reduced. Sugar and derivatives and molasses retarder have a wide range of raw materials. It is cheap and therefore widely used. The dosage is generally 0. 01% ~ 0. 3%.

Glucosodium: wide adaptability, high cost performance, has certain effects of reducing water and retarding condensation, controlling losses, and delaying condensation time. The market price varies, generally between 3800-5800;

Citric acid: The pH value is acidic and the setting time is slightly longer. It is a super retardant. The current market price is about 8,000 yuan;

Dextrin: It can increase the cohesion of the concrete mixture. It can effectively improve concrete segregation, bleeding and other phenomena. The market price is 4,600 yuan;

White sugar: It is also one of them. It has a better effect when used in polycarboxylic acid. The current market price is about 7,000 yuan.

Most organic retarder have surface activity. They adsorb at the solid-liquid interface and change the surface properties of solid particles; or they absorb a large number of water molecules through the hydrophilic groups in the molecules to form a thicker water film layer. The crystals change from mutual contact to shielding, changing the structure formation process; or through certain functional groups in their molecules and free calcium to form insoluble calcium salts that are adsorbed on the surface of mineral particles, thus inhibiting the hydration process of cement and slowing down the process. coagulation effect. Most inorganic retarders can form double salts (such as ettringite) with cement, which precipitate on the surface of cement mineral particles to inhibit cement hydration and meet an industrial demand.

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