What is the Effect of Adding Calcium Chloride in Shotcrete Operation?

How Much Should be Used?

• The functions of calcium chloride in shotcrete operation:

1. Accelerate coagulation and hardening

  Calcium chloride can react with mineral components in cement. For example, it can accelerate the hydration reaction of tricalcium silicate and dicalcium silicate, etc., to generate more hydrated calcium silicate gels and calcium hydroxide. In the shotcrete process, especially in the case that the shotcrete needs to play a supporting role as soon as possible in underground engineering, calcium chloride can promote the rapid solidification and hardening of concrete, shorten the initial and final coagulation time, so as to form a supporting structure with a certain strength faster, improve construction efficiency and safety.

2. improve the early strength

  Because calcium chloride promotes the hydration reaction of cement, the concrete can obtain higher strength at an early stage. For the concrete layer formed by shotcrete, the early strength improvement helps it to withstand external forces such as surrounding rock pressure more quickly. In the early support of underground engineering such as tunnel and mine roadway, higher early strength can effectively prevent the loosening and collapse of surrounding rock, and create favorable conditions for subsequent construction.

3. Lower the freezing point

  Calcium chloride can reduce the freezing point of the liquid phase in concrete when spraying in some cold areas. When the ambient temperature is low, it can make the water in the concrete remain liquid at low temperatures, so as to continue to participate in the hydration reaction of the cement. This function can ensure the normal construction and hardening of shotcrete in low temperature environment, reduce the impact of freezing damage on the performance of concrete, such as avoiding the damage of concrete structure caused by ice expansion.
However, it is necessary to pay attention to the amount of calcium chloride when using it, because excessive use may lead to reduced strength of concrete in the later stage, corrosion of steel bars and other problems.

• The amount of calcium chloride used in shotcrete operations

1. Engineering types and requirements

①Rapid support of underground engineering: In the shotcrete operation of some underground tunnels, mine roadways and other projects that need to form support structures quickly, if the early strength is very high, the amount of calcium chloride may be close to 5%. But this high dosage requires closer attention to the potential impact on concrete durability.

②General building structure repair shotcrete: for ordinary building structure repair shotcrete engineering, calcium chloride dosage is generally controlled at 2%-3%, while meeting the construction schedule requirements, to ensure the balance of concrete quality, reduce the adverse impact on the strength and reinforcement of the later period.

2. Cement varieties and quality

①Different brands of cement: cement produced by different manufacturers has different mineral composition and properties. Some cement itself has high reactivity, the amount of calcium chloride can be appropriately reduced; For cement with lower activity, it may be necessary to increase the amount of calcium chloride, but it is generally not more than 5%.

②Cement grade: the fineness and other indicators of high-grade cement are better, and the hydration reaction is relatively easy to carry out. When using high-grade cement for shotcrete, the amount of calcium chloride can be lower limit; On the contrary, low-grade cement may require slightly more calcium chloride, but it should also be carefully controlled within a reasonable range.

3. Environmental conditions

①Temperature effect: In the low-temperature environment (such as close to 0 ℃ or lower) for the mortar spraying operation, in order to better play the role of calcium chloride to reduce the freezing point and promote hydration, you can appropriately increase the dosage of calcium chloride, but it should not exceed the upper limit of the regulations, and at the same time pay attention to the monitoring of the long-term performance of the concrete. At room temperature, the dosage can be selected according to the conventional range.

②Humidity: high humidity environment, with sufficient water in the concrete, calcium chloride dosage can be according to the normal range; in a dry environment, water evaporation is fast, so it may be necessary to ensure the quality of the premise, according to the actual situation of fine-tuning of calcium chloride dosage, to avoid the rapid loss of water to avoid the impact of hydration reaction and the role of calcium chloride to play.