Keywords

1 Introduction

With the rapid development of the national economy, all kinds of major infrastructure and livelihood projects have been started, resulting in a rapid increase in the amount of concrete as a major building material [1]. As the commonly used high-quality mineral admixtures in cement concrete, fly ash, mineral powder and other resources have also been consumed in large quantities and are on the point of exhaustion [2], which brings great pressure to the stable and healthy development of the concrete industry. Therefore, it has become an urgent problem to find new admixtures that are easy to obtain, of high quality and low price, and with abundant reserves. China has a large amount of iron tailings industrial solid waste, due to its low comprehensive utilization rate, resulting in a large amount of accumulation, not only occupy land resources, pollute the environment, but also prone to collapse, landslide and debris flow and other potential safety risks [3,4,5,6]. Iron tailings micro-powder is the tailings produced after iron concentrate is extracted from iron ore plants, and the particle size of which is less than 0.075 mm after screening and grinding [7,8,9,10]. If it can be used as an admixture for concrete production, it can not only expand the selection of admixtures for concrete, but also help to save resources and turn waste into treasure. For this reason, scholars at home and abroad have carried out exploration and research.

Song Shaomin et al. [11] found that when the content of fine powder of iron tailings was 20%, the amount of admixture would not be increased to meet the workability requirements of concrete, and the compressive strength at 28d age would meet the design value requirements. Zhang Hongru et al. [12] showed that the mechanical properties of UHPC can be significantly improved under autoclaved curing and constant temperature water culture by using iron tailings micro-powder instead of quartz powder to prepare UHPC. Huang Zexuan et al. [13] studied the influence of mixed iron tailings fine powder and fly ash on the deformation and durability of concrete. When the content of iron tailings fine powder does not exceed 50%, the drying shrinkage of concrete can be reduced and the frost resistance can be improved. Kexin Huang et al. [14] studied the influence of different amounts of iron tailings fine powder on the mechanical properties and durability of high-strength concrete under high temperature environment. According to Zhou shuang et al. [15], the slump of C40 self-compacting concrete increases slowly, while the compressive strength decreases gradually with the increase of iron tailings micro-powder content. Liu Xuan et al. [16] used skarn type iron tailings micro-powder as raw material to prepare cementing material, and found that the iron tailings had the best mechanical strength when the micro-powder content was 40%. Huang Jingjing et al. [17] found that a large amount of iron tailings micro-powder in the cement system would reduce the hydration gelling property of the material, while autoclaved curing could improve the crystallinity of C-S-H gelling. Mingjing Yang [18] with Cheng et al. [19] found that by increasing the specific surface area of the fine powder of iron tailings by mechanical grinding to improve its hydration reactivity, it can meet the performance requirements of being used as concrete mineral admixtures.

To sum up, iron tailings have been widely used as cementing material components and concrete mineral admixtures. However, due to the different geological conditions and discharge technologies of iron tailings ponds in different regions, the properties of iron tailings are quite different. In this paper, we try to use iron tailings as mineral admixtures, and compare limestone powder with fly ash. Firstly, the physicochemical properties of the three admixtures are compared and analyzed. Then, it replaces the cement of equal quality to study the different influence rules on the fluidity, mechanical strength and activity index of cement mortar, and on this basis, the experimental study on the influence of the types of admixtures on the performance of different grades of C30 ~ C40 concrete is carried out, so as to preliminatively determine the feasibility of iron tailings fine powder as mineral admixtures.

2 Materials and Methods

2.1 Test Raw Materials

  1. (1)

    Cement: adopt Shaanxi Tongchuan Shengwei P·O 42.5 grade.

  2. (2)

    admixture: fine powder fineness of iron tailings is 25.9%, mobility ratio is 99%; Fly ash obtained from Tongchuan Huaneng Power Plant Grade II fly ash, fineness of 21.3%, water requirement ratio of 97%; The fineness of limestone powder is 24.8%, and the water requirement ratio is 102%. The mineral powder adopts Delon S95 grade, with a specific surface area of 426 m2/kg and a density of 2.9 g/cm3.

  3. (3)

    Fine aggregate: natural river sand is used, the fineness modulus is 2.5, the apparent density is 2650 kg/m3, and the loose packing density is 1690 kg/m3.

  4. (4)

    Coarse aggregate: continuous graded gravel with a particle size of 5–25 mm is used, the apparent density is 2650 kg/m3, the loose packing density is 1580 kg/m3, and the crushing index is 6%.

  5. (5)

    Water reducing agent: polycarboxylic acid high efficiency water reducing agent produced by China Construction West Construction New Material Technology Co., LTD., with a solid content of 12.6%;

  6. (6)

    Water: tap water.

2.2 Test Methods

The fluidity of cement mortar is tested according to the “Method for Determination of cement mortar fluidity” (GB/T2419-2005). The compressive and flexural strength of cement mortar is determined by referring to the Test Method for cement mortar Strength (ISO method) (GB/T17671-1999). The activity index of mineral admixtures was tested according to the Technical Specification for Application of Mineral Admixtures (GB/T 51003-2014). According to the Standard of Test Method for Mechanical Properties of Ordinary Concrete (GB/T 50081–2002), the cube specimens with dimensions of 100 × 100 × 100 mm were formed, and the compressive strength of concrete of different ages was tested. Non-contact method and electric flux method were used to test the self-shrinking and chloride ion penetration resistance of concrete, and the specific test methods were carried out according to the “Standard for Long-term Performance and Durability Test Method of ordinary Concrete” (GB/T 50082-2009).

3 Results and Discussion

3.1 Characteristics Analysis of Different Kinds of Mineral Admixtures

The main chemical compositions of the three admixtures of iron tailings fine powder, fly ash and ground limestone powder are compared in Table 1, and their XRD spectra are shown in Fig.  1. According to the XRD pattern analysis in Fig. 1, quartz accounts for the largest proportion in the mineral composition of iron tailings micro-powder and fly ash, and the main component of quartz is SiO2, which is also consistent with the highest chemical component content of SiO2 in Table 3 and Table 4. In addition, the main chemical components of the two are roughly the same, which are mainly Si, Al and Fe (Fig. 2). The Fe content of iron tailings is 26.6%, and the Fe content of fly ash is 7.8%. According to the analysis of Fig. 3 and Table 1, the mineral composition of ground limestone powder is mainly composed of CaCO3 (calcite), which corresponds to the highest CaO content in the chemical composition. According to the above analysis, similar to fly ash, the fine iron tailings powder is mainly siliceous powder, while the ground limestone powder is calcareous powder.

Table 1 Main chemical composition of admixture
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Fig. 1
A spectral graph of X R D pattern of limestone powder. The top irregular line represents limestone powder. The second line corresponds to iron tailings powder and the bottom line represents fly ash. The peaks are labeled with letters A to F.

XRD pattern of limestone powder

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Fig. 2
Four microscopic images of the detailed textures of 1 siliceous, 2 sericite, and 1 siliceous, 2 sericite, and 3 epidote with 50 and 200 times magnifications.

Microsilty rock facies analysis of iron tailings

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Fig. 3
A line graph of compressive strength versus age plots 4 concave-down increasing trends. The trends pass through the following, J C, (0, 40), (30, 55), and (60, 60), J F, (5, 20), (30, 40), and (60, 50), J S, (5, 20), (30, 35), and (60, 37), and J Z, (5, 20), (30, 35), and (60, 36). Values are estimated.

Compressive strength

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According to the microsilty rock facies analysis of iron tailings in Fig. 2, it can be seen that the tailings are mainly composed of clay minerals, epidotes, quartz, metal minerals, etc., and have the characteristics of flake blastoblastic structure and plate structure. The mineral is mainly composed of fine clay minerals, and the arrangement has a certain orientation. The particle size distribution of clay minerals is about 0.003 × 0.003 mm, colorless and transparent under a single polarizing lens, some of them are transformed into sericite, the flash is obvious, and the interference color becomes secondary blue violet. The particle size distribution of epidote is about 0.05 × 0.05 mm. Under the microscope, the epidote is yellow-green, the interference color is blue-violet, and the height is protrude. The quartz particles are colorless inside, and the particle size distribution is about 0.01 × 0.01 mm, which is unevenly distributed inside the rock. The feldspar filling inside the rock contains microcrystalline quartz with a particle size less than 0.02 × 0.02 mm, which belongs to the mineral with alkali-silica activity.

3.2 Comparison of the Influence of Different Mineral Admixtures on the Properties of Cement Mortar

The total mass of the cementing material is 450 g, and three kinds of admixtures such as fly ash, limestone powder and tussah stone powder are used to replace 30% cement dosage respectively. The mixing ratio used in the test is shown in Table 2.

Table 2 Test ratio of cement mortar/g
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Table 3 Fluidity of mortar with different admixtures
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Table 4 Concrete mix ratio
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Influence of the type of admixture on the loss of fluidity of mortar during warp time

In the test, when the admixture content is adjusted to reach the same initial flow of mortar, the admixture content and flow loss during warp time of three different admixtures are compared, and the results are shown in Table 3.

As can be seen from Table 3 above, when the initial fluidity of mortar is the same, the admixture dosage of limestone powder and iron tailings micro-powder is higher than that of fly ash. From the perspective of fluidity, the compatibility of fly ash system and water reducer is the best. The fluidity loss of the three admixtures was different at different times. The fluidity loss rates of fly ash and iron tailings were similar at 1 h (10.7%, 14.3%), 2 h (19.6, 17.9%) and 3 h (30.4, 26.8%), respectively. However, the 1 h, 2 h and 3 h flow loss of limestone powder blended mortar is lower than that of the other two types of admixture powders, which are 3.6, 7.1 and 7.1%, respectively. Through comprehensive comparison of fluidity and fluidity loss, it can be seen that the addition of limestone powder has a good effect on improving the fluidity loss of colloidal sand. This is because the fineness of limestone powder is larger than that of cement, which can make the discontinuous gradation of micro-aggregate more reasonable. The water in the original cement void is replaced by limestone powder, and the free water increases to make up for the loss of fluidity. Because the fly ash contains more glass beads in its structure, it can improve the flow, and the admixture content is lower when the initial flow degree is the same, and the retarded components contained in the admixture are correspondingly reduced, which is the main reason for the large loss of the flow rate during the period. The chemical composition of iron tailings micro-powder contains a high proportion of Fe2O3, which has a certain adsorption effect on water reducing agent, so the flow loss rate is larger with the extension of time.

Influence of admixture type on strength and activity index

The mechanical strength and activity index of pulverized iron tailings, fly ash and limestone powder at different curing ages are shown in Figs. 3, 4 and 5.

Fig. 4
A line graph of flexural strength versus age plots 4 concave-down increasing trends. The trends pass through the following, J C, (0, 7), (30, 9), and (60, 9.1), J F, (0, 5), (30, 8), and (60, 9.5), J S, (0, 5), (30, 7.5), and (60, 7.3), and J Z, (0, 5), (30, 6.8), and (60, 7.5). Values are estimated.

Flexural strength

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Fig. 5
A line graph of the activity index in percentage versus age in days plots 4 concave-down increasing trends with fluctuations between 5 and 25 days for J F, J S, and J Z.

Activity index

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It can be seen from Figs. 3 and 4 that the compressive strength and folding strength of the mortar mixed with three different admixtures increase with the extension of curing time, and the compressive strength and folding strength of the experimental groups are basically similar at the early age of 14d. With the further development of the age, the compressive strength and folding strength of single fly ash increase the fastest after the age of 14d. Compared with the single limestone powder and iron tailings powder group, the compressive strength and bending strength of 28d colloidal sand are increased by 9.3 and 10.2%, respectively, and 8.0 and 14.1%, respectively. However, the strength of the three admixtures at each age is lower than that of the pure cement reference group. In combination with Fig. 5, it can be seen that the activity index of the three admixtures and the change law of cement strength with the development of age are basically consistent. The reason may be that the filling effect and the crystal nucleus effect of the three admixtures play a major role in the early hydration stage, resulting in little difference in compressive and flexion strength of the three cemels before the age of 14d. However, after the age of 14d, the pozzolash effect of the admixtures begins to play a role, and more C-S-H gels are generated by the second hydration reaction, contributing to the strength. Due to the high activity index of fly ash and stronger pozzolash effect, more hydration products are generated in the later stage of hydration, while the low activity index of iron tailings and limestone powder belong to inert admixture, which is also consistent with the test results of the strength growth characteristics of the three admixtures (Fig. 6).

Fig. 6
A line graph plots compressive strength versus age. The trends pass through the following, A 1, (3, 19), (28, 45), and (60, 52), A 2, (3, 19), (28, 42), and (60, 46), and A 3, (3, 19), (28, 45), and (60, 46). Values are estimated.

C30 concrete under the same curing conditions

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3.3 Comparison of Effects of Different Mineral Admixtures on Concrete Properties

C30–C40 strength grade concrete was selected, and three admixtures with equal mass of 60 kg fly ash, limestone powder and iron tailings fine powder were added respectively. In the test, the water-binder ratio and water consumption were fixed. In order to meet the workability requirements, the water-reducing agent content was changed to adjust the output slump of concrete in each test group within the range of 200 ± 30 mm.

Influence of admixture type on compressive strength of concrete

The concrete with different water-binder ratio, different curing methods and different admixtures was cured to 3, 7, 28 and 60 days respectively, and the curve of compressive strength change with age was obtained, as shown in Figs. 7, 8, 9, 10 and 11.

Fig. 7
A line graph plots compressive strength versus age. The trends pass through the following, A 1, (3, 20), (28, 50), and (60, 58), A 2, (3, 19), (28, 50), and (60, 55), and A 3, (3, 20), (28, 50), and (60, 55). Values are estimated.

C30 concrete under standard curing

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Fig. 8
A line graph plots compressive strength versus age. The trends pass through the following, B 1, (3, 19), (28, 45), and (60, 55), B 2, (3, 19), (28, 40), and (60, 50), and B 3, (3, 20), (28, 42), and (60, 52). Values are estimated.

C40 concrete under the same curing conditions

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Fig. 9
A line graph plots compressive strength versus age. The trends pass through the following, B 1, (3, 22), (28, 50), and (60, 64), B 2, (3, 20), (28, 40), and (60, 49), and B 3, (3, 22), (28, 50), and (60, 51). Values are estimated.

C40 concrete under standard curing

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Fig. 10
A line graph plots compressive strength versus age. The trends pass through the following, C 1, (3, 23), (28, 55), and (60, 64), C 2, (3, 25), (28, 49), and (60, 57), and C 3, (3, 23), (28, 47), and (60, 55). Values are estimated.

C50 concrete under the same curing conditions

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Fig. 11
A line graph plots compressive strength versus age. The trends pass through the following, C 1, (3, 28), (28, 60), and (60, 68), C 2, (3, 28), (28, 55), and (60, 62), and C 3, (3, 28), (28, 55), and (60, 56). Values are estimated.

C50 concrete under standard curing

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The performance of compressive strength of concrete with different types of admixtures is inconsistent, mainly because the activity of the three admixtures is quite different. Combined with the data curve in Fig. 6, it can be seen that the activity of iron tailings fine powder and limestone powder is close to that of fly ash, and the activity index of both are inert admixtures, which only play the role of micro-aggregate filling in cement slurry, and their contribution to concrete strength is relatively limited. The strong pozzolanic effect of fly ash will generate more hydration products in the hydration reaction process, and combined with the dense filling effect on the particle structure of the cementing material, the compressive strength of concrete mixed with fly ash will increase more.

Influence of type of admixture on shrinkage performance of concrete

C35 concrete with three different admixtures in Table 4 was selected as the test ratio, and the non-contact method was used to measure the shrinkage and deformation properties of concrete specimens after hardening under unconstrained and specified temperature and humidity conditions. The shrinkage rate of three kinds of admixture concrete at different ages is shown in Fig. 12.

Fig. 12
A line graph of shrinkage versus age plots 3 increasing trends with fluctuations for B 1 to B 3. B 1 has higher values, followed by B 2 and B 3.

Shrinkage rate of concrete

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According to the change curve of concrete shrinkage rate shown in the figure, the shrinkage rate of the three kinds of admixture concrete increases with the extension of curing age. Compared with fly ash concrete, the addition of limestone powder and iron tailings fine powder has a better inhibition effect on the self-shrinkage of concrete, and its self-shrinkage value at 12d age is 0.057% and 0.04%, respectively. It is only 50% and 35% of the shrinkage of fly ash doped concrete. This may be due to the fact that the hydration reactivity of limestone powder and iron tailings fine powder is lower than that of cement, fly ash and other cementing materials. After replacing cement of equal quality, with the development of curing age, most of the fine powder particles of limestone powder and iron tailings do not react with water, but stably exist in the void structure of cement slurry in the form of inert filler. Its volume does not change, which is conducive to improving the self-shrinking performance of concrete.

Influence of the type of admixture on the chloride ion penetration resistance of concrete

In the test, C35 concrete with three different admixtures in Table 4 was selected as the test ratio, and the electric flux method was used to evaluate the chemical damage resistance and permeability resistance of hardened concrete. The electric flux values of three kinds of admixture concrete at different ages are shown in Fig. 13.

Fig. 13
A line graph plots electric flux versus age. The trends pass through the following, B 1, (1, 3100), (7, 1500), and (28, 700), B 2, (1, 3300), (7, 1800), and (28, 1000), and B 3, (1, 3350), (7, 1800), and (28, 1000). Values are estimated.

Electric flux of concrete

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It can be seen from the law of numerical changes in Fig. 13 that the electric flux of C35 strength grade concrete prepared with different types of admixtures decreases with the increase of age. The electric flux values of the concrete mixed with limestone powder and iron tailings have little difference at each age, while the electric flux values of the concrete mixed with fly ash at each age are lower than the first two, and the electric flux values of the three are 951C, 986C and 713C at the age of 28d. According to JGJ/T 193 evaluation standard, the electric flux results of three different admixtures are divided into “Q-iv” grade, that is, the electric flux is in the range of “500C ≤ Q < 1000C”. The results show that the chloride ion penetration resistance of the concrete prepared with iron tailings is equivalent to that of the concrete mixed with fly ash and limestone powder, and can meet the application requirements of the concrete's chloride ion erosion resistance.

3.4 SEM Microscopic Analysis

In order to further analyze the influence of three different admixtures on the concrete microstructure, SEM photos and EDS spectra of C35 concrete samples doped with fly ash, limestone powder and iron tailings micro-powder were selected after 28d hydration and magnified by 5000 times. See Fig. 14 for the photos of the micro-structure analysis of concrete with different types of admixtures.

Fig. 14
Three S E M images and E D S spectral graphs of fly ash, limestone powder, and iron tailings micro-powder. In all spectral graphs, the maximum peak is for O.

SEM photos and EDS spectra of the sample after 28 days of hydration

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As can be seen from Fig. 14a, the concrete doped with fly ash has more C-S-H gels in clusters and a small amount of sheet Ca(OH) 2, which is because the secondary hydration reaction of fly ash consumes a large amount of Ca(OH) 2 in the system. In Fig. 14b, c, it can be seen that there are a large number of plate-like Ca(OH)2 crystals and a small amount of granular C-S-H gel in the concrete mixed with limestone powder and iron tailings fine powder, and the amount of hydration products generated is not enough to fill the internal pores, and the overall structure is relatively loose. The microstructure analysis shows that fly ash has pozzolanic effect, which can promote the hydration of cement, and it also has micro-aggregate effect, which has more advantages in enhancing the density of structure. Similar to iron tailings, limestone powder only has weak hydration reactivity, and when added to the system, it mainly plays the role of particle filling, so the dense filling effect on the structure is not as good as fly ash, which is also consistent with the macro performance of concrete.

4 Conclusion

  1. (1)

    The chemical composition of iron tailings is similar to that of fly ash, and the main chemical composition is SiO2, which belongs to siliceous powder and has certain alkali-silicon activity, and can be used as mineral admixtures in concrete.

  2. (2)

    Under the condition that the initial fluidity of cement mortar is the same by changing the admixture content, the 1, 2 and 3 h flow loss of cement mortar of iron tailings and fly ash is similar, but greater than that of cement mortar with limestone powder; For the same amount of iron tailings, fly ash and limestone powder, the compressive and folding strength of the three camels is basically similar before the age of 14d, while the strength of the camels with fly ash increases greatly at the age of 28d and 56d, and is higher than the other two. The activity index of each age is consistent with the change of strength.

  3. (3)

    Under the same age and curing conditions, the relationship between the compressive strength of C30–C40 concrete mixed with three different admixtures is as follows: the compressive strength of fly ash concrete is the highest, the compressive strength of iron tailings powder is the second, and the compressive strength of limestone powder concrete is the lowest; Under the same curing age and the same mix ratio, the strength of concrete under standard curing conditions is generally higher than that under the same curing conditions, and all meet the design requirements of different grades of concrete strength.

  4. (4)

    The non-contact shrinkage test of concrete shows that the shrinkage rate of concrete mixed with fly ash is the highest, followed by that of limestone powder, and that of iron tailings is the lowest, which has good anti-shrinkage performance.

  5. (5)

    Compared with limestone powder and fly ash concrete, the 28d electric flux value of the concrete doped with iron tailings is in a lower range, and it has good chloride ion penetration resistance.

  6. (6)

    Combined with the microscopic scanning electron microscopy image analysis, it can be seen that the internal hydration product structure of the concrete with iron tailings micro-powder and limestone powder is not as dense as that of fly ash concrete, which is mutually confirmed with the macro performance.