Analysis of the Possibility of Using Cenospheres in ...

The topic of research included in this article was the evaluation of the influence of cenospheres on selected parameters of mortar cement. Samples were designed as CEM I 42.5 R Portland cement with the application of different additive amounts. In the experimental work, the consistency, compressive strength, and bending strength were tested after 28 and 56 days of maturation, and after heating temperatures of 20, 300, 500, and 700 °C. The compressive strength was tested on half beams (40 × 40 × 160 mm). Using the obtained results, the properties of the mortars were compared. The research confirmed the possibility of producing cenosphere-modified cement mortars. Cenospheres used in the preparation of cement mortar negatively affected the bending and compressive strength with increasing temperature (20, 300, 500, 700 °C) and increasing content of this additive (10, 20, 30%).

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1. Introduction

Coal is currently one of the world’s basic energy resources, and will remain so for the next few years [1]. In Poland, about 90% of electricity comes from coal. Its combustion in industrial processes is a major source of CO2, SO2, NOx and dust emissions. To maintain sustainable development, and thus a clean environment, increasingly stringent requirements on pollutant emissions standards have been imposed by the EU [2]. Emerging clean coal technology (CTW) processes are associated with the production of waste that requires management or disposal. Due to the dynamic development of CTW processes and technological progress, the waste in question is treated as a valuable product. Because furnaces operate at high temperatures (1000–1800 °C), the organic matter of the coal combusted is decomposed into molten slag of morphologically variable size and shape [3]. This variability leads to the formation of various types of particles in coal fly ash (CFA), such as plerospheres, ferrospheres, and cenospheres. illustrates cenospheres. A necessary condition for the formation of aluminosilicate microspheres is the presence of gases trapped inside the molten ash droplet [4,5].

The properties of aluminosilicate microspheres depend on the type and quality of coal, and the temperature and time in the combustion and cooling zones. Plerospheres are large microspheres with encapsulated particles filled with fine clusters of spherical particles along with minerals and gases [5], while ferrospheres are spherical particles rich in iron [6]. Cenospheres (CS), similar to fly ash, are mainly aluminosilicate spheres filled with gases from the coal combustion process: carbon dioxide and nitrogen with a Si/Al ratio from 1.5 to 3.5 [7]. Cenospheres in the presence of a large amount of mullite [8,9] show low thermal expansion, considerable thermal stability, and high creep and crack resistance [10]. Their quality depends primarily on the impurities generated in the basic process, i.e., the amount of slag, ash, and unburned mazut or coal [11], while their efficiency depends on cooling rate, melt viscosity, combustion temperature, and falling distance [12,13]. Cenospheres provide an excellent insulating and filling material, used in composite technologies (special rubber mixtures, refractory and insulating materials, ecological barriers, etc.) for the production of light, high strength concrete [14,15,16]. The important features of cenospheres are low bulk density of about 400 kg/m3 [17,18], real density of about 2.3 kg/m3 [19], low thermal conductivity at room temperature of 0.60 (W/(mK)), and a high melting point of 1200 °C [20,21,22]. The outer diameter of the CS can vary from 1 to 500 µm, with the majority of particles having a size of 60–200 μm, while their total fraction mass in CFA is about 1–2% [23]. It is known from the literature that the water absorption of porous CS is about 18 times greater than that of sand [4,23].

The mineralogical and chemical composition of cenospheres, depending on the concentration of unburned carbon, can change their color from brown to gray or black. Taking into account the ASTM C618 standard, depending on the raw coal, CS are divided into class C from lignite combustion or class F from anthracite combustion. The main role in the activity of CS is played by the combination of silicon, aluminum, and iron oxides, representing 70% of activity for class F and 50% for class C [24,25]. Considering the morphology, elemental composition, and viscosity of slag liquid, researchers have divided cenospheres of A-Si and Fe-Al.-Si alloys into two types: magnetic and non-magnetic [26]. Some CS contain large amounts of Fe exhibiting magnetic properties on the surface, while others are mainly composed of Al and Si [26].

Cenospheres are used in many fields, including building and construction materials [27], plastics [28], ceramics [29], construction [9], coatings [30], lightweight construction materials [31], polymer fillers [32], and energy storage devices [33,34]. Due to their resistance to high temperatures, they are used as a refractory material [35]. In the field of building materials, CS enable a decrease in concrete density while maintaining mechanical strength [36], making them an appropriate production material for lightweight concrete [37]. Additionally, the spherical CS particles act like miniature ball bearings in fresh concrete mix. When added to the mixture in an amount between 1 and 5%, they increase its workability [38].

Souza et al. [39] used CS in the production of lightweight concrete with high tensile strength. Concrete was produced with CS replacing aggregate in quantities of 33, 67 and 100% by volume. It was observed that the addition of CS improved the concrete’s specific strength. Likov et al. [39] found that the crystallization of hydrated products increased with increasing maturation temperature. Hanif et al. [40] developed a formula for a cement composite using aerogel and cenospheres. The obtained material exhibited decreased density. Properties such as particle size and bulk density accelerate the cement hydration process [41]. Satpathy et al. [27] stated that in order to obtain the assumed mechanical properties, it is necessary to determine the optimal percentage of aggregate replacement by cenospheres. Additional thermal and acoustic insulation can be provided by the introduction of spherical CS particles as an additive to concrete, with a positive effect on the insulation properties of plasters, coatings [42], and mortars used in construction [42]. An increase of 100% in the acoustic insulation coefficient was achieved by adding up to 40% CS volume to the cement matrix [43].

A popular use for cenospheres is the production of lightweight structures made of mortar and concrete [38]. The CS particles may be smaller than or similar in size to those of Portland cement and sand, and their shapes may reduce the size and number of open pores. In addition, at higher temperatures (80 °C) CS behave like pozzolana, absorbing free Ca(OH)2 from hydrated Portland cement by filling the pores between the cement surface and the CS particles with insoluble silicate hydrates [21,44,45,46]. The pozzolanic activity strengthens the interfacial bonds between the aggregate and the cement matrix. Despite their reactivity in an alkaline environment, they do not exhibit any harmful alkaline–silica reaction effects in mortars [47].

Baronins et al. [48] observed that Portland cement containing cenospheres up to 40% by volume showed a density reduction of 23% compared with cement mortars without CS, and reduced the water absorption capacity to 3%. Mortar with lower water absorption can positively affect the frost resistance and durability of the composite. In addition, it was found that increasing the CS concentration in the range of 0 to 40 percent volume increased the total volume of the open micrometric pores. In cold climates, this in turn may cause cracks in structures to form more quickly during the processes of freezing and thawing.

The main causes of degradation in cement composites at high temperatures are micro-cracks resulting from different properties of the cement matrix and thermal properties of the aggregate [49]. Arizmendi-Morquecho [50] conducted research into the properties of cenosphere-based protective coatings when exposed to high temperatures. The specified thermal expansion coefficient of the microsphere in the temperature range 20–1000 °C reached a value half that of concretes (6.13 × 10−6/°C), indicating better thermal compatibility of the cement matrix and the aggregate. The research allowed formulation of a hypothesis about the increased fire resistance of cement composites made with cenospheres. The use of a new lightweight cement composite with low thermal conductivity and high strength may be a promising direction in increasing fire resilience during construction. Cenosphere-based cement composites have been tested at room temperature [51,52,53] and at high temperatures [48,54,55,56,57].

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Szymkuć and Tokłowicz [58,59] conducted experimental tests on a light cement composite with the addition of cenospheres. After more than 90 days of maturation, the test samples were heated to temperatures of 105, 200, 400, 600, 700, 800, and 1000 °C. The tests demonstrated a significant improvement in compressive strength compared with traditional concrete. When heated to 60 °C, the samples retained 92% of the initial strength measured at a temperature of about 20 °C, and about 60% when heated to 1000 °C. The obtained material was characterized by low density (1450 kg/m3) and reduced thermal conductivity (0.60 W/(mK)) at a temperature of 20 °C. Additionally, numerical analysis was carried out on selected elements of the structure. The results showed an increase in the fire-resistance time for individual parts that were composed of lightweight cement composites that included cenospheres. This indicates the possibility of using lightweight composites in the construction and fire engineering of buildings.

Pursuant to Regulation (EC) No. 1272/2008 (CLP), cenospheres are not classified as hazardous in terms of toxicological and ecotoxic risks [59]. Additionally, they are not classified as hazardous waste according to Commission Decisions 2000/532/EC and 2001/118/EC. If possible, CS should be recovered for further use as part of recycling processes [60,61].

The concept of this research work included evaluating the possibility of using cenospheres as a cement substitute in the production of cement mortars. This research was also aimed at determining the influence of elevated temperatures on designed mortars’ mechanical properties. Annealing temperatures were chosen to correspond to the different phases of fire in rooms of different heights. The selection of temperatures for observation was determined by the points where structural changes of the mortars occurred. The aim of the experiment was to determine the effects of the applied mortar additives on bending strength under compression, under normal and initial thermal load conditions. The heating of the samples was carried out in accordance with the adopted temperature distribution curve, similar to the model curve used in the fire-resistance tests for the structural elements of buildings) [62,63,64].