This article discusses some of the problems of inferior machine-made sand existing in the market, as well as the production process and quality control methods of high-quality machine-made sand.
1. Main quality issues of machine-made sand
At present, due to the influence of rock materials and production processes, some machine-made sand on the market commonly has quality problems such as poor gradation, excessive stone powder content, and large particle edges and corners, which have a greater impact on the performance of concrete.
a. The gradation of machine-made sand shows the characteristics of "more at both ends and less in the middle". The sieve residue of particles with a particle size greater than 2.360 mm is as high as 40%, and the sieve residue of particles with a particle size less than 0.075 mm is as high as 15%. Due to poor gradation , the aggregate gap rate is large, and the concrete strength and working performance are poor.
b. Excessive stone powder content will lead to quality problems such as reduced strength of concrete and increased shrinkage and deformation.
c. The particles have larger edges and corners. Compared with river sand concrete with rounded particles, machine-made sand concrete has poorer workability, fluidity and compactness.
2. High-quality machine-made sand production technology and quality control
a. Preferred mines
The quality of the parent rock determines the quality of the manufactured sand from the source. In the selection of sand field mines, it is necessary to do a good job in surveying and select mines with thin covering soil layers, less mud in the interlayers, high strength of the parent rock, and good rock integrity.
The parent rock should not have potential alkali-aggregate reactivity, and limestone, dolomite, granite, basalt, etc. should be used. , these are clean, hard texture, no weak particles and no weathered stone.
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The lithology of the exposed strata in Guizhou Province is mainly limestone, with a certain amount of basalt in the west, shallow metamorphic rocks in the southeast, and abundant sources of machine-made sand and gravel.
The selection of railway sand and gravel mines should follow the principles of high quality, environmental protection and economy. A quality inspection team was set up to explore, detect and analyze sand and gravel sources, and select a mine with shallow overburden and a recoverable volume 1.5 times greater than the design demand. The mine is mainly sand-limestone, with reserves of 5 million tons.
b. “5S” high-quality sand and gravel aggregate processing technology
Some machine-made sand on the market adopts the production process of "secondary crushing + screening", which lacks the adjustment of aggregate gradation, control of stone powder content and particle shape.
1) Level 5 crushing, 2 times of shaping,
First, use a jaw crusher to pre-break (coarsely crush) the parent rock for the first time to reduce the diameter of the parent rock and reduce the production pressure of the impact crusher;
Then use an impact crusher to crush the masterbatch for the second time (intermediate crushing) to further develop the micro-cracks in the aggregate produced by the jaw crusher. Finally, the aggregate is crushed again from the micro-cracks to complete the second aggregate crushing and the first secondary plastic surgery;
Finally, the method of "stone on stone" or "stone on iron" is used to crush the aggregate for the third time (fine crushing) and the second shaping.
2) Secondary desliming
Before coarse crushing, the rod feeder and desliming screen are used to deslim the parent rock and soil with a particle size less than 30 mm for the first time, and the aggregates with a particle size less than 10 mm are deslimed for the second time before medium crushing. , remove soil and stone chips from the aggregate, and ensure that the MB value of the aggregate is less than 0.5.
3) Grading adjustment
The circulating screening system is set up in the medium and fine crushing stages. Part of the aggregate can be returned through the circulation system through the GSP-regulated silo and re-crushed into smaller particles. The production ratio of aggregates of different particle sizes is adjusted according to needs, and then the content of small particles is adjusted through air selection to control the powder, which solves the problem of "more at the two ends and less in the middle".
4) Air separation and powder control
The finely crushed aggregates are first subjected to gravity air separation to control the powder content to 10%. About %, before the finished sand enters the silo, adjust the wind pressure of the fan to extract stone powder as needed, and adjust the stone powder content again to meet the requirements for machine-made sand and stone powder content in concrete of different strengths.
5) Automatic humidification
The finished machine-made sand is automatically humidified after the air separation process is completed, and the moisture content of the machine-made sand is controlled at 2.5. %～3.5%. The stable moisture content facilitates concrete production and quality control, can effectively avoid dust during production and transportation, and prevent aggregates from segregating during loading, unloading, and transportation.
3. Environmental protection
In terms of mining, we must do a good job in environmental protection, restoration and management, green reclamation, energy conservation and emission reduction, soil and water conservation, and geological disaster prevention. According to the construction requirements of green mines.
Dust control uses spray or water bag covering methods for blasting.
Choose crushing and screening equipment with excellent noise reduction performance and low dust generation, and select production equipment types.
In terms of the design and layout of the dust removal process, a dust collection device is installed at each dust-producing part, and then box-type and V-shaped dust collectors are used for centralized dust removal. Colored steel tiles are used for fully enclosed isolation of semi-finished products and finished product warehouses.
In terms of waste reuse, the two fallen mud and stones are separated again, the stones are used as roadbed filler and raw materials for concrete bricks for secondary processing, and the mud is piled separately for later vegetation restoration, thereby improving material utilization and increasing economic benefits. , Reduce the land occupied by waste residue storage and reduce the safety hazards of waste residue accumulation.
Facing the depletion of natural river sand resources, the quality of inferior machine-made sand on the market cannot meet the outstanding requirements of high-performance concrete sand for railway engineering. You can choose to mine along the railway, or you can use tunnel waste residue to produce high-quality machine-made sand on the spot to replace the shortage of river sand for engineering construction. This can not only greatly reduce construction costs, but also reduce the impact of waste residue on the environment. It has great economic value and environmental protection significance.