Why is the recycled aggregate of construction waste popular?

In recent years, there has been a shortage of sand and gravel, and the price of sand has soared. With the support of the state for the recycling of construction waste, recycled aggregate has become popular. It is reported that the conversion rate of construction waste into aggregate can reach 85%, that is, 1 ton construction waste can produce 0.85 ton recycled aggregate, and the price is much cheaper than natural aggregate, so it has attracted a lot of investors. Some friends hesitated and doubted the quality of recycled aggregates. Below we will analyze the difference between the two in terms of particle size, impurity content, density, water absorption, crush value and firmness.

The preparation of recycled aggregate is basically similar. Firstly, the waste concrete is hammered and broken, metal and steel bars are selected by magnetic sorting, and wood and plastic are removed by debris sorting. Then, through the jaw crusher, the fine particles below 5mm are screened out, and the particles of 5-40mm are obtained, and then the eccentric rotating equipment is used to make them collide and grind, remove the mortar and cement slurry, and obtain the recycled aggregate, which is prepared in large quantities. , Simple, we denote the aggregate obtained in this step as regenerated aggregate 1, pass regenerated aggregate 1 through a 5mm vibrating screen to remove fine particles such as cement and mortar, and finally obtain regenerated aggregate 2.

This article will study and compare regenerate aggregate 1, regenerate aggregate 2 and natural aggregate.

1.Particle size and impurity content

Regenerate aggregate includes regenerated coarse aggregate and regenerated fine aggregate. Particles size less than 5mm are called regenerated fine aggregates, and those larger than 5mm are regenerated coarse aggregates. The composition of recycled fine aggregate is complex, the content of impurities is large, and it is quite different from natural sand. Generally, too fine recycled fine aggregate cannot be used to prepare concrete.

The recycled coarse aggregate particles are generally stones with a part of the cement mortar on the surface, a small part of the stones completely separated from the mortar, some broken bricks, and a small part is mortar particles. The debris in the aggregate mainly includes metals, plastics, asphalt, wood, glass, grass roots, leaves and branches and other substances that do not belong to concrete, mortar, bricks or stones.

The content of needle-like particles of recycled aggregate and natural aggregate is basically the same, and the content of debris is obviously different.

The content of impurities in the aggregate, the content of needle-like particles (%)
Natural Regenerate Aggregate1 Regenerate Aggregate 2(Screened0
Impurities in the Aggregate 0 1.2 0.9
Needle-like Particles 3 2 2

2.  Apparent Density

The apparent density and bulk density of the recycled aggregate are smaller than those of natural aggregate. The apparent density of recycled aggregate 1 exceeds 88% of that of natural aggregate, the bulk density exceeds 83%. 95% of the aggregate, and the bulk density exceeds 92%. In this way, it can be roughly judged from the density of aggregate that recycled aggregate 2 is much better than the recycled aggregate 1. The aggregate screened by the vibrating screen is very similar to the natural aggregate only in terms of density.

5-10mm Natural 2620 1480 44 1.1 0.2 0.001 Qualified
Recycled 1 2390 1280 46 9.6 0.34 0.001 Qualified
Recycled1 screened 2590 1420 45 7.7 0.20 0.002 Qualified
10-20mm Natural 2660 1530 42 0.8 0.2 0.001 Qualified
Recycled 1 2340 1270 46 6.3 0.29 0.001 Qualified
Recycled1 screened 2540 1410 44 4.4 0.37 0.001 Qualified
20-40mm Natural 2630 1500 43 0.6 0.1 0.001 Qualified
Recycled 1 2340 1250 47 9.2 0.36 0.001 Qualified
Recycled1 screened 2540 1430 44 3.1 0.23 0.001 Qualified

3. Water Absorption

The content of cement mortar in recycled aggregate is relatively high, and the surface is much rougher than that of natural aggregate. Due to the large number of micro-cracks in the disintegration and crushing of concrete blocks, the water absorption rate is much larger than that of natural aggregate.

It can be seen from the above table that the water absorption rate of recycled aggregate 1 is 7-8 times larger than that of natural aggregate in the best case, and the water absorption rate of screened recycled aggregate 2 is larger than that of natural aggregate in the best case by 5-8 times. 6 times, and the maximum time exceeds l0 times.

This is mainly because this recycled material contains a lot of brick slag, which brings great trouble to the design of the proportion of recycled aggregate. In domestic experiments, the addition of superplasticizer or high-efficiency plasticizer is commonly used, and additional Water can be treated in two ways. Using the method of adding additional water to mix recycled concrete, the water consumption is about 5% more than the theoretical water consumption of the natural aggregate concrete mix ratio design.

4. Crushing and Solidity Value

Classification Crushing Value Solidity
5-10mm Natural / 3
Recycled 1 / 8.5
Recycled 2 Screened / 5.6
10+20mm Natural 4-6 2
Recycled 1 15 12.6
Recycled 2 Screened 10 6.7
20-40mm Natural / 2
Recycled 1 / 14.2
Recycled 2 Screened / 5.6

The composition of recycled aggregate is complex, including raw stones, sand and slag wrapped in mortar, and a small amount of debris. Compared with natural aggregate, the crushing loss of recycled aggregate is larger. However, the data in the table above shows that recycled aggregate 2 has It meets the requirements of Class I crushing index (<12%) in GB/T25177-2010 “Recycled Coarse Aggregates for Concrete”, and Recycle 1 meets the requirements of Class II (<20%).

From the results of the firmness test, the quality loss of the recycled aggregate is larger than that of the natural aggregate after soaking and drying in saturated sodium sulfate solution and drying for 5 times. It meets the requirements of class II in GB/T25177-2010 (<10.0%), and the recycling 1 meets the requirements of class III (<15.0%). The durability test in the produced recycled concrete requires further inspection and testing.

It shows that there are many difference between natural aggregate and recycled aggregate. But the aggregate screened by the vibrating screen has low debris content and high bulk density, which is very close to the indicators of natural aggregate. It shows that after reasonable and proper processing, recycled aggregates that meet the corresponding specifications can be obtained. On the premise of ensuring quality, recycled concrete prepared from recycled aggregate can reduce the cost of ready-mixed concrete, save raw materials, reduce the mining of mineral resources, turn industrial waste and construction waste into treasure, and is conducive to the sustainable development of resources and environment.

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