Accelerating agent is the admixture that directs concrete to set and harden rapidly, is an essential component of shotcrete, it has a wide range of applications in the construction industry; Its use can make the cement slurry in a few minutes to the initial setting, and in ten minutes to reach the final setting. Here is to introduce the type, characteristics, preparation and application of accelerator. However, at present, the research on liquid no (low) accelerator in our country is still in the development stage. Although there are many products, the comprehensive performance are uneven, and the stability and adaptability are poor, so it needs to be further improved.

• Types of accelerators:                                                                       

Commonly used liquid accelerants are sodium silicate, sodium aluminate, aluminum sulfate, potassium aluminum sulfate and other types of accelerants. Among them, sodium silicate liquid accelerator is made of sodium silicate (Na2SiO3) as the main component, combined with potassium dichromate, sodium nitrite and triethanolamine. This kind of accelerator is highly alkaline. Sodium aluminate is more soluble in water and is strongly alkaline, so this kind of accelerator is also used as alkaline accelerator. Liquid alkali free (low) accelerator can be divided into the following categories:

1. Aluminum sulfate type

Aluminum sulfate liquid accelerator is composed of anhydrous aluminum sulfate and other components, which belongs to a liquid low alkali accelerator. There are many kinds of such accelerants, and the composition and preparation technology are different. The low base liquid accelerator with aluminum sulfate and sodium aluminate as the main components has little influence on the late strength of cement and has good adaptability to different types of cement. And to aluminum sulfate and aluminum hydroxide as the main components of the low base liquid accelerator is simplified aluminum sulfate and sodium aluminate reaction into aluminum hydroxide regeneration into polyaluminum sulfate reaction steps, but this kind of liquid accelerator stability is poor, easy to precipitate, crystallization, not conducive to long-term storage. In addition, aluminum sulfate can also be combined with fluoride, magnesium sulfate and other liquid alkali – free (low) accelerator.

2. Aluminum potassium sulfate type

A liquid low alkali accelerator can be prepared by combining potassium aluminum sulfate with sodium fluoride, water reducing agent and thickener. This kind of accelerator is generally to add non-alkali ingredients, such as Nitto Chemical Industry Co., Ltd. using alkali metal sulfate or carbonate mixed with a water-soluble aluminum salt synthesis accelerator.

3.Organic and inorganic compound accelerator

Organic and inorganic compound accelerating agent can solve many problems of inorganic accelerating agent, such as by increasing the cohesivity of cement slurry so that the recovery rate of concrete in construction is greatly reduced. The organic and inorganic compound liquid alkali-free accelerating agent has been successfully developed by using nitrate and alkanolamine as the basic components of liquid alkali-free accelerating agent and adding carboxylic acid and polyhydroxyl compounds. Later, the environment-friendly organic and inorganic compound liquid alkali-free accelerating agent is mainly composed of sodium fluoride, triethanolamine, aluminum sulfate and polyacrylamide, in which aluminum sulfate and sodium fluoride are quick-setting components; The effect of triethanolamine is to improve the early compressive strength of cement mortar quickly. The main function of polyacrylamide is to improve the bond strength of concrete, increase the thickness of a injection, so as to reduce the rebound rate, reduce the number of repeated injection, and finally achieve the purpose of reducing the construction cost.

• The characteristics of accelerator:

1.The production process is pollution-free and non-corrosive

The production process of traditional powdery accelerator will consume a lot of raw materials and energy, and produce a lot of dust and waste gas in the process of grinding, pelleting, calcination and packaging, which leads to serious environmental pollution. The liquid accelerator can avoid the production of dust and waste gas in the production process and reduce environmental pollution. At the same time, the traditional alkaline accelerator is easy to cause the alkali aggregate reaction of concrete, which will cause serious damage to concrete. And liquid (low) alkali accelerator is generally neutral or slightly acidic, to avoid the introduction of alkaline substances, greatly reducing the harm to human health and damage to concrete.

2.Low jet rebound rate, cost saving

Liquid non-(low) alkali accelerator generally contains viscosifying ingredients, in the construction process can effectively reduce the rebound rate of concrete mix, reduce the waste of raw materials, save construction costs, but also reduce the alkaline wet shotcrete construction on the health of the construction personnel harm degree. Generally speaking, the springback rate of shotcrete mixed with liquid no (low) alkali accelerator is about 15%, while the springback rate of shotcrete mixed with traditional powder accelerator is about 30%. It can be seen that the use of liquid free (low) alkali accelerator greatly reduces the rebound rate of concrete injection, thus reducing the construction cost.

3. High degree of mechanization and fast project progress

Liquid alkali free (low) accelerator is mainly used in wet spray technology, the technology is highly mechanized, can greatly reduce labor consumption, but also can greatly reduce the tunnel support time, accelerate the construction progress.

• Problems existing in liquid alkali-free &low alkali accelerator:

1. Adaptability

In recent years, liquid (low) alkali accelerator compared with the traditional powder accelerator and alkaline accelerator, rapid development, its product performance is increasingly stable, but there are still some problems, such as some products with cement, admixtures and admixtures type change and different working environment and performance instability.

2.The problem of unstable coagulation time

Different from traditional accelerators, liquid alkali – free (low -) alkali accelerators have low alkaline content. Although the use of liquid alkali free (low) accelerator in the construction process is conducive to the control of spray layer thickness and ensure the smooth construction process, but the reduction of alkali content in the accelerator often leads to the extension of concrete setting time. Therefore, in the research and development process of liquid alkali free (low) alkali accelerator, attention should be paid to its influence on the strength of concrete, but also to its control of coagulant time.

3.The problem of late strength loss of concrete

Liquid alkali – free (low) alkali – free accelerator has significantly improved the strength of concrete in early stage and avoided the strength loss in late stage. Some liquid accelerants have no adverse effect on the 28d strength of cement mortar, but there are still some products have the problem that although the early strength increases quickly, the late strength loss is larger. Therefore, effective measures can be taken in the research and development of liquid alkali free (low) accelerator to ensure the rapid development of the early strength of concrete and solve the problem of high strength loss in the later period of concrete.

 In addition, there are also problems such as low yield, high price and large dosage of liquid (low) alkali accelerator, which will become the focus of future research and development of such products.

Liquid non-(low) alkali accelerator is gradually replacing the traditional powder accelerator with its early strength development fast, late strength loss, no dust, low alkali content, small rebound rate, convenient construction, little harm to human body and many other advantages. This kind of accelerator will be more widely used in the future engineering construction, and researchers should conduct more in-depth and systematic research on the problems existing in adaptability, stability, strength development and cost.

• Preparation method and application of accelerator agent

Alkali free concrete accelerator by sulfur aluminate, ester plasticizer, catalyst and other chemical composition synthesis, does not contain chloride ions, do not contain alkali metal K+, Na+ ions, does not rust reinforcement, does not pollute the environment and harm the body of the operator; In jet cement slurry, cement mortar, concrete mixed with high performance alkali free accelerator, can speed up the cement setting and hardening speed, improve the early strength, late strength impact is small, even do not reduce; And greatly improve the cohesion and bonding strength of concrete and mortar, rebound rate reduced by more than 80%, effectively reduce the material loss caused by rebound, improve economic benefits; As the rebound rate is greatly reduced, the air pollution and damage to the human body of alkaline cement dust is greatly reduced, protect the environment, speed up the construction schedule; With micro expansion shrinkage anti-cracking effect, greatly improve the impermeability and waterproof performance, at the same time has the function of corrosion resistance, than the ordinary accelerator corrosion coefficient increased by more than 50%; Alkali-free concrete accelerator is the preferred material for high performance shotcrete, alkali aggregate active concrete, durable concrete and environmental protection engineering.

The high performance alkali-free accelerator is suitable for the study of the effect of alkali-free accelerator and low alkali accelerator on the strength of concrete. The design method of mix ratio of wet shotcrete can be improved by combining with the test results of large slab strength.

• Accelerating agent variety selection

At present, wet shotcrete liquid accelerants commonly used in engineering construction can be divided into two kinds: no alkali accelerants and low alkali accelerants. Alkali-free accelerating agent refers to the alkali setting speed agent with Na2O content less than 1%, which belongs to environmental protection products. Under good construction environment, the use of alkali-free accelerator can ensure the strength of concrete, so that the strength of concrete develops normally with age, the probability of cracks in concrete is lower, and the appearance integrity of the building is more lasting. Low alkali accelerator refers to the alkali content in 8~20% of the accelerator. When the construction environment is poor, the use of low alkali accelerator can volatilize into the air in large quantities, causing skin burns and respiratory system stimulation. Not only that, the concrete is also less strong, more prone to cracks, groundwater leakage, affecting the appearance and durability of the building. When the content of active aggregate in the low alkali accelerator is 3%~4%, the low alkali accelerator will bring in a lot of alkali, and the building is more likely to appear alkaline aggregate reaction. Therefore, the reasonable selection of accelerator variety is of great significance to engineering quality.

• Relationship between the variety of accelerator and compressive strength

The content of 42.5 ordinary Portland cement and water reducing agent is 0.7%, the ratio of water to cement in the cement sand strength test is 0.52, and the ratio of cement to sand is 1:2.5. After mixing cement mortar and colloidal sand for 3min, alkali free accelerator and low alkali accelerator are added respectively. After adding the accelerant, stir again for 30 seconds to obtain the formed specimen. The compressive strength experiment was conducted on the specimen, and the experimental results showed that the compressive strength of specimen 1d was lower after the addition of non-alkali accelerator mortar, while the compressive strength of specimen 1d was higher after the addition of low alkali accelerator mortar. The 28d compressive strength of the specimen was close to 100% when alkali-free accelerator and water reducing agent were used together. However, the 28d compressive strength of specimens with low alkali accelerator and water reducing agent is lower, and the compressive strength decreases with the increase of parameters. Therefore, although the low alkali accelerator can make the compressive strength of concrete high in the short term, the low alkali accelerator has a greater impact on the late strength of concrete, and the use of alkali free accelerator concrete has a better late strength.

• Analysis of influence of accelerant on crack resistance of mortar

The mortar with a certain mix ratio was loaded into the anti-crack ring, and the shaped ring specimen was placed in the cement constant temperature curing box for demudding treatment (24h). The demoulding process is to put the specimen into a dry shrinkage room, maintain the indoor temperature at (20±2) ℃ and the relative humidity at (65±5) %, observe the initial crack of the concrete specimen, the number of cracks and the maximum crack width within 48 hours after cracking, and compare the anti-crack performance of different coagulants. Parameters of concrete specimen: content of water reducing agent 0.8%, water-cement ratio 0.5, cement-sand ratio 1:20.

Above, the types, characteristics, preparation methods and applications of accelerators are introduced. The practice has proved that the accelerator has the excellent properties of quick setting, strengthening, micro expansion, anti-cracking, anti-seepage, anti-corrosion and anti-corrosion. In particular, alkali-free liquid accelerator is widely used in railway, highway, civil air defense, fire-resistant engineering, protective engineering and other fields.