The stone powder in the crushed sand concrete has three effects: hydration, filling, water retention and thickening.
crushed sand refers to rock, mine tailings, or industrial waste particles with particle size not exceeding 4.75mm and made by soil removal treatment, mechanical crushing, and screening, excluding soft, weathered particles. The fineness modulus is generally between 2.6 and 3.6. It is medium-coarse sand, containing a certain amount of stone dust, with irregular particle gradation, rough surface, and sharp angles.
It is generally believed that stone powder is harmful to concrete. Some even waste time, energy, and money to find ways to remove the stone powder in the sand. In fact, the addition of appropriate stone powder in concrete is beneficial.
This article will introduce in detail the role of stone powder in crushed sand on concrete properties.
Hydration effect of stone powder in crushed sand on concrete properties
It is studied that the calcium alumina formed in the early stage of hydration will be converted to monosulfur-type calcium sulfoaluminate in the later stage. This will reduce the strength of cement stone. The problem can be effectively solved by adding stone powder containing calcium carbonate. In addition, the stone powder is based on calcium carbonate. Calcium carbonate can react with C3A hydration to form hydrated calcium carbonate aluminate, which improves the strength of concrete.
Filling effect of stone powder in crushed sand on concrete properties
Stone powder can fill the voids in concrete, acting as a filler for concrete. In this way, it increases the compactness of concrete, thus playing the role of inert admixture. For the low amount of cementitious materials, the poor performance of the mix, as long as using medium and low strength grade of crushed sand concrete, it can be effectively compensated.
The stone powder of C50 ~ C55 strength grade of mountain sand concrete (sand ratio 37% ~ 45%) cannot exceed 7%. This is because the content of stone dust and the total amount of cementitious materials (concrete fines content) can not exceed 550kg/m3. (Note: Mountain sand belongs to a kind of crushed sand. Here, it especially refers to the rock particles made of carbonate rocks with nominal particle size less than 5.00mm by de-soiling mining, mechanical crushing, and sieving.)
Water retention and thickening effect of stone powder in crushed sand on concrete properties
There is stone powder in the crushed concrete, which can reduce the risk of concrete mix segregation and water secretion. Because the stone powder can absorb the water in the concrete, which invariably increases the water consumption of concrete per square. Therefore, the higher the stone powder content, the higher the viscosity of the concrete. Moreover, adding stone powder can also reduce the shrinkage of concrete and compensate for the water used in the later hydration of concrete. Because, even if the concrete is hardened, the water previously absorbed by the stone powder will be gradually released.
It was found that the stone powder content should be moderate. The main component of stone powder in crushed sand is calcium carbonate. However, the hydration is not unlimited and is also limited by the composition of the cement. If the stone powder content is too high, it is not conducive to the bonding of the aggregate to the cement stone. Because the free state of stone powder will appear in the cement stone or interface transition zone, thus reducing the concrete performance.
In addition, if the stone powder content exceeds a certain limit value, it is not conducive to the durability performance of concrete. Because the water retention undertaken by a single square of stone powder is significantly reduced, the dry shrinkage becomes significantly larger. Comprehensive various studies, stone powder content should be controlled at 10% to 15% for concrete below C50 grade, while the stone powder content should not exceed 10% for concrete above C50.
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