The difference between retarder and accelerator, and under what circumstances should they be used?

Accelerating setting agent

Accelerating setting agent is an admixture that can quickly set and harden concrete. The main types of accelerators are inorganic salts and organic substances. The commonly used accelerating setting agents in my country are inorganic salts. Inorganic salt accelerators can be roughly divided into three categories according to their main components: accelerators with sodium aluminate as the main component; accelerators with calcium aluminate, calcium fluoroaluminate, etc. as the main components; accelerators with silicic acid It is an accelerator containing salt (NaSiO2) as the main component. The main models include Red Star 1, 711, 782, 8604, WJ-l, J85 and so on.

1. Commonly used accelerating setting agent

Red Star 1 type accelerating setting agent is composed of alumina clinker (main component is sodium aluminate), sodium carbonate and quicklime according to mass 1: 1:O. A powder formulated with a ratio of 5. The appropriate dosage is 2.5% to 4.0% of the cement mass.

7ll type accelerating setting agent is made by grinding alumina clinker and anhydrous gypsum in a mass ratio of 3:1. The appropriate dosage is 3% to 5% of the cement mass.

Type 782 accelerating setting agent is composed of alum mud, alumina clinker and quicklime with a mass ratio of r, 4.5%: 14.5%: 11% Proportionally formulated. The appropriate dosage is 5% to 7%.

Several other accelerators also use aluminum-oxygen clinker as the main component.

After the accelerating setting agent is mixed into concrete, the concrete can be initially set within 5 minutes and final set within 10 minutes. Strength can be generated in 1 hour, and the ld strength is increased by 2 to 3 times, but the strength will decrease in the later period. The strength at 28 days is about 80% to 90% of that without doping. The temperature rises and the quick-setting effect is improved. As the water-cement ratio of concrete increases, the quick-setting effect will be reduced. Therefore, the water-cement ratio of concrete mixed with quick-setting agent is generally about O·4. After adding accelerator, the dry shrinkage of concrete tends to increase, and the elastic modulus, shear strength, bonding strength, etc. decrease.

Coagulant is used when the project requires concrete to quickly reach a certain strength. It can quickly stimulate the performance of the material. However, it is just counterproductive and will definitely affect the later development. Strength, that is to say, the strength that could reach 100 can only reach 80 or 70, but in terms of three-day strength, adding an accelerating agent may reach 50 or 60 in three days without accelerating the setting of the concrete. If it is not an early-strength type, it may only have a strength of 20.30, so today’s developers generally use early-strength cement plus accelerators to achieve extremely high building strength at the beginning, and completely ignore it in the future. The same goes for retarder. Due to the influence of temperature, moisture, humidity, etc., the hydration speed of cement is different. The use of retarder can effectively suppress a series of problems such as low strength caused by excessive hydration.

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