Cenospheres are wastes from coal-fired power generation. These wastes can be incorporated into cementitious materials to produce lightweight materials. In addition, the incorporation of these wastes can be used to improve the thermal and acoustic properties of the cementitious material. However, the incorporation of these coal cells alters other properties. Therefore, it is critical to understand how the incorporation of coal-centered microbeads into cement composites affects various properties. This paper provides a critical review of the properties of cementitious materials containing hollow microbeads. Firstly, the origin, physical properties and chemical properties of hollow microbeads are briefly discussed. The effects of hollow microbeads on physical, mechanical and durability are also discussed.
Cenospheres are aluminosilicate spheres, a glass-like material filled with a mixture of oxygen, nitrogen and carbon dioxide. They range in size from 10 to 600 microns and therefore appear to the human eye like dust.
They can be extracted from subsurface sediments dating back 65 million years or, more commonly, from fly ash, a byproduct of coal-fired power stations. As a result, many power plants adjust their combustion parameters to obtain the desired quantity and quality of recoverable coal cores.
Although they are a product of fossil fuel resources, they are often considered more ecologically friendly chemical feedstocks because they can be used in waste products.
Generally speaking, where there is fly ash, there are cenospheres, which means that the world's largest producers are located in China, Russia, the United States and India. Nevertheless, there is still a large amount of fly ash available, with more than 50,000 coal-fired power plants worldwide producing over 1 billion tons of fly ash per year.
The primary method for recovering coal cells is the wet process, as they float due to their low density and hollow nature. They are then collected by suction or mechanically on the surface of a settling tank and then dried. Frictional electricity [through the use of charge separation] or centrifugal systems are also in use, depending on the amount to be processed, which then allows them to be separated from the heavier particles containing carbon and iron."
As a rough guide, the ceramic particles in fly ash have three types of structures. The first is hollow and is called hollow beads. The second is solid and is called duster, while the third type of particles are called plerospheres and are hollow particles with a small diameter containing larger dusters and cenospheres.
They vary in color, from white to dark gray, depending on the raw material used, the size, the combustion process used, the wall thickness, and the material from which they are made. Thus, the smaller hollow beads have a higher content of silicon dioxide (SiO2) and aluminum oxide (Al2O3) and a completely empty internal structure, while the larger hollow beads have a lower content of SiO2 and Al2O3 but a more elastic internal structure. Their chemical composition also gives them the alternative name of aluminosilicate microspheres.
The demand for cementitious materials with lower density and enhanced thermal properties has led to the development of lightweight cementitious materials. Compared to normal weight cementitious materials, lightweight cementitious materials are lighter in weight, save material, have better thermal properties and are cheaper. One method of producing lightweight cementitious materials is the use of lightweight aggregates/fillers. Several types of lightweight aggregates/fillers exist, such as expanded glass, pumice, etc., which can be incorporated into cementitious materials. However, the production of most of these aggregates/fillers is energy intensive and/or puts great pressure on the natural deposits of the material.
It is worth mentioning that cenospheres can also be used in applications other than binder materials. Cenospheres can be used in aerospace materials, plastics, rubber, thermal and sound insulation materials, etc. However, despite the many promising advantages of using hollow beads, one of the main disadvantages is their high cost. The use of higher content of coal cells leads to a significant increase in the total cost of the binder material. However, with the more rapid evolution and commercial/mass application of cenospheres expected in the coming years, the cost of hollow coils is expected to decrease in the near future.
Cenospheres have similar characteristics and origin to fly ash. However, the cenospheres are larger and lighter in size compared to fly ash. The use of cenospheres in cementitious materials has been found to significantly reduce weight, thermal conductivity, cost and environmental footprint. In addition, incorporation of coal cells into binder materials has been found to improve electrical properties, impermeability, compatibility and shrinkage resistance.
Since cenospheres are mainly used as fillers in cementitious materials, their physical properties have a significant impact on the corresponding properties of the materials. Due to the spherical shape of the cenospheres, they have a low surface area to volume ratio. It was also found that the spherical shape of the cenospheres produced higher shrinkage resistance and workability when incorporated into the cementitious material. The thickness of the coal cell shell is about 3% to 11% of the diameter of the coal cell.
Due to the hollow structure of the cenospheres, they have a low bulk density in the range of 200 kg/m3 to 1000 kg/m3. The porosity and water absorption of the coal cells are about 66% and 1%, respectively. The Mohs hardness of cenospheres is reported to be about 5, while the crushing strength is in the range of 1.6-3.2 MPa. Similar to fly ash, cenospheres are off-white in color. The thermal conductivity of cenospheres is reported to be about 0.07-0.11 W/mK.
Although cenospheres are used as fillers for cements, their chemical composition plays a huge role in their properties, especially when they are subjected to long periods and high temperatures and alkaline environments. All cenospheres are mainly composed of aluminates and silicates. Regardless of the origin of the cenospheres, the combined silicate and aluminate composition is greater than 80%.
The glassy phase of the cenospheres consists mainly of aluminosilicates, while the crystalline phase consists of quartz and mullite with traces of magnetite, calcite, potassium and hematite. Coal cells have also been reported to exhibit volcanic ash reactions, which increase at higher temperatures as the size of the coal cell becomes smaller. Due to the chemical nature of cenospheres, they can also be used to produce alternative binders, such as alkali-activated binders.
1. The use of cenospheres as an internal curing agent should be extensively investigated to reduce the shrinkage of cementitious materials due to their ability to absorb large amounts of water.
2. It has been found that incorporating coal cells into cementitious materials can improve the thermal properties. Future research on the use of hollow cenospheres in cementitious materials could focus on further enhancing the thermal properties of the materials by incorporating phase change materials into the hollow beads prior to incorporation into cementitious materials.
3. The performance of cementitious materials made by incorporating cenospheres and using alternative binders (e.g. alkali-activated binders) should be investigated. Since the aluminate and silicate monomers in the hollow beads may be activated by alkali, the use of an alkali-activated binder next to the hollow beads may provide higher material properties.
4. Due to the limited research on the durability properties of cementitious materials incorporated with cenospheres, it is recommended that more research be conducted in this area. Performance evaluation of cementitious materials incorporating cenospheres under extreme conditions (e.g., freeze-thaw and wet-dry cycles) may provide a better understanding of the effect of cenospheres on material resistance.
5. Due to the degraded mechanical and durability properties of cementitious materials incorporated with cenospheres, more research and development should be conducted to develop sustainable and innovative methods to improve the performance of cementitious materials incorporated with cenospheres. Such methods may include modification/treatment of coal cells, use of supplementary cementitious materials, and nanomaterials, etc.
With the increasing use of cenospheres in cementitious materials and other applications, it is critical to conduct a variety of research and development focused on producing and creating a cheaper supply of cenospheres. The reduction in cost of coal cells will drive the increased use of coal cells in cementitious materials and will lead to the production of economical and sustainable cementitious materials.