Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2024.47.0028 Acta Polytechnica CTU Proceedings 47:28–34, 2024 © 2024 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague PARAMETERS OF BACK FILLING MATERIAL PREPARED FROM MINE TAILING, FLY ASH, AND MINE WATER Karel Dvořák∗, Andrea Jančíková Brno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic ∗ corresponding author: karel.dvorak@vut.cz Abstract. The article investigates the influence of mine tailing, fly ash, and mine water on strength parameters as mine paste filling material performing the strength tests on different recipes of mortar mixes with individual types of fillers and wastewater from extraction of mine tailings. Recipe is prepared in proportions corresponding to the standard mortar and compared with a reference recipe according to the fresh mortar consistency test using a shaking table according to ČSN EN 1015-3. Resulting measured values on 40 × 40 × 160 mm solids samples for which the evaluation of the volume weight was carried out in the range of 2, 7, 14, 28, 56 and 90 days according to EN 1015-10 (722400). The results showed that that it is possible to prepare relatively fluid mixtures with extremely low binder dosage. And not only the water coefficient but also the granulometry of the raw material plays an important role in the long-term strengths of samples. Keywords: Fly ash, mine water utilization, mine backfill mixture. 1. Introduction The mining industry can be divided into two broad categories, namely metallic and non-metallic. Pre- cious metals are extracted from mined ores following a well-defined series of mineral processing and met- allurgical procedures. This series leads to the gen- eration of large quantities of solid industrial wastes from mine development, such as fragmented waste rock and mined bedrock [1]. Another category of solid waste generated from mineral processing (separation of valuable minerals from ores) is called tailings. It follows that mining and mineral processing is associ- ated with the generation of significant quantities of waste. Coal ash from thermal power plants during coal mining, tailings from precious metal mining and metallurgical slag from copper mining are significant waste streams generated [2, 3]. The generation of such a huge number of segregated tailings adversely affects the environment, including air, water, and soil [4, 5]. A vast area of surface land is subsequently required to adequately store and manage this waste in the form of segregated tailings. Underground disposal of these industrial wastes (tailings, fly ash cinders, red mud etc.) generated from mining and related activities in the form of mine backfill is an effective solution [6–8]. The voids created after ore extraction in underground mines can be refilled with specific waste material referred to as backfill [9]. Backfill- ing of mines helps to prevent subsidence, improves the stability of underground structures, and prevents ore grinding also increases the recoverability of the mined waste and as a result increases production with many other benefits [8–11]. Backfilling can be divided into two broad categories, the former being unconsoli- dated backfill and the latter being cemented reinforced backfill [12]. Unconsolidated backfill, waste is added without any binder. While in cemented backfill, a hy- draulic binder is added, usually this binder is Portland cement which is added to the solid waste material to create background for mechanical properties of the paste [12, 13]. One form of cemented backfill used in underground mines is a paste with well-defined mechanical properties [12–15]. Paste backfill is widely applied worldwide because of its techno-economic and its technology success which is evident from the fact of increased demand over the last 40 years [14]. In recent years, several modifications and investigations have been made regarding improvements in the de- sign and environmental friendliness of paste backfill. Significant progress in this area has been achieved using alternative binders, the use and modification of backfill materials, investigation of their strengths, rheology of leaching probability, and the economic use of other wastes. This work was devoted to the study of the basic mechanical properties and the effect of different mine waters used for the preparation of tailings backfill mixtures with binder in the form of fluidized fly ash activated by grinding [8–14]. Mine tailings, which are deposited after the extrac- tion of minerals, are the largest source of waste in the processing of mining raw materials. Worldwide, approximately 5-7 billion tons of waste are generated annually in the form of mined tailings [7, 8]. This waste has traditionally been disposed of in tailings ponds, repositories, which is associated with serious en- vironmental, geotechnical, and economic problems [15– 17]. Cement paste backfilling is increasingly used to fill excavated voids in underground spaces [6–8, 10]. Backfilling with cement paste recycles the processed tailings. into underground mine cavities, reducing the volume that needs to be deposited on the surface. This process reduces the potential environmental impacts 28 https://doi.org/10.14311/APP.2024.47.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 Parameters of back filling material associated with the issue [8, 18–21]. Material recycled in this way can act as a permanent support system or temporary working platform for others [15, 16]. Cement pastes treated in this way, in the form of a high-density slurry, consist of mined tailings, a low proportion of cement binder (3-7 %) and processed mine water to meet the design mechanical properties. To minimize the cost of the binder used, other alter- native binders and materials are very often used, the most common being high temperature fly ash with poz- zolanic activity and silica fume from the metallurgical industry. To achieve better pumpability and lower wa- ter coefficient while still maintaining good workability of these mixtures, superplasticizers are also very often used [1–3, 13–16]. The transport of backfill mixtures in underground mines is most often carried out by gravity or mediated by pumping through pipelines or boreholes. For this reason, the prepared slurry should be thickened to achieve a non-sedimentary character for easy pumping into mine cavities created during deep mining operations [11, 12, 19]. Hence, the fluid- ity of the paste backfill is an essential parameter in the design of cement slurry. The rheological properties (i.e., yield strength and viscosity) are related to the so- called amount of energy consumed to ensure fluidity. Mine backfill mixtures should be homogeneous, mo- bile, suitable for transport by pumping over distances of hundreds of meters, and workable for a minimum of 120 min. After curing, the strength should ideally reach at least 4 MPa after 90 days, however, in condi- tions without static function, a strength of about half, 2-3 MPa may be acceptable [18–22]. Several studies have been carried out in recent years to understand the rheological behavior of backfill pastes and their main influencing factors. These studies show that the rheological behavior is influenced by external (e.g., temperature and time) and internal (e.g., cement con- tent, addition of slag, fly ash or superplasticizers, and type of water used) factors. Although the effects of these external and internal factors are widely doc- umented in the literature, the reported results are not consistent because the physical, chemical, and mineralogical properties of tailings vary from mine to mine [8–14]. The aim of this research was to ab- solutely minimize the amount of binder component, which is only supposed to have the function of ensur- ing the cohesion of the cured composite. The use of fluidized power plant fly ash, which is not used at all to produce conventional concretes, is also interesting. 2. Material and methods To create models of non-static landfill mixtures, residues after separation of heavy and light recover- able components from the Cínovec, Měděnec and Zlaté Hory landfills were used, for which specific weight tests were carried out, where the measured values of specific weights for the tailings are given in Table 1. The cement paste binder has been completely re- placed by a binder based on grinding-activated fly ash, Specific weight [g cm−3] Měděnec 3.56 Cínovec 2.74 Zlaté hory 2.82 Table 1. The resulting measured values of the specific weight of the separated tailings. which is a dry, clinker-free hydraulic binder material commonly used as an active admixture in shotcrete. Composition of the fly ash used: • above 50 % SiO2 with Al2O3 • up to 20 % free lime, CaO • up to 8 % sulfur dioxide, SO3 The specific weight of the fly ash was measured in the laboratory using a Helium pycnometer Accupyc 1340 (micromeritics gas pycnometer), which reached 2.78 g cm−3. The specific weight is significantly lower than that of cement (3.1 g cm−3). And its fineness of grinding was also measured similarly to cement according to ČSN EN 196-6 using the ZEB Pc-Blaine- Star instrument. Specific surface area of the fly ash came out to be 8 200 cm2 g−1. On the contrary the specific surface area of the fly ash is significantly higher than that of cement (3 000 – 4 500 cm2 g−1). Results are given in Table 2. Specific Specific weight surface area [g cm−3] [cm2 g−1] Fly Ash 2.78 8 200 Table 2. Determination of specific weight and specific surface area. Furthermore, the normal consistency and the onset of hardening of the binder based on fly ash activated by grinding were determined similarly to cement accord- ing to EN 196-3 (722100), Determination of setting times and volume stability. The result is given in the following Table 3. and Table 4. Amount Resulting consistency [g] [mm] Demineralized water 215 3 Fly Ash 500 Table 3. Determination of normal consistency. In addition to the independent properties of the binder consisting of fly ash and filler, which were the tailings residues after heavy and light metal separation, the applicability of mine water for the production and subsequent observation of the durability of the tailings mixtures was tested. To study the influence 29 Karel Dvořák, Andrea Jančíková Acta Polytechnica CTU Proceedings Amount Beginning of hardening [g] [min] Demineralized water 215 40 Fly Ash 500 Table 4. Determination of the beginning of hardening. of mine water, water from the mining area of Cínovec and Zlaté Hory (acidic water and alkaline water) was used. The pH value was monitored with a AD8000 Professional Multi-Parameter pH – Bench Meter, with the resolution of 0.01 pH and accuracy of ±0.01 seen in Table 5. Seven mortar pastes based on fly ash binder were created to monitor the process of gaining their strengths. The mixture was always composed of 1 350 g of residues after separation of heavy and light usable components from landfills 225 g of Fly Ash and either demineralized water (reference) or mine water (Water Cínovec, Water Zlaté Hory acidic and Water Zlaté Hory alkaline). pH Cínovec 8.5 Zlaté Hory Acidic 3.4 Zlaté Hory Alkaline 7.8 Table 5. resulting measured pH values for each water type. To perform the strength tests, the recipes of mortar mixes for individual types of fillers and wastewater from the extraction of tailings were designed and com- pared with a reference recipe prepared in proportions corresponding to the standard mortar according to the fresh mortar consistency test using a shaking ta- ble according to ČSN EN 1015-3. The monitored parameters were: • Volumetric weight of fresh mortar according to EN 1015-10 (722400), • Flexural tensile strength and compressive strength at curing times of 2, 7, 14, 28, 56 and 90 days according to EN 1015-11 (722400), determination of flexural and compressive strength of hardened mortars. Due to the requirement for flowability of the mixes, the consistency of fresh mortar was chosen using a shaking table and levelling them to a 180 mm spillage. To achieve a 180 mm spillage, more water had to be added to the mixes than for normal consistency due to wetting of the separated aggregate. The proposed formulations of the stockpile mixes and the reference mixes are shown in Table 6, Table 7 and Table 8. 3. Results and discussion Resulting measured values on 40 × 40 × 160 mm solids samples for which the evaluation of the volume weight Amount Resulting consistency [g] [mm] CEN Standard Sand EN 196-1 1 350 Demineralized water 250 Spillage 180/180 Fly Ash 225 Table 6. Proposed reference recipe for the mortar mixture. Cínovec Zlaté Hory [g] [g] Tailings 1 350 1 350 Demineralized water 420 580 Fly Ash 225 225 Table 7. Proposed recipes for the mortar mixture with demineralized water. was carried out in the range of 2, 7 14 and 28 days according to EN 1015-10 (722400), Test methods for mortars for masonry – Part 10: Determination of volume weight of dry hardened mortar. The values obtained for the volume weights are summarized in Table 9 and then evaluated in the bar graph in Fig- ure 1 for each individual mixture consisting of the residues after separation from the respective mine deposit together with the respective water and a fly ash-based binder activated by grinding. Resulting measured values on 40 × 40 × 160 mm solids samples for which the strength tests were evalu- ated in the time intervals 2, 7 14, 28, 56 and 90 days according to EN 1015-11 (722400), Test methods for mortars for masonry – Part 11: Determination of tensile flexural and compressive strength of hardened mortars. The achieved strength test values are sum- marized in Table 10 and Table 11 and then evaluated in the bar charts in Figure 2 and Figure 3 for each in- dividual mix consisting of the residue after separation from the respective mine deposit together with the respective water and a fluidized fly ash-based binder activated by grinding. The results of the fundamental mechanical proper- ties of the backfill mixtures, determined on selected formulations with very high-water coefficient, showed that it is possible to prepare relatively fluid mixtures with extremely low binder dosage. Not only the wa- ter coefficient but also the granulometry of the raw material plays an important role in the long-term strengths. Here, the flotation waste is in principle significantly handicapped, however, at least minimum strengths were achieved here as well. The fluidity was ensured only by the water coefficient without the addition of thickening or plasticizing additives. These agents were deliberately not used here, as they would have made the whole technology significantly more expensive. The effect of the acidic mixing water on the mechanical properties did not show any significant effect and for this reason can be considered suitable 30 vol. 47/2024 Parameters of back filling material Měděnec [g] Cínovec [g] Zlaté Hory [g] Tailings 1 350 1 350 1 350 Fly Ash 225 225 225 Demineralized water 420 (Water Cínovec) 420 (Water Cínovec) 620 (ZH Acidic) 620 (ZH Alkaline) Table 8. Proposed recipes for the mortar mixture with the use of post-mining water. After 2 Days After 7 Days After 14 Days After 28 Days Reference 1 900 1 950 1 970 1 990 Cínovec + Demi.W. 1 700 1 730 1 800 1 810 Zlaté Hory + Demi.W. 1 620 1 760 1 750 1 770 Cínovec 1 780 1 690 1 730 1 750 Měděnec 2 040 2 170 2 110 2 150 Zlaté Hory + Acidic W. 1 770 1 760 1 720 1 750 Zlaté Hory + Alkaline W. 1 850 1 850 1 790 1 860 Table 9. Achieved volumetric weights of given recipes. Figure 1. Graph showing the volume weights from Table 9 of the tested samples at 2, 7, 14 and 28 days. Figure 2. Graph showing the tensile strength in bending from Tables 10 and 11 of the tested samples at 2, 7, 14 and 28, 56 and 90 days. 31 Karel Dvořák, Andrea Jančíková Acta Polytechnica CTU Proceedings Figure 3. Graph showing the Compressive strength from Tables 10 and 11 of the tested samples at 2, 7, 14 and 28, 56 and 90 days. Properties [MPa] Reference Cínovec + Demi.W. Zlaté Hory + Demi W. 2 days Tensile strength in bending 0.3 0.2 0.3 Compressive strength 0.4 0.2 0.1 7 days Tensile strength in bending 0.9 0.4 0.3 Compressive strength 3.3 0.9 0.4 14 days Tensile strength in bending 2.1 0.9 0.7 Compressive strength 7.3 2.8 1.6 28 days Tensile strength in bending 3.4 1.2 1.0 Compressive strength 8.1 3.2 1.7 56 days Tensile strength in bending 3.3 1.4 1.0 Compressive strength 8.9 3.7 1.9 90 days Tensile strength in bending 3.5 1.5 1.2 Compressive strength 8.9 4.1 2.4 Table 10. Achieved volumetric weights of given recipes. Properties [MPa] Cínovec Měděnec ZH + Acidic W. ZH + Alkaline W. 2 days Tensile strength in bending 0.1 0.3 0.3 0.3 Compressive strength 0.2 0.2 0.2 0.2 7 days Tensile strength in bending 0.4 0.6 0.3 0.3 Compressive strength 1.3 0.9 0.3 0.4 14 days Tensile strength in bending 0.9 1.1 0.6 0.7 Compressive strength 2.9 2.9 1.4 1.6 28 days Tensile strength in bending 1.2 1.2 1.0 1.0 Compressive strength 3.3 3.4 1.5 1.8 56 days Tensile strength in bending 1.4 1.5 1.0 1.1 Compressive strength 3.6 3.8 1.8 1.9 90 days Tensile strength in bending 1.6 1.5 1.2 1.1 Compressive strength 4.3 4.2 2.4 2.6 Table 11. Achieved strength properties of the formulations with mine waters. 32 vol. 47/2024 Parameters of back filling material for these backfill mixtures without a static function. 4. Conclusion In conclusion, it can be stated that: • According to the measured results, it can be con- cluded that it is possible to prepare relatively well- flowing mixtures with very high-water coefficients without the addition of plasticizers and with ex- tremely low dosage of binder based on fly ash mix- tures under placement conditions without static function. • FBC fly ash appears to be a promising binder for the preparation of backfill mixtures. • The presence of free lime in fly ash can compensate for the significantly lower pH of the mine water. • The lower pH of the water used to produce the tailing mixtures did not have a negative effect nei- ther on volume weights nor strength in bending nor Compressive strength of the tested samples in given times. • It will thus be possible to use untreated mine water to produce mixtures. 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Procedia Engineering 65:296–301, 2013. https://doi.org/10.1016/j.proeng.2013.09.046 34 https://doi.org/10.1016/j.conbuildmat.2021.125120 https://doi.org/10.1016/j.conbuildmat.2021.125120 https://doi.org/10.1016/j.proeng.2017.02.186 https://doi.org/10.1016/j.conbuildmat.2021.124061 https://doi.org/10.1016/j.conbuildmat.2021.124061 https://doi.org/10.1016/j.proeng.2013.09.046 Acta Polytechnica CTU Proceedings 47:28–34, 2024 1 Introduction 2 Material and methods 3 Results and discussion 4 Conclusion Acknowledgements References