How To Control The Stone Powder Content In Machine-made Sand?

Rocks, tailings or industrial waste are processed by soil removal and are made by crushing and screening, with a particle size of less than 4.75 mm, but excluding soft and weathered particles, commonly known as machine-made sand. Particles with a particle size of less than 75 μm in machine-made sand are called stone powder.

The role of stone powder in machine-made sand concrete

1. Hydration

Studies have shown that the ettringite formed in the early stage of hydration will transform into monosulfide calcium sulfoaluminate in the later stage, which will reduce the strength of cement stone, but adding stone powder containing calcium carbonate can effectively solve the problem; In addition, the stone powder is based on Calcium carbonate is the main one, and calcium carbonate can undergo a hydration reaction with C3A to form hydrated calcium aluminum carbonate, thereby improving the strength of concrete.

2. Filling effect

Stone powder can fill the voids in concrete and act as a filler in concrete to increase the compactness of concrete, thus acting as an inert admixture. For the characteristics of the small amount of cementitious materials and poor performance of the mixture, as long as the medium and low-strength grade machine-made sand concrete is used, it can be effectively compensated.

3. Water retention and thickening effect

The presence of stone powder in machine-made concrete reduces the risk of segregation and bleeding of the concrete mix. Because stone powder can absorb water in concrete, it virtually increases the unilateral water consumption of concrete, so the higher the stone powder content, the greater the viscosity of concrete; Moreover, the addition of stone powder can also reduce the shrinkage of concrete and compensate for the water used for the hydration of concrete later, because, Even if the concrete hardens, the moisture previously absorbed by the stone powder will gradually be released.

Although stone powder plays an important role in machine-made sand concrete, the more the better. The study found that the stone powder content should be appropriate. The main component of stone powder in machine-made sand is calcium carbonate, but the hydration is not unlimited and is also limited by the composition of cement. If the content of stone powder is too high, it is not conducive to the bonding of aggregate and cement stone, because free stone powder will appear in the cement stone or in the interface transition zone, thereby reducing the performance of concrete.

Control of stone powder content in machine-made sand

According to the requirements of the construction design specifications, in order to make the stone powder content meet the requirements, the following methods are adopted.

(1) Dry screening: Dry screening is adopted in the secondary screening workshop, and the sand less than 5 mm is directly transported into the finished sand bin through the belt conveyor to reduce the loss of stone powder. During the screening process, part of the stone powder is mixed and lost in the dust, and then recycled by the dust collector.

(2) Mixed cavity production: There are two cavity types for vertical shaft impact to sand making: crushing between stones and crushing between stone and iron. The artificial sand and gravel powder produced by the crushing between stone and iron cavity type has a higher content, but the wear-resistant iron sticks to wear faster and the cost is higher. The artificial gravel powder produced by crushing between stones cavity has low content, and the cost is low because there is no wear-resistant iron stick. The combination of the two cavity types can reasonably control the stone powder content.

Equipment recommendation

Vanguard Machinery's VSI6X series vertical shaft impact crusher has been used in many large-scale crushing and sand making projects at home and abroad. The machine-made sand produced in the production site has a good particle shape and reasonable gradation. The finished sand is often used in high-quality concrete production.

(3) Mixed production: Combine vertical shaft impact crusher and rod mill to increase stone powder content.

(4) Dry production: Artificial sand dry production is not used very much in domestic projects. The main process is that after the aggregate is crushed by a crusher to make sand, it is directly sieved to screen out the mixture larger than 5 mm, and the mixture smaller than 5 mm The sand is directly transported into the finished sand bin through the belt conveyor, which can reduce the loss of stone powder.

(5) Stone powder recycling: Stone powder recovery equipment is used to recycle the stone powder lost in the process of method screening, dehydration production and dry production, and then the recycled stone powder is evenly mixed in the finished sand bin.

After a period of irregular inspection, the content of stone powder was tested to be 10% to 15%. Therefore, the stone powder content of sand making can be controlled at 10-15% by adopting methods such as dry screening, mixed chamber production, mixed production, dry production and stone powder recycling.