1. Introduction
Construction and demolition wastes (CDWs) comprise the largest waste fraction in industrialized countries. They represent one third of the total waste volume generated by industrial activities, which in the EU-28 is about 3 billion tons per year [
1]. CDWs derive from construction and demolition activities and mainly contain concrete, masonry, asphalt and metals [
2]. It is estimated that approximately 54% of CDW is comprised of ceramic materials (i.e., bricks and tiles), while approximately 12% is concrete [
3]. Their disposal requires large areas and often causes severe impacts to the environment [
1,
4]. On the other hand, the energy requirements for the production of concrete are almost 5% of the global energy consumption. Moreover, the production of 1 ton of cement results in emissions of approximately 1 ton of carbon dioxide (CO
2), while aggregate production also poses severe problems to surrounding areas [
2,
5,
6].
In recent years, a large number of studies pertinent to the recycling of CDW and the production of eco-friendly concrete have been carried out. A promising alternative for the management of CDW appears to be alkali-activation for the production of secondary materials, often called inorganic polymers (IPs) or geopolymers. These materials exhibit advanced physico-chemical properties, including good fire and chemical resistance, and can be used in the construction sector [
4,
7,
8]. IPs are synthesized by using various wastes and alkaline solutions [
6,
7,
9,
10,
11]. The alkaline solutions, mainly sodium or potassium hydroxide and silicate solutions, act as activators to initiate polymerization of Si and Al present in the raw materials and for the formation of Si–O–Al–O bonds [
12]. Raw materials rich in silica and alumina, including bricks, ceramic waste, metakaolin, fly ash, kaolin and slag can be used for the production of IPs [
2,
12]. Apart from the construction sector, the produced IPs may be used for the immobilization of hazardous elements or as adsorbents in various industrial processes [
13,
14,
15,
16,
17]. The properties of the produced IPs may be determined by the use of various analytical techniques [
3,
18].
Alkaline activation has been recently attracted attention as a promising option for the management of CDW and the production of IPs with beneficial mechanical, thermal and physico-chemical properties [
19,
20]. Apart from the use of single CDW constituents, other waste streams, such as slags, leaching residues and sediments may be used in order to obtain optimum raw material mixtures and IPs with tailored properties [
15,
20,
21,
22]. The degree of reactivity of each waste stream differs and depends on the type and content of the contained aluminosilicates. More details on the reactivity of a wide range of natural Al–Si minerals that could serve as potential source materials for the synthesis of geopolymers can be found in an excellent earlier study [
23].
Slag is a brittle material with varying grain size (0.075–4 mm). Large amounts of slags are produced worldwide as by-products of steel and non-ferrous metal production. Part of the produced slag, namely ground granulated blast-furnace slag (GGBFS), is used in the cement industry, while the remaining quantities are disposed of in various sites. Improper disposal practices may result in solubilization of the contained hazardous elements and cause damage to environmental receptors [
24,
25]. Valorization of metallurgical slags through alkali-activation appears to be a viable management option [
20,
26]. The role of iron during alkali activation of iron-rich raw materials for the production of geopolymers has been investigated in previous studies and the results are often contradictory. It has been indicated that possible incorporation of ferric iron in the tetrahedral network of geopolymers may result in products with increased strength. In such systems, the behavior or iron can be elucidated through
57Fe Mössbauer spectroscopy and/or
in-situ X-ray total scattering and subsequent pair distribution function analysis. The results of these investigations may also predict the long-term behavior of the produced geopolymers [
9,
27,
28,
29].
The co-valorization of different waste streams is in line with the principles of industrial symbiosis and circular economy [
30,
31]. Thus, this research study aims to fully investigate the optimum alkali-activation synthesis conditions and the associated properties of IPs produced using brick wastes and metallurgical slag as raw materials. In this context, several factors have been studied, including oxide molar ratios in the initial paste, and curing and ageing conditions.
2. Materials and Methods
Brick wastes, clean of other contaminants, such as concrete, collected from demolished buildings (Chania, Crete, Greece), and metallurgical slag (S), produced during ferronickel production at the ‘‘LARCO S.A” plant (Larymna, Central Greece), were used for the production of IPs. Brick waste was pulverized using a Sepor type rod mill (5΄΄ Series, Sepor, Los Angeles, CA, USA), while metallurgical slag was with the use of a Bico type pulverizer (Type UA, Fritsch, Dresden, Germany). Chemical composition in the form of oxides was determined using an X-ray fluorescence energy dispersive spectrometer (XRF-EDS) (Bruker AXS (D8 Advance type), Bruker, Karlsruhe, Germany), Bruker-AXS S2 Range type (Bruker, Karlsruhe, Germany). Loss on ignition (LOI) was determined by heating the materials at 1050 °C for 4 h and particle size analysis of the raw materials was determined by a laser particle size analyzer (Mastersizer S, Malvern Instruments, Malvern, UK). The chemical composition and particle size distribution of the raw materials used are presented in
Table 1.
It is observable from this table that both raw materials contain high amounts of SiO
2 (59.1 and 32.7 wt % for brick and slag, respectively) and Al
2O
3 (10.2 and 3.7 wt % for brick and slag, respectively) which indicates their potential for alkali activation. Furthermore, the CaO content in bricks is much higher compared to slag (17.8 and 3.7 wt %, respectively), while the Fe
2O
3 content in slag (43.8 wt %) is almost six times higher than the respective content in brick waste (7.4 wt %). The main mineralogical phases present in brick waste are quartz (SiO
2) and calcite (CaCO
3), while slag is mostly dominated by iron oxides and quartz [
9,
20].
The mixing proportions used for the synthesis of IPs are shown in
Table 2. The activating solution used consisted of sodium hydroxide solution, 6–10 mol L
−1 (M), produced by dissolving the required amount of NaOH anhydrous pellets in distilled water and sodium silicate solution (Na
2O = 7.5–8.5 wt %, SiO
2 = 25.5–28.5 wt %). Sodium hydroxide was selected over potassium hydroxide solution, since the IPs produced during preliminary tests obtained better mechanical properties. It has been shown in earlier studies carried out in our laboratory that the use of NaOH solution causes higher Si and Al dissolution from similar raw materials [
20]. The effect of different H
2O/Na
2O (15.5, 18.1 and 22.0) and SiO
2/Na
2O molar ratios (1.0, 1.2 and 1.4) in the activating solution was also investigated and is shown in
Table 2.
First, raw materials were mixed with the activating solution in a laboratory mixer for 15 min so that a homogeneous paste was obtained. Then, the paste was cast in cubic steel molds 5 × 5 × 5 cm
3. The molds were vibrated for 5 min to eliminate the presence of air voids within the reactive paste and then remained at room temperature for either 6 h (brick-based specimens) or 24 h (brick-slag specimens) to enable hardening of the paste, which mainly depends on the amount of water present, the particle size of the raw materials and the strength of the activating solution. Specimens were then demolded, sealed in plastic bags to avoid evaporation of water and heated at 60, 80 and 90 °C for 24 h in an oven (ON-02G). After an ageing period of 7 or 28 days, the compressive strength of the produced IPs was determined using a Matest type compression and flexural machine (C123N, Matest S.p.A, Treviolo, Bergamo, Italy) with dual range 500/15 kN. The experimental conditions used were based on previous studies carried out in the laboratory for the production of IPs with the use of various industrial wastes [
13,
32]. In each test, the solid to liquid (S/L) ratio was slightly modified so that a paste with proper flowability was produced; solids include the raw materials while liquids include the activating solution (NaOH solution of a given molarity and Na
2SiO
3 solution). Tests and measurements were carried out in triplicate and mean values are given in the following tables and figures.
The produced IPs were characterized using analytical techniques such as X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), mercury intrusion porosimetry (MIP), thermogravimetric (TG) analysis and scanning electron microscopy (SEM). XRD analysis was performed using an X-ray diffractometer (Bruker AXS, D8-Advance, Bruker, Karlsruhe, Germany) with a Cu tube and a scanning range from 4° to 70° 2theta (θ), with a step of 0.02°, and 0.2 s/step measuring time. Qualitative analysis was carried out using the DiffracPlus Software (EVA v. 2006\, Bruker, Karlsruhe, Germany) and the PDF database. FTIR analysis was carried out on KBr pellets with a PerkinElmer 1000 spectrometer (PerkinElmer, Akron, OH, USA), in the spectra range of 400 to 4000 cm
−1. Pellets were produced by mixing each sample with KBr at a ratio 1:100 wt %. The porosity (%) of selected specimens was determined using a Micromeritics AutoPore 9400 porosimeter (Micromeritics, Atlanta, GA, USA). Water absorption of selected specimens was determined according to EN 13755 [
33]. Thermogravimetric analysis was performed up to a maximum temperature of 900 °C, using a heating rate 10 °C/min and a nitrogen atmosphere with a 100 mL/min flow rate. Scanning electron microscopy (SEM) was carried out to evaluate the morphology of the raw materials and the IPs produced. Specimens were evaluated under low-vacuum mode using a JEOL-6380LV scanning microscope (JEOL Ltd., Tokyo, Japan) equipped with an Oxford INCA energy dispersive spectroscopy (EDS) microanalysis system (Oxford Instruments, Abingdon, UK) for elemental analysis.
IPs produced under the optimum synthesis conditions were then subjected to firing at 400, 600 and 800 °C for 1 h, using a laboratory furnace (N-8L Selecta) and also immersed in various solutions, namely distilled water and acidic solutions (1 M HCl and 1 M H2SO4) for a period of 7 and 30 days, to assess their durability in different environments. Compressive strength, weight loss and volumetric shrinkage of the IPs were also determined.
4. Conclusions
The present paper shows that brick waste and metallurgical slag can be successfully alkali-activated for the production of IPs with high mechanical strength and good durability performance.
Brick-slag IPs obtained the highest compressive strength (48.9 MPa) when the initial mixture containing equal quantities of each raw material was alkali-activated with a H2O/Na2O molar ratio in activating solution equal to 18.1, cured at 90 °C and aged for 7 days. Regarding brick-based IPs, the highest compressive strength (43.4 MPa) was also obtained under the same conditions.
High temperature firing (800 °C) of the produced IPs caused decrease in their compressive strength by 33.5% to 32.5 MPa; however, this value is still considered quite high. The most noticeable effect of acidic attack on IPs was shown after their immersion in 1 M HCl solution for 30 days, as the compressive strength decreased by 34.4% to 32.1 MPa. On the other hand, specimens immersed in distilled water after 30 days exhibited much higher compressive strength; i.e., 42.2 MPa.
Finally, based on the experimental results, it is deduced that alkali activation is an advantageous process for the co-valorization of large volumes of CDW and metallurgical slags and the production of sustainable building materials or binders with good mechanical properties and durability. However, research efforts should take always into account the high variation in chemical and mineralogical composition of each raw materials’ stream that affects their reactivity in alkali-activated systems, thus the quality of the final products.