**2. Materials and Methods**

FA used in this work was from "Nikola Tesla B" power plant (Serbia). The sFCCC sample was obtained from a local petrol refinery. Portland cement CEM I 52.5 N (CEM, Našice cement, Nexe d.d., Našice, Croatia) was used as a minor component of the new binder.

As-received FA and sFCCC were mechanically activated in a planetary ball mill (Fritsch Pulverisette 05 102, Fritsch GmbH, Idar-Oberstein, Germany). The mechanical activation of FA was conducted in stainless steel bowls (500 cm<sup>3</sup> in volume) using 13 mm diameter steel balls, with a 1:3 FA to balls ratio, at 380 rpm and lasting for 15 min. The sFCCC sample was ground for 20 min at 200 rpm in corundum grinding bowls (500 cm3), and the sFCCC to corundum grinding balls (5 mm in diameter) mass ratio was 1:3.

Particle size distribution (PSD) of the materials was analyzed using Mastersizer 2000 (Malvern Panalytical, Malvern, UK).

The chemical composition of the binder components was determined by energy dispersive X-ray fluorescence spectrometer ED2000 (Oxford Instruments, Abingdon, UK).

X-ray diffraction analysis (XRD, Rigaku Smart Lab, Rigaku, Tokyo, Japan), with Cu anticathode operating at 40 kV and 30 mA, was used to determine mineral composition of the starting materials and the new binder pastes. The XRD analyses were conducted in the 5–55 ◦2*θ* range, with 0.01◦ step and 2◦/min recording speed.

As-received and grinded FA and sFCCC, as well as resulting binder samples, were examined using a scanning electron microscope (SEM, VEGA TS 5130 MM, Tescan, Brno, Czech Republic) equipped with a backscattered detector (BSE, Tescan). Before the analyses, the samples were Au-coated.

The new binders were prepared by dry mixing the waste materials and CEM in 70:30 mass ratios for 5 min. Paste samples were made by mixing the binder with water (Table 1), provided that the standard consistency was achieved [9]. The setting time of the pastes was determined following the recommendations given in EN 196-3 [9].


**Table 1.** Binder samples denotation and composition.

Binder mortars were prepared by mixing the binder with water and standard sand [10]. Water to binder (w/b) ratios of the new binder mortars were aimed at providing workability similar to the workability of the CEM mortar prepared with w/b = 0.50 [10,11]. Therefore, the w/b ratio of the mortar based only on FA (binder FA70) was 0.57 and the w/b ratio of the mortar prepared with both of the waste materials (FCCC21) was 0.58.

Paste and mortar samples were cured in a humid chamber (relative humidity ~90%, temperature 20 ± 2 ◦C) until testing.

The compressive strength of the mortars was determined using Matest testing machine E161 (Matest, Treviolo, Italy) [9].

#### **3. Results and Discussion**

*3.1. Characterization of the Binder Components*

3.1.1. Effects of the Mechanical Activation on Morphology and Particle Size Distribution of FA and sFCCC

The as-received FA sample consisted mainly of large particles (~100 μm), irregular in shape (Figure 1a). The grinding of the FA sample in the planetary ball mill for only 15 min provided a material with fine particles, mostly smaller than 10 μm (Figure 1b).

**Figure 1.** SEM micrographs: (**a**) as-received FA; (**b**) ground FA.

The sFCCC sample obtained from the local petrol refinery contained spherical particles, ~20 μm in diameter, and smaller irregular particles (Figure 2a). After the mechanical activation of the as-received sFCCC sample, most of the spherical particles were broken. However, SEM analysis of the ground sFCCC sample showed the presence of agglomerates (Figure 2b).

**Figure 2.** SEM micrographs: (**a**) as-received sFCCC; (**b**) ground sFCCC.

Particle size distribution analyses of the as-received and ground FA and sFCCC demonstrated that mechanical activation, for only 15 min, resulted in a substantial decrease in particle sizes of both of the materials (Figure 3a,b). It has already been demonstrated that mechanical activation increases the reactivity of these waste materials [12–15]. The fact that the PSD of the mechanically activated FA and sFCCC was almost similar to the PSD of CEM (Figure 3c) could also have a positive impact on the mixing of the powders when preparing the binder [16].

**Figure 3.** Particle size distribution: (**a**) as-received and ground FA; (**b**) as-received and ground sFCCC; (**c**) binder components (cumulative curve).

#### 3.1.2. Chemical and Mineral Composition of the Binder Components

Table 2 shows the chemical composition of the materials used for the preparation of the new binder (the CEM sample and the ground FA and sFCCC samples, hereinafter referred to as FA and sFCCC). The FA produced by Serbian power plants originates from burning lignite and usually belongs to class F [17,18]. The FA sample used in this work consisted mostly of SiO2 and Al2O3, with a relatively high content of CaO (Table 2). The sFCCC sample was rich in Al2O3 and contained approximately equal amounts of SiO2 and Al2O3 (~42 mass %), while the chemical composition of the CEM sample was typical for the material.


**Table 2.** Chemical composition of the starting materials.

XRD analyses also showed that the mineral composition of the FA, sFCCC and CEM samples was typical for these materials (Figure 4). Clinker minerals and anhydrite were detected in the CEM sample, while quartz peaks were the most prominent in the XRD graph of the FA sample. Crystalline zeolite faujasite was detected in the sFCCC sample. The presence of the amorphous phases in the both of the waste materials was indicated by the shape of the baselines in the 15–35 ◦2*θ* range (Figure 4).

**Figure 4.** Mineral composition of the binder components.
