Manufactured sand
(Manufactured sand)

Natural river sand is a non-renewable resource and is increasingly scarce. As each country has more and more stringent environmental protection requirements, the restrictions on river sand mining in various places have become more stringent. Therefore, the use of manufactured sand (M-sand) instead of natural river sand has become an irreversible trend in the global concrete industry.

Manufactured sand is also called M-sand, which is made by mechanical crushing and screening. Rocks, mine tailings or industrial waste slag particles with a particle size of less than 4.75 mm.

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The research on manufactured sand is now mainly focused on the impact of manufactured sand gradation, stone powder content, parent rock characteristics, grain shape, MB value and other indicators on concrete performance and mechanism research, research on the design of manufactured sand concrete mix, and manufactured sand concrete durability sexual research. However, the application of manufactured sand and the performance of manufactured sand concrete largely depend on the different production processes of manufactured sand. The crushing methods of sand making machines mainly include jaw type, cone type, impact type, counter-roll type and impact type.

According to the advantages and disadvantages of the shape of the manufactured sand particles, they are as follows:

Vertical shaft impact crusher> Impact crusher> Cone crusher> Jaw crusher> Roller crusher

At present, there are few comparative studies on the impact of production technology on the performance of manufactured sand concrete, which is not conducive to the promotion and application of advanced manufactured sand production technology in engineering.

In this paper, two production processes of ordinary sand production and dry sand production are used to prepare concrete with two strength grades of C30 and C55. The impact of manufactured sands of different production processes on the working performance, mechanical properties and durability of concrete is comparatively studied.

Read Also: How to Choose the Process of Manufactured Sand

Test

1 Raw materials

Cement: P•O42.5 cement, main performance indicators:

The main performance indicators of cement
The main performance indicators of cement

Fly ash: Grade I fly ash, fineness (45μm square hole sieve residue) is 7.2%, water demand ratio is 93%, density is 2.2g/cm³.

Coarse aggregate: 5-20 mm continuous graded limestone crushed stone, with a bulk density of 1461 kg/m³, and a dry water absorption rate of 0.9% on the saturated surface.

Fine aggregate: Manufactured sand produced by the traditional ordinary sand making process and the new dry sand making process.

Read Also: How to Improve Aggregate Quality

The two production process gradations are as follows:

  • Ordinary sand making process:
    Feeding: vibration feeding, the feed size is less than 20 mm
    Sand making: two-stage crusher, jaw crusher and cone crusher, with a particle size of 0-5 mm
    Screening: single-layer screen
    Dust removal: spray system dust suppression
    Transportation: Chute type
  • New dry sand making process
    Feeding: vertical frequency conversion vibrating feeder + iron remover, the feed size is less than 20 mm
    Sand making: vertical shaft impact crusher, output particle size 0-5 mm
    Screening: circular vibrating screen (with inclination), 2 layers of screens, screen diameter ≤5 mm
    Dust removal: pulse bag dust collector, according to need to control the content of stone powder, spray system (adjustable nozzle direction and spray water volume)
    Transportation: belt conveyor (to avoid sand segregation during transportation)

The traditional ordinary sand making process mainly uses a jaw crusher and cone crusher to obtain man-made sand by screening after two-stage crushing, while the new dry sand making process uses a vertical impact crusher, taking into account the control of stone powder content and the measures to prevent segregation of manufactured sand.

The stone powder content in the ordinary process manufactured sand is 15.9%, the MB value is 1.5, and the fineness modulus is 3.0. Dry sand production process, the stone powder content of manufactured sand is 13.8%, MB value is 0.5, and fineness modulus is 2.9.

Water-reducing agent: polycarboxylic acid-based high-performance water-reducing agent, the water reduction rate is 40%, the bleeding rate ratio is ≤20%, and the slump retention value (1h) is ≥140 mm.

water

Concrete mix ratio

C30 and C55 concrete were prepared according to JGJ55-2011 “General Concrete Mix Proportion Design Regulations”.

Test results and analysis

Workability of concrete

For C30 strength grade concrete, when the concrete slump reaches the same level, the ordinary production process manufactured sand needs to be mixed with a water-reducing agent 2.21 kg/m³, while the dry process machine-made sand only needs to be mixed with a water-reducing agent 1.54 kg/m³, saving 30% of water reducing agent, even if 30% of water reducing agent is used less. The slump of concrete prepared with dry process manufactured sand is 10 mm larger and the expansion degree is 55 mm larger than that of ordinary process manufactured sand concrete.

For C55 strength grade concrete, when the slump reaches the same level, the ordinary production process machine-made sand needs to be mixed with a water-reducing agent 2.90kg/m³. The dry-process manufactured sand only needs to add 2.23kg/m³ of water-reducing agent, which saves 23% of the water-reducing agent. At this time, the slump of concrete prepared with dry-process manufactured sand is 15mm larger than that of ordinary manufactured-sand concrete. The expansion degree is 65mm larger.

This is mainly because the MB value of the manufactured sand produced by the dry production process is 0.5, which is much lower than that of the ordinary process manufactured sand, which makes the adsorption effect of the external additive smaller than that of the ordinary process machine-made sand.

In addition, the dry sand making process uses an impact crusher, and the manufactured sand has undergone collision and shaping between particles. The edges and corners of the manufactured sand are less and closer to a round shape, which is beneficial to improve the working performance of concrete.

Compared with these two strength grades of concrete, the low-strength grade concrete dry process manufactured sand can save more water-reducing agent compared with the ordinary process manufactured sand. This is mainly because the low-strength grade concrete has relatively less cementitious materials. The advantage of better quality of time-made sand is more prominent.

Mechanical properties of concrete

The influence of manufactured sand from different production processes on the mechanical properties of concrete:

Compressive strength

For C30 strength grade concrete, the two production process sands have little effect on the compressive strength of concrete. The compressive strength of concrete prepared with dry-process sand (3d, 7d, 28d and 56d) is higher than that of ordinary process manufactured sand: 6.3%, -2.5%, 9.4% and 3.1%.

For C55 strength grade concrete, the impact of the two production processes of machine-made sand on the compressive strength of concrete is more obvious. The compressive strength of concrete prepared with dry-process machine-made sand (3d, 7d, 28d and 56d) is higher than that of ordinary process manufactured sand: 5.5 %, 9.6%, 10.1% and 5.8%.

Splitting tensile strength

For C30 strength grade concrete, the two kinds of production process sands have little effect on the splitting tensile strength of concrete at early ages, and the effect tends to be obvious after 28d age. The splitting tensile strength of concrete prepared with dry-processed sands (3d, 7d, 28d and 56d) are respectively higher than the ordinary process sand: 2.4%, 2.9%, 18.5% and 5.0%.

For C55 strength grade concrete, the effect of the two production processes on the splitting tensile strength of concrete is the same as that of C30 strength grade concrete, and it has little effect at the early age. The effect is more obvious after 28d. Dry-made sand The split tensile strength of the prepared concrete is higher than that of the ordinary process manufactured sand in order: 1.1%, 4.0%, 8.5% and 9.5%.

In addition, the split tensile strength of C55 strength grade concrete increased significantly at an early age, and tended to stabilize with the extension of time, while the split tensile strength of C30 strength grade concrete increased steadily before the 56d age.

Compressive modulus of elasticity

For C30 strength grade concrete, the compressive elastic modulus of concrete (7d, 28d and 56d) prepared with dry-processed sand is higher than that of ordinary process manufactured sand concrete at each age: 2.3%, 0.2% and 3.9%.

The C55 strength grade concrete also has the same law. The compressive elastic modulus of concrete (7d, 28d and 56d) prepared with dry process sand is higher than that of ordinary process manufactured sand concrete at each age: 18.2%, 2.3% and 3.2%.

The compressive strength, splitting tensile strength and compressive elastic modulus of concrete prepared with dry-process manufactured sand are all larger than those of ordinary manufactured sand concrete. This is mainly because the performance of concrete prepared with dry-process manufactured sand is better than Ordinary process machine-made sand concrete, the concrete is easier to be compacted, and the mechanical properties are better.

The dry sand production process uses an impact crusher. Due to the collision between the particles, the weak parts of the manufactured sand particles are knocked off. Compared with the ordinary process manufactured sand, there are fewer weak links in the concrete. In addition, due to the collision between the particles, the manufactured sand particles tend to be more rounded, and when formulated in concrete, the interface transition zone between manufactured sand and cement paste is less than that of ordinary process manufactured sand.

Durability of concrete

Electric flux

The electric fluxes of concrete prepared with two kinds of process sand are basically the same at 28d and 56d ages. Compared with C55 strength grade concrete, the electric flux of C55 concrete is smaller than C30 concrete at 28d age, but at 56d age. Instead, the period is larger than that of C30 concrete. This is mainly because C30 concrete is mixed with 18% fly ash. The early activity of fly ash is not high. In the later stage, it gradually reacts with the cement hydration product Ca (OH) 2 and increases the compactness of the concrete.

Carbonization resistance

For C30 strength grade concrete, the anti-carbonization performance of concrete prepared with dry-process manufactured sand is better than that of ordinary process manufactured sand concrete at all ages, and the carbonization depths of 3d, 7d, 14d and 28d are reduced respectively: 2.2%, 3.6%, 17.3% and 12.5%.

The impact of the C55 strength grade concrete on the anti-carbonization performance is the same as that of the C30 strength grade concrete. The concrete prepared with dry process sand for each age is better. The carbonization depths of 3d, 7d, 14d and 28d are reduced respectively: 9.5%, 5.7%, 5.1% and 6.5%. This is mainly due to the fact that the content of dry sand-making manufactured sand and gravel powder is less than that of ordinary manufactured sand, and the prepared concrete has better carbonization resistance.

Summary

The MB value of dry-process manufactured sand is smaller than that of ordinary-process manufactured sand, and the dry-process manufactured sand has a better grain shape, so that the concrete workability of C30 and C55 strength grades prepared with dry-process manufactured sand is better than that of ordinary-process manufactured sand. In the case of little difference in workability, the manufactured sand of the dry process can save the amount of water-reducing agent.

Because the dry process has fewer weak points of manufactured sand, the workability of concrete is better, and the transition zone of the aggregate slurry interface is less. The modulus of elasticity is higher than that of ordinary manufactured sand concrete.

The impact of manufactured sand of different production processes on the electric flux of concrete is similar, and the anti-carbonization performance of concrete prepared with manufactured sand of dry process is better than that of the ordinary process of manufactured sand at all ages.

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