The quality of the machine sand is greatly affected by the production process and the properties of the parent rock. Some of the sand production equipment has a better sand grain shape, controllable gradation and excellent performance. There are also some sand making equipment produced by the mechanism sand fineness modulus is large, the particles are multi-triangle or square, the surface is rough, the particles are sharp and angular, the gradation is poor, the coarse particles and fine particles (stone powder) are more, the middle Less particles. In addition, the machine sand produced by the mother rock with higher hardness has more needle-like shapes and poorer particle shape.

Mechanism sand powder content

Particles with a particle size of less than 75 μm in the machine sand are called stone powder. Due to the production process, the stone powder content in the finished machine sand generally exceeds the stone powder content limit specified in GB/T14684-2011 “Construction Sand”. The stone powder in the machine sand is different from the mud powder in the river sand. The proper amount of stone powder can improve the workability, workability and durability of the concrete.

(1) Influence of stone powder on concrete workability

Stone powder has both positive and negative effects on concrete workability. On the one hand, stone powder will adsorb certain water and admixture to increase the water consumption of concrete; on the other hand, stone powder can improve the poor grade of sand due to mechanism, easy to isolate and bleed. The disadvantages are increased concrete consistency and improved concrete workability. The stone powder content in the machine sand should not be too high or too low, and control a suitable range: for concrete with a strength grade lower than C30, the cementitious material is less, and the mechanical sand contains 10% to 15% of stone powder to supplement the cementitious material. Increase the amount of slurry and improve the workability of concrete; but for concrete with strength grade higher than C60, the concrete itself has more cementing materials and the concrete viscosity is larger. The content of stone powder should be properly controlled to reduce the viscosity of concrete and the content of stone powder. It should be controlled at 7% to 10%; the ultra-high-strength concrete with strength grade greater than C80 should have a stone powder content of less than 3% to 5%.

(2) Impact on concrete strength and durability

Fine stone powder particles can improve the gradation of cementitious materials, reduce the porosity of cementitious materials, and improve the compactness of concrete microstructure. In addition, stone powder can promote the hydration of cement, which is beneficial to the mechanical properties and durability of concrete. For ordinary strength sand concrete, the chloride ion penetration resistance of concrete increases with the increase of stone powder content in the mechanism sand, while the concrete frost resistance decreases with the increase of stone powder content, especially when the stone powder content is higher than 10% ( When the volume ratio of stone powder to cement exceeds 1:3.47), the frost resistance of concrete deteriorates significantly. However, for high-strength concrete, for concrete workability, when the stone powder content is more than 7%, the concrete slump is reduced, and for the durability against chloride ion permeability and frost resistance, the stone powder content is increased from 3.5% to 14%. None of the performance degradation of the mechanical sand high-strength concrete.

Mechanical sand particle characteristics

The particle characteristics of the machine sand have an important influence on the performance of the concrete.

(1) Impact on concrete workability

The concrete prepared by continuous grading aggregates has better working performance than the discontinuous grade aggregate concrete. The particle grading of the mechanism sand is generally expressed as “two big and small middle”, that is, more than 2.36mm and <0.15mm. , while the middle particles are less (especially 0.3mm ~ 1.18mm), sometimes a certain grain level breaks. When the concrete is prepared, the amount of the cementing material and the amount of the admixture are too large. If the preparation is unreasonable, the concrete mixture slump is prone to "grass hat-like" and the workability is poor. The mechanism sand is divided into 1.18mm sieve holes, and the mechanical sand particle composition is divided into two parts: particle size ≥1.18mm and particle size <1.18mm. The particle size ≥1.18mm affects the fluidity of the concrete. 1.18mm particles have an effect on the cohesiveness and water retention of concrete. By comparing with the natural sand grading, it is found that it is more beneficial to improve the workability of the mechanism sand concrete by appropriately increasing the particles of 0.15-0.3mm and 0.3-0.6mm in the mechanical sand.

(2) Influence on concrete strength and durability

The surface roughness of the machine sand is greater than that of the river sand, which can improve the bond strength between the cement stone and the aggregate, and is beneficial to improve the compressive strength and volume stability of the concrete. However, the rough surface of the mechanism sand has a more significant influence on the porosity of the concrete. The use of the mechanism sand increases the porosity of the concrete and reduces the compressive strength and durability of the concrete.