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How Much Mechanism Sand and Stone Powder Should be Mixed in Concrete?

Thomas Huang posted this article in June 30, 2020

Foreword
With the increasing scarcity of natural sand, the application of machine-made sand in concrete is becoming more and more extensive. The use of machine-made sand instead of natural sand in concrete will become an inevitable trend. This article focuses on the effects of different sand ratios and different stone powder content on the performance of concrete, and summarizes the relevant laws to achieve a better use of machined sand.
The characteristics of machine-made sand and the action mechanism of stone powder
Characteristics of machine-made sand
(1) The surface is rough and has many corners.
(2) The fineness modulus is large, the particle gradation is concentrated, and the particles passing through the 0.315mm sieve are between 8% and 13%.
(3) Large porosity.
Mechanism of stone powder
(1) Filling effect of micro aggregates. The stone powder particles are very small, they can act as micro-aggregate in concrete, fill into tiny pores, and participate in hydration reaction at the same time. Physical filling and hydration reaction product filling work together, which is more effective than pure physical filling of inert micro-aggregate. Good, make the concrete more dense, thereby increasing the strength of the concrete.
(2) Water retention and thickening effect. On the one hand, the stone powder can absorb the water in the concrete and increase the water consumption per unit cubic meter of the concrete to a certain extent. As the content of the stone powder increases, the viscosity of the concrete continues to increase, effectively reducing the risk of segregation and bleeding of the concrete mixture; On the other hand, during the hardening of the concrete, the stone powder will release the water it absorbs, which is used to compensate for the later hydration water of the concrete, thereby reducing the shrinkage of the concrete.
Raw materials and test methods
Raw materials
(1) Cement: P.O42.5R;
(2) Fine aggregate: machine-made sand for granite processing (Grade II sand, fineness modulus of 3.6, and powder content of 9.6%);
(3) Coarse aggregate: continuous granite crushed stone of 5~31.5mm;
(4) Stone powder: stone powder produced during the production of machine-made sand;
(5) Admixture: Polycarboxylic acid Point-400S water reducer, solid content 13.6%, water reduction rate 20%;
(6) Tap water.
experimental method
Test results and analysis
Experimental mix ratio
Taking C30 machine-made sand pumping concrete as the research object, the effects of the machine-made sand ratio and stone powder content on the workability and strength of the concrete are mainly studied
Effect of self-made sand on concrete performance
Using the actual production mechanism sand to prepare II grade sand (fineness modulus is 2.8, the gradation is the same as natural sand), which is 38%, 39%, 40%, 41%, 42%, 43% Sand ratio to prepare concrete, then conduct concrete mixture performance and compressive strength test
Under the same particle gradation, the concrete prepared with self-made mechanical sand is worse than the natural sand prepared concrete. The analysis should be that the surface of the machined sand particles is rough, and the edges and corners are many, resulting in poor performance of the concrete. For natural sand, from the perspective of pumping requirements, other measures must be taken to improve the fluidity of machine-made sand; from the strength point of view, the strength of machine-made sand concrete is higher than that of natural sand. The analysis is also a characteristic of machine-made sand. It improves the degree of bonding with cementitious materials and thus contributes to its concrete strength. Therefore, when considering the mix ratio of pumped concrete with machine-made sand, the water-cement ratio will increase, and the sand rate will also increase. According to the specific use of machine-made sand, the sand rate is selected from 40% to 42%. reasonable.
Effect of actually produced machine-made sand on concrete performance
Using the actually produced machine-made sand, the concrete was prepared with sand ratios of 38%, 39%, 40%, 41%, 42%, and 43% respectively, and then the concrete mixture performance and compressive strength test were conducted
Compared with self-produced machine-made sand, the actual production of machine-made sand has reduced the performance of concrete. Its powder has played a role in absorbing water in the concrete. At the same time, the particle gradation of machine-made sand is relatively concentrated, which leads to segregation and bleeding. Also because the actual production of machine-made sand particles is discontinuous, the voids in the concrete are not effectively filled, which leads to a decrease in the density of the concrete and the strength of the concrete is lower than the strength of the self-made machine-made sand.
Influence of different stone powder content on concrete performance
Concrete was prepared with 6%, 8%, 10%, 12%, 14%, 16% and 18% stone powder content, and then the performance and compressive strength test of concrete mixture
As the content of stone powder increases, the cohesiveness of concrete improves. After the content of stone powder exceeds a certain range, the slump of concrete shows a decreasing trend, which will not be conducive to the pumping construction of concrete. The analysis should be that the water absorption of stone powder leads to the initial stage Water is not effectively used; within a certain range, with the increase of stone powder content, its concrete strength gradually increases. The analysis should play a role for the stone powder in the concrete. During the hardening of the concrete, the stone powder plays a role of micro-aggregate filling. It fills the gaps of concrete and improves the density of concrete. The water-retaining effect of stone powder also provides a guarantee for the hydration reaction of concrete and thus increases the strength of concrete. In summary, the powder content of machine-made sand pumped concrete is within 10%~14%.
Conclusion and suggestion
(1) The pumped concrete is prepared with machine-made sand. Because the surface of machine-made sand particles is rough, with many corners and a large amount of stone powder, it can be better combined with cementitious materials, which is conducive to the improvement of concrete strength, but these characteristics of machine-made sand Greatly reduces the fluidity of concrete, resulting in reduced pumpability. Therefore, other technical measures are needed to improve the performance of concrete in production and use to facilitate construction. Effective construction supervision shall be carried out during the construction process to ensure the construction level and efficiency.
(2) In the production of machine-made sand, the particle gradation of sand should be strictly controlled. The fineness modulus of machine-made sand should not be greater than 3.2. The particle gradation should meet the requirements of Zone II of the building sand standard.
(3) Machine-made sand is used for pumping concrete. To achieve good workability, it must contain a certain amount of stone powder. The content of stone powder is 10% to 14%.

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