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Matters needing attention in slag powder production of concrete

Author: admin Release time: June 12, 2017 Number of visits:

Research and development of high performance concrete is a hot topic in civil engineering. At present, the common technical route of preparing high performance concrete is: general cement + micro powder admixture + superplasticizer. Among them, silica fume, finely ground fly ash, zeolite powder and so on are mostly used as micro powder admixtures. Due to the limited silica fume resources in China and the low activity of zeolite powder and fly ash, domestic researchers have focused on the development and application of slag powder admixture in recent years.

With the development of concrete technology, more and more attention has been paid to the durability of concrete. One of the ways to prepare durable concrete is to add mineral admixtures including slag powder.

Slag powder can replace part of cement in concrete, which can improve the strength of concrete and improve the working performance of concrete, so it can be used as a substitute product to reduce the amount of cement in ready mixed concrete. Slag powder as an important admixture in ready mixed concrete has been widely used. But in the application process, we must pay attention to the following problems:

1. When using ball milled slag powder, the detection should be strengthened and the fineness of slag powder should be strictly controlled. The fineness of slag powder of large vertical mill slag powder production line should be controlled in the range of 400m2 / kg ~ 500m2 / kg. Because of its production technology, the fineness of slag powder is very stable. However, the fineness of ball milled slag powder is difficult to reach more than 400m2 / kg, and it is difficult to be stable for a long time. Once the fineness of slag powder is greatly reduced, it will bring many problems to the concrete, such as: the cohesiveness decreases, segregation and bleeding appear; the setting time is prolonged; the early strength decreases, even the 28d strength decreases in different degrees. Therefore, it is necessary to strengthen the detection and strictly control the fineness when using the ball milled slag powder.

2. When the amount of slag powder is more than 40%, it is better to reduce the volume of slag powder by more than 50%. The total amount of substitution should not exceed 50%. Fly ash should be controlled within 20% and slag powder should be controlled more than%. In the initial use, fly ash is better controlled within 10%, slag powder is controlled within 20%, mass concrete can be appropriately relaxed.

3. When the fly ash is mixed with different grades, the appropriate composite proportion should be selected.

When slag powder is used in concrete mixing plant, it is often used in combination with fly ash. This is because fly ash is cheaper than slag powder, and single slag powder concrete is unfavorable to concrete cost. Although the cost can be greatly reduced by adding fly ash alone, the amount of fly ash is limited. In addition, fly ash and fly ash can make full use of their advantages. When slag powder is combined with grade II fly ash, the replacement amount of fly ash should be controlled within 15%, and slag powder should be controlled within more than%. Because of the sufficient supply of fly ash in grade I concrete, a large amount of fly ash is used in the mixing plant. But the quality stability of grade II fly ash is very poor, which brings a lot of inconvenience to the preparation of concrete. The quality stability of slag powder is far better than that of grade II fly ash, which brings us a good choice for preparing concrete. As long as the appropriate mix proportion and appropriate amount of admixture are found out through the test, the concrete with good workability and no obvious increase in cost can be prepared. If conditions permit, slag powder should be used as much as possible. The combination of slag powder and grade I fly ash should be a better combination. Fly ash can be controlled within 20%, slag powder can be controlled within 40%. The ratio between them can be adjusted according to different strength levels and different technical requirements.

4. Pay attention to the maintenance of slag powder (or slag powder and fly ash mixed) concrete

When the curing temperature is appropriate and the humidity is high, the water evaporation in concrete is less, the hydration is sufficient, the void ratio and pore size are reduced. At the same time, because the hydration products block the water molecular channel, the number of open pores decreases, and the number of closed pores that can play the role of "reserve" increases. Therefore, the establishment of a good curing system is conducive to improve the frost resistance of concrete.

Slag powder (or slag powder and fly ash) concrete has more stringent requirements for curing conditions. Therefore, the technical personnel of the concrete mixing station should strengthen the communication with the construction party to ensure the concrete curing conditions. When slag powder or slag powder and fly ash are mixed together, it is necessary to strengthen the maintenance. Only by sufficient curing can the function of admixtures be brought into play.

5. Attention should be paid to adjusting the setting time of concrete. The setting time of ground slag on concrete is better than that of ordinary concrete without ground slag. The initial setting time and final setting time of concrete are 1-2.5h later than that of the reference concrete. Therefore, the commercial concrete company should pay attention to adjust the setting time of concrete, especially in winter construction, adjust the mix proportion of concrete, control the amount of slag powder in concrete and use early strength water reducing agent. In summer construction, slag powder can be properly added to delay the setting time of concrete.

6. Attention should be paid to the adjustment of concrete water consumption. When ground slag and superplasticizer are used in combination, it has the function of auxiliary water reducing. Compared with the concrete with ordinary cement only containing superplasticizer, the water consumption can be reduced under the same initial slump of concrete.