Ultra-high performance concrete with metal mine tailings and its properties: a review

2.1 Mineral and chemical composition

The types of MMT mainly include iron mine tailings, copper mine tailings, gold mine tailings, lead-zinc mine tailings, and molybdenum mine tailings, with the above five types of tailings accounting for approximately 85 % of all MMT (Zhao et al. 2021). Different MMT have diverse mineral compositions, but the main mineral composition is similar, with quartz predominating and usually accompanied by minerals such as sodium feldspar, hornblende, calcite, and pyroxene (Adiguzel et al. 2022). The main chemical composition of different MMT is similar, with SiO₂, Fe₂O₃, Al₂O₃, and CaO predominate, accompanied by small amounts of MgO, SO₃, K₂O, and even lower content of other oxides. The chemical composition content of several main types of MMT is shown in Table 1, with the highest SiO₂ content. Usually, the SiO₂ crystal structure within MMT is relatively stable (Carrasco et al. 2017; Han et al. 2017; Liu et al. 2019), and the particles are inert; in a few cases, amorphous SiO₂ exists, with low volcanic ash activity (Tuan et al. 2011; Uchechukwu and Ezekiel 2014), i.e., reacting with Ca(OH)₂ to form C–S–H gels. Certain MMT contain small amounts of SO₃, which can accelerate the consumption of C₂S and C₃S, and promote the rate of hydration reactions (Wang et al. 2021). The chemical composition average content of the main types of MMT is shown in Figure 1.

Figure 1:

Average content of chemical composition for main types of MMT.

2.2 Physical characteristics

MMT are fine-grained, generally in the form of powder or fine sand. The main differences between the particle morphology of MMT and typical raw materials (e.g., cement, nature sand) are as follows: MMT particles are more angular and sharp, with many needle-like, flake-like, and other irregularly shaped particles. The surface of MMT particles is rough, with many pores and grooves. Compared with natural sand, MMT sand has finer particle size, higher specific surface area, and stronger water absorption. The particle morphology comparison of powdered and fine sandy MMT with cement and nature sand are shown in Figure 2.

Figure 2:

The particle morphology of cement, natural sand, and several MMT: (a) the powder form (Ling et al. 2021; Lu et al. 2021; Wang et al. 2018); (b) the fine sand form (Ling et al. 2021; Lu et al. 2021; Wang et al. 2018).

2.3 Application methods in UHPC

The particle size distribution of some MMT and UHPC common raw materials is shown in Figure 3. According to the characteristics of MMT particles in powder form and fine sand form, MMT powder can be used to replace binder materials, and MMT sand can be used to replace fine aggregate to prepare UHPC. For convenient narration, this paper abbreviates the former as “MMT-P-UHPC” and the latter as “MMT-S-UHPC.”

Figure 3:

Particle size of some MMT and UHPC common raw materials (Ahmed et al. 2021a,b; Wang 2018).

In MMT-P-UHPC, the maximum particle size of MMT powder generally does not exceed 150 μm. It is possible to use MMT powder directly in its original state as a substitute for binder materials or to activate MMT powder before use. Activation is designed to disrupt the stable crystal structure, stimulate volcanic ash activity, and improve the cementation of MMT (Carrasco et al. 2017; Liu et al. 2019; Wang et al. 2021). Activation methods include physical activation, chemical activation, thermal activation, and compound activation. Table 2 summarizes some MMT-P-UHPC, the literature statistics show that mechanical activation is the more common method.

In MMT-S-UHPC, the maximum particle size of MMT sand generally does not exceed 600 μm. When exceeds 600 μm, the larger the particle size, the more detrimental to UHPC property. The reason is that as the particle size increases, the difference in elastic modulus between the MMT sand and the cement slurry matrix increases, the stress in the interface transition zone becomes concentrated, leading to many microcracks near the MMT sand. Table 3 summarizes some MMT-S-UHPC.

From this section onward, the effects of incorporating MMT on the workability, mechanical properties, durability, leaching toxicity, and microstructure of UHPC will be gradually presented. It is important to note that there are some complex differences in the properties of MMT due to geographical differences, the nature of the mine, etc. Whereas the influence of MMT on the properties of UHPC is the result of multi-factors coupling, this paper merely analyses the influence of the primary properties (i.e., primary factors) of most MMT on the properties of UHPC, possibly ignoring the influence of individual unique properties (i.e., secondary factors).

3.1 Decrease workability

In most studies, incorporating MMT powder or sand will decrease the UHPC workability, the greater the amount, the more severe the decrease. The reasons are as follows:

1. Poor particle morphology of MMT increases the viscosity of the fresh slurry. The rough and angular surface of the MMT particles increases the friction between particles and the shear force inside the slurry (Mora and Kwan 2000), and many needle-like, flake-like, and other irregularly shaped particles also interlock in the slurry (Mora and Kwan 2000). Irregular particles reduce the solid materials’ packing density (Wong and Kwan 2008), resulting in larger voids between particles, and require more water to fill them during mixing. (ii) The MMT sand has strong water absorption, which reduces the free water content in fresh slurry. Compared with natural sand, MMT sand has a high specific surface area, with many pores and grooves on the particle surface, which absorb more water during mixing and decrease the workability. (iii) MMT in a damp state has a detrimental effect on the UHPC workability. Because the total free water added during mixing needs to subtract the water contained in the MMT in a damp state, some water in the MMT is difficult to release during mixing, resulting in a low total amount of free water in the fresh slurry. Wang et al. (2018) used lead-zinc mine tailings in the wet state to prepare UHPC founding that the workability decreased and the fresh slurry become viscous; however, used lead-zinc mine tailings in the dry state shows no negative effect on workability.

3.2 Increase workability

In individual references, e.g., Huang et al. (2021) used iron mine tailings powder to replace a small amount of cement, Zhu et al. (2015a,b) used iron mine tailings sand to replace quartz sand and natural sand, respectively, in the preparation of UHPC and found that the workability increases. The reasons may be due to the following:

1. Cement has a finer average particle size (usually an order of magnitude finer) than MMT powder, with a larger specific surface area which will absorb more water. When using MMT powder to replace cement, there may be more free water in the fresh slurry, which has a positive effect on workability. However, the positive effects triggered by this situation are usually minimal and generally overridden by the degradation effects caused by poor particle morphology of MMT on workability. (ii) The proper micro-powder can act as a “micro-aggregate filler” effect (Adiguzel et al. 2022; Wang et al. 2021). Micro-powder in MMT generally refers to particles with a particle size of less than 75 μm. Micro-powder that does not participate in hydration reactions can fill the voids between cement and aggregates, releasing more free water and improving flowability. Attention should be attached to that micro-powder maybe contains impurity components, water reducer will adsorb on the micro-powder and impurity surface, diluting the water reducer effect. If the MMT micro-powder content is not well controlled, it is not beneficial to the workability of UHPC.

3.3 Relationship between workability and compressive strength

The relationship between workability variation and the compressive strength variation of MMT-UHPC was analyzed, as shown in Figure 5. The linear fit correlation coefficient R² is 0.72 based on the limited reference data available. Although the fitting result presents a certain dispersion, there is a positive correlation between workability variation and compressive strength variation.

Figure 5:

Relationship between workability variation and compressive strength variation of MMT-UHPC.

4.1 Compressive strength

Compressive strength is one of the most fundamental properties of UHPC. Generally, the overall performance can be judged based on compressive strength. Table 5 summarizes the studies by different researchers on compressive strength of MMT-UHPC, and some research results are plotted in Figure 6.

Figure 6:

Effect of incorporating MMT on the compressive strength of UHPC: (a) MMT-P-UHPC; (b) MMT-S-UHPC.

4.1.2 Effect of tailings sand

The effect of MMT sand on the compressive strength of UHPC is shown in Figure 6b. The compressive strength of most UHPC firstly increase and then decrease as the content of MMT sand increase. The maximum increase is 14.3 % (Lu et al. 2021), and the maximum decrease is 29.3 % (Wang et al. 2018).

4.1.2.1 Effect of dosage

MMT sand have strong water absorption, and a high content reduces the free water content required for the hydration process (Uchechukwu and Ezekiel 2014; Zhu et al. 2015a,b), resulting in inadequate hydration reaction and reduced compressive strength.

4.1.2.2 Effect of particle size

When the particle size of MMT sand is smaller than that of common fine aggregates (Ahmed et al. 2021a,b; Gu et al. 2022a,b; Mu 2021; Zhao et al. 2014; Zhao et al. 2021), the particles can form a close packing after mixing and increase the packing density (Wang et al. 2021). UHPC has maximum compactness and the compressive strength is improved when MMT sand replace the common fine aggregates by “Optimal replacement” in Table 5. When the particle size of MMT sand is similar to or larger than that of common fine aggregates, the filling effect is not ideal after incorporation, i.e., a tightly packed system cannot be formed and the UHPC compressive strength remains unchanged or decreases.

The larger the particle size of the MMT sand, the more detrimental it is to the strength of UHPC. As shown in Figure 7, Zhu et al. (2015a,b) investigated the UHPC in which all fine aggregates were iron mine tailings with different fineness and found that the compressive and flexural strength increased as the particle size of MMT decreased. Akçaoğlu et al. (2004) demonstrated that with an increase in the particle size of MMT sand, there was a greater difference in elastic modulus between MMT sand and the cement slurry matrix, leading to stress concentration occurred in the interface transition zone. Consequently, more micro-cracks occurred near the MMT sand, resulting in a decrease in strength.

Figure 7:

Influence of MMT particle size on UHPC strength (Zhu et al. 2015b).

4.1.2.3 Effect of morphology

The compressive strength is also affected by the surface texture and morphology of the aggregates (Donza et al. 2002). On the one hand, the rough surface of the MMT sand can increase the cohesive force between aggregate and slurry, reducing the strength loss to a certain extent. But on the other hand, the poor morphology of MMT sand particles with many internal defects can hinder the combination of adjacent hydration products, resulting in more pores. To improve this, the amount of inert micro-powder particles in MMT sand can be controlled to play the role of micro-aggregate filling, optimize the internal pore structure, and improve the compactness.

4.1.2.4 Effect of hardness

MMT sand has lower average stiffness and hardness than quartz sand (Ahmed et al. 2021a,b) or natural sand (Zhao et al. 2014), so the UHPC strength will reduce if the MMT sand cannot compensate for the loss of strength due to lower hardness. Some MMT sand has a high content of low crystalline quartz and soft minerals, and the soft minerals on the surface result in a weak bond between the matrix and the MMT particles (Alsalman et al. 2017). The MMT particle-matrix weak bond results in a lower UHPC modulus of elasticity (Perkins 1999), which is extremely detrimental to the UHPC strength (Zhao et al. 2014).

4.2 Flexural strength

Flexural strength is an index to measure flexural resistance. Table 6 summarizes the studies by different researchers on the flexural strength of MMT-UHPC, and some research results are plotted in Figure 8. Because of the complex microstructure of UHPC, the flexural strength is more sensitive to microscopic features (e.g., micro-cracks in the material) than the compressive strength (Cao et al. 2000; Toutanji and Bayasi 1999), so the variation degree in flexural strength shows significant dispersion in Figure 8a. The MMT sand has little effect on the UHPC flexural-compressive ratio in Figure 8b, which shows the good performance of MMT sand as an alternative to common aggregates.

Figure 8:

Effect of incorporating MMT sand on the flexural properties of UHPC: (a) flexural strength; (b) flexural–compression ratio.

In addition to being affected by the amount and size of tailings sand, the incorporation of steel fibers plays an important role in the flexural strength of MMT-UHPC. The reasons are as follows: (i) The average particle size of MMT sand is smaller than common aggregates, which can make steel fibers have better dispersion, and enhance the bond strength between matrix and steel fibers (Indhumathi et al. 2022). (ii) The irregularity of MMT particles makes the contact with steel fibers more complex, further exerting the reinforcing and toughening effect of the steel fibers (Vakili et al. 2022; Wang 2018) and improving the toughness of the matrix. (iii) The steel fibers act as a bridge in the matrix (Ma et al. 2018) and continue to carry the load after the matrix reached its peak load. Thus, the ultimate flexural strength of MMT-UHPC is dominated by fiber dispersion and fiber-matrix interactions.

4.3 The relationship between flexural strength and compressive strength

Due to the paucity of data on MMT-P-UHPC, this paper only analyzes the relationship between flexural strength and compressive strength of MMT-S-UHPC. The relevant results are shown in Figure 9.

Figure 9:

Relationship between flexural strength and compressive strength of MMT-S-UHPC.

The empirical formulas for the flexural and compressive strength of ordinary concrete are as follow:

f _t,f  = 0.81f_c^0.5 (1) (Standards Europe 1990)

f _t,f  = 0.54f_c^0.5 (2) (Standards America 2019a, Standards America 2019b)

where f_t,f is the flexural strength in MPa, and f_c is the compressive strength in MPa.

Based on the empirical formulas above, the data collected were fitted using the following equations:

f _t,f  = af_c^0.5 (3)

The relationship between the flexural and compressive strengths of metal tailings UHPC was obtained as:

f _t,f  = 2.33f_c^0.5 R² = 0.45 (4)

The correlation coefficient was small and the equation was adjusted to fit using as:

f _t,f  = af_cⁿ (5)

The relationship between the flexural strength and compressive strength of metal tailings UHPC obtained after optimization is approximate as:

f _t,f  = 0.02f^1.5 R² = 0.77 (6)

The optimized linear fit correlation coefficient R² is 0.77 based on the limited reference data available, which shows a positive correlation between the flexural and compressive strength of MMT-S-UHPC.

5.1 Impermeability

The deterioration of UHPC durability is mostly formed by the involvement of moisture; therefore, the impermeability can reflect durability and is an important indicator for evaluating durability. The water absorption and chloride ion permeability experiments can well reflect the impermeability of UHPC. Table 7 summarizes the studies by different researchers on the impermeability of MMT-UHPC, where the electric flux is an indicator of the chloride ion permeability.

In the case of a certain amount of water, the replacement of cement with MMT powder increases the water–cement ratio, making the cement easily dispersed and increasing the spacing between cement particles. The hydration products cannot effectively fill the interparticle spaces, resulting in an increasing in total porosity (Wang et al. 2021). Hence, the water absorption and chloride ion permeability increase, and the impermeability decrease.

The replacement of fine aggregates by MMT sand allows for a close packing with common aggregates and a more homogeneous and compact UHPC matrix, which has a positive effect on impermeability. In addition, incorporating MMT sand helps to convert large pores into small pores and reduce the total porosity (Protasio et al. 2021). Inert MMT particles, which are not involved in the hydration reaction, can also fill the pores in the UHPC matrix and optimize the pore structure (Yu et al. 2012). Therefore, as the content of metal mines tailings sand increases, the water absorption and electric flux decrease. In reference (Wang et al. 2021), with the increase of MMT sand content, the electric flux eventually increases. The reason is that the MMT contain conductive components, which will affect the magnitude of the current in the rapid chloride ion migration tests (Onuaguluchi and Eren 2012; Shettima et al. 2016).

The maximum water absorption or electrical flux of MMT-UHPC in Table 7 meets the requirements of high impermeability (water absorption <1.5 % (Standards America 2013); electrical flux <100C (Standards America 2019), which shows that MMT-UHPC have extreme high impermeability. For construction projects in coastal areas with a design life of 50 years, the water absorption of concrete should not exceed 2 % (Ahmed et al. 2021a,b; Xiong et al. 2022). Chan and Sun (2013) found that low water absorption had a positive effect on the durability of concrete. The water absorption of MMT-UHPC is less than 1.5 %, so MMT-UHPC can be used to prepare structures, pavements, or bridges that are more durable than the concrete made with common sand, especially for projects in coastal areas.

5.2 Corrosion resistance

Since MMT-UHPC has extremely high impermeability, low water absorption, and strong resistance to chlorine ion permeability, the corrosive ions in water do not easily enter the UHPC. However, certain properties of metal mine tailings may affect the corrosion resistance of UHPC. Metal oxides and other chemicals contained in metal mine tailings may react chemically with other components in cement or concrete, affecting corrosion resistance. This may lead to a decrease in the durability of cement or concrete, accelerating its deterioration and damage. Metal mine tailings may contain certain radioactive elements such as uranium, nickel, etc., and the release of which may have an effect on the corrosiveness of cement or concrete. The release of such radioactive elements may lead to changes in the structure of the cement or concrete, making it more susceptible to corrosion.

The current research on the corrosion resistance of MMT-UHPC still needs to be improved. For example, there are fewer experimental studies on sulfate corrosion, dry-wet cycle, frost resistance, etc. Therefore, it is difficult to systematically summarize data on the corrosion resistance of MMT-UHPC. When MMT are used in place of cement or fine aggregate in the preparation of UHPC, rigorous chemical analysis and engineering design must be performed to ensure that the addition of MMT will not adversely affect the corrosion resistance of UHPC.

5.3 Drying shrinkage

The MMT-UHPC dry shrinkage increase first and then tend to be stable along with time prolongs, and the drying shrinkage experimental results are shown in Figure 10. The dry shrinkage of UHPC with iron mine tailings powder replacing cement is shown in Figure 10a, and it can be seen that the appropriate amount of iron mine tailings powder plays a role of micro-filling, which can optimize the internal pore structure and improve the compactness, effectively reduce and offset the stress in the capillary pores. The dry shrinkage of UHPC with iron mine tailings sand replacing artificial sand is shown in Figure 10b, and it can be seen that iron mine tailings sand and artificial sand mixed in a reasonable proportion can reduce drying shrinkage.

Figure 10:

The dry shrinkage rate experimental results of UHPC incorporating MMT: (a) utilizing iron mine tailings powder as cement alternative (Ye et al. 2022); (b) utilizing iron mine tailings sand as artificial sand alternative (Zhao et al. 2021).

5.4 Autogenous shrinkage

The autogenous shrinkage experimental results are shown in Figure 11. Autogenous shrinkage develops into three stages alone with time prolonged: early rapid growth stage, mid-term rebound stage, and later gradual growth stage. These three stages reflect a series of physical and chemical interactions in the UHPC system.

Figure 11:

Effect of incorporating MMT on the autogenous shrinkage of UHPC: (a) utilizing lead-zinc mine tailings powder as cement alternative (Wang et al. 2018); (b) utilizing iron mine tailings sand as river sand alternative (Zhao et al. 2021).

Partial replacement of cement by MMT powder can decrease early autogenous shrinkage of UHPC and inhibit early cracking. Inasmuch the replacement of cement by MMT powder reduces the chemical shrinkage and drying shrinkage caused by the hydration process, the reduction to cement content makes the increase to water–cement ratio, and the evolution of autogenous shrinkage decreases with the free water content increases (Ghafari et al. 2016; Yalcinkaya and Yazici 2017). As shown in Figure 11a, it is evident that using MMT powder to partially replace cement in the preparation of UHPC leads to a significant reduction in early autogenous shrinkage.

The results of the UHPC autogenous shrinkage experiment for MMT sand replacing natural sand are shown in Figure 11b. Replacing natural sand with iron mine tailings sand did not influence the early autogenous shrinkage of UHPC, instead increased the later autogenous shrinkage. The reason is that the iron mine tailings sand absorb much free water, resulting in lower internal pressure of the UHPC matrix and shrinkage increase. Studies (Lv et al. 2019; Shen et al. 2018; Yang et al. 2019) have confirmed that aggregates water absorption affects the autogenous shrinkage of concrete, with the higher the aggregates water absorption, the greater the UHPC autogenous shrinkage. MMT sand usually have a higher water absorption than common fine aggregates and, therefore, MMT-S-UHPC has a greater autogenous shrinkage.

7.1 Hydration

The type of UHPC hydration products did not alter by the incorporation of MMT powder or sand. The XRD diffraction patterns of hydration products after 28 days of MMT-UHPC are shown in Figure 13. The diffraction peak intensity of AFt, C₃S, and C₂S gradually decreases as the substitution rate of MMT increases.

Figure 13:

The XRD diffraction patterns of hydration product at 28 days: (a) utilizing lead-zinc mine tailings powder as cement alternative (Wang 2018); (b) utilizing gold mine tailings sand as river sand alternative (Zhao et al. 2021).

The hydration heat tests of the UHPC utilizing lead-zinc mine tailings powder as cement alternative are shown in Figure 14 (Wang et al. 2018; Wang et al. 2021). The hydration exothermic system includes several processes such as the pre-induction period, induction period, acceleration period, deceleration period, and stabilization period. The figure shows that the hydration reaction exothermic mainly occurs within the first 3 days. When a certain amount of cement is replaced by MMT powder, the hydration products per unit mass decrease, resulting in a significant reduction in the peak value of hydration heat release. Reducing cumulative heat release helps to prevent cracks, so replacing a certain amount of cement with MMT powder has a beneficial effect on the structural stability of UHPC. The first exothermic peak in the diagram corresponds to the rapid involvement of C₃A in the hydration reaction and the formation of ettringite, and the second exothermic peak corresponds to the hydration of C₃S.

Figure 14:

Hydration heat test of the UHPC utilizing lead-zinc mine tailings powder as cement alternative.

The addition of MMT powder does not advance or delay the time to reach the maximum hydration exothermic heat. This is attributed to the nucleation effect of the micro-powders attached to the aggregate, which counteracts the cement dilution effect.

7.2 Interfacial transition zone (ITZ)

The SEM images of UHPC prepared with iron ore tailings sand entirely as fine aggregates are shown in Figure 15 (Zhao et al. 2014; Zhang et al. 2020). In Figure 15a, a large amount of AFt is observed within the interfacial transition zone (ITZ) between iron ore tailings sand and paste, and the AFt is easily fractured and cracked under load. In Figure 15b, oriented arrangement CH is observed within the ITZ, where the CH is prone to fracture and will initiate cracks under load. The reason for the CH formation is that the high specific surface area and porous morphology of the iron ore tailings particles can absorb more water, providing growth conditions for CH and forming a porous ITZ (Wang et al. 2019). MMT particles can be activated to stimulate volcanic ash activity after activation treatment. A certain amount of activated MMT powder can be added to MMT sand to make CH react with SiO₂ and consume CH to generate C–S–H gel and then weaken the damage of CH to the system.

Figure 15:

SEM images of UHPC utilizing iron mine tailings sand entirely as fine aggregates alternative: (a) from (Zhang et al. 2020); (b) from (Zhao et al. 2014).

7.3 Pore structure

The threshold pore diameter can affect the permeability and diffusivity of cementitious materials, which is considered a key indicator of the material durability (Tejas and Pasla 2023). According to the International Union of Pure and Applied Chemistry (IUPAC), the pore size of concrete can be classified into three classes: <2 nm, 2–50 nm, and >50 nm. It is generally accepted that pores larger than 100 nm are harmful (Zhang et al. 2020). The pore structure of MMT-UHPC mainly consists of macropores with pore sizes from tens to hundreds of micron and micropores with pore sizes under 100 nm.

The macropores mainly come from air bubbles entrained in the freshly mixed slurry, which is associated with flowability. The incorporation of MMT sand or powder will lead to the reduction of UHPC flowability and the increase of the slurry viscosity, and air bubbles formed during the mixing process are more likely to be sandwiched in it, forming larger pores and thus making the UHPC pores slightly coarser. In addition, MMT sand has a higher water absorption rate than common fine aggregates, and the surface of its particles more easily forms a water film that can inhale air, leading to an increase in the air content of the slurry and a higher porosity. As shown in Figure 16, the low magnification images reveal the presence of numerous pores within the UHPC incorporating iron mine tailings sand.

Figure 16:

Low magnification images of the UHPC in which all fine aggregates are iron mine tailings sand: (a) from (Zhang et al. 2020); (b) from (Zhao et al. 2014).

The creation of micropores can be attributed to the formation of capillary and gel pores during the hydration process of cementitious materials. When MMT powder replaces cement excessively, it is difficult to generate enough hydration products to fill the pores created during the hydration process due to the reduction of cement content. The incorporated MMT powder particles cannot effectively fill the capillary pores, increasing the micropores volume in the UHPC matrix.

Table 9 summarizes the research results on the porosity of MMT-UHPC. It can be seen that in most references, the higher the incorporation of MMT, the greater the porosity of UHPC.

The performance of MMT-UHPC is closely related to its pore structure (Das and Kondraivendhan 2012; Huang et al. 2021). Based on the literature data collected (Zhang et al. 2020; Zhao et al. 2014; Zhao et al. 2021), the relationship between porosity and compressive strength was plotted in Figure 17. The literature data was fitted with the linear fitting correlation coefficients R² of 0.86, 0.82, and 0.81, as shown in Figure 17a, indicating that porosity and compressive strength exhibited a good negative linear relationship. The relationship between the compressive strength variation and the porosity variation was analyzed with a linear fitting correlation coefficient R² of 0.62 based on the limited reference data available, as shown in Figure 17b. The fitted results, although somewhat scattered, still indicate a negative correlation between the porosity variation and the compressive strength variation of MMT-UHPC. Consequently, if further improvement of the MMT-UHPC compressive strength is needed, research can be done to reduce porosity.

Figure 17:

Relationship between porosity and compressive strength of MMT-UHPC.

8.1 Conclusions

MMT-UHPC is a relatively emerging research field with a research course of about 20 years so far. This review presents the relevant literature and provides a detailed summary of research findings. The following conclusions can be drawn:

8.2 Prospect

Based on the current state of research, the application of MMT in UHPC is in a developmental stage and a lot of research is needed to fill a series of blank areas. Some further research directions are suggested as follows:

1. There have been few quantitative analyses of the relationship among the mechanical properties of MMT-UHPC, such as the study of the functional relationship between elastic modulus and compressive strength, constitutive relation, etc. should be carried out.

2. The research on metal tailings UHPC is still at the material property test stage, and full-scale component tests have not been reported. The transition from laboratory study to practical application needs to be achieved.

3. The actual service environment of MMT-UHPC may be a mutual coupling of carbonation, chloride ion erosion, sulfate corrosion, freeze–thaw cycles, and other factors. Durability tests and related simulation analyses of coupled conditions under various environmental factors should be carried out to facilitate accurate service life prediction of MMT-UHPC structures.

4. Further grasp the formation mechanism and evolution process of MMT-UHPC microstructure, and study the correlation between microstructure and macroscopic performance.