Tuylu S
(Department of Mining Engineering, Istanbul University-Cerrahpasa, Istanbul, 34500, Turkey)
Abstract: In the cemented paste backfill (CPB) method, which can also be used for fortification purposes in mines, different additive materials with pozzolanic properties can be employed as substitutes instead of cement that is the main binder. One of the most popular pozzolanic materials that can be employed instead of cement is fly ash, which is thermal power plant tailings. But the compositions of fly ash and tailings used in high amounts in the CPB method, as well as the chemical structures that these materials form by interacting with the cement binder, affect the mechanical properties of the material depending on time. In this study, fly ash with 4 different chemical compositions (TFA, SFA, YFA, and CFA) was used as a cement substitute in CPB. By substituting fly ash with different chemical compositions in different proportions, CPB samples were created and their strength was elucidated according to 28, 56, and 90-day curing times. The results of the study revealed that TFA with the highest CaO/SiO2 and SO3 ratios remained stable at the strength values of 6 MPa (total 9% binder) and 10 MPa (total 11% binder) in the long term. However, CFA with the lowest CaO/SiO2, SO3, and the highest SiO2+Al2O3+Fe2O3 ratios resulted in the greatest strength increase at a 20% substitution rate (11% of the total binder). Nevertheless, it was found that the SFA, which is in Class F, increased its strength in the early period based on the CaO rate.
Key words: tailings disposal; cemented paste backfill; fly ash; chemical properties; strength
Tailings that occur after ore dressing are commonly stored in tailings impoundments. In particular, uncontrolled and risky traditional tailings impoundments can lead not only to the use of the land around the mine but also to some problems in terms of environmental and lively life. For these reasons, the paste backfill method has been adopted as an effective tailings disposal method in terms of sustainable mining by storing large amounts of tailings in underground mine cavities. Up to 60% of mine tailings can be stored underground. The backfill mass can usually be used as a temporary or permanent heel. In this way, it provides additional structural fortifications for continuous excavations and can minimize mineral loss. The created cemented paste backfills (CPB) mixture must fit the requirement of a minimum 4 MPa if it is to be used for ceiling fortification and 1 MPa uniaxial compressive strength (UCS) if it is to be used only for unloaded backfill.
A UCS of 100 kPa is commonly adopted as the liquefaction potential limit, and the required UCS for paste backfill depends on roof support, ground or pillar support, and working platform.
CPB technology, which is becoming increasingly important in tailings disposal management, is widely applied in underground mining enterprises around the world due to its important environmental, technical, and economic benefits. CPB is a new material created with three basic materials: process tailings filtered or dewatered in the range of 70% and 85% by weight, hydraulic binders used in the range of 3% -10% by weight, and mixture water used to ensure the liquefaction of the paste in the pipeline and uniform diffusion at the stope. The solid percentage in the mixture should be at a rate that can ensure the liquefaction and maximum tailings volume required for transfer and diffusion. The variability in the solid ratio is important for the pumpability of the paste backfill material, and therefore a solid ratio suitable for the slump range of 15-25 cm is selected.
The basic binding material used in CPB applications is usually portland cement (PC). At the same time, cement is a significant part of CPB operating costs. For this reason, various studies have been conducted on the artificial or natural pozzolans that are the secondary binders to provide the desired strength and stability while reducing the PC ratio to keep operating costs to a minimum. Fly ash, one of these pozzolans, reacts chemically with Ca(OH), the primary binding hydration product, to form calciumsilicate-hydrate (C-S-H), which is the secondary hydration products, contributing to the hardening and strength gain of the backfill material. Therefore, in cemented mixtures, a partial replacement of fly ash can be made instead of PC, thus contributing to the reduction of the cost of the binder. But since the chemical compounds of fly ash do not show a uniform structure, it is necessary to investigate the appropriate type and ratio of fly ash according to the purpose of use. The chemical composition of fly ash depends on various variables, including the quality of coal burned in thermal power plants and the impurities contained in it, the method of collection, the duration, and the temperature of combustion. Fly ashes are divided into Class C and Class F according to their chemical composition, but in some studies, it has been noted that the classification of fly ashes according to their chemical structure is not appropriate because some fly ashes have characteristics of more than one class or characteristics that are different enough to form a class. It is stated that the partial use of C-Class fly ash instead of cement in paste backfill greatly increases the strength in long-term curing. However, some Class C fly ashes with high calcium and aluminum content can cause preterm hardening. Class F fly ash, on the other hand, hydrates more slowly and produces lower hydration temperatures than cement, which reduces the tendency to collapse loss, preterm hardening, and thermal cracking. When appropriate fly ash is used, resistance to alkali-silica reaction (ASR) and sulfate attack can be improved, which negatively affects the mechanical properties of CPB. Sulfates in the CPB mixture can react with the hydration product 3CaOAlO(CA) to form by-products such as gypsum and ettringite, which expand and reduce the strength of the material. The content of cemented mixtures also changes the amount of products formed by sulfate attack, and if the amount of SOincreases, the main obtained product becomes more gypsum. By-products formed as a result of sulfate attack and ASR effect reduce the compressive strength of CPB by creating a clearance volume. The use of fly ash increases the amount of oxides (SiO, AlO, and FeO) in the binder while reducing the amount of alkali (CaO, KO, and NaO), thereby reducing the risk of ASR formation. Fly ash is less risky in terms of ASR formation as it has a lower amount of alkali than cement. However, since Class F fly ashes have a lower amount of alkali than Class C, their use is more appropriate in terms of the risk of ASR formation. In a study of fly ash taken from different thermal power plants, the chemical and mineralogical structures of fly ash affect the hydration and pozzolanic properties. For this reason, the amount of fly ash used in mixtures should also be evaluated accordingly, as it can have different effects on the strength of the mixture, especially according to its chemical and mineralogical structure.
When the related studies are examined through the literature; in CPB mixtures where tailings containing high content of sulfate or predominantly pyrite mineral is used, it is seen that fly ash is used together with other pozzolanic materials as a cement substitute, especially in order not to adversely affect the strength in the case of sulfate attacks. But no studies are showing the effect of changes in the chemical composition of fly ash on the mechanical properties of CPB. One of the main reasons for this is that the binders used in the CPB mixture are not exactly known how to react with tailings whose physical and chemical properties vary. Different CaO/SiOratios in the composition of fly ash from thermal power plants in different regions directly affect CPB strength. Therefore, it seems that the mechanical properties of CPB mixtures that have different physical and chemical properties, especially formed with a sulfur-containing waste, need to be investigated by conducting additional tests to improve their performance. In this study, the mechanical properties of CPB mixtures created using fly ash with different chemical compositions were examined and it was revealed how the differences in chemical composition affected the strength property of the material mechanism that is proposed here.
2.1 Materials
When preparing paste backfill mixtures, Pb-Zn mine process tailings in Balya/Balıkesir region, fly ash from Catalagzi/Zonguldak (CFA), Soma/Manisa (SFA), Yatagan/Muğla (YFA) and Tufanbeyli/Adana (TFA) thermal power plants, CEM I 42.5 R portland cement and tap water were used. Fly ashes were selected from thermal power plants where lignite or hard coal from different regions are used as fuel (Fig.1). The chemical properties of the materials used in the mixture are given in Table 1.
Fig.1 Location of the mine and thermal power plants
Table 1 Important chemical compounds of tailings, cement, and fly ash
Component/% Fe2O3SiO2Al2O3CaOMgOK2ONa2O*S/**SO3 Tailings 13.6 36.2 8.1 23.3 2.5 2.6 0.2 *7.5 Cement 3.4 19.9 4.8 61.6 1.3 0.9 0.3 **3.7 CFA 6.4653.3426.124.06 2.274.14 0.49 **0.68 SFA 3.9945.0423.3619.041.721.31 0.43 **4.88 YFA 6.4346.2323.0412.982.812.36 0.62 **2.85 TFA 4.5 33.7 19.8 22 2.2 1.6 0.5 **13.7
Fly ashes are classified as Class C if their SiO+AlO+FeOcontent totals ≥ 50% and Class F if they are ≥ 70%. According to this, CFA with 86%, SFA 72%, and YFA with 76% values are in class F, TFA with 58% is in Class C. However, these fly ashes are divided into three groups depending on their chemical structures; Silicate-alumina (SiO-AlO) based fly ash is generally in class F and composed of hard coal, as well as, silicate-calcite (SiO-CaO) based class C fly ashes and sulfur-calcite (CaO-SO) based fly ashes are classified mostly in the class C fly ashes and usually composed of lignite. Class C fly ash has cement properties in addition to pozzolanic properties and can resist the acid potential of mine tailings due to its high calcium content. Especially when CaO content in fly ashes is examined, it is seen in Table 1 that SFA and TFA are closer together. Fly ash with a high amount of CaO is expected to show better pozzolanic properties. Also, Wanget al
(2003) identified the basicity ratio in ashes as CaO/SiO. It is stated that if this ratio is high, the resistance against sulfate attacks that may occur in CPB will be provided and the negative impact of CPB strength can be prevented.2.2 Test methods
Monteiro and Kurtis (2003) stated that the use of 25% and 45% fly ash instead of cement reduces the expansion between hydrated PC and sulfate ions through chemical reactions. Also, pozzolans can also be harmful when used too much, they can increase the water needs of the mixture because they have a grained structure thinner than cement, and they can also reduce the rate of hardening and gaining strength as they delay the starting time of the setting. Therefore, to better understand the effects of chemical compositions of fly ash on strength in this study, fly ash substitution rates were determined as 20%-30%-40%-50% when preparing CPB samples. In addition, in CPB mixtures, the total solid matter ratio was 80% by weight and the total binder (PC+fly ash) ratio was applied up to 9% and 11% by weight. The total solid matter and binder ratio was determined by taking into account the study of Tuylu (2020). In this study, Pb-Zn tailings from the same region were used and it was stated that samples with cement binders of approximately 9% and 11% reached sufficient strength for fortification purposes. In addition, mixtures of different chemical content were created using different binder rates, and thus the effects of changes in the amount of cement on pozzolanic materials were studied. CPB mixtures created with different binder rates are also different in chemical content. Paste backfill mixtures prepared according to the determined mixture proportions were poured into cylinder sample molds with a diameter of 5 cm and a height of 10 cm. These samples were tested for uniaxial compressive strength (UCS) according to ASTM C39at the end of 28, 56, and 90-day curing periods (80% humidity, 22 ℃ temperature), and the results were evaluated according to reference samples. Optimal fly ash ratios in different mixtures were determined according to 90-day curing times because pozzolanic properties usually occur over a long period, and then the effects of changes in the chemical content of the most suitable mixtures on strength were studied.
3.1 Determination of optimal fly ash ratios testing
The UCS results of CPB samples prepared with different quantities of fly ash at 9% and 11% binder rates for 28-56-90 day curing times are given in Figs.2-5.
As shown in Fig.2, CFA usage in terms of UCS values remains below all reference values at a 9% binder rate, while it is above reference designs in certain amounts, at a rate of 11% binder. The highest strengths of CFA use according to 28 days of curing time (early strength) are 2.56 MPa (9% binder ratio) and 2.97 MPa (11% binder rate) at a 30% substitution rate. On the 56th and 90th days, it is understood that pozzolanic properties become better effective depending on time, especially in samples with a binder rate of 11%, with an increase in calcium hydroxide (CH) due to the amount of cement. The fact that high siliceous and alumina content in Class F fly ash forms C-S-H products with hydrated binding properties can be expressed as the reason for this situation. In addition, the highest strength amounts on the 90th day in samples created with CFA are provided at 50% fly ash substitution rate (3.66 MPa) for 9% binder rate and 20% fly ash substitution rate (9.82 MPa) for 11% binder rate.
Fig.2 UCS results of CPB samples with CFA
As can be seen from Fig.3, although SFA is in Class F, it achieved preterm strength (28-day curing time) due to its high CaO content (19%), which fared better than reference designs in terms of UCS values. But after 56 days, it is usually seen that there is a decrease in strength. During the 90-day curing period, only 30% and 40% SFA substitution at a 9% binder rate remained below the reference strength, while all SFA ratios at 11% binder failed to exceed the reference strength. It is believed that ASR is formed in the long term due to the high alkaline ratio in the SFA content, and therefore the strength decreases in the long term. On the 90th day, a 20% fly ash substitution rate (6.96 MPa) with a 9% binder ratio provided the highest resistance of samples in which SFA is used, while a 40% fly ash substitution rate (7.35 MPa) with an 11% binder ratio provided the best.
Fig.3 UCS results of CPB samples with SFA
As shown in Fig.4, YFA usage passed over reference strengths, with values of 5.94, 6.33, 5.78 MPa at 20, 30, and 40 percent substitution rates, respectively, during the 56-day curing period for the 9% binder ratio, while it was able to pass over reference strength, with a substitution rate of 30% in just 28 days of curing time for the 11% binder ratio. The strength of the samples created with YFA, has decreased significantly, especially after 56 days like the SFA. In these long-term declines, it is thought to be effective for YFA to be in class F, as the SFA, and to contain more than 10% CaO. In addition, the highest strength in samples created with YFA in the 90-day curing period was provided by a 30% fly ash substitution rate with 4.63 MPa in 9% binder and 5.84 MPa in 11% binder.
Fig.4 UCS results of CPB samples with YFA
As presented in Fig.5, reference strengths were passed over in TFA-substituted samples at both 9% and 11% binder rates and at all substitution rates only at the end of the 90-day curing period. In addition, although strength values decreased with the substitution increase (5.90, 5.81, 5.75, 5.73, 10.33, 10.00, 9.85, and 9.65 MPa at 9% binder, respectively, according to the substitution increase), this decrease was very few. According to the 90-day curing time, the TFA substituted samples achieved the highest strength at a substitution rate of 5.90 MPa at a 9% binder and 10.33 MPa at 11% binder at a 20% substitution rate. But since the change in 90-day strength values is very few, it is understood that TFA can be used in CPB mixtures up to 50% substitution economically if early and mediumterm strength is sufficient.
Fig.5 UCS results of CPB samples with TFA
As a result, when UCS values were examined according to fly ash substitution rates of samples with a binder rate of 9%; fly ash substitution rates that give the highest UCS values were determined as 50% CFA, 20% SFA, 30% YFA, and 20% TFA. In samples with a binder rate of 11%, these rates were determined as 20% CFA, 40% SFA, 30% YFA, and 20% TFA.
3.2 Effect of chemical content on strength
The results of the samples that gave the highest UCS results at a binder rate of 9% according to different types of fly ash are given in Fig.6. In addition, CaO/SiOand SOvalues of fly ashes of different chemical compositions, which are thought to directly affect CPB strengths, are also shown in the figures.
As shown in Fig.6, CPB samples with 9% binder and 20% SFA substitution (5.3-7.3 MPa) gave the highest UCS values at 28, 56, and 90-day curing times. Also, 50% CFA substitution showed similar results to SFA usage based on the amount of increase between cure times. Early strength values are low in mixtures where CaO content is low. This is seen in Fig.6, and accordingly, UCS values, which are low in the early strength, have increased due to the pozzolanic property, which is effective in medium and long-term curing times. It is understood from this increase that the CH source required for pozzolanic materials, which is usually formed by hydrating Portland cement, is sufficient. In 30% YFA samples, sulfate attacks are believed to occur after 56 days of treatment, and, accordingly, UCS values decrease. While the strength of 20%TFA was stable for up to 56 days, it then showed an increase of about 2 MPa. Although TFA contains high SOcompared to ASTM C618, it is understood that the ettringite and gypsum structures that may form in the material when used in a CPB mixture contribute to the increase of long-term strength (Fig.7). In other words, for samples containing TFA, it can be stated that the gaps that may occur in the material as a result of chemical processes such as ASR are filled with formations such as ettringite and gypsum.
Fig.6 Fly ash ratios giving the optimum strength according to the curing time in 9% binder
As a result of the hydration of cement, calcium silicate hydrate (C-S-H binding gel) and calcium hydroxide (CH) occur. Depending on the time, these components also turn into ettringite (needle-shaped crystals formed on gypsum aluminates) by the action of sulfate. Sulfur ions, especially from the pyrite mineral and fly ash, penetrate the mixture, reacting with calcium hydroxide (CH) and calcium aluminate hydrates (C-A-H) obtained by hydrating the cement, forming products called gypsum and ettringite, respectively. In CPB samples containing TFA, ettringite, and gypsum structures formed at the end of the 90-day treatment period are seen in Fig.7.
Fig.7 SEM image of CPB sample with TFA according to 90 days curing time
When the strength of CPB samples substituted with fly ash and 9% binder was examined, it was determined that SFA-substituted paste filler mixtures could be used for fortification (≥4 MPa) purposes, but after 56 days, the strength increase did not occur. The SFA has shown good strength in the short and mediumterm because the CaO value in its content is greater than other Class F fly ashes. Akyazili (2009) noted that fly ashes with an excess CaO content had a high strength in their early days, but showed excessive volume expansion with the effect of ASR, and their strength decreased in later periods due to the free state of CaO. In CPBs used in TFA, despite the high CaO content, the expected high strength in the early period was not seen. This is due to the high amount of SOin TFA content.
The results of samples giving the highest UCS results in 11% binder rate according to a different type of fly ash are given in Fig.8.
Fig.8 Fly ash ratios giving the optimum strength according to the curing time in 11% binder
Fig.8 shows that with the increase in the amount of cement, there are also significant increases in strength. According to the 28-day curing period, it was determined that paste filling mixtures at all substitution rates of fly ashes other than CFA can be used for fortification (≥4 MPa) purposes. Also, 20% CFA substitution provided the most efficient pozzolanic activity according to the 90-day cure period. Here, a high amount of SiOin the content of CFA reacts in mixtures for long periods, which causes the pozzolanic effect to be better. In the SFA substitution, it was determined that strength had fallen after 56 days. A similar situation occurred for 30% YFA substitution with a lower strength of 56 and 90 days. Also, 20% TFA showed the best performance at 90-day strength. 30% YFA and 20% TFA substitution rates worked best in the same substitution rates in 11% binder as in 9% binder. In addition, according to strength values, 30% YFA increased in the range of about 1 MPa and 20% TFA in the range of 3-5 MPa depending on the increase in the amount of cement. Therefore, it was determined that when CPB samples had the appropriate fly ash/cement ratio, strength values were improved depending on their chemical content. However, with the increase in the amount of cement, the use of CFA and TFA has been shown to activate the pozzolanic properties of the material better.
Hassaniet al
(2007) stated that portland cement alone could not be enough to ensure the long-term stability of CPB mixtures made up of high sulfurcontaining tailings, while CPB samples with Class C fly ash gave better strength results than cement. This is especially clear in the TFA fly ash substitute. In addition, the use of class F fly ash with a low CaO rate, such as CFA, is often seen to reduce early strengths. However, it was determined that the high amounts of SiO, AlO, and FeOin CFA content increased the pozzolanic reaction in the following periods and that the mixtures created using CFA with a sufficient amount of cement gave better strength results than reference designs. Eker (2019) conducted a study with a copper mine tailing containing 33% sulfur and stated that the use of 40% F-type fly ash instead of cement increased the long-term strength of CPB samples. Gorakhki and Bareither (2018) studied three separate mining (copper, soda ash, and garnet) tailings, and stated that the chemical content of three different fly ash used instead of cement has a direct effect on the strength of the CPB material and that the strength of the CPB material increases in direct proportion with the increase in the CaO/SiOratio of the fly ash. In addition, Yilmaz (2018) studied Class C fly ash and slag and determined that the chemical content of slag and fly ash used instead of cement acts on the strength of CPB samples and therefore the additives used as substitutes for PC in the CPB mixture must be in the optimal chemical composition to ensure the desired strength. The fact that the fly ashes of different chemical composition in this study differ in their CPB strength supports this situation. Each of the backfill components affects the short, medium, and long-term stability of the paste backfill. Especially as the amount of cement increases, the strength of the paste backfill is expected to increase. But in the face of sulfate attacks caused by the ratio of sulfur in the tailings, it is also necessary to use additives that are resistant to sulfate, such as fly ashes of different types of chemical composition.In this study, compared to CPB samples with 9% and 11% binders (cement+fly ash) in general, it was determined that their strength at different curing times showed significant variability due to different chemical compositions of the same class of fly ash. In case of sulfate attacks caused by sulfur-weighted tailings used as fillers in mines, it can be stated that fly ashes can be used together with cement, especially taking into account CaO/SiOand SOvalues. As the amount of cement gradually decreases inversely proportional to the increase in fly ash substitution, the amount of CaO in paste backfill decreases, while SOamount decreases in YFA and CFA and increases in SFA and TFA. It was determined that the long-term expected pozzolanic effect against sulfate attacks was achieved in CPB samples with 9% binder and TFA substitution, as well as, 11% binder with TFA and YFA substitution. Therefore, it is understood that the proportion of cement and fly ash, which will be added instead of cement, directly affects the duration of CPB strengths.
Considering the ratio of fly ash in the total mass, it is understood that using the right type and proportion of fly ash in paste backfill mixtures will lead to a significant reduction in cement consumption and therefore costs. However, it has been revealed that Pb-Zn tailings in the work site can be used as cemented paste backfill with fly ash doped in accordance with the physical and chemical properties used in underground work for fortification purposes. It is thought that the usability of different pozzolanic materials in the paste backfill made with Pb-Zn tailings should be investigated in subsequent studies and that alternative mixture ratios may contribute to sustainable mining.
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