How the Process of Silica Sand Mining

Silica sand mining aggregate is a general term for materials such as sand, pebble (gravel) stone, crushed stone, block stone, and aggregate in water conservancy and construction projects, serving as the primary building material for concrete and masonry structures. This article summarizes common questions regarding concrete aggregates.

Question 1: What key factors should be monitored when receiving coarse and fine aggregates?

For coarse aggregates, primary attention must be paid to particle size, gradation, grain shape, stone powder content, and mud content. Each delivery vehicle requires a preliminary visual inspection; unqualified loads must not be unloaded. Additionally, various indicators must be tested in batches in accordance with regulatory requirements.
For fine aggregates, control must focus on fineness modulus and mud content. Each delivery vehicle requires a preliminary visual inspection; unqualified vehicles must not be unloaded. Batch testing must also be conducted according to specifications.

Question 2: Why is it critical to limit the needle and flake content of coarse aggregates in pumped concrete?

High needle-like content results in lower flexural strength of the individual particles and reduced bonding strength between aggregates, leading to decreased overall concrete strength. In ready-mixed concrete, excessive needle and flake content compromises aggregate shape, thereby reducing concrete fluidity. Furthermore, needles and flakes are prone to causing blockages within the pumping pipes, cau

sing the pump to block, and even cause pipe bursts. Therefore, pumped concrete requires a needle content of ≤10%, with even stricter requirements for high-strength concrete.

Question 3: What kind of sand is needed for ready-mixed concrete?

Ready-mixed concrete requires medium sand. In addition to meeting standard specifications for particle size distribution, clay content, and mud content, it is crucial that at least 15% of the sand passes through a 0.315mm sieve. This factor significantly impacts the pumpability of the concrete. If this percentage is too low, it can easily cause pump blockages and result in poor water retention and bleeding in the concrete.

Question 4: What impact will the sand have if it is too fine?

If the sand is too fine, the water demand of the concrete will increase. Moreover, concrete prepared with fine sand exhibits extremely poor pumpability and slump retention, leading to decreased concrete strength and a higher susceptibility to cracking.

Question 5: What to do if there is only fine sand?

If the sand source consists primarily of fine sand, it can be blended with machine-made sand to prepare pumped concrete. For instance, fine sand with a fineness modulus below 2.0 can be mixed with machine-made sand having a fineness modulus of 3.0–3.2 at an approximate ratio of 6:4. Evaluate its fluidity and pumpability, and finalize the mix ratio through experimental testing.

Question 6: What are the consequences of high sand and mud content?

Excessive sand and mud content increases the water demand for concrete, reduces workability, leads to greater shrinkage, lowers concrete strength, and makes the structure prone to cracking. Therefore, it is essential to control the sand and mud content to ≤3% for C30–C50 concrete. High-strength concrete requires even stricter limits on mud content.

Question 7: How does mud in sand and gravel affect concrete?

Beyond the general impacts associated with mud in aggregates, it can severely compromise concrete strength. For example, mud lumps weaken the effective cross-section of the concrete; during ground pouring, these lumps may float upward, and after shrinking

, they will form pits and other defects on the surface.

Question 8: Why is the strength of pebble concrete 3-4 MPa lower than that of gravel concrete at the same mix ratio?

The rough surface of coarse aggregates enhances the interfacial bond strength between the cement paste and the aggregate. Based on years of experimental data, concrete made with river pebbles typically contains more weathered stones, has a lower crushing index compared to crushed stone, and features smoother surfaces, resulting in weaker interface bonding. Consequently, the compressive strength of pebble concrete is generally 3-4 MPa lower than that of crushed stone concrete when using the same mix proportions.

Question 9: Why must the moisture content of sand and gravel be measured every shift?

The dosage of sand and gravel in ready-mixed concrete ranges from approximately 800 to 1100 kg/m³. A variation of just 1% in moisture content introduces an additional 8–11 kg of water into the concrete mix. Since sand is often sourced directly from rivers, its moisture content can fluctuate significantly. Failure to frequently monitor this moisture level and adjust the mixing water accordingly leads to substantial variations in concrete slump, pumpability, and final strength.

Question 10: What

is the alkali-aggregate reaction?

The alkali in concrete chemically reacts with aggregates containing active silica. Once the alkali-silica gel forms, it absorbs water and expands; this expansion stress causes the concrete to crack. This process is known as the alkali-aggregate reaction.