To improve the resource utilization of recycled aggregate concrete (RAC) and make use of the unique pozzolanic activation characteristics of iron ore tailing (IOT), the constitutive curves of tailing recycled concrete (TRC) before and after carbonization were analyzed theoretically, experimentally and microscopically. Firstly, according to the experimental data, the damage constitutive and related damage parameters of TRC were theoretically established by Weibull probability distribution function. Secondly, the comprehensive damage parameter b under different working conditions was studied. Finally, the damage mechanism was formed by EDS and SEM. The results showed that the damage constitutive model based on Weibull probability distribution function was in good agreement with the experimental results. Under each carbonization period, the b first decreased and then rose with the increase of tailings content. When its content was 30%, the b values of TRC were minimized, which were 22.14%, 20.99%, 25.39% lower than those of NAC, and 41.09%, 34.89%, 35.44% lower than those of RAC, indicating that IOT had a relatively good optimization effect on the constitutive curve of RAC. The microscopic analysis results also proved that the IOT addition with a proper amount would improve the matrix structure of RAC and increased its compactness, but when the content was higher, it would also cause harmful cracks in its matrix structure and reduced its density. Therefore, the optimal tailing content was about 30%. This paper provided a new method for damage constitutive calculation and analysis of TRC before and after carbonization.

As a residual waste after ore separation from concentrate, the tailings are also known as a misplaced resource [

At the same time, the rapid expansion of urban scale also caused the accumulation of construction wastes. However, its recycling rate in China is also less than 20%, far lower than 95% in developed countries. In view of the increasing concrete amount, it is an effective way to replace coarse aggregate to prepare RAC, which owns obvious environmental, social and economic benefit [

Therefore, a new type of green concrete-TRC came into being at the present stage, which had been rarely researched by scholars. Moon et al. [

Related literatures [

Based on this, the damage constitutive curves of TRC before and after carbonization were analyzed by Weibull probability density function. At the same time, their damage degradation mechanism was studied microcosmically by EDS and SEM, which provided a theoretical reference for promoting the wide application of TRC.

Based on the quasi-brittle and meso-heterogeneous mechanical properties of concrete, the Weibull probability density function is selected to simulate the stress-strain curve of TRC. According to the strain equivalence principle proposed by the French scholar Lemaitre [

where,

Combining the relationship between effective stress and Cauchy stress, it is concluded that:

where,

Substituting

As can be seen from the above formulas, when

In order to apply Weibull probability distribution theory, the following assumptions need to be made:

TRC is macroscopically isotropic, and the damage develops equivalently in all directions;

The concrete can be considered to innumerable tiny elements macroscopically as a combination, each of which contains random distribution of small defects and cracks.

Based on the above two assumptions, if any element in concrete is selected, its density and strain obey the Weibull distribution, its probability density function is as follows:

where, a and b are damage parameters related to its physical and mechanical properties.

At the same time, D owns a physical quantity to characterize the damage degree of materials, which is related to the number of defects affecting the strength of micro-elements directly. The damage factor D and the micro-element ε obey the statistical law as below:

Combining

By substituting

And the derivative of strain ε can be obtained as below:

Then the boundary conditions at the peak strain are used:

Taking the natural logarithm twice on both sides of the above equation:

Simultaneously, by substituting

Then, the deformation is available:

Combining

By substituting

Guo et al. [_{max}, σ = σ_{max}; (d) ε = ε_{max}, dσ/dε =0. The following is to verify the applicability of the above damage constitutive law.

Derivation of the above equation can be obtained:

Available from the

Due to _{max}, σ = σ_{max}, the conditions (c) and (d) are also satisfied. Therefore,

For the research convenience, taking

In the initial loading stage, subjective and objective factors cause large fluctuations of _{c} is relatively small [_{0} is defined as:

_{0}. Point B and OC are the peak point and the on-line secant modulus, while θ is the angle between OC and the horizontal axis. As can be seen from the above figure, D_{0} can also be expressed as:

The figure area surrounding the constitutive curve and the coordinate axis represents the energy dissipation capacity of the specimen in a certain state. The larger the area is, the stronger the energy dissipation capacity is [_{0} makes the damage factor at the peak state, which can directly reflect the overall energy dissipation capacity of its rising section under different conditions. It can be obtained as follows:

Therefore, the ultimate equation of the damage constitutive curve and damage factor are:

From the above analysis, it showed that the damage constitutive equation and the value of D was only related to the elastic modulus, peak stress and peak strain of the materials. So the corresponding damage constitutive curve could be obtained by the relevant physical quantities measured by the experiment. Combining

From the above figures, when b is small, the ascending part of the damage constitutive curve is steeper, while the descending part is relatively flat, and the area around the curve and the horizontal axis is larger, which indicates that the energy dissipation capacity is also better. As b increases, the curve gradually closes and the descending section changes faster, indicating that the concrete with lower ductility and energy consumption. Combined _{max} is smaller or larger, the damage factors show a relatively gentle growth trend on the whole, and the damage factor increases in the middle stage. The larger the b is, the shorter the damage section is, which indicates that the process from the initial damage to the completion is shorter, the energy consumption is poorer, and the concrete presents obvious brittleness.

In the test, Qinling Portland cement (P.O. 42.5) was adopted with its water requirement for normal consistency, Fineness modulus, Initial setting and Final setting time being correspondingly 28%, 2.8 min, 160 min and 280 min. The coarse aggregate, including natural coarse aggregates (NCA) and recycled coarse aggregates (RCA), were common materials in the market with the size of 5 mm–20 mm and continuous gradation. NCA was artificial gravel, while RCA being in service for nearly 30 years was produced by a company in Xi’an, whose matrix strength of NCA was C30 and its old mortar content of RCA remained vary from 8.2% to 14.36%. In addition, RCA used in the experiment was manufactured after a series of procedures. The IOT used were from a tailings pond in Shangluo City. Then, its main physical indicators were measured in experiment through screening, drying, bagging and other procedures. The particle gradation diagrams of main materials meeting the requirements of relevant Chinese specifications were presented in

Diameter of sieve pore/mm | 2.36 | 4.75 | 9.5 | 16 | 19 | 26.5 |
---|---|---|---|---|---|---|

Cumulative sieve residue (NCA)/% | 100 | 99 | 94 | 65 | 5.4 | 0 |

Cumulative sieve residue (RCA)/% | 100 | 98 | 91 | 50 | 1.2 | 0 |

Diameter of sieve pore/mm | 0.15 | 0.30 | 0.60 | 1.18 | 2.36 | 4.75 |
---|---|---|---|---|---|---|

Cumulative sieve residue (sand)/% | 98 | 84 | 56 | 24 | 8 | 2.5 |

Cumulative sieve residue (IOT)/% | 90 | 65.4 | 35.9 | 16.7 | 3.18 | 0.73 |

To facilitate analysis, the replacement rate of RCA was selected as 30% [

In the atmospheric environment, it also contains about 0.03% CO_{2} gas, and 0.05% in industrial and densely populated areas [_{2} has increased from 310 μmol/mol in 1960 to 400 μmol/mol. According to the United Nations Intergovernmental Panel on climate change (IPCC), it will exceed 800 μmol/mol by the end of the 21st century [_{2} has brought a series of major problems related to the national livelihood, and the carbonation of concrete is a key problem to be solved.

The stress-strain curves of nine group specimens before and after carbonization were carried out in TH-W rapid carbonization box, and the carbonation process was shown in

Sequence of steps | Making and curing | Drying | Covering | Carbonizing | Loading and testing |
---|---|---|---|---|---|

Parameter setting | Standard curing for 28 d | Drying for 48 h at 60°C | Covering 5 surfaces with paraffin and leaving a carbonized surface | CO_{2} levels: (20 ± 3)% |
Loading rate: 0.5 MPa/s |

In the test process, the failure modes under various working conditions were observed and analyzed. When the tailings content was high, the typical shear failure occurred due to the diagonal cracks. Through the experimental phenomenon, it also could be observed that the longer the carbonization cycle was, the higher the tailings content was, the more quick the formation of main cracks were, and the more concrete that fell off after the failure was, the more obvious the brittleness of the test block was. Then, the measured stress-strain curves under different carbonization cycles (0 d, 28 d, 90 d) could be obtained, as shown in

Generally speaking, the stress-strain constitutive curves of TRC had a similar trend to that of NAC. It was to say, it owned obvious ascending and descending sections with the single peak curve. Intuitively, with the increase of carbonization cycle, the performance difference of RAC with different tailing content decreased, in other words, the dispersion of ascending and descending sections become smaller. For example, the slope of descending sections tended to be the same under different tailing contents, whose results were similar to those using high silicon materials to RAC in the paper of Wang et al. [

Usually, to reduce the contact effect between the test block and the press steel plate being leakiness, a preload was applied before the formal loading, but in the existing specifications, the detailed magnitude of the preload is not given, so the preload of about 1% is applied in this test. Due to insufficient preload, some blocks have negative stiffness at the initial stage. In order to maintain the authenticity of the test results, the negative stiffness was not deleted in the later stage, as shown in

The damage constitutive model obtained in Section 1 was used to simulate and analyze the experimental results. Due to the influence of some uncontrollable factors such as the concrete discreteness in the experiment process, the one in a group with the fullest and most consistent on the theoretical stress-strain characteristics was selected as the research object. Taking NAC as an example, the fitting curve related to the damage constitutive was shown in

Except for the maximum deviation of some points being close to 30% in the descending section, it could be clearly seen from the above figures that a small deviation of all other points were within ±5% on the whole. Similarly, the damage curves and correlation coefficients of remaining specimens (0 d, 28 d, 90 d) could be obtained. In a signed section, the typical stress-strain damage curves and correlation coefficients of carbonization for 0 d were selected as representatives, as shown in _{0} and their correlation coefficients were shown in

Number | D_{0} | Correlation coefficient | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

0 d | 28 d | 90 d | 0 d | 28 d | 90 d | 0 d | 28 d | 90 d | 0 d | 28 d | 90 d | |

NAC-1 | 0.00376 | 0.00435 | 0.00379 | 5.48231 | 5.04147 | 4.11393 | 1.20010 | 1.21940 | 1.27517 | 0.99322 | 0.96302 | 0.96783 |

RAC-1 | 0.00266 | 0.00372 | 0.00321 | 7.24532 | 6.11745 | 4.75412 | 1.14799 | 1.17759 | 1.23410 | 0.92379 | 0.90483 | 0.97554 |

RAC-2 | 0.00418 | 0.00458 | 0.00358 | 6.69874 | 5.75432 | 4.44342 | 1.16121 | 1.18980 | 1.25239 | 0.87573 | 0.91476 | 0.85737 |

RAC-3 | 0.00395 | 0.00380 | 0.00480 | 4.26837 | 3.98334 | 3.06951 | 1.26397 | 1.28537 | 1.38512 | 0.86726 | 0.98135 | 0.89862 |

RAC-4 | 0.00472 | 0.00441 | 0.00423 | 2.91045 | 2.76592 | 2.64358 | 1.41017 | 1.43554 | 1.45976 | 0.95323 | 0.97112 | 0.93486 |

RAC-5 | 0.00503 | 0.00470 | 0.00400 | 3.76738 | 3.51764 | 2.94736 | 1.30433 | 1.32881 | 1.40395 | 0.90845 | 0.86724 | 0.95935 |

RAC-6 | 0.00394 | 0.00390 | 0.00374 | 4.02542 | 3.74214 | 3.35762 | 1.28208 | 1.30634 | 1.34693 | 0.90524 | 0.91456 | 0.96822 |

RAC-7 | 0.00419 | 0.00409 | 0.00355 | 5.45987 | 4.27596 | 3.62337 | 1.20127 | 1.26347 | 1.31783 | 0.91224 | 0.90444 | 0.84611 |

RAC-8 | 0.00516 | 0.00382 | 0.00360 | 7.53119 | 6.11135 | 4.97852 | 1.14211 | 1.17778 | 1.22246 | 0.98921 | 0.87559 | 0.90466 |

As shown in _{0} showed obvious changes. Owing to the influence of objective factors, the _{0} showed the opposite law, which illustrated that the carbonization could make up for some defects of TRC and improve its energy dissipation characteristics. Simultaneously, it also explained that its capacity of the rising section had been improved to varying degrees under carbonization. The longer the carbonization cycle was, the greater the increase was.

Under the same carbonation cycle, different tailing contents also caused regular changes with _{0} parameters, whose influence rule on b parameters were shown in

At the same time, it could also be proved that the RCA made its overall performance lower than NAC. As the carbonation cycle increased, the b value of RAC had increased by 32.16%, 21.34%, and 15.56%, compared with its values of NAC. It could also be seen that the longer the carbonization cycle was, the smaller the b value of RAC was, and the stronger the energy consumption capacity of the test block was.

In order to quantitatively express the variation of parameters, the formula fitting of b with tailing content in the same carbonization cycle was studied. The fitting curves and correlation coefficients were also shown in

where, u was the substitution rate of iron tailing enlarged by 100 times.

It could also be seen that b changed approximately linearly with the increase of tailing content. The longer the carbonization period was, the smaller the absolute value of the linear change slope was. In other words, it also showed that the carbonization could improve the performance of TRC, whose results were consistent with the conclusions of Skocek et al. [

The internal micro-structure of concrete directly determined its macro-mechanical performance. Therefore, it was necessary to study the internal micro-structure. In this paper, the microstructure of TRC was analyzed by EDS and SEM to find out its mechanism of carbonation damage.

The EDS Atlas of NAC, RAC and TRC were listed in

Referring to the related literatures [_{2} and the needle-like or rod-like ettringite with different content. The hydration degree with NAC and RAC (_{2} was relatively small, which indicated that the tailing activity made the hydration reaction of cement forward to some extent.

From

When the tailing content was small, the carbonization products could fill the pore and optimize the ITZ. While it was large, carbonization destroyed the matrix structure of concrete, resulting in a large number of dry shrinkage micro-cracks in the matrix structure. The reason was that CaCO_{3} generated by carbonization filled the gap in the ITZ, improving the compactness of concrete and its energy dissipation capacity, the longer the carbonation period was, the higher the improvement range was, as shown in

In conclusion, the appropriate tailing amount (30%) could reduce the porosity, improve the pore structure and optimize the corresponding mechanical properties. Simultaneously, carbonation could also improve the compactness of RAC, but when the tailing content was larger and the carbonization cycle remained was longer, the matrix structure was easy to crack. Therefore, in the future application of TRC, it was necessary to strictly control the amount of IOT to use limited resources to obtain the maximum energy consumption capacity. However, in view of the concrete discreteness, it is necessary to use more micro-means to study the pore morphology, pore diameter and porosity in the future, so as to quantitatively express this change and provide theoretical basis for better use of RAC.

Based on the fine powder and pozzolanic activation characteristics of IOT, the rapid carbonization test was carried out on TRC, and the stress-strain constitutive curves were analyzed by Weibull probability distribution function. Then, the micro-damage mechanism of TRC was studied. The main conclusions were as follows:

According to the principle of strain equivalence and Weibull probability distribution, the stress–strain damage constitutive curves and related damage factors of TRC were deduced theoretically, which laid a foundation for the theoretical analysis of TRC.

The stress-strain curves of TRC before and after carbonization were tested with 9 mix ratios. Based on the above theory, the damage constitutive curves were simulated and analyzed. The correlation coefficients could be kept above 85%, which were in good agreement with the experimental results.

Under each carbonation cycle, when the tailing content was 30%, the b values of TRC were minimized, which was 22.14%, 20.99%, 25.39% lower than that of NAC, and 41.09%, 34.89%, 35.44% lower than that of RAC, indicating that IOT had a relatively good optimization effect on the constitutive curve of RAC.

Through microscopic analysis, a small amount of IOT could fill and promote the hydration of cementitious materials, resulting in a denser matrix structure and a stronger overall energy dissipation capacity. When the IOT content was too high, the gradation of aggregates was changed by adding IOT, which increased the harmful pores of matrix materials, so as to enhance the brittleness of the test block and reduce its energy dissipation capacity to different degree.

During carbonization, its products could also fill the harmful pores of RAC and improve the ITZ, thus improving its constitutive curve to become full and enhancing its energy dissipation capacity, the longer the carbonization cycle was, the greater the increase range was. However, when the tailing content was bigger and the carbonization cycle was longer, the hydration and carbonization degree became higher, resulting in the larger demand of free water, which caused the matrix structure to more crack, thus changing the constitutive curve and reducing its energy consumption capacity. After comprehensive consideration, when the tailing content was 30%, the matrix structure owned the densest, and the constitutive curve remained the fullest, also with the best energy consumption.