1. Introduction
Marbles are natural stones which are formed as a result of recrystallization of limestone and dolomite under high temperature and pressure [
1]. They are used since ancient times for the construction of cultural heritage monuments, historical buildings and numerous other applications. Their economic value depends both on their morphological and structural characteristics. Worldwide, there exist various types and significant reserves of marbles which they account for about 50% of the total natural stones production [
2,
3]. Due to the processing requirements and environmental limitations, only 10% of the quarrying material is used in the market [
4]. Cutting and polishing processes in marble plants result in the production of two types of waste, namely marble waste slurry and marble waste powder. Thus far, the ornamental stone industry has not developed an efficient valorization process for these waste types, therefore they are either stored in warehouses for future use in other applications, or disposed of in landfills, causing environmental problems [
5,
6].
Marble waste can be used as replacement of aggregates or additive in concrete production [
7], as well as raw material for up to 30 wt. % substitution of red clay during the production of bricks for the improvement of their properties [
8,
9]. Thus, the higher use of marble waste in construction applications, via a technically and economically feasible technology, can result in significant economic and environmental benefits for the quarrying industry [
10].
Alkali activation is a promising technology for the valorization of various waste types and the production of secondary materials, termed alkali-activated materials (AAMs), inorganic polymers or geopolymers. AAMs have amorphous/semi-crystalline structure and their production is based on the dissolution of silicon and aluminum from raw materials with the use of NaOH or KOH and silicate solutions, followed by the formation of a reactive paste, which hardens after mild temperature curing. The factors that affect the properties of the produced AAMs are the content of the aluminosilicates, the particle size of the precursors and the synthesis conditions. Over the last 20 years, AAMs with excellent physical, chemical and thermal properties have been produced [
11,
12,
13,
14,
15,
16,
17].
The valorization of the wastes produced by the marble and the entire dimension-stone industry via alkali activation has attracted so far limited interest by the industry and the research community. Wang et al. [
18] investigated the potential of the addition of marble and granite waste as aggregate, for the production of blast furnace slag-based inorganic polymers. Based on their results, marble addition enhanced the bonding strength of the produced inorganic polymers compared to granite-based ones because of its higher dissolution obtained in the polymeric matric. The use of marble waste was also investigated along with other materials such as travertine and volcanic tuff for the production of AAMs under dry and wet curing conditions [
5]. In this study, the produced AAMs acquired similar compressive strength under both conditions for all the starting mixtures tested; however, the highest compressive strength (46 MPa) was obtained under dry curing at 20 °C. Colangelo et al. [
19] investigated the addition of marble waste as filler in a binder mixture consisting of metakaolin-based inorganic polymer, epoxy resin and expanded polystyrene, for the production of thermal insulating materials. The results showed that milled marble waste, with size smaller than 300 μm, resulted in the production of AAMs with improved mechanical properties and reduced drying shrinkage. Coppola et al. [
20] developed AAMs using as raw materials marble sludge and waste glass powder and investigated the effect of different curing conditions, using dry and humid environment, on their morphology and properties. The results showed that the dissolution of calcium ions from marble waste promoted the formation of (N)-C-S-H gel and thus the air-cured AAMs acquired high compressive strength, almost 45 MPa. Finally, Simão et al. [
21] used calcite-rich stone cutting waste, namely from marble and volcanic stone, for the production of metakaolin-based inorganic polymers with beneficial mechanical properties. In this study, the calcite-rich waste was used as an effective replacement of metakaolin, and resulted in the production of specimens which acquired compressive strength of 29 MPa, mainly due to the lower water requirements of the starting alkaline mixture.
The present study investigates the valorization potential of marble wastes, mixed with metakaolin, via alkali activation. The addition of metakaolin, due to its high Al and Si content, aims to substantially improve the poor inherent alkali activation potential of marble waste and enable the production of AAMs with beneficial properties.
2. Materials and Methods
Marble waste with an initial size of <32 mm was obtained from a marble processing plant located in the island of Crete (Greece), while metakaolin was produced by calcination of commercial kaolin (Fluka, Switzerland) at 750 °C for 2 h. Metakaolin was used in this study for the regulation of the ratio of SiO
2/Al
2O
3 in the starting mixture of the raw materials in order to facilitate the formation of aluminosilicate phases during alkali activation [
15]. Marble waste was first crushed in a Fritsch-type jaw crusher (Fritsch, Germany) to a particle size <8 mm and then pulverized using a Sepor-type rod mill (Sepor, Los Angeles, CA, USA) and a Bico-type pulverizer (Fritsch, Dresden, Germany) to obtain a fine-grained material. Three different particle size fractions were obtained, after 60 and 90 min of grinding in the rod mill and after 2 min of grinding in the pulverizer [
22], in order to evaluate the effect of the particle size on the compressive strength of the produced AAMs. The particle size analysis of marble waste and metakaolin was determined with the use of a laser particle size analyzer Mastersizer-S (Malvern Instruments, Malvern, UK) and is given in
Table 1. It is seen from these data that both materials were pulverized to a very fine size. For example, the d
50 (50% passing) of marble waste was <18 μm for all the three sample fractions obtained, while that of metakaolin was <9 μm.
The chemical composition of the raw materials was determined with the use of an X-ray fluorescence energy dispersive spectrometer (XRF-EDS) S2 Range type (Bruker, Karlsruhe, Germany) and is shown in
Table 2. As seen from these data, metakaolin may act as source of silicon and aluminum (55.9 wt. % SiO
2 and 38.5 wt. % Al
2O
3), elements which are required for alkali activation, whereas marble waste has, as anticipated, high content of CaO (53.1 wt. %).
The activating solution consisted of sodium silicate solution (Na
2O = 7.5–8.5 wt. %, SiO
2 = 25.5–28.5 wt. %, Merck, Germany) and NaOH solution (6–10 mol/L). The NaOH solution was produced by dissolving anhydrous pellets of NaOH in distilled water so that the desired molarity was obtained.
Table 3 shows the mixing proportions of the raw materials and reagents used as well as the SiO
2/Al
2O
3 molar ratio in the starting mixture, the Na
2O/SiO
2 and H
2O/Na
2O molar ratios in the activating solution and the overall liquid/solid (L/S) ratio. Specific proportions of marble waste and metakaolin were used in order to obtain suitable SiO
2/Al
2O
3 ratios in the precursors, based on the results of earlier studies carried out in our lab as well as by using literature data. [
11,
23]. The resulting paste was obtained after mechanical mixing of the raw materials with the activating solution for 10 min. The activating solution was prepared by mixing the solutions used and was left overnight prior to use. As seen in
Table 3, three different H
2O/Na
2O molar ratios were used in the experimental series in order to obtain a slurry with suitable fluidity and workability, while at the same time maintaining in all cases the lowest possible L/S ratio, which in our case varies slightly between 0.49 and 0.54 [
24]. It is mentioned that in other earlier studies carried out in our labs with the use of other wastes, e.g., slags and construction and demolition wastes, this ratio was much lower, i.e., 0.20–0.30 [
12,
17]. The produced paste was poured in steel cubic molds (5 × 5 × 5 cm³) and remained at room temperature for either 2 h (metakaolin-based (MK) specimens) or 4 h (marble waste-metakaolin (MW/MK) specimens) to facilitate sufficient initial setting and hardening. The specimens were then demolded, placed in sealed plastic bags and cured at 40, 60 and 90 °C for 24 h in a laboratory oven ON-02G (Jeio Tech, Billerica, MA, USA). After curing, the specimens were removed from the oven, aged at room temperature for 7 days and their compressive strength was determined using a Matest compression machine with dual range 500/15 kN (Matest, Bergamo, Italy). Measurements were carried out in triplicate and in the paper mean values are given. It is mentioned that deviation of measurements was marginal and did not exceed ±3.5 %.
The mineralogy, structure and morphology of selected AAMs was determined by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM). XRD analysis was performed using an X-ray diffractometer (D8-Advance, Bruker AXS, Bruker, Karlsruhe, Germany) with a Cu tube, a scanning range from 4° to 70° 2-theta (θ) with a step 0.02° and a measuring time of 0.2 s/step. Qualitative analysis was carried out using the DiffracPlus Software (Bruker AXS, Bruker, Karlsruhe, Germany) and the PDF database. Fourier Transform Infrared Spectroscopy (FTIR) was performed using KBr pellets with a PerkinElmer 1000 spectrometer (PerkinElmer, Akron, OH, USA). MIP analysis was performed using a Micromeritics AutoPore 9400 porosimeter (Micromeritics, Atlanta, GA, USA). Scanning Electron Microscopy (SEM) was carried out using a JEOL 6380LV (JEOL, Tokyo, Japan) microscope equipped with an Oxford INCA energy dispersive X-ray spectrometer (EDS) microanalysis system (Oxford Instruments, Abingdon, UK). Prior to SEM analysis, AAM specimens were stored in an anaerobic glove box and then embedded in an epoxy resin (EpoFix, Struers, Ballerup, Denmark). The epoxy-embedded samples were ground and polished using diamond abrasives. The apparent density and the water absorption of selected AAMs were also determined according to BS EN 1936:2006 [
25] and BS EN 13755:2008 [
26] standards, respectively.
In order to study their structural integrity, selected AAMs were (i) subjected to firing at 200, 400 and 600 °C for 2 h, in a laboratory furnace N-8L Selecta (JP Selecta, Abrera, Spain), (ii) immersed in deionized water and 1 mol/L (M) NaCl solution for 7 and 30 days and (iii) subjected to 7 and 15 freeze–thaw cycles, according to ASTM 1262-10 [
27]. AAMs were subjected to freezing at minus 17 °C for 4 h and thawing at room temperature for 18 h (one complete cycle). After each test, the weight loss and the compressive strength of the specimens were determined.
4. Conclusions
This study showed that marble waste can be valorized, with the addition of metakaolin, via alkali activation for the production of AAMs with beneficial physical and mechanical properties. The highest compressive strength was obtained by the AAMs produced when marble waste was mixed with metakaolin at a ratio of 0.3 and alkali activated under the optimum synthesis conditions, namely H2O/Na2O molar ratio 16 in the activating solution, curing temperature 40 °C, curing time 24 h and ageing time 7 days.
The addition of metakaolin as precursor during alkali activation was considered due to its known high alkali activation potential. The properties and the microstructure of MW/MK AAMs are mainly due to the synergistic action of the filler effect of calcium carbonate present in marble waste and the precipitation of some calcium bearing phases.
The produced AAMs showed a fairly good performance after immersion in 1 mol/L NaCl solution and especially in deionized water. More specifically, the AAMs after immersion in deionized water for 30 days retained an acceptable compressive strength value (24.3 MPa). On the other hand, when the AAMs subjected to firing at 200 °C they lost a substantial percentage of their compressive strength while after firing at temperatures higher than 400 °C their integrity was deteriorated fast.
In addition, the structural integrity of the produced AAMs was severely affected after subjection to freeze–thaw tests, and the compressive strength loss after 7 cycles reached almost the value of 40%. This is attributed to the high porosity (25.1%) and water absorption (14.2%) values of the initial specimens.
Additional studies are under way to improve alkali activation of marble waste and maximize its valorization potential. These studies explore the use of other precursors including mixtures of marble waste and other waste types with much higher Si and Al content which exhibit higher alkali activation potential, such as metallurgical slags or construction and demolition wastes, to improve the properties of the produced AAMs. Substitution of metakaolin with other precursors may result in a process with reduced cost and also enable the valorization of other waste streams that are produced worldwide in very large quantities. Also, the incorporation of natural fibers in the AAMs produced after alkali activation may result in specimens with beneficial properties. In any case, the selection of an additive that will improve the alkali activation potential of marble wastes depends on the desired properties of the produced specimens that define their final use as for example binder or construction element exhibiting among others suitable compressive strength, porosity or fire resistance.