*2.2. Methods*

#### 2.2.1. Methods for Raw Materials

The mineralogical and textural characteristics of the used raw materials were examined in polished thin sections with a polarizing microscope according to EN-932-3 [38] standard for petrographic description of aggregates. Thin sections were prepared to study the mineralogical composition and textural characteristics of the studied materials. The thin sections were examined under a petrographic microscope (Leitz Ortholux II POL-BK Ltd., Midland, ON, Canada) for mean grain size and grain shape. The bulk mineral composition of the studied samples was also determined by X-ray Diffraction (XRD), using a Bruker D8 advance diffractometer, with Ni-filtered CuKα radiation. Random powder mounts were prepared by gently pressing the powder into the cavity holder. The scanning area for bulk mineralogy of specimens covered the 2θ interval 2–70◦, with a scanning angle step size of 0.015◦ and a time step of 0.1 s. The mineral phases were determined using the DIFFRACplus EVA 12® software (Bruker-AXS, Gmbtl, Karlsruhe, Germany) based on the ICDD Powder Diffraction File of PDF-2 2006. Furthermore, loss on ignition (LOI) of the studied samples was determined according to the ASTM D7348-13 standard [39]. Additionally, an XRF (X-ray Fluorescence) spectrometer and a sequential spectrometer cited at the Laboratory of Electron Microscopy and Microanalysis (University of Patras, Greece) were used for the determination of the major and trace elements of the studied zeolite. An amount of 0.8 g of dried ground sample was mixed with 0.2 g of wax (acting as binder) and was pressed to a pellet under 15 tones. Pressed pellets were analyzed with a RIGAKU ZSX PRIMUS II spectrometer, which is equipped with Rh-anode. In order to examine the mineralogical and textural characteristics of the tested slags, a scanning electron microscope (JEOL JSM-6300 SEM) equipped with energy dispersive (EDS: Model: 6699, Det. Area: 10 mm2, Resol.: 138 eV) using the INCA software was used. The scanning electron microscope used is located in the Laboratory of Electron Microscopy and Microanalysis (University of Patras, Greece). Operating conditions were accelerating voltage 25 kV and beam current 3.3 nA, with a 4 μm beam diameter. The total counting time was 60 s and dead-time 40%. Synthetic oxides and natural minerals were used as standards for the analyses, where the detection limits are ~0.1% and accuracy better than 5% was obtained. Moreover, the specific gravity of the raw materials (slags) used as aggregates were calculated according to ASTM C1567 standard [40].

### 2.2.2. Methods for the Produced Concrete

Twelve concrete cylindrical specimens were made from two different types of slags and three standard concretes were made containing natural aggregates (limestones) according to ACI-211.1-91 [41]. The aggregates were crushed through standard sieves a separated into the size classes of 2.00–4.75, 4.45–9.5 and 9.5–19.1 mm. After 24 h, the samples were removed from the mold and were cured in water for 28 days. Curing temperature was 20 ± 3 ◦C. In the present study, concrete mixtures were prepared with variations in the quantitative proportions of the different types of slags as shown in Figure 1 and Table 1.

These specimens were tested in a compression testing machine. The compressive strength of concrete is calculated by the division of the value of the load at the moment of failure over the area of specimen both at 7 and 28 days. In this work, our aim was to study the process of hydration of cement at 7 and 28 days because they are the key days to come up to a reliable conclusion on a micro scale regarding the relationship of cement paste with aggregates in concrete, especially comparing different mixtures of raw materials. The compression test was elaborated according to ASTM C42/C42M-12 [42] in a compressive strength Pilot machine-Controls (Model C13C02) with maximum load 1500 kN (Figure 2a). The cylindrical specimens used for this test had 50 mm diameter and 55 mm high (Figure 2b). After the compressive strength test, the textural characteristics of concretes were examined. Polished thin sections were studied in a polarizing microscope according to ASTM C856–17 [43]. The surface texture of aggregate samples was studied by using Secondary Electron Images (SEI) according to BS 812 Part 1 [44] which outlines six qualitative categories, e.g., glassy, smooth, granular, rough, crystalline, honeycomb and porous. Furthermore, two physical properties of the produced concrete specimens were calculated, the water absorption as well as the density according to ASTM Standard C642 [45].

**Figure 1.** Results of the aggregates' concentration of concrete samples.



(**a**) (**b**)

In this stage, the examination of the concrete textural features was carried out when using polished thin sections in a petrographic microscope (ASTM C856–17) [43]. A 3D depiction of the petrographic characteristics of the concrete as well as of the studied slags was carried out by the 3D Builder software using thin sections.
