Compared with natural sand, sand has significant own characteristics. It's mostly a triangle or a particle shape Rectangular body, rough surface, sharp edges, generally satisfactory degree smaller than natural sand. Currently, mostly sand and you can adjust the level with a certain amount of powder in the sand, fineness modulus mostly about 2.8-3.5, due to sand production mechanism of the country rock lithology, different production processes and equipment, production out of the sand quality vary greatly. Mainly affected by the quality of sand grain shape, fineness modulus, grading, dust content and other effects. In the current standards on the back of the main provisions of the three factors, in order to ensure the stability of quality sand.

 

1, sand grain shape

British Standard BS882: 1992 "for concrete stone" in the shape of the stone is divided into six categories: round, irregular, angular cone, flat, long bar and prolate. Sand grain mainly pyramidal shape and irregular, and with different types of rock, sand equipment changes. Shape of the particles exist a significant impact on the close-packed, expect more practical application round particles, it is not only beneficial to the close-packed, more conducive to the work of the concrete. In general, the particle shape due to the sand making equipment: rod mill, hammer and impact, such as superior to counterattack, cone and scrollwheel, Jaw, roll and gyrating worst, but the former manufacturing costs higher. Beijing adopted a sand quarry counterattack and vertical shaft impact production, particle shape Founder rate, fineness modulus can be controlled to 2.4 to 3.0, powder content of 2 to 18%, using the dry process and solve the dust problem . But now the use of vertical shaft crusher sand production is increasing.

 

2, sand fineness modulus

For custom sand production, and its fineness modulus can be controlled. In the production process, by adjusting the ratio of different screening to achieve the desired fineness modulus. Stone chips less than 4.75mm for the processing of gravel screening, because of its particle size distribution is not strictly in accordance with the relevant standards formulated, so very different fineness modulus, to control the use of carefully chosen. For the fineness modulus inappropriate sand, can be used to determine the proportion of mixed sand or fine sand through experiments in natural. Previously mentioned water conservancy and hydropower projects, sand fineness modulus mostly controlled at 3.0. Longtan Hydropower Station as sand fineness modulus of 2.9; GOUPITAN station sand fineness modulus of 2.8; heaven pumped storage power station sand fineness modulus of 2.5. Sand used in concrete commodities generally coarse sand. Beijing construction market at present there are two sand: one entirely replace natural sand sand, this mechanism is small sand fineness modulus, generally below 3.0. Such as the sand of the Beijing Prosper Center uses fineness modulus of 3.0; HengKun concrete produced sand fineness modulus of concrete products used in a variety of 2.4-3.0. Second, the use of mixed sand and natural sand, its high fineness modulus, generally above 3.0, 4.4 more than the individual. The proportion of coarse particles too large fineness modulus showed a larger aggregate, while a smaller proportion 0.315mm below. This mechanism sand itself is difficult to achieve the most densely packed state, thus increasing the total amount of cementitious material in concrete and affect various properties of concrete. For too coarse sand, and is generally used to supplement natural silt less than 0.315mm part to control the mix of sand fineness modulus of sand within range. If you want to completely replace the sand to achieve the purpose of natural sand, sand must be by choice and control the production process to produce the fineness modulus is less than 3.0, with an excellent level of quality sand.

 

3, graded sand

For granular materials and powder materials, whether or Andreassen from fuller curve model point of view, there is a best profile. Grain shape for the same aggregate, the total existence of excellent gradation. Excellent grading means that aggregate itself through optimal ratio of the size of the particles to be close to the most densely packed state, when the porosity is small. Sand gradation range from a few um to 4.75mm, wherein the average particle size of the powder is generally 30-40um and play the role of micro-filling. When the sand particles from 4.75mm, 2.36 mm, 1.18 mm ... 0.08 mm filling each other, and finally micro gap requires a certain amount of powder to fill. DR Dinger the powder material utilization Andreassen computer simulation model, the results showed that the powder material can be up to 100% of the closest packing state. Therefore, from grading and accumulation point of view, the amount of dust content is beneficial. Due to the current national standards limiting powder content is too low, a lot of sand production companies use water or winnowing way to remove excessive powder, at the same time destroy the mechanism sands. When the dust content of less than 3%, fineness modulus greater, then sand gradation than the poor in general, the performance of the porosity is relatively large. Discovery research process: some mechanism sand powder content of 2.5%, 4.3 fineness modulus, its loosely packed porosity of 52.6%; 14.8% dust content, fineness modulus of 2.7 loose sand piled porosity of 34.3% . The former is difficult to completely replace the natural sand in the concrete, which completely replace the natural sand production of C30 grade concrete products with better performance and mechanical properties than natural sand concrete. Graded sand directly affect the amount of cementitious material in concrete. Graded fine aggregate after their close-packed, requiring only a small amount of cementitious material filling the voids and bonded aggregate particles. When aggregate gradation is poor, this time with a larger porosity require more cementitious material to fill the voids and increase costs. Of course, with the increasing degree of closely packed, along with the increase of friction, impact on the concrete work between the aggregate particles. Therefore, there is an optimal particle size distribution, while meeting the coordination amount of cementitious material and working performance. When the dust content in the sand and graded less poor, then the concrete mixture is more prone to segregation, bleeding phenomenon, manifested in low strength concrete slurry lean more apparent. When the powder is better graded, with the increase of concrete powder content, the concrete viscosity becomes larger. For bleeding phenomenon produced concrete, stone powder by adding concrete ways to improve performance.