Concrete is generally composed of six components: ① cement, ② water, ③ coarse aggregate (mainly stone), ④ fine aggregate (mainly sand), ⑤ mineral admixture (mainly fly ash or other admixtures), ⑥ additive (such as expansion agent, water reducer, retarder, etc.).
In concrete, cement is very important component. Aggregate and sand are also indispensable.
In concrete, stone and sand play a skeleton role and are called aggregates. Sand is called fine aggregate and stone is called coarse aggregate.
The stones are piled up in the ordinary concrete to form a tight framework, and the sand, cement and water are mixed into mortar to fill the gaps of the framework.
The cement and water form cement slurry, which is wrapped on the surface of the aggregate and fills the interspace. Before the concrete hardens, the cement slurry, additive and admixture give the mixture certain fluidity, playing a lubricating role, which is convenient for the construction operation. After the cement slurry is hardened, stone and sand will be cemented into a solid whole.
Generally, stone and sand do not participate in the chemical reaction between cement and water. Their main functions are to save cement, bear load, and limit the shrinkage of hardened cement.
Admixtures and additives can not only improve the performance of concrete, but also save cement.
1, stone (coarse aggregate)
The strength and material of stones will both affect the strength and qulaity of concrete.
2, sand (fine aggregate)
The mud content of sand, the material of parent rock, and the content of harmful substances in sand will affect the strength and setting time of concrete in varying degrees.
3, cement
The selection of cement material and grade affects the strength of concrete and the hydration heat of concrete. The quality of related finished products plays a major role in the quality of finished concrete.
4, water
The PH value, quality and sulfate content of water affect the strength and quality of concrete.
5, mineral admixtures (mainly fly ash or other admixtures)
Different admixtures affect the workability, strength curve and appearance of concrete.
6, additive (such as expansion agent, water reducing agent, retarder, etc.)
The type and amount of additive affect the setting time, strength and physical properties of concrete.
The technical requirements for fine aggregate for concrete include the following aspects:
Particle gradation and fineness
The particle gradation of sand refers to the matching proportion of coarse and fine particles in the sand. When the particles of different sizes are well matched, the interspace between the sand particles is the least.
The fineness degree of sand refers to the overall fineness of the sand after the coarse and fine particles are mixed together, usually divided into coarse sand, medium sand and fine sand.
When other conditions are the same, the total surface area of fine sand is larger, while the total surface area of coarse sand is smaller. In concrete, the surface of sand needs to be wrapped by cement slurry, and the interspace between sand particles needs to be filled by cement slurry. In order to save cement and improve strength, the total surface area of sand and the interspace between sand particles should be reduced as much as possible. Therefore, it is better to choose coarse sand or medium sand with good gradation.
When selecting sand for concrete, the particle gradation and fineness of sand shall be considered at the same time. Zone II sand should be preferred when preparing concrete and particles smaller than 0.315mm in the sand should not be less than 15%.
Harmful impurities and alkali activity
The sand for concrete shall be clean and less harmful impurities. The mud block, mud, mica, organic matter, sulfide, sulfate, etc. contained in the sand will have a negative impact on the performance of the concrete. The content of harmful impurities should not exceed relevant specifications.
For sand used in concrete of important projects, alkali activity test shall also be carried out to determine the applicability of sand.
Robustness
The robustness of sand refers to the ability of sand to resist cracking under the influence of climate, environmental change or other physical factors. The robustness of the sand shall be tested with sodium sulfate solution. The mass loss of the sample after five cycles shall comply with the provisions of relevant standards.
The commonly used coarse aggregate for ordinary concrete includes gravel and pebble. The technical requirements for coarse aggregate are as followed:
Particle gradation and max particle size
The particle gradation of crushed stone for concrete can be divided into continuous grain gradation and single grain gradation.
Among them, the aggregate of single grain size is generally used to combine aggregate with continuous grain gradation, or mix with the aggregate with continuous grain gradation to improve the gradation. If single grain gradation aggregate has to be used due to resource constraints, measures should be taken to avoid segregation of concrete.
The upper limit of nominal particle size in coarse aggregate is called maximum particle size. When the aggregate particle size increases, its specific surface area decreases and the amount of cement in concrete also decrease. Therefore, under the premise of meeting the technical requirements, the maximum particle size of coarse aggregate should be selected as large as possible.
Strength and robustness
The strength of coarse aggregate can be expressed by rock compressive strength and crushing index. When the concrete strength grade is C60 and above, the rock compressive strength shall be tested.
For coarse aggregate used in concrete with frost resistance requirements, we should test the robustness.
Harmful impurities and needle-like particles
The clay, silt, fine dust, sulfate, sulfide and organic matter contained in the coarse aggregate are harmful substances, and their content should meet the relevant requirements. In addition, it is forbidden to mix calcined dolomite or limestone in the coarse aggregate.
For coarse aggregate used in concrete of important projects, alkali activity test shall also be carried out to determine the applicability.
Too much needle-like particles in coarse aggregate will reduce the workability and strength of concrete, so the content of needle and flake particles in coarse aggregate should meet relevant standards.
It can be seen that the amount and quality of sand and stone have a significant impact on the performance and quality of concrete. To produce high quality concrete, we need to ensure the quality of sand and stone.
Therefore, we should control the quality of sand and stone from the source and select reliable production equipment and manufacturers. SBM provides various types and models of equipment for high quality sand and gravel production. And we can arrange site visit for customers in China and abroad. Contact SBM if you are interested in crushing and sand making equipment.
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