How to Choose Fine Aggregate

Fine aggregate is generally made of natural sand, which is a mixture of different mineral particles of different sizes and formed after weathering of the rock. Generally, there is river sand, sea sand, and mountain sand. The quality requirements of the fine aggregate used in the preparation of concrete are as follows:

1.Harmful impurities

The fine aggregate of the concrete is required to be clean and free of impurities to ensure the quality of the concrete. The sand often contains some harmful impurities, such as mica, clay, silt, silt, etc., which adhere to the surface of the sand, hinder the bonding of cement and sand, and reduce the strength of concrete; At the same time, the water consumption of the concrete is increased, thereby increasing the shrinkage of the concrete and reducing the frost resistance and impermeability. Some organic impurities, sulfides, and sulfates all have a corrosive effect on cement.

The sand used in important engineering concrete should be tested for alkali activity. When it is judged to be potentially hazardous after inspection when preparing concrete, cement with an alkali content of less than 0.6% or an admixture that can inhibit the alkali-aggregate reaction, such as fly ash, should be used. When using water reducing admixture for concrete containing potassium and sodium ions, special tests must be carried out.

In general, sea sand can be used to prepare concrete and reinforced concrete. However, due to the high salt content of sea sand, it has a corrosive effect on steel bars. Therefore, for reinforced concrete, the chloride ion content in sea sand should not exceed 0.06% (using dry sand). Weighted percentage meter). Sea sand is not suitable for prestressed concrete. If sea sand must be used, it should be washed with freshwater, and its chloride ion content shall not exceed 0.02%. Some impurities such as soil, shells, and debris can be removed by washing and sieving before use. Especially when preparing high-strength concrete, it should be stricter. When high-grade cement is used to prepare low-strength concrete because the water-cement ratio (the mass ratio of water to cement) is large, the amount of cement is small, and the workability of the mixture is not good. At this time, if there are more fine soil powders in the sand, as long as the mixing time is slightly extended, the workability of the mixture can be improved.

2.Particle shape and surface characteristics

The particle shape and surface characteristics of the fine aggregate will affect its bonding with cement and the fluidity of the concrete mixture. The particles of mountain sand mostly have edges and corners, rough surfaces, and good adhesion with cement. The concrete mixed with it has higher strength, but the fluidity of the mixture is poor. River sand and sea sand mostly have round particles, smooth surfaces, and poor bonding with cement. When used to mix concrete, the strength of concrete is lower, but the fluidity of the mixture is better.

3.Grain size and thickness of the sand

The gradation of sand particles means the collocation of sand particles. The voids between sand grains in concrete are filled by cement slurry. To save cement and improve strength, the voids between sand grains should be minimized.

The degree of sand thickness refers to the overall thickness of sand particles of different particle sizes mixed together, usually divided into coarse sand, medium sand, and fine sand. Under the same quality conditions, the total surface area of fine sand is larger, while the total surface area of coarse sand is smaller. In concrete, the surface of the sand needs to be covered with cement slurry. The larger the total surface area of the sand, the more cement slurry needs to cover the surface of the sand. Therefore, it is generally said that mixing concrete with coarse sand saves the amount of cement slurry required for mixing with fine sand.

When the sand contains more coarse-grained sand and fills the voids with appropriate medium-grained sand and a small amount of fine-grained sand, the porosity and total surface area can be smaller. This kind of sand is ideal, not only cement The amount of slurry is less, and it can also improve the compactness and strength of concrete.

4.Sand firmness

The robustness of sand refers to the ability of sand to resist cracking under the action of climate, environmental changes, or other physical factors. For concrete sand with fatigue resistance, abrasion resistance, and impact resistance requirements, or concrete sand for underground structures that are corrosive media or often in the water level change area, the solidity mass loss rate should be less than 8%.

Kingsun is a professional concrete admixtures manufacturer and supplier.