There are so many concrete admixtures, such as retarder and reducing agent, what are their functions?

1. The working principle of concrete water-reducing agent: Water-reducing agent is usually a surfactant, which is an anionic surfactant. It is adsorbed on the surface of cement particles so that the particles display electrical properties. The particles repel each other because they have the same charge, causing the cement particles to be dispersed and release excess water between particles to produce water reduction. On the other hand, after adding the water reducing agent, an adsorption film is formed on the surface of the cement particles, which affects the hydration rate of the cement, makes the growth of cement stone crystals more perfect, reduces the capillary gaps for water evaporation, and the network structure is denser, improving the Improves the hardness and structural compactness of cement mortar.

2. Action mechanism of concrete retarder

Most organic retarder has surface Activity, they produce adsorption on the solid-liquid interface, changing the surface properties of solid particles; or adsorbing a large number of water molecules through the hydrophilic groups in the molecules to form a thicker water film layer, causing the crystals to change from mutual contact to shielding, changing the structure The formation process; or through certain functional groups in its molecules and free Ca to form insoluble calcium salts that are adsorbed on the surface of mineral particles, thereby inhibiting the hydration process of cement and achieving a retarding effect. Most inorganic retarders can form double salts (such as ettringite) with cement, which precipitate on the surface of cement mineral particles and inhibit cement hydration. The mechanism of retarder is relatively complex, and is usually the result of the combined effect of the above multiple retarder mechanisms

3. The mechanism of action of concrete early strength agent

Early strengthening agents are divided into organic and inorganic categories according to their different chemical compositions. Organic ones include calcium formate, triethanolamine, triisopropanolamine, urea, etc.; inorganic ones include sulfate, chloride salt, etc. Different early strengthening agents work on different principles.

a. Sulfate early strength agent: such as anhydrous sodium sulfate, which dissolves in water and reacts with calcium hydroxide produced by cement hydration to generate calcium oxide and sulfuric acid. calcium. The newly generated calcium sulfate has extremely fine particles and is much more active than doped calcium sulfate, so it reacts with C3A to form hydrated calcium sulfoaluminate much faster. Sodium hydroxide is an active agent that can increase the solubility of C3A and gypsum, accelerate the amount of calcium sulfoaluminate in cement, and lead to the setting and hardening of cement and the improvement of early strength. However, sulfate early-strength agents have a certain corrosive effect on the steel bars in concrete, including chloride early-strength agents, and attenuate the later strength of cement mortar. Therefore, the current dosage of chloride and sulfate early-strength agents is gradually decreasing.

b. Calcium formate is an additive that has no corrosive effect on steel bars. Its molecular formula is C2H2CaO4. It mainly accelerates the formation of tricalcium silicate in cement. Hydration increases the early strength of cement mortar. The effect of calcium formate on the strength of cement mortar mainly depends on the content of tricalcium silicate in the cement. With less tricalcium silicate, its early strength effect is good, it will not attenuate the later strength of cement mortar, and it has certain antifreeze properties at low temperatures.

4. Action mechanism of concrete air-entraining agent

1) Interface active effect: stirring After the air-entraining agent is added during the concrete process, the air-entraining agent molecules quickly adsorb on the interfaces of each phase in the cement-water-air system. On the cement-water interface, a single-molecule (or multi-molecule) directional adsorption film is formed in which the hydrophobic groups point toward the cement particles, while the hydrophilic groups point toward the water; on the bubble film (i.e., the water-air interface), a hydrophobic group points toward the water. air, while the hydrophilic groups point toward the directional adsorption layer of water. Due to the adsorption of surfactants, the free energy of the entire system is greatly reduced, making it easy to introduce small bubbles during the stirring process.

2) Foaming effect: Bubbles can be divided into three types: bubbles, foam and sol bubbles. Bubbles in concrete are colloidal bubbles. Clean water will not bubble. Even if it is violently stirred or oscillated, the water will be stirred into fine bubbles and become turbid. However, after standing, the bubbles will immediately float up and burst. However, when an air-entraining agent is added to the water, a large number of bubbles will be introduced through shaking or stirring. The reason is: the liquid surface has a tendency to automatically shrink, and bubbling is a process in which the interface area increases significantly. When the surface tension remains unchanged, it will inevitably lead to a large increase in the free energy of the system. It is a thermodynamically unstable system and will causing the bubbles to shrink and burst. However, in the presence of air-entraining agents, since it can be adsorbed to the air-liquid interface, it reduces the interface energy, that is, reduces the surface tension, making foaming easier.

3) Foam stabilizing effect: Adding some surfactants to concrete can also introduce a large number of tiny bubbles during the mixing process, but when the concrete is left to stand for a certain period Over time, or after transportation, loading and unloading, and pouring, the air content of concrete has greatly decreased, and most of the bubbles have overflowed and disappeared. However, the air-entraining agent is different. After being mixed, it can not only introduce a large number of tiny bubbles into the concrete during the mixing process , and these bubbles can exist relatively stably, which is an important guarantee for the existence of pores with a certain structure in hardened concrete.

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