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.