How to control the content of stone powder in machine-made sand?

Particles with a particle size of less than 4.75mm, but do not include soft and weathered particles, produced from rocks, tailings or industrial waste residues after crushing and screening mechanically after soil removal treatment, commonly known as machine-made sand. The particles with a particle size of less than 75μm in machine-made sand are called stone powder.

Is stone powder in the machine-made sand useful? How to control the content of stone powder? Here are the answers.

4 forms of stone powder in machine-made sand

(1) Free powder: The particles of stone powder do not adhere to each other and are not adsorbed on the surface of sand grains, and can move freely under the action of wind and gravity.

(2) Agglomerate powder: The stone powder particles are tightly agglomerated to form a larger particle size agglomerate of stone powder, and the particles adhere to each other and aggregate. This type of stone powder aggregate is difficult to remove by traditional powder selection equipment due to the large particle size and mass of the agglomerates.

(3) Adhesive powder: There are stone powder particles attached to the surface of the sand with a larger particle size. When the surface of the sand grain is relatively smooth, the stone powder particles are easy to remove under mechanical force, and when the surface of the sand grain is uneven, the stone powder particles and the sand grains adhere tightly to each other, which is difficult to separate by general mechanical methods.

(4) Crevice powder: There are often natural or mechanically broken crevices with a width of tens to hundreds of microns on the surface of sand grains. These crevices are often filled with a large number of stone powder particles. This is the tightest way to adhere to stone powder.

Function of stone powder in machine-made sand concrete

1, hydration

Studies have shown that the ettringite formed in the early stage of hydration will be transformed into monosulfur calcium sulfoaluminate in the later stage, which will reduce the strength of the cement, but adding stone powder containing calcium carbonate can effectively solve the problem; in addition, the main component of stone powder is calcium carbonate, and calcium carbonate can react with C3A to form hydrated calcium aluminate, thereby increasing the strength of concrete.

2, filling effect

Stone powder can fill the voids in concrete and act as filler for concrete to increase the density of concrete, thereby acting as an inert admixture. As for the characteristics of low amount of cementing material and poor performance of mixture, it can be effectively compensated by using medium and low strength machine-made sand concrete.

3, water retention and thickening effect

The machine-made sand concrete contains stone powder, which can reduce the risk of segregation and bleeding of the concrete mixture. Because stone powder can absorb the water in concrete, it virtually increases the water consumption of per cubic meter of concrete. Therefore, the higher the content of stone powder, the greater the viscosity of the concrete is. In addition, adding stone powder can also reduce the shrinkage of concrete and compensate for the later hydration water of concrete. Because, even if the concrete has hardened, the water absorbed by stone powder will gradually release.

Although stone powder plays an important role in machine-made sand concrete, it is not the more the better. Studies have found that the amount of stone powder should be appropriate. The main component of stone powder in machine-made sand is calcium carbonate, but the hydration effect 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, because free stone powder will appear in the cement or in the transition zone of the interface, thereby reducing the performance of concrete.

Control of stone powder content in machine-made sand

According to the requirements of the construction design specification, in order to achieve the required stone powder content, here are some methods to control the stone powder content:

(1) Dry screening method: The dry screening method is adopted in the secondary screening workshop, and the sand smaller than 5mm is directly transported by the belt conveyor into the finished sand warehouse, reducing the loss of stone powder. In the screening process, part of the stone powder is mixed in the dust and lost, and then the dust collector is used to recover it.

(2) Mixed cavity production: sand making machine has two cavity types in the working process: rock-on-rock and rock-on-iron. The stone powder content in machine-made sand produced by rock-on-iron crushing cavity is higher, but the wear-resistant guard plate wears faster and the cost is higher. The stone powder content in machine-made sand produced by rock-on-rock crushing cavity is lower and the cost is also lower. Combination of the two crushing methods can reasonably control the stone powder content.

(3) Mixed production: Combine sand making machine and rod mill in the production plant to increase stone powder content.

(4) Dry production method: The main process of artificial sand dry production is that aggregate after crushing and sand making process, is directly sent to the vibrating screen, where the mixture larger than 5mm is screened out, and the sand smaller than 5mm is directly transported into the finished sand bin through the belt conveyor, which can reduce the loss of stone powder.

(5) Stone powder recovery: Adopt stone powder recovery equipment to recover the stone powder lost in the process of screening, dehydration and dry production, and then mix the recovered stone powder evenly in the finished sand bin.

Adopting the above mentioned methods, the content of stone powder in sand production can be controlled at 10-15%.