Experimental study on the effect of volcanic residue on the performance of recycled lightweight aggregate concrete

Recycled lightweight aggregate concrete prepared with waste brick recycled light aggregate has high water absorption, large apparent density and poor frost resistance. The technical measures of regen-erating lightweight aggregate concrete with modified waste bricks from volcanic slag are put forward. The effects of volcanic slag on the properties of waste lightweight aggregate concrete were studied. The experi-mental results show that volcanic slag can significantly reduce the apparent density of recycled lightweight aggregate concrete and improve its frost resistance.


Introduction
In recent years, with the rapid development of urban construction, housing construction, the construction of new buildings and the removal of old buildings resulted in a large amount of construction waste. Recycled concrete technology can realize the effective recycling of waste concrete, which is of great significance for protecting the environment, saving resources and developing ecological buildings. In recent years, many domestic and foreign constructors use of this recycled aggregate instead of natural aggregate preparation of new concrete (commonly known as recycled concrete) research. However, there are relatively few studies on the recycled aggregate of waste bricks. As the waste bricks recycled aggregate has the characteristics of large porosity and high water absorption, according to the design method of ordinary concrete mix design, the slump of recycled concrete is low and the apparent density is larger. Volcanic residue is a natural light aggregate, with volcanic slag modified waste brick recycled aggregate concrete, can improve its performance.

Raw materials and their properties.
Recycled aggregate: Recycled aggregate for a building old red brick wall after removal by the broken processing the main technical performance are shown in Table 1; Cement: Jilin honored cement limited company. Production tripod deer card P.O 42.5 R ordinary Portland cement; Fly ash: meeting the national standards of grade II; Volcanic slag: Huinan County, Jilin Province, the main performance parameters in Table 2; Water reducing agent: Air-entraining type water-reducing agent, water reduction rate of 20%; Water: tap water.

Preparation and testing of test pieces
The size of the recycled aggregate concrete specimens in each group was 100mm × 100mm × 100mm, a total of 6 specimens were produced in each group. The compression test was carried out on a YES-2000 compression test machine. The frost resistance test was carried out with reference to"concrete quality control standard GB50164-92". The strength loss rate and mass loss rate of each group of concrete specimens after 100 freeze-thaw cycles were measured.

Effect of Volcanic Slag Content on Compressive Strength of Recycled Aggregate Concrete for Construction Waste
The effect of volcanic coarse aggregate content on the compressive strength of recycled aggregate concrete is shown in Figure 1, The effect of volcanic fine aggregate content on the compressive strength of recycled aggregate concrete is shown in Figure 2. It can be seen from Figure 1 fine aggregate is still using recycled waste brick fine aggregate, coarse aggregate part or all of the use of volcanic residue instead of coarse aggregate. Volcano slag coarse aggregate content of 10% of the recycled aggregate concrete compressive strength and the whole building waste recycled aggregate concrete compared to almost no difference. But the volcanic slag due to volcanic slag cylinder pressure is low, when the volcanic coarse aggregate content of 10% or more. The compressive strength of recycled aggregate concrete is gradually reduced and the degree of reduction is more than 5%. Figure 2 can be seen that the coarse aggregate is still recycled red brick aggregate, and fine aggregate part or all of the use of volcanic residue to replace. When the volcanic fine aggregate content of 10%-30%, the construction of recycled concrete concrete compressive strength slightly improved, and when the volcanic fine aggregate content of over 30%, the recycled aggregate concrete compressive strength is continued to decrease.

Apparent Density of Recycled Lightweight Aggregate Concrete in Modified Slag
The results of the determination of the apparent density of the reconstituted light aggregate concrete of the modified waste brick of the volcanic slag are shown in Table 4  It can be seen from Table 4 that the apparent density of reconstituted lightweight aggregate concrete with flammable slag modified is less than the apparent density of recycled lightweight aggregate concrete.

Effect of Volcano Slag on the Frost Resistance of Recycled Aggregate Concrete for Construction Waste
The freezing and thawing tests of recycled aggregate concrete with coarse aggregate and volcanic slag fine aggregate are shown in Table 5. It can be seen from the table, the incorporation of volcanic slag is clearly mentioned waste brick recycled lightweight aggregate concrete frost resistance. The porous structure of volcanic slag and the effect of "micro pump" of volcanic slag on the improvement of concrete structure have great influence on improving the frost resistance of concrete, the interface between the volcanic slag microstructure and the volcanic slag and the concrete cement transition zone is shown in Figure.  Volcanic slag itself, the apparent density of small, in the mixed concrete, it was in the concrete was floating state, and waste brick recycled aggregate in the mixing was sinking state. The mixing of the two improves the homogeneity of the light aggregate concrete, reduces the loss of water, prevents the early plastic cracking, and greatly improves the frost resistance of the lightweight aggregate concrete. The porosity of the volcanic slag determines that it has significant water absorption and backwater in the cement slurry. It can be seen from Figure 4, volcanic slag and the interface transition zone cement stone structure matrix is more compact. Volcanic slag has a "self-curing" effect in the concrete, making the cement at the interface of the volcanic slag more fully hydrated. When the blended slag waste slag recycled light aggregate concrete, for the waste brick recycled lightweight aggregate concrete as a whole cement hydration more fully, the cement structure is more compact, thereby enhancing the concrete frost resistance.

Conclusion
Volcanic slag can effectively reduce the apparent density of recycled lightweight aggregate concrete, The apparent density of the concrete prepared by replacing the waste aggregate with the volcanic slag is reduced from 1776 kg/m3 to 1694 kg/m3. In addition, volcanic slag greatly improves the frost resistance of waste concrete recycled aggregate concrete.