The machine-made sand is mechanically broken and sieved. The water-permeable concrete has a particle size of less than 4.75 mm, but does not include soft rock and weathered rock particles. Some gravel manufacturers sell the graves of the stone for simple screening and sell them according to the mechanism sand. This so-called mechanism sand grading is very unreasonable. Generally, the two ends are large and the middle is small. The fineness modulus is generally 3.0~3.5. It belongs to medium coarse sand, and there are too many coarse particles. The particles above 2.36mm are seriously exceeded, generally exceeding the standard. More than 15%; <0.15 particles exceed the standard by about 10%; intermediate particles (0.30mm~1.18mm) are less than 20%, and the technical requirements of the mechanical sand grading composite I zone or zone II sand. The prepared concrete has poor water retention and is easy to separate from bleeding. When using concrete sand produced by the waste material of the stone factory to prepare concrete, the gradation of fine aggregate should be adjusted by the method of compounding with river sand, or the sand ratio should be appropriately increased and the amount of mineral admixture should be appropriately reduced. Concrete company
(1) Influence of mechanical sand on concrete performance
(1) Influence of mechanical sand on concrete performance
Compared with natural sand, the surface of the mechanism sand has many edges and corners, and the surface is rough, and there is a certain amount of stone powder. Generally, the slump of the mechanism sand concrete is small, and the bleeding performance of the concrete can be improved to some extent by increasing the content of the stone powder. Poly performance. These functions of stone powder are particularly evident in the preparation of some low-grade concrete, but in high-strength and high-performance concrete, because of the large amount of cementitious material itself, excessive stone powder content or gradation of mechanical sand will lead to concrete. Work performance is getting worse.
(2) Influence of mechanical sand on mechanical properties of concrete
The strength of machine-made sand concrete is higher than that of river sand concrete. The stone powder in the machine sand can play a role of nucleation in cement hydration, which induces crystallization of cement hydration products, accelerates cement hydration and participates in hydration reaction of cement to form hydrated calcium carbonate, and prevents it. The ettringite is converted to a monosulfide type hydrated calcium sulphoaluminate. The presence of stone powder in the machine sand can significantly improve the pore characteristics of the concrete, thereby improving the cement slurry-aggregate interface structure, and the concrete crystal phase has different degrees of change.
(3) Influence of mechanical sand on durability of concrete
Due to the physical properties of the mechanism sand itself, the surface is rough and can bond well with the slurry, which enhances the compactness of the cement stone. The more stone powder contained in the mechanism sand can improve the compactness of the concrete and increase the cement. The bonding of stone and aggregate interface; the stone powder in the mechanism sand can accelerate the hydration of C3S, and can react with C3A and C4AF to form crystalline hydrate, improve the pore structure of cement stone and improve the impermeability.
(2) Problems in the application of mechanical sand
(1) Mix ratio design
Now when formulating the mechanism sand concrete, it simply refers to the preparation method of ordinary concrete, lacks the design concept of the sand concrete mix ratio, and some sensitive factors in the preparation of the mechanism sand concrete (such as admixture type and amount, mineral admixture and There is no substantial research on the relationship between sand ratio, stone powder content, etc. and each other. Concrete price
(2) work performance
Compared with natural sand, machine-made sand has the characteristics of graded difference, fineness modulus, rough surface, sharp and angular particles, etc. In addition, the water absorption of the mechanism sand is greater than that of natural river sand, generally natural river. When the saturated surface of the sand is dry, the water content is about 1%, while the saturated surface of the mechanical sand is close to 2% when it is dry. This causes the water consumption of the concrete to be high and the slump loss to be large. The increase in the bond between the aggregate and the cement increases the resistance of the concrete mixture, and the mechanical sand concrete has poor workability and poor fluidity. When formulating the mechanism sand concrete, the sand rate should be appropriately increased to increase the water consumption to improve the workability of the concrete.
(III) Effect of stone powder in mechanical sand on concrete performance
In the process of mechanical sand production, particles having a particle size of less than 75 μm are inevitably produced, and these powder particles may be either stone powder or mud powder. Stone powder and mud powder are two distinct substances. On the one hand, the particle size of stone powder and mud powder are relatively small, which can play a filling role in concrete slurry; the structure of the other surface is completely different, and the stone powder structure is dense. There is only physical adsorption of water on the surface, and the mud powder is a layered loose structure similar to sponge. The water absorption rate is high. After the water is adsorbed, it usually expands, which affects the strength and durability of the concrete. The decalcification, illite and kaolin have strong adsorption effects on the admixture. The stone powder is usually the fine particles formed in the process of mechanical sand production, which is consistent with the chemical properties of the parent rock and has low adsorption of the admixture. Although mud powder and stone powder are similar in particle size, there are essential differences between the two, and the impact on concrete is also very different. With the increase of mud powder content in the mechanism sand, the ability of mud powder to adsorb water and admixture increases accordingly. The concrete water consumption is increased and the slump loss is accelerated. When the mud powder content is large, it also has an adverse effect on the durability of concrete such as strength, frost resistance and impermeability.
(1) Effect of stone powder in mechanical sand on concrete workability
To a certain extent, stone powder with a particle size of less than 75 μm can be regarded as an inert mineral admixture. The stone powder in the mechanism sand can adjust the gradation of the cementitious material, especially in the case of fine cement fineness. The condensed material lacks particles of 45μm~75μm, and the mechanism sand powder can effectively supplement this part of the particles and improve the void ratio of the volume of the cementitious material.
In the process of commercial concrete production, the mechanism sand with suitable stone powder content should be selected according to the change of concrete strength grade. If the content of stone powder is too small, the volume of the slurry should not be increased to improve the workability, and the purpose of concrete bleeding and segregation can be reduced. The content of stone powder should not be too high. If it is too high, the water demand will increase and the workability of concrete will decrease. Therefore, it is necessary to grasp the problem of one degree, not too high or too low, to control a suitable range: for concrete with strength grade lower than C30, there is less cementing material, and 10%~15% of stone powder in machine sand can be added. Cementing materials, increase the amount of slurry, improve the workability of concrete; but for concrete with strength grade higher than C60, concrete itself has more cementing materials, concrete viscosity is larger, should properly control the content of stone powder to reduce concrete viscosity The content of stone powder should be controlled at 7%~10%; the content of stone powder of ultra-high strength concrete with strength grade greater than C80 should be less than 3%~5%.
(2) Effect of stone powder in mechanical sand on mechanical properties of concrete
The stone powder content of the machine sand has a certain influence on the compressive strength of the concrete. For a certain grade of concrete, as the content of the sandstone powder increases, the compressive strength of the concrete increases first and then decreases. This is because the stone powder content of the mechanism sand plays a two-fold role in the concrete: on the one hand, a small amount of mechanical sand can effectively fill the gap between the cement stone and the aggregate, and improve the compactness of the concrete, with the mechanism sand content. The increase shows that the concrete strength increases; on the other hand, as the content of the sandstone powder increases, when the stone powder content exceeds the optimum content of the strength grade, it will adversely affect the particle gradation of the concrete, plus The specific surface area of the stone powder is too large, causing the water demand of the concrete to become large, and the workability is deteriorated.
(3) Effect of stone powder content in mechanical sand on concrete impermeability
The stone powder in the machine sand can effectively improve the particle gradation of the concrete material, reduce the void ratio of the concrete material, and prevent the capillary pores generated during the hydration process of the cement. The larger the stone powder content, the stronger the ability to block the water permeable passage, and the more favorable it is. Improve the impermeability of concrete; on the other hand, the large specific surface area of stone powder can effectively improve the cohesiveness and water retention of medium and low-strength concrete, improve the segregation resistance and reduce the microscopic defects of cement stone.
(4) Effect of mechanism sand and gravel powder content on crack resistance
The increase of the content of the sandstone powder will increase the shrinkage of the concrete, which is not conducive to the control of plastic cracking of the concrete. For low-strength grade concrete, the mechanical sandstone content exceeds 10%. With the increase of stone powder content, the time of concrete crack generation is advanced, and the number and width of cracks increase. For the high-strength grade concrete mechanism sandstone content of more than 7%, the concrete crack resistance is worse.