In the construction of modern tunnel engineering, shotcrete is an important guarantee to ensure the construction speed, and accelerating agent is the key of shotcrete technology. Concrete accelerator is an admixture which can make concrete set and harden quickly.The main species are inorganic salts and organic substances.Powdery solid, its dosage only accounts for 2% ~ 3% of cement content in concrete, but can make concrete in 5min initial setting, concrete accelerator within 12min setting.In order to achieve the purpose of emergency repair or rapid setting of concrete in the shaft.It is an indispensable additive in shotcrete construction method.Their function is to accelerate the hydration and hardening of cement, forming enough strength in a very short time to ensure the special requirements of construction.
1. Hydroxide of alkali earth metal carbonate and alkali earth metal
Powdery alkaline earth metal carbonates or hydroxides were rarely used in shotcrete construction before.They are the most commonly used accelerators of this type of concrete, with a conventional dosage of 2.5% to 6% of the weight of cement. They mainly promote the hydration of C3S.Generally add a small amount of aluminum carbonate, can affect the setting time of cement.However, the effects can only be observed when large doses are added.The reaction between the accelerator and cement is mainly affected by cement chemical composition, fineness, mineral additives and ambient temperature.
2. Alkali silicate, sodium silicate, potassium silicate type accelerator
Mainly used for wet mix shotcrete, they are usually liquid, and a large amount of mixing (>10% cementitious material weight).The soluble silicate precipitates as a result of the reaction to form calcium silicate.When used in large doses, these coagulants reduce the adhesion force with the base, and eventually, lead to the decline of concrete strength and severe dry shrinkage.Because of these reasons, the maximum dosage of this accelerator does not exceed 15%, and the ultimate strength loss is limited to less than 30%.
3. Sodium aluminate, potassium aluminate, aluminate concrete accelerator
Can use dry mix already, can use wet mix shotcrete project again, commonly used dosage is 2.5% ~ 5.5% commonly.Potassium aluminate has a better effect than sodium aluminate, but it also costs more.They are mainly directly involved in the hydration of Portland cement and accelerate the setting of cement, binding with gypsum, preventing the formation of ettringite on the surface of cement particles, and making C3A react immediately, producing the initial strength required for most shotcrete.Their effects are usually influenced by the chemical composition, fineness of the cement and the mineral additives it contains, but this effect is smaller than that seen with carbonate additives.The final strength will be lost by 20% to 25%.There is no doubt that aluminate accelerating agent has the best application effect in wet mixing shotcrete engineering and has good construction effect on thick lining board and even roof board.However, its high alkali content and the hazard to health caused by underground construction are the main factors limiting its application.In addition, there are some problems in the engineering application of sulphuric acid and active aggregate, the latter may occur alkali aggregate reaction.
4. Alkali free liquid concrete accelerator
Liquid alkali - free accelerator, its application scope is not very extensive.This kind of coagulant can solve some common problems of alkaline accelerator, such as the harm to underground construction environment, the harm to alkali aggregate reaction, the harm to high pH value in the use process and the loss of late strength of shotcrete.