Sulphate induced changes in the reactivity of cemented tailings backfill

Strength development and self-desiccation of saline cemented paste backfill

Given that many mines around the world are located in areas where fresh water is scarce, and companies are being held to increasingly stringent sustainability and environmental responsibility standards, many mines are looking to use locally available saline groundwater or seawater as mixing water in cemented paste backfill (CPB). However, the impacts of this decision on key engineering properties of CPB (e.g. strength and self-desiccation) that affect its mechanical stability need to be better understood to allow confident selection of this practical and more sustainable solution. Thus, the effect of mixing water salinity and binder type on the strength (unconfined compressive strength, UCS) development and self-desiccation (measured by suction and volumetric water content) of CPB is explored in this research. NaCl concentrations from 0 to 300 g/L were used in CPB made with silica tailings and Portland cement type I (PC). Concentrations of 10 and 35 g/L were found to moderately increase UCS, while a concentration of 100 g/L had comparable UCS to non-saline CPB and a concentration of 300 g/L was found to significantly decrease UCS over all curing times. The overall trend is 10 g/L > 35 g/L > 0 g/L > 100 g/L > 300 g/L. The UCS of the 60-day-old CPB with a NaCl of 300 g/L is significantly lower, registering a 26% decrease compared to the UCS of the 60-day-old CPB without salt. In contrast, the UCS of the 60-day-old CPBs containing 10 g/L and 35 g/L of salt exhibits a notable improvement, being 15% and 10% higher, respectively, than the UCS of the 60-day-old CPB without salt. Water content and suction monitoring were conducted up to 28 days of curing time, and it was found that suction only slightly contributed to UCS gain of the saline CPB, and high salt contents (100 and 300 g/L) significantly inhibited the self-desiccation ability of CPB due to inhibition of cement hydration by the excessive amount of salt. The increase in strength of both saline and non-saline samples was attributed primarily to the increase in cement hydration products, while the increased strength of the samples with salinities of 10 and 35 g/L was mainly attributed to the enhancement of the binder hydration due to the low amount of salt and the presence of Friedel's salt in the pores. The effect of PC replacement by 25 to 75% with slag on CPB with 35 g/L mixing water salinity was also studied. Slag replacement of 50% and higher resulted in significantly higher UCS over most curing times. Suction likely moderately contributed to UCS of the saline CPB with slag, in addition to the presence of Friedel's salt in the pores and the acceleration of cement and slag hydration by the presence of NaCl.

Lithium Slag and Solid Waste-Based Binders for Cemented Lithium Mica Fine Tailings Backfill

To mitigate the adverse effects of fine-grained lithium mica tailings and other solid wastes generated from the extraction of lithium ore mining, as well as the limitations of traditional cement-based binders for lithium mica fine tailings, this study explores the feasibility of using a binder composed of ordinary Portland cement, lithium slag, fly ash, and desulfurization gypsum to stabilize lithium fine tailings into cemented lithium tailings backfill. Compared with traditional cementitious binders, an extensive array of experiments and analyses were conducted on binders formed by various material proportion combinations, employing uniaxial compressive strength tests, microstructural morphology, grayscale analyses, and flowability tests. The results show the following: (1) In this study, an LSB binder exhibiting superior mechanical properties compared to traditional cementitious binders was identified, with an optimal OPC:LS:FA:DG ratio of 2:1:1:1. (2) In the context of cemented lithium mica fine tailings, the LSB-CLTB material exhibits higher unconfined compressive strength and lower self-weight compared to OPC-CLTB materials. At a binder content of 10 wt%, the UCS values achieved by the LSB-CLTB material at curing periods of 7 days, 14 days, and 28 days are 0.97 MPa, 1.52 MPa, and 2.1 MPa, respectively, representing increases of 40.6%, 34.5%, and 44.8% over the compressive strength of OPC-based materials under the same conditions. (3) The LSB binder not only exhibits enhanced pozzolanic reactivity but also facilitates the infilling of detrimental pores through its inherent particle size and the formation of AFt and C-(A)-S-H gels via hydration reactions, thereby effectively improving the compressive strength performance of fine-grained tailings backfill.

Physico-chemical and micro-structural behavior of cemented mine backfill: Effect of pH in dam tailings

The tailings created during ore processing have been a serious problem for mining companies and environment since it is a challenging task to effectively manage these highly voluminous/dangerous tailings. Therefore, several tailings disposal methods like tailings dams are needed for sustainable mining operations. The tailings accumulated in the dams reflect a critical raw material source since they might contain key base/precious metals, such as Au, Ag, Co, Ni, Cu and Zn. This study deals with the use-ability of dam tailings in cemented mine/paste backfill (CMB/CPB), considering the physico-chemical and micro-structural aspects. The backfill mixtures were manufactured at 76 wt% solid and 5 wt% cement contents, exposed to cure for up to 56 days, and tested for determining their strength (UCS), geo-chemical (i.e., pH, redox potential, and conductivity) and microstructure (i.e., XRD, TGA, and SEM) characteristics. Results disclosed that the strength of backfill was improved by the augmented basicity/age while only backfills made with sulfide-rich tailings had a noticeable drop in strength. This can be enlightened by the types of tailings (aged and fresh), and the hydration products shaped owing to the interaction of these tailings mixed with cement. While the values of pH detected by chemical tests were amplified up to 14 days, some decreased up to 56 days due to acid formations and erosions. This is the key function of CPB's deterioration physically, chemically, and microstructurally. Lastly, the outcomes of this study will allow us to further explore/assess the effects of dam tailings' potential usages on quality/performance of backfill mixtures.

Experimental Study on Microstructure and Erosion Mechanisms of Solid Waste Cemented Paste Backfill under the Combined Action of Dry-Wet Cycles and Sulphate Erosion

Solid waste cemented paste backfill (SWCPB) meets the needs of coal mining area management. SWCPB is a cementitious paste backfill material without added cement and is made only from oil shale residue (OSR), steel slag (SS), soda residue (SR) and water. In this study, mine water characteristics were simulated by combining dry–wet cycling experiments with sulphate erosion experiments. SWCPB was assessed regarding appearance, mass loss, and unconfined compressive strength (UCS), and the erosion products were microscopically analysed with X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The mechanism for erosion of the SWCPB by sulphate-rich mine water was comprehensively analysed and revealed. Research showed that the erosion mechanism was divided into two parts: chemical and physical erosion. Low concentrations of sodium sulphate promoted hydration, thereby contributing to the increased mass and strength of SWCPB. At high sodium sulphate concentrations, the erosion mainly consumed Ca(OH)₂ within the material, and the main generated erosion products were gypsum and ettringite (AFt). This was accompanied by the destructive effects of Na₂SO₄ crystal expansion, which resulted in damage and the reduced workability of the SWCPB. The whole erosion process was continuous, mainly due to transformations of pits, pores and cracks. The conclusions of this study may provide appropriate guidance for application of SWCPB materials in the treatment of coal mine backfills. In addition, the corresponding theoretical analysis of the erosion mechanism for SWCPB materials is provided.

Experimental study of high-flow and low-expansion backfill material

High-flow low-expansion backfill materials have been developed to improve difficult slurry pipeline transport and poor roof-contact effect of many filling materials. The fly ash content was fixed at 80%, with 8.5% - 9.5% mineral powder content, 8.5% - 9.5% lime, 2% - 3% desulfurized gypsum, 0.9% - 1.2% sodium carbonate, and 0.01% - 0.02% aluminum powder content. The prepared backfill material processed good fluidity, with the expansion rate of the hardened material reaching 2% - 3%, and compressive strength on 90 d reaching 4 MPa—5.5 MPa. SEM observations indicated that as the aluminum content increased, ettringite on bubble walls transformed from a fine-needle to needle-rod shape. Secondly, the hydration products of the system were mainly hydrated calcium silicate gel and ettringite, which interconnected and promoted the formation of the structure. The backfill material has extensive sources of raw materials, low cost, simple filling process, and good filling effect.

Lithium slag and fly ash-based binder for cemented fine tailings backfill

This research work was an exploration of the feasibility of utilizing a lithium slag (LS) and fly ash (FA)-based binder for cemented fine tailings backfill (CFTB). Extensive experiments were conducted with different combinations of LS and ordinary Portland cement (OPC), along with FA as an additive. The unconfined compressive strength (UCS), micromorphology and slump values were analyzed. The results showed that (i) the LS and FA had a significant influence on the strength of binders. The OPC-LS-FA ratio of 2:1:1 appeared to be optimal with the highest strength and was referred as the LS and FA-based binder (LFB). (ii) The LFB significantly improved the UCS of the CFTB. The UCS values of CFTB specimens curing for 7,28 and 56 days reached 0.95 MPa,2.28 MPa and 3.37 MPa, respectively, with a 10 wt% content of LFB. The strength satisfied the strength requirement of backfill for supporting the surrounding rock of stopes in the Yinshan lead-zinc mine (0.8 MPa, 2.0 MPa, 3.0 MPa). (iii) The pore-filling effect of the secondary hydration products, which was mainly produced by LFB, played a significant role in the early stage (<7 days), while the pozzolanic activity worked mostly in the mid-long period (>28 days). (iv) The LFB reduced the slump value of CFTB slurry by 2.6%-9.4% compared with OPC when the mass concentration increased from 58% to 64%, which was acceptable to satisfy the requirements of better fluidity and less transportation resistance in the Yinshan lead-zinc mine. Therefore, the LFB could be utilized as an alternative cementitious material for CFTB, which also provides a safe and economical approach to recycle LS and FA in an underground mine.

Compressive Strength Characteristics of Cemented Tailings Backfill with Alkali-Activated Slag

With the use of glauberite mineral (GM) and sodium hydroxide (SH) alkaline catalysts to stimulate slag powder’s internal cementation activity and incorporate the two fine-grained solid wastes, such as quicklime (Q) and desulfurized ash (DA), a new cementitious material suitable for mine tailings was developed to replace traditional ordinary Portland cement (OPC) for reducing cement-related costs. A series of uniaxial compressive strength (UCS) tests were carried out on cemented tailings backfill (CTB) samples containing different activators. The results showed that (1) the highest UCS values of 14-day and 28-day cured CTB samples were 1.259 MPa and 2.429 MPa, respectively, and the effect of different activator types was in the order of SH > GM > DA > Q and SH > GM > Q > DA; (2) the relationship between UCS and activator dosages followed the function y = ax3 − bx2 + cx − d. Compared with the OPC 32.5 R cemented samples, the minimum strength growth factor was 1.45, and the maximum reached 2.03; (3) the optimal proportion of DA slag formula was 4.5% or 5.0% Q, 19% DA, 2.5% GM, and 0.7% SH. The aforesaid new cementitious materials met the mine’s UCS requirements with a relatively low cost (17.04–17.20 €/ton) and solved the stacking problem of solid wastes on the surface well. Ultimately, this study provides a useful reference for the development of mineral binders.

Evaluation of Viscosity, Strength and Microstructural Properties of Cemented Tailings Backfill

In this study, the particle size distribution and chemical composition of gold mine tailings were examined experimentally. A series of viscosity and uniaxial compressive strength (UCS) tests were used to study the relations between the viscosity of cemented tailings backfill (CTB) slurry, the solid content (SD), and the cement-to-tailings ratio (c/t). Relations between UCS performance of CTB and SD, c/t, and curing time (CT) were discussed while examining the microstructure of 28-day cured backfill with different solid contents. Results illustrate that a major increase in CTB viscosity by increasing the SD leads to the formation of tailings grains for a skeleton formation, which is formed due to consolidation and gravitational forces. The CTB’s strength increases with the increase of c/t, SD, and CT, due to a decrease in water-to-cement ratio and porosity, and an increase in hydration products over time. The SEM micrographs show how CTB’s microstructure is affected by the SD, generating ettringites and calcium silicate hydrates in the backfill matrix. The findings of this study will lead to an efficient CTB mix design for reaching the higher performance in underground mining structures, thereby reducing expenses related to the backfill.

The formation of unsaturated zones within cemented paste backfill mixtures-effects on the release of copper, nickel, and zinc

Flooding of cemented paste backfill (CPB) filled mine workings is, commonly, a slow process and could lead to the formation of unsaturated zones within the CPB fillings. This facilitates the oxidation of sulfide minerals and thereby increases the risk of trace metal leaching. Pyrrhotitic tailings from a gold mine (cyanidation tailing (CT)), containing elevated concentrations of nickel (Ni), copper (Cu), and zinc (Zn), were mixed with cement and/or fly ash (1-3 wt%) to form CT-CPB mixtures. Pyrrhotite oxidation progressed more extensively during unsaturated conditions, where acidity resulted in dissolution of the Ni, Cu, and Zn associated with amorphous Fe precipitates and/or cementitious phases. The establishment of acidic, unsaturated conditions in CT-CBP:s with low fractions (1 wt%) of binders increased the Cu release (to be higher than that from CT), owing to the dissolution of Cu-associated amorphous Fe precipitates. In CT-CPB:s with relatively high proportions of binder, acidity from pyrrhotite oxidation was buffered to a greater extent. At this stage, Zn leaching increased due the occurrence of fly ash-specific Zn species soluble in alkaline conditions. Irrespective of binder proportion and water saturation level, the Ni and Zn release were lower, compared to that in CT. Fractions of Ni, Zn, and Cu associated with acid-soluble phases or amorphous Fe precipitates, susceptible to remobilization under acidic conditions, increased in tandem with binder fractions. Pyrrhotite oxidation occurred irrespective of the water saturation level in the CPB mixtures. That, in turn, poses an environmental risk, whereas a substantial proportion of Ni, Cu, and Zn was associated with acid-soluble phases.