**1. Introduction**

Cement is widely used as a basic component of concrete. Due to the rapid construction development, the demand for cement and natural aggregates has, exceptionally, been increased. More specifically, in 2014, about 40 billion tons of aggregates and 4 billion tons of cement were required for constructions all over the world [1,2]. As a result, a great amount of carbon dioxide (CO2) is released in the air. This is the reason why several researchers, i.e., [3,4], have turned their attention to producing environmentally friendly applications, and at the same time, they have reduced CO2 emissions during their production. The reaction of cement and water produces volume shrinkage, which may lead to structural cracking of cement-based materials [5]. This is detrimental not only for the mechanical properties and durability of cement-based materials, but it may also lead to shorter service life of constructions, resulting in a great loss of natural resources. The problem of shrinkage of cement is becoming more and more serious, as by increasing the concrete strength, the amount of cement per unit volume of concrete increases, too [6,7]. Cementitious concrete is the most-used manmade material among all, and comprises a mixture of mortar, aggregates, and water [2,4,8]. The main component of concrete is the material which binds the aggregate particles together, commonly comprising a mixture of cement and water [4,8]. Concrete structures can be described as a three-phase system composed of hardened cement paste, aggregate, and the interface between aggregate particles and cement paste [8,9].

**Citation:** Giannakopoulou, P.P.; Rogkala, A.; Lampropoulou, P.; Kalpogiannaki, M.; Petrounias, P. Evaluation of Cement Performance Using Industrial Byproducts Such as Nano MgO and Fly Ash from Greece. *Appl. Sci.* **2021**, *11*, 11601. https:// doi.org/10.3390/app112411601

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 22 October 2021 Accepted: 4 December 2021 Published: 7 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

One of the effective ways to reduce cracking is by adding expansion agent in cement concrete or by using the expansion generated during the hydration process of expansion agent to compensate the shrinkage of cement concrete [5,6]. Nowadays, the expansion agents mainly used in several applications primarily include sulfoaluminate-type, CaOtype, and MgO-type expansion agents. In comparison with the first two kinds of expansion agents, MgO expansion agent (nano MgO) displays a lot of advantages, including less water requirement for hydration, stable hydration product Mg(OH)2, adjustable design of expansion process, and so on, in order to widely to be used in modern concrete [10]. Since the 1970s, numerous researchers have begun to study MgO expansion agent (MEA), which has been applied to a wide range of buildings, such as dams. The expansion produced by MEA hydration is used to compensate the temperature drop shrinkage and dry shrinkage of mass concrete, effectively improving the crack resistance and durability of the structure [11,12].

Cements with high MgO content have gained more and more popularity in the last decade, perhaps due to the augmented concern about climate change, with the intention and need of reducing the CO2 emissions regarding the production of conventional Portland cements. Some authors believe that it is possible to produce such type of cements with a high MgO content and reduced CO2 emissions [13]. In recent decades, the major motivation for the development and uptake of MgO-based cements has been driven from an environmental standpoint. The lower temperatures required for the production of MgO compared to the conversion of CaCO3 to PC and the energy savings associated with this reduced temperature have led researchers to envision MgO-based cements as being central to the future of ecofriendly cement production. Equally, the ability of MgO to absorb CO2 from the atmosphere to form a range of carbonates and hydroxycarbonates lends itself well to the discussion of "carbon-neutral" cements, which could potentially absorb close to as much CO2 during their service life as was emitted during their manufacture. These two interconnected aspects have led to a recent explosion in interest, both academic and commercial, in the area of MgO-based cements.

Magnesium is the eighth most abundant element in the Earth's crust, at ~2.3% by weight, present in a range of rock formations such as dolomite, magnesite, and silicate. Magnesium is also the third-most abundant element in solution in seawater, with concentrations of ~1300 ppm [5]. The mineral magnesite comprises a widely used source of magnesium oxide (MgO), because magnesium is considered a critical element by the EU. However, alternative sources, such as several Mg-rich silicates, may become more relevant in the near future. The transition of magnesite to magnesia has been achieved by heat treatment at temperatures above 600 ◦C. This transition from carbonate to oxide by release of carbon dioxide (CO2) initially produces a porous microstructure with low bulk density. Moreover, this material is characterized as a strong reactive due to its low magnesia particle sizes with high surface areas which can be retained if not heated further. Such high surface area variants of magnesia are known as "active magnesia" in cementrelated research. Exposing this porous form to higher temperatures (>1200 ◦C) solidifies the structure by a sintering mechanism and drastically reduces the reactive surface area as well as the affected sites that promote wetting and dissociation of magnesia in acid phosphate solutions. Magnesia-based cements, by definition, use MgO as a building block rather than the CaO which represents more than 60% of the elemental composition of PC. Due to the substantially different chemistry of MgO compared to that of CaO, one cannot simply change the feedstock for conventional Ca-based cements to produce a directly corresponding material using the same infrastructure. Comparing the respective (MgO, CaO)–Al2O3–SiO2 ternary phase diagrams, vast differences in chemistry and phase formation are shown. More specifically, no magnesium silicate phases are formed in high temperatures that have hydraulic properties akin to those formed in the calcium-rich region of the CaO–SiO2–Al2O3 system: Ca3SiO5, Ca2SiO4, and Ca3Al2O6 are critical hydraulic phases in PC, but have no magnesian analogues.

Currently, the MEA used in the market is mainly produced by the calcining of magnesite. However, with the continuous mining of mineral resources, the problems of resource exhaustion and environmental pollution are becoming more and more threatening. Following governments' instructions regarding the mining of nonrenewable mineral resources, magnesite resources are decreasing [14] while, at the same time, a large amount of magnesite is discarded annually. These gradually accumulated large amounts of magnesite not only occupy cultivated land but also cause waste of resources. Magnesite is a raw material used in different industrial applications. It is used mainly after heat treatment for the production of caustic magnesia and dead burned magnesia, whilst untreated magnesite has a few industrial applications too (e.g., in the production of fertilizers, electrodes, and environmental protection). The main magnesite deposits in Greece appear in Chalkidiki and Evoia Island. Regarding the case of Evoia's magnesite deposits, they have been started to be exploited since 1893 mainly for the production of magnesia-rich raw materials and refractory final products. In this area, the proven, probable, and possible reserves have been approximately estimated as 35, 45, and 60 million tons, respectively [15]. The concession areas cover ~404 km2 while all mines are within a 14 km radius around the refractory industry plants. The last is 2 km away from the loading port. According to the current activities, the annual capacity is 120,000 MT of caustic magnesia and dead burned magnesia, while above 250,000 tons of stockpile waste have been disposed of in the area during last year's activities. They consist of the fine ore (−40 mm) which is rejected before the beneficiation processing and specifically before the hand-sorting stage of magnesite. This tailing contains significant amounts of magnesite, and in this work, the feasibility of its utilization as MEA is studied. In the Greek market, materials of similar quality are found, but they are produced from natural mineral raw materials and not from byproducts.

The real contribution of such initiatives does not result in a notable decrease in negative externalities. This fact can be assigned to a limited acceptance of "greener" building materials [16]. The relatively poor adoption of modified binders represents a barrier towards a more sustainable construction industry [17]. However, the change of this paradigm needs to be completed not only by the technical parameters but also must be combined with other scientific disciplines involved in the sustainability principles [18]. In a nutshell, new design and development materials must go hand in hand with corresponding leadership, convincing communication, and complex assessment of ecofriendly materials to overcome major barriers in the conservative building industry. Numerous researchers who study green construction materials and applications often use micropetrographic analytical methods [19].

The aim of this study is to evaluate the impact of MEA combined with fly ash on final physicomechanical performance of the produced mortar, providing a new way to solve the environmental pollution of magnesite tailings and fly ashes. For this scope, magnesite tailings and fly ash from Greece were used where the influence of different percentages of MEA combined with fly ash on the physicomechanical performance of the produced cements were studied.

#### **2. Geological Setting of Magnesite Tailings**

Magnesite tailings are located in central and northern Evoia and are occupied by Jurassic ophiolitic rocks such as harzburgites and basalts (Figure 1). These rocks overlay a mélange (the Pagondas complex), comprising Upper Triassic to Jurassic sedimentary and volcanic rocks, which was developed as an accretionary prism [20]. The entire sequence of the ophiolitic rocks and the mélange is thrust on the Pelagonian carbonate platform. The ultramafic rocks are covered by Pliocene terrigenous and minor marine and lacustrine sediments, and Quaternary deposits [21]. The overthrust of the Evoia ophiolite is structurally overlain by a dismembered metamorphic sole which constitutes evidence for a late Upper Jurassic emplacement age [22,23]. Magnesite is presented as cryptocrystalline, mostly nodular, and appears as open space fillings along joints of ultramafic rocks, forming stockworks, as veins controlled by brittle faults, indicating that magnesite was deposited

during opening of the veins in an extensional geodynamic regime, and as nodular forms that resulted from replacement of ultramafic rock. Those nodules are mainly centimetersized and contain relics of chromite. Magnesite also predates deposition of dolomite and silica polymorphs where a vertical mineral zonation is evident: magnesite modal percentages decrease towards the surface, whereas dolomite and quartz gradually become more abundant. Moreover, vuggy silica textures are presented in the uppermost parts. Brecciation of early-formed magnesite and cementation by magnesite deposited at a later stage is also observed. Both the deposits of stockwork type of mineralization and the main veins have not been deformed. Significant displacements by younger faults affecting the deposits are not observed.
