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Matters needing attention in concrete mixed with mineral pow
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  • 作者:admin
  • 发布时间:2020-01-18 15:38
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(I) Precautions for adding mineral powder
 
(1) Pay attention to the amount of mineral powder
 
For single-doped mineral powder, 30% to 50% is generally suitable. When double-blended, fly ash is within 20% and mineral powder is within 30%. In the initial use, it is best to control the fly ash within 10% and the ore powder within 20%, and then adjust it gradually by trial. If the amount of slag powder is too high, the viscosity is large, which is not conducive to concrete construction, and it will also lengthen the concrete setting time, especially for those vertical structures, because the concrete is in a plastic state for a long time, it is easy to cause plastic cracks. For large-volume concrete, the ore powder reduces the heat of hydration, and the content can be appropriately increased, but it cannot exceed 70%.
 
(2) Strengthen the inspection and strictly control the fineness of mineral powder
 
The fineness of the ore powder produced by the vertical mill high-fine ore production line is controlled within the range of 400-500m2 / kg, while the fineness of the ball mill ore powder is difficult to reach more than 400m2 / kg. If the grinding time is extended, it can barely reach 400m2 / kg. Above kg, it is difficult to be stable for a long time. Once the fineness of ore powder decreases, it will bring problems such as plastic decline, segregation, bleeding, prolonged setting time, and reduced strength to the concrete. Therefore, when using the ball mill ore powder, the detection should be strengthened to strictly control the fineness of the ore powder.
 
(3) When compounding, choose appropriate compounding ratio for different grades of fly ash
 
1. Compounding of mineral powder and class II fly ash
 
When the mineral powder is used in combination with class II fly ash, the blending amount should not be too high, and only a suitable ratio can be found through experiments. Generally, the replacement amount of fly ash is controlled within 15%, and the ore powder should be controlled within 30%. Because grade II fly ash tends to increase the viscosity of concrete, it is not advisable to formulate high-strength concrete when the two are mixed.
 
2. Compounding of mineral powder and Class I fly ash
 
The combination of mineral powder and Class I fly ash should be the best combination. The amount of fly ash is controlled within 20%, and the amount of ore powder can be controlled within 40%. When trial mixing, adjustments can be made according to different strength levels and technical requirements.
 
(4) Maintenance of concrete when mixed
 
The concrete mixed with slag and fly ash is not sufficiently cured (humidity is less than 80%), the cement hydration is almost difficult to carry out, the strength development is very slow, and the low early strength is likely to cause plastic concrete cracks. For most construction sites, due to construction progress, structural forms, maintenance methods, and personnel quality, maintenance has not been taken seriously, especially for vertical structures, which are not easy to crack due to poor maintenance.
 
(5) Pay attention to adjusting the setting time of concrete
 
Compared with ordinary concrete without slag, the setting time of slag to concrete has a certain retarding effect. The initial and final setting time of concrete is delayed by 1 to 2 hours compared with the benchmark concrete. Therefore, attention should be paid to adjusting the setting time of concrete, especially in winter construction. The concrete mix ratio should be adjusted to control the amount of slag powder in the concrete and use early strength water reducing agent.
 
(6) How to design the amount of slag and fly ash in the concrete mix ratio?
 
Fly ash generally uses "excess" to replace the cement method to ensure that the strength of the concrete is up to standard; finely ground ore powder usually uses the "equal amount" to replace the cement method to prepare the concrete.
 
1. For "single-mixed" mineral powder, the appropriate amount of mineral powder can be determined according to the principle of equal replacement and according to the following methods; for ground structures and concrete structures with higher early strength requirements, the dosage is generally 20-30% ; For underground structures and concrete structures with medium strength requirements, the dosage is generally 30 to 50%; for large volume concrete or concrete structures with strict temperature rise **, the dosage is generally 50 to 65%; for high durability The performance requirements of special concrete structures (such as offshore anti-corrosion structures, sewage treatment facilities, etc.) can reach 50 to 70%.
 
2. When "dual blending" fly ash and ore powder, due to the large impact of fly ash content and quality fluctuations, the correct addition of each component can only be determined through specific tests based on the above basic principles.
 
(Two) some suggestions for using mineral powder
 
(1) Mineral powder generally needs to be reexamined for the activity index and fluidity ratio, but these two indicators have a great relationship with the cement used for inspection. When different contrast cements are used for the same mineral powder, the test results will be very different; Even if the same cement is used, the results may be different due to different batches. Therefore, commercial concrete companies should use the actual cement for comparison, and test and analyze more, especially before selecting cement produced by different manufacturers, they must first use the cement as a comparative cement for testing.
 
(2) Pay attention to the maintenance of slag powder (or slag powder and fly ash mixed concrete). When the curing temperature is suitable and the humidity is high, the water in the concrete will evaporate less, the hydration will be sufficient, and the porosity and average pore size will decrease. At the same time, because the hydration products block the water molecular channels, the number of open pores is reduced, and the number of closed pores that can play a "reserve" role is increased. Therefore, establishing a good curing system is conducive to improving the frost resistance of concrete.
 
(3) Pay attention to the adjustment of concrete water consumption. The water consumption has an important impact on the strength of concrete and its durability such as resistance to salt and alkali erosion, impermeability, resistance to reinforcement corrosion, and resistance to carbonization. When the mineral powder is combined with the superplasticizer, it has the function of assisting water reduction. Compared with the ordinary cement mixed with the superplasticizer, the water consumption can be reduced while ensuring the same initial slump of the concrete. The use of mineral powder as an admixture for concrete can improve the comprehensive properties of concrete, but if the method of using mineral powder is unreasonable, it will not only achieve the expected results, but will have a counter-effect. Therefore, the mineral powder should be reasonably applied to maximize the role of the mineral powder in the commercial concrete.