1. Introduction
The mining industry is a dominant industry for national economic development in many countries worldwide and provides an important material basis for the survival and development of human society [
1]. However, a large amount of mine solid wastes such as waste rock and tailings are also generated in the production process [
2], among which tailings account for approximately 80% of the total amount of waste. According to the relevant statistics [
3], the total amount of stockpiles of tailings in China has exceeded 20 billion tons, and the annual emissions have reached more than 1.6 billion tons in recent years. With the continuous improvement of the technical level of mineral processing, the optional grade of ore is decreasing, further increasing the emissions of tailings [
4,
5]. If the tailings cannot be disposed of and managed reasonably, it will cause land occupation, environmental pollution, and geological hazards such as landslides and mudslides, which will have a lasting and harmful impact on the social economy and environment [
6]. At the same time, improper disposal of tailings also restricts the green, safe, efficient and sustainable development of modern mines [
7].
At present, except for approximately 11% of tailings used for underground mine backfilling [
8,
9], most tailings are stored in the form of a tailings slurry in traditional tailings ponds [
10,
11]. Because the concentration of tailings discharged into the tailings pond is low (20–40 wt%), the water content is high, the height of the tailing dam is large, and the safety factor of the tailings pond is low, which produces a potential high potential energy hazard. It is difficult to maintain the physical integrity and stability of the tailings dam for a long time, and the maintenance cost is high, which is a further challenge [
7,
12,
13]. Especially in heavy rains, floods, typhoon seasons, and earthquake-prone areas, accidents are easily caused such as dam breaks, landslides and seepage in tailings the ponds [
14,
15], causing great threats and damage to people’s lives and property. In September 2008, the dam of an iron ore tailings dam in Shanxi Province collapsed, resulting in 277 deaths, four people missing, and 33 injured, while direct economic losses reached 96.192 million yuan [
16]; in November 2015, the Samarco iron ore tailings dam accident in Brazil killed 19 people, and caused the leakage of six million cubic meters of residual waste, polluting 650 km of rivers and remitting tailings into the Atlantic Ocean, causing the most serious environmental disaster in Brazil’s history [
15,
17]; in 2019, the tailings dam of the Córrego do Feijão mine in Brazil leaked and collapsed, 235 people were killed and 35 people were missing, and the accident caused serious pollution to the surrounding ecology.
At present, with increasingly stringent environmental regulations [
18], government departments have attached great importance to safety in the production industry. The modern mining industry has been forced to explore safe and reliable tailings surface treatment methods to reduce or eliminate the drawbacks of traditional tailings storage [
19,
20], to more effectively meet environmental and regulatory requirements [
7]. Tailings consolidation surface disposal is an emerging tailings surface treatment technology with great development potential [
21]. The technology dehydrates the tailings slurry discharged from the mineral processing plant by using concentration or filtration equipment, and then uniformly mixes the dewatered tailings, hydraulic cementing materials (such as ordinary Portland cement, fly ash, blast furnace slag or their mixtures) [
22,
23] and water in a certain proportion. Transported by a belt conveyor or truck [
7], the mixtures are stacked at a designated location. Artificial hardening composite materials with certain engineering characteristics are formed due to condensation and hardening during the hydration of the cementation agents. The tailings consolidation disposal technology is beneficial for the recycling of ore dressing water [
24], which can reduce the water content in the tailings, improve the safety factor of the consolidated pile, reduce or eliminate the large construction cost of the tailings dam, and facilitate the timely storage of the tailings and gradual rehabilitation of the site. This technology has advantageous safety, environmental protection, and economic benefits.
Due to the short amount of time that this technology has been proposed and applied, previous studies have mostly focused on dehydration, cementing materials and economic costs. However, the lack of systematic research on the engineering characteristics of the post-stacked consolidated bodies has led to a lack of a comprehensive understanding of the technology. To some extent, it limits the application and promotion of this technology. According to the concept and advantages of this technology, it can be seen that the tailings will be consolidated into an artificial pile when they are placed in the designated position. First, the formed pile should have a certain stability; that is, the consolidated body must have sufficient mechanical strength to ensure structural safety. Second, whether the consolidated pile can exert its engineering characteristics for a long time depends on its durability. Durability mainly includes frost resistance and erosion resistance. Although their mechanisms and manifestations are different, they are all determined by the impermeability of the consolidated body. Finally, the consolidated body is required to be environmentally friendly. Tailings containing sulfide minerals in contact with water and oxygen can produce acid mine drainage (AMD), which can lead to the acidification of the surrounding water and land resources, as well as cause the release and migration of heavy metal ions. This requires the consolidated body to have a low permeability to prevent the oxidation of sulfide minerals [
25,
26]. In summary, strength and permeability are two key parameters for assessing the performance of the consolidated body. The consolidated body is an engineering material with certain engineering properties due to the coagulation and hardening of the added cementation material and can be regarded as a cement-based material with a high water-cement ratio and tailings as the aggregates. According to the characteristics of cement-based materials, its macro performance [
27] is determined by its microstructure characteristics, and the microscopic pore structure is one of its important properties [
28,
29]. As the hydration reaction progresses, it evolves and affects the macroscopic properties such as strength [
30,
31,
32,
33] and permeability [
34,
35,
36].
However, in the past, the study of consolidated bodies has mainly focused on the strength as the index to measure its macro performance, ignored the study of consolidation permeability, and rarely reported the systematic study of the macro performance of the consolidated bodies combined with the microstructure evolution of the consolidated bodies. To study the engineering properties of the consolidated body more comprehensively, this paper studies the evolution law of the strength and permeability of the consolidation body with respect to the curing age through unconfined compressive strength (UCS) and permeability tests from a macroscopic perspective. Through scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests, the hydration process and the evolution of the microscopic pore structure characteristics of the consolidation body were analyzed from the microscopic perspective. By studying the consolidation body from the macroscopic and microscopic perspectives, this research provides a basis for further exploring the relationship between the macroscopic properties and the microscopic pore structure, and reveals the mechanism of the macroscopic performance of the consolidator. Furthermore, the engineering properties of the consolidated body could be improved by optimizing the microstructure.
4. Conclusions
This paper presents the macroscopic performance and microstructural properties evolution of CCT samples with increasing curing age. The tests performed include UCS tests, permeability tests, SEM observations, and MIP tests. During the curing period, the UCS, permeability, and microstructure markedly changed. Based on the results obtained within the limits of this study, the following conclusions and suggestions for future research can be summarized as follows:
(1) In general, the UCS of the CCT samples increases nonlinearly and the growth rate decreases with increasing curing age. This indicates that the structural stability of the consolidated pile gradually increases as the curing age increases. This will ensure the safety performance of a project to a certain extent. On a semilogarithmic scale, there is a linear relationship between the UCS and curing age, and the linear relationship can predict the UCS of the CCT samples in a reasonable manner.
(2) The CCT sample permeability decreased rapidly in a short period of curing age, i.e., between 0 and 7 days of curing age, gradually stabilized during the curing period of 7 to 28 days, and after 28 days curing reached a stable value. This permeability evolution trend helps to improve the environmental and corrosion resistance of the consolidated body. From the obtained experimental results, regression models were found to predict the permeability evolution of the CCT sample.
(3) The SEM observations reflect that with the hydration process, the morphology of the hydration products and microstructures changed significantly. SEM observations showed that strength, permeability, and the micro-pore structure evolution are related to increasing precipitation of the hydration product in the CCT samples.
(4) The pore structure evolution reported from the SEM signifies that as the curing age increases, the compactness of the CCT sample increases and the pore structure is refined, i.e., the CCT sample is refined. The total pore volume and the critical pore size decreased, and the proportion of pore volume for the large pores (>200 nm) decreased, whereas the small pore volume (<200 nm) increased. This is due to the growth of the hydration products filling the larger pores.
(5) Above all, the test results indicated that the cement consolidation applied to the surface tailings management is effective and the SEM observations and MIP tests allow us to better understand the evolution of the strength and permeability. The microscopic experimental results show that the strength and permeability are a macroscopic reflection of the microstructure. However, further studies are needed to quantitatively investigate the relationships between the macro behavior and microstructure properties during evolution.