Hollow Fly Ash Particles Research Articles

Light-weight and high thermal insulation building material; a comparative study between cenosphere & fly ash

The increasing demand for light-weight sustainable materials have led researchers to find the alternatives to the available natural resources used as construction materials. Cenospheres are hollow fly ash particles which are obtained from fly ash waste by burning coal in thermal plants. Cenospheres have been widely used as filler materials in various industries like aircraft, automation etc, due to its light-weight and high thermal resistivity, However, the use of cenospheres in construction industry has been limited, Therefore, in this research an attempt has been made to replace OPC with cenosphere particles. The cement was replaced from 0 to 30% to produce a light-weight and high thermal insulation concrete. This concrete was studied for the properties like fresh, heat of hydration mechanical, durability and thermal conductivity. It was observed that the heat of hydration significantly reduced by the use of cenosphere particles. The density of the concrete was also reduced with the increment in the content of cenosphere in concrete, however with the decrease in the density the mechanical strength of concrete also reduced but was within the design strength of concrete. It was also observed that the cenosphere contribute to the late gain in the strength of the concrete similar to the fly ash concrete. The thermal conductivity of the concrete reduced by more than 50% with the incorporation of the cenosphere in the concrete. From this research it can be concluded that cenosphere can be used as a replacement to cement in the concrete to produce a light-weight and high thermal insulated concrete.

Cenosphere-based PCM microcapsules with bio-inspired coating for thermal energy storage in cementitious materials

Phase change materials (PCMs) provide passive storage of thermal energy in buildings to flatten heating and cooling load profiles and minimize peak energy demands. After microencapsulated into a protective shell, they can be directly added to cementitious materials to add thermal energy storage capacity to the material. However, existing studies show that the strength of the produced materials can be drastically reduced by the added PCM microcapsules. To overcome this drawback, this study develops a novel PCM microcapsule using the cenosphere as the protective shell, together with a bio-inspired dopamine coating. As hollow fly ash particles, cenosphere shells are much harder and stronger than commonly used polymer based protective shells. While the strong binding ability of dopamine to diverse surfaces through covalent and non-covalent interactions provides better bonding between the PCM microcapsule and the surrounding cement paste. For these reasons, the strength of the produced cement mortar can be greatly improved by this microcapsule. An experimental study shows that a 35% increase in compressive strength has been achieved by adding 10 wt% of the new PCM microcapsule into the cement mortar. Microstructure examination and hydration products analysis revealed that the dopamine coating facilitates the hydration of the cement and produces a denser and stronger interface between the microcapsule and the cement paste. As a result, a novel PCM microcapsule is developed which not only integrates latent heat storage capacity to the cement mortar but also enhance the compressive strength of the mortar, which cannot be achieved by any other PCM microcapsules.

A Brief Review of Fly Ash as Reinforcement for Composites with Improved Mechanical and Tribological Properties

Fly ash (FA) is one of the particulate wastes generated during combustion of coal in thermal power plants. Around 110 million tons of FA is generated in the USA every year, and 60% of it is deposited in landfills. Utilization of FA can create value for this waste material and also help the environment. FA is essentially a mixture of metal oxides that can be used as a filler reinforcement in metal and polymer composites. FA as a filler material reduces the amount of metals and polymer, reducing embodied energy. Hollow FA particles, called cenospheres, can provide the advantage of low density in composites as well as higher hardness and strength. FA also reduces the coefficient of thermal expansion. This article provides a brief review to capture the state of the art on the mechanical and tribological behavior of composites reinforced with FA to identify the possible benefits of using this waste material. The corrosion performance of metal matrix FA composites is also explored. Future perspectives in this field are discussed based on the potential applications of FA-filled composites.

Frost resistance of concrete with high contents of fly ash - A study on how hollow fly ash particles distort the air void analysis

Cenospheres are hollow fly ash particles. When performing air void analysis on a contrast enhanced plane section, air inclusions in cenospheres are counted as air voids. In the present study, air void analyses for air entrained concrete mixtures with fly ash (up to 50% of binder mass) were corrected based on chord counting for non-air entrained paste samples with various contents of fly ash. The correction only lead to a small reduction of the total air content, but it increased the spacing factor up to 25%. The concrete mixtures were also exposed to salt frost scaling testing. The amounts of scaling were unacceptable for several mixtures with high dosages of fly ash. Inferior strength or inadequate air void structure could not explain this. Additional testing pointed to that chemical surface degradation aggravated the physical frost attack for concrete mixtures with high contents of fly ash.

Converting hollow fly ash into admixture carrier for concrete

This study proposes a low-cost method to convert cenospheres into an admixture carrier for concrete manufacturing. Cenospheres are hollow fly ash particles generated in coal burning power plants, having an aluminosilicate shell with high strength and stiffness. The large inner volume of the cenospheres can be used to carry and release admixtures in concrete. However, directly using cenospheres as the carrier is not possible because the inner pore is not accessible to the admixture. To address this problem, chemical etching is employed to produce perforating holes through the shell. Liquid admixtures can be easily loaded into and later released from the produced perforated cenospheres (PCs). A series of characterization tests were carried out to understand the working mechanism of the chemical etching method and its effect on the properties of the PCs. It has been found that chemical etching dissolved a small amount of amorphous materials from the cenosphere shell. As a result, the cenoshpere shell was weakened, as indicated by the reduction of the bulk crushing strength of the PCs. Nevertheless, the bulk crushing strength of all produced PCs are sufficient to guarantee that they can survive the mixing and initial stress in fresh concrete. To experimentally confirm the feasibility of using the PCs as an admixture carrier for concrete, PCs were added into cement mortar as the internal water carrier, which successfully mitigated the autogeneous shrinkage in a low water-to-cement ratio concrete. Scanning electron microscopy analysis of the cement mortar confirms that PCs not only survived the mixing of concrete, but also were dispersed and bonded well to the cement mortar. This study suggests that PCs may provide a versatile tool to integrate various admixtures in concrete.

Integrating phase change materials into concrete through microencapsulation using cenospheres

Phase change materials (PCMs) can enhance the building energy efficiency through thermal energy storage and thermal regulation. Microencapsulated PCMs (MEPCMs) provide a better utilization of PCMs with building materials. This study proposes a novel method to encapsulate PCMs into cenospheres which are hollow fly ash particles generated in coal burning power plants with size ranging from a few micrometers to hundreds of micrometers. The shell of the cenosphere inherently has some small pores which are sealed by a thin layer of glass-crystalline film. By removing this film through chemical etching, these holes can be exposed, providing paths for PCMs moving into the internal void of cenospheres. A thin layer of silica is coated on the PCM loaded cenospheres to prevent the possible leakage of liquid PCMs. The produced PCM microcapsules are referred to as CenoPCM, which can be directly added into traditional construction and building materials such as concrete to produce thermally active concrete. Prototype thermally active cement mortar integrated with the produced CenoPCM capsules have also been manufactured and characterized for its mechanical and microstructural properties. The characterizations showed that there was only minor reduction in strength and the mortar remained strong enough for building application. From this work, it is found that the produced CenoPCM capsules have great potential to be added into construction materials for reducing energy consumptions in buildings.

Effect of fly ash on the pore structure of cement paste under a curing period of 3 years

The service life and durability of concrete structures strongly depend on the transport properties of the concrete. These transport properties are highly related to their microstructure, especially the pore structure of the hydrated cement paste. In this paper the microstructure development of cement paste and cement paste blended with fly ash is investigated, from solid phase to pore phase, at a long-term curing period up to 3years. The solid phases are observed by environmental scanning electron microscope. The pore structure of blended cement paste is determined by mercury intrusion porosimetry. The results indicate that the addition of fly ash increases the total porosity of cement paste, not only at early ages, even at curing age of 3years. The voids in hollow fly ash particles act as ink-bottle pores with progress of the pozzolanic reaction of fly ash, resulting in the increase of the total porosity of blended cement paste.

Internal curing of high performance concrete using cenospheres

This study explores a novel internal curing agent, perforated cenospheres. Cenospheres are hollow fly ash particles produced from coal burning power plants. The shell of the cenospheres is inherently porous that is sealed by a thin layer of glass-crystalline film. By removing this film through chemical etching, the pores on the shell can be exposed, perforating the cenospheres and providing paths for water propagating into the internal volume of cenospheres. The perforated cenosphere were found to have water absorption as high as 180wt%. The loaded water can be readily released from the cenospheres under high relative humidity (95%). When incorporating saturated cenospheres into cement mortar for internal curing, the autogenous shrinkage of the mortar was almost eliminated. The internal curing also improved the compressive strength of the cement mortar. All these results suggest that perforated cenospheres can be used as an efficient internal curing agent for HPC.

Analysis of AC Breakdown Characteristics of Fly Ash Cenosphere/Epoxy Composite

In this study, AC breakdown properties of cenosphere/epoxy composite are investigated. The thermal properties of the composite are also discussed. The cenosphere/epoxy composites were prepared by epoxy resin filled with 0~40 wt% of hollow fly ash particles (cenospheres). The AC breakdown strength of the composites was measured at 25~125℃. The permittivity, glass transition temperature, and coefficient of thermal linear expansion of the composites were also measured. The experimental results of the cenosphere/epoxy composite were compared with those of the silica/epoxy composite which are widely used as insulating materials in power apparatus. The cenosphere/epoxy composite showed the similar or better performance than silica/epoxy composite in terms of breakdown strength, glass transition temperature, and coefficient of thermal expansion.

Three dimensional microstructure and image-based simulation of a fly ash/al syntactic foam using X-ray micro-computed tomography

An aluminum matrix syntactic foam, incorporated with hollow-structured fly ash particles, was fabricated by pressure infiltration technique. X-ray micro-computed tomography was used to characterize its heterogeneous microstructure three dimensionally (3D). The quantification of some microstructure features, such as content and size distribution of hollow fly ash particles, was acquired in 3D. The tomographic data were exploited as a rapid method to generate a microstructurally accurate and robust 3D meshed model. The thermal transport behavior has been modeled using a commercial finite-element code to conduct steady state analyses. Simulation of the thermal conductivity showed good correlation with experimental result.

Compressive and ultrasonic properties of polyester/fly ash composites

The addition of hollow fillers having appropriate mechanical properties can decrease the density of the resulting composite, called syntactic foams, while concurrently improving its mechanical properties. In this study, hollow fly ash particles, called cenospheres, are used as fillers in polyester matrix material. Cenospheres are a waste by-product of coal combustion and, as such, are available at very low cost. In this study, the composites were synthesized by settling cenospheres in a glass tube filled with liquid polyester resin and subsequently curing the resin. This process resulted in a functionally graded structure containing a gradient in the cenosphere volume fraction along the sample height. Uniform radial sections were cut from each composite and were characterized to observe the relationship between cenosphere volume fraction and compressive properties of the composite. The composite was also tested using ultrasonic non-destructive evaluation method. Results show that the modulus of the composites increases with increasing cenosphere volume fraction. The modulus of composites containing more than 4.9 vol% cenosphere was found to be higher than the matrix resin. In general, the modulus of composites increased from 1.33 to 2.1 GPa for composites containing from 4.9–29.5 vol% cenospheres. The specific strength of the composite was found to be as high as 2.03 MPa/(kg/m3) compared to 0.96 MPa/(kg/m3) for the neat resin. Numerous defects present in fly ash particles caused a reduction in the strength of the composite. However, the reduction in the strength was found to be only up to 22%. Increase of over 110% in the specific modulus and only a slight decrease in the strength indicates the possibility of significant saving of weight in the structures using polyester/fly ash syntactic foams.

Thermal Expansion of Aluminum–Fly Ash Cenosphere Composites Synthesized by Pressure Infiltration Technique

The coefficients of thermal expansion (CTEs) of commercially available pure aluminum and aluminum alloy composites containing hollow fly ash particles (cenospheres) of average size 125 mm are measured using a dilatometer. Three types of composites are made using the pressure infiltration technique at applied pressures and infiltration times of 35 kPa for 3 min, 35 kPa for 7 min, and 62 kPa for 7 min. The volume fractions of the fly ash cenospheres in the composites are around 65%. The CTE of the composites is measured to be in the range of 13.1×10-6-11×10-6/°C, which is lower than that of pure aluminum (25.3×10-6/°C). The infiltration processing conditions are found to influence the CTE of the composites. A higher applied pressure and a longer infiltration time lead to a lower CTE. The theoretical value of the CTE of fly ash cenospheres is estimated to be 6.1×10-6/°C.

Compressive characteristics of A356/fly ash cenosphere composites synthesized by pressure infiltration technique

Loose beds of hollow fly ash particles (cenospheres) were pressure infiltrated with A356 alloy melt to fabricate metal-matrix syntactic foam, using applied pressure up to 275 kPa. The volume fractions of cenospheres in the composites were in the range of 20–65%. The processing variables included melt temperature, gas pressure and particles size of fly ash. The effect of these processing variables on the microstructure and compressive properties of the synthesized composites is characterized. Compressive tests performed on these metal-matrix composites containing different volume fractions of hollow fly ash particles showed that their yield stress, Young's modulus, and plateau stress increase with an increase in the density. Variations in the compressive properties of the composites in the present study were compared with other foam materials.

Age-hardening characteristics of aluminum alloy-hollow fly ash composites

The aging characteristics of aluminum alloy A356 and an aluminum alloy A356 containing hollow spherical fly ash particles were studied using optical microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, hardness tests, and compressive tests. The variation of hardness and compressive strength as a function of aging time for the composite have been reported. Since the density of the composite is lower than that of the base alloy due to the presence of hollow particles, the composites have a higher specific strength and specific hardness compared to the matrix. Even though the hardness of the as-cast composite was higher than that of the base alloy, no significant change in the aging kinetics was observed, due to the presence of spherical fly ash particles in the matrix. Aging times of the order of 104 to 105 seconds were required to reach the peak hardness (92 HRF) and compressive strength (376 MPa) in both the A356-5 wt pct fly ash composite and the matrix alloy. The possible effects of shape and hollowness of particles, the interface between the matrix and the particles, the low modulus of the particles, and the microcracks formed on the surface of hollow fly ash particles on the kinetics of the age hardening of aluminum alloy A356 are discussed.