Environmental-friendly mortar produced with treated and untreated coal wastes as cement replacement materials

This study aims to examine the influence of untreated coal waste (UCW) and treated coal waste (TCW) as supplementary cementitious materials (SCMs) on the environmental, mechanical, durability, and microstructural characteristics of mortar mixes. UCW preparation procedure consists of sequential steps of crushing and grinding. Afterward, UCW is thermally activated through incinerating at 750 °C to be promoted to TCW. Experimental work includes mixing mortar mixtures by partially replacing cement with the coal waste binders (UCW and TCW) at different incorporation levels of 4, 8, 12, and 16% of cement weight. Toxicity characteristic leaching procedure (TCLP) test was applied to investigate the environmental impacts of coal wastes. TCLP test results pointed out that heavy metals including manganese, cadmium, lead, and chromium could successfully entrap in the cement matrix. The compressive and flexural strengths as mechanical characteristics of mortar mixtures were determined at 3, 7, 28, 90, and 180 curing days. Moreover, the mortar specimens were immersed in 3% sulfuric acid (H2SO4) for 60 and 150 days. Durability results showed that the H2SO4 attack resistance of binary cement mortars containing 4% coal waste binders performed better than the plain mortar. Based on the scanning electron microscopy (SEM) images, ettringite was found as the main hydration product of binary cement after 28 days; however, the existence of calcium silicate hydrate (CSH) and calcium hydroxide (Ca(OH)2) in the cement matrix after 90 days explains the more compact microstructure attained by using coal waste as cement replacement materials compared to control mixtures.


Environmental impacts of coal waste
Nowadays, the production of waste materials is increasing, threatening human health and the environment around the world. Industry and technology development and the world population growth accelerated global solid waste materials (Karimipour et al. 2020). Coal as a fundamental energy asset is anticipated to account for half of the worldwide energy utilization in 2030 (Modarres et al. 2018). Based on the US energy information Administration (EIA), five countries, including the USA, Russia, Australia, China, and India, hold about 75% of the world's proved coal reserves. The world produces over 7.4 billion tons of coal per year, of which about 15-20% is stockpiled as coal waste (Mejia-Ballesteros et al. 2019). Coal waste is a solid by-product and is generally made out of clay minerals and carbon (Frías et al. 2012). Coal mining waste has become a severe ecological and environmental concern (Karimipour et al. 2020), especially acid mine drainage.
Acid mine drainage (AMD) runoff is the most dangerous water contamination of coal mines. AMD comprises a substantial quantity of iron, SO 4 2− , and various quantities of poisonous heavy metals such as arsenate and copper with a toxic structure that contaminates water (Adhikari and Mal 2021). Indeed, the problem of coal waste pollution is caused by pyrite oxidation. In essence, the oxidation of pyrite is the main reason for acidic water through pyretic exposure. Pyrite oxide and AMD runoff were observed in the surrounding water samples exposed to coal wastes (Hesami et al. 2016). The impact of coal waste on the surrounding environment is destructive because it forms deforestation, occupation of land, air pollution, and seepage of noxious waste into groundwater.
A study conducted by Ardejani et al. (2011) on coal waste landfills revealed physical changes in the surface water (Doulati Ardejani et al. 2010). Also, the leaching test findings indicated the concentration of noxious heavy metals. Furthermore, the SO 4 concentration was higher than the allowable limit. It should be mentioned that AMD sludge has also been reported neighboring water of coal mines. Besides, coal waste comprises hazardous heavy metals, including chromium (Cr), cadmium (Cd), lead (Pb), nickel (Ni), zinc (Zn), copper (Cu), and manganese (Mn). As an example of the toxic impact of these heavy metals, Mn has a neurotoxicological effect on human health, particularly on embryos and neonates (Adhikari and Mal 2021).

Supplementary cementitious materials (SCMs) as a sustainable solution
The composition of greenhouse gases consists of 81% carbon dioxide (CO 2 ), 10% methane, 7% nitrous oxide, and 2% fluorinated gases (USEPA 2018). Globally, 35 billion metric tons of CO 2 was released in 2020 and is projected to increase to 43 billion metric tons by 2050 (Statista 2021). Thus, CO 2 has a significant impact on global warming. The production of cement consumes energy extensively and is solely responsible for 7% of the overall atmospheric CO 2 emissions (Afrakoti et al. 2020), resulted from 1.6 billion tons of cement production worldwide (Afrakoti et al. 2020). Supplementary cementitious materials (SCMs) are known as a sustainable solution that can be replaced partially with cement (Hesami et al. 2016). Partially replacing cement with SCMs reduces environmental pollution, decreases the cost of cementitious products, and consumes by-products or wastes; therefore, it helps progress toward sustainable construction (Sakir et al. 2020). SCMs are usually incorporated with clinker or as a partial cement substitute. In addition, they can simply be combined with clinker. The application of these materials can result in environmentally friendly, high strength, and durable mixes.
Million tons of by-products is stockpiled counted as solid waste materials. Using these waste materials as a partial replacement of binder in concrete is an economical, ecological, and green remedy to save natural resources (Li et al. 2020b). Previous studies have examined the cement substitution by SCMs, such as fly ash (Abdel-Gawwad et al. 2021), rice husk ash (Moayedi et al. 2019), waste ceramic dust (Kočí et al. 2016), coal waste (Afrakoti et al. 2020), and other human-made wastes materials (Gouran et al. 2021). It is worth mentioning that the particle size of SCMs has a direct impact on the mechanical properties of hardened mortar. For instance, the mortar incorporated with 8% ultrafine fly ash (3.4 µm) as a cement substitution delivered 23% higher compressive strength than the control mortar and revealed a higher strength up to 15% cement substitution (Sakir et al. 2020).

Literature review of coal waste
Coal waste has previously been utilized as coarse and fine aggregates in concrete with fibers (Karimipour et al. 2020) and without fibers (Karimaei et al. 2020), as well as the partial replacement of cement in various applications, including soil stabilization (Afrakoti et al. 2020), road subgrade (Shirin et al. 2020), asphalt pavements (Ameli et al. 2020), concrete blocks (Dos Santos et al. 2013), fired bricks (Taha et al. 2018, concrete pavements (Shamsaei et al. 2019), and mortar mixtures (Frías et al. 2012). Considering previous researches, using untreated coal waste in soil stabilization enhanced mechanical properties over time (Afrakoti et al. 2020). Frías et al. (2012) used the incinerated coal waste (i.e., treated coal waste) at 10% and 20% cement substitution in mortar mixes (Frías et al. 2012). Their research showed that the mortar mixture containing 10% treated coal waste improved compressive strength negligibly after 7 days of curing in comparison with the plain specimen; conversely, at 28 and 90 days, an average lower strength of 10% and 11% was observed, respectively. Additionally, this by-product was investigated by Hesami et al. (2016) in roller compacted concrete pavement as partial replacement of cement at incorporation levels of 5, 10, and 20%. They reported specimens with 5% untreated coal waste (i.e., raw coal waste) outperformed compressive strength in comparison with the plain sample after 7, 28, and 90 days of curing. Contrary, Shamsaei et al. (2019) reported a reduction in mechanical properties of mixes with untreated coal waste as SCM up to 5% replacement level.
The notion of applying treated and untreated coal waste materials as SCMs in the cement industry is still a novel research area and needs more studies to predict the environmental and technical performance of asphalt or concrete mixtures using coal waste. According to the literature, there is a research gap in understanding the performance of this new SCM at early and late ages, specifically determining an environmentally sustainable and durable optimum content to assure its construction benefits. Therefore, in this study, experimentally using coal waste binders as cement replacement materials were studied in order to produce a sustainable and environmentally friendly mortar without sacrificing durability and mechanical properties.
In this study, coal waste materials were gathered from the Zarand coal washing plants located in the Kerman province, southeast of Iran. Currently, more than 6 million tons of coal waste materials is piled up in the dumping areas and the sedimentary basins near the coal washing plant. Annually, the amounts of the added wastes are projected to be more than 400,000 tons (Yaghubpour and Hakkakzadeh 2010), and the coal waste volume is exponentially rising. The current experimental research was conducted with consideration to the environmental impact evaluation, mechanical properties (compressive and flexural strengths) of mortar mixtures during curing time, 3, 7, 28, 90, and 180 days, durability examination of 60-day and 150-day mortar mixtures, and microstructural analysis of 28-and 90-day mixtures to assure the feasibility of using untreated and treated coal waste in construction projects. Ultimately, the goal of this research is to produce a green mortar, considering the mechanical and durability improvement of mortar mixtures through using human-made coal waste.

Experimental procedure
The experimental process of this research is illustrated in Fig. 1. The main laboratory tests of this research have focused on (1) environmental impact assessment to evaluate the amount of heavy metals on the mortar leakage, (2) mechanical properties (i.e., compressive and flexural strengths) to test hardened mortars, (3) durability property of mortars containing coal waste materials to examine their resistance when exposed to the sulfuric acid solution, (4) microstructure analysis to evaluate the effect of coal waste binders on cement hydration.

Materials
The fine aggregate (sand) properties were chosen corresponding to the ASTM C778, and its physical properties are listed in Table 1. In addition, the sand gradation is displayed in Fig. 2. Type II Portland cement was utilized compatible with ASTM C150. Table 2 represents the chemical and physical properties of the studied cementitious binders. The chemical composition was determined with respect to ASTM E1621 by the X-ray fluorescence spectrometry (XRF) method. The particle size distributions of cement and the SCMs are plotted in Fig. 3. The particle sizes of UCW were roughly coarser than cement particles. In contrast, TCW had finer particles compared to cement, which supports pozzolanic reactivity, particularly lead to a significant enrichment in the pozzolanic reactivity (Tironi et al. 2013).

Activating process of treated coal waste
The weight loss of a substance such as rice husk ash after burning at high temperatures up to 1000 °C is described as loss on ignition (LOI) (Hesami et al. 2016). According to Table 2, the LOI of untreated coal waste was 37.6%, which did not meet the boundaries indicated in ASTM C618. In this study, to decrease the LOI level, coal waste was ignited at various temperatures of 450, 750, 950, and 1200 °C for 2 h. Afterward, LOI test was applied for the ignited coal waste powders. Since the LOI content became steady at temperatures above 750 °C, therefore, this temperature was designated as the activated temperature for coal waste.

Pozzolanic activity analysis of SCMs
The Chapelle test is a straightforward method to analyze pozzolanic activity based on NF P 18-513. In this approach, for a pozzolanic material, a minimum amount of 700 mg calcium hydroxide per gram of a pozzolan is required to be consumed (Ferraz et al. 2014). The results showed that the UCW could not meet the requirement and have a lower value in comparison with the allowable limit. Although the raw coal waste (untreated coal waste) could not pass the requirement to be considered as pozzolan since it showed low reactivity, it contributed to the cement hydration process as a filler. Considering the Chapelle test results, TCW consumed more calcium hydroxide values than a standard

Mix design and specimen preparation of mortar samples
Cement pastes and mortar mixtures were prepared according to ASTM C305. Table 3 presents the replacement proportions of cement with UCW and TCW. A total of nine mixtures were designed and tested, including control mixtures. The substitution dosages of cement with UCW and TCW were obtained as 4, 8, 12, and 16%. Mortar prisms were prepared by a sand/binder ratio of 3/1 and a water/binder ratio of 0.5. At least three identical samples were produced in each batch to test, and the average value was reported. For mechanical tests, cubic and prism samples were used. The compressive strengths were assessed by casting mortar in 135 cubes of 50 × 50 × 50 mm 3 , then cured at 20 ± 1 °C, and finally stored at 95% relative humidity in a standard curing box for 24 h. After 24 h, mortar specimens were removed from the molds and cured in saturated limewater at 20 ± 2 °C until specified testing times of 3, 7, 28, 90, and 180 days. Similarly, the flexural strengths were evaluated by producing 135 prisms of 40 × 40 × 160 mm 3 cured for 3, 7, 28, 90, and 180 days. After performing the mechanical tests, a part of the mortar sample was immersed into ethanol to end the hydration of cement composites for scanning electron microscope (SEM) tests. SEM is a qualitative technique for microstructure analysis. For the durability test, 54 cubic specimens of 50 × 50 × 50 mm 3 were prepared. After 28 days of normal curing, the samples were fully soaked in 3% H 2 SO 4 (pH = 0.6) solutions at a constant temperature of 20 ± 2 °C to simulate erosion damage until the exposure ages of 60 and 150 days. It should be mentioned that the pH of the solution was monitored daily, and if needed, the periodical addition of concentrated acid to each batch was performed to keep the pH at 0.6.

Toxicity characteristics leaching procedure (TCLP) test
The toxicity characteristics leaching procedure (TCLP) test is a standard technique developed by the US environmental protection agency so as to indicate the ability of waste materials to leach heavy metals into groundwater (USEPA 1992). In this method, the buffer of 1 M sodium acetate was used as an extraction fluid, and the pH was held at 4.99. In a glass bottle, a 20:1 solid-ratio extraction solution was taken and mounted in a rotary shaker at 30 ± 2 rpm at 25 °C for 18 h. Afterward, using Whatman No. 42-grade filter paper,   the solutions were filtered and then analyzed. A substance is known as a hazardous material on the basis of the TCLP test if any found metals are detected at concentrations above 100 times the drinking water standard (Disfani et al. 2012). For instance, the allowable limit of Mn in drinking water is 0.1 mg/L (Adhikari and Mal 2021), which is acceptable for non-hazardous materials to be up to 10 mg/L.

Mechanical tests
In this study, mechanical properties (compressive and flexural strengths) of cement mortar incorporating UCW and TCW were reported at ages of 3, 7, 28, 90, and 180 days in accordance with ASTM C109 and ASTM C348, respectively. The tests were accomplished using an automatic electronic universal testing machine. Concerning ASTM C109, the loading rate must be between 0.9 and 1.8 kN/s. Therefore, a loading rate of 1.2 kN/s was employed for the compressive test. Considering ASTM C348, the loading rate of 2.64 ± 0.11 kN/s must be applied. The flexural strength test was carried out with a loading rate of 2.7 kN/s.

Sulfuric acid resistance test
To evaluate the durability of mortar samples, the resistance to sulfuric acid (H 2 SO 4 ) attack test was conducted consistent with ASTM C267. The examined parameters were the residuals of compressive strength and mass of the samples after exposing to sulfuric acid. Previous results showed that the concentration of 3% sulfuric acid was selected to model the aggressive atmosphere of the sewer structure (Abdel-Gawwad et al. 2021). The residual mass and the compressive strength were recorded after 60 and 150 days of acid solution exposure.

Scanning electron microscopy (SEM) test
SEM images were employed to discuss the microstructure of hardened mortar specimens containing waste cementitious materials (i.e., UCW and TCW) at 28 and 90 days. To take SEM micrographs, an LEO 1400 microscope was used, as shown in Fig. 4. For the preparation of SEM specimens, 10 × 10 × 3 mm 3 dried paste was prepared and used.

TCLP test results
To assure that using coal waste in mortar is environmentally friendly, firstly, its environmental impact should be controlled before investigating the technical properties. Noteworthy, TCLP is employed recently to estimate the heavy metal concentration of coal mining dumps (Taha et al. 2018). In this investigation, the TCLP test was utilized to examine the environmental effect of coal waste binders (i.e., UCW and TCW) before and after use in the mortar mixtures. Figure 5 shows the TCLP results of UCW (raw coal waste), TCW (ignited coal waste), and AMD (acid mine drainage sludge) of the studied coal washing plant in comparison with the allowable levels specified by EPA SW846-1311 (USEPA 1992). The concentrations of seven heavy metals, including Mn, Cd, Pb, Cu, Ni, Cr, and Zn, were measured. Based on Fig. 5, the concentration of Mn, Pb, Cr, and Cd was more than the USEPA thresholds. It is worth mentioning that the TCW leachate concentrations of heavy metals were higher in comparison with those of the UCW and AMD runoff. TCLP test was also performed for crushed hardened mortar specimens after 28 days of curing, and the results are depicted in Fig. 6. The heavy metal concentrations were all below the permissible limits for mortar mixtures. The concentration of heavy metals in mortar samples containing TCW was higher compared to those containing UCW, analogous to coal waste binder samples. It should be noted that some of the leaching concentrations of control mixes did not detect in the test due to the small amount of heavy metal.

Effect of cement matrix on entrapping the heavy metals
Toxic heavy metals of waste materials could be entrapped physically and bond chemically by the cement matrix (Khater and Ghareib 2020). Likewise, a comparison of TCLP results of the waste cementitious materials pre- (Fig. 5) and post-employing (Fig. 6) in the mortar mixes indicated that the cement hydrate products could substantially stabilize and solidify the heavy metals. According to  (Argane et al. 2016), it has been stated that the potential of the cement matrix in stabilizing a significant quantity of heavy metals can also improve the mechanical properties. Figure 7 depicts the concentration of heavy metal and the percentage of the heavy metals' removal ratio with reference to the primary content for Mn, Cd, Cr, and Pb since these heavy metals had higher concentrations than the USEPA threshold (Fig. 5). As shown in Fig. 7, the removal ratio in heavy metals concentration was mostly more than 85% (Figs. 7(a), (b), (e), (f), (g)), and (h)). The maximum solidification was observed for Cr by more than 90% removal ratio (Figs. 7(e), and (f)), and the minimum stabilization was reported for Cd with a 57% reduction in mixes containing UCW (Fig. 7(c)); however, mixes containing TCW showed a better performance in entrapping Cd by more than 65% removal rate (Fig. 7(d)).
Although TCW was detected with higher heavy metal compared to UCW (Fig. 5), the heavy metal removal of mixes with TCW performed better than mixes with UCW (Fig. 6). The analysis between the results of TCLP in the coal waste binders pre-and post-use in the mortar samples found that the cement hydrate products in the mortar mixes could considerably solidify and stabilize the UCW and TCW heavy metals and limit the secondary possible problem of using them, which is the leakage of heavy metals.

Compressive strength
Compressive strength is a major property of concrete structures for design purposes. The compressive strength of mortar mixtures at 3, 7, 28, 90, and 180 days is demonstrated in Fig. 8. As displayed in the figure, the maximum compressive strength was attained for TCW-8 containing 8% TCW, equaling to 19.2, 27.5, 34.6, 43.0, and 48.8 MPa at ages of 3, 7, 28, 90, and 180 days, respectively. Noticeably, mortar mixtures containing 4% UCW (UCW-4) and 4% TCW (TCW-4) also outperformed plain mortar at all ages, especially at the age of 180 days by about 12% and 15%, respectively. The minimum 28-day compressive strength is recommended to be equal to or more than 17.2 MPa based on ASTM C270 standard specification. Concerning this criterion, all mixes satisfied the minimum compressive strength requirement; thus, in terms of compressive strength concerns, mortar mixes containing waste cementitious materials are qualified to be used for construction purposes.
It should be notified that the test results also specified that increasing UCW content from 4 to 16% dropped the compressive strength values drastically over time. The strength drop of coal wastes in the mixtures might result from heavy metals (Frías et al. 2012) since the presence of Cr and Pb concentrations in the coal waste was more than the regulatory limits specified by the US environmental protection agency (Modarres et al. 2018).
On the other hand, the continuous strength reduction of UCW mixtures was not observed for TCW mixtures since the compressive strength presents a peak at the mixture with 8% TCW (TCW-8) because of higher strength value compared to 4% TCW and control mixture. This strength enhancement is associated with several factors, such as the ongoing progression of the pozzolanic reaction that results in compressive strength enhancement (Frías et al. 2012). Due to the fact that TCW is a pozzolanic material (see Sect. 2.1.3), the better mechanical performance of TCW-4 and TCW-8 could be correlated with it. The filler effect is another factor that can explain the betterment of the compressive strength of hardened mortar with 4% UCW compared to the plain mixture. This improvement in the mechanical properties has previously been reported for coal waste concrete mixtures (Modarress et al. 2018). In essence, SCM particles provide nucleation centers for the precipitation of hydrated products that cause a denser paste (Avila-López et al. 2015). Last but not least, the particle size of SCMs affects strength gain (Sakir et al. 2020) because finer particles can provide more nucleation sites to fill the pores to accelerate the hydration reactions (Engbert and Plank 2021). TCW has finer particles compared to UCW and cement materials (Fig. 3); thus, this might be another reason for the better performance of TCW mixes versus UCW and control mixtures.

Flexural strength
Flexural strength is considered an important parameter in designing concrete structures, such as bridges, concrete pavement, and building structures (Fakhri and Saberi 2016). The flexural strength test results at 3, 7, 28, 90, and 180 days are illustrated in Fig. 9. Based on this figure, the reported flexural strengths were between 1.53 and 6.42 MPa. As illustrated in the figure and similar to compressive strength values, TCW-8 presents the maximum flexural strength among the mixes. Furthermore, UCW-4 and TCW-4 have higher flexural strength with respect to the control mixture. Figure 9 designates that rising the degree of cement replacement Fig. 8 Compressive strength of mortar samples at different cement replacement levels and curing time of SCMs from 4 to 16% reduces flexural strengths. As an example, UCW content increases from 4 to 16%, the flexural strength dropped on average of 37% at all ages. However, similar to compressive strength, a peak was observed for mixes with TCW where flexural strength of TCW-8 was reported higher than TCW-4, about 10% at all ages. The minimum mortar flexural strength was observed in mixtures with 16% UCW (UCW-16).

Effect of coal waste materials on the toughness of hardened mortar
For the judgment of the toughness of hardened mortar, compressive/flexural strength ratio is an important factor (Wang et al. 2005). Higher compressive/flexural strength (toughness) ratio represents higher toughness of mortar (Parghi and Shahria Alam 2016). The toughness ratios of mortar mixes for various cementitious material content are displayed in Fig. 10. As shown in the figure, the toughness ratio rises with increasing the cementitious materials ratios up to 16%, which contrasts with previous studies (Wang et al. 2005). The highest toughness for UCW mixes was observed after 28 days and for TCW mixes after 180 days. This difference might be due to the pozzolanic property of TCW in comparison with UCW that delays the strength gaining for mixes with TCW. The toughness ratio of the studied mortar was between 6.8 and 9.5, which is in the range of 3.5 to7.5 (Wang et al. 2005), and 4 to 10 (Parghi and Shahria Alam 2016).

Effect of curing time on the mechanical properties
The logarithmic functions were plotted to find the connection between the curing period and compressive and flexural strengths, as displayed in Figs. 11,12,13,14. According to the figures, the logarithmic function could robustly predict the compressive and flexural strengths (R 2 > 0.95). The curves for mortar mixtures with 4% UCW (UCW-4), 4% TCW (TCW-4), and 8% TCW (TCW-8) show higher slopes than the control mix, especially for the mixtures with TCW at later curing times. This increase means that the differences between the mechanical properties of UCW-4, TCW-4, and TCW-8 samples were considerably accelerated over time compared to the control mixture. Furthermore, it could be associated with the delayed cement hydration stemmed from the pozzolanic reaction of SCMs with an exceptional surface area that reacts with calcium hydroxide (Ca(OH) 2 ) in the pore solution of cement paste to produce more hydration products (Pitarch et al. 2021).
Moreover, the mortar mixtures that incorporated treated coal waste represent higher strength when compared with the mixtures incorporating cement and untreated coal waste  For instance, the difference of compressive strength for the mixes with 4% TCW (TCW-4) was about 6% and 3% compared to the mixes with 4% UCW UCW-4) at 90 and 180 days, respectively. This difference for flexural strength was reported 7% and 6% at 90 and 180 days, respectively. Considering the literature (Engbert and Plank 2021), higher mechanical properties of mortar mixtures with UCW and TCW binders might be associated with the filler effect and nucleation sites, respectively, which accelerate the hydration reactions.

Durability investigation
Durability is one of the most critical components of concrete structures. Durable concrete resists the attack of acids and sulfates to which concrete may be exposed (Vafaei et al. 2021). Nevertheless, ordinary concrete cannot resist acid attacks for a considerable period. When a concrete structure is exposed to an acidic environment, a neutralization reaction between Ca(OH) 2 and hydrogen ion in the binders occurs (Usman and Sam 2017). Then, calcium cations are leached from the calcium-comprising elements, like calcium  (Scrivener and Young, 1997). Therefore, the alkalinity reduction of concrete matrix and increase in the total porosity lead to terminations of the hydration products that cause the concrete failure (Chen et al. 2013) and causing a significant reduction in the durability and mechanical characteristics of concrete (Scrivener and Young, 1997). Nonetheless, in the presence of pozzolanic materials in concrete, the Ca(OH) 2 amount is diminished, and the microstructure is augmented due to micro-filling and pozzolanic impacts (Usman and Sam 2017). Hence, the impermeability of acid solution that aggravates the harmful impact of acid on concrete is drastically lowered.

Residual compressive strength
After immersion in the acid solution, the relative compressive strength of the mortars was obtained regarding the identical samples cured in limewater. The residual compressive strength is illustrated in Fig. 15. According to this figure, the compressive strength for all the samples declined after 150 days. Nonetheless, the rate of strength reduction of mixes with treated coal waste binders was less than the mortars with untreated coal waste. In general, cement mortars containing TCW had higher resistance to sulfuric acid attack than the mixes with UCW and control mortars at both exposure periods. After 150 days of exposure, the compressive strength of mortar mixes without coal waste binders (control), with 4% untreated coal waste (UCW-4), and with 4% treated coal waste (TCW-4), as shown in Fig. 15, decreased to 33, 38, and 40%, respectively.
The higher resistance to the sulfuric acid attack of mortars with 4% coal waste (UCW-4 and TCW-4) compared to the control mortar can be attributed to the filler effect and pozzolanic activity of untreated and treated coal waste, respectively. Even though sulfuric acid exposure changes the microstructure of the cement matrix by developing micro-cracks (Khan et al. 2019), the formations of more hydrates reinforce the microstructure and therefore lessen the rate of acid permeability into the mortar containing pozzolan (Usman and Sam 2017). Correspondingly, the improvement in the cement matrix of mortars incorporated with pozzolanic materials such as fly ash, rice husk ash, and silica fume compared to plain mortar in the acidic environment has been reported previously (Abdel-Gawwad et al. 2021). Figure 16 represents the residual masses of plain and mortars with coal waste binders immersed in the acid solution. All mortars showed a decline in their weight after the first 60 days of contact with the acid solution and continued up to 150 days. Nonetheless, this decline varies with increasing the mortar binder. Similar to the compressive strength scenario, mortars containing the UCW binders exhibited a higher decline in mass than that one with TCW mortars over time. It is worth mentioning that the plain mortar had the highest drop in mass. In this regard, at 150 days, the residual mass of control, UCW-4, and TCW-4 mortar specimens were 45, 54, and 56%, respectively.

Residual mass
There are many reasons that can directly influence the mass drop of the samples. The first reason could be related to the partial leaching out of bulk cement paste stemmed from the dissolution action of sulfuric acid on the cement paste (Usman and Sam 2017). Second, the hydration products shaped in the cement microstructure are decomposed because of the reaction of calcium with acidic anions, leading to a rise in total porosity and accelerating acid attack (Duan et al. 2015). In fact, H 2 SO 4 -exposed mortar containing anion sulfate results in developing the formation of gypsum. Subsequently, the induction of internal decomposing stresses and surface spalling could be expected (Sata et al. 2012). In other words, the lesser formation of gypsum in the hydration system results from the reduction of calcium hydroxide related to the higher pozzolanic activity of supplementary cementitious materials (Usman and Sam 2017), leading to a reduction in mass of pastes. Also, the sum of oxide composition (Al 2 O 3 + SiO 2 + CaO) of the pozzolanic materials positively impacts the acid resistance of the hardened mortars (Vafaei et al. 2021), which is considerably higher in the chemical composition of coal waste binders since the values for cement, untreated coal waste, treated coal waste are 27.91, 52.92, and 84.74% of weight, respectively (based on Table 2). Figures 17 and 18 signify the SEM images of the hardened mortar specimens at the age of 28 and 90 days, respectively. Based on Fig. 17(a), the microstructure of the control mortar is different from the mixture containing coal waste materials. In the microstructure of the mix incorporated with 4% UCW (Fig. 17(b)), the existence of ettringite (needle-like crystals) in the microstructure was considerable after 28 days compared to the control mixture ( Fig. 17(a)). However, a spherical and denser microstructure could be seen for UCW-4 mortar mixes after 90 days of curing ( Fig. 18(b)) in comparison with the control mixture ( Fig. 18(a)) due to the presence of calcium silicate hydrate (gel-like flocks) and calcium hydrate (fibrous-like crystals). Previously, the formation of crystals of ettringite was observed in mortars containing supplementary cementitious materials as well (Li et al. 2020b), especially in the cement pastes with coal waste as a supplementary binder (Afrakoti et al. 2020). It should be noted that the enhancement of mechanical strength is attained by a considerably more compact microstructure where pore space between unreacted particles is completely filled with dense webs of ettringite crystallites resulted from SCMs (Nguyen et al. 2021). Furthermore, the improvement of CaO content, as well as the presence of a suitable proportion of calcium to pozzolanic materials, diminishes non-condensing crystals of ettringite and CH abundantly for mixes containing UCW (Afrakoti et al. 2020), as can be observed in Fig. 18(b) for UCW-4 mixture after 90 days of curing. On the other hand, the addition of UCW improves the connection between the cement paste and the sand grains, which plays an essential role in permeability, durability, and strength properties (Afrakoti et al. 2020).

Microstructural observation
As illustrated in Figs. 17(c) and (d), a considerable amount of ettringite also can be seen for TCW-4 and TCW-8 after 28 days of curing in comparison with UCW-4 ( Fig. 18(b)) and control mortar ( Fig. 17(a)). The microstructure of mortar mixes containing TCW binders is improved after 90 days (Figs. 18(c) and (d)) because of more hydration products such as calcium silicate hydrate and calcium hydrate. It has been reported that finer particles like TCW could enhance the packing density of the cement matrix by developing the number of nucleation sites, led to an increase in the hydration products (Li et al. 2020b). The higher packing density can cause better mechanical properties of concrete mixes (Filimonov et al. 2021).
Moreover, the maximum packing density can be reached by dense particle size distribution, varying the particle size ratio between the large and small particles. It is possible that replacing cement with coal waste materials of the studied mortar mixes has modified the gradation of binders in the mortar mixes and resulted in improving the packing density and cement paste performance. The SEM images of mortar mix with coal waste binders attested to the improved structure of cement paste in comparison with the mixtures without these additives. Therefore, binary mixes containing coal waste powders formed a more integrated microstructure, especially for treated coal waste. It is noteworthy that the boosted mechanical properties of mortar mixes incorporated with treated coal waste binders are aligned with the microstructural analysis.

Conclusions
The ability of the cement matrix in stabilizing heavy metals has a positive effect on the mechanical properties stated previously as well (Argane et al. 2016 ). On the other hand, other factors are involved in the betterment of mechanical test results, such as the ongoing progression of the pozzolanic reaction (Frías et al. 2012 ), and the filler effect (Modarress et al 2018). Indeed, coal waste particles provide nucleation sites for the precipitation of hydrated products (Avila-López et al. 2015). However, it was unknown whether these enhancements can improve the resistance of mortar mixes containing coal waste materials to the acid attack. Therefore, in this research, the effect of raw (untreated) coal waste and ignited (treated) coal waste as cement replacement materials in mortar, considering durability tests along with environmental, mechanical, and microstructural tests, was studied. Herein, the following conclusions can be obtained: • The environmental impact evaluation confirmed that the heavy metals were well solidified and stabilized by cement hydrate products. • Mortar mixes containing 4% UCW or TCW represented better mechanical and durability performance, including compressive strength, flexural strength, and sulfuric acid resistance. • The reduction in mass and compressive strength of mortars produced with the binary binders containing 4% UCW (or 4% TCW) were lower than the plain mortar produced with ordinary cement in response to sulfuric acid attack. Hence, coal waste binders could be considered durable and practically be used in acidic environments such as concrete sewer pipes and wastewater treatment facilities. • Considering environmental, mechanical, and durability assessment, the optimum replacement percentage of cement with coal waste binders (UCW and TCW) was established as 4%. • SEM observation provided evidence that typical hydration products including ettringite (needle-like crystals), calcium hydroxide (fibrous-like crystals), and calcium silicate hydrate (gel-like flocks) were observed in the cement matrix of mixes containing coal waste binders. The amount of ettringite in mixes containing UCW was considerable in comparison with the TCW and control mixture after 28 days. However, it was reduced after 90 days of curing.
In conclusion, utilizing untreated and treated coal waste in mortar can essentially lessen cement production. This coal waste utilization controls the damaging environmental impacts of toxic materials in this waste. Furthermore, the concept of circular economy, which means to avoid wasting natural resources, has been considered by recycling coal waste into cement replacement materials. In this regard, coal could be kept in use and the maximum value could be extracted from it. Indeed, the environmental, mechanical, durability, and microstructural advantages of the supplementary cementitious materials certainly are aligned with the ultimate goal of sustainable and cleaner production.

Conflict of interest
The authors have no conflict of interest to declare that are relevant to the content of this article.