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Article

Study on the Effects of Microwave Heating Time and Power on the Mechanical Properties of Cemented Tailings Backfill

by
Pengchu Ding
1,*,
Shiheng Yan
1,
Qinqiang Guo
2,
Liwu Chang
1,
Zhen Li
1,
Changtai Zhou
3,
Dong Han
4 and
Jie Yang
5
1
College of Intelligent Construction and Civil Engineering, Zhongyuan University of Technology, Zhengzhou 450007, China
2
Department of Henan Yukuang Resources Development Group Co., Ltd., Zhengzhou 450007, China
3
Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong 999077, China
4
Department of Northwest Geological Exploration Institute, China Metallurgical Geology Bureau, Xi’an 710119, China
5
Institutes of Science and Development, Chinese Academy of Sciences, Beijing 100190, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 944; https://doi.org/10.3390/min14090944
Submission received: 12 August 2024 / Revised: 11 September 2024 / Accepted: 14 September 2024 / Published: 15 September 2024
(This article belongs to the Topic New Advances in Mining Technology)
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Abstract

:
With the escalating demand for advanced and eco-friendly processing technologies in mining engineering, the potential applications of microwave heating technology in the treatment of cement tailings backfill (CTB) are expanding significantly. This research comprehensively investigates the mechanisms through which microwave irradiation duration and power influence the mechanical properties of CTB with varying concentrations and cement-to-sand ratios. The aim is to reveal the influencing patterns through experimental methods, providing scientific evidence for optimizing CTB treatment processes. This paper conducted microwave heating tests, uniaxial compression tests, and SEM-EDS tests on CTB. The research results indicate that heating time and power significantly enhance the early strength of CTB, with a more pronounced effect on CTB with higher concentrations and higher cement–sand ratios. When the heating time is 7 min and the heating power is 340 W, the cement hydration reaction is maximally promoted, thereby increasing the density and strength growth rate of CTB. However, excessively long heating time or overly high heating power may cause microcracks or thermal stress concentration within the CTB, adversely affecting the strength growth rate of CTB. Optimal thermal exposure duration and microwave power settings facilitate the activation of cementitious materials and the nucleation of calcium-silicate-hydrate (C-S-H) phases, thereby accelerating the compressive strength evolution of cemented tailings backfill (CTB). The outcomes of this research offer valuable insights into the deployment of microwave heating methodologies in underground mine backfilling, which are pivotal for augmenting the economic viability and environmental sustainability of mining operations.

1. Introduction

In the field of mining engineering, cemented tailings backfill (CTB) technology has become a primary technique for underground mining [1,2]. This technology utilizes tailings generated during the mining and beneficiation processes as the main filling material. By incorporating a specific ratio of binding agents and water, and achieving a homogeneous mixture to produce a cemented backfill composite, this material is utilized for the reclamation of voids in subterranean mining operations [3]. This technology effectively utilizes tailings resources, reduces the storage of tailings, and lowers the risks of environmental pollution and tailings dam failures. By filling the mined-out areas, it provides support, reduces the risk of surface subsidence and geological disasters such as earthquakes, ensures safe mining operations, and aims to improve resource utilization and promote the development of a circular economy [4]. The mechanical properties of CTB are of great significance for ensuring safe mining operations and maximizing resource utilization. The straightforwardness and applicability of uniaxial compressive strength (UCS) in mining operations have led numerous researchers to regard UCS as a reliable parameter for assessing the mechanical characteristics of cement-treated base (CTB) [5]. Fall et al. [6] studied the effects of various curing temperatures (2, 20, 35, and 50 °C) on the mechanical properties of CTB and pointed out that excessively high temperatures have an adverse effect on the strength development of CTB.
In recent years, with technological advancements, microwave irradiation technology has increasingly been integrated into the mining sector, demonstrating significant efficacy in processes such as ore comminution and dewatering [7,8,9,10]. As an innovative thermal treatment technique, microwave irradiation functions by exploiting the interaction between microwave electromagnetic fields and the molecular structure of materials, thereby enhancing molecular agitation and generating thermal energy [11,12,13]. Compared to conventional thermal methods, microwave irradiation offers benefits such as accelerated processing times and superior energy efficiency [14,15,16].
Despite the relatively recent initiation of research into the application of microwave heating technology in mining engineering, it has experienced significant advancement in recent years. This technology boasts a unique thermal mechanism and exceptional energy conversion efficiency [17], as well as superior penetration and controllability, which confer considerable benefits in rock treatment processes. Numerous researchers have investigated the impact of microwave heating on the comminution of minerals, revealing that microwave pre-treatment can markedly enhance the efficiency of mineral crushing [18,19,20,21,22]. Furthermore, other studies have examined the utilization of microwaves in the extraction of valuable metals from ores, demonstrating that microwave heating can optimize the leaching process and increase metal recovery rates [23,24]. Wu et al. [25] elucidated the mechanisms through which microwave heating augments the compressive strength of concrete. Haque et al. [26] explored the variations in the early and late strength of mortar and concrete specimens subjected to microwave heating, indicating that microwave heating can expedite the curing process. Additionally, Koh et al. [27] conducted a comparative analysis of the costs associated with microwave curing versus traditional steam curing of concrete and evaluated the feasibility of microwave curing.
Despite the extensive interest in microwave heating technology within the domains of rock and concrete, investigations into its impact on the mechanical properties of CTB remain relatively scant. As a critical structural element in mining operations, the mechanical integrity of CTB is intrinsically linked to the mine’s safety and stability [28]. The application of microwave heating technology in the thermal treatment of backfill materials exhibits notable advantages and promising potential, notably in enhancing the dewatering and curing processes of CTB, thereby improving its mechanical characteristics [3]. Consequently, comprehensive research on the implications of microwave heating on CTB’s mechanical properties is imperative, not only to advance the theoretical framework of microwave heating applications in mining but also to offer practical engineering guidance.
The parameters of microwave heating duration and power output are pivotal in determining the efficacy of the treatment [29,30,31,32], as variations in these parameters can significantly affect the mechanical properties of CTB. Additionally, the concentration and cement–sand ratio are crucial factors influencing CTB’s mechanical behavior [33]. This study endeavors to examine the evolution of CTB’s mechanical properties across a spectrum of concentrations and cement–sand ratios under various microwave heating durations and power settings through experimental methodologies. The objective is to ascertain the optimal microwave treatment parameters that will enhance the structural stability and load-bearing capacity of CTB.
The implementation of microwave heating technology is anticipated to augment the mechanical properties of CTB, thereby bolstering the safety and economic efficiency of mining activities. By fine-tuning the microwave heating duration and power, it is feasible to exert precise control over the internal structure and properties of CTB, addressing diverse engineering demands. This has profound implications for mine environmental sustainability and resource recovery, with the research outcomes poised to furnish invaluable insights for the application of microwave heating technology in the mining sector.

2. Experimental Program and Procedure

2.1. Materials

The materials used in the preparation of CTB specimens include tailings, binders and water.

2.1.1. Tailings

The tailings analyzed in this research were sourced from the MiaoLing Gold Mine located in Luoyang City, China. The chemical characteristics of these tailings are detailed in Table 1. The analytical results were obtained from the Analysis and Testing Center of Zhengzhou University. Particle size distribution was assessed utilizing a Bettersize 2600 laser particle size analyzer (Dandong Baite Instrument Co., Ltd., Dandong, China). Key metrics, including the coefficient of uniformity (Cu) and the coefficient of curvature (Cc), are presented in Table 2. The results of the grain size analysis for the tailings are illustrated in Figure 1.

2.1.2. Binder

Ordinary Portland cement of 42.5 grade served as the binding agent. The chemical composition of the cement is detailed in Table 3.

2.1.3. Mixing Water

Tap water was used as the mixing water.

2.2. Specimen Preparation and Mix Proportion

Based on the actual production conditions of the Miaoling Gold Mine, the experimental parameters were designed as shown in Table 4. It should be noted that the control group has a heating power or heating time of 0. The constituents of the CTB were blended in accordance with the specified mix ratio utilizing a mortar mixer (JJ-5) to achieve a uniform composite. Subsequently, the prepared CTB mixture was cast into plastic molds (diameter = 50 mm, height = 100 mm) to fabricate CTB specimens. The plastic mold is made of acrylic, which has a melting point of 130–140 °C. During the experiment, the maximum surface temperature of the sample was 76.8 °C, which did not cause the plastic mold to melt and release toxic gases. The CTB was divided into two parts, one of which was not subjected to microwave heating, while the other part, with the same mix proportion, underwent microwave heating after the slurry was added to the mold. In the evaluation of the experimental outcomes, the UCS growth rate is defined as the comparative strength of the microwave-treated specimens versus the control specimens that were not exposed to microwave heating at the same age. The microwave heating apparatus utilized in this experiment is depicted in Figure 2. This device features adjustable output power, with a maximum capacity of 1000 W. During the microwave heating process, the apparatus is positioned above the mold containing the specimen for a predetermined duration, after which the surface temperature of the specimens is measured. Subsequently, the specimens were cured in a controlled temperature and humidity environment (20 ± 1 °C, with 95% relative humidity) for periods of 3, 7, and 28 days. The procedural flow of the experiment is illustrated in Figure 3. It is important to note that, while investigating the correlation between concentration and microwave exposure time, the cement-to-sand ratio was maintained at 1:8, with the power set at 340 W. In assessing the relationship between the cement-to-sand ratio and microwave exposure duration, the concentration was fixed at 72%, and the power was also set at 340 W. For the analysis of the relationship between concentration and microwave power, the cement-to-sand ratio was established at 1:8, while the heating duration was designated at 7 min. Lastly, when exploring the connection between the cement-to-sand ratio and microwave power, the concentration was held constant at 72%, with the heating duration also set at 7 min.

2.3. Experimental Test Program

In this section, surface temperature tests, uniaxial compressive strength (UCS) tests and Scanning Electron Microscopy–Energy-Dispersive Spectroscopy (SEM-EDS) tests were conducted on CTB, revealing the intrinsic relationship between them.

2.3.1. Surface Temperature Tests

The thermal characteristics of the specimens were evaluated utilizing the ST8550 thermal imaging device, which operates within a temperature measurement spectrum of −25 to 550 °C, thereby satisfying the experimental requirements.

2.3.2. UCS Tests

In compliance with ASTMC39 [34], the UCS tests were conducted utilizing a SAAS electronic universal testing apparatus, which has a maximum load capacity of 300 kN. The testing procedure employed a displacement loading technique, with a predetermined loading rate of 0.5 mm/min. The experimental design comprised 16 distinct groups, each containing 20 samples, resulting in a total of 48 duplicate specimens. For each category of CTB, three identical samples were subjected to testing, and the average UCS value was computed to derive the final compressive strength [35,36], amounting to an overall total of 576 samples evaluated.

2.3.3. SEM-EDS Tests

Following the completion of the UCS assessments on the CTB, a cubic specimen measuring 0.5 cm on each side was extracted from the center of each CTB sample for examination via Scanning Electron Microscopy (SEM). A rubber pipette was utilized to eliminate particulate matter from the surface of the specimen, ensuring a pristine cross-section for analysis [37]. Subsequently, the sample was subjected to a thin metallic conductive coating in a vacuum chamber to augment surface visibility [38,39]. The microstructural characteristics of the CTB were analyzed using a field emission electron microscope (Quanta FEG 250, FEI, Hillsboro, OR, USA). Furthermore, an EDS integrated with SEM was employed to ascertain the mineralogical composition of the CTB.

3. Results and Discussion

3.1. The Effect of Microwave Heating Time on the Growth Rate of CTB Strength

3.1.1. The Effect of Microwave Heating Time on the UCS Growth Rate of CTB with Different Concentrations

Figure 4 shows the strength growth rate of CTB with concentrations of 68%, 70%, 72%, and 74% at microwave heating times of 4, 7, 10, and 13 min, relative to CTB that was not subjected to microwave heating, at 3, 7, and 28 days.
As illustrated in Figure 4a, at a curing age of 3 days, the rate of strength development for various concentrations of CTB initially ascends and subsequently descends with an increase in microwave heating duration. For instance, analyzing the strength development rate of the 72% concentration CTB post microwave treatment, at a heating duration of 4 min (with a maximum surface temperature of the samples (MSTS) reaching 51.9 °C), microwave heating introduces a specific amount of thermal energy that enhances the hydration reactions of the cementitious materials, yielding a strength development rate of 45.3%. When the heating duration extends to 7 min (MSTS at 58.8 °C), the hydration process of the cementitious materials is further accelerated, resulting in a strength development rate of 69.5%. However, at 10 min of heating (MSTS at 62.6 °C), the beneficial impact of microwave heating on the hydration reaction of CTB diminishes, leading to a reduced strength development rate of 58.8%. Finally, at a heating duration of 13 min (MSTS at 73.8 °C), the extended heating causes substantial moisture loss in the CTB, resulting in a further decline in the strength development rate to 54.6%. Microwave heating enhances the hydration process of cement by amplifying the kinetic activity of water molecules, which generates heat and accelerates the reaction kinetics of cement particles. Brief heating periods can effectively promote the hydration reaction of cement, leading to a rapid increase in compressive strength. Conversely, extended heating durations may elevate the internal temperature of the CTB excessively, causing significant moisture evaporation from the specimen within a short timeframe. This not only disrupts the normal hydration progression but also induces the formation of numerous voids within the specimen, ultimately diminishing the UCS growth rate. As illustrated in Figure 4a, it is evident that an increase in concentration correlates with an enhanced strength growth rate of the CTB. This phenomenon occurs because cement serves as the binding agent in the mixture; when the CTB concentration is elevated, the cement content increases, resulting in a dense packing of cement particles and robust bonding forces, which, in turn, promotes a higher rate of strength development. At a CTB concentration of 74%, the strength growth rate peaks at 83.2% following 7 min of microwave heating (MSTS at 63.2 °C). In contrast, at lower CTB concentrations, the reduced cement content leads to larger interstitial spaces between cement particles and inadequate bonding forces, resulting in a diminished strength growth rate. For a CTB concentration of 68%, the maximum strength growth rate achieved is 54.4% with 7 min of microwave heating (MSTS at 53.6 °C).
Analysis of Figure 4a–c reveals that, exemplified by a 72% concentration CTB, microwave heating duration positively influences the compressive strength of CTB at various curing stages. Notably, this enhancement is most pronounced at a 3-day curing period, achieving a peak strength augmentation of 83.2%. This phenomenon is attributable to the early-stage microwave-induced rapid internal temperature elevation within the CTB, which accelerates the cement hydration process, thereby generating a greater quantity of hydration products, such as C-S-H gel. This acceleration fosters faster cement paste hardening, yields a denser microstructure, and diminishes CTB porosity, culminating in a substantial early strength improvement. For CTB with a 7-day curing period, strength continues to rise with prolonged heating, albeit at a reduced rate compared to the 3-day period. This reduction is due to the partial completion of cement hydration over time, which attenuates the efficacy of microwave heating. In the case of CTB with a 28-day curing period, the strength increment is less significant. This is because the protracted hydration reaction is nearly complete, and the cement has been largely consumed, rendering the microwave heating’s facilitative effect on cement hydration negligible. It is also noteworthy that for a 68% concentration CTB subjected to 13 min of microwave heating, the strength increment is merely 5.4%, comparable to that of CTB without microwave treatment.

3.1.2. The Effect of Microwave Heating Time on the UCS Growth Rate of CTB with Different Cement–Sand Ratios

Figure 5 shows the strength growth rate of CTB with cement–sand ratios of 1:4, 1:8, 1:12, and 1:20 at microwave heating times of 4, 7, 10, and 13 min, relative to CTB that was not subjected to microwave heating, at 3, 7, and 28 days.
From Figure 5a, it is evident that the duration of microwave heating significantly enhances the compressive strength of cemented tailings backfill (CTB) with varying cement-to-sand ratios. Initially, as the microwave heating time increases, the strength augmentation rate of CTB rises, followed by a decline. For instance, consider the CTB with a cement-to-sand ratio of 1:4: upon 4 min of microwave heating (MSTS at 52.6 °C), the strength growth rate achieved 65.5%; at 7 min of heating (MSTS at 59.4 °C), the rate peaked at 83.9%; extending the heating to 10 min (MSTS at 65.3 °C) reduced the rate to 72.5%; and at 13 min (MSTS at 75.6 °C), the rate further decreased to 67.4%. This trend mirrors that of CTB with varying cement concentrations, indicating that 7 min is the optimal microwave heating duration. This period maximizes the hydration reaction, thereby enhancing CTB strength without inducing significant overheating.
Furthermore, Figure 5a reveals that microwave heating more effectively boosts the strength of CTB with a higher cement-to-sand ratio. The strength increases more rapidly within shorter heating periods (4–7 min) and diminishes more gradually with prolonged heating (10–13 min). Conversely, for CTB with a lower cement-to-sand ratio, strength increments are slower during shorter heating durations and decrease more rapidly with extended heating times. This discrepancy is attributed to the higher cement content in the CTB with a higher cement-to-sand ratio, which, during microwave heating, produces more hydration products. These products form a dense network enveloping the tailings particles, enhancing the compactness of the CTB. The abundant hydration products also fill the pores within the CTB, thereby reducing porosity and increasing compressive strength. For CTB with a lower cement-to-sand ratio, inadequate hydration products are formed during microwave heating, leading to potential internal cracking from heat accumulation over extended periods, negatively impacting strength enhancement.
Figure 5a–c indicates that the strength growth rate of CTB with different cement-to-sand ratios progressively declines with curing age, yet remains positive, demonstrating that microwave heating enhances the strength of CTB across various curing durations. However, the extent of this enhancement varies. Taking CTB with a cement-to-sand ratio of 1:4 and 7 min of microwave heating as an example, the strength growth rate is 83.9% at 3 days, 55.3% at 7 days, and 16.2% at 28 days. This can be attributed to the rapid internal temperature rise in CTB due to microwave heating at 3 days of curing, which accelerates the hydration reaction of cement particles. Specific mineral components in the cement (such as C3S, C2S, etc.) react more swiftly to form hydration products, significantly boosting CTB strength. By 7 days of curing, the strength of microwave-heated CTB remains higher than that of non-microwave-heated CTB, though the growth rate declines compared to 3 days. At 28 days of curing, the strength difference between microwave-heated and non-microwave-heated CTB narrows further. At this stage, the hydration reaction is essentially complete, and CTB strength stabilizes, diminishing the relative advantage of microwave heating. Overall, the primary impact of microwave heating time is on the early hydration reaction of CTB. In the long term, the ultimate strength of CTB is predominantly determined by the extent of cement hydration and the volume of hydration products formed.

3.2. The Effect of Microwave Heating Power on the UCS Growth Rate of CTB

3.2.1. The Effect of Microwave Heating Power on the UCS Growth Rate of CTB at Different Concentrations

Figure 6 shows the strength growth rate of CTB with concentrations of 68%, 70%, 72%, and 74% at microwave heating powers of 60, 200, 340, and 480 W, relative to CTB that was not subjected to microwave heating, at 3, 7, and 28 days.
From Figure 6a, it can be seen that, taking the CTB with a concentration of 72% and a curing age of 3 days as an example for analysis, when the microwave power is 60 W (MSTS at 55.8 °C), the strength growth rate is 45.4%; when the microwave power is 200 W (MSTS at 59.5 °C), the strength growth rate is 59.8%; when the microwave power is 340 W (MSTS at 64.4 °C), the strength growth rate further increases to 69.5%; and when the microwave power reaches 480 W (MSTS at 74.2 °C), the strength growth rate starts to decrease to 54.3%, which is lower than the enhancement effects of 200 and 340 W. This is because low-power microwave heating can promote the hydration reaction of cement. As the microwave power increases, the hydration reaction of the cement increases significantly, and microwave heating causes thermal expansion in the CTB, thereby altering its microstructure, reducing its porosity, and increasing its strength. However, high-power microwaves cause the slurry to overheat, leading to rapid evaporation of moisture, which affects cement hydration. From Figure 6a, it can also be seen that under the same microwave heating power, the strength growth rate of high-concentration CTB is greater than that of low-concentration CTB. This is because an increase in concentration means more cement is involved in the reaction, generating more hydration products, and thus the strength increases more significantly.
From Figure 6a–c, it can be seen that microwave heating power has a certain enhancement effect on the strength of CTB at different ages, but the effect is more pronounced on the strength of CTB at early curing ages. Taking the CTB with a concentration of 72% as an example for analysis, at a curing age of 3 days, the strength growth rate first increases and then decreases, reaching the highest value of 69.5% at a heating power of 340 W. At a curing age of 7 days, the strength growth rate still shows a trend of first increasing and then decreasing, reaching the highest value of 44.6% at a heating power of 340 W. At a curing age of 28 days, although the strength growth rate shows a trend of first increasing and then decreasing, it reaches the peak strength growth rate of 17.7% at a microwave heating power of 200 W, and at a microwave power of 480 W, the strength growth rate is only 8.6%, which is lower than the strength growth rate of 14.5% at a microwave power of 60 W. This is because the increase in microwave heating power effectively accelerates the hydration reaction, increasing the density of the CTB. However, excessively high heating power causes a large amount of heat to accumulate inside the CTB in a short period, leading to the evaporation of some moisture and affecting the continued hydration reaction, thus reducing the strength growth rate of the CTB. At a curing age of 28 days, the effect of microwave heating is not obvious because the cement hydration reaction is basically complete. Therefore, moderate microwave heating power can significantly improve the strength of CTB during the early curing stage, while its impact on the strength growth of CTB during the later curing stage is minimal.

3.2.2. The Effect of Microwave Heating Power on the UCS Growth Rate of CTB with Different Cement–Sand Ratios

Figure 7 shows the strength growth rate of CTB with cement–sand ratios of 1:4, 1:8, 1:12, and 1:20 at microwave heating powers of 60, 200, 340, and 480 W, relative to CTB that was not subjected to microwave heating, at 3, 7, and 28 days.
Figure 7a illustrates that, taking a CTB with a cement–sand ratio of 1:4 and a 3-day curing period as an example, when subjected to microwave heating at 60 W (MSTS at 56.3 °C), the strength increment rate stands at 67.4%. At 200 W (MSTS at 59.7 °C), this rate increases to 72.3%. With an elevation to 340 W (MSTS at 66.7 °C), a peak strength growth rate of 83.9% is attained; however, at 480 W (MSTS at 75.8 °C), the rate diminishes to 68.4%. Hence, the strength increment rate initially rises and subsequently declines with increasing microwave power, peaking at 340 W. This phenomenon is attributable to the fact that elevated microwave power yields more heat within the CTB over the same heating duration. Optimal high temperatures enhance the hydration reaction of the cement, while excessively high temperatures expedite moisture loss from the CTB, adversely affecting its strength augmentation. Additionally, Figure 7a reveals that a higher cement–sand ratio amplifies the impact of microwave power on strength enhancement. This is because a CTB with a higher cement–sand ratio contains more cement, and increased microwave heating power significantly accelerates the cement hydration reaction, producing more hydration products and thereby bolstering the CTB’s strength.
From Figure 7a–c, it can be seen that microwave heating power has a certain enhancement effect on the strength of CTB at different ages, but the effect is more pronounced on the strength of CTB during the early curing stages. Taking the CTB with a cement–sand ratio of 1:4 as an example for analysis, at a curing age of 3 days, the strength growth rate first increases and then decreases, reaching the highest value of 83.9% at a heating power of 340 W. At a curing age of 7 days, the strength growth rate still shows a trend of first increasing and then decreasing, reaching the highest value of 55.3% at a heating power of 340 W. At a curing age of 28 days, although the strength growth rate shows a trend of first increasing and then decreasing, it reaches a peak strength growth rate of 22.4% at a microwave heating power of 200 W, and at a microwave power of 480 W, the strength growth rate is 9.4%, which is lower than the growth rate of 18.1% at a microwave power of 60 W. This is because early-stage CTB contains more moisture, and the internal heat accumulation of non-microwave-heated CTB is slower; so, microwave heating has a greater impact on the early strength growth rate. The strength growth trend of CTB cured for 7 days is similar to that of CTB cured for 3 days but to a lesser extent. The strength growth rate of CTB cured for 28 days further weakens because as the curing period increases, the non-heated CTB also continues to undergo hydration reactions, thereby further increasing the strength of the non-heated CTB. Overall, microwave heating power has a certain strength enhancement effect on CTB at different curing ages, but it has a more pronounced effect on the strength of early-stage CTB and does not significantly affect the final strength in the later stages.

3.3. SEM-EDS Results

Figure 8 presents the SEM and EDS images of CTB. Given that microwave heating has minimal impact on long-term strength, this section focuses solely on CTB samples cured for 3 and 7 days. SEM images of the CTB without microwave treatment are depicted in Figure 8a,e for the 3- and 7-day curing periods, respectively. Figure 8c,g illustrates the SEM images of the CTB subjected to microwave heating at a power of 340 W for 7 min, corresponding to the 3- and 7-day curing durations. The EDS images corresponding to these samples are shown in Figure 8b,d,f,h.
SEM analysis reveals that the CTB, absent of microwave exposure, exhibits numerous pores which diminish over extended curing durations. Microwave-treated CTB displays a significant reduction in pore presence, concomitant with an increase in ettringite content. Prolonged curing under microwave conditions leads to the formation of extensive interwoven ettringite regions, critical to CTB mechanical strength. These findings indicate that microwave irradiation expedites the hydration process, facilitating rapid ettringite crystal growth, thereby effectively occluding pores and enhancing the CTB’s structural coherence.
EDS analysis shows that microwave-treated CTB shares similar peak patterns with non-microwave-treated CTB; however, the peak intensities of individual ions vary. For instance, after 3 days of microwave curing, Ca2+ peak intensity is elevated, attributed to the formation of Ca(OH)2 during the initial hydration phase of Portland cement. Conversely, after 7 days of microwave treatment, Ca2+ peak intensity is marginally lower compared to non-microwave-treated CTB due to accelerated cement activation and enhanced Al2O3 dissolution. The resultant Al2O3 can interact with Ca2+ to form C-S-H crystals, reducing Ca2+ peak intensity. This observation implies that microwave treatment not only hastens the initial hydration but also alters the long-term microstructural evolution of CTB. The formation of C-S-H crystals is pivotal for mechanical strength development in cementitious materials. Therefore, the interaction between Ca2+ and Al2O3 under microwave conditions may result in a more rapid and uniform distribution of these strength-enhancing phases. These phenomena collectively contribute to the increased strength of CTB, corroborating the analyses presented in Section 3.1 and Section 3.2.

4. Conclusions

(1) Microwave irradiation markedly improves the early compressive strength of CTB. Over an extended period, the ultimate strength of CTB is predominantly influenced by the extent of cement hydration and the volume of hydration products generated. The efficacy of microwave treatment in enhancing strength is particularly notable in CTB with elevated cement concentrations and a high cement-to-sand ratio.
(2) With a heating time of 7 min, microwave heating can effectively promote the evaporation of moisture between tailing particles and the hydration reaction of the binder, thereby enhancing the density and strength of CTB. Excessive heating time can cause thermal stress within the CTB, expand the internal structure, and induce microcracks, adversely affecting the strength growth of CTB.
(3) When the heating power is lower than 340 W, the mechanical properties of CTB gradually improve with increasing power. Moderate energy input can accelerate the cement hydration process and improve CTB stability. However, when the heating power is too high, the sharply increased temperature may cause thermal damage within the CTB, reducing its strength growth rate.
(4) Reasonable heating time and power are conducive to cement activation and the formation of C-S-H crystals, thereby enhancing the strength growth rate of CTB. Excessive heating time or power inhibits the synthesis of C-S-H crystals, accelerates moisture evaporation, alters the structure and properties of CTB, and reduces its strength growth rate.
(5) The findings of this investigation can be utilized in backfill mining operations. The implications of these conclusions offer valuable insights for the implementation of microwave heating technology in the reclamation of excavated areas within mines, thereby enhancing both the economic viability and environmental sustainability of mining practices.

Author Contributions

Conceptualization, C.Z., D.H. and J.Y.; Methodology, S.Y., L.C. and Z.L.; Validation, S.Y. and Z.L.; Investigation, S.Y. and Z.L.; Writing—Original Draft, P.D. and Q.G.; Writing—Review and Editing, P.D. and Q.G.; Supervision, P.D., S.Y., L.C. and Z.L.; Formal Analysis, S.Y. and Z.L.; Funding Acquisition, P.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (No. 52374110); the Key scientific and technological projects of Henan province (No. 242102320337); and the Basic Research Fund of Zhongyuan University of Technology (No. K2022QN008).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Qinqiang Guo is employee of Henan Yukuang Resources Development Group Co., Ltd. The paper reflects the view of the scientist and not the company.

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Minerals 14 00944 g001
Figure 1. Grain size distribution of the tailings.
Figure 1. Grain size distribution of the tailings.
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Figure 2. Microwave heating device.
Figure 2. Microwave heating device.
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Figure 3. Experimental process diagram.
Figure 3. Experimental process diagram.
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Figure 4. Variation in the UCS growth rate of CTB with different concentrations with changes in microwave heating time. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
Figure 4. Variation in the UCS growth rate of CTB with different concentrations with changes in microwave heating time. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
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Figure 5. The growth rate of UCS of CTB with different cement–sand ratios varies with changes in microwave heating time. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
Figure 5. The growth rate of UCS of CTB with different cement–sand ratios varies with changes in microwave heating time. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
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Figure 6. The growth rate of UCS of CTB at different concentrations varies with changes in microwave heating power. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
Figure 6. The growth rate of UCS of CTB at different concentrations varies with changes in microwave heating power. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
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Figure 7. The growth rate of UCS of CTB with different cement–sand ratios varies with changes in microwave heating power. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
Figure 7. The growth rate of UCS of CTB with different cement–sand ratios varies with changes in microwave heating power. (a) curing 3 d; (b) curing 7 d; (c) curing 28 d.
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Figure 8. The SEM and EDS images of CTB at 3 and 7 days. (a) SEM image of the curing 3d CTB without microwave heating; (b) EDS image of the curing 3 d CTB without microwave heating; (c) SEM image of the curing 7 d CTB without microwave heating; (d) EDS image of the curing 7 d CTB without microwave heating 3 d; (e) SEM image of the curing 3d CTB subjected to microwave heating; (f) EDS image of the curing 3 d CTB subjected to microwave heating; (g) SEM image of the curing 7 d CTB subjected to microwave heating; (h) EDS image of the curing 7 d CTB subjected to microwave heating.
Figure 8. The SEM and EDS images of CTB at 3 and 7 days. (a) SEM image of the curing 3d CTB without microwave heating; (b) EDS image of the curing 3 d CTB without microwave heating; (c) SEM image of the curing 7 d CTB without microwave heating; (d) EDS image of the curing 7 d CTB without microwave heating 3 d; (e) SEM image of the curing 3d CTB subjected to microwave heating; (f) EDS image of the curing 3 d CTB subjected to microwave heating; (g) SEM image of the curing 7 d CTB subjected to microwave heating; (h) EDS image of the curing 7 d CTB subjected to microwave heating.
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Table 1. Chemical composition of the tailings.
Table 1. Chemical composition of the tailings.
Chemical CompositionsSiO2Al2O3CaOMgOTFeSPbZnLoss
wt.%64.5812.262.462.434.361.720.120.164.83
Table 2. Grain size distribution analysis and gradation characteristics of tailings.
Table 2. Grain size distribution analysis and gradation characteristics of tailings.
Element Unitd10/μmd30/μmd50/μmd60/μmd90/μmCuCc
Tailings44.6193.04143.1173.1302.23.881.12
Table 3. Chemical composition of the cement (adopted from Ref. [33]).
Table 3. Chemical composition of the cement (adopted from Ref. [33]).
OxidesCaOSiO2Al2O3Fe2O3MgOSO3K2ONa2OTiO2Loss
Mass (%)64.1319.194.503.331.821.061.040.410.242.28
Table 4. Variables for mix proportions.
Table 4. Variables for mix proportions.
Solid Content (wt%)Binder-to-Tailings RatioMicrowave Heating Time/minMicrowave Heating Power/W
681:4460
701:87200
721:1210340
741:2013480
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Ding, P.; Yan, S.; Guo, Q.; Chang, L.; Li, Z.; Zhou, C.; Han, D.; Yang, J. Study on the Effects of Microwave Heating Time and Power on the Mechanical Properties of Cemented Tailings Backfill. Minerals 2024, 14, 944. https://doi.org/10.3390/min14090944

AMA Style

Ding P, Yan S, Guo Q, Chang L, Li Z, Zhou C, Han D, Yang J. Study on the Effects of Microwave Heating Time and Power on the Mechanical Properties of Cemented Tailings Backfill. Minerals. 2024; 14(9):944. https://doi.org/10.3390/min14090944

Chicago/Turabian Style

Ding, Pengchu, Shiheng Yan, Qinqiang Guo, Liwu Chang, Zhen Li, Changtai Zhou, Dong Han, and Jie Yang. 2024. "Study on the Effects of Microwave Heating Time and Power on the Mechanical Properties of Cemented Tailings Backfill" Minerals 14, no. 9: 944. https://doi.org/10.3390/min14090944

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